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Diseases of striated muscle linked to intermediate filament (IF) proteins are associated with defects in the organization of the contractile apparatus and its links to costameres, which connect the sarcomeres to the cell membrane. Here we study the role in skeletal muscle of synemin, a type IV IF protein, by examining mice null for synemin (synm-null). Synm-null mice have a mild skeletal muscle phenotype. Tibialis anterior (TA) muscles show a significant decrease in mean fiber diameter, a decrease in twitch and tetanic force, and an increase in susceptibility to injury caused by lengthening contractions. Organization of proteins associated with the contractile apparatus and costameres is not significantly altered in the synm-null. Elastimetry of the sarcolemma and associated contractile apparatus in extensor digitorum longus myofibers reveals a reduction in tension consistent with an increase in sarcolemmal deformability. Although fatigue after repeated isometric contractions is more marked in TA muscles of synm-null mice, the ability of the mice to run uphill on a treadmill is similar to controls. Our results suggest that synemin contributes to linkage between costameres and the contractile apparatus and that the absence of synemin results in decreased fiber size and increased sarcolemmal deformability and susceptibility to injury. Thus synemin plays a moderate but distinct role in fast twitch skeletal muscle., (Copyright © 2015 the American Physiological Society.)

Background: The underlying molecular mechanisms of the remodeling after myocardial infarction (MI) remain unclear. The purpose of this study was to investigate the role of a survival pathway (PI3K/Akt) and an apoptosis pathway (calcineurin/BAD) in the remodeling after MI in a large animal model., Methods: Ten Dorset hybrid sheep underwent 25% MI in the left ventricle (LV, n=10). Five sheep were used as sham control. The regional strain was calculated from sonomicrometry. Apoptosis and the activation of the PI3K/Akt and calcineurin/BAD pathways were evaluated in the non-ischemic adjacent zone and the remote zone relative to infarct by immunoblotting, immunoprecipitation, and immunofluorescence staining., Results: Dilation and dysfunction of LV were present at 12 weeks after MI. The regional strain in the adjacent zone was significantly higher than in the remote zone at 12 weeks (36.6 ± 4.0% vs 9.5 ± 3.6%, p<0.05). Apoptosis was more severe in the adjacent zone than in the remote zone. The PI3K/Akt and calcineurin/BAD pathways were activated in the adjacent zone. Dephosphorylation and translocation of BAD were evident in the adjacent zone. Regional correlation between the strain and the expression of calcineurin/BAD indicated that the activation was strain-related (R(2)=0.46, 0.48, 0.39 for calcineurin, BAD, mitochondrial BAD, respectively, p<0.05)., Conclusions: The PI3K/Akt survival and calcineurin/BAD apoptotic pathways were concomitantly activated in the non-ischemic adjacent zone after MI. The calcineurin/BAD pathway is strain related and its imbalanced activation may be one of the causes of progressive remodeling after MI., (Copyright © 2011 Elsevier Ireland Ltd. All rights reserved.)

Intermediate filaments (IFs) in cardiomyocytes consist primarily of desmin, surround myofibrils at Z disks, and transmit forces from the contracting myofilaments to the cell surface through costameres at the sarcolemma and desmosomes at intercalated disks. Synemin is a type IV IF protein that forms filaments with desmin and also binds α-actinin and vinculin. Here we examine the roles and expression of the α and β forms of synemin in developing rat cardiomyocytes. Quantitative PCR showed low levels of expression for both synemin mRNAs, which peaked at postnatal day 7. Synemin was concentrated at sites of cell-cell adhesion and at Z disks in neonatal cardiomyocytes. Overexpression of the individual isoforms showed that α-synemin preferentially localized to cell-cell junctions, whereas β-synemin was primarily at the level of Z disks. An siRNA targeted to both synemin isoforms reduced protein expression in cardiomyocytes by 70% and resulted in a failure of desmin to align with Z disks and disrupted cell-cell junctions, with no effect on sarcomeric organization. Solubility assays showed that β-synemin was soluble and interacted with sarcomeric α-actinin by coimmunoprecipitation, while α-synemin and desmin were insoluble. We conclude that β-synemin mediates the association of desmin IFs with Z disks, whereas α-synemin stabilizes junctional complexes between cardiomyocytes.

