Synchronous occurrence of spontaneous localized calcium release from the sarcoplasmic reticulum generates action potentials in rat cardiac ventricular myocytes at normal resting membrane potential

Under certain conditions, spontaneous release of Ca²⁺ from the sarcoplasmic reticulum occurs in resting mammalian myocardium. In single rat ventricular myocytes, such spontaneous Ca²⁺ release appears localized rather than homogeneous. When the increase in cytosolic Ca²⁺ is present in a single locus within a cell, it causes a small depolarization, which, at the normal resting potential, is subthreshold for generating an action potential. However, when spontaneous Ca²⁺ release occurs simultaneously at more than a single discrete locus, the resultant sarcolemmal depolarization is augmented to levels that can induce an action potential, even when this depolarization begins at the normal resting membrane potential. Thus, the synchronous occurrence of multifocal localized increases in cytosolic Ca²⁺ due to spontaneous Ca²⁺ release from the sarcoplasmic reticulum within ventricular myocytes is a mechanism for 'abnormal automaticity'.