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 (ICl(Ca)) in frog (Xenopus laevis) oocytes. ICl(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 ICl(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 (Ifast) and slow (Islow) components of ICl(Ca) with inhibition constants of 22 microM and 68 microM, respectively. NPPB block of ICl(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 ICl(Ca), and may prove useful in the study of exogenously expressed Cl channels.