Factors affecting cytidine uptake and utilization by the rat uterus

The influence of cytidine dosage on uptake and distribution of [3H]cytidine by the uterus of the adult ovariectomized control rat was examined 20 min after injecting the nucleoside. Intracellular accumulation of free cytidine was observed only at a dosage of 4.2 mumol/100g. At lower dosages, the nucleoside was phosphorylated as rapidly as it entered uterine cells, thereby preventing its accumulation within cells. [3H]Cytidine (4.2 mumol/100 g) was used in time-course studies in ovariectomized control and estradiol-treated rats. From this study the following differences between uteri of control and estradiol-treated animals were noted: 1) both nucleotides and RNA were five to six times more highly labeled in treated than control animals after a five minute labeling period, 2) this difference between control and treated animals was reduced to twofold when the labeling period was extended to 20 min, 3) free nucleoside had begun to accumulate intracellularly in treated animals, but had not equilibrated with uterine extracellular space in controls 5 min after [3H]cytidine injection, and 4) at 20 min, the free nucleoside of the uterus was labeled about equally in control and treated rats. It is concluded that estradiol causes a rapid equilibration of circulating cytidine with the uterus and that increases in labeling of RNA and precursor nucleotides 2 h after hormone treatment are largely a consequence of this rapid equilibration.