The ␤␥ subunits of GTP-binding proteins (G ␤␥ ) activate the muscarinic K ؉ channel (K ACh ) in heart by direct binding to both of its component subunits. K ACh channels can also be gated by internal Na ؉ ions. Both activation mechanisms show dependence on hydrolysis of intracellular ATP. We report that phosphatidylinositol 4,5-bisphosphate (PIP 2 ) mimics the ATP effects and that depletion or block of PIP 2 retards the stimulatory effects of G ␤␥ subunits or Na ؉ ions on channel activity, effects that can be reversed by restoring PIP 2 . Thus, regulation of K ACh channel activity may be crucially dependent on PIP 2 and phosphatidylinositol signaling. These striking functional results are in agreement with in vitro biochemical studies on the PIP 2 requirement for G ␤␥ stimulation of G protein receptor kinase activity, thus implicating phosphatidylinositol phospholipids as a potential control point for G ␤␥ -mediated signal transduction.

In freshly dissociated uterine myocytes, the outward current is carried by K+ through channels highly selective for K+. Typically, nonpregnant myocytes have rather noisy K+ currents; half of them also have a fast-inactivating transient outward current (ITO). In contrast, the current records are not noisy in late pregnant myocytes, and ITO densities are low. The whole-cell IK of nonpregnant myocytes respond strongly to changes in [Ca2+]o or changes in [Ca2+]i caused by photolysis of caged Ca2+ compounds, nitr 5 or DM-nitrophene, but that of late-pregnant myocytes respond weakly or not at all. The Ca2+ insensitivity of the latter is present before any exposure to dissociating enzymes. By holding at −80, −40, or 0 mV and digital subtractions, the whole-cell IK of each type of myocyte can be separated into one noninactivating and two inactivating components with half-inactivation at approximately −61 and −22 mV. The noninactivating components, which consist mainly of iberiotoxin-susceptible large-conductance Ca2+-activated K+ currents, are half-activated at 39 mV in nonpregnant myocytes, but at 63 mV in late-pregnant myocytes. In detached membrane patches from the latter, identified 139 pS, Ca2+-sensitive K+ channels also have a half-open probability at 68 mV, and are less sensitive to Ca2+ than similar channels in taenia coli myocytes. Ca2+-activated K+ currents, susceptible to tetraethylammonium, charybdotoxin, and iberiotoxin contribute 30–35% of the total IK in nonpregnant myocytes, but <20% in late-pregnant myocytes. Dendrotoxin-susceptible, small-conductance delayed rectifier currents are not seen in nonpregnant myocytes, but contribute ∼20% of total IK in late-pregnant myocytes. Thus, in late-pregnancy, myometrial excitability is increased by changes in K+ currents that include a suppression of the ITO, a redistribution of IK expression from large-conductance Ca2+-activated channels to smaller-conductance delayed rectifier channels, a lowered Ca2+ sensitivity, and a positive shift of the activation of some large-conductance Ca2+-activated channels.

Muscarinic potassium channels (KAch) are composed of two subunits, GIRK1 and GIRK4 (or CIR), and are directly gated by G proteins. We have identified a novel gating mechanism of KAC h, independent of G-protein activation. This mechanism involved functional modification of KACh which required hydrolysis of physiological levels of intracellular ATP and was manifested by an increase in the channel mean open time. The ATP-modifled channels could in turn be gated by intracellular Na +, starting at ~3 mM with an ECs0 of ~40 raM. The Na+-gating of KAC h was operative both in native atrial cells and in a heterologous system expressing recombinant channel subunits. Block of the Na+/K + pump (e.g., by cardiac glycosides) caused significant activation of KAC h in atrial cells, with a time course similar to that of Na + accumulation and in a manner indistinguishable from that of Na+-mediated activation of the channel, suggesting that cardiac glycosides activated KACh by increasing intracellular Na + levels. These results demonstrate for the first time a direct effect of cardiac glycosides on atrial myocytes involving ion channels which are critical in the regulation of cardiac rhythm.