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Proarrhythmic cardiotoxicity remains a substantial barrier to drug development as well as a major global health challenge. In vitro human pluripotent stem cell-based new approach methodologies have been increasingly proposed and employed as alternatives to existing in vitro and in vivo models that do not accurately recapitulate human cardiac electrophysiology or cardiotoxicity risk. In this study, we expanded the capacity of our previously established 3D human cardiac microtissue model to perform quantitative risk assessment by combining it with a physiologically based pharmacokinetic model, allowing a direct comparison of potentially harmful concentrations predicted in vitro to in vivo therapeutic levels. This approach enabled the measurement of concentration responses and margins of exposure for 2 physiologically relevant metrics of proarrhythmic risk (i.e. action potential duration and triangulation assessed by optical mapping) across concentrations spanning 3 orders of magnitude. The combination of both metrics enabled accurate proarrhythmic risk assessment of 4 compounds with a range of known proarrhythmic risk profiles (i.e. quinidine, cisapride, ranolazine, and verapamil) and demonstrated close agreement with their known clinical effects. Action potential triangulation was found to be a more sensitive metric for predicting proarrhythmic risk associated with the primary mechanism of concern for pharmaceutical-induced fatal ventricular arrhythmias, delayed cardiac repolarization due to inhibition of the rapid delayed rectifier potassium channel, or hERG channel. This study advances human-induced pluripotent stem cell-based 3D cardiac tissue models as new approach methodologies that enable in vitro proarrhythmic risk assessment with high precision of quantitative metrics for understanding clinically relevant cardiotoxicity. [ABSTRACT FROM AUTHOR]

Recent advances in human induced pluripotent stem cell (hiPSC)-derived cardiac microtissues provide a unique opportunity for cardiotoxic assessment of pharmaceutical and environmental compounds. Here, we developed a series of automated data processing algorithms to assess changes in action potential (AP) properties for cardiotoxicity testing in 3D engineered cardiac microtissues generated from hiPSC-derived cardiomyocytes (hiPSC-CMs). Purified hiPSC-CMs were mixed with 5–25% human cardiac fibroblasts (hCFs) under scaffold-free conditions and allowed to self-assemble into 3D spherical microtissues in 35-microwell agarose gels. Optical mapping was performed to quantify electrophysiological changes. To increase throughput, AP traces from 4x4 cardiac microtissues were simultaneously acquired with a voltage sensitive dye and a CMOS camera. Individual microtissues showing APs were identified using automated thresholding after Fourier transforming traces. An asymmetric least squares method was used to correct non-uniform background and baseline drift, and the fluorescence was normalized (ΔF/F0). Bilateral filtering was applied to preserve the sharpness of the AP upstroke. AP shape changes under selective ion channel block were characterized using AP metrics including stimulation delay, rise time of AP upstroke, APD30, APD50, APD80, APDmxr (maximum rate change of repolarization), and AP triangulation (APDtri = APDmxr−APD50). We also characterized changes in AP metrics under various ion channel block conditions with multi-class logistic regression and feature extraction using principal component analysis of human AP computer simulations. Simulation results were validated experimentally with selective pharmacological ion channel blockers. In conclusion, this simple and robust automated data analysis pipeline for evaluating key AP metrics provides an excellent in vitro cardiotoxicity testing platform for a wide range of environmental and pharmaceutical compounds. [ABSTRACT FROM AUTHOR]

Environmental factors play a substantial role in determining cardiovascular health, but data informing the risks presented by environmental toxicants is insufficient. In vitro new approach methodologies (NAMs) offer a promising approach with which to address the limitations of traditional in vivo and in vitro assays for assessing cardiotoxicity. Driven largely by the needs of pharmaceutical toxicity testing, considerable progress in developing NAMs for cardiotoxicity analysis has already been made. As the scientific and regulatory interest in NAMs for environmental chemicals continues to grow, a thorough understanding of the unique features of environmental cardiotoxicants and their associated cardiotoxicities is needed. Here, we review the key characteristics of as well as important regulatory and biological considerations for fit-for-purpose NAMs for environmental cardiotoxicity. By emphasizing the challenges and opportunities presented by NAMs for environmental cardiotoxicity we hope to accelerate their development, acceptance, and application. [ABSTRACT FROM AUTHOR]

