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Increases in action potential duration (APD), genetic or acquired, and arrhythmias are often associated; nonetheless, the relationship between the two phenomena is inconstant, suggesting coexisting factors. β-adrenergic activation increases sarcoplasmic reticulum (SR) Ca 2+ -content; angiotensin II (ATII) may increase cytosolic Ca 2+  and ROS production, all actions stimulating RyRs opening. Here we test how APD interacts with β-adrenergic and AT-receptor stimulation in facilitating spontaneous Ca 2+  release events (SCR).   Methods:  Under “action potential (AP) clamp”, guinea-pig cardiomyocytes (CMs) were driven with long (200 ms), normal (150 ms), and short (100 ms) AP waveforms at a CL of 500 ms; in a subset of CMs, all the 3 waveforms could be tested within the same cell. SCR were detected as inward current transients (I TI ) following repolarization; I TI  incidence and repetition within the same cycle were measured under increasing isoprenaline concentration ([ISO]) alone, or plus 100 nM ATII (30 min incubation+superfusion).   Results:  I TI  incidence and repetition increased with [ISO]; at longer APs the [ISO]-response curve was shifted upward and I TI  coupling interval was reduced. ATII increased I TI  incidence more at low [ISO] and under normal (as compared to long) APs. Efficacy of AP shortening in suppressing I TI  decreased in ATII-treated myocytes and at higher [ISO].   Conclusions:  AP prolongation sensitized the SR to the destabilizing actions of ISO and ATII. Summation of ISO, ATII and AP duration effects had a “saturating” effect on SCR incidence, thus suggesting convergence on a common factor (RyRs stability) “reset” by the occurrence of spontaneous Ca 2+  release events.

Action Potential Prolongation, β-Adrenergic Stimulation, and Angiotensin II as Co-factors in Sarcoplasmic Reticulum Instability