THE ANTIFUNGAL IMIDAZOLE CLOTRIMAZOLE AND ITS MAJOR IN-VIVO METABOLITE ARE POTENT BLOCKERS OF THE CALCIUM-ACTIVATED POTASSIUM CHANNEL IN MURINE ERYTHROLEUKEMIA-CELLS

Clotrimazole (CLT), a member of the antifungal imidazole family of com pounds, has been found to inhibit both calcium (Ca2+)-activated Rb-86 and potassium (K) fluxes of human red cells and to inhibit red cell bi nding of I-125-charybdotoxin (ChTX) [11]. We have now used patch-clamp techniques to demonstrate reversible inhibition of whole cell KCa2+ c urrents in murine erythroleukemia (MEL) cells by submicromolar concent rations of CLT. Inhibition was equivalent whether currents were elicit ed by bath application of the Ca2+ ionophore A23187 or by dialyzing ce lls with a pipette solution containing micromolar concentrations of fr ee Ca2+ The extent of inhibition of whole cell MEL KCa2+ currents was voltage-dependent, decreasing with increasing test potential. We also determined the single channel basis of the CLT inhibition in MEL cells by demonstrating the inhibition of a calcium-activated, ChTX-sensitiv e K channel by CLT in outside-out patches. The channel was also blocke d by the des-imidazolyl metabolite of CLT, 2-chlorophenyl-bisphenyl-me thanol (MET II) [15], thus demonstrating that the imidazole ring is no t required for the inhibitory action of CLT. Single KCa2+ channels wer e also evident in inside-out patches of MEL cells. Block of K current by CLT was not unique to MEL cells. CLT also inhibited a component of the whole cell K current in PC12 cells. Channel specificity of block b y CLT was determined by examining its effects on other types of voltag e-sensitive currents. CLT block showed the following rank order of pot ency: K currents in PC12 cells > Ca2+ currents in PC12 cells much grea ter than Na currents in sympathetic neurons. These results demonstrate that direct inhibition of single KCa2+ by CLT can be dissociated from inhibition of cytochrome P-450 in MEL cells.