Phenothiazines and haloperidol block Ca-activated K channels in rat forebrain synaptosomes

Ca-dependent, K-stimulated 86Rb efflux, a measure of Ca-activated K conductance in rat brain synaptosomes, was blocked by phenothiazines and haloperidol. Micromolar concentrations of the phenothiazines, fluphenazine and trifluoperazine, and haloperidol, a non-phenothiazine antipsychotic and calmodulin antagonist, selectively inhibited the Ca-activated K channels. The IC50 values of all three agents for inhibition of the Ca-activated K channels was on the order of 0.5-1 microM. Measurements of K-stimulated 45Ca uptake indicated that the effects of these agents on Ca-activated K channels was not due to inhibition of Ca influx through voltage-gated Ca channels. Sulpiride, a potent antipsychotic with weak anti-calmodulin activity, was a relatively weak inhibitor of Ca-activated K channels. Calmidazolium (compound R-24571) and W7, two non-phenothiazine calmodulin antagonists, did not selectively inhibit Ca-activated K channels. Biphasic dose response curves for inhibition of the Ca-dependent, K-stimulated 86Rb efflux by the phenothiazines raise the possibility that there may be two kinds of Ca-activated K channels in rat brain presynaptic terminals, with different sensitivities to the phenothiazines. These results demonstrate that two phenothiazines and haloperidol are potent and relatively selective inhibitors of Ca-activated K channels in nerve endings. This inhibition does not appear to be mediated by calmodulin or by dopamine receptors.