Regulation of thyrotropin secretion in rats bearing the Walker 256 carcinoma.

Previous studies have shown that rats bearing transplantable Walker 256 carcinoma in the thigh have decreased serum total and free T4 and T3 concentrations as well as a decrease in the concentration of hepatic nuclear T3 receptors. Despite these findings, the tumor-bearing (T) rats appeared euthyroid, since the serum TSH concentration remained unchanged, and the induction of the thyroid hormone-sensitive hepatic enzymes a-glycerophosphate dehydrogenase and malic enzyme appeared normal. To evaluate the mechanisms whereby serum TSH was maintained in the normal range in T rats in association with decreased serum T4 and T3 concentrations, we studied the regulation of TSH secretion in groups of control (C) and T rats. The pituitary TSH contents were similar in the two groups of animals, and an iv injection of TRH (5 μg/100 g BW) resulted in a 10-fold increase in plasma TSH concentrations in the control group 15 min after injection. The TSH concentrations achieved after TRH injection were significantly greater ...

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