G proteins activate ATP-sensitive K⁺ channels by antagonizing ATP-dependent gating

To determine whether G proteins activate cardiac ATP-sensitive K⁺ (K_ATP) channels by regulating intracellular ATP (ATP_i)-dependent gating, currents were measured in inside-out patches. When ATP_i closed K_ATP channels, activators of endogenous G proteins, GTP (plus adenosine or acetylcholine), GTPγS, or AIF₄^- stimulated channels, an effect prevented by GDPβS. In the absence of ATP_i, G protein activators were ineffective. Intracellular nucleoside diphosphates restored K_ATP channel openings after the "rundown" of spontaneous activity. Only when ATP_i suppressed nucleoside diphosphate-induced openings, GTPγS or AIF₄^- enhanced K_ATP channel activity. Active forms of exogenous G protein subunits (G_αi-1, G_αi-2, or G_α0 activated only K_ATP channels closed by ATP_i. G proteins stimulate cardiac K_ATP channels apparently by antagonizing ATP_i-dependent inhibitory gating. Regulation of ligand-dependent gating represents a distinct type of G protein modulation of ion channels.