Verapamil suppresses cardiac alternans and ventricular arrhythmias in acute myocardial ischemia via ryanodine receptor inhibition.

T-wave alternans (TWA) is a potent arrhythmia substrate under the conditions of acute myocardial ischemia. Abnormal intracellular calcium cycling contributes to the genesis of cardiac alternans. Ryanodine receptor (RyR) is a pivotal Ca2+ cycling protein central to Ca2+ signaling in the heart. Here, we investigated the potential role of RyR in cardiac alternans and ventricular arrhythmias in acute myocardial ischemia. Transmembrane action potentials were simultaneously recorded from epicardium and endocardium together with a transmural ECG and isometric contraction force in the arterially perfused left ventricular wedge preparations. Calcium alternans were induced by incremental frequency of field stimulation in rat ventricular myocytes. TWA, mechanical alternans and ventricular arrhythmias were reproducibly induced by rapid pacing in the acute ischemic wedge preparations. Compared with control group, calcium alternans ratio and spontaneous calcium release were increased in acute ischemic myocytes. Verapamil, a phenylalkylamine calcium channel blocker, can successfully abolish spontaneous calcium release, TWA, and ventricular arrhythmias. The inhibition effect of verapamil could be diminished by low concentration of ryanodine (10 nmol/L). However, nifedipine, a dihydropyridine calcium channel blocker, could not block TWA or arrhythmias. Moreover, verapamil, but not nifedipine, significantly decreased ROS production in ischemic myocytes. Collectively, our results indicate that verapamil can significantly inhibit the development of cardiac alternans and ventricular arrhythmias in acute myocardial ischemia, and the mechanism was related to the inhibition of RyR and the protective function to oxidative stress.