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Cardiac biomarkers, especially high-sensitivity cardiac troponin C or I (hs-cTnC or hs-cTnI, respectively), are vital for diagnosing acute myocardial infarction (AMI). Despite the specificity of hs-cTn as a biomarker, the creatine kinase-myocardial band (CK-MB) is commonly used alongside hs-cTn in emergency departments (EDs). We analyzed 23,771 simultaneous hs-cTn (hs-cTnT or hs-cTnI) and CK-MB requests for 17,185 patients in tertiary hospital ED in 2022. The objective of this study was to assess their practical value in diagnosing AMI in real-world settings. Among all 17,185 patients tested, 98.0% underwent hs-cTnT and CK-MB tests, and substantially fewer underwent hs-cTnI testing. We observed concordance between the initial hs-cTn and CK-MB results in 71.3% of patients. Of 131 AMI cases, 57 were positive for both biomarkers, 63 for hs-cTn only, and none for CK-MB alone. CK-MB positivity was often found in the absence of AMI. Discrepancies between the hs-cTnT and hs-cTnI results occurred in 30.0% of patients. Indiscriminate CK-MB testing for diagnosing AMI in EDs should be reconsidered. Efficient use of CK-MB is important for reducing costs and ensuring optimal patient care.

Background: Impaired left ventricular relaxation, high filling pressures, and dysregulation of Ca 2+ homeostasis are common findings contributing to diastolic dysfunction in hypertrophic cardiomyopathy (HCM). Studies have shown that impaired relaxation is an early observation in the sarcomere-gene-positive preclinical HCM cohort, which suggests the potential involvement of myofilament regulators in relaxation. A molecular-level understanding of mechanism(s) at the level of the myofilament is lacking. We hypothesized that mutation-specific, allosterically mediated, changes to the cTnC (cardiac troponin C)-cTnI (cardiac troponin I) interface can account for the development of early-onset diastolic dysfunction via decreased PKA accessibility to cTnI., Methods: HCM mutations R92L-cTnT (cardiac troponin T; Arg92Leu) and Δ160E-cTnT (Glu160 deletion) were studied in vivo, in vitro, and in silico via 2-dimensional echocardiography, Western blotting, ex vivo hemodynamics, stopped-flow kinetics, time-resolved fluorescence resonance energy transfer, and molecular dynamics simulations., Results: The HCM-causative mutations R92L-cTnT and Δ160E-cTnT result in different time-of-onset diastolic dysfunction. R92L-cTnT demonstrated early-onset diastolic dysfunction accompanied by a localized decrease in phosphorylation of cTnI. Constitutive phosphorylation of cTnI (cTnI-D 23 D 24 ) was sufficient to recover diastolic function to non-Tg levels only for R92L-cTnT. Mutation-specific changes in Ca 2+ dissociation rates associated with R92L-cTnT reconstituted with cTnI-D 23 D 24 led us to investigate potential involvement of structural changes in the cTnC-cTnI interface as an explanation for these observations. We probed the interface via time-resolved fluorescence resonance energy transfer revealing a repositioning of the N-terminus of cTnI, closer to cTnC, and concomitant decreases in distance distributions at sites flanking the PKA consensus sequence. Implementing time-resolved fluorescence resonance energy transfer distances as constraints into our atomistic model identified additional electrostatic interactions at the consensus sequence., Conclusions: These data show that the early diastolic dysfunction observed in a subset of HCM is attributable to allosterically mediated structural changes at the cTnC-cTnI interface that impair accessibility of PKA, thereby blunting β-adrenergic responsiveness and identifying a potential molecular target for therapeutic intervention., Competing Interests: None.

Background: Cardiac troponin (cTn) T and cTnI are considered cardiac specific and equivalent in the diagnosis of acute myocardial infarction. Previous studies suggested rare skeletal myopathies as a noncardiac source of cTnT. We aimed to confirm the reliability/cardiac specificity of cTnT in patients with various skeletal muscle disorders (SMDs)., Methods: We prospectively enrolled patients presenting with muscular complaints (≥2 weeks) for elective evaluation in 4 hospitals in 2 countries. After a cardiac workup, patients were adjudicated into 3 predefined cardiac disease categories. Concentrations of cTnT/I and resulting cTnT/I mismatches were assessed with high-sensitivity (hs-) cTnT (hs-cTnT-Elecsys) and 3 hs-cTnI assays (hs-cTnI-Architect, hs-cTnI-Access, hs-cTnI-Vista) and compared with those of control subjects without SMD presenting with adjudicated noncardiac chest pain to the emergency department (n=3508; mean age, 55 years; 37% female). In patients with available skeletal muscle biopsies, TNNT/I1-3 mRNA differential gene expression was compared with biopsies obtained in control subjects without SMD., Results: Among 211 patients (mean age, 57 years; 42% female), 108 (51%) were adjudicated to having no cardiac disease, 44 (21%) to having mild disease, and 59 (28%) to having severe cardiac disease. hs-cTnT/I concentrations significantly increased from patients with no to those with mild and severe cardiac disease for all assays (all P <0.001). hs-cTnT-Elecsys concentrations were significantly higher in patients with SMD versus control subjects (median, 16 ng/L [interquartile range (IQR), 7-32.5 ng/L] versus 5 ng/L [IQR, 3-9 ng/L]; P <0.001), whereas hs-cTnI concentrations were mostly similar (hs-cTnI-Architect, 2.5 ng/L [IQR, 1.2-6.2 ng/L] versus 2.9 ng/L [IQR, 1.8-5.0 ng/L]; hs-cTnI-Access, 3.3 ng/L [IQR, 2.4-6.1 ng/L] versus 2.7 ng/L [IQR, 1.6-5.0 ng/L]; and hs-cTnI-Vista, 7.4 ng/L [IQR, 5.2-13.4 ng/L] versus 7.5 ng/L [IQR, 6-10 ng/L]). hs-cTnT-Elecsys concentrations were above the upper limit of normal in 55% of patients with SMD versus 13% of control subjects ( P <0.01). mRNA analyses in skeletal muscle biopsies (n=33), mostly (n=24) from individuals with noninflammatory myopathy and myositis, showed 8-fold upregulation of TNNT2 , encoding cTnT (but none for TNNI3 , encoding cTnI) versus control subjects (n=16, P Wald <0.001); the expression correlated with pathological disease activity ( R =0.59, P t-statistic <0.001) and circulating hs-cTnT concentrations ( R =0.26, P t-statistic =0.031)., Conclusions: In patients with active chronic SMD, elevations in cTnT concentrations are common and not attributable to cardiac disease in the majority. This was not observed for cTnI and may be explained in part by re-expression of cTnT in skeletal muscle., Registration: URL: https://www., Clinicaltrials: gov; Unique identifier: NCT03660969.

To compare the performance of high-sensitivity cardiac troponin I and T (hs-cTnI; hs-cTnT) in diagnosing obstructive coronary artery disease (CAD 50 ) in patients with suspected chronic coronary syndrome (CCS). A total of 706 patients with suspected CCS, referred for Coronary Computed Tomography Angiography, were included. cTn concentrations were measured using the Singulex hs-cTnI (limit of detection [LoD] 0.08 ng/L) and Roche hs-cTnT (LoD 3 ng/L) assays. Obstructive coronary artery disease (CAD 50 ) was defined as ≥ 50% coronary stenosis. Cardiovascular risk was determined by the NORRISK2-score. Median age of the patients was 65 (range 28-87) years, 35% were women. All patients had hs-cTnI concentrations above the LoD (median 1.9 [Q1-3 1.2-3.6] ng/L), 72% had hs-cTnT above the LoD (median 5 [Q1-3 2-11] ng/L). There was a graded relationship between hs-cTn concentrations and coronary artery calcium. Only hs-cTnI remained associated with CAD 50 in adjusted analyses (OR 1.20 95% Confidence Interval [1.05-1.38]), p = 0.009). The C-statistics for hs-cTnI and hs-cTnT were 0.65 (95% CI [0.60-0.69]) and 0.60 (0.56-0.64). The highest specificity and negative predictive values for CAD 50 were in the lowest NORRISK2-tertile. hs-cTn concentrations provide diagnostic information in patients with suspected CCS, with superior performance of hs-cTnI compared to hs-cTnT in regard to CAD 50 . The diagnostic performance appeared best in those with low cardiovascular risk., (© 2022. The Author(s).)