A-kinase anchoring proteins (AKAPs) bind the regulatory subunits of protein kinase A (PKA) and localize the holoenzyme to discrete signaling microdomains in multiple subcellular compartments. Despite emerging evidence for a nuclear pool of PKA that rapidly responds to activation of the PKA signaling cascade, only a few AKAPs have been identified that localize to the nucleus. Here we show a PKA-binding domain in the amino terminus of Chd8, and demonstrate subcellular colocalization of Chd8 with RII. RII overlay and immunoprecipitation assays demonstrate binding between Chd8-S and RIIα. Binding is abrogated upon dephosphorylation of RIIα. By immunofluorescence, we identified nuclear and perinuclear pools of Chd8 in HeLa cells and rat neonatal cardiomyocytes. We also show high levels of Chd8 mRNA in RNA extracted from post-natal rat hearts. These data add Chd8 to the short list of known nuclear AKAPs, and implicate a function for Chd8 in post-natal rat cardiac development.

Protein kinase A (PKA)-dependent phosphorylation is regulated by targeting of PKA to its substrate as a result of binding of regulatory subunit, R, to A-kinase-anchoring proteins (AKAPs). We investigated the effects of disrupting PKA targeting to AKAPs in the heart by expressing the 24-amino acid regulatory subunit RII-binding peptide, Ht31, its inactive analog, Ht31P, or enhanced green fluorescent protein by adenoviral gene transfer into rat hearts in vivo. Ht31 expression resulted in loss of the striated staining pattern of type II PKA (RII), indicating loss of PKA from binding sites on endogenous AKAPs. In the absence of isoproterenol stimulation, Ht31-expressing hearts had decreased +dP/dtmax and -dP/dtmin but no change in left ventricular ejection fraction or stroke volume and decreased end diastolic pressure versus controls. This suggests that cardiac output is unchanged despite decreased +dP/dt and -dP/dt. There was also no difference in PKA phosphorylation of cardiac troponin I (cTnI), phospholamban, or ryanodine receptor (RyR2). Upon isoproterenol infusion, +dP/dtmax and -dP/dtmin did not differ between Ht31 hearts and controls. At higher doses of isoproterenol, left ventricular ejection fraction and stroke volume increased versus isoproterenol-stimulated controls. This occurred in the context of decreased PKA phosphorylation of cTnI, RyR2, and phospholamban versus controls. We previously showed that expression of N-terminal-cleaved cTnI (cTnI-ND) in transgenic mice improves cardiac function. Increased cTnI N-terminal truncation was also observed in Ht31-expressing hearts versus controls. Increased cTnI-ND may help compensate for reduced PKA phosphorylation as occurs in heart failure.

Subcellular compartmentalization of the cAMP-dependent protein kinase (PKA) by protein kinase A-anchoring proteins (AKAPs) facilitates local protein phosphorylation. However, little is known about how PKA targeting to AKAPs is regulated in the intact cell. PKA binds to an amphipathic helical region of AKAPs via an N-terminal domain of the regulatory subunit. In vitro studies showed that autophosphorylation of type II regulatory subunit (RII) can alter its affinity for AKAPs and the catalytic subunit (PKA(cat)). We now investigate whether phosphorylation of serine 96 on RII regulates PKA targeting to AKAPs, downstream substrate phosphorylation and calcium cycling in primary cultured cardiomyocytes. We demonstrated that, whereas there is basal phosphorylation of RII subunits, persistent maximal activation of PKA results in a phosphatase-dependent loss of RII phosphorylation. To investigate the functional effects of RII phosphorylation, we constructed adenoviral vectors incorporating mutants which mimic phosphorylated (RIIS96D), nonphosphorylated (RIIS96A) RII, or wild-type (WT) RII and performed adenoviral infection of neonatal rat cardiomyocytes. Coimmunoprecipitation showed that more AKAP15/18 was pulled down by the phosphomimic, RIIS96D, than RIIS96A. Phosphorylation of phospholamban and ryanodine receptor was significantly increased in cells expressing RIIS96D versus RIIS96A. Expression of recombinant RII constructs showed significant effects on cytosolic calcium transients. We propose a model illustrating a central role of RII phosphorylation in the regulation of local PKA activity. We conclude that RII phosphorylation regulates PKA-dependent substrate phosphorylation and may have significant implications for modulation of cardiac function.

