Increased Ca2+leak and spatiotemporal coherence of Ca2+release in cardiomyocytes during beta-adrenergic stimulation

beta-Adrenergic receptor (beta-AR) stimulation of cardiac muscle has been proposed to enhance Ca2+ release from the sarcoplasmic reticulum (SR) through ryanodine receptors (RyRs). However, the anticipated increase in RyR Ca2+ sensitivity has proven difficult to study in intact cardiomyocytes, due to accompanying alterations in SR Ca2+ content, inward Ca2+ current (I-Ca) and diastolic cytosolic Ca2+ concentration ([Ca2+](i)). Here, we studied whole-cell Ca2+ release and spontaneous Ca2+ leak (Ca2+ sparks) in guinea-pig ventricular myocytes with confocal Ca2+ imaging before and during beta-AR stimulation by isoproterenol (Iso), but under otherwise nearly identical experimental conditions. The extent of SR Ca2+ loading was controlled under whole-cell voltage-clamp conditions. UV flash-induced uncaging of Ca2+ from DM-nitrophen was employed as an invariant trigger for whole-cell Ca2+ release. At matched SR Ca2+ content, we found that Iso enhanced the spatiotemporal coherence of whole-cell Ca2+ release, evident from spatially intercorrelated release and accelerated release kinetics that resulted in moderately (similar to 20%) increased release amplitude. This may arise from higher RyR Ca2+ sensitivity, and was also reflected in spontaneous SR Ca2+ leak. At comparable SR Ca2+ content and cytosolic [Ca2+](i), we observed a similar to 4-fold increase in Ca2+ spark frequency in Iso that also appeared in quiescent cells within 2 min without increased SR Ca2+ content. This was likely to have been mediated by Ca2+/calmodulin-dependent protein kinase (CaMKII), rather than cAMP dependent protein kinase (PKA). We conclude that Iso increases the propensity of RyRs to open, both in response to rapid elevations of [Ca2+](i) and at diastolic [Ca2+](i). While this could be beneficial in enhancing and synchronizing systolic whole-cell SR Ca2+ release, the same behaviour could also be proarrhythmogenic during diastole.