Pharmacological changes in cellular Ca2+ homeostasis parallel initiation of atrial arrhythmogenesis in murine langendorff-perfused hearts

1. Intracellular Ca(2+) overload has been associated with established atrial arrhythmogenesis. The present experiments went on to correlate acute initiation of atrial arrhythmogenesis in Langendorff-perfused mouse hearts with changes in Ca(2+) homeostasis in isolated atrial myocytes following pharmacological procedures that modified the storage or release of sarcoplasmic reticular (SR) Ca(2+) or inhibited entry of extracellular Ca(2+). 2. Caffeine (1 mmol/L) elicited diastolic Ca(2+) waves in regularly stimulated atrial myocytes immediately following addition. This was followed by a decline in the amplitude of the evoked transients and the disappearance of such diastolic events, suggesting partial SR Ca(2+) depletion. 3. Cyclopiazonic acid (CPA; 0.15 micromol/L) produced more gradual reductions in evoked Ca(2+) transients and abolished diastolic Ca(2+) events produced by the further addition of caffeine. 4. Nifedipine (0.5 micromol/L) produced immediate reductions in evoked Ca(2+) transients. Further addition of caffeine produced an immediate increase followed by a decline in the amplitude of the evoked Ca(2+) transients, without eliciting diastolic Ca(2+) events. 5. These findings correlated with changes in spontaneous and provoked atrial arrhythmogenecity in mouse isolated Langendorf-perfused hearts. Thus, caffeine was pro-arrhythmogenic immediately following but not > 5 min after application and both CPA and nifedipine pretreatment inhibited such arrhythmogenesis. 6. Together, these findings relate acute atrial arrhythmogenesis in intact hearts to diastolic Ca(2+) events in atrial myocytes that, in turn, depend upon a finite SR Ca(2+) store and diastolic Ca(2+) release following Ca(2+)-induced Ca(2+) release initiated by the entry of extracellular Ca(2+).