To continue our studies on the influence of T3 on TSH regulation in the Walker 256 carcinoma-bearing rat model of nonthyroidal disease, we measured the effect of T3 on pituitary content of βTSH mRNA and rat (r) TSH in hypothyroid control (C) and tumor-bearing (T) rats. The effect of T3 on TSH regulation was compared to effects on GH mRNA and rGH in the same animals. mRNA content was normalized to a pool of pituitaries from euthyroid rats (=1.0). βTSH mRNA increased 18-fold in both hypothyroid C and T rats and then decreased similarly with increasing T3 infusion to a value of 0.1. GH mRNA content decreased to 0.11 ± 0.01 in hypothyroid C rats, but to only 0.38 ± 0.02 in T rats (P < 0.001). The pituitary contents of GH mRNA and rGH in hypothyroid T rats was significantly greater than those in C rats at all T3 infusion rates. These data together with our previous report of decreased nuclear T3 in T rats suggest that regulation of βTSH mRNA by T3 is intact in T rats, but occurs at a lower concentration of nuclear T3. In contrast, the GH mRNA response is enhanced, displaying differential regulation of these two T3-responsive gene products in this model of nonthyroidal illness.
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