The M3 receptor-mediated K(+) current (IKM3), a G(q) protein-coupled K(+) channel.

Stimulation of muscarinic acetylcholine receptors (mAChRs) can activate an inward rectifier K(+) current (I(KACh)), which is mediated by the M(2) subtype of mAChR in cardiac myocytes. Recently, a novel delayed rectifier-like K(+) current mediated by activation of the cardiac M(3) receptors (designated I(KM3)) was identified, which is distinct from I(KACh) and other known K(+) currents. While I(KACh) is known to be a G(i) protein-gated K(+) channel, the signal transduction mechanisms for I(KM3) activation remained unexplored. We studied I(KM3) with whole-cell patch clamp and macropatch clamp techniques. Whole cell I(KM3) activated by choline persisted with minimal rundown over 2 h in presence of internal GTP. When GTP was replaced by guanyl-5'-yl thiophosphate, I(KM3) demonstrated rapid and extensive rundown. While I(KACh) (induced by ACh) was markedly reduced in cells pretreated with pertussis toxin, I(KM3) was unaltered. Intracellular application of antibodies targeting alpha-subunit of G(i/o) protein suppressed I(KACh) without affecting I(KM3). Antibodies targeting the N and the C terminus, respectively, of G(q) protein alpha-subunit substantially depressed I(KM3) but failed to alter I(KACh). The antibody against beta-subunits of G proteins inhibited both I(KACh) and I(KM3). I(KM3) activated by choline in the cell-attached mode of macropatches persisted in the cell-free configuration. Application of purified G(q) protein alpha-subunit or betagamma-subunit of G proteins or guanosine 5'-O-(thiotriphosphate) to the internal solution activated I(KM3)-like currents in inside-out patches. Our findings revealed a novel aspect of receptor-channel signal transduction mechanisms, and I(KM3) represents the first G(q) protein-coupled K(+) channel. We propose that the G protein-coupled K(+) channel family could be divided into two subfamilies: G(i) protein-coupled K(+) channel subfamily and G(q) protein-coupled K(+) channel subfamily.