Light-induced changes of extracellular ions and volume in the isolated chick retina-pigment epithelium preparation

To better understand the mechanisms of extracellular space volume regulatio n and their possible effects on retinal function, light-induced changes in the concentrations of the principal extracellular ions (Na+, K+, Ca2+, and Cl-) were measured with ion-sensitive microelectrodes in the chick retina-p igment epithelium-choroid preparation. Changes of extracellular space volum e were assessed by measuring the concentration of an impermeant marker, tet ramethylammonium. In the inner retina, transient ON/OFF Na+ decrease was ab out twice as large as K+ increase, and the charge difference was compensate d by a decrease in Cl- concentration. The ion changes were accompanied by e xtracellular space-volume decreases here. In the subretinal space, [Na+](o) increase was about twice as large as K+ decrease, yet [Cl-](o) also decrea sed; this was accompanied by a sustained extracellular space-volume increas e. The ionic changes in the inner retina are consistent with a model of ext racellular space-volume regulation which assumes that neuronal depolarizati on causes net uptake of NaCl, cell swelling, and extracellular space shrink age. However, to prevent the apparent violation of electroneutrality in the subretinal space, our simple model should be expanded to include the invol vement of unidentified anion(s). Substantial changes in the subretinal spac e volume may influence interaction between the neural retina and pigment ep ithelium. Among ionic changes, only the light-induced [K+](o) decrease arou nd the photoreceptors and the [Ca2+](o) increase near the photoreceptor bod ies and synaptic terminals are large enough (-25% and 7.5%, respectively) t o be likely candidates for integrated intercellular signaling.