Differential voltage-dependent modulation of the ACh-gated K(+) current by adenosine and acetylcholine

Inhibitory regulation of the heart is determined by both cholinergic M(2) receptors (M(2)R) and adenosine A(1) receptors (A(1)R) that activate the same signaling pathway, the ACh-gated inward rectifier K(+) (K(ACh)) channels via G(i/o) proteins. Previously, we have shown that the agonist-specific voltage sensitivity of M(2)R underlies several voltage-dependent features of I(KACh), including the ‘relaxation’ property, which is characterized by a gradual increase or decrease of the current when cardiomyocytes are stepped to hyperpolarized or depolarized voltages, respectively. However, it is unknown whether membrane potential also affects A(1)R and how this could impact I(KACh). Upon recording whole-cell currents of guinea-pig cardiomyocytes, we found that stimulation of the A(1)R-G(i/o)-I(KACh) pathway with adenosine only caused a very slight voltage dependence in concentration-response relationships (~1.2-fold EC(50) increase with depolarization) that was not manifested in the relative affinity, as estimated by the current deactivation kinetics (τ = 4074 ± 214 ms at -100 mV and τ = 4331 ± 341 ms at +30 mV; P = 0.31). Moreover, I(KACh) did not exhibit relaxation. Contrarily, activation of the M(2)R-G(i/o)-I(KACh) pathway with acetylcholine induced the typical relaxation of the current, which correlated with the clear voltage-dependent effect observed in the concentration-response curves (~2.8-fold EC(50) increase with depolarization) and in the I(KACh) deactivation kinetics (τ = 1762 ± 119 ms at -100 mV and τ = 1503 ± 160 ms at +30 mV; P = 0.01). Our findings further substantiate the hypothesis of the agonist-specific voltage dependence of GPCRs and that the I(KACh) relaxation is consequence of this property.