Basolateral membrane chloride transport in isolated epithelia of frog skin

Isotopic methodology was used to characterize Cl^- transport in isolated epithelia of frog skin (northern Rana pipiens) bathed in Cl^- -rich Ringer solution and short-circuited. Cl^- content of epithelia measured when loaded to ³⁶Cl specific activity equilibrium averaged 139.6 neq/mg dry wt. The kinetics of ³⁶Cl efflux was biexponential and consistent with binding or compartmentalization of ~ 30% of tissue Cl^- within the intracellular pool. Because efflux of ³⁶Cl to the apical solution was immeasurable, it was concluded that apical membranes were virtually impermeable to Cl^- and that basolateral membranes were highly permeable to Cl^- with a mean unidirectional Cl^- efflux of 21.7 μA/cm². Both furosemide (1 mM) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (5 x 10^-4 M) inhibited markedly the basolateral membrane chloride fluxes within seconds, as measured in chamber experiments. As inhibition of Cl^- flux occurred in the absence of a change of the electrical parameters of apical and basolateral membranes, the mechanisms of Cl^- transport appeared to be electroneutral and, for the most part at least, not coupled to the fluxes of Na⁺ and K⁺. Transepithelial Cl^- fluxes averaged near 1 μA/cm², proceeding via transport routes in parallel to the cells of the stratified epithelium. No correlation existed between the 'shunt' resistance measured in the presence of 100 μM amiloride (> 1,000 Ω·cm²) and the partial conductance to Cl^-.