In contrast to the other heterotrimeric GTP-binding proteins (G proteins) [G.sub.s] and [G.sub.i], the functional role of Go is still poorly defined. To investigate the role of G[[alpha].sub.o] in the heart, we generated transgenic mice with cardiac-specific expression of a constitutively active form of G[[alpha].sub.o1] * (G[[alpha].sub.o] *), the predominant G[[alpha].sub.o] isoform in the heart. G[[alpha].sub.o] expression was increased 3- to 15-fold in mice from 5 independent lines, all of which had a normal life span and no gross cardiac morphological abnormalities. We demonstrate enhanced contractile function in G[[alpha].sub.o] * transgenic mice in vivo, along with increased L-type [Ca.sup.2+] channel current density, calcium transients, and cell shortening in ventricular G[[alpha].sub.o] *-expressing myocytes compared with wild-type controls. These changes were evident at baseline and maintained after isoproterenol stimulation. Expression levels of all major [Ca.sup.2+] handling proteins were largely unchanged, except for a modest reduction in [Na.sup.+]/[Ca.sup.2+] exchanger in transgenic ventricles. In contrast, phosphorylation of the ryanodine receptor and phospholamban at known PKA sites was increased 1.6- and 1.9-fold, respectively, in G[[alpha].sub.o] * ventricles. Density and affinity of [beta]-adrenoceptors, cAMP levels, and PKA activity were comparable in G[[alpha].sub.o] * and wild-type myocytes, but protein phosphatase 1 activity was reduced upon G[[alpha].sub.o] * expression, particularly in the vicinity of the ryanodine receptor. We conclude that G[[alpha].sub.o] * exerts a positive effect on [Ca.sup.2+] cycling and contractile function. Alterations in protein phosphatase 1 activity rather than PKA-mediated phosphorylation might be involved in hyperphosphorylation of key [Ca.sup.2+] handling proteins in hearts with constitutive [G[alpha].sub.o] activation. G proteins; signal transduction; calcium; contraction; transgenic mice

Cardiac hypertrophy is a common response to injury and hemodynamic stress and an important harbinger of heart failure and death. Herein, we identify the Kruppel-like factor 15 (KLF15) as an inhibitor of cardiac hypertrophy. Myocardial expression of KLF15 is reduced in rodent models of hypertrophy and in biopsy samples from patients with pressure-overload induced by chronic valvular aortic stenosis. Overexpression of KLF15 in neonatal rat ventricular cardiomyocytes inhibits cell size, protein synthesis and hypertrophic gene expression. KLF15-null mice are viable but, in response to pressure overload, develop an eccentric form of cardiac hypertrophy characterized by increased heart weight, exaggerated expression of hypertrophic genes, left ventricular cavity dilatation with increased myocyte size, and reduced left ventricular systolic function. Mechanistically, a combination of promoter analyses and gel-shift studies suggest that KLF15 can inhibit GATA4 and myocyte enhancer factor 2 function. These studies identify KLF15 as part of a heretofore unrecognized pathway regulating the cardiac response to hemodynamic stress. heart | transcription

The given document is an abstract from the Journal of Clinical & Translational Science. It includes summaries of three different programs aimed at developing early career faculty in the field of clinical and translational research. The first program discussed is the Post-doctoral Master of Science in Clinical and Translational Research program at the University of Puerto Rico, which has been successful in training Hispanic researchers who have made significant contributions to the field. The second program is the Advance RI-K Scholar Career Development Program, a one-year intensive program in an IDeA state that supports early career faculty in preparing for mentored career development awards. The third program focuses on resident training in research fundamentals using an online, asynchronous course. These programs have all been effective in providing training and support to early career researchers in their respective fields. [Extracted from the article]

