Single Channel Studies of Inward Rectifier Potassium Channel Regulation by Muscarinic Acetylcholine Receptors

Negative regulation of the heartbeat rate involves the activation of an inwardly rectifying potassium current (I(KACh)) by G protein–coupled receptors such as the m2 muscarinic acetylcholine receptor. Recent studies have shown that this process involves the direct binding of G(βγ) subunits to the NH(2)- and COOH-terminal cytoplasmic domains of the proteins termed GIRK1 and GIRK4 (Kir3.1 and Kir3.4/CIR), which mediate I(KACh). Because of the very low basal activity of native I(KACh), it has been difficult to determine the single channel effect of G(βγ) subunit binding on I(KACh) activity. Through analysis of a novel G protein–activated chimeric inward rectifier channel that displays increased basal activity relative to I(KACh), we find that single channel activation can be explained by a G protein–dependent shift in the equilibrium of open channel transitions in favor of a bursting state of channel activity over a long-lived closed state.