Changes in fluid compartments and ionic composition in the isolated chick retina during SD

Total content of water, extracellular spaces (ES), na+, K+, and C1- in the isolated chick retina were measured in the presence (test) or absence (control) of spreading depression (SD). During SD in medium with 0.5 mM or 2 mM MgSO4, there is an increase in the intracellular concentration of Na+ and C1- and a decrease in the intracellular concentration of K+. A decrease in the ES was only found in the medium with 2 mM MgSO4 together with a diminished outmovement of K+. We suggest that a decrease in the ES is due to an increased absorption of K+ by the Muller cells, causing its swelling and consequently a decrease of the ES. The addition of sucrose (17 mM) to the incubation medium as the extracellular marker markedly decreased the intracellular concentration of C1- in control retinas, blocked the inward movement of this ion to the tissue during SD and also changed the K+ movement during the phenomenon in medium with 2 mM MgSO4. We suggest that C1- is an important ion in the ionic balance of the Muller cells and that sucrose must have its site of action at these cells.