Cytidine metabolism in photoreceptor cells of the rat.

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Abstract
During brief (30-min) incubations, isolated rat retinas accumulated [3H]cytidine converted it to 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, Ag grains representing RNA were concentrated over the nucleus in dark and light, while Ag 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 PL. The 3H-labeled CDG content, was increased 2- to 4-fold in light in the presence of actinomycin D, and autoradiograms show a heavy concentration of Ag 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 Pl synthesis. Apparently, competition may exist in light for cytidine or CTP and availability of cytidine for CDG synthesis may have a regulatory role in Pl metabolism within the photoreceptor cells.