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

Abstract

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, Sicsinger, 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 μM)-stimulated K + currents of 308 ± 26 nA and 298 ± 29 nA, respectively, which were both Ba 2+- and pertussis toxin-sensitive. Activation of the M 3 receptor produced 2382 ± 478 nA of current which was insensitive to Ba 2+ and pertussis toxin, but was 85% inhibitable by the Cl - channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (10-20 μM) consistent with coupling to an endogeneous Ca 2+-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.