Background and Aims: Current methods (highly sensitive and ultra-sensitive) of cardiospecific troponins detection have enabled early diagnosis of myocardial infarction (MI) and selection of optimal treatment tactics for patients early from admission. The use of these methods in real clinical practice helps to choose the most optimal treatment tactics for patients in the early stages after admission, and this significantly improved the further prognosis of patients suffering from MI. However, there are a number of problems that arise when using highly sensitive or ultra-sensitive methods for determining cardiospecific troponins: frequent and unexplained increases in serum levels of cardiospecific troponins in a number of pathological conditions unrelated to MI; insufficient knowledge and understanding of the mechanisms of release and increase in the levels of cardiospecific troponins; poorly understood features and mechanisms of circulation and elimination of cardiospecific troponins; the presence of conflicting information about the influence of several factors (gender, age and circadian characteristics) on the levels of cardiospecific troponins in blood serum; undisclosed diagnostic potential of cardiospecific troponins in non-invasive human biological fluids. These problems cause great difficulties and increase the risk of errors in the differential diagnosis of MI, and also do not allow to fully unlock the diagnostic potential of cardiospecific troponins. In general, these problems are associated with a lack of understanding of the fundamental mechanisms of the metabolism of cardiospecific troponins. The main purpose of this narrative review is to summarize and provide detailed information about the metabolism of cardiospecific troponins and to discuss the potential impact of metabolic features on the diagnostic value of cardiospecific troponins and their diagnostic capabilities., Materials and Methods: This narrative review is based on the analysis of publications in the Medline, PubMed, and Embase databases. The terms "cardiac troponins", "troponin T", and "troponin I" in combination with "mechanisms of increase", "mechanisms of release", "circulation", "proteolytic cleavage", "elimination", "circadian rhythms", "saliva", and "urine" were used to search publications., Results: It has been reported that the metabolic features (mechanisms of release, circulation, and elimination) of cardiospecific troponins may have an important influence on the diagnostic value of cardiospecific troponins in a number of physiological and pathological conditions that cause cardiomyocyte damage. The main mechanisms of cardiac troponin release are: cardiomyocyte apoptosis; myocardial cell regeneration and renewal; increased cell membrane permeability; release of troponins by vesicular transport; increased proteolytic degradation of cardiospecific troponin molecules within the cell which may facilitate their release from intact myocardial cells or in the initial phase of those pathological conditions that increase the activity of enzymes that degrade cardiospecific troponins. Besides, the formation of small fragments (troponin molecules) may facilitate their penetration into other body fluids such as urine and/or oral fluid which may provide researchers and practitioners with a new diagnostic opportunity. It should be noted that in addition to release mechanisms, cardiospecific troponin elimination mechanisms may play an important diagnostic role. The contribution of release and elimination mechanisms to different pathologies may differ significantly. Circadian rhythms of cardiospecific troponins may be associated with fluctuations in the activity of those organ systems which influence the mechanisms of cardiospecific troponin release or elimination. Such major systems include: neuroendocrine, urinary, and hemostasis., Conclusion: Cardiospecific troponins metabolism has an important influence on diagnostic value and diagnostic capabilities. Further study of the features of cardiac troponin metabolism (mechanisms of release, circulation and elimination) is required to improve diagnosis and differential diagnosis of diseases causing cardiomyocyte damage. The data on the influence of circadian rhythms of cardiospecific troponins on the diagnostic value and the possibility of determining cardiospecific troponins in body fluids that can be obtained by noninvasive methods are very interesting. However, so far this information and valuable capabilities have not been applied in clinical practice because of the paucity of studies conducted., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.)

The metabolic processes of endo- and exogenous compounds play an important role in diagnosing and treating patients since many metabolites are laboratory biomarkers and/or targets for therapeutic agents. Cardiac troponins are one of the most critical biomarkers to diagnose cardiovascular diseases, including acute myocardial infarction. The study of troponin metabolism is of great interest as it opens up new possibilities for optimizing laboratory diagnostics. This article discusses in detail the key stages of the cardiac troponins metabolism, in particular the mechanisms of release from a healthy myocardium, mechanisms of circulation in the bloodstream, possible mechanisms of troponin penetration into other biological fluids (oral fluid, cerebrospinal fluid, pericardial and amniotic fluids), mechanisms of elimination of cardiac troponins from the blood, and daily changes in the levels of troponins in the blood. Considering these aspects of cardiac troponin metabolism, attention is focused on the potential value for clinical practice.

Background:  Perioperative myocardial infarction/injury (PMI) diagnosed by high-sensitivity troponin (hs-cTn) T is frequent and a prognostically important complication of non-cardiac surgery. We aimed to evaluate the incidence and outcome of PMI diagnosed using hs-cTnI, and compare it to PMI diagnosed using hs-cTnT., Methods: We prospectively included 2455 patients at high cardiovascular risk undergoing 3111 non-cardiac surgeries, for whom hs-cTnI and hs-cTnT concentrations were measured before surgery and on postoperative days 1 and 2. PMI was defined as a composite of perioperative myocardial infarction (PMI Infarct ) and perioperative myocardial injury (PMI Injury ), according to the Fourth Universal Definition of Myocardial Infarction. All-cause mortality was the primary endpoint., Results: Using hs-cTnI, the incidence of overall PMI was 9% (95% confidence interval [CI] 8-10%), including PMI Infarct 2.6% (95% CI 2.0-3.2) and PMI Injury 6.1% (95% CI 5.3-6.9%), which was lower versus using hs-cTnT: overall PMI 15% (95% CI 14-16%), PMI Infarct 3.7% (95% CI 3.0-4.4) and PMI Injury 11.3% (95% CI 10.2-12.4%). All-cause mortality occurred in 52 (2%) patients within 30 days and 217 (9%) within 1 year. Using hs-cTnI, both PMI Infarct and PMI Injury were independent predictors of 30-day all-cause mortality (adjusted hazard ratio [aHR] 2.5 [95% CI 1.1-6.0], and aHR 2.8 [95% CI 1.4-5.5], respectively) and, 1-year all-cause mortality (aHR 2.0 [95% CI 1.2-3.3], and aHR 1.8 [95% CI 1.2-2.7], respectively). Overall, the prognostic impact of PMI diagnosed by hs-cTnI was comparable to the prognostic impact of PMI using hs-cTnT., Conclusions: Using hs-cTnI, PMI is less common versus using hs-cTnT. Using hs-cTnI, both PMI Infarct and PMI Injury remain independent predictors of 30-day and 1-year mortality., (© 2021. The Author(s).)

Checkpoint inhibitor-associated myocarditis (ir-myocarditis) and myositis (ir-myositis) may occur concurrently among patients on checkpoint inhibitor immunotherapy. While cardiac-specific troponin I (cTnI) and troponin T (cTnT) are regarded to have similar sensitivities and specificities in conditions such as acute coronary syndrome, the cardiac specificity of cTnT has been challenged following observation that patients with neuromuscular diseases, including myositis, may have elevated cTnT without apparent clinical evidence of myocardial injury. Consequently, in the context of concurrent ir-myositis, cTnI may be a more appropriate biomarker for diagnosing and monitoring ir-myocarditis. To illustrate this point, we report a case of a patient with severe ir-myositis while on adjuvant programmed cell death protein 1 inhibitor immunotherapy for stage III melanoma, with accompanying elevation in cTnT., (Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.)

Background: Cardiac troponin I (cTnI) and cTnT are the established biomarkers of cardiomyocyte damage and the recommended biomarkers for the diagnosis of acute myocardial infarction (MI). High-sensitivity immunochemical diagnostic systems are able to measure the cTn concentrations in the blood of a majority of healthy people. At the same time, the concentration of cTn may be increased not only after MI but also because of other pathologies that might affect myocardium. This effect reduces the clinical specificity of cTn for MI and may complicate the diagnosis., Content: This review summarizes the existing information regarding the causes and mechanisms that lead to the increase of cTn concentration in blood and the forms of cTn that are present in circulation after MI or other types of myocardial injury., Summary: Different etiologies of disease associated with increases of cTn above the 99th percentile and various mechanisms of troponin release from myocardium could result in the appearance of different forms of cTn in blood and provide the first clinical evidence of injury. Additional research is needed for the careful characterization of cTn forms that are present in the blood in different clinical settings. That knowledge may lead to the development of immunochemical systems that would differentiate certain forms of troponins and possibly certain types of cardiac disease., (© American Association for Clinical Chemistry 2020.)

Background: Troponin composition characterization has been implicated as a next step to differentiate among non-ST elevation myocardial infarction (NSTEMI) patients and improve distinction from other conditions with troponin release. We therefore studied coronary and peripheral troponin compositions in relation to clinical variables of NSTEMI patients., Methods: Samples were obtained from the great cardiac vein (GCV), coronary sinus (CS), and peripheral circulation of 45 patients with NSTEMI. We measured total cTnI concentrations, and assessed both complex cTnI (binary cTnIC + all ternary cTnTIC forms), and large-size cTnTIC (full-size and partially truncated cTnTIC). Troponin compositions were studied in relation to culprit vessel localization (left anterior descending artery [LAD] or non-LAD), ischemic time window, and peak CK-MB value., Results: Sampling occurred at a median of 25 hours after symptom onset. Of total peripheral cTnI, a median of 87[78-100]% consisted of complex cTnI; and 9[6-15]% was large-size cTnTIC. All concentrations (total, complex cTnI, and large-size cTnTIC) were significantly higher in the CS than in peripheral samples (P < 0.001). For LAD culprit patients, GCV concentrations were all significantly higher; in non-LAD culprit patients, CS concentrations were higher. Proportionally, more large-size cTnTIC was present in the earliest sampled patients and in those with the highest CK-MB peaks., Conclusions: In coronary veins draining the infarct area, concentrations of both full-size and degraded troponin were higher than in the peripheral circulation. This finding, and the observed associations of troponin composition with the ischemic time window and the extent of sustained injury may contribute to future characterization of different disease states among NSTEMI patients., (© American Association for Clinical Chemistry 2020.)

