Regulation of Rat Pancreatic Glucocorticoid Receptors by Thyroxine during Development*

The effect of T4 on the development of pancreatic glucocorticoid receptors was studied in normal and adrenalectomized rat pups. Daily injection of T4 (0.1 microgram/g BW) to intact pups starting 3 days before death at 10, 15, and 20 days of age resulted in a precocious increase in pancreatic glucocorticoid-binding capacities. Intact pups made hypothyroid by propylthiouracil feeding exhibited lower glucocorticoid-binding capacities in their pancreata. Scatchard analysis demonstrated an increase in the number of glucocorticoid-binding sites in the pancreata of T4-treated intact rats compared to that in normal intact rats. In hypothyroid groups the number of glucocorticoid-binding sites was much lower than that in normal intact rats. The Kd values, however, were unchanged in hypothyroid, hyperthyroid, and control groups. Rat pups who underwent adrenalectomy at 12 days of age had undetectable plasma corticosterone levels and showed an increase in their pancreatic glucocorticoid-binding capacity 3 days after operation. Replacement of corticosterone resulted in a binding level similar to that in the sham-operated group. However, injection of T4 alone to adrenalectomized pups led to a further increase in pancreatic glucocorticoid-binding capacity above that due to adrenalectomy alone. When both T4 and corticosterone were given together to adrenalectomized pups their pancreatic glucocorticoid-binding capacities increased to levels above those in the adrenalectomized group, but lower than those in pups receiving T4 alone. Our results suggest that T4 modulates the development of rat pancreatic glucocorticoid receptors and, at least in part, acts via pathways independent of adrenal function.