INHIBITION BY WORTMANNIN OF M-CURRENT IN BULLFROG SYMPATHETIC NEURONS

1 The actions of wortmannin, an inhibitor of myosin light chain kinase (MLCK), on M-type potassium current of dissociated bullfrog sympathetic neurones have been examined. 2 The amplitude of M-current was measured by whole cell recordings from cells pretreated with wortmannin (0.01-10 mu M) or the wortmannin vehicle, dimethylsulphoxide (0.0001-0.1 vol%), for 30 min. Internal (recording pipette) solutions having three different pCa values (6, 7 and 8) were used for the measurements. 3 Irrespective of the pCa, M-current was not detectable when the cells were pretreated with 10 mu M wortmannin. Wortmannin, 3 mu M, produced 85-95% inhibition of the M-current. Pretreatment with 10-30 nM wortmannin was without effect on M-current. 4 The M-current inhibition by wortmannin at concentrations of 0.1-1 mu M depended on the pCa of the internal solution. Inhibition occurred only when the calcium-rich (pCa = 6) internal solution was used. 5 Pre-treatment of the cells with wortmannin (10 mu M) did not affect rapidly-inactivating A-type or delayed rectifier-type potassium currents nor did it alter inwardly rectifying sodium-potassium current (I-H). 6 These observations show that M-current inhibition by wortmannin has two pharmacological profiles. One is calcium-dependent and occurs at lower concentrations (0.1-1 mu M), and is attributed to inhibition of MLCK by wortmannin. At higher concentrations (3-10 mu M), wortmannin has an additional, calcium-independent action, inhibiting the M-current by an unknown mechanism.