Background Detailed insights in temporal evolution of high-sensitivity cardiac troponin following acute coronary syndrome (ACS) are currently missing. We aimed to describe and compare the post-ACS kinetics of high-sensitivity cardiac troponin I (hs-cTnI) and high-sensitivity cardiac troponin T (hs-cTnT), and to determine their intra- and interindividual variation in clinically stable patients. Methods and Results We determined hs-cTnI (Abbott) and hs-cTnT (Roche) in 1507 repeated blood samples, derived from 191 patients with ACS (median, 8/patient) who remained free from adverse cardiac events during 1-year follow-up. Post-ACS kinetics were studied by linear mixed-effect models. Using the samples collected in the 6- to 12-month post-ACS time frame, patients were then considered to have chronic coronary syndrome. We determined (differences between) the average hs-cTnI and average hs-cTnT concentration, and the intra- and interindividual variation for both biomarkers. Compared with hs-cTnT, hs-cTnI peaked higher (median 3506 ng/L versus 494 ng/L; P <0.001) and was quicker below the biomarker-specific upper reference limit (16 versus 19 days; P <0.001). In the post-6-month samples, hs-cTnI and hs-cTnT showed modest correlation ( r spearman =0.60), whereas the average hs-cTnT concentration was 5 times more likely to be above the upper reference limit than hs-cTnI. The intraindividual variations of hs-cTnI and hs-cTnT were 14.0% and 18.1%, while the interindividual variations were 94.1% and 75.9%. Conclusions Hs-cTnI peaked higher after ACS and was quicker below the upper reference limit. In the post-6-month samples, hs-cTnI and hs-cTnT were clearly not interchangeable, and average hs-cTnT concentrations were much more often above the upper reference limit than hs-cTnI. For both markers, the within-patient variation fell largely below beween-patient variation. Registration URL: https://www.trialregister.nl; unique identifiers: NTR1698 and NTR1106.

The early concentration kinetic profiles of cardiac troponin in patients with non-ST-elevated myocardial infarction (NSTEMI) measured by high-sensitivity cardiac troponin I (hs-cTnI) and T (hs-cTnT) assays have not been described. In intermediate-to-high-risk of NSTEMI patients we measured serial cTn concentrations on ED arrival, at 1, 2, 3, 6-12, 24 and 48-hours with hs-cTnI and hs-cTnT assays. Log-normal curves were fitted to concentrations from time from symptom onset, and the time to rule-out decision thresholds estimated (hs-cTnI: 2 ng/L and 5 ng/L; hs-cTnT: 5 ng/L). Among 164 patients there were 58 NSTEMI. The hs-cTnI to hs-cTnT ratio increased linearly over the first 6-12 h following symptom onset. The estimated times from symptom onset to the 2 ng/L and 5 ng/L thresholds for hs-cTnI were 1.8 (0.1-3.1) and 1.9 (1.1-3.5) hours, and to the 5 ng/L threshold for hs-cTnT 1.9 (1.1-3.8) hours. The estimated time to exceed 5 ng/L was ≥3 hours in 32.6% (95%CI: 20.0% to 48.1%) cases for hs-cTnI and 33.3% (19.6% to 50.0%) for hs-cTnT. cTnI concentrations increased at a much more rapid rate than cTnT concentrations in patients with NSTEMI. Concentrations of a high proportion of patients took longer than 3 hours from symptom onset to exceed the 5 ng/L rule-out decision threshold., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Background: High-sensitivity cardiac troponin (hs-cTn) assays enhance detection of lower circulating troponin concentrations, but the impact on outcomes in clinical practice is unclear. Our objective was to compare outcomes of chest pain patients discharged from emergency departments (EDs) using hs-cTn and conventional troponin (cTn) assays., Methods: We conducted an observational study of chest pain patients aged 40-105 years who presented to an ED from April 1, 2013, to March 31, 2017, and were discharged home. We compared 30-day and 1-year outcomes of EDs that used hs-cTn versus cTn assays. The primary outcome was a composite of all-cause death, myocardial infarction or unstable angina. Comparisons were conducted with (1) no adjustment; (2) adjustment for demographic, socioeconomic, and hospital characteristics; and (3) full clinical adjustment., Results: Among the 394,910 patients, 62,138 (15.7%) were evaluated at hs-cTn EDs and 332,772 (84.3%) were evaluated at cTn EDs. Patients discharged from hs-cTn EDs were less likely to have diabetes, hypertension, or prior heart disease. At 30 days, the unadjusted primary outcome rate was lower in hs-cTn EDs (0.9% vs 1.0%, P < .001). The 30-day hazard ratios for the primary outcome were 0.84 (95% CI 0.77-0.92) for no adjustment and 0.98 (95% CI 0.88-1.08) for full adjustment. Over 1 year, patients discharged from hs-cTn EDs had significantly fewer primary outcomes (3.7% vs 4.1%, P < .001) and lower hazard ratio (0.93; 95% CI 0.89-0.98) even after full adjustment., Conclusions: Hs-cTn testing was associated with a significantly lower adjusted hazard of myocardial infarction, angina, and all-cause hospitalization at 1 year but not 30 days., Competing Interests: Disclosures Dr Kavsak has received grants/reagents/consultant/advisor /honoraria from Abbott Laboratories, Abbott Point of Care, Abbott Diagnostics Division Canada, Beckman Coulter, Ortho Clinical Diagnostics, Randox Laboratories, Roche Diagnostics, and Siemens Healthcare Diagnostics. McMaster University has filed patents with Dr Kavsak listed as an inventor in the acute cardiovascular biomarker field. Dr Udell is supported in part by a Heart and Stroke Foundation of Canada National New Investigator/Ontario Clinician Scientist Award, Women's College Research Institute, and the Department of Medicine, Women's College Hospital; Peter Munk Cardiac Center, University Health Network; and Department of Medicine and Heart and Stroke Richard Lewar Center of Excellence in Cardiovascular Research, University of Toronto. Dr Ko and Dr Austin are supported by Mid-Career Investigator Awards from the Heart and Stroke Foundation, Ontario Provincial Office., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Background: Although cardiac troponin I (cTnI) and troponin T (cTnT) form a complex in the human myocardium and bind to thin filaments in the sarcomere, cTnI often reaches higher concentrations and returns to normal concentrations faster than cTnT in patients with acute myocardial infarction (MI)., Methods: We compared the overall clearance of cTnT and cTnI in rats and in patients with heart failure and examined the release of cTnT and cTnI from damaged human cardiac tissue in vitro., Results: Ground rat heart tissue was injected into the quadriceps muscle in rats to simulate myocardial damage with a defined onset. cTnT and cTnI peaked at the same time after injection. cTnI returned to baseline concentrations after 54 h, compared with 168 h for cTnT. There was no difference in the rate of clearance of solubilized cTnT or cTnI after intravenous or intramuscular injection. Renal clearance of cTnT and cTnI was similar in 7 heart failure patients. cTnI was degraded and released faster and reached higher concentrations than cTnT when human cardiac tissue was incubated in 37°C plasma., Conclusion: Once cTnI and cTnT are released to the circulation, there seems to be no difference in clearance. However, cTnI is degraded and released faster than cTnT from necrotic cardiac tissue. Faster degradation and release may be the main reason why cTnI reaches higher peak concentrations and returns to normal concentrations faster in patients with MI., (© American Association for Clinical Chemistry 2020. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Background: Increasing numbers of patients are presenting worldwide to emergency departments with suspected myocardial infarction. The use of point-of-care troponin assays might enable faster decision-making in this high-risk population and reduce the burden on emergency facilities. Here, we evaluate the diagnostic performance of a point-of-care high-sensitivity troponin I assay., Methods: We conducted a prospective cohort study including patients presenting to the emergency department with suspected myocardial infarction from July 2013 to July 2016. A diagnostic algorithm for a high-sensitivity troponin I point-of-care assay was developed in a derivation data set with 669 patients and validated in an additional 610 patients., Results: The derived 0/1 h algorithm for the point-of-care assay consisted of an admission troponin I <4 ng/L and a δ from 0 h to 1 h <3 ng/L for rule out and an admission troponin I ≥90 ng/L or a δ from 0 h to 1 h ≥20 ng/L for rule in of non-ST-elevation myocardial infarction. Application to the validation cohort showed a negative predictive value of 99.7% (95% CI, 98.1%-100.0%) and 48.0% of patients ruled out, whereas 14.6% were ruled in with a positive predictive value of 86.5% (95% CI, 77.6%-92.8%). The diagnostic performance of the point-of-care high-sensitivity assay was highly comparable to guideline-recommended use of a laboratory-based high-sensitivity troponin assay., Conclusions: The clinical application of a 0/1 h diagnostic algorithm based on a high-sensitivity troponin I point-of-care assay is safe, and diagnostic performance is comparable to a laboratory-based high-sensitivity troponin I assay., (© 2019 American Association for Clinical Chemistry.)

