Increased Ca(2+) leak and spatiotemporal coherence of Ca(2+) release in cardiomyocytes during beta-adrenergic stimulation.

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