title: Molecular correlates of repolarization alternans in cardiac myocytes.
abstract: Arrhythmogenic action potential alternans (APD-ALT) is thought to arise from beat to beat alteration in cellular Ca(2+) 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 Ca(2+) 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 Ca(2+) cycling between endocardial and epicardial myocytes as evidenced by weaker Ca(2+) release (32% lower Ca(2+) amplitude, and 16% longer rise time), and slower Ca(2+) reuptake (24% larger Ca(2+) decay time constant, and 9% longer Ca(2+) transient duration) in endocardial compared to epicardial myocytes. Taken together these data indicate that myocytes that are most susceptible to APD-ALT exhibit impaired Ca(2+) release and reuptake. Moreover, transmural differences in Ca(2+) 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 Ca(2+) cycling proteins may underlie spatial heterogeneity of APD-ALT that has been closely linked to cardiac arrhythmogenesis.
authors: The Heart and Vascular Research Center, MetroHealth Campus, Case Western Reserve University, Cleveland, OH 44109-1998, USA.
authors: Xiaoping
authors: X
authors: Wan
authors: Kenneth R
authors: KR
authors: Laurita
authors: Etienne J
authors: EJ
authors: Pruvot
authors: David S
authors: DS
authors: Rosenbaum
mesh_terms: D000200:Action Potentials; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000209:etiology*; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000032:analysis / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000032:analysis / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000032:analysis / Q000378:metabolism; D002304:Cardiac Pacing, Artificial; D004699:Endocardium / Q000166:cytology; D006168:Guinea Pigs; D006329:Heart Conduction System / Q000503:physiopathology*; D006352:Heart Ventricles / Q000166:cytology; D032383:Myocytes, Cardiac / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010496:Pericardium / Q000166:cytology; D019837:Ryanodine Receptor Calcium Release Channel / Q000032:analysis / Q000378:metabolism; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019831:Sodium-Calcium Exchanger / Q000032:analysis / Q000378:metabolism; D000099211:Phospholamban
chemical_list: D002118:Calcium; D020746:Calcium Channels, L-Type; D002135:Calcium-Binding Proteins; D000252:Calcium-Transporting ATPases; D019837:Ryanodine Receptor Calcium Release Channel; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019831:Sodium-Calcium Exchanger; D000099211:Phospholamban