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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.

1. Conventional and ion‐selective double‐barrelled microelectrodes were used in an in vitro preparation of bovine retinal pigment epithelium (RPE)‐choroid to measure the changes in membrane voltage, resistance and intracellular Cl‐ activity (aCli) produced by small, physiological changes in extracellular potassium concentration ([K+]o). These apical [K+]o changes approximate those produced in the extracellular (subretinal) space between the photoreceptors and the RPE following transitions between light and dark. 2. Changing apical [K+]o from 5 to 2 mM in vitro elicited membrane voltage responses with three distinct phases. The first phase was generated by an apical membrane hyperpolarization, followed by a (delayed) basolateral membrane hyperpolarization (DBMH); the third phase was an apical membrane depolarization. The present experiments focus on the membrane and cellular mechanisms that generate phase 2 of the response, the DBMH. 3. The DBMH was abolished in the presence of apical bumetanide (100 microM); this response was completely restored after bumetanide removal. 4. Reducing apical [K+]o, adding apical bumetanide (500 mM), or removing apical Cl‐ decreased aCli by 25 +/‐ 6 (n = 8), 28 +/‐ 1 (n = 2) and 26 +/‐ 5 mM (n = 3), respectively; adding 100 microM apical bumetanide decreased aCli by 12 +/‐ 2 mM (n = 3). Adding apical bumetanide or removing apical bath Cl‐ hyperpolarized the basolateral membrane and decreased the apparent basolateral membrane conductance (GB). 5. DIDS (4,4'‐diisothiocyanostilbene‐2,2'‐disulphonic acid) blocked the RPE basolateral membrane Cl‐ conductance and inhibited the DBMH and the basolateral membrane hyperpolarization produced by apical bumetanide addition or by removal of apical Cl‐o. The present results show that the DBMH is caused by delta[K]o‐induced inhibition of the apical membrane Na(+)‐K(+)‐2Cl‐ cotransporter; the subsequent decrease in aCli generated a hyperpolarization at the basolateral membrane Cl‐ channel.