Background: Cardiac troponin I (cTn I) and cardiac troponin T (cTn T) are currently widely used as diagnostic biomarkers for myocardial injury caused by ischemic heart diseases in clinical and forensic medicine. However, no previous meta-analysis has summarized the diagnostic roles of postmortem cTn I and cTn T. The aim of the present study was to meta-analyze the diagnostic roles of postmortem cTn I and cTn T for cardiac death in forensic medicine, present a systematic review of the previous literature, and determine the postmortem cut-off values of cTn I and cTn T., Methods: We searched multiple databases for the related literature, performed a meta-analysis to investigate the diagnostic roles of postmortem cardiac troponins, and analyzed the receiver operating characteristic (ROC) curve to determine their postmortem cut-off values., Results and Conclusions: The present meta-analysis demonstrated that postmortem cTn I and cTn T levels were increased in pericardial fluid and serum in cardiac death, especially in patients with acute myocardial infarction (AMI). We determined the postmortem cut-off value of cTn I in the pericardial fluid at 86.2 ng/mL, cTn I in serum at 9.5 ng/mL, and cTn T in serum at 8.025 ng/mL.

Mutations in thin filament regulatory proteins that cause hypertrophic cardiomyopathy (HCM) increase myofilament Ca 2+ sensitivity. Mouse models exhibit increased Ca 2+ buffering and arrhythmias, and we hypothesized that these changes are primary effects of the mutations (independent of compensatory changes) and that increased Ca 2+ buffering and altered Ca 2+ handling contribute to HCM pathogenesis via activation of Ca 2+ -dependent signaling. Here, we determined the primary effects of HCM mutations on intracellular Ca 2+ handling and Ca 2+ -dependent signaling in a model system possessing Ca 2+ -handling mechanisms and contractile protein isoforms closely mirroring the human environment in the absence of potentially confounding remodeling. Using adenovirus, we expressed HCM-causing variants of human troponin-T, troponin-I, and α-tropomyosin (R92Q, R145G, and D175N, respectively) in isolated guinea pig left ventricular cardiomyocytes. After 48 h, each variant had localized to the I-band and comprised ∼50% of the total protein. HCM mutations significantly lowered the K d of Ca 2+ binding, resulting in higher Ca 2+ buffering of mutant cardiomyocytes. We observed increased diastolic [Ca 2+ ] and slowed Ca 2+ reuptake, coupled with a significant decrease in basal sarcomere length and slowed relaxation. HCM mutant cells had higher sodium/calcium exchanger activity, sarcoplasmic reticulum Ca 2+ load, and sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA2) activity driven by Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) phosphorylation of phospholamban. The ryanodine receptor (RyR) leak/load relationship was also increased, driven by CaMKII-mediated RyR phosphorylation. Altered Ca 2+ homeostasis also increased signaling via both calcineurin/NFAT and extracellular signal-regulated kinase pathways. Altered myofilament Ca 2+ buffering is the primary initiator of signaling cascades, indicating that directly targeting myofilament Ca 2+ sensitivity provides an attractive therapeutic approach in HCM., (© 2018 Robinson et al.)

Background and Aims: Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are dominant inherited muscular dystrophies with multiple systemic involvement, often producing cardiac injury. This study sought to determine the clinical significance of elevated high sensitivity cardiac troponin T and I (hs-cTnT and hs-cTnI), and N-terminal pro B-type natriuretic peptide (NT-pro-BNP) in this population., Methods: Sixty DM patients (35 men and 25 women; mean age: 45.1 years, range: 12-73 years) underwent clinical cardiac investigations and measurements of serum hs-cTnT, hs-cTnI, creatine kinase (CK), and NT-proBNP. Left ventricular (LV) ejection fraction (EF) was assessed by echocardiography., Results: Genetic analysis revealed that 46 of the 60 patients were DM1, and 14 DM2. Blood measurements showed persistent elevation of hs-cTnT and CK in 55/60 DM patients (91.73%). In contrast, hs-cTnI values were persistently normal throughout the study. Only 2 patients showed an EF <50%, being the overall range of this population between 40% and 79%. We found ECG abnormalities in 19 patients. Of these patients, 13 showed first or second-degree atrio ventricular (AV) blocks (PR interval ≥ 200 ms), 4 showed a left bundle branch block (LBBB) prolonged (QRS duration ≥120 ms), and 2 had an incomplete bundle branch block (QRS duration between 110 and 119 ms). After excluding patients with EF <50%, NT-pro-BNP measurement > 125 pg/mL was an independent predictor of ECG abnormalities., Conclusions: NT-pro-BNP levels may be considered to be used clinically to identify DM patients at increased risk of developing myocardial conduction abnormalities., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Background: Cardiac-specific troponin T (cTnT) and troponin I (cTnI) are considered diagnostically equal in patients with acute coronary syndrome (ACS). The aim of this systematic review was to compare the prevalence and prognostic strength of elevations of cTnT and cTnI in patients with other conditions than ACS., Methods: A systemic review was conducted in concordance with the PRISMA guidelines. The studies were identified by searching PubMed, EMBASE and Cochrane Central Register, from May to August 2016. Studies measuring both cTnT and cTnI in populations without ACS were eligible., Results: Twenty-nine studies were included (n = 25,859). Seventeen studies reported on prognostic information with follow-up time ranging for 30 d-5 years. Elevation above the 99th percentile (reference value for a healthy population) in non-ACS population was reported to be 0-39% for cTnI and 40-100% for cTnT. Elevation of cTnT tends to be a superior predictor for all-cause mortality and elevation of cTnI tends to be a superior predictor for cardiovascular related mortality., Discussion: In the absence of ACS, elevation of cTnT is more frequent than elevation of cTnI., Conclusion: Both cTnT and cTnI elevations have important prognostic information regarding morbidity, cardiac mortality and all-cause mortality.

Context: Troponin T upstream open reading frame peptide (TnTuORF) may be useful as a novel biomarker in acute cardiac syndromes., Objective: The study examined the early release kinetics of TnTuORF., Materials and Methods: We analyzed the time course of the release of cardiac troponins I and T and TnTuORF in patients (n = 31) with hypertrophic obstructive cardiomyopathy undergoing transcoronary ablation of septal hypertrophy (TASH)., Results: Fifteen minutes after TASH, the levels of both troponins increased significantly (cTnT median: 18 ng/L versus 27 ng/L; cTnI median: 15 ng/L versus 25 ng/L). TnTuORF showed no variation., Discussion: We observed a significantly greater increase in cTnI compared with cTnT., Conclusion: Our results demonstrate that troponin assays allow early detection of myocardial injury, whereas TnTuORF levels remain unchanged in this setting.

Objectives: Cardiac troponins are specific for the heart, but not for the acute coronary syndrome. We wanted to assess how common elevated cardiac troponin concentrations were, in a population with significant non-cardiac disease., Design & Methods: We measured both hs-cTnT and hs-cTnI on all samples submitted to the laboratory during one 24h period, and assessed the magnitude of the cTn concentration with the location and severity of disease of the patient., Results: Community patients and patients from the maternity ward had the lowest cTn concentrations with results above the 99th percentile being only 0-2% of the total. As expected, the highest proportion of results >99th percentile came from Coronary Care and Intensive Care. However, substantial numbers of persons on Medical and Surgical wards, without a primary diagnosis of cardiac disease, also had cTn >99th percentile. Particularly for cTnT, there was a highly significant odds ratio predicting mortality when results above and below the 99th percentile were compared., Conclusions: Significant illnesses apart from the acute coronary syndrome are important causes of a rise in cTn to above the 99th percentile, and appear to reflect the total body burden of disease. Even when the high hs-cTn concentration is not due to the acute coronary syndrome, there is a significant association with all-cause mortality., (Copyright © 2016 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.)

Objective: To compare the changes in muscle enzyme between children with myocarditis and Duchene/Becker muscular dystrophy (DMD/BMD), and to seek the explanations for variation.
, Methods: The retrospective analysis for 83 myocarditis children (myocarditis group) and 69 DMD/BMD children (DMD/BMD group), who were collected from Department of Pediatric of Shengjing Hospital affiliated to China Medical University since January 2008 to May 2015, was carried out. At the same time, 24 healthy children from the Department of Pediatric Development served as a control group. The examination indexes included creatine kinase (CK), creatine kinase-isoenzyme MB (CK-MB), creatine kinase isoenzyme MB mass (CK-MB mass), cardiac troponin I (cTnI) and high-sensitive-cTnT (hs-cTnT).
, Results: 1) In the myocarditis group, the CK increased from 100 to 1 000 U/L, reached a peak after 5 days, which lasted for a week and then dropped to the normal; the CK-MB reached a peak after 5 to 7 days and dropped to the normal a month later; the CK-MB mass reached a peak on the first day and dropped to the normal after 3 weeks; the cTn reached to a peak after 5 days and dropped to the normal after about 17 days; hs-cTnT reached to a peak on the first day and dropped to the normal after about 19 days. 2) In the DMD/BMD group, the CK increased significantly and 27 cases had a CK value of more than 10 000 U/L. After the treatment for 1 to 2 weeks, their enzyme rose again after a slight drop. In terms of cTnI, 6 cases showed a moderate increase, 5 of them couldn't drop to the normal level until more than 3 weeks later; the hs-cTnT increased in the 45 cases, which lasted for more than 3 weeks in the 31 cases of them and showed a tendency of persisting increase.
, Conclusion: The cTnI and hs-cTnT rise significantly and possess wider observation window than CK and CK-MB mass in myocarditis children, with more sensitive and specific changes. The myocardial damage can occur before myasthenia and keep this trend for a long time in the DMD/BMD children. The trend of cTnI change in myocarditis children is similar to hs-cTnT, while hs-cTnT in DMD/BMD children is more sensitive than cTnI.

