Inhibition of acetylcholine-activated K+ currents by U73122 is mediated by the inhibition of PIP2-channel interaction

1 We have investigated the effect of U73122, a specific inhibitor of phospholipase C (PLC), on acetylcholine-activated K+ currents (I-KACh) in mouse atrial myocytes. 2 In perforated patch clamp mode, I-KACh was activated by 10 muM acetylcholine. When atrial myocytes were pretreated with U73122 or U73343, I-KACh was inhibited dose-dependently (half-maximal inhibition at 0.12 +/-0.0085 and 0.16 +/-0.0176 muM, respectively). The current-voltage relationships for I-KACh in the absence and in the presence of U73122 showed that the inhibition occurred uniformly from -120 to +40 mV, indicating a voltage-independent inhibition. 3 When U73122 was applied after I-KACh reached steady-state, a gradual decrease in I-KACh, was observed. The time course of the current decrease was well fitted to a single exponential, and the rate constant was proportional to the concentration of U73122. 4 When K-ACh channels were directly activated by adding I mm GTP gammaS to the bath solution in inside-out patches, U73122 (1 muM) decreased the open probability significantly without change in mean open time. When K-ACh channels were activated independently of G-protein activation by 20 mm Na+, open probability was also inhibited by U73122. 5 Voltage-activated K+ currents and inward rectifying K+ currents were not affected by U73122. 6 These findings show that inhibition by U73122 and U73343 of K-ACh, channels occurs at a level downstream of the action of G beta gamma or Na+ on channel activation. The interference with phosphatidylinositol 4,5-bisphosphate (PIP2)-channel interaction can be suggested as a most plausible mechanism.