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

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‐ 3 H] myo ‐inositol, 32 P i , [2‐ 3 H]glycerol, and [ methyl ‐ 3 H]choline into retinal phospholipids in light compared to that in darkness. [2‐ 3 H] myo ‐inositol was incorporated primarily into phos‐phatidylinositol. 32 P i 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. Likewise, [2‐ 3 H]glycerol incorporation into all retinal phospholipids and diglycerides was greater in light than in the dark. There was no effect of light on the incorporation of [2‐ 3 H]ethanolamine into phosphatidylethanolamine or of [3‐ 3 H]serine into phosphatidylserine, although these phospholipids were labeled to a greater extent in light with [2‐ 3 H]glycerol. There was no effect of light on the incorporation of [ 3 H]palmitic acid into diglycerides and phospholipids, with the exception of phosphatidylino‐sitol. Light also had no effect on the uptake of [2‐ 3 H]glycerol, [2‐ 3 H]inositol, or [ methyl ‐ 3 H]choline into the retina. We conclude from these studies that light stimulates the phosphoinositide effect in the rat retina. Although some of the results are consistent with a stimulation of de novo synthesis of all lipid classes, our studies with [ 3 H]palmitate, [2‐ 3 H]ethanolamine, and [3‐ 3 H]serine do not support this conclusion. A possible explanation for the increased incorporation of [2‐ 3 H]glycerol, or 32 P i , and [ methyl ‐ 3 H]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.