Cardiac troponin is a dynamic complex of troponin C, troponin I, and troponin T (TnC, TnI, and TnT, respectively) found in the myocyte thin filament where it plays an essential role in cardiac muscle contraction. Mutations in troponin subunits are found in inherited cardiomyopathies, such as hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). The highly dynamic nature of human cardiac troponin and presence of numerous flexible linkers in its subunits suggest that understanding of structural and functional properties of this important complex can benefit from the consideration of the protein intrinsic disorder phenomenon. We show here that mutations causing decrease in the disorder score in TnI and TnT are significantly more abundant in HCM and DCM than mutations leading to the increase in the disorder score. Identification and annotation of intrinsically disordered regions in each of the troponin subunits conducted in this study can help in better understanding of the roles of intrinsic disorder in regulation of interactomes and posttranslational modifications of these proteins. These observations suggest that disease-causing mutations leading to a decrease in the local flexibility of troponins can trigger a whole plethora of functional changes in the heart.

Tn is a unique translational biomarker in cardiology whose potential has not been diminished in the new era of high sensitive assays. cTns can be valuable markers in cardiac diseases as well as in infectious diseases and respiratory diseases. Furthermore, the role of cTns is growing in the routine evaluation of cardioxicity and in determining the efficacy/safety ratio of novel cardioprotective strategies in clinical settings. cTns can detect myocardial injury not only in a wide spectrum of laboratory animals in experimental studies in vivo, but also in isolated heart models or cardiomyocytes in vitro. The crucial issue regarding the cross-species usage of cardiac troponin investigation remains the choice of cardiac troponin testing. This review summarizes the recent proteomic data on aminoacid sequences of cTnT and cTnI in various species, as well as selected analytical characteristics of human cardiac troponin high-sensitivity assays. Due to the highly phylogenetically conserved structure of troponins, the same bioindicator can be investigated using the same method in both clinical and experimental cardiology, thus contributing to a better understanding of the pathogenesis of cardiac diseases as well as to increased effectiveness of troponin use in clinical practice. Measuring cardiac troponins using commercially available human high-sensitivity cardiac troponin tests with convenient antibodies selected on the basis of adequate proteomic knowledge can solve many issues which would otherwise be difficult to address in clinical settings for various ethical and practical reasons. Our survey could help elaborate the practical guidelines for optimizing the choice of cTns assay in cardiology., (© 2016 International Union of Biochemistry and Molecular Biology.)

Heat shock protein (Hsp27) renders cardioprotection from stress situations but little is known about its role in myofilaments. In this study we have evaluated the relationship between Hsp27 and troponin response after naloxone-induced morphine withdrawal. Rats were treated with two morphine (75 mg) pellets during six days. Precipitated withdrawal was induced by naloxone on day seven. Hsp27 expression, Hsp27 phosphorylated at serine 82 (Ser82), cardiac troponin T (cTnT), cardiac troponin I (cTnI) and µ-calpain were evaluated by immunoblotting in left ventricle. Hsp, cTnT and cTnI was also evaluated by immunofluorescence procedure. Our results show that enhancement in Hsp27 expression and phosphorylation induced by naloxone-precipitated morphine withdrawal occurs with concomitant increases of cTnT and µ-calpain expression, whereas cTnI was decreased. We also observed co-localization of Hsp27 with cTnT in cardiac tissues. These findings provide new information into the possible role of Hsp27 in the protection of cTnT degradation by µ-calpain (a protease mediating proteolysis of cTnT and cTnI) after morphine withdrawal., (Copyright © 2015 Elsevier B.V. All rights reserved.)

The heart is characterized by a remarkable degree of heterogeneity, the basis of which is a subject of active investigation. Myofilament protein post-translational modifications (PTMs) represent a critical mechanism regulating cardiac contractility, and emerging evidence shows that pathological cardiac conditions induce contractile heterogeneity that correlates with transmural variations in the modification status of myofilament proteins. Nevertheless, whether there exists basal heterogeneity in myofilament protein PTMs in the heart remains unclear. Here we have systematically assessed chamber-specific and transmural variations in myofilament protein PTMs, specifically, the phosphorylation of cardiac troponin I (cTnI), cardiac troponin T (cTnT), tropomyosin (Tpm), and myosin light chain 2 (MLC2). We show that the phosphorylation of cTnI and αTm vary in the different chambers of the heart, whereas the phosphorylation of MLC2 and cTnT does not. In contrast, no significant transmural differences were observed in the phosphorylation of any of the myofilament proteins analyzed. These results highlight the importance of appropriate tissue sampling-particularly for studies aimed at elucidating disease mechanisms and biomarker discovery-in order to minimize potential variations arising from basal heterogeneity in myofilament PTMs in the heart., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Primary cardiac involvement, an under-recognised manifestation of the idiopathic inflammatory myopathies (IIM) and systemic sclerosis (SSc)-spectrum disorders, is associated with significant mortality. Within these two conditions, traditional skeletal muscle enzyme testing may not effectively distinguish between skeletal and cardiac muscle involvement, especially in patients with subclinical cardiac disease. Accurate biomarkers are thus required to screen for cardiac disease, to better inform both therapeutic decision-making and treatment response. The widespread uptake of cardiac troponin testing has revolutionised the management of acute coronary syndromes. While cardiac troponin I (cTnI) appears specific to the myocardium, cardiac troponin T (cTnT) is also expressed by skeletal muscle, including regenerating skeletal muscle tissue. There is increasing interest about the role of cardiac troponins as a putative biomarker of primary cardiac involvement in IIM and SSc-spectrum disorders. Herewith we discuss subclinical cardiac disease in IIM and SSc-spectrum disorders, the respective roles of cTnI and cTnT testing, and the re-expression of cTnT within regenerating skeletal muscle tissue. There remains wide variation in access to cardiac troponin testing nationally and internationally. We propose two pragmatic clinical pathways using cardiac troponins, preferably measuring concomitant cTnT followed by confirmatory (cardiac) cTnI to screen patients for subclinical cardiac disease and/or low-grade skeletal muscle disease activity, and also an agenda for future research., (Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.)

Skeletal muscles are the most abundant tissues in the human body. They are composed of a heterogeneous collection of muscle fibers that perform various functions. Skeletal muscle troponin (sTn) regulates skeletal muscle contraction and relaxation. sTn consists of 3 subunits, troponin I (TnI), troponin T (TnT), and troponin C (TnC). TnI inhibits the actomyosin Mg(2+)-ATPase, TnC binds Ca(2+), and TnT is the tropomyosin (Tm)-binding subunit. The cardiac and skeletal isoforms of Tn share many similarities but the roles of modifications of Tn in the two muscles may differ. The modifications of cardiac Tn are known to alter muscle contractility and have been well-characterized. However, the modification status of sTn remains unclear. Here, we have employed top-down mass spectrometry (MS) to decipher the modifications of human sTnT and sTnI. We have extensively characterized sTnT and sTnI proteoforms, including alternatively spliced isoforms and post-translationally modified forms, found in human skeletal muscle with high mass accuracy and comprehensive sequence coverage. Moreover, we have localized the phosphorylation site of slow sTnT isoform III to Ser1 by tandem MS with electron capture dissociation. This is the first study to comprehensively characterize human sTn and also the first to identify the basal phosphorylation site for human sTnT by top-down MS.

Purpose of Review: We aim to help clinicians to use and interpret high-sensitivity cardiac troponins (cTns) in different acute care settings. This guidance is timely and relevant as high-sensitivity cTns are currently replacing conventional cTn assays in most parts of the world., Recent Findings: cTn I and T are structural proteins unique to the heart. Detection of cTn in peripheral blood indicates cardiomyocyte injury. Although acute myocardial infarction is a very common, dangerous, but treatable and therefore clinically important cause of cardiomyocyte injury, multiple other acute conditions are associated with substantial amounts of cardiomyocyte injury and corresponding elevations in cTn. These include acute heart failure, tachyarrhythmias, pulmonary embolism, sepsis, shock, and noncardiac surgery. Recent advances in assay technology have led to more sensitive and precise cTn assays that now allow the detection and exact quantification of cardiomyocyte injury also in many predominately noncardiac acute conditions., Summary: In all of these, elevated levels of high-sensitivity cTn are associated with increased mortality risk. In some of these, concepts are evolving as to how the pathophysiological signal of cardiomyocyte injury could be used to alter patient management and potentially improve outcomes.

