MODULATION OF THE INWARDLY RECTIFYING POTASSIUM CHANNEL IRK1 BY THE M1 MUSCARINIC RECEPTOR

Modulation of the inwardly rectifying potassium channel (IRK1) by the mi muscarinic receptor was studied with the whole-cell patch-clamp rec ording technique with the use of a mammalian expression system. After transfection with IRK1 and mi muscarinic receptor genes, tsA cells exp ressed a cesium-sensitive inwardly rectifying potassium conductance th at was reduced on application of the muscarinic receptor agonist carba chol. This reduction was reversible on washout of carbachol and could be completely inhibited by the muscarinic receptor antagonist atropine . Conversely, stimulation of the m2 muscarinic receptor, when coexpres sed with IRK1, resulted in no change in IRK1 current amplitude. Phorbo l-12,13-dibutyrate, an activator of protein kinase C (PKC), mimicked t he effect of m1 muscarinic receptor stimulation by inhibiting the IRK1 conductance. Preincubation with staurosporine or the specific PKC inh ibitor calphostin C, before application of carbachol, fully prevented the inhibition of IRK1 by m1 muscarinic receptor stimulation. Administ ration of 8-bromo-cAMP, an activator of protein kinase A, and thapsiga rgin, a stimulator of intracellular calcium release, had no effect on IRK1, suggesting that these second messengers were not involved in the mi muscarinic receptor-induced response. Therefore, the data indicate that the m1 muscarinic receptor inhibits IRK1, presumably via stimula tion of PKC. As IRK1 is widely distributed throughout the central nerv ous system, it is possible that such an action on IRK1 underlies the i nhibitory effects of muscarinic receptor stimulation on inwardly recti fying potassium conductances observed in the brain.