Multicellular spheroids generated through cellular self-assembly provide cytoarchitectural complexities of native tissue including three-dimensionality, extensive cell-cell contacts, and appropriate cell-extracellular matrix interactions. They are increasingly suggested as building blocks for larger engineered tissues to achieve shapes, organization, heterogeneity, and other biomimetic complexities. Application of these tissue culture platforms is of particular importance in cardiac research as the myocardium is comprised of distinct but intermingled cell types. Here, we generated scaffold-free 3D cardiac microtissue spheroids comprised of cardiac myocytes (CMs) and/or cardiac fibroblasts (CFs) and used them as building blocks to form larger microtissues with different spatial distributions of CMs and CFs. Characterization of fusing homotypic and heterotypic spheroid pairs revealed an important influence of CFs on fusion kinetics, but most strikingly showed rapid fusion kinetics between heterotypic pairs consisting of one CF and one CM spheroid, indicating that CMs and CFs self-sort in vitro into the intermixed morphology found in the healthy myocardium. We then examined electrophysiological integration of fused homotypic and heterotypic microtissues by mapping action potential propagation. Heterocellular elongated microtissues which recapitulate the disproportionate CF spatial distribution seen in the infarcted myocardium showed that action potentials propagate through CF volumes albeit with significant delay. Complementary computational modeling revealed an important role of CF sodium currents and the spatial distribution of the CM-CF boundary in action potential conduction through CF volumes. Taken together, this study provides useful insights for the development of complex, heterocellular engineered 3D tissue constructs and their engraftment via tissue fusion and has implications for arrhythmogenesis in cardiac disease and repair. [ABSTRACT FROM AUTHOR]

The heart is a complex pluricellular organ composed of cardiomyocytes and non-myocytes including fibroblasts, endothelial cells and immune cells. Myocytes are responsible for electrical conduction and contractile force generation, while the other cell types are responsible for matrix deposition, vascularization, and injury response. Myocytes and non-myocytes are known to communicate and exert mutual regulatory effects. In concert, they determine the structural, electrical and mechanical characteristics in the healthy and remodelled myocardium. Dynamic crosstalk between myocytes and non-myocytes plays a crucial role in stress/injury-induced hypertrophy and fibrosis development that can ultimately lead to heart failure and arrhythmias. Investigations of heterocellular communication in the myocardium are hampered by the intricate interspersion of the different cell types and the complexity of the tissue architecture. In vitro models have facilitated investigations of cardiac cells in a direct and controllable manner and have provided important functional and mechanistic insights. However, these cultures often lack regulatory input from the other cell types as well as additional topographical, electrical, mechanical and biochemical cues from the cardiac microenvironment that all contribute to modulating cell differentiation, maturation, alignment, function and survival. Advancements in the development of more complex pluricellular physiological platforms that incorporate diverse cues from the myocardial microenvironment are expected to lead to more physiologically relevant cardiac tissue-like in vitro models for mechanistic biological research, disease modelling, therapeutic target identification, drug testing and regeneration. [ABSTRACT FROM AUTHOR]

Pulmonary hypertension (PH) eventually leads to right ventricular (RV) fibrosis and dysfunction that is associated with increased morbidity and mortality. Although angiotensin II plays an important role in RV remodeling associated with hypoxic PH, the molecular mechanisms underlying RV fibrosis in PH largely remain unresolved. We hypothesized that PKC-p38 signaling is involved in RV collagen accumulation in PH and in response to angiotensin II stimulation. Adult male Sprague- Dawley rats were exposed to 3 wk of normoxia or hypoxia (10% FIO2) as a model of PH. Hypoxic rats developed RV hypertrophy and fibrosis associated with an increase in PKC βII and δ protein expression and p38 dephosphorylation in freshly isolated RV cardiac fibroblasts. Further mechanistic studies were performed in cultured primary cardiac fibroblasts stimulated with angiotensin II, a key activator of ventricular fibrosis in PH. Angiotensin II induced a reduction in p38 phosphorylation that was attenuated following chemical inhibition of PKC βII and δ. Molecular and chemical inhibition of PKC βII and δ abrogated angiotensin II-induced cardiac fibroblast proliferation and collagen deposition in vitro. The effects of PKC inhibition on proliferation and fibrosis were reversed by chemical inhibition of p38. Conversely, constitutive activation of p38 attenuated angiotensin II-induced increase of cardiac fibroblast proliferation and collagen accumulation. PKC βII- and δ-dependent inactivation of p38 regulates cardiac fibroblast proliferation and collagen deposition in response to angiotensin II, which suggests that the PKC-p38 signaling in cardiac fibroblasts may be involved and important in the pathophysiology of RV fibrosis in PH. [ABSTRACT FROM AUTHOR]

