Comparative Studies of the Effects of Drugs on X-Ray-Induced G2 Delay

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Abstract
The mitotic cell selection technique was used to monitor the effects of various drugs, primarily inhibitors of RNA synthesis, on X-ray-induced G2 delay. Chinese hamster ovary cells located in the cycle prior to about 52 min before division, i.e., prior to the X-ray transition point (X-TP), were delayed for 110-130 min from progression into mitosis (G2 delay) after receiving 100 rad. Cells past this X-TP were unaffected in their progression. However, the addition of actinomycin D (AMD, 2 μg/ml), caffeine (19-194 μg/ml), theophylline (18-180 μg/ml), or cordycepin (5-30 μg/ml) immediately before or after irradiation greatly reduced the G2 delay and shifted the X-TP away from division. The magnitude of this protective effect increased with concentration, with a maximum effect of about 90 min for reduction of G2 delay and about 11 min for shifting the X-TP away from division. Less protective effect was observed from the addition of dimethylsulfoxide (DMSO, $10^{4}\ \mu {\rm g}/{\rm ml}$), which reduced G2 delay by only 28 min and had no effect on the X-TP when DMSO was added immediately after irradiation. Unlike the drugs above, DMSO offered more protection when added before irradiation than when added after irradiation. In contrast to the protective effects discussed above, the addition of 2-mercapto-1-(β-4-pyridethyl) benzimidazole (MPB, 25-75 μg/ml) or lucanthone (5-20 μg/ml) immediately before irradiation increased G2 delay and shifted the X-TP closer to division. Studies of the effects of the drugs on incorporation of tritiated uridine or tritiated leucine into acid insoluble material indicated no correlation between reduction of G2 delay and rates of overall RNA or protein synthesis. For example, caffeine, which had the greatest effect on reducing G2 delay, had very little effect on either RNA or protein synthesis. Furthermore, AMD and cordycepin, both of which reduced G2 delay, and MPB and lucanthone, both of which increased G2 delay, had marked inhibitory effects on RNA synthesis with only minimal effects on protein synthesis. These results are discussed in relation to other reported metabolic effects of these drugs in an attempt to search for correlations relevant to identifying the lesion involved in G2 delay.