Age dependent changes in the metabolism of 3H-thymidine and 3H-uridine in young rats.

The metabolism of subcutaneously injected 3H-thymidine and 3H-uridine was studied in 5, 15 and 30-day-old rats and some age-dependent differences were found. The total availability of 3H-uridine was found to decrease when the rats became older. In the case of 3H-thymidine, however, there was only minor differences in the total availability. The catabolism of both 3H-thymidine and 3H-uridine, as measured by the appearance of 3H-water, was more efficient when the rats became older. In the case of 3H-thymidine, however, the increased catabolic capacity seems to be compensated by a decreased proliferative growth so the total availability will be constant. It was also observed that the amount of intermediate catabolic products in the blood was much larger after an injection of 3H-uridine than after 3H-thymidine.