Recruitment of Multiple Spontaneous Ca2+ Release Initiation Sites Promotes Ca2+ Waves in Myocytes of Intact Rat Heart Under Conditions of Ca2+ Overload

Abstract

Spontaneous Ca2+ release (SCR) in the form of Ca2+ waves is responsible for cardiac myocyte depolarization and delayed afterdepolarizations (DADs) that can produce triggered beats. Whether or not a cell reaches threshold is determined by the magnitude and rate of spread of the Ca2+ wave in the cell and the resultant activation of inward current via forward mode Na-Ca exchange. In this study, we combined experimental observations with computer simulations in order to investigate the mechanisms by which the characteristics of Ca2+ wave activation influences DAD magnitude. Ca2+ waves were measured in individual myocytes in the left ventricular subepicardium in rat hearts using confocal microscopy (fluo-4 Ca2+ fluorescence). Extracellular Ca2+, [Ca2+]e, was raised to increase sarcoplasmic reticulum (SR) Ca2+ load and induce Ca2+ waves. With increasing [Ca2+]e, the number of SCR sites increased along with the incidence of Ca2+ waves within myocytes. Interestingly, Ca2+ wave velocity at higher [Ca2+]e was considerably heterogeneous_both faster as well as nearly equivalent to that at normal [Ca2+]e, the average at higher [Ca2+]e being only moderately faster. Computer simulations demonstrated that the recruitment of multiple SCR sites is a highly effective means of increasing the magnitude and rate of cytoplasmic Ca2+. The rapid delivery of Ca2+ to the Na-Ca exchanger increases DAD magnitude and the probability of producing a triggered beat. Our results suggest that it is the recruitment of multiple SCR initiation sites that determines DAD magnitude and whether or not depolarization can reach threshold. This mechanism is likely to contribute to arrhythmogenesis under conditions of SR Ca2+ overload and in genetically-based disease states in which ryanodine receptor function is altered, such as in catecholaminergic polymorphic ventricular tachycardia.