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University of California, School of Optometry, Berkeley 94720.

1. Conventional and ion-selective double-barrelled microelectrodes were used in an in vitro bovine retinal pigment epithelium (RPE)-choroid preparation to measure the changes in membrane voltage, resistance and intracellular K+ and Cl- activities produced by small, physiological changes in extracellular potassium ([K+]o). 2. In the intact eye, light-induced changes in [K+]o occur in the extracellular (or subretinal) space that separates the neural retina and the RPE apical membrane. These [K+]o changes can be approximated in vitro by decreasing apical bath [K+]o from 5 to 2 mM. 3. This in vitro change in [K+]o simultaneously decreased intracellular Cl- and K+ activities (aCli and aKi) by 25 +/- 6 mM (n = 8) and 19 +/- 7 mM (n = 4) (mean +/- S.D.), respectively. In control Ringer solution (5 mM [K+]o) aCli and aKi were 65 +/- 10 mM (n = 28) and 65 +/- 8 mM (n = 6), respectively. 4. The [K+]o-induced decreases in aCli and aKi were both significantly inhibited, either by blocking the apical membrane K+ conductance with Ba2+ or the basolateral membrane Cl- conductance with DIDS (4,4'-diisothiocyano-stilbene-2,2'-disulphonic acid). 5. Transepithelial current pulses were used to determine the relative basolateral membrane Cl- conductance, TClBAS, was approximately 0.6 (n = 3), and the relative apical membrane K+ conductance, TKAP, was approximately 0.7 (n = 2). Step changes in basal bath [K+]o were used to estimate the relative basolateral membrane K+ conductance, TKBAS, was approximately 0.34 (n = 3). 6. These data show that the apical membrane K+ conductance and the basolateral membrane Cl- conductance are electrically coupled. In vivo, this coupling could have significant functional importance by modulating the relative hydration of the subretinal space, regulating RPE cell volume, and buffering the chemical composition of the subretinal space.

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