Expression and Impact of Adenosine A(3) Receptors on Calcium Homeostasis in Human Right Atrium

Increased adenosine A(2A) receptor (A(2A)R) expression and activation underlies a higher incidence of spontaneous calcium release in atrial fibrillation (AF). Adenosine A(3) receptors (A(3)R) could counteract excessive A(2A)R activation, but their functional role in the atrium remains elusive, and we therefore aimed to address the impact of A(3)Rs on intracellular calcium homeostasis. For this purpose, we analyzed right atrial samples or myocytes from 53 patients without AF, using quantitative PCR, patch-clamp technique, immunofluorescent labeling or confocal calcium imaging. A(3)R mRNA accounted for 9% and A(2A)R mRNA for 32%. At baseline, A(3)R inhibition increased the transient inward current (I(TI)) frequency from 0.28 to 0.81 events/min (p < 0.05). Simultaneous stimulation of A(2A)Rs and A(3)Rs increased the calcium spark frequency seven-fold (p < 0.001) and the I(TI) frequency from 0.14 to 0.64 events/min (p < 0.05). Subsequent A(3)R inhibition caused a strong additional increase in the I(TI) frequency (to 2.04 events/min; p < 0.01) and increased phosphorylation at s2808 1.7-fold (p < 0.001). These pharmacological treatments had no significant effects on L-type calcium current density or sarcoplasmic reticulum calcium load. In conclusion, A(3)Rs are expressed and blunt spontaneous calcium release at baseline and upon A(2A)R-stimulation in human atrial myocytes, pointing to A(3)R activation as a means to attenuate physiological and pathological elevations of spontaneous calcium release events.