Pathology of experimental radiation pancarditis. II. Correlation between ultrastructural changes of the myocardial mitochondria and succinic dehydrogenase activity in rabbit heart receiving a single dose of X-ray irradiation.

Twenty-three rabbits were given a single dose of 3,000R X-irradiation over the heart to study the correlation between the ultrastructural changes of myocardial mitochondria and succinic dehydrogenase activity. The study was made from 1 hour until 1 month after irradiation. The myocardial mitochondria showed, at 1 hour, regressive changes such as irregular arrangement, swelling, vacuolation, decrease and disappearance of cristae, formation of giant mitochondria, and whorled myelin figures. These changes became maximum at 12 hours-2 days. Regeneration of mitochondria had begun at 2 days. At 4-7 days, structurally normal mitochondria had markedly proliferated accompanied by large mitochondria with concentrically arranged cristae and small mitochondria around the nuclei and beneath the sarcolemma of myocardial cells. The myocardial succinic dehydrogenase activity, on the other hand, showed the lowest value at 12 hours--2 days, and recovered almost to the normal value at 7 days. At 2 weeks--1 month when myocardial fibrosis developed, however, the value again mildly decreased. Cardiac death due to radiation therapy may be initiated by the mitochondrial hypofunction in radiation myocardial fibrosis, because the irradiation injures the mitochondrial membrane resulting in morphological and functional changes.