The need to assess heart failure at an early stage highlights the importance of accurate microarray analysis using small tissue samples. To test our ability to obtain high quality RNA from biopsy-sized cardiac specimens, amplification was performed on RNA from biopsy-sized samples of left ventricle (LV) tissue from one explanted failing human heart and one non-failing heart. Two methods were used: one-cycle (1C) amplification of 1.6 microg of RNA, and two-cycle (2C) amplification of 50 ng of RNA. The resulting cRNA was hybridized to Affymetrix GeneChip arrays. Over 65% of all differentially expressed genes for failing vs non-failing hearts were concordant between 1C and 2C RNA amplification. Differentially expressed genes between 1C and 2C RNA amplification in our study were highly correlated (R(2) = 0.957 and changes in gene expression agreed with prior studies on genes and heart failure; e.g., decreased alpha-myosin heavy chain and alpha-tropomyosin, as well as increased expression of insulin-like growth factor). Two cycles of amplification from cardiac biopsies will permit accurate transcription profiling of heart failure at pre-symptomatic stages. Ability to measure gene expression from nanogram amounts of RNA will provide new opportunities to predict progression to symptomatic heart failure, and to identify potential targets for therapy.

Targeting of protein kinase A (PKA) by A-kinase anchoring proteins (AKAPs) contributes to high specificity of PKA signaling pathways. PKA phosphorylation of myofilament and cytoskeletal proteins may regulate myofibrillogenesis and myocyte remodeling during heart disease; however, known cardiac AKAPs do not localize to these regions. To identify novel AKAPs which target PKA to the cytoskeleton or myofilaments, a human heart cDNA library was screened and the intermediate filament (IF) protein, synemin, was identified as a putative RII (PKA regulatory subunit type II) binding protein. A predicted RII binding region was mutated and resulted in loss of RII binding. Furthermore, synemin co-localized with RII in SW13/cl.1-vim+ cells and co-immunoprecipitated with RII from adult rat cardiomyocytes. Synemin was localized at the level of Z-lines with RII and desmin in adult hearts, however, neonatal cardiomyocytes showed differential synemin and desmin localization. Quantitative Western blots also showed significantly more synemin was present in failing human hearts. We propose that synemin provides temporal and spatial targeting of PKA in adult and neonatal cardiac myocytes.

Regulator of G-protein-signaling (RGS) proteins play a key role in the regulation of G-protein-coupled receptor (GPCR) signaling. The characteristic hallmark of RGS proteins is a conserved approximately 120-aa RGS region that confers on these proteins the ability to serve as GTPase-activating proteins (GAPs) for G(alpha) proteins. Most RGS proteins can serve as GAPs for multiple isoforms of G(alpha) and therefore have the potential to influence many cellular signaling pathways. However, RGS proteins can be highly regulated and can demonstrate extreme specificity for a particular signaling pathway. RGS proteins can be regulated by altering their GAP activity or subcellular localization; such regulation is achieved by phosphorylation, palmitoylation, and interaction with protein and lipid-binding partners. Many RGS proteins have GAP-independent functions that influence GPCR and non-GPCR-mediated signaling, such as effector regulation or action as an effector. Hence, RGS proteins should be considered multifunctional signaling regulators. GPCR-mediated signaling is critical for normal function in the cardiovascular system and is currently the primary target for the pharmacological treatment of disease. Alterations in RGS protein levels, in particular RGS2 and RGS4, produce cardiovascular phenotypes. Thus, because of the importance of GPCR-signaling pathways and the profound influence of RGS proteins on these pathways, RGS proteins are regulators of cardiovascular physiology and potentially novel drug targets as well.

Besides the core structure conserved in all troponin I isoforms, cardiac troponin I (cTnI) has an N-terminal extension that contains phosphorylation sites for protein kinase A under beta-adrenergic regulation. A restricted cleavage of this N-terminal regulatory domain occurs in normal cardiac muscle and is up-regulated during hemodynamic adaptation (Z.-B. Yu, L.-F. Zhang, and J.-P. Jin (2001) J. Biol. Chem. 276, 15753-15760). In the present study, we developed transgenic mice overexpressing the N-terminal truncated cTnI (cTnI-ND) in the heart to examine its biochemical and physiological significance. Ca(2+)-activated actomyosin ATPase activity showed that cTnI-ND myofibrils had lower affinity for Ca(2+) than controls, similar to the effect of isoproterenol treatment. In vivo and isolated working heart experiments revealed that cTnI-ND hearts had a significantly faster rate of relaxation and lower left ventricular end diastolic pressure compared with controls. The higher baseline relaxation rate of cTnI-ND hearts was at a level similar to that of wild type mouse hearts under beta-adrenergic stimulation. The decrease in cardiac output due to lowered preload was significantly smaller for cTnI-ND hearts compared with controls. These findings indicate that removal of the N-terminal extension of cTnI via restricted proteolysis enhances cardiac function by increasing the rate of myocardial relaxation and lowering left ventricular end diastolic pressure to facilitate ventricular filling, thus resulting in better utilization of the Frank-Starling mechanism.