Aim: It is unknown whether cardiac troponin (cTn) I or cTnT is the preferred biomarker in the early diagnosis of acute myocardial infarction without ST segment elevation (NSTEMI)., Methods and Results: In a prospective multicentre study, we measured cTnI and cTnT using clinically available high-sensitivity assays (hs-cTnI Abbott and hs-cTnT Roche) and compared their diagnostic and prognostic accuracies in consecutive patients presenting to the emergency department with acute chest pain. The final diagnosis was adjudicated by two independent cardiologists using all information pertaining to the individual patient. The mean follow-up was 24 months. Among 2226 consecutive patients, 18% had an adjudicated final diagnosis of NSTEMI. Diagnostic accuracy at presentation as quantified by the area under the receiver-operating-characteristics curve (AUC) for NSTEMI was very high and similar for hs-cTnI [AUC: 0.93, 95% confidence interval (CI) 0.92-0.94] and hs-cTnT (0.94, 95% CI: 0.92-0.94) P = 0.62. In early presenters (<3 h since chest pain onset) hs-cTnI showed a higher diagnostic accuracy (AUC: 0.92, 95% CI: 0.89-0.94) when compared with hs-cTnT AUC (0.89, 95% CI: 0.86-0.91) (P = 0.019), while hs-cTnT was superior in late presenters [AUC hs-cTnT 0.96 (95% CI: 0.94-0.96)  vs. hs-cTnI 0.94 (95% CI: 0.93-0.95); P = 0.007]. The prognostic accuracy for all-cause mortality, quantified by AUC, was significantly higher for hs-cTnT (AUC: 0.80; 95% CI: 0.78-0.82) when compared with hs-cTnI (AUC: 0.75; 95% CI: 0.73-0.77; P < 0.001)., Conclusion: Both hs-cTnI and hs-cTnT provided high diagnostic and prognostic accuracy. The direct comparison revealed small but potentially important differences that might help to further improve the clinical use of hs-cTn., (Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2014. For permissions please email: journals.permissions@oup.com.)

We previously reported a point mutation substituting Cys for Arg(111) in the highly conserved troponin T (TnT)-contacting helix of cardiac troponin I (cTnI) in wild turkey hearts (Biesiadecki et al. J Biol Chem 279: 13825-13832, 2004). This dominantly negative TnI-TnT interface mutation decreases the binding affinity of cTnI for TnT, impairs diastolic function, and blunts the β-adrenergic response of cardiac muscle (Wei et al. J Biol Chem 285: 27806-27816, 2010). Here we further investigate cellular phenotypes of transgenic mouse cardiomyocytes expressing the equivalent mutation cTnI-K118C. Functional studies were performed on single adult cardiomyocytes after recovery in short-term culture from isolation stress. The amplitude of contraction and the velocities of shortening and relengthening were lower in cTnI-K118C cardiomyocytes than wild-type controls. The intracellular Ca(2+) transient was slower in cTnI-K118C cardiomyocytes than wild-type cells. cTnI-K118C cardiomyocytes also showed a weaker β-adrenergic response. The resting length of cTnI-K118C cardiomyocytes was significantly greater than that of age-matched wild-type cells, with no difference in cell width. The resting sarcomere was not longer, but slightly shorter, in cTnI-K118C cardiomyocytes than wild-type cells, indicating longitudinal addition of sarcomeres. More tri- and quadrinuclei cardiomyocytes were found in TnI-K118C than wild-type hearts, suggesting increased nuclear divisions. Whole-genome mRNA array and Western blots detected an increased expression of leukemia inhibitory factor receptor-β in the hearts of 2-mo-old cTnI-K118C mice, suggesting a signaling pathway responsible for the potent effect of cTnI-K118C mutation on early remodeling in cardiomyocytes., (Copyright © 2014 the American Physiological Society.)

Investigations of cardiomyopathy mutations in Ca(2+) regulatory proteins troponin and tropomyosin provide crucial information about cardiac disease mechanisms, and also provide insights into functional domains in the affected polypeptides. Hypertrophic cardiomyopathy-associated mutations TnI R145G, located within the inhibitory peptide (Ip) of human cardiac troponin I (hcTnI), and TnT R278C, located immediately C-terminal to the IT arm in human cardiac troponin T (hcTnT), share some remarkable features: structurally, biochemically, and pathologically. Using bioinformatics, we find compelling evidence that TnI and TnT, and more specifically the affected regions of hcTnI and hcTnT, may be related not just structurally but also evolutionarily. To test for functional interactions of these mutations on Ca(2+)-regulation, we generated and characterized Tn complexes containing either mutation alone, or both mutations simultaneously. The most important results from in vitro motility assays (varying [Ca(2+)], temperature or HMM density) show that the TnT mutant "rescued" some deleterious effects of the TnI mutant at high Ca(2+), but exacerbated the loss of function, i.e., switching off the actomyosin interaction, at low Ca(2+). Taken together, our experimental results suggest that the C-terminus of cTnT aids Ca(2+)-regulatory function of cTnI Ip within the troponin complex., (Copyright © 2014 Elsevier Inc. All rights reserved.)

The C-terminal end-segment of Troponin I (TnI) corresponding to the last 27-33 amino acids is the most conserved structure of TnI and interacts with tropomyosin in a Ca(2+)-regulated manner, suggesting a role in muscle relaxation. Mutations in the C-terminal end-segment of cardiac TnI cause restrictive cardiomyopathy. Here we demonstrate that mouse cardiac TnI containing R193H or R205H mutation have significantly conformational changes in the region interfacing with troponin T (TnT) and increased binding affinity for TnT. These restrictive cardiomyopathy mutations also exhibit increased binding affinity for troponin C at pCa 4. The effects of R193H mutation were more profound than that of R205H. Tertiary troponin complex was reconstituted using the TnI mutants and a mini TnT lacking tropomyosin-binding sites to examine the interaction between the C-terminal end-segment of TnI and tropomyosin. The results showed that, R193H, but not R205H, caused a moderate but statistically significant increase in the binding affinity for tropomyosin at pCa 9. Similar trend was observed at pCa 5.5 but not pCa 4. These results provide novel evidence for the function of the C-terminal end-segment of TnI, where mutations with conformational effects alter TnI's interaction with other troponin subunits and tropomyosin to cause diastolic dysfunction., (Copyright © 2013. Published by Elsevier Inc.)

Aim: Cardiac troponin I (cTnI) and T (cTnT) are the most important biomarkers in the diagnosis of acute myocardial infarction (AMI). Nevertheless, they can be elevated in the absence of AMI. It is unclear if such elevations represent irreversible cardiomyocyte-damage or leakage from viable cardiomyocytes. Our objective is to evaluate whether cTn is released from viable cardiomyocytes in response to ischemia and to identify differences in the release of cTn and its molecular forms., Methods and Results: HL-1 cardiomyocytes (mouse) were subjected to ischemia (modeled by anoxia with glucose deprivation). The total contents and molecular forms of cTn were determined in culture media and cell lysates. Cell viability was assessed from the release of lactate dehydrogenase (LDH). Before the release of LDH, the intracellular cTn content in ischemic cells decreased significantly compared to control (52% for cTnI; 23% for cTnT) and was not matched by a cTn increase in the medium. cTnI decreased more rapidly than cTnT, resulting in an intracellular cTnT/cTnI ratio of 25.5 after 24 h of ischemia. Western blots revealed changes in the relative amounts of fragmented cTnI and cTnT in ischemic cells., Conclusions: HL-1 cardiomyocytes subjected to simulated ischemia released cTnI and cTnT only in combination with the release of LDH. We find no evidence of cTn release from viable cardiomyocytes, but did observe a significant decrease in cTn content, before the onset of cell death. Intracellular decrease of cTn in viable cardiomyocytes can have important consequences for the interpretation of cTn values in clinical practice., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Background: Hypertrophic cardiomyopathy (HCM) is a common genetic disorder caused mainly by mutations in sarcomeric proteins and is characterized by maladaptive myocardial hypertrophy, diastolic heart failure, increased myofilament Ca(2+) sensitivity, and high susceptibility to sudden death. We tested the following hypothesis: correction of the increased myofilament sensitivity can delay or prevent the development of the HCM phenotype., Methods and Results: We used an HCM mouse model with an E180G mutation in α-tropomyosin (Tm180) that demonstrates increased myofilament Ca(2+) sensitivity, severe hypertrophy, and diastolic dysfunction. To test our hypothesis, we reduced myofilament Ca(2+) sensitivity in Tm180 mice by generating a double transgenic mouse line. We crossed Tm180 mice with mice expressing a pseudophosphorylated cardiac troponin I (S23D and S24D; TnI-PP). TnI-PP mice demonstrated a reduced myofilament Ca(2+) sensitivity compared with wild-type mice. The development of pathological hypertrophy did not occur in mice expressing both Tm180 and TnI-PP. Left ventricle performance was improved in double transgenic compared with their Tm180 littermates, which express wild-type cardiac troponin I. Hearts of double transgenic mice demonstrated no changes in expression of phospholamban and sarcoplasmic reticulum Ca(2+) ATPase, increased levels of phospholamban and troponin T phosphorylation, and reduced phosphorylation of TnI compared with Tm180 mice. Moreover, expression of TnI-PP in Tm180 hearts inhibited modifications in the activity of extracellular signal-regulated kinase and zinc finger-containing transcription factor GATA in Tm180 hearts., Conclusions: Our data strongly indicate that reduction of myofilament sensitivity to Ca(2+) and associated correction of abnormal relaxation can delay or prevent development of HCM and should be considered as a therapeutic target for HCM.

