Homeostasis of the retinal micro-environment: I. Magnesium, potassium and calcium distributions in the avian eye

Abstract

The concentrations of Mg 2+, K + and Ca 2+ in the intraocular fluids (IOFs) and blood plasma of chickens and pigeons were determined by atomic absorption spectrophotometry. The Mg 2+ concentration in the IOFs of both species was greatest in the liquid vitreous adjacent to the retina followed by aqueous > blood plasma > plasma dialysate. In contrast the concentration of K + in the IOFs of both chickens and pigeons was greater in the aqueous than in the liquid vitreous. The concentrations of Ca 2+ in all IOF compartments of chicken eyes were virtually identical and were lower than that of blood plasma. The concentrations of Mg 2+ in the IOFs of the chicken, especially in the liquid vitreous, was remarkably stable; experimentally lowering or raising the plasma Mg 2+ concentration over a relatively wide range had little or no effect on the Mg 2+ concentration in the IOFs of these animals. We can conclude that a high Mg 2+ and low K + concentration in the extracellular fluids of the retina is maintained in the avian eye, as in the mammalian eye, by active transport processes across the blood-retinal barrier systems. Because the avian retina is completely avascular, the site of these homeostatic transport processes must be the epithelium of the retinal choroid and/or the pecten. These findings support the concept that the contribution of the vitreous to the homeostasis of the retinal micro-environment is inversely related in vertebrates to the degree of retinal vascularization.