To determine whether G proteins activate cardiac ATP-sensitive K+ (K-A
TP) channels by regulating intracellular ATP (ATP(i))-dependent gating
, currents were measured in inside-out patches. When ATP(i) closed K-A
TP channels, activators of endogenous G proteins, GTP (plus adenosine
or acetylcholine), GTP gamma S, or AlF4- stimulated channels, an effec
t prevented by GDP beta S. In the absence of ATP(i), G protein activat
ors were ineffective. Intracellular nucleoside diphosphates restored K
-ATP channel openings after the ''rundown'' of spontaneous activity. O
nly when ATP(i) suppressed nucleoside diphosphate-induced openings, GT
P gamma S or AlF4- enhanced K-ATP channel activity. Active forms of ex
ogenous G protein subunits (G(alpha i-1), G(alpha i-2), or G(alpha o))
activated only K-ATP channels closed by ATP(i). G proteins stimulate
cardiac K-ATP channels apparently by antagonizing ATP(i)-dependent inh
ibitory gating. Regulation of ligand-dependent gating represents a dis
tinct type of G protein modulation of ion channels.