Abstract Body: Cardiac arrhythmias are leading cause of death in the United States. Evidences indicates that many cardiac events occur in a time-of-day–dependent manner. However, the intrinsic electrophysiological mechanisms underlying these diurnal variations in arrhythmia susceptibility remain incompletely understood. We hypothesized that cardiac electrophysiological properties exhibit 24-hour fluctuations and that arrhythmia susceptibility increases during periods associated with shorter refractory periods.

In this ex vivo study, 24 rat hearts were used. Rat hearts were excised at six Zeitgeber time (ZT) points across the 24-h circadian cycle (n=4/time point). Dual optical mapping was performed to simultaneously quantify transmembrane voltage (Vm) and intracellular calcium (Ca++) dynamics. In parallel cohorts, tissues were collected and rapidly flash frozen for molecular study of important downstream regulatory proteins.

Our results shows that cardiac electrophysiology exhibited circadian variation in rat whole hearts, with action potential duration (APD80) (fig 1) and functional refractory period (FRP) showing clear diurnal oscillations with the largest differences observed between ZT6 and ZT14 (fig 2). Calcium transient duration (CaTD80) showed minimal circadian variation, while arrhythmia inducibility was significantly higher during the active phase under both baseline (fig 3) and isoproterenol stimulation.

Our findings suggest that cardiac electrophysiology, mediated by intrinsic circadian clock regulation of ion channels, contributes to time-of-day differences in electrical activity and arrhythmia vulnerability. These observations highlight the potential for time-of-day–guided therapeutic interventions in arrhythmia prevention.