Synchronization of Spontaneous Calcium Release Waves Among Myocytes in Intact Heart Determines the Magnitude of Delayed Afterdepolarizations and Triggered Activity

Abstract

Intracellular Ca waves occur as the result of spontaneous Ca release (SCR) during Ca overload. Ca waves activate Na-Ca exchange, causing delayed afterdepolarizations (DAD) which can achieve threshold and produce triggered extrasystoles. It is not known how these single cell events depolarize enough myocytes in intact heart to produce a triggered beat. We combined experimental observations with computer simulations to explain how SCR synchronization among myocytes brings a critical tissue mass to threshold. Confocal microscopy was used to measure SCR waves in groups of myocytes in the LV epicardium of rat hearts loaded with fluo-4AM. Contraction was abolished with cytochalasin-D and blebbistatin. Raising extracellular [Ca] and rapid pacing protocols were used to increase sarcoplasmic reticulum (SR) Ca load and induce Ca waves. As Ca load increased, the number of myocytes giving waves increased. Both the wave latency and the variability (SD) of wave latency decreased with increasing Ca load. Similar results were obtained in isolated rat ventricular myocytes, indicating that the reduction in latency interval and variability represent intrinsic properties of SR release in Ca overload. Computer simulations demonstrate that decreasing wave latency and variability determine the rate and magnitude of increased cytoplasmic [Ca] and therefore determine the timing and magnitude of the DAD. The synchrony of SCR waves among myocytes therefore determines the likelihood of achieving threshold and producing a triggered beat. These results demonstrate that intrinsic properties of SR Ca release are responsible for Ca wave synchronization during Ca overload, causing DADs to reach threshold and produce triggered arrhythmias.