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 Gβγ or of cytosolic Na+. Na+ activation is fast, Gβγ-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 mm), 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 Gβγ scavenger, hence it is Gβγ-dependent; but it does not require GTP. We hypothesized that Na+ elevates the cellular concentration of free Gβγ by promoting the dissociation of the Gαβγ heterotrimer into free GαGDP and Gβγ. Direct biochemical measurements showed that Na+ causes a moderate decrease (∼2-fold) in the affinity of interaction between GαGDP and Gβγ. Furthermore, in accord with the predictions of our model, slow Na+ activation was enhanced by mild coexpression of Gαi3. Our findings reveal a previously unknown mechanism of regulation of G proteins and demonstrate a novel Gβγ-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 Gβγ.