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 (M2R) 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 M2R 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 (similar to 1.2-fold EC50 increase with depolarization) that was not manifested in the relative affinity, as estimated by the current deactivation kinetics (tau = 4074 +/- 214 ms at -100 mV and tau = 4331 +/- 341 ms at +30 mV; P = 0.31). Moreover, I-KACh did not exhibit relaxation. Contrarily, activation of the M2R-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 (similar to 2.8-fold EC50 increase with depolarization) and in the I-KACh deactivation kinetics (tau = 1762 +/- 119 ms at -100 mV and tau = 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.

Automated summary

The inhibitory regulation of the heart is determined by both M-2 and A(1) receptors, which activate K-ACh channels. The study found that M2R has voltage sensitivity, while A(1)R has minimal voltage dependence. This finding further supports the hypothesis of agonist-specific voltage dependence in GPCRs.