GTP-binding protein Gq mediates muscarinic-receptor-induced inhibition of the inwardly rectifying potassium channel IRK1 (Kir 2.1)

Abstract

The inwardly rectifying potassium channel IRK1, a member of the Kir 2.0 family, is inhibited by m1 muscarinic receptor stimulation. In this study the mechanism of action underlying the muscarinic response was investigated by identification of the subtype of heterotrimeric G-protein involved in transduction of the signal. tsA201 cells were simultaneously transfected with cDNAs encoding IRK1, m1 and the Gα subunit of either G q, G 12 or G 13. The whole-cell patch–clamp technique was used to study the effects of G-protein transfection. Antibodies generated against the C-terminal regions of Gα q/11 and Gα 12 were used to confirm G-protein expression by Western blot. When challenged with carbachol, IRK1 currents recorded from cells co-transfected with Gα q were potently inhibited compared with controls. Conversely, co-transfection with Gα 12 or Gα 13 subunits had no effect on muscarinic-receptor-induced inhibition of IRK1. Concentration response curves revealed that carbachol was 16 times more potent at inhibiting IRK1 currents in cells co-transfected with Gα q as compared with Gα 12 co-transfected cells. Immunoblotting illustrated low levels of endogenous Gα q/11 and Gα 12 in untransfected tsA cells. Transfection with Gα q or Gα 12 cDNAs greatly increased the levels of G-protein expression in both cell populations. G-protein expression did not interfere with m1 muscarinic receptor expression levels. These findings suggest that the m1 muscarinic-receptor-induced inhibition of IRK1 is mediated by the heterotrimeric G-protein, Gα q, in tsA cells.