NPPB BLOCK OF CA++-ACTIVATED CL- CURRENTS IN XENOPUS OOCYTES

NPPB (5-nitro-2-(3-phenylpropylamino)-benzoate), a member of the novel class of Cl- channel blockers related to diphenylamine-2-carboxylate, was studied for its effect on the Ca++-activated Cl- current (I(Cl)(Ca)) in frog (Xenopus laevis) oocytes. I(Cl)(Ca) was activated by bath application of 5 mM Ca++ to oocytes that had been Ca++ permeabilized with the Ca++ ionophore A23187. In order to prevent the inactivation of I(Cl)(Ca) that occurs with repetitive applications of Ca++, oocytes were also treated with H-7 (1,5-isoquinolinesulfonyl-1,2 methylpiperazine), an inhibitor of protein kinases. NPPB reversibly blocked both the fast transient (I(fast)) and slow (I(slow)) components of I(Cl)(Ca) with inhibition constants of 22-mu-M and 68-mu-M, respectively. NPPB block of I(Cl)(Ca) was voltage dependent and potentiated by depolarization of the oocyte membrane. Our results indicate that NPPB is a potent blocker of the oocyte endogenous I(Cl)(Ca), and may prove useful in the study of exogenously expressed Cl channels.