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

Abstract

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. In perforated patch clamp mode, I_KACh was activated by 10 μM 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 μM, 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. 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. When K_ACh channels were directly activated by adding 1 mM GTPγS to the bath solution in inside‐out patches, U73122 (1 μM) 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. Voltage‐activated K⁺ currents and inward rectifying K⁺ currents were not affected by U73122. These findings show that inhibition by U73122 and U73343 of K_ACh channels occurs at a level downstream of the action of Gβγ or Na⁺ on channel activation. The interference with phosphatidylinositol 4,5‐bisphosphate (PIP₂)‐channel interaction can be suggested as a most plausible mechanism. British Journal of Pharmacology (2001) 134, 1066–1072; doi:10.1038/sj.bjp.0704347