Na+ promotes the dissociation between GαGDP and Gβγ, activating G protein-gated K+ channels

G protein-gated K+ channels (GIRK, or Kir3) are activated by the direct binding of Gbetagamma or of cytosolic Na+. Na+ activation is fast, Gbetagamma-independent, and probably via a direct, low affinity (EC50, 30-40 mm) binding of Na+ to the channel. Here we demonstrate that an increase in intracellular Na+ concentration, [Na+](in), within the physiological range (5-20 nM), activates GIRK within minutes via an additional, slow mechanism. The slow activation is observed in GIRK mutants lacking the direct Na+ effect. It is inhibited by a Gbetagamma scavenger, hence it is Gbetagamma-dependent; but it does not require GTP. We hypothesized that Na+ elevates the cellular concentration of free Gbetagamma by promoting the dissociation of the Galphabetagamma heterotrimer into free Galpha(GDP) and Gbetagamma. Direct biochemical measurements showed that Na+ causes a moderate decrease (similar to2-fold) in the affinity of interaction between Galpha(GDP) and Gbetagamma. Furthermore, in accord with the predictions of our model, slow Na+ activation was enhanced by mild coexpression of Galpha(i3). Our findings reveal a previously unknown mechanism of regulation of G proteins and demonstrate a novel Gbetagamma-dependent regulation of GIRK by Na+. We propose that Na+ may act as a regulatory factor, or even a second messenger, that regulates effectors via Gbetay.