There is increasing evidence that subcellular targeting of signaling molecules is an important means of regulating the protein kinase A (PKA) pathway. Subcellular organization of the signaling molecules in the PKA pathway insures that a signal initiated at the receptor level is transferred efficiently to a PKA substrate eliciting some cellular response. This subcellular targeting appears to regulate the function of a highly specialized cell such as the cardiac myocyte. This review focuses on A-kinase anchoring proteins (AKAPs) which are expressed in the heart. It has been determined that, of the approximately 13 different AKAPs expressed in cardiac tissue, several of these are expressed in cardiac myocytes. These AKAPs bind several PKA substrates and some appear to regulate PKA-dependent phosphorylation of these substrates. AKAP tethering of PKA may be essential for efficient regulation of cardiac muscle contraction. The ability of an AKAP to anchor PKA may be altered in the failing heart, thus compromising the ability of the myocyte to respond to stimuli which elicit the PKA pathway.

Altered expression of different classes of genes has been shown to differentiate between failing and nonfailing human hearts. However, characterization of proteins and the post-translational modifications that regulate their functions is required for understanding both the physiology of cardiac muscle and the mechanisms leading to pathological states associated with cardiac diseases. We present in this paper, an analysis of the human cardiac transcriptome, proteome and phosphoproteome. Data from two sources (i) experiments performed in our laboratory and (ii) bioinformatics searches of public databases (SWISS-PROT, NCBI, Cardiac Gene Expression Knowledge Base, Gene Ontology Consortium and Affymetrix) are reported in a relational database that allows user-designed specific queries. Microarray experiments were performed with Affymetrix Hu95Av2. Cardiac proteins were digested with trypsin. An 11 step cation exchange procedure produced fractions for analysis in separate reversed phase high-performance liquid chromatography-tandem mass spectrometry (MS/MS) experiments. Immobilized metal affinity chromatography was used to select the phosphopeptides from the same tryptic peptide mixture. They were then further investigated by MS/MS. Gel-free approaches were used to detect 267 proteins and 47 phosphopeptides. Our human cardiac database contains 447 entries. We propose the use of this platform, built with data derived from nonfailing hearts, as a template for initiating the effort to characterize the human cardiac proteome and its associated post-translational modifications.

Background: We investigated the occurrence of apoptosis during and after resolution of cardiac allograft rejection. Apoptosis could play different roles in graft survival depending on the target cells; thus, we also determined the cell types involved., Methods: Endomyocardial biopsy specimens were evaluated during the first 6 months after transplantation as follows: group I, no current or prior rejection; group II, during an episode of moderate rejection; and group III, histologic resolution after an episode of moderate rejection., Results: Groups II and III showed significantly increased apoptotic activity, indicated by increased caspase-8 and caspase-3 activity; however, activated caspase-3 was undetectable in group I. Activated caspase-3 was detected only in groups II and III. Terminal deoxynucleotide transferase-mediated dUTP nick-end labeling was detected in groups II and III but not group I and predominantly in inflammatory cells., Conclusions: Increased caspase activity and apoptosis of infiltrating cells not only occurs during acute cardiac allograft rejection but persists after histologic resolution. Thus, programmed cell death occurs beyond the period of histologic resolution and may play a role in regulation of the rejection process.

Protein kinase A anchoring proteins (AKAPs) tether cAMP-dependent protein kinase (PKA) to specific subcellular locations. The muscle AKAP, mAKAP, co-localizes with the sarcoplasmic reticulum Ca2+ release channel or ryanodine receptor (RyR). The purpose of this study was to determine whether anchoring of PKA by mAKAP regulates RyR function. Either mAKAP or mAKAP-P, which is unable to anchor PKA, was expressed in CHO cells stably expressing the skeletal muscle isoform of RyR (CHO-RyR1). Immunoelectron microscopy showed that mAKAP co-localized with RyR1 in disrupted skeletal muscle. Following the addition of 10 microm forskolin to activate adenylyl cyclase, RyR1 phosphorylation in CHO-RyR1 cells expressing mAKAP increased by 42.4 +/- 6.6% (n = 4) compared with cells expressing mAKAP-P. Forskolin treatment alone did not increase the amplitude of the cytosolic Ca2+ transient in CHO-RyR1 cells expressing mAKAP or mAKAP-P; however, forskolin plus 10 mm caffeine elicited a cytosolic Ca2+ transient, the amplitude of which increased by 22% (p < 0.05) in RyR1/mAKAP-expressing cells compared with RyR1/mAKAP-P-expressing cells. Therefore, localization of PKA by mAKAP at RyR1 increases both PKA-dependent RyR phosphorylation as well as efflux of Ca2+ through the RyR. Therefore, RyR1 function is regulated by mAKAP targeting of PKA, implying an important functional role for PKA phosphorylation of RyR in skeletal muscle.

