β-Adrenergic Enhancement of Sarcoplasmic Reticulum Calcium Leak in Cardiac Myocytes Is Mediated by Calcium/Calmodulin-Dependent Protein Kinase

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Abstract
Enhanced cardiac diastolic Ca leak from the sarcoplasmic reticulum (SR) ryanodine receptor may reduce SR Ca content and contribute to arrhythmogenesis. We tested whether β-adrenergic receptor (β-AR) agonists increased SR Ca leak in intact rabbit ventricular myocytes and whether this depends on protein kinase A or Ca/calmodulin-dependent protein kinase II (CaMKII) activity. SR Ca leak was assessed by acute block of the ryanodine receptor by tetracaine and assessment of the consequent shift of Ca from cytosol to SR (measured at various SR Ca loads induced by varying frequency). Cytosolic [Ca] ([Ca]i) and SR Ca load ([Ca]SRT) were assessed using fluo-4. β-AR activation by isoproterenol dramatically increased SR Ca leak. However, this effect was not inhibited by blocking protein kinase A by H-89, despite the expected reversal of the isoproterenol-induced enhancement of Ca transient amplitude and [Ca]i decline rate. In contrast, inhibitors of CaMKII, KN-93, or autocamtide-2–related inhibitory peptide II or β-AR blockade reversed the isoproterenol-induced enhancement of SR Ca leak, and CaMKII inhibition could even reduce leak below control levels. Forskolin, which bypasses the β-AR in activating adenylate cyclase and protein kinase A, did not increase SR Ca leak, despite robust enhancement of Ca transient amplitude and [Ca]i decline rate. The results suggest that β-AR stimulation enhances diastolic SR Ca leak in a manner that is (1) CaMKII dependent, (2) not protein kinase A dependent, and 3) not dependent on bulk [Ca]i.