Differential response to L-triiodothyronine of anterior pituitary growth hormone messenger ribonucleic acid (mRNA) and β-thyrotropin mRNA in a hypothyroid walker 256 carcinoma-bearing rat model of nonthyroidal disease

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.