LIGHT-EVOKED MODULATION OF BASOLATERAL MEMBRANE C1- CONDUCTANCE IN CHICK RETINAL-PIGMENT EPITHELIUM - THE LIGHT PEAK AND FAST OSCILLATION

1. We studied the ionic mechanism of the light-peak voltage of the DC electroretinogram (DC ERG) in an in vitro preparation of chick neural retina-retinal pigment epithelium (RPE)-choroid. The light peak originates from a depolarization of the RPE basolateral (basal) membrane, associated with an increase in its conductance. Using conventional and Cl--selective microelectrodes, we tested the hypothesis that the light-peak voltage is generated by an increase in Cl- conductance (g(Cl)) of the basolateral (basal) membrane. 2. Perfusion of the RPE basal membrane with 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS), a known blocker of g(Cl) in chick RPE, suppressed both the light-peak depolarization and the accompanying conductance increase of the basal membrane. 3. Using sustained transepithelial current to clamp the basal membrane potential at different levels, we estimated the reversal potential of the light peak. At membrane potentials above the equilibrium potential for Cl- (E(Cl) = -40 +/- 10 mV mean +/- SE), light-peak polarity was reversed. Current-voltage (I-V) curves measured in the dark and at the peak of the light peak also gave a reversal potential in the same range as E(Cl). In addition, shifting E(Cl) by changing intracellular Cl- (a(Cl)i) via passage of transepithelial current or perfusing the apical side of the RPE with the Cl- uptake blocker, furosemide, shifted the light-peak reversal potential in the same direction as the change in E(Cl). 4. The transference number for Cl-, T(Cl), was estimated from step decreases in basal Cl- and increased from 0.20 +/- 0.01 in the dark to 0.31 +/- 0.01 during the light peak. These results indicate an average increase of 55% in the relative conductance of the basal membrane for Cl-. 5. Light-evoked changes in a(Cl)i, measured with Cl--selective microelectrodes, were too small to account for the change in basal membrane potential during the light peak. These data strongly support the hypothesis that the light peak originates from an increase in RPE basal membrane permeability to Cl-. 6. We also obtained support for the model of Joseph and Miller that the fast-oscillation trough of the DC ERG, generated by a delayed basal membrane hyperpolarization of the RPE, originates from light-evoked modulation of the Cl- transport pathway. Perfusing either the apical side of the RPE with furosemide or the basal side with DIDS suppressed the fast oscillation. The delayed basal hyperpolarization reversed polarity at membrane potentials positive to E(Cl). In addition, a decrease in a(Cl)i was evoked by light that could account for the time course and amplitude of the delayed basal hyperpolarization. We conclude, as Joseph and Miller proposed, that a light-evoked decrease in a(Cl)i, expressed at the basal membrane Cl- conductance, generates the fast-oscillation trough of the DC ERG.