Effects of Thyroidectomy and Thyroxine Therapy on Biosynthesisand Release of Luteinizi g Hormone y Rat Anterior Pituitary Glands in vitro’

We examined the role of thyroid hormones in regulating the synthesis and release of luteinizing hormone (LU) by anterior pituitary glands (AP) of ovariectomized (Ovx) rats. Immediately after ovariectomy, rats were thyroidectomized (Tx) or sham-Tx. Starting 30 days later, they were injected with either saline solution or thyroxine (T4) for 9 days. Twenty-four hours after the last injection, the APs were removed and incubated in vitro with 3.6 X 10 ’ M gonadotropin-releasing hormone (GnRH) and the radioactive precursors of LH, I’4Cl-alanine and (1-I]-glucosamine. Radiolabeled LH was isolated by immunoprecipitation with specific anti-LH-(3 serum. The levels of T4 in the serum as well as the immunoreactive LU (IR-LH) in the serum, medium, and AP5 were measured by radioiinmunoassay. After thyroidectomy T4 was undetectable in the serum of Ovx rats. T4 replacement in Ovx-Tx rats restored the serum T4 to approximately normal levels. Serum LII concentrations were higher in Ovx-Tx rats than in Ovx rats. Depending on dose, T4 therapy either depressed or had no significant effect on serum LU in Ovx-Tx rats. The total lR-LH content in the pituitary was higher and the release of lR-LH by AP5 in response to GnRH was greater in Ovx-Tx rats than in Ovx-Sham Tx rats. Thyroidectomy stimulated the incorporation of l’4C1-alanine and l3H].glucosamine into LH; however, the in vitm release of de novo synthesized radiolabeled LU was not altered by thyroid status. Thyroidectomy did not modify the uptake of 14C1-alanine and (3Hl-glucosamine, whereas it reduced the incorporation of f3Hl-glucosamine into pituitary glycoprotein. These results suggest that thyroid hormones directly affect LH synthe-sis