Cytidine metabolism in photoreceptor cells of the rat

Abstract

During brief (30-min) incubations, isolated rat retinas accumulated [3H]cytidine, converted it to cytidine triphosphate (CTP), and incorporated it into RNA and cytidine diphosphate-diacylglyceride (CDG), a phospholipid precursor of phosphatidylinositol (PI). Labeled CTP, RNA, and CDG contents were found to be 2- to 3-fold higher in photoreceptor cells than in cells of the inner retina. Autoradiograms showed that, within photoreceptor cells, silver grains representing RNA were concentrated over the nuclei in dark and light, while silver grains representing CDG were concentrated over the inner segments only after incubation in dark. The formation of labeled CTP and the synthesis of RNA were enhanced in light, while labeled CDG levels became reduced in light concurrent with an increase in the incorporation of labeled inositol into PI. The 3H-labeled CDG content, however, was increased 2- to 4-fold in light in the presence of actinomycin D, and autoradiograms show a heavy concentration of silver grains over the inner segments of photoreceptor cells. These findings establish a role for cytidine nucleotides in photoreceptor cell metabolism and in light-dependent increases in RNA and PI synthesis. Furthermore, the observations indicate that a competition may exist in light for cytidine or CTP and suggest that availability of cytidine for CDG synthesis may have a regulatory role in PI metabolism within the photoreceptor cells.