AJP - Cell Physiology, Vol 262, Issue 2 C374-C383, Copyright © 1992 by American Physiological Society

ARTICLES

Direct evidence for a basolateral membrane Cl- conductance in toad retinal pigment epithelium

S. Fujii, R. P. Gallemore, B. A. Hughes and R. H. Steinberg
Department of Physiology, University of California, San Francisco 94143.

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 (Vap) and basal (Vba) membranes averaged -60 +/- 2 and -45 +/- 2 mV, respectively (n = 40). Intracellular Cl- activity (aiCl = 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 Vba by 12 +/- 1 mV (n = 17) and increased the apparent basal membrane resistance. By sequential measurement of aiCl and subepithelial Cl- activity during a step decrease in basal Cl-, we constructed the change in Cl- equilibrium potential (ECl) across the basal membrane. Estimation of the change in basal membrane electromotive force during the change in ECl gave an average value for the Cl- transference number (TCl) of 0.45. Further evidence for a Cl- conductance was obtained by measuring changes in aiCl induced by transepithelial current. Depolarizing Vba elevated aiCl, whereas hyperpolarizing Vba 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 aiCl were reduced by basal perfusion with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (250-500 microM), a blocker of Cl- channels in some epithelia.

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