Background: Transgenic mice with transient cardiac expression of constitutively active Galpha q (Gαq-TG) exhibt progressive heart failure and ventricular arrhythmias after the initiating stimulus of transfected constitutively active Gαq becomes undetectable. However, the mechanisms are still unknown. We examined the effects of chronic administration of olmesartan on heart failure and ventricular arrhythmia in Gαq-TG mice. Methodology/Principal Findings: Olmesartan (1 mg/kg/day) or vehicle was chronically administered to Gαq-TG from 6 to 32 weeks of age, and all experiments were performed in mice at the age of 32 weeks. Chronic olmesartan administration prevented the severe reduction of left ventricular fractional shortening, and inhibited ventricular interstitial fibrosis and ventricular myocyte hypertrophy in Gαq-TG. Electrocardiogram demonstrated that premature ventricular contraction (PVC) was frequently (more than 20 beats/min) observed in 9 of 10 vehicle-treated Gαq-TG but in none of 10 olmesartan-treated Gαq-TG. The collected QT interval and monophasic action potential duration in the left ventricle were significantly shorter in olmesartan-treated Gαq-TG than in vehicle-treated Gαq-TG. CTGF, collagen type 1, ANP, BNP, and β-MHC gene expression was increased and olmesartan significantly decreased the expression of these genes in Gαq-TG mouse ventricles. The expression of canonical transient receptor potential (TRPC) 3 and 6 channel and angiotensin converting enzyme (ACE) proteins but not angiotensin II type 1 (AT1) receptor was increased in Gαq-TG ventricles compared with NTG mouse ventricles. Olmesartan significantly decreased TRPC6 and tended to decrease ACE expressions in Gαq-TG. Moreover, it increased AT1 receptor in Gαq-TG. Conclusions/Significance: These findings suggest that angiotensin II type 1 receptor activation plays an important role in the development of heart failure and ventricular arrhythmia in Gαq-TG mouse model of heart failure. [ABSTRACT FROM AUTHOR]

Background: Beneficial effects of nicorandil on the treatment of hypertensive heart failure (HF) and ischemic heart disease have been suggested. However, whether nicorandil has inhibitory effects on HF and ventricular arrhythmias caused by the activation of G protein alpha q (Gαq) -coupled receptor (GPCR) signaling still remains unknown. We investigated these inhibitory effects of nicorandil in transgenic mice with transient cardiac expression of activated Gαq (Gαq-TG). Methodology/Principal Findings: Nicorandil (6 mg/kg/day) or vehicle was chronically administered to Gαq-TG from 8 to 32 weeks of age, and all experiments were performed in mice at the age of 32 weeks. Chronic nicorandil administration prevented the severe reduction of left ventricular fractional shortening and inhibited ventricular interstitial fibrosis in Gαq- TG. SUR-2B and SERCA2 gene expression was decreased in vehicle-treated Gαq-TG but not in nicorandil-treated Gαq-TG. eNOS gene expression was also increased in nicorandil-treated Gαq-TG compared with vehicle-treated Gαq-TG. Electrocardiogram demonstrated that premature ventricular contraction (PVC) was frequently (more than 20 beats/min) observed in 7 of 10 vehicle-treated Gαq-TG but in none of 10 nicorandil-treated Gαq-TG. The QT interval was significantly shorter in nicorandil-treated Gαq-TG than vehicle-treated Gαq-TG. Acute nicorandil administration shortened ventricular monophasic action potential duration and reduced the number of PVCs in Langendorff-perfused Gαq-TG mouse hearts. Moreover, HMR1098, a blocker of cardiac sarcolemmal KATP channels, significantly attenuated the shortening of MAP duration induced by nicorandil in the Gαq-TG heart. Conclusions/Significance: These findings suggest that nicorandil can prevent the development of HF and ventricular arrhythmia caused by the activation of GPCR signaling through the shortening of the QT interval, action potential duration, the normalization of SERCA2 gene expression. Nicorandil may also improve the impaired coronary circulation during HF. [ABSTRACT FROM AUTHOR]

