Agonist-independent inactivation and agonist-induced desensitization of the G protein-activated K+ channel (GIRK) in Xenopus oocytes.
The G-protein-activated K+ channels of the GIRK (Kir 3) family are activated by Gbetagamma subunits of heterotrimeric Gi/Go 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.
Vorobiov
D
D
Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat Aviv, Israel.
Levin
G
G
Lotan
I
I
Dascal
N
N
D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D004361:Drug Tolerance; D004553:Electric Conductivity; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D019204:GTP-Binding Proteins / Q000502:physiology*; D009865:Oocytes / Q000502:physiology*; D011188:Potassium / Q000008:administration & dosage; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D011493:Protein Kinase C / Q000378:metabolism; D043585:Receptor, Muscarinic M2; D011976:Receptors, Muscarinic / Q000502:physiology; D012964:Sodium / Q000008:administration & dosage / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014982:Xenopus laevis
D015221:Potassium Channels; D043585:Receptor, Muscarinic M2; D011976:Receptors, Muscarinic; D012964:Sodium; D011493:Protein Kinase C; D019204:GTP-Binding Proteins; D000109:Acetylcholine; D013755:Tetradecanoylphorbol Acetate; D011188:Potassium