Our previous observations that x-irradiation induced profound changes in cytoplasmic ultrastructures have been extended. Parallel investigations on four types of mouse tumors (IBAH, DBAG, Adj-PC-5, DBrB), differing in growth rates, were carried out in respect to the ultrastructural integrity of their mitochondria, their ability to carry out oxidative metabolism, and their response to x-irradiation. Mitochondria of the most rapidly proliferating DBAG and DBrB tumors possessed the lowest oxidative and phosphorylative capacities as determined by P:O ratios. The mitochondria of these tumors were few in number and of inferior quality, as deduced from electron micrographs. The P:O values of liver and kidney were significantly higher than those of the tumors, corresponding to the superior quality and quantity of mitochondria. Similarly, the ATP, HMP, and cytochrome oxidase levels were also significantly lower in the tumors than in the livers and kidneys. Exposures to 3000 R induced extensive swelling of mitochondria, breakages of their cristae, and dilatation of vesicles of the endoplasmic reticulum in the tumor cells. Conversely, exposures up to 10,000 R failed to induce significant effects on the mitochondrial oxidative metabolism of the tumors. On the basis of electron microscopic findings, it is inferred that the intrinsic quality of the cytoplasmic constitutents, including mitochondria, plays a greater role in cellular radiosensitivity than the metabolic events.

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