Adenosine A2A receptors are expressed in human atrial myocytes and modulate spontaneous sarcoplasmic reticulum calcium release

Abstract

Background: Alterations in the cyclic AMP-dependent regulation of the cardiac ryanodine receptor (RyR2) have been proposed to account for increased spontaneous calcium release from the sarcoplasmic reticulum (SR) in patients with heart failure, ventricular tachyarrhythmias and atrial fibrillation. While the adenosine A(2A) receptor (A(2A)R) is known to regulate cyclic AMP levels, expression and function of this receptor in human cardiac myocytes has not been investigated.

Methods: PCR, western blotting and immunofluorescence were used to identify the A(2A)R, and functional effects of A(2A)R stimulation were measured with confocal calcium imaging and patch-clamp technique.

Results: The A(2A)R is expressed in the human right atrium and distributed in a banded pattern along the Z-lines, overlapping with the ryanodine receptor. A(2A)R stimulation caused a protein kinase A dependent increase in spontaneous SR calcium release in isolated human atrial myocytes. The A(2A)R agonist CGS21680 increased the frequency of calcium sparks from 0.12+/-0.03 to 0.31+/-0.08 sparks.mum min(-1) (p<0.05) and calcium waves from 0.65+/-0.31 to 5.11+/-1.84 waves.min(-1) (p<0.03). Moreover, spontaneous Na-Ca exchange currents (I(NCX)) increased from 1.19+/-0.17 to 2.50+/-0.42 min(-1) (p<0.001). In contrast, CGS21680 did not alter caffeine inducible calcium release (6.98+/-0.52 vs. 6.82+/-0.57 amol pF(-1), p=0.6) or the spontaneous I(NCX) amplitude (0.32+/-0.05 vs. 0.29+/-0.04 pA pF(-1), p=0.2). Current-voltage relationship and amplitude of the L-type calcium current (1.62+/-0.18 vs. 1.80+/-0.18 pA pF(-1)) were not altered, but calcium release dependent inactivation was faster with CGS21680 (13.4+/-0.7 vs. 15.8+/-1.0 ms, p<0.001).

Conclusions: Adenosine A(2A) receptors are expressed in the human atrial myocardium and modulate the frequency of spontaneous calcium release from the SR.