Distribution of tritiated compounds (tritiated thymidine and tritiated water) in the mother-fetus system and its consequences for the radiotoxic effect of tritium.

The incorporation and distribution of tritiated thymidine (3H-TdR) and tritiated water (HTO) have been measured in newborn rats exposed to various levels of tritium by continuous infusion into pregnant rats from day 9 until term. In the animals exposed to HTO, the tritium activity was homogeneously distributed, while 3H-TdR led to accumulation of DNA-bound and homogeneously distributed tritium. The incorporated activity and the specific activity of DNA from ovaries which showed a reduction of total oocyte number by approximately 50% were used to estimate the dose absorbed by the ovarian cell nuclei in both systems. From the absorbed dose, a factor of 3.7 was calculated for the "internal relative biological effectiveness" of DNA-bound tritium as compared to homogeneously distributed 3H under the restrictive assumption that the static description of the system at birth reflects the situation during the time of dynamic development of the ovaries when the toxic effect occurs. The influence of these dynamic factors of changing nuclear size and tritium incorporation during the sensitive period is weighed against the possibility that the continuous 3H-TdR infusion during pregnancy might represent a model in which DNA-bound tritium shows a higher effectiveness than homogeneously distributed tritium.