Troponin complex is a component of skeletal and cardiac muscle thin filaments. It consists of three subunits - troponin I, T, and C, and it plays a crucial role in muscle activity, connecting changes in intracellular Ca2+ concentration with generation of contraction. In spite of more than 40 years of studies, many aspects of troponin functioning are still not completely understood, and several models describing the mechanism of muscle contraction exist. Being a key factor in the regulation of cardiac muscle contraction, troponin complex is utilized in medicine as a target for some cardiotonic drugs used in the treatment of heart failure. A number of mutations in troponin subunits are associated with development of different types of cardiomyopathy. Moreover, for the last 25 years cardiac isoforms of troponin I and T have been widely used for immunochemical diagnostics of pathologies associated with cardiomyocyte death (myocardial infarction, myocardial trauma, and others). This review summarizes the existing evidence on the structure and function of troponin complex subunits, their role in the regulation of cardiac muscle contraction, and their clinical applications.

The pathological progression of hypertrophic cardiomyopathy (HCM) is sexually dimorphic such that male HCM mice develop phenotypic indicators of cardiac disease well before female HCM mice. Here, we hypothesized that alterations in myofilament function underlies, in part, this sex dimorphism in HCM disease development. Firstly, 10-12month female HCM (harboring a mutant [R403Q] myosin heavy chain) mice presented with proportionately larger hearts than male HCM mice. Next, we determined Ca(2+)-sensitive tension development in demembranated cardiac trabeculae excised from 10-12month female and male HCM mice. Whereas HCM did not impact Ca(2+)-sensitive tension development in male trabeculae, female HCM trabeculae were more sensitive to Ca(2+) than wild-type (WT) counterparts and both WT and HCM males. We hypothesized that the underlying cause of this sex difference in Ca(2+)-sensitive tension development was due to changes in Ca(2+) handling and sarcomeric proteins, including expression of SR Ca(2+) ATPase (2a) (SERCA2a), β-myosin heavy chain (β-MyHC) and post-translational modifications of myofilament proteins. Female HCM hearts showed an elevation of SERCA2a and β-MyHC protein whereas male HCM hearts showed a similar elevation of β-MyHC protein but a reduced level of cardiac troponin T (cTnT) phosphorylation. We also measured the distribution of cardiac troponin I (cTnI) phosphospecies using phosphate-affinity SDS-PAGE. The distribution of cTnI phosphospecies depended on sex and HCM. In conclusion, female and male HCM mice display sex dimorphic myofilament function that is accompanied by a sex- and HCM-dependent distribution of sarcomeric proteins and cTnI phosphospecies., (Copyright © 2013 Elsevier Inc. All rights reserved.)

Aims: The pure form of familial dilated cardiomyopathy (DCM) is mainly caused by mutations in genes encoding sarcomeric proteins. Previous measurements using recombinant proteins suggested that DCM mutations in thin filament proteins decreased myofibrillar Ca(2+) sensitivity, but exceptions were reported. We re-investigated the molecular mechanism of familial DCM using native proteins., Methods and Results: We used the quantitative in vitro motility assay and native troponin and tropomyosin to study DCM mutations in troponin I, troponin T, and α-tropomyosin. Four mutations reduced myofilament Ca(2+) sensitivity, but one mutation (TPM1 E54K) did not alter Ca(2+) sensitivity and another (TPM1 D230N) increased Ca(2+) sensitivity. In thin filaments from normal human and mouse heart, protein kinase A (PKA) phosphorylation of troponin I caused a two- to three-fold decrease in myofibrillar Ca(2+) sensitivity. However, Ca(2+) sensitivity did not change with the level of troponin I phosphorylation in any of the DCM-mutant containing thin filaments (E40K, E54K, and D230N in α-tropomyosin; R141W and ΔK210 in cardiac troponin T; K36Q in cardiac troponin I; G159D in cardiac troponin C, and E361G in cardiac α-actin). This 'uncoupling' was observed with native mutant protein from human and mouse heart and with recombinant mutant protein expressed in baculovirus/Sf9 systems. Uncoupling was independent of the fraction of mutated protein present above 0.55., Conclusion: We conclude that DCM-causing mutations in thin filament proteins abolish the relationship between myofilament Ca(2+) sensitivity and troponin I phosphorylation by PKA. We propose that this blunts the response to β-adrenergic stimulation and could be the cause of DCM in the long term.

Aims: We studied the relationship between myofilament Ca(2+) sensitivity and troponin I (TnI) phosphorylation by protein kinase A at serines 22/23 in human heart troponin isolated from donor hearts and from myectomy samples from patients with hypertrophic obstructive cardiomyopathy (HOCM)., Methods and Results: We used a quantitative in vitro motility assay. With donor heart troponin, Ca(2+) sensitivity is two- to three-fold higher when TnI is unphosphorylated. In the myectomy samples from patients with HOCM, the mean level of TnI phosphorylation was low: 0.38 ± 0.19 mol Pi/mol TnI compared with 1.60 ± 0.19 mol Pi/mol TnI in donor hearts, but no difference in myofilament Ca(2+) sensitivity was observed. Thus, troponin regulation of thin filament Ca(2+) sensitivity is abnormal in HOCM hearts. HOCM troponin (0.29 mol Pi/mol TnI) was treated with protein kinase A to increase the level of phosphorylation to 1.56 mol Pi/mol TnI. No difference in EC(50) was found in thin filaments containing high and low TnI phosphorylation levels. This indicates that Ca(2+) sensitivity is uncoupled from TnI phosphorylation in HOCM heart troponin. Coupling could be restored by replacing endogenous troponin T (TnT) with the recombinant TnT T3 isoform. No difference in Ca(2+) sensitivity was observed if TnI was exchanged into HOCM heart troponin or if TnT was exchanged into the highly phosphorylated donor heart troponin. Comparison of donor and HOCM heart troponin by mass spectrometry and with adduct-specific antibodies did not show any differences in TnT isoform expression, phosphorylation or any post-translational modifications., Conclusion: An abnormality in TnT is responsible for uncoupling myofibrillar Ca(2+) sensitivity from TnI phosphorylation in the septum of HOCM patients.

Cardiomyocyte-like cells have been reported in thoracic veins of rodents and other mammals, but their differentiation state and relationship to the muscle mass in the heart remain to be characterized. Here we investigated the distribution, ultrastructure, expression and developmental regulation of myofilament proteins of mouse and rat pulmonary and azygos venous cardiomyocytes. Tracing cardiomyocytes in transgenic mouse tissues using a lacZ reporter gene driven by a cloned rat cardiac troponin T promoter demonstrated scattered distribution of cardiomyocytes discontinuous from the atrial sleeves. The longitudinal axis of venous cardiomyocytes is perpendicular to that of the vessel. These cells contain typical sarcomere structures and intercalated discs as shown in electron microscopic images, and express cardiac isoforms of troponin T, troponin I and myosin. The expression of troponin I isoform genes and the alternative splicing of cardiac troponin T in thoracic venous cardiomyocytes are regulated during postnatal development in precise synchrony with that in the heart. However, the patterns of cardiac troponin T splicing in adult rat thoracic venous cardiomyocytes are slightly but clearly distinct from those in the atrial and ventricular muscles. The data indicate that mouse and rat thoracic venous cardiomyocytes residing in extra-cardiac tissue possess a physiologically differentiated state and an intrinsically pre-set developmental clock, which are apparently independent of the very different hemodynamic environments and functional features of the vessels and heart., (© 2012 The Authors Journal compilation © 2012 FEBS.)

Acute myocardial infarction (AMI) is the leading cause of death worldwide, with early diagnosis still being difficult. Promising new cardiac biomarkers such as troponins and creatine kinase (CK) isoforms are being studied and integrated into clinical practice for early diagnosis of AMI. The cardiac-specific troponins I and T (cTnI and cTnT) have good sensitivity and specificity as indicators of myocardial necrosis and are superior to CK and its MB isoenzyme (CK-MB) in this regard. Besides being potential biologic markers, cardiac troponins also provide significant prognostic information. The introduction of novel high-sensitivity troponin assays has enabled more sensitive and timely diagnosis or exclusion of acute coronary syndromes. This review summarizes the available information on the potential of troponins and other cardiac markers in early diagnosis and prognosis of AMI, and provides perspectives on future diagnostic approaches to AMI.