The native reference peptide (NRP) method has been adapted to the measure of the degree of protein nitration at a specific tyrosine residue. In these experiments, human serum albumin was modified in a myeloperoxidase-mediated reaction in the presence of nitrite, with nitration detected predominantly at one site, Y162. The time-dependent increase in nitration at this site was measured based on the increasing abundance of the peptide 162YnLYEIAR168 and the corresponding decrease in the 162YLYEIAR168 peptide in in-gel trypsin digests. The peptide 66LVNEVTEFAK75, also formed in the tryptic digest, was used as the native reference peptide. Quantitation was achieved by determining the chromatographic peak area of the two analyte peptides relative to the native reference peptide by LC/tandem mass spectrometric analyses with selected reaction monitoring. The NRP results were validated by correlation to the time-dependent increase in total protein-nitrotyrosine content determined by Western blot analysis. The precision and limit of detection of the assay were also evaluated and were found to be approximately 10% (relative standard deviation) and 5 fmol on-column, respectively. These results demonstrate the utility of the NRP method for quantitative analyses of posttranslation modifications, in terms of broad applicability, ease of experimental design, sensitivity, and precision.

Multiple pathways are responsible for transducing mechanical and hormonal stimuli into changes in gene expression during heart failure. In this study our goals were (i) to develop a sound statistical method to establish a comprehensive cutoff point for identification of differentially expressed genes, (ii) to identify a gene expression fingerprint for heart failure, (iii) to attempt to distinguish different etiologies of heart failure by their gene expression fingerprint, and (iv) to identify gene clusters that show coordinated up- or down-regulation in human heart failure. We used oligonucleotide microarrays to profile seven nonfailing (NF) and eight failing (F) human hearts with a diagnosis of end-stage dilated cardiomyopathy. Biological and experimental variability of the hybridization data were analyzed, and then a statistical analysis procedure was developed, including Student's t test after log-transformation and Wilcoxon Mann-Whitney test. A comprehensive cutoff point composed of fold change, average difference, and absolute call was then established and validated by TaqMan PCR. Of 6,606 genes on the GeneChip, 103 genes in 10 functional groups were differentially expressed between F and NF hearts. A dendrogram identified a gene expression fingerprint of F and NF hearts and also distinguished two F hearts with distinct etiologies (familial and alcoholic cardiomyopathy, respectively) with different expression patterns. K means clustering also revealed two potentially novel pathways associated with up-regulation of atrial natriuretic factor and brain natriuretic peptide and with increased expression of extracellular matrix proteins. Gene expression fingerprints may be useful indicators of heart failure etiologies.

We have developed and validated a method that uses liquid chromatography/electrospray ionization-mass spectrometry to quantify site-specific protein phosphorylation. The method uses selected ion monitoring to determine the chromatographic peak areas of specific tryptic peptides from the protein of interest. The extent of phosphorylation is determined from the ratio of the phosphopeptide peak area to the peak area of an unmodified reference peptide that acts as internal standard, correcting for variations in protein amounts and peptide recovery in the digest preparation procedure. As a result, we refer to this protocol as the native reference peptide method. Mole of phosphate at the selected site per mole of protein is obtained from this ratio, using calibration curves of synthetic peptides to determine relative responses. Our method begins with protein separation by SDS-PAGE and is carried out on amounts of peptide produced by an in-gel digestion of single Coomassie blue-stained bands. To illustrate the utility of the method and provide validation, we used cardiac troponin I as analyte and monitored the time course of a protein kinase C betaII reaction. Those analyses appropriately demonstrate the time-dependent increase of phosphorylation at a PKC-preferred site, Ser44 in the peptide 41ISASPR45 and the concomitant consumption of the nonphosphorylated peptide. We believe that this method provides a novel tool to directly measure specific phosphorylation sites in proteins in different physiological states and expect that the method will be adaptable not only to a variety of samples types (i.e., culture cells, tissues, etc.) but to a variety of posttranslation modifications as well.

