Effect of Cytidine on Membrane Phospholipid Synthesis in Rat Striatal Slices

Abstract: Using rat striatal slices, we examined the effect of cytidine on the conversion of ³H]choline to ³H]-phosphatidylcholine (³H]PC), and on net syntheses of PC, phosphatidylethanolamine (PE), and phosphatidylserine, when media did or did not also contain choline, ethanolamine, or serine. Incubation of striatal slices with cytidine (50–500 µM) caused dose-dependent increases in intracellular cytidine and cytidine triphosphate (CTP) levels and in the rate of incorporation of ³H]choline into membrane ³H]PC. In pulse-chase experiments, cytidine (200 µM) also increased significantly the conversion of ³H]choline to ³H]PC during the chase period. When slices were incubated with this concentration of cytidine for 1 h, small (7%) but significant elevations were observed in the absolute contents (nmol/mg of protein) of membrane PC and PE (p < 0.05), but not phosphatidylserine, the synthesis of which is independent of cytidine-containing CTP. Concurrent exposure to cytidine (200 µM) and choline (10 µM) caused an additional significant increase (p < 0.05) in tissue PC levels beyond that produced by cytidine alone. Exposure to choline alone at a higher concentration (40 µM) increased the levels of all three membrane phospholipids (p < 0.01); the addition of cytidine, however, did not cause further increases. Concurrent exposure to cytidine (200 µM) and ethanolamine (20 µM) also caused significantly greater elevations (p < 0.05) in tissue PE levels than those caused by cytidine alone. In contrast, the addition of serine (500 µM) did not enhance cytidine's effects on any membrane phospholipid. Exposure to serine alone, however, like exposure to sufficient choline, increased levels of all three membrane phospholipids significantly (p < 0.01). These data show that exogenous cytidine, probably acting via CTP and the Kennedy cycle, can increase the synthesis and levels of membrane PC and PE in brain cells.