Specificity of coupling of muscarinic receptor isoforms to a novel chick inward-rectifying acetylcholine-sensitive K+ channel

The G-protein-gated inward-rectifying K+ channel GIRK1 has been demonstrated in heart and brain, These tissues also both express the M(2), M(3), and M(4) muscarinic acetylcholine receptors (mAChR) (Gadbut, A. P., and Galper, J. B. (1994) J. Biol. Chem. 269, 25823-25829). Only the M(2) mAChR has been demonstrated to couple to GIRK1 (Kubo, Y., Reuveny, E., Slesinger, P. A., Jan, Y. N., and Jan, L. Y. (1993) Nature 264, 802-806). In this study we determined the specificity of coupling of the M(3) and M(4) mAChR to a new GIRK1 cloned from a chick brain cDNA library, This clone codes for a 492-amino acid protein that is 93% identical to rat GIRK1 and is expressed in brain, atrium, and ventricle, but not skeletal muscle. In Xenopus laevis oocytes co-expression of GIRK1 with either the chick M(2) or M(4) mAChR gave carbamylcholine (10 mu M)-stimulated K+ currents of 308 +/- 26 nA and 298 +/- 29 nA, respectively, which were both Ba2+- and pertussis toxin-sensitive, Activation of the M(3) receptor produced 2382 +/- 478 nA of current which was insensitive to Ba2+ and pertussis toxin, but was 85% inhibitable by the Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (10-20 mu M) consistent with coupling to an endogeneous Ca2+ activated Cl- channel via a phosphatidylinositol-dependent mechanism. Coexpression of the cardiac inward rectifier CIR with chick M(2) or M(4) mAChR and GIRK1 increased currents more than 10-fold, but had no effect on specificity of coupling. These data demonstrate a new function for the M(4) mAChR and a high degree of specificity for coupling of each receptor subtype to GIRK1.