Molecular correlates of repolarization alternans in cardiac myocytes

Arrhythmogenic action potential alternans (APD-ALT) is thought to arise from beat to beat alteration in cellular Ca2+ cycling. Previously, we found that spatial heterogeneity in APD-ALT between ventricular myocytes is key to the mechanism linking APD-ALT to cardiac arrhythmogenesis. However, the cellular and molecular basis for APD-ALT is poorly understood. To test the hypothesis that spatial heterogeneities in expression and function of calcium cycling proteins underlies heterogeneities in APD-ALT, endocardial and epicardial myocytes were isolated from left ventricular free wall of 20 guinea pig hearts. APD-ALT and Ca2+ transient alternans (Ca-ALT) were measured simultaneously as stimulus rate was increased progressively. Endocardial myocytes exhibited greater susceptibility to cellular alternans than epicardial myocytes as evidenced by a significantly lower pacing rate threshold for APD-ALT (113 ± 9 bpm vs. 151 ± 8 bpm, respectively, P < 0.05) and for Ca-ALT (110 ± 8 bpm vs. 149 ± 8 bpm, respectively, P < 0.05). APD-ALT never occurred without Ca-ALT, whereas Ca-ALT was readily induced in the absence of APD-ALT by repetitive constant action potential waveform, suggesting that Ca-ALT was not secondary to APD-ALT. Importantly, there were significant voltage-independent differences in Ca2+ cycling between endocardial and epicardial myocytes as evidenced by weaker Ca2+ release (32% lower Ca2+ amplitude, and 16% longer rise time), and slower Ca2+ reuptake (24% larger Ca2+ decay time constant, and 9% longer Ca2+ transient duration) in endocardial compared to epicardial myocytes. Taken together these data indicate that myocytes that are most susceptible to APD-ALT exhibit impaired Ca2+ release and reuptake. Moreover, transmural differences in Ca2+ cycling function was associated with significantly reduced endocardial expression of ryanodine release channel (by 22%) and SERCA2 (by 40%), suggesting a potential molecular basis for spatially heterogeneous APD-ALT. Moreover, transmural differences in expression and function of key SR Ca2+ cycling proteins may underlie spatial heterogeneity of APD-ALT that has been closely linked to cardiac arrhythmogenesis.