Agonist-independent inactivation and agonist-induced desensitization of the G protein-activated K⁺ channel (GIRK) in Xenopus oocytes

Abstract

 The G-protein-activated K⁺ channels of the GIRK (Kir 3) family are activated by G_βγ subunits of heterotrimeric G_i/G_o proteins. Atrial GIRK currents evoked by acetylcholine (ACh)¹ via muscarinic m2 receptors (m2R) display prominent desensitization. We studied desensitization of basal and ACh-evoked whole-cell GIRK currents in Xenopus oocytes. In the absence of receptor and/or agonist, the basal GIRK activity showed inactivation which was prominent when the preparation was bathed in a low-Na⁺, high-K⁺ extracellular solution (96 mM [K⁺]_out and 2 mM [Na⁺]_out) but did not occur in a normal physiological solution. Ion substitution experiments showed that this basal, agonist-independent inactivation was caused by the decrease in [Na⁺]_out rather than by the increased [K⁺]_out. We hypothesize that it reflects a depletion of intracellular Na⁺. ACh-evoked GIRK currents desensitized faster than the basal ones. The agonist-induced desensitization was present when the preparation was bathed in all solutions tested, independently of [Na⁺]_out. A protein kinase C (PKC) activator inhibited the GIRK currents both in high and low [Na⁺]_out, apparently mimicking agonist-induced desensitization; however, a potent serine/threonine protein kinase blocker, staurosporine, blocked only a minor part of desensitization. We conclude that basal inactivation and agonist-induced desensitization are separate processes, the former caused by changes in Na⁺ concentrations, and the latter by unknown factor(s) with only a minor contribution of PKC.