The contribution of caffeine-modifiable repair process to the radio sensitivity of a radioresistant HeLa strain (RC-355) has been investigated in comparison with control HeLa strain (CC-24). Both the final slope and the shoulder of X-ray survival curve for log-phase cells were affected by caffeine posttreatment. When the treatment with 10 mM caffeine delayed, an increase in survival was observed with increasing interval between irradiation and the treatment. During first several hours of the repair interval, the steepness of the final slope of survival curve decreased rapidly, and rate of the decrease was found to be higher in RC-355 than in CC-24 cells. Longer time (24 hours or more) before the initiation of caffeine treatment was required for the complete recovery of the shoulder. When the cells were incubated in plateau-phase after irradiation, an appreciable increase in survival was observed in comparison with when plated immediately following X-ray. The increase was found to be greater for RC-355 than for CC-24. The results suggest that the radioresistant RC-355 cells repaired more X-ray-induced PLD than CC-24 cells did.

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A calculation method of frequency distributions for the microdosimetric quantity J (number of effective primary events produced in a target), h_J, is presented. This method is briefly compared with other approaches developed by Kellerer and Chmelevsky, and Wilson and Paretzke for numerical calculations of microdosimetric quantities. The hj distributions are calculated for nanometer-size targets in water irradiated with ⁶⁰Co γ rays as a practical application. It was found that the h_J distribution remarkably depends on two parameters, Dσ₀ and 1/W'', where D is the absorbed dose, σ₀ is the mean geometrical cross section of the target, 1 is the mean target thickness, and W'' is the mean energy to produce an effective primary event in the target. Our h_J distributions are compared with those in the conventional target theory. Finally, the quantity z'' and the frequency distribution for z'', h_z'', which correspond to the specific energy z and the probability density f(z) defined in ICRU Report 36, are derived from J and the h_J distribution, respectively. The h_z'' distribution depends on the values of Dσ₀ and 1/W'' and the frequency mean of z'', z'', is equal to the absorbed dose D.

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Three generations of rats were long-term exposed to HTO in drinking water at activity of 37.0 kBq/ml or to food containing OBT at activity of 48.1 kBq/g. The rats consumed tritiated water and tritiated food ad libitum. In the experiment the F₁ and F₂ generation of rats were exposed continuously from conception to the 21-st or 120-th day of age and rats of F₃ generation during 22 days of their intrauterine life. It was found that the amount of tritium incorporated into the major rat brain phospholipids and gangliosides after administration of tritiated food was higher than after administration of tritiated water. Tritium activity in all studied phospholipids and gangliosides was the highest in 21-day-old rats exposed during both-pregnancy and lactation.

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Survival studies were carried out after 8 Gy whole body gamma radiation (WBGR) in rats treated either with Solcoseryl, β-mercaptopropionyl glycine (MPG), 2-aminoethylisothiuronium bromide hydrobromide (AET), 5-hydroxy L-tryptophan (5-HTP), MPG+5-HTP or AET+5-HTP. The drugs were tested after either oral or i.p. administration in fasted rats. Solcoseryl and the combinations of 5-HTP with AET or MPG rendered excellent protection against 8 Gy but lower survival against 10 Gy WBGR. Oral ingestion of these combinations also gave effective protection against 8 Gy whole body gamma radiation.

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The inhibitory effect of N⁶-butyrylcordycepin (7904), one of purine nucleoside analogues, on potentially lethal damage repair (PLDR) in vivo was tested using two experimental tumors, EMT6 sarcoma and SCCVII carcinoma, by an in vivo-in vitro assay. The drug had been reported to be the most potent PLDR inhibitor in vivo. In both tumors, PLDR was apparent in the experimental dose range of 15-35 Gy. However, 100 mg/kg and 200 mg/kg of the 7904 had no inhibitory effect on PLDR in EMT6 tumors, and 200 mg/kg had little effect in SCCVII tumors. Measurement of drug concentrations in the blood by high-performance liquid chromatography showed a low peak level and a relatively short half-life. This was considered to be a cause of the ineffectiveness in vivo.

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The embryos of the fish Oryzias latipes at 3 days after fertilization were irradiated with 10 Gy gammarays after the environmental stresses such as heat-shock, the release from hypoxia, cadmium or arsenite treatment, and change in germ cell number at hatching was determined. Heat-shock (37-42°C, 30 min) could develop radiation resistance increasingly with an increase in the temperature of the heat-shock. However, the release from hypoxia and cadmium or arsenite treatment, which are known to induce heat-shock proteins as well as thermotolerance did not affect the radio sensitivity at all.

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