ICaL inhibition prevents arrhythmogenic Ca2+ waves caused by abnormal Ca2+ sensitivity of RyR or SR Ca2+ accumulation

Arrhythmogenic Ca-2 waves result from uncontrolled Ca-2 release from the sarcoplasmic reticulum (SR) that occurs with increased Ca-2 sensitivity of the ryanodine receptor (RyR) or excessive Ca-2 accumulation during -adrenergic stimulation. We hypothesized that inhibition of the L-type Ca-2 current (I-CaL) could prevent such Ca-2 waves in both situations. Ca-2 waves were induced in mouse left ventricular cardiomyocytes by isoproterenol combined with caffeine to increase RyR Ca-2 sensitivity. I-CaL inhibition by verapamil (0.5 M) reduced Ca-2 wave probability in cardiomyocytes during electrostimulation, and during a 10 s rest period after ceasing stimulation. A separate type of Ca-2 release events occurred during the decay phase of the Ca-2 transient and was not prevented by verapamil. Verapamil decreased Ca-2 spark frequency, but not in permeabilized cells, indicating that this was not due to direct effects on RyR. The antiarrhythmic effect of verapamil was due to reduced SR Ca-2 content following I-CaL inhibition. Computational modelling supported that the level of I-CaL inhibition obtained experimentally was sufficient to reduce the SR Ca-2 content. Ca-2 wave prevention through reduced SR Ca-2 content was also effective in heterozygous ankyrin B knockout mice with excessive SR Ca-2 accumulation during -adrenergic stimulation. I-CaL inhibition prevents diastolic Ca-2 waves caused by increased Ca-2 sensitivity of RyR or excessive SR Ca-2 accumulation during -adrenergic stimulation. In contrast, unstimulated early Ca-2 release during the decay of the Ca-2 transient is not prevented, and merits further study to understand the full antiarrhythmic potential of I-CaL inhibition.