Cardiac hypertrophy is a well-established risk factor for cardiovascular morbidity and mortality. Activation of Gq/11-mediated signaling is required for pressure overload-induced cardiomyocyte (CM) hypertrophy to develop. We previously showed that among Regulators of G protein Signaling, RGS2 selectively inhibits Gq/11 signaling and its hypertrophic effects in isolated CM. In this study, we generated transgenic mice with CM-specific, conditional RGS2 expression (dTG) to investigate whether RGS2 overexpression can be used to attenuate Gq/11-mediated signaling and hypertrophy in vivo. Transverse aortic constriction (TAC) induced a comparable rise in ventricular mass and ANF expression and corresponding hemodynamic changes in dTG compared to wild types (WT), regardless of the TAC duration (1-8 wks) and timing of RGS2 expression (from birth or adulthood). Inhibition of endothelin-1-induced Gq/11-mediated phospholipase C β activity in ventricles and atrial appendages indicated functionality of transgenic RGS2. However, the inhibitory effect of transgenic RGS2 on Gq/11-mediated PLCβ activation differed between ventricles and atria: (i) in sham-operated dTG mice the magnitude of the inhibitory effect was less pronounced in ventricles than in atria, and (ii) after TAC, negative regulation of Gq/11 signaling was absent in ventricles but fully preserved in atria. Neither difference could be explained by differences in expression levels, including marked RGS2 downregulation after TAC in left ventricle and atrium. Counter-regulatory changes in other Gq/11-regulating RGS proteins (RGS4, RGS5, RGS6) and random insertion were also excluded as potential causes. Taken together, despite ample evidence for a role of RGS2 in negatively regulating Gq/11 signaling and hypertrophy in CM, CM-specific RGS2 overexpression in transgenic mice in vivo did not lead to attenuate ventricular Gq/11-mediated signaling and hypertrophy in response to pressure overload. Furthermore, our study suggests chamber-specific differences in the regulation of RGS2 functionality and potential future utility of the new transgenic model in mitigating Gq/11 signaling in the atria in vivo. [ABSTRACT FROM AUTHOR]

Cardiac fibroblasts play a key role in fibrosis development in response to stress and injury. Angiotensin II (ANG II) is a major profibrotic activator whose downstream effects (such as phospholipase Cβ activation, cell proliferation, and extracellular matrix secretion) are mainly mediated via Gq-coupled AT1 receptors. Regulators of G protein signaling (RGS), which accelerate termination of G protein signaling, are expressed in the myocardium. Among them, RGS2 has emerged as an important player in modulating Gq-mediated hypertrophic remodeling in cardiac myocytes. To date, no information is available on RGS in cardiac fibroblasts. We tested the hypothesis that RGS2 is an important regulator of ANG II-induced signaling and function in ventricular fibroblasts. Using an in vitro model of fibroblast activation, we have demonstrated expression of several RGS isoforms, among which only RGS2 was transiently upregulated after short-term ANG II stimulation. Similar results were obtained in fibroblasts isolated from rat hearts after in vivo ANG II infusion via minipumps for 1 day. In contrast, prolonged ANG II stimulation (3-14 days) markedly downregulated RGS2 in vivo. To delineate the functional effects of RGS expression changes, we used gain- and loss-of-function approaches. Adenovirally infected RGS2 had a negative regulatory effect on ANG II-induced phospholipase Cβ activity, cell proliferation, and total collagen production, whereas RNA interference of endogenous RGS2 had opposite effects, despite the presence of several other RGS. Together, these data suggest that RGS2 is a functionally important negative regulator of ANG II-induced cardiac fibroblast responses that may play a role in ANG II-induced fibrosis development. [ABSTRACT FROM AUTHOR]

