Effect of Decreased Hepatic Nuclear L-Triiodothyronine Receptors on the Response of Hepatic Enzymes to LTriiodothyronine in Tumor-Bearing Rats*

Abstract

The activity of hepatic α-glycerophosphate dehydrogenase (α-GPD) and malic enzyme (ME) was measured in rats bearing the Walker 256 carcinoma to assess the relationship between the reduction in the nuclear L-T3 (T3) receptor concentration which occurs in tumor-bearing (T) rats and the biological effects of thyroid hormones. T3 receptor concentration was uniformly decreased when tumors exceeded 9.5% of body weight. No change in basal α-GPD activity was observed, but ME activity was decreased to between 29–61% of control. The decrease in ME activity did not appear to be related to decreased nuclear T;) receptor concentration since ME was also decreased in T rats with tumors that were too small to effect a decrease in the concentration of nuclear T3 receptors or plasma T4 and T3. The response of both ME and α-GPD to T3 administration in T rats was comparable to that of control despite reduced nuclear T3 receptor and plasma hormone concentrations. When non-T rats were subjected to food restriction to simulate ...