Inhibition of Ca2+-activated K+ current by clotrimazole in rat anterior pituitary GH(3) cells

The ionic mechanism of clotrimazole, an imidazole antimycotic P-450 inhibit or, was examined in rat anterior pituitary GH, cells. In perforated-patch w hole-cell recording experiments, clotrimazole reversibly caused an inhibiti on of the Ca2+-activated K+ current in a dose-dependent manner. The IC50 va lue of the clotrimazole-induced inhibition of I-K(Ca) was 3 mu M. In the ou tside-out configuration of single channel recording, application of clotrim azole (10 mu M) into the bath medium did not change the single channel cond uctance of large conductance Ca2+-activated K+(BKCa) channels, but it suppr essed the channel activity significantly. The change in the kinetic behavio r of BKCa channels caused by clotrimazole in these cells is found to be due to a decrease in mean open time and an increase in mean closed time. Other structurally distinct P-450 inhibitors (e.g. ketoconazole or econazole) al so effectively suppressed the amplitude of I-K(Ca). Clotrimazole (10 mu M) blocked both the inactivating and non-inactivating components of the voltag e-dependent K+ outward current (I-K(V)), but it produced a slight reduction of L-type Ca2+ inward current (I-CA,I-L) without altering the current-volt age relationship of I-CA,I-L. Clotrimazole (10 mu M) also increased the fir ing rate of action potentials. These results provide direct evidence that c lotrimazole is capable of suppressing the activity of BKCa channel in GH(3) cells. Because of the non-selective inhibitory effect of clotrimazole on I -K(Ca) and I-K(V), this inhibition is mainly, if not entirely, due to a dir ect channel blockade. Thus, the present study implies that the blockade of these ionic channels by clotrimazole would affect hormonal secretion and ne uronal excitability. (C) 1999 Elsevier Science Ltd. All rights reserved.