The voltage-sensitive cardiac M2 muscarinic receptor modulates the inward rectification of the G protein-coupled, ACh-gated K+ current

The acetylcholine (ACh)-gated inwardly rectifying K + current ( I KACh ) plays a vital role in cardiac excitability by regulating heart rate variability and vulnerability to atrial arrhythmias. These crucial physiological contributions are determined principally by the inwardly rectifying nature of I KACh . Here, we investigated the relative contribution of two distinct mechanisms of I KACh inward rectification measured in atrial myocytes: a rapid component due to K ACh channel block by intracellular Mg 2+ and polyamines; and a time- and concentration-dependent mechanism. The time- and ACh concentration-dependent inward rectification component was eliminated when I KACh was activated by GTPγS, a compound that bypasses the muscarinic-2 receptor (M 2 R) and directly stimulates trimeric G proteins to open K ACh channels. Moreover, the time-dependent component of I KACh inward rectification was also eliminated at ACh concentrations that saturate the receptor. These observations indicate that the time- and concentration-dependent rectification mechanism is an intrinsic property of the receptor, M 2 R; consistent with our previous work demonstrating that voltage-dependent conformational changes in the M 2 R alter the receptor affinity for ACh. Our analysis of the initial and time-dependent components of I KACh indicate that rapid Mg 2+ -polyamine block accounts for 60–70% of inward rectification, with M 2 R voltage sensitivity contributing 30–40% at sub-saturating ACh concentrations. Thus, while both inward rectification mechanisms are extrinsic to the K ACh channel, to our knowledge, this is the first description of extrinsic inward rectification of ionic current attributable to an intrinsic voltage-sensitive property of a G protein-coupled receptor.