DIRECT EVIDENCE FOR A BASOLATERAL MEMBRANE CL- CONDUCTANCE IN TOAD RETINAL-PIGMENT EPITHELIUM

There is now evidence that a Cl- conductance on the basal membrane of the retinal pigment epithelium (RPE) is involved in the generation of both the fast oscillation and the light peak of the direct-current electroretinogram as well as being critical for transepithelial fluid and salt movement. In the present study, we characterized the basolateral membrane Cl- conductance of an in vitro preparation of toad RPE-choroid using conventional and Cl- selective microelectrodes. Under control conditions, the potential across the apical (V(ap)) and basal (V(ba)) membranes averaged -60 +/- 2 and -45 +/- 2 mV, respectively (n = 40). Intracellular Cl- activity (a(Cl)i = 20 +/- 1 mM) was distributed above equilibrium across both membranes, consistent with active accumulation of Cl-. A sixfold decrease in Cl- in the basal bath depolarized V(ba) by 12 +/- 1 mV (n = 17) and increased the apparent basal membrane resistance. By sequential measurement of a(Cl)i and subepithelial Cl- activity during a step decrease in basal Cl-, we constructed the change in Cl- equilibrium potential (E(Cl)) across the basal membrane. Estimation of the change in basal membrane electromotive force during the change in E(Cl) gave an average value for the Cl- transference number (T(Cl)) of 0.45. Further evidence for a Cl- conductance was obtained by measuring changes in a(Cl)i induced by transepithelial current. Depolarizing V(ba) elevated a(Cl)i, whereas hyperpolarizing V(ba) had the opposite effect, consistent with conductive Cl- movement across the basal membrane. Both the amplitude of the Cl- diffusion potential and the current-induced changes in a(Cl)i were reduced by basal perfusion with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (250-500-mu-M), a blocker of Cl- channels in some epithelia.