Interrelationship between estrogen and thyroxine on the release of luteinizing hormone and gonadotropin-releasing hormone in vitro

The present experiments were designed to study the interaction between estradiol benzoate (EB) and thyroxine (T4) given in vivo on the responsiveness of pituitary luteinizing hormone (LH) to gonadotropin-releasing hormone (GnRH) and the release of GnRH in vitro. Ovariectomized-thyroidectomized (Ovx-Tx) rats were injected s.c. with saline or T4 (2 micrograms/100 g b.wt), and oil or EB (0.1 microgram) once daily for 40 days following a 2 x 2 factorial design. All animals were then decapitated and blood samples were collected. Anterior pituitaries (APs) were incubated in vitro with and without 0.1 ng GnRH at 37 degrees C for 4 h. Mediobasal hypothalami (MBHs) were excised and then incubated with and without APs from Ovx donor rats. Concentrations of LH and GnRH in the medium and that of LH in the serum were measured by radioimmunoassay. The LH level in media containing MBHs and donor APs was used as the index of bioactive GnRH release. In Ovx-Tx rats, T4 injections reduced the serum LH concentration, the pituitary LH response to GnRH, and the bioactive as well as the immunoreactive GnRH release. The serum LH levels and the spontaneous as well as the GnRH-stimulated release of LH in vitro were suppressed in Ovx-Tx rats following administration of EB. By contrast, the serum LH concentration, as well as pituitary LH response to GnRH and GnRH release in vitro, were higher in the group treated with both T4 and EB than in that treated with saline and EB. These results suggest that the differential changes in the LH secretion after thyroidectomy of Ovx versus non-Ovx rats are due to an antagonistic effect between T4 and estrogen on the response of pituitary LH to GnRH, and the release of GnRH.