The effects of the Cl- channel blocker, 4-nitro-2-(3-phenylpropylamino)benzoate (NPPB) on active transepithelial Cl- transport were measured in the isolated bullfrog cornea. With a Cl(-)-free Ringers, stromal-side 10(-5) M NPPB elicited a maximum depolarization of the membrane voltage from -72 +/- 6 to -48 +/- 9 mV (n = 6, P less than 0.05) and reduced the magnitude of the depolarization induced by a 10-fold increase in K+ concentration. Subsequent exposure to 10(-4) M ouabain decreased the membrane voltage from -41 +/- 6 mV to -25 +/- 2 mV (n = 6, P less than 0.05). After stimulation with 10(-5) M amphotericin B of a short-circuit current, Isc, largely accounted for by tear to stroma K+ diffusion, this Isc was effectively inhibited by 10(-5) M NPPB on the stromal-side. This decrease reflected a fall in basolateral membrane K+ conductance. In NaCl Ringers, inhibition of the essentially Cl(-)-originated Isc either on the tear- or stromal-sides required instead 10(-4) M NPPB. NPPB depolarized the membrane voltage from -55 +/- 7 to -38 +/- 6 mV (n = 14, P less than 0.05). The direction of the change in the fractional apical membrane resistance (fRo) depended upon its initial value; in those corneas with a lower value it increased whereas if they had a higher fRo, 10(-4) M NPPB consistently caused fRo to fall. However, following exposure to 5 x 10(-3) M Ba2+ and a fall in fRo, NPPB consistently caused fRo to increase significantly from 30 +/- 8 to 53 +/- 4% (n = 5). Therefore, inhibition of active Cl- transport by 10(-4) M NPPB may be associated with declines in: (1) a basolateral membrane K+ conductance that is distinct from a Ba2(+)-sensitive pathway; (2) an apical membrane Cl- conductance. Neither of these effects may be the result of a direct effect of NPPB on a conductance pathway because: (1) the drug was equipotent from either bathing solution; (2) following a one hour washout the Isc had not fully recovered to its control value.
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