Expression of an atrial G-protein-activated potassium channel in Xenopus oocytes

Injection of rat atrial RNA into Xenopus oocytes resulted in the expression of a guanine nucleotide binding (G) protein-activated K⁺ channel. Current through the channel could be activated by acetylcholine or, if RNA encoding a neuronal 5HT1A receptor was coinjected with atrial RNA, by serotonin (5HT). A 5HT-evoked current (I_5HT) was observed in oocytes injected with ventricle RNA fractions (of 2.5-5.5 kb) and 5HT1A receptor RNA. I_5HT displayed strong inward rectification with very little conductance above the K⁺ equilibrium potential, was highly selective for K⁺ over Na⁺, and was blocked by 5-300 μM Ba²⁺. I_5HT was suppressed by intracellular injection of the nonhydrolyzable analog of GDP, guanosine 5′-[β-thio]diphosphate, but not by treatment with pertussis toxin (PTX), suggesting coupling of the receptor to the G-protein-activated K⁺ channel via a PTX-insensitive G protein, possibly endogenously present in the oocyte. Coexpression of the α subunit of a PTX-sensitive G protein, G_i2, rendered I_5HT sensitive to PTX inhibition. Native oocytes displayed a constitutively active inwardly rectifying K⁺ current with a lower sensitivity to Ba²⁺ block; expression of a similar current was also directed by atrial or ventricle RNA of 1.5-3 kb. Xenopus oocytes may be employed for cloning of the G-protein-activated K⁺ channel cDNA and for studying the coupling between this channel and G proteins.