Inwardly rectifying K⁺ currents of alveolar type II cells isolated from fetal guinea-pig lung: regulation by G protein- and Mg²⁺-dependent pathways

Abstract

 K⁺ currents in alveolar type II cells, isolated from fetal guinea-pig lung, were studied using the whole-cell patch-clamp technique. Inwardly rectifying (IR) K⁺ currents were observed when cells were bathed in symmetrical KCl-rich solutions. When extracellular K⁺ was replaced by Na⁺, inward currents were greatly decreased and the zero-current potential moved from 0 mV to –69 mV, indicating high K⁺ selectivity. In recordings with an intracellular KCl-rich solution, containing 1.12 mM Mg²⁺ and 10^–8 M free Ca²⁺, IR K⁺ currents slowly diminished with time. Addition of the irreversible G protein activator, guanosine 5’-O-(3-thiotriphosphate) (GTP [γ-S]), to the intracellular solution accelerated the rate of current run-down. In experiments where the intracellular solution was Mg²⁺ free, current run-down was abolished. The rate of current run-down was found to increase with increasing free intracellular [Mg²⁺]. Raising the intracellular free [Ca²⁺] to 10^–6 M under Mg²⁺-free conditions had no effect on the K⁺ currents. Extracellular Ba²⁺ blocked the IR K⁺ currents in a concentration- and voltage-dependent manner. Tolbutamide, a blocker of ATP-sensitive K⁺ (K_ATP) channels, had no effect on the currents. The single channel underlying the whole-cell IR K⁺ currents displayed inward rectification and had a conductance of 31 pS in symmetrical KCl-rich solutions. We demonstate that mRNA coding for IRK1 is expressed in this cell preparation. Possible functions for this channel are discussed.