H. Cho et al., Inhibition of acetylcholine-activated K+ currents by U73122 is mediated bythe inhibition of PIP2-channel interaction, BR J PHARM, 134(5), 2001, pp. 1066-1072
1 We have investigated the effect of U73122, a specific inhibitor of phosph
olipase C (PLC), on acetylcholine-activated K+ currents (I-KACh) in mouse a
trial myocytes.
2 In perforated patch clamp mode, I-KACh was activated by 10 muM acetylchol
ine. 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 inhibitio
n 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 decr
ease in I-KACh, was observed. The time course of the current decrease was w
ell 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 chan
nels were activated independently of G-protein activation by 20 mm Na+, ope
n 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, channe
ls occurs at a level downstream of the action of G beta gamma or Na+ on cha
nnel activation. The interference with phosphatidylinositol 4,5-bisphosphat
e (PIP2)-channel interaction can be suggested as a most plausible mechanism
.