In heart failure, chronic catecholaminergic stimulation increases diastolic Ca2+ leak from ryanodine receptors (RyRs) of sarcoplasmic reticulum (SR), possibly due to the phosphorylation of RyRs through the activation of protein kinase A (PKA) or Ca2+/calmodulin-dependent protein kinase II (CaMKII). In the present study, we attempted to identify which activated kinase is responsible for the enhanced Ca2+ leak caused by beta-adrenergic stimulation. Trabeculae obtained from the hearts of adult male C57BL/6J mice were treated with isoproterenol and then permeabilized with saponin. To examine SIR functions, Ca2+ in SR was released with caffeine and measured with fluo-3. The Ca2+ leak in isoproterenol-treated preparations was significantly increased when the PKA-dependent phosphorylation of RyR was increased without the involvement of CaMKII-dependent phosphorylation. Both the increase in Ca2+ leak and the phosphorylation of RyR were blocked by a PKA inhibitor. Our results show that beta-adrenergic stimulation increases Ca2+ leak from SR through PKA-dependent phosphorylation of RyR. (C) 2009 Elsevier Inc. All rights reserved.