Nuclear actin - which is immunologically distinct from cytoplasmic actin - has been documented in a number of differentiated cell types, and cardiac isoforms of troponin I (cTnI) and troponin T (cTnT) have been detected in association with nuclei of adult human cardiac myocytes. It is not known whether these and related proteins are present in undifferentiated stem cells, or when they appear in cardiomyogenic cells following differentiation. We first tested the hypothesis that nuclear actin and cardiac isoforms of troponin C (cTnC) and tropomyosin (cTm) are present along with cTnI and cTnT in nuclei of isolated, neonatal rat cardiomyocytes in culture. We also tested the hypothesis that of these five proteins, only actin is present in nuclei of multipotent, bone marrow-derived mesenchymal stem cells (BM-MSCs) from adult rats in culture, but that cTnC, cTnI, cTnT and cTm appear early and uniquely following cardiomyogenic differentiation. Here we show that nuclear actin is present within nuclei of both ventricular cardiomyocytes and undifferentiated, multipotent BM-MSCs. We furthermore show that cTnC, cTnI, cTnT and cTm are not only present in myofilaments of ventricular cardiomyocytes in culture but are also within their nuclei; significantly, these four proteins appear between days 3 and 5 in both myofilaments and nuclei of BM-MSCs treated to differentiate into cardiomyogenic cells. These observations indicate that cardiac troponin and tropomyosin could have important cellular function(s) beyond Ca(2+)-regulation of contraction. While the roles of nuclear-associated actin, troponin subunits and tropomyosin in cardiomyocytes are not known, we anticipate that the BM-MSC culture system described here will be useful for elucidating their function(s), which likely involve cardiac-specific, Ca(2+)-dependent signaling in the nucleus., (Copyright © 2011 International Society of Differentiation. Published by Elsevier B.V. All rights reserved.)

Cardiac troponin (cTn) I and T are structural proteins unique to the heart. Detection of cTn in peripheral blood indicates cardiomyocyte damage. As acute myocardial infarction (AMI) is the most important cause of cardiomyocyte damage, cTns have become an integral part in the diagnosis of AMI. For this indication, cTns are superior to all other biomarkers and therefore are the preferred marker for the diagnosis of AMI. However, cTn indicates and provides an estimate of cardiomyocyte damage irrespective of its cause. The major limitation of contemporary cTn assays is that they are often not elevated during the initial hours of AMI. Recent advances in assay technology have led to more sensitive and precise cTn assays that will have a profound impact on clinical practice. High-sensitive cTn (hs-cTn) assays have two differentiating features from contemporary cTn assays: (i) detection of cTn in a majority of healthy persons and (ii) precise definition of what is 'normal' (=the 99th percentile). Recent multicentre studies have shown that hs-cTn assays improve the early diagnosis of patients with suspected AMI, particularly the early rule-out. To achieve best clinical use, cTn has to be interpreted as a quantitative variable. Rising and/or falling levels differentiate acute from chronic cardiomyocyte damage. The terms 'troponin-positive' and 'troponin negative' should therefore be avoided. 'Detectable' levels will become the norm and will have to be differentiated from 'elevated' levels. The differential diagnosis of a small amount of cardiomyocyte damage and therefore minor elevations of cTn is broad and includes acute and chronic cardiac disorders. The differential diagnosis of larger amount of injury and therefore more substantial elevations of cTn is largely restricted to AMI, myocarditis, and a rare patient with tako-tsubo cardiomyopathy.

In addition to the core structure conserved in all troponin I isoforms, cardiac troponin I (cTnI) has an ∼30 amino acids NH(2)-terminal extension. This peptide segment is a heart-specific regulatory structure containing two Ser residues that are substrates of PKA. Under β-adrenergic regulation, phosphorylation of cTnI in the NH(2)-terminal extension increases the rate of myocardial relaxation. The NH(2)-terminal extension of cTnI is also removable by restrictive proteolysis to produce functional adaptation to hemodynamic stresses. The molecular mechanism for the NH(2)-terminal modifications to regulate the function of cTnI is not fully understood. In the present study, we tested a hypothesis that the NH(2)-terminal extension functions by modulating the conformation of other regions of cTnI. Monoclonal antibody epitope analysis and protein binding experiments demonstrated that deletion of the NH(2)-terminal segment altered epitopic conformation in the middle, but not COOH-terminal, region of cTnI. PKA phosphorylation produced similar effects. This targeted long-range conformational modulation corresponded to changes in the binding affinities of cTnI for troponin T and for troponin C in a Ca(2+)-dependent manner. The data suggest that the NH(2)-terminal extension of cTnI regulates cardiac muscle function through modulating molecular conformation and function of the core structure of cTnI.

Background: Cardiac troponins I (cTnI) and T (cTnT) have received international endorsement as the standard biomarkers for detection of myocardial injury, for risk stratification in patients suspected of acute coronary syndrome, and for the diagnosis of myocardial infarction. An evidence-based clinical database is growing rapidly for high-sensitivity (hs) troponin assays. Thus, clarifications of the analytical principles for the immunoassays used in clinical practice are important., Content: The purpose of this mini-review is (a) to provide a background for the biochemistry of cTnT and cTnI and (b) to address the following analytical questions for both hs cTnI and cTnT assays: (i) How does an assay become designated hs? (ii) How does one realistically define healthy (normal) reference populations for determining the 99th percentile? (iii) What is the usual biological variation of these analytes? (iv) What assay imprecision characteristics are acceptable? (v) Will standardization of cardiac troponin assays be attainable?, Summary: This review raises important points regarding cTnI and cTnT assays and their reference limits and specifically addresses hs assays used to measure low concentrations (nanograms per liter or picograms per milliliter). Recommendations are made to help clarify the nomenclature. The review also identifies further challenges for the evolving science of cardiac troponin measurement. It is hoped that with the introduction of these concepts, both laboratorians and clinicians can develop a more unified view of how these assays are used worldwide in clinical practice.

The troponin complex plays an essential role in the thin filament regulation of striated muscle contraction. Of the three subunits of troponin, troponin I (TnI) is the actomyosin ATPase inhibitory subunit and its effect is released upon Ca(2+) binding to troponin C. The exon-8-encoded C-terminal end segment represented by the last 24 amino acids of cardiac TnI is highly conserved and is critical to the inhibitory function of troponin. Here, we investigated the function and calcium regulation of the C-terminal end segment of TnI. A TnI model molecule was labeled with Alexa Fluor 532 at a Cys engineered at the C-terminal end and used to reconstitute the tertiary troponin complex. A Ca(2+) -regulated conformational change in the C-terminus of TnI was shown by a sigmoid-shape fluorescence intensity titration curve similar to that of the CD calcium titration curve of troponin C. Such corresponding Ca(2+) responses are consistent with the function of troponin as a coordinated molecular switch. Reconstituted troponin complex containing a mini-troponin T lacking its two tropomyosin-binding sites showed a saturable binding to tropomyosin at pCa 9 but not at pCa 4. This Ca(2+) -regulated binding was diminished when the C-terminal 19 amino acids of cardiac TnI were removed. These results provided novel evidence for suggesting that the C-terminal end segment of TnI participates in the Ca(2+) regulation of muscle thin filament through interaction with tropomyosin., (© 2011 The Authors Journal compilation © 2011 FEBS.)

Objectives: Seventeen male participants (mean (SD) (range): age 33.5 (6.5) years (46-26 years), body mass 80 (9.2) kg (100-63 kg), height 1.81 (0.06) m (1.93- 1.70 m)) ran a marathon to investigate the relationship between systolic function (using cardiac magnetic resonance (CMR)) and diastolic function (using echocardiography) against biomarkers of cardiac damage., Methods: Echocardiographic and cardiac troponin I (cTnI)/N-terminal pro-B-type natriuretic peptide (NTproBNP) data were collected 24 h premarathon, immediately postmarathon and 6 h postmarathon. CMR data were collected 24 h premarathon and at 6 h postmarathon., Results: Body mass was significantly reduced postmarathon (80 (9.2) vs 78.8 (8.6) kg; p<0.001). There was a significant E/A reduction postmarathon (1.11 (0.34) vs 1.72 (0.44); p<0.05) that remained depressed 6 h postmarathon (1.49 (0.43); p<0.05). CMR demonstrated left ventricular end-diastolic and end-systolic volumes were reduced postmarathon, with a preserved stroke volume. Left ventricular ejection fraction 6 h postmarathon significantly increased (64.4% (4.2%) vs 67.4% (5%); p<0.05). There were significant elevations in cTnI (0.00 vs 0.04 (0.03) μg/l; p<0.05) and NTproBNP (37.4 (24.15) ng/l vs 59.34 (43.3) ng/l; p<0.05) immediately postmarathon. Eight runners had cTnI elevations immediately postmarathon above acute myocardial infarction cutoff levels (≥0.03 μg/l). No correlations between cTnI/NTproBNP and measures of diastolic function (E, A, E/A, isovolumic relaxation time, E deceleration time and E/E') or measures of systolic function (stroke volume or ejection fraction) were observed immediately postmarathon or 6 h postmarathon., Conclusions: Biomarkers of cardiac damage after prolonged exercise are not associated with either systolic or diastolic functional measures.