A hallmark of atrial fibrillation is an excess of spontaneous calcium release events, which can be mimicked by β1- or β2-adrenergic stimulation. Because β1-adrenergic receptor blockers (β1-blockers) are primarily used in clinical practice, we here examined the impact of β2-adrenergic stimulation on spontaneous calcium release and assessed whether the R- and S-enantiomers of the non-selective β- blocker carvedilol could reverse these effects. For this purpose, human atrial myocytes were isolated from patients undergoing cardiovascular surgery and subjected to confocal calcium imaging or immunofluorescent labeling of the ryanodine receptor (RyR2). Interestingly, the β2-adrenergic agonist fenoterol increased the incidence of calcium sparks and waves to levels observed with the non-specific β-adrenergic agonist isoproterenol. Moreover, fenoterol increased both the amplitude and duration of the sparks, facilitating their fusion into calcium waves. Subsequent application of the non β-blocking R-Carvedilol enantiomer reversed these effects of fenoterol in a dose-dependent manner. R-Carvedilol also reversed the fenoterol-induced phosphorylation of the RyR2 at Ser-2808 dose-dependently, and 1 µM of either R- or S-Carvedilol fully reversed the effect of fenoterol. Together, these findings demonstrate that β2-adrenergic stimulation alone stimulates RyR2 phosphorylation at Ser-2808 and spontaneous calcium release maximally, and points to carvedilol as a tool to attenuate the pathological activation of β2-receptors.
