Beta-Adrenergic Stimulation Increases the Intra-SR Ca Termination Threshold for Spontaneous Ca Release in Cardiac Myocytes

In the heart, beta-adrenergic stimulation is associated with pro-arrhythmic Ca waves that occur as the result of the sarcoplasmic reticulum (SR) Ca content reaching a critical threshold level. Recently, we have shown that beta-adrenergic stimulation increases the intra-SR Ca threshold for Ca wave initiation, potentially serving as a protective mechanism against pro-arrhythmic Ca release during beta-adrenergic stimulation (Domeier et al., 2012). However, data regarding the termination of such release and details on the regulation of this process have yet to be elucidated. In this study we directly and dynamically measured the intra-SR Ca level ([Ca]SR) at which spontaneous Ca waves terminate (termination threshold) under control conditions and during beta-adrenergic stimulation. Application of the beta-adrenergic receptor agonist isoproterenol (ISO; 1 μM) resulted in an increase in basal [Ca]SR. Importantly, in the presence of ISO the [Ca]SR at which spontaneous Ca waves terminated was also increased compared to control conditions. In addition, the depletion amplitude of spontaneous Ca waves was decreased in the presence of ISO compared to control conditions. When [Ca]SR was subsequently lowered in the presence of ISO to that observed under control conditions (by reducing extracellular Ca and partially inhibiting SERCA with cyclopiazonic acid or thapsigargin), the [Ca]SR at which spontaneous release terminated was still increased compared to control conditions. Likewise, the depletion amplitude remained decreased compared to control conditions. These data indicate that during beta-adrenergic stimulation in the heart, both the intra-SR Ca threshold at which spontaneous Ca waves initiate and terminate is increased, while the amount of Ca released during Ca waves is decreased. The Ca wave termination level may represent an important mode of altering diastolic Ca wave amplitude, and thus, the arrhythmogenic potential of the cell during acute beta-adrenergic stimulation.