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 10-5M NPPB elicited a maximum depolarization of the membrane voltage from -72 ± 6 to -48 ± 9 mV (n = 6, P < 0.05) and reduced the magnitude of the depolarization induced by a 10-fold increase in K+concentration. Subsequent exposure to 10-4M ouabain decreased the membrane voltage from -41 ± 6 mV to -25 ± 2 mV (n = 6, P < 0.05). After stimulation with 10-5M amphotericin B of a short-circuit current, Isc, largely accounted for by tear to stroma K+diffusion, this Iscwas effectively inhibited by 10-5M NPPB on the stromal-side. This decrease reflected a fall in basolateral membrane K+conductance. In NaCl Ringers, inhibition of the essentially Cl--originated Isceither on the tear- or stromal-sides required instead 10-4M NPPB. NPPB depolarized the membrane voltage from -55 ± 7 to -38 ± 6 mV (n = 14, P < 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, 10-4M NPPB consistently caused fRoto fall. However, following exposure to 5·10-3M Ba2+and a fall in fRo, NPPB consistently caused fRoto increase significantly from 30 ± 8 to 53 ± 4% (n = 5). Therefore, inhibition of active Cl-transport by 10-4M 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 Ischad not fully recovered to its control value. © 1990.
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