Effect of X-irradiation on the transport of thymidine into asynchronous and S phase CHO cells

An investigation has been carried out into the effects of moderate (1000 rad) and high (5000 rad) doses of X-irradiation on the transport od 3H-thymidine into both asynchronous and S-phase CHO cells. Various concentrations of 3H-thymidine were added at 10 min and 2.5 hours post-irradiation and cellular incorporation measured after thymidine exposure periods of up to 10 minutes. Thymidine transport into both asynchronous and S-phase cells was not affected by doses of 1000 and 5000 rad when the measurement was made 10 min after irradiation. When transport was studied 2.5 hours after irradiation of asynchronous cultures, there was no effect by 1000 rad but a significant reduction in transport at all concentrations with 5000 rad. For a 10 min pulse with 0.75 μm 3H-thymidine 2.5 hours after irradiation, incorporation into the acid-insoluble fraction was reduced to 47% of control with 5000 rad and 80% of control with 1000 rad. Acid-insoluble thymidine incorporation into synchronous cultures from a 10 minute pulse was not affected by 1000 or 5000 rad (9 hours after mitosis) when measured 10 minutes post-irradiation. For cells irradiated 8 hours after mitosis and pulsed 2.5 hours after irradiation, the uptake of thymidine was reduced to 50% of control after 5000 rad, whereas no reducton was apparent at 1000 rad, although the acid-insoluble incorporation was reduced to 51% of control for 1000 rad and to 45% for 5000 rad. Radiation-induced effects on transport therefore have little relevance to the decrease in acid-insoluble incorporation. Possible mechanisms are discussed. (U.K.)