Background: Our previous study showed that diacylglycerol kinase zeta (DGKzeta), which degenerates diacylglycerol (DAG), inhibits ventricular structural remodeling and rescues activated G protein (alpha)q (G(alpha)q)-induced heart failure. However, whether DGKzeta inhibits atrial remodeling is still unknown.Objective: This study aimed to elucidate the effects of DGKzeta on atrial remodeling.Methods: A transgenic mouse (G(alpha)q-TG) with cardiac expression of activated G(alpha)q and a double transgenic mouse (G(alpha)q/DGKzeta-TG) with cardiac overexpression of DGKzeta and activated G(alpha)q were created.Results: During electrocardiogram (ECG) recording for 10 min, atrial fibrillation was observed in 5 of 11 anesthetized G(alpha)q-TG mice but not in any wild-type (WT) and G(alpha)q/DGKzeta-TG mice (P <.05). All of the ECG parameters measured were prolonged in the G(alpha)q-TG compared with WT mice. Interestingly, in G(alpha)q/DGKzeta-TG mice, although the PR and RR intervals were still prolonged, the P interval, QRS complex, and QT interval were not different from those in WT mice. In Langendorff-perfused hearts, the incidence of atrial tachyarrhythmia induced by rapid atrial pacing was greater in G(alpha)q-TG hearts than in G(alpha)q/DGKzeta-TG hearts (P <.05). Action potential duration prolongation and impulse conduction slowing were observed in G(alpha)q-TG atria compared with G(alpha)q/DGKzeta-TG atria. Dilatation of the left atrium with thrombus formation was observed in 9 G(alpha)q-TG hearts but not in any G(alpha)q/DGKzeta-TG hearts. Moreover, the degree of extensive interstitial fibrosis in the left atrium was greater in G(alpha)q-TG hearts than that in G(alpha)q/DGKzeta-TG hearts (P <.05).Conclusion: These results show that DGKzeta inhibits G(alpha)q-induced atrial remodeling and suggest that DGKzeta is a novel therapeutic target for atrial fibrillation. [ABSTRACT FROM AUTHOR]

The aim of the present work was to analyze coronary endothelial function in the transgenic mouse model of dilated cardiomyopathy (Tgαq*44 mice). Coronary vasodilatation, both NO-dependent (induced by bradykinin) and PGI2-dependent (induced by acetylcholine), was assessed in the isolated hearts of Tgαq*44 and FVB mice. Cardiac function was analyzed in vivo (MRI). In Tgαq*44 mice at the age of 2–4 months cardiac function was preserved and there were no alterations in endothelial function. By contrast, in Tgαq*44 mice at the age of 14–16 months cardiac function was significantly impaired and NO, but not PGI2-dependent coronary function was altered. Interestingly, the basal level of PGI2 in coronary circulation increased fourfold as compared to FVB mice. Cardiac O2 − production increased 1.5-fold and 3-fold in Tgαq*44 vs. FVB mice at the age of 2–6 and 14–16 months, respectively, and was inhibited by apocynin. Interestingly, inhibition of NADPH oxidase or NOS-3 normalized augmented PGI2 production in Tgαq*44 mice. There was also an increased expression of gp91phox in Tgαq*44 vs. FVB hearts, without evident alterations in the expression of COX-1, COX-2, NOS-3 and PGI2-synthase. In the mouse model of dilated cardiomyopathy, endothelial dysfunction in coronary circulation is present in the late but not the early stage of heart failure pathology and is characterized by a decrease in NO bioavailability and a compensatory increase in PGI2. Both the decrease in NO activity and the increase in PGI2 activity may result from excessive O2 − production by cardiac NADPH oxidase in Tgαq*44 hearts. [ABSTRACT FROM AUTHOR]

In contrast to the other heterotrimeric GTP-binding proteins (G proteins) Gs and Gi, the functional role of Go is still poorly defined. To investigate the role of Gαo in the heart, we generated transgenic mice with cardiac-specific expression of a constitutively active form of Gαo1* (Gαo*), the predominant Gαo isoform in the heart. Gαo expression was increased 3- to 15-fold in mice from 5 independent lines, all of which had a normal life span and no gross cardiac morphological abnormalities. We demonstrate enhanced contractile function in Gαo" transgenic mice in vivo, along with increased L-type Ca2+ channel current density, calcium transients, and cell shortening in ventricular Gαo*-expressing myocytes compared with wild-type controls. These changes were evident at baseline and maintained after isoproterenol stimulation. Expression levels of all major Ca2+ handling proteins were largely unchanged, except for a modest reduction in Na+/Ca2+ exchanger in transgenic ventricles. In contrast, phosphorylation of the ryanodine receptor and phospholamban at known PKA sites was increased 1.6- and 1.9-fold, respectively, in Gαo* ventricles. Density and affinity of β-adrenoceptors, cAMP levels, and PKA activity were comparable in Gαo* and wild-type myocytes, but protein phosphatase 1 activity was reduced upon Gαo expression, particularly in the vicinity of the ryanodine receptor. We conclude that Gαo* exerts a positive effect on Ca2+ cycling and contractile function. Alterations in protein phosphatase 1 activity rather than PKA-mediated phosphorylation might be involved in hyperphosphorylation of key Ca2+ handling proteins in hearts with constitutive Gαo activation. [ABSTRACT FROM AUTHOR]

Provides information on a study that investigated the physiological role of G protein Betagamma-subunits for heart rate control and arrythmia vulnerability in vivo using electrocardiographic telemetry. Background on the function of the potassium channel in the pacemaker cells of the sinoatrial node; Methods used; Results and discussion.

