AbstractAim:  The multifunctional signal molecule calmodulin kinase II (CaMKII) has been associated with cardiac arrhythmogenesis under conditions where its activity is chronically elevated. Recent studies report that its activity is also acutely elevated during acidosis. We test a hypothesis implicating CaMKII in the arrhythmogenesis accompanying metabolic acidification.Methods:  We obtained monophasic action potential recordings from Langendorff‐perfused whole heart preparations and single cell action potentials (AP) using whole‐cell patch‐clamped ventricular myocytes. Spontaneous sarcoplasmic reticular (SR) Ca2+release events during metabolic acidification were investigated using confocal microscope imaging of Fluo‐4‐loaded ventricular myocytes.Results:  In Langendorff‐perfused murine hearts, introduction of lactic acid into the Krebs‐Henseleit perfusate resulted in abnormal electrical activity and ventricular tachycardia. The CaMKII inhibitor, KN‐93 (2 μm), reversibly suppressed this spontaneous arrhythmogenesis during intrinsic rhythm and regular 8 Hz pacing. However, it failed to suppress arrhythmia evoked by programmed electrical stimulation. These findings paralleled a CaMKII‐independent reduction in the transmural repolarization gradients during acidosis, which previously has been associated with the re‐entrant substrate under other conditions. Similar acidification produced spontaneous AP firing and membrane potential oscillations in patch‐clamped isolated ventricular myocytes when pipette solutions permitted cytosolic Ca2+ to increase following acidification. However, these were abolished by both KN‐93 and use of pipette solutions that held cytosolic Ca2+ constant during acidosis. Acidosis also induced spontaneous Ca2+ waves in isolated intact Fluo‐4‐loaded myocytes studied using confocal microscopy that were abolished by KN‐93.Conclusion:  These findings together implicate CaMKII‐dependent SR Ca2+ waves in spontaneous arrhythmic events during metabolic acidification.