BASOLATERAL MEMBRANE C1- AND K-ADAPTED CHICK RETINAL-PIGMENT EPITHELIUM( CONDUCTANCES OF THE DARK)

1. We characterized the basolateral membrane Cl- and K+ conductances o f the dark-adapted chick neural retina-retinal pigment epithelium (RPE )-choroid preparation. Conventional microelectrodes were used to measu re apical (V(ap)) and basolateral (V(ba)) membrane voltage, and double -barreled Cl- and K+ selective microelectrodes were used to follow the time course and magnitude of ion concentration changes outside the ba solateral (basal) membrane. 2. In response to a fivefold decrease in b asal [Cl-]o, V(ba) rapidly depolarized by 6.4 +/- 0.7 (SE) mV, and the apparent resistance of the basolateral membrane (R(ba)) increased. Th e Cl- channel blocker 4,4'-diisothiocyanostilbene-2,2-disulfonate (DID S) suppressed the V(ba) depolarization by 40% and blocked the R(ba) in crease. Estimates of the relative Cl- conductance (transference number , T(Cl)) from the DIDS-sensitive component of the Cl- diffusion potent ial gave an average value for T(Cl) of 0.22 +/- 0.03. 3. Further evide nce for a Cl- conductance was obtained by measuring changes in intrace llular Cl- activity (a(Cl)i) induced by transtissue current. Depolariz ing V(ba) elevated a(Cl)i, whereas hyperpolarizing V(ba) had the oppos ite effect, consistent with conductive Cl- movement across the basal m embrane. T(Cl) estimated from these data averaged 0.23 +/- 0.02. 4. In response to a sixfold increase in basal [K+]o, V(ba) depolarized 6.1 +/- 0.8 mV. The amplitude of this K+ diffusion potential was inhibited 44 and 67% by 5 and 10 mM Ba2+, respectively. T(K) was estimated to b e 0.61 +/- 0.05. 5. The rapid c-wave membrane hyperpolarizations in re sponse to the light-evoked decrease in subretinal [K+]o were used to c alculate the equivalent resistances of the apical membrane (R(ap)), ba solateral membrane (R(ba)), and the paracellular shunt pathway (R(s)). They were 152 +/- 10, 615 +/- 38, and 138 +/- 7 OMEGA . cm2 (n = 11 t issues), respectively. From these data the equivalent electromotive fo rce for the basal (E(ba)) and apical (E(ap)) membranes were estimated to be -45 +/- 2 and -77 +/- 1 mV, respectively. This estimate of E(ba) is in the range of that predicted from our estimates of T(Cl) and T(K ), indicating that, in the dark-adapted chick retina, the resting cond uctance of the basal membrane can largely be accounted for by the Cl- and K+ conductances described here.