SEX STEROIDS MODULATE THE PITUITARY PARAMETERS INVOLVED IN THE REGULATION OF TSH SECRETION IN THE RAT

The sex-related differences observed in the regulation of TSH secretion was further investigated by determination of the densities of T3 nuclear and TRH membrane receptors as well as the activity of 5'-deiodinase (5'D) in the anterior pituitary gland of adult male and female rats. The respective modulatory roles of androgens and estrogens on these parameters were evaluated by similar determinations carried out in castrated and in estrogen-treated male rats. The density of pituitary T3 and TRH receptors and the activity of 5'D type II were significantly greater in the female than in the male rats. The E2-treated male rats disclosed a female profile, viz. also greater densities of T3 and TRH receptors when compared with control male rats (2.3 ± 0.2 vs 1.8 ± 0.2 fmol T3/mg gland and 9.4 ± 0.8 vs 6.0 ± 0.8 fmol TRH/mg gland, mean ± SEM), whereas no changes were found in the castrated rats. The E2-treated rats and the castrated rats exhibited an increased pituitary activity of 5'D, type II (0.87 ± 0.10 and 0.66 ± 0.05, respectively, vs control 0.34 ± 0.07 pmol rT3 · h-1 · (mg protein)-1), suggestive of a stimulatory effect of E2 and of an inhibitory effect of androgens on this parameter. In contrast, no differences in hepatic 5'D were found between all groups, illustrating the well-known tissue-specific regulation of 5'D. These results demonstrate that the sex difference in the density of pituitary T3 and TRH receptors and the activity of 5'D in the adult rat is mainly due to a modulatory effect of estrogens, which may be responsible for the sex-dependent regulation of TSH secretion.