Objectives: This study was undertaken to test the hypothesis that transplant coronary vasculopathy (CV) is associated with increased myocardial protein expression of both tissue factor (TF) and integrin alphavbeta3., Background: The vitronectin receptor (integrin alphavbeta3) and TF have recently been found to play a key role in apoptotic cell death and vascular endothelial cell injury., Methods: A total of 77 heart transplant recipients underwent simultaneous endomyocardial biopsy and intravascular ultrasound (IVUS) at one year of transplant. Patients with pre-existing donor coronary atherosclerosis (n = 35) or with acute rejection (grade >1A, n = 10) at the time of the IVUS were excluded from the analysis. The remaining 32 patients constitute the cohort of the present study. A computerized biopsy score was derived based on the duration and severity of cellular rejection. Both TF and alphavbeta3 expression in the heart biopsy specimens were evaluated by immunoperoxidase histochemistry and Western blot analysis., Results: Patients with CV (n = 24) had increased expression of alphavbeta3 (2.7-fold, p = 0.003) and TF (7.9-fold, p = 0.04) compared with patients without evidence of vasculopathy (n = 8). In the absence of myocardial fibrosis, alphavbeta3 expression correlated significantly with the cellular rejection score (r = 0.58, p = 0.02)., Conclusions: Transplant vasculopathy is associated with increased expression of both TF and alphavbeta3. The significant correlation of alphavbeta3 with cellular rejection suggests an important role for this integrin in serving as a mechanistic link between cellular rejection and vasculopathy.

The vitronectin receptor (integrin alphavbeta3), a cell-surface adhesion receptor, has been shown to play a significant role in endothelial cell migration, apoptosis, atherosclerosis, and T-lymphocyte activation. This study was undertaken to test the hypothesis that cardiac allograft rejection is associated with increased expression of alphavbeta3. We also determined whether fibronectin receptor (alpha5beta1) and tissue factor are up-regulated in the presence of acute cellular rejection. We evaluated endomyocardial biopsy specimens with histologic evidence of different degrees of acute cellular rejection (grade 0, n = 10; grade 1A, n = 10; grade 2, n = 10; grade 3A, n = 10). Biopsies were obtained 2-4weeks after cardiac transplantation. Immunoperoxidase staining was performed for alphavbeta3, tissue factor, and alpha5beta1, and protein levels were further determined by Western blot analysis. Specimens with grade 2 and grade 3A rejection showed positive staining of alphavbeta3 in lymphocytic aggregates and vascular endothelial cells. By immunoblotting, we identified significantly increased expression of alphavbeta3 in the presence of acute rejection, grade 2 (3-fold, p = 0.01) and grade 3A (3.6-fold, p = 0.005) compared to grade 0 and 1 A specimens. There was no evidence of increased expression of alpha5beta1 or tissue factor. Acute cellular rejection, a process characterized by T-lymphocyte activation and release of inflammatory cytokines, is associated with increased expression of alphavbeta3.

An increase in extracellular adenosine triphosphate (ATP) is arrhythmogenic in rat cardiac myocytes and extracellular ATP levels are elevated during cardiac ischemia. To gain insight into the mechanism by which the arrhythmic contractions are generated, we investigated changes in subcellular elemental content by electron probe microanalysis (EPMA) in isolated adult rat cardiac myocytes stimulated by the ATP analog, 2-methylthio-ATP (2-M-S-ATP). We also measured the effects of 2-M-S-ATP stimulation on myocyte cell shortening. In electrically stimulated myocytes, 2-M-S-ATP stimulation generated arrhythmic contractions and also increased the amplitude of cell shortening. However, only the arrhythmic contractions were reversed by 2-M-S-ATP washout. EPMA of freeze-dried cryosections of rapidly frozen 2-M-S-ATP-stimulated myocytes showed increased cytosolic Na and Cl, decreased K, but no significant change in mitochondrial Ca upon 2-M-S-ATP stimulation. The arrhythmias were abolished upon 2-M-S-ATP washout, and the observed changes in cytosolic elemental content also reversed upon agonist washout, thus suggesting that the increased Na and Cl, and decreased K, are specifically associated with the ATP-dependent spontaneous contractile activity. The observed increase in intracellular Na upon 2-M-S-ATP stimulation may explain our observation of prolonged relaxation time of 2-M-S-ATP-stimulated contractions. This may be due to inhibition of Ca(2+) efflux via the Na(+) Ca(2+) exchanger.