Regional effects of sodium aspartate and sodium glutamate on protein synthesis in the retina.

Isolated retinas from Xenopus laevis tadpoles or juvenile frogs were incubated in Ringer's bicarbonate-glucose medium in which sodium chloride was replaced with equimolar amounts of either sodium aspartate (NaAsp) or sodium glutamate (NaGlu). At all concentrations tested (0.1 to 50 mM), both acidic amino acids caused a dramatic decrease in the incorporation of 3H-leucine into proteins in the inner retina. Higher concentrations of these acids (50 mM) caused an increase in the incorporation of the label by photoreceptors. Polyacrylamide gel disc electrophoresis of whole retinal proteins showed a selective increase in incorporation of the label into rhodopsin, a protein unique to rod photoreceptor cells. Quantitative autoradiographic and biochemical studies showed that NaAsp had no effect on the uptake of 3H-leucine by the retinal cells, suggesting that the effect is directly on protein synthesis. When retinas were removed from NaAsp treatment, protein synthetic capacity recovered. and the degree of recovery was related to the concentration of NaAsp in the preincubation medium. These studies demonstrate that acidic amino acids in concentration utilized in electrophysiologic studies to isolate the photoreceptor generated a-wave of the electroretinogram also have a profound effect on retinal protein synthesis. Whether these two actions are related remains to be determined.