Cyclic AMP-induced chloride permeability in the apical membrane of Necturus gallbladder epithelium.

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Abstract
The effects of theophylline, 8-Br-cAMP, and cAMP on Nocturus gallbladder epithelium were investigated using microelectrode techniques. Each of these substances depolarized the cell membranes by .apprx. 15 mV and decreased the apparent ratio of apical to basolateral membrane resistances to a value not significantly different from 0. Examination of the ionic selectivity of the apical membrane by ion substitutions in the mucosal bathing medium revealed a large increase in Cl permeability with no apparent changes in K and Na permeabilities. Intracellular Cl activity (aCli) was measured using Cl-sensitive liquid ion-exchanger microelectrodes. Under control conditions, aCli was .apprx. 20 mM, 2.5 times higher than the value expected for equilibrium distribution .**GRAPHIC**. After addition of 8-Br-cAMP, aCli decreased within < 60 s to .apprx. 13 mM, a value not significantly different from .**GRAPHIC**. Virtually identical results were obtained with theophylline. Under control conditions, luminal Cl removal caused aCli to fall at an initial rate of 1.8 mM/min, whereas in tissues exposed to 8-Br-cAMP or theophylline a rate of 11.6 mM/min was observed. The apical membrane Cl transference number was estimated from the change of aCli upon exposure to 8-Br-cAMP as well as from the changes in apical membrane potential during variation of the luminal Cl concentration. The results, 0.91 and 0.88, respectively, are indicative of a high Cl permeability of the apical membrane during cAMP. This effect may explain, at least in part, the complete inhibition of fluid absorption produced by theophylline in this tissue. Enhancement of apical membrane Cl permeability may account for a variety of cAMP effects in epithelial tissues.