53. Comparative Biokinetics of Tritium in Rats during Continuous lngestion of Tritiated Water and Tritiated Food

Hiroshi Takeda, Kiriko Miyamoto Shoichi. Fuma, Kei Yanagisawa, Nobuyoshi Ishii. and Noriko Kuroda

Keywords: tritiated water, tritiated wheat, biokinetics, dose estimatton, rat

In the present study, we investigated the dynamics of tritium in rats during about 100 days' continuous ingestion of tritiated water and tritiated wheat. Wistar strain male rats weighing about 450 g were used. Sixty rats were separated into two groups. Each group of rats were continuously ingested tritiated water as drinking water or tritiated wheat as food. Throughout the expernment, rats were housed in metabolic cages (]VIetabolica, Sugiyama gen Iriki Instrument Co.) in order to measure consumption of water and food and to collect daily samples of urine and feces from each animal. Tritiated water was purchased from NEN and di Juted to an appropriate concentration with distilled water to prepare the tritiated drinking water and tritiated food. The concentration of tritium in drink ing water was adjusted to 6.3 kBq/ml. For prepara tion of tritiated food, wheat was grown in a 1/50 m² plastic Wagner's pot in a metabolic chamber After flowering the plant was sometimes irrigated with tritiated water. At the time of ripening, the plants were harvested and the edible part was used for administration to rats as a tritiated food. Before administration to rats, the tritiated wheat was put verized, freeze-dried and mixed with the powdered standard chew. The concentration of tritium in the food including tritiated wheat was 0.18 kBq/g. The rats were sacrificed by decapitation under ether an esthesia on the 1 2 3, 4, 5, 6, 8, TO and 14th week after beginning of the ingestion. Various ttssue sampies were taken from the sacrificed animals and immediately the activity of total tritium in the fresh wet tissues was determined. A part of each tissue sample was lyophilized for determination of OBT in dry tissue sample. All samples were combusted in an oxidizer and tritium activity in the combustton water was measured with a liquid scintillatton counter.

Time-variations of the concentration of total trit ium and OBT in the tissues of rats during the continuous ingestion of tritiated water showed that the concentrations of total tritium increased rapidly and attained an equilibrium state at 2-3 weeks after start of the exposure to tritiated water and the equi librium state of OBT concentrations was observed at about the 4th week. In the case of the chronic exposure to tritiated food, the equilibrium state for both total tritium and OBT was not observed for TO weeks after start of the exposure in almost all tissues except for liver, in which the equilibrium was attained at 4 weeks after start of the exposure.

R-value, defined as the ratio of specific activity (T/H) of tritium in dry tissues to specific activity (T/H) of tritium in daily ingested drinking water or food, is an important factor for evaluating the extent of tritium incorporation into the tissue organic constituents of organism. In this study, R-values for dif ferent tissues of rats after 98 days' continuous ingestion of tritiated water and tritiated food were c)alculated. For this calculation, we used the values of hydrogen contents for organic component of vari ous tissues given by Pietrzak-Flis et al. Hydrogen content for daily ingested drinking water should be 11.l% and Chat for tritiated food was estimated to be 7.O% from the list of its composition. Results are shown in Table 7. We found that the R-values for the exposure to tritiated water ranged from 0.15 for heart to 0.24 for testis, while the R-values for the exposure to tritiated food ranged from 0.36 for brain to 0.78 for lung. Thus, the R-values for the exposure to tritiated food were obviously higher than those for the exposure to tritiated water. In the prevuous study, we have also estimated the R-values for the 22 days' chronic exposure to tritiated water and tritiated food (wheat). Compared with this previous result, the present R-values for the exposure to tritiated water did not differ so significantly, but those for the exposure to tritiated food were evu dently greater. Nevertheless, they were never over 1.0, indicating that the specific activity of tritium in the tissue organic constituents of rat did not exceed the specific activity of tritium in the food as well as that in the drinking water.

The radiation dose rates from total tritium in the different tissues at the end of chronic exposure to tritiated water or tritiated food were calculated. OBT contribution to the total dose rates was also calculated using the data on the dose rates from OBT and the water content for individual tissue. Table 8 shows the calculated dose rates and OBT contribution. The ratios of the dose rates from the ingestion of tritiated food to those from the nngestion of tritiated water are also given. The total dose rates from the exposure to tritiated food were higher than those from the exposure to tritiated water by a factor of 1.3 to 4.5 but the factors were within 2 in the majority of tissues except for small intestine and adipose tissue.

In our previous study in which tritiated water and tritiated wheat were continuously ingested for 3 weeks, the dose rates from tritiated foods were higher than those from tritiated water by a factor of 2.1 to 3.6. The results of the present study, therefore, indicated that the difference in the dome rates between tritiated water and tritiated foods would not increase by lengthening the period of chronic ex posure.

Table 7. Ratio of Specific Activity (T/H) of Tritium in the Dry Tissue to Specific Activity (T/H) of Tritium in the Daily Ingested Tritiated Food in the 14thWeek of Chronic Exposure

Table 8. Radiation Dose Rates from Total Tritium and OBT Constribution to the Dose Rates in the 14th Week of Chronic Exposure to Tritiated Water and Tritiated Wheat.

^a : The dose rates were calculated assuming tritium exposure to be 3.7 MBq/g body weight.
^b : OBT contribution is expressed as a percentage of total dose rate, which is shown in parentheses