K- TRANSPORT MECHANISMS IN BOVINE PIGMENT-EPITHELIUM THAT COULD MODULATE SUBRETINAL SPACE VOLUME AND COMPOSITION( AND CL)

1. Conventional and ion-selective double-barrelled microelectrodes wer e 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, ligh t-induced changes in [K+]o occur in the extracellular (or subretinal) space that separates the neural retina and the RPE apical membrane. Th ese [K+]o changes can be approximated in vitro by decreasing apical ba th [K+]o from 5 to 2 mM. 3. This in vitro change in [K+]o simultaneous ly decreased intracellular Cl- and K+ activities (a(Cl)i and a(K)i) by 25 +/- 6 mM (n = 8) and 19 +/- 7 mM (n = 4) (mean +/- S.D.), respecti vely. In control Ringer solution (5 mM [K+]o) a(Cl)i and a(K)i were 65 +/- 10 mM (n = 28) and 65 +/- 8 mm (n = 6), respectively. 4. The [K+] o-induced decreases in a(Cl)i, and a(K)i were both significantly inhib ited, either by blocking the apical membrane K+ conductance with Ba2or the basolateral membrane Cl- conductance with DIDS (4,4'-diisothioc yano-stilbene-2,2'-disulphonic acid). 5. Transepithelial current pulse s were used to determine the relative basolateral membrane Cl- conduct ance, T(Cl)BAS, was approximately 0.6 (n = 3), and the relative apical membrane K+ conductance, T(K)AP was approximately 0.7 (n=2). Step cha nges in basal bath [K+]o were used to estimate the relative basolatera l membrane K+ conductance, T(K)BAS, was approximately 0.34 (n = 3). 6. These data show that the apical membrane K+ conductance and the basol ateral membrane Cl- conductance are electrically coupled. In vivo, thi s coupling could have significant functional importance by modulating the relative hydration of the subretinal space, regulating RPE cell vo lume, and buffering the chemical composition of the subretinal space.