The present study was performed to compare the effects of the new positive inotropic phosphodiesterase III inhibitors pimobendan, adibendan, and saterinone on the isometric force of contraction in electrically driven ventricular trabeculae carneae isolated from explanted failing (end-stage myocardial failure) with those from nonfailing (prospective organ donors) human hearts. In preparations from nonfailing hearts the phosphodiesterase inhibitors, as well as the aβ-adrenoceptor agonist isoprenaline, the cardiac glycoside dihydroouabain, and calcium, which were studied for comparison, revealed pronounced positive inotropic effects. The maximal effects of pimobendan, adibendan, and saterinone amounted to 56%, 36% and 45%, respectively, of the maximal effect of calcium. In contrast, in preparations from failing hearts the phosphodiesterase III inhibitors failed to significantly increase the force of contraction and the effect of isoprenaline was markedly reduced. The effects of dihydroouabain and calcium were almost unaltered. The diminished effects of isoprenaline were restored by the concomitant application of phosphodiesterase inhibitors. To elucidate the underlying mechanism of the lack of effect of the phosphodiesterase III inhibitors in the failing heart we also investigated the inhibitory effects of these compounds on the activities of the phosphodiesterase isoenzymes I-III separated by DEAE-cellulose chromatography from both kinds of myocardial tissue. Furthermore, the effects of pimobendan and isoprenaline on the content of cyclic adenosine monophosphate (determined by radioimmunoassays) of intact contracting trabeculae were studied. The lack of effect of the phosphodiesterase inhibitors in failing human hearts could not be explained by an altered phosphodiesterase inhibition, since the properties of the phosphodiesterase isoenzymes I-III and also the inhibitory effects of the phosphodiesterase inhibitors on these isoenzymes did not differ between failing and nonfailing human myocardial tissue. Instead, it may be due to a diminished formation of cyclic adenosine monophosphate in failing hearts, presumably caused mainly by a defect in receptor-adenylate cyclase coupling at least in idiopathic dilated cardiomyopathy. Both the basal and the pimobendan-stimulated or isoprenaline-stimulated contents of cyclic adenosine monophosphate of intact contracting trabeculae from failing hearts were decreased compared with the levels in nonfailing hearts. However, under the combined action of isoprenaline and pimobendan the cyclic adenosine monophosphate level reached values as high as with each compound alone in nonfailing preparations, and in addition the positive inotropic effect of isoprenaline was restored. These findings may have important clinical implications. Along with the elevated levels of circulating catecholamines the positive inotropic effects of the phosphodiesterase inhibitors may be maintained in patients with heart failure. Furthermore, the concomitant application of a β-adrenoceptor agonist and a phosphodiesterase inhibitor might be beneficial in terminal heart failure refractory to conventional therapeutic regimens. [ABSTRACT FROM AUTHOR]

A study was carried out to investigate whether or not an impairment of the adenosine mediated negative inotropic effect in the presence of beta adrenoceptor stimulation plays a role in the pathogenesis of the hereditary cardiomyopathy of the Syrian hamster. In electrically driven papillary muscles isolated from the hearts of cardiomyopathic (strain BIO 8262) and age matched healthy control Syrian hamsters the effects of isoprenaline, adenosine, and adenosine in the presence of isoprenaline were studied within the first 30 days of life (the prenecrotic stage of the disorder). In both cardiomyopathic and control hamsters adenosine antagonised the positive inotropic effect of isoprenaline, whereas adenosine alone had no or, only a weak, inhibitory effect on the force of contraction. The effects in both groups were similar. The effect of isoprenaline on the force of contraction also did not differ in the two groups.The data show that in both cardiomyopathic and control hamsters adenosine reduces the force of contraction during beta adrenergic stimulation. The potency or efficacy of adenosine did not differ in the two groups. An impaired adenosine mediated feedback control of the heart does not therefore seem to play a role in the pathogenesis of the hereditary dystrophic cardiomyopathy of the Syrian hamster. [ABSTRACT FROM PUBLISHER]