Effect of Light on the Metabolism of Lipids in the Rat Retina

Abstract

The effect of light on the in vitro incorporation of a variety of radioactive precursors into glycerolipids was tested in isolated retinas of albino rats. There was an increase in the incorporation of [2-3H]myo-inositol, 32Pi, [2-3H]glycerol and [methyl-3H]choline into retinal phospholipids in light compared to that in darkness. [2-3H]myo-Inositol was incorporated primarily into phosphatidylinositol. 32Pi was incorporated primarily into the phosphoinositides, although there were significant increases in the specific activities of all retinal phospholipids in light compared to those in darkness. [2-3H]glycerol incorporation into all retinal phospholipids and diglycerides were greater in light than in the dark. There was no effect of light on the incorporation of [2-3H]ethanolamine into phosphatidylethanolamine or of [3-3H]serine into phosphatidylserine, although the phospholipids were labeled to a greater extent in light with [2-3H]glycerol. There was no effect of light on the incorporation of [3H]palmitic acid into diglycerides and phospholipids, with the exception of phosphatidylinositol. Light had no effect on the uptake of [2-3H]glycerol, [2-3H]inositol, or [methyl-3H]choline into the retina. Evidently, light stimulates the phosphoinositide effect in the rat retina. A possible explanation for the increased incorporation of [2-3H]glycerol or 32Pi, and [methyl-3H]choline into retinal phospholipids is that the metabolic events that follow photon capture alter the size and/or turnover rate of the choline, choline phosphate, ATP and glycerol-3-phosphate pools, thus altering the specific radioactivities of these precursors. This would result in different specific activities of the lipid products in the absence of any direct effect of light on the rate of synthesis.