Intrinsic gating properties of a cloned G protein-activated inward rectifier K⁺ channel

The voltage-, time-, and K⁺-dependent properties of a G protein- activated inwardly rectifying K⁺ channel (GIRK1/KGA/Kir3.1) cloned from rat atrium were studied in Xenopus oocytes under two-electrode voltage clamp. During maintained G protein activation and in the presence of high external K⁺ (V(K) = 0 mV), voltage jumps from V(K) to negative membrane potentials activated inward GIRK1 K⁺ currents with three distinct time-resolved current components. GIRK1 current activation consisted of an instantaneous component that was followed by two components with time constants τ(f) ~50 ms and τ(s) ~400 ms. These activation time constants were weakly voltage dependent, increasing approximately twofold with maximal hyperpolarization from V(K). Voltage-dependent GIRK1 availability, revealed by tail currents at -80 mV after long prepulses, was greatest at potentials negative to V(K) and declined to a plateau of approximately half the maximal level at positive voltages. Voltage-dependent GIRK1 availability shifted with V(K) and was half maximal at V(K)-20 mV; the equivalent gating charge was ~1.6 e^-. The voltage-dependent gating parameters of GIRK1 did not significantly differ for G protein activation by three heterologously expressed signaling pathways: m2 muscarinic receptors, serotonin 1A receptors, or G protein β1γ2 subunits. Voltage dependence was also unaffected by agonist concentration. These results indicate that the voltage-dependent gating properties of GIRK1 are not due to extrinsic factors such as agonist-receptor interactions and G protein-channel coupling, but instead are analogous to the intrinsic gating behaviors of other inwardly rectifying K⁺ channels.