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 demons trated 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., Reuven y, 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% ide ntical to rat GIRK1 and is expressed in brain, atrium, and ventricle, but not skeletal muscle. In Xenopus laevis oocytes co-expression of GI RK1 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, respe ctively, which were both Ba2+- and pertussis toxin-sensitive, Activati on of the M(3) receptor produced 2382 +/- 478 nA of current which was insensitive to Ba2+ and pertussis toxin, but was 85% inhibitable by th e 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 speci ficity 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.