Utilization of labelled uridine, cytidine and orotic acid for determination of ribonucleic acid synthesis in mouse liver

3H]uridine and 3H]orotic acid were equally utilized for labelling of RNA in mouse liver. Incorporation of 3H]cytidine was 2-3 times as high as that of 3H]-labelled uridine or orotic acid. These results differ from findings in rat liver, where both cytidine and orotic acid are better utilized for RNA labelling than is uridine. The ratio between liver RNA 3H]-activity and volatile 3H]-activity was 2, 3 and 13, respectively, at 300 min after injection of labelled uridine, orotic acid and cytidine, indicating an efficient chanelling of cytidine into liver anabolic pathways.