Alterations of nuclear thyroid hormone receptors in cerebral cortex in vivo

Investigation was conducted under in vivo conditions in the adult male rat to determine the basic characteristics of the nuclear thyroid hormone receptors in the cerebral cortex. Equilibrium with cortical nuclei of an intravenous dose of triiodothyronine (T3) occurred 3 h after injection and showed a t1/2 of 1 h for dissociation. Saturation of receptors occurred at 0.5--06. ng/mg DNA. The endogenous level of binding in the normal rat was 0.07--0.1 ng/mg DNA, representing a 15% occupancy of total receptors at a serum concentration of 66 ng/dl. These characteristics were then examined under several pathophysiological conditions. In the hypothyroid rat, an apparent 37% increase in total binding sites occurred. Under either fasting conditions or insulin or glucagon administration declines in serum T3 were noted, and the endogenous binding also decreased in parallel. Only glucagon produced a significant reduction in total binding sites. Under the hypoxic condition produced by maintenance under a 7% oxygen atmosphere, a slight increase in apparent total binding sites was found with no change in endogenous binding level. Severe narcosis resulted in no effects on T3 binding parameters. These results demonstrate specific alterations of thyroid hormone receptors that may be important physiologically.