Inhibition of acetylcholine‐activated K + currents by U73122 is mediated by the inhibition of PIP 2 ‐channel interaction

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 μ M 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 μ M , 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 m M GTPγS to the bath solution in inside‐out patches, U73122 (1 μ M ) decreased the open probability significantly without change in mean open time. When K ACh channels were activated independently of G‐protein activation by 20 m M 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 (PIP 2 )‐channel interaction can be suggested as a most plausible mechanism.British Journal of Pharmacology (2001) 134 , 1066–1072; doi: 10.1038/sj.bjp.0704347