To determine whether or not the cytotoxic effects of hyperthermia are directed primarily to malignant cells, we examined changes in thermosensitivity during hepatocarcinogenesis in rats, as induced by 3′‐methyl‐4‐dimethylaminoazobenzene (3′‐Me‐DAB). The findings were compared with those in livers of rats fed a commercial diet. The cell viability was determined using the succinate dehydrogenase inhibition (SDI) test. The succinate dehydrogenase (SD) activity of liver cells, when exposed to heat (43°C) for 2, 5, or 10 hr, decreased in a time‐dependent manner, in each tissue. The decrease in SD activity was evident in 3′‐Me‐DAB liver for 5 hr of heat treatment on day 57, compared with findings in the normal liver. Significant differences were present for 2, 5, and 10 hr on days 93 and 136. Thus a chemically induced hepatoma is more sensitive to heat than are the normal cells. As this thermosensitivity gradually increased during the hepatocarcinogenesis, the malignant cells are particularly vulnerable to hyperthermia.
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