Increased Ca 2+ leak and spatiotemporal coherence of Ca 2+ release in cardiomyocytes during β‐adrenergic stimulation

β‐Adrenergic receptor (β‐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 β‐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 a ∼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.