The role of growth hormone in the control of gonadotropin secretion in adult male rats

Although it is known that GH plays an important role in normal growth and development, its influence on the control of gonadotropin secretion is poorly understood. To address this issue, we have treated adult male rats with bovine GH via osmotic pumps (250 mu g/day for 2 weeks; Exp design I) or immunized rats against ovine GH (100 mu g/month for 6-7 months; Exp design II) and evaluated their neuroendocrine function. Vehicle-treated animals served as controls. Two experiments were conducted to evaluate the gonadotropin responses to: 1) GnRH (in saline) in gonad-intact rats and 2) testosterone propionate (TP; in oil) in castrated rats. Saline-or oil-injected rats served as controls. Circulating GH antibodies, LH, FSH, PRL, testosterone, and insulin-like growth factor I levels were measured by RIAs. Plasma LH levels were decreased (P < 0.025) in rats treated with GH. The plasma LH and FSH responses to GnRH treatment were similar in rats treated with either saline or GH. The suppressive effect of TP on LH secretion was attenuated (P < 0.025) in GH-treated rats on day 8 after castration. The FSH response to TP administration was similar in both subgroups of rats. Administration of GH decreased (P < 0.01) PRL secretion. Plasma testosterone levels were not altered by GH treatment. As expected, GH antibodies were detected and plasma insulin-like growth factor I levels were decreased (P < 0.001) in rats immunized against GH. The basal LH and FSH levels were higher (LH, P < 0.005; FSH, P < 0.025) in rats previously immunized against GH. The percent increase in plasma LH levels after GnRH treatment was decreased in GH-immunized animals. Furthermore, the percent increase in circulating FSH levels was higher in GH-immunized rats than in adjuvant-injected control rats. Administration of TP to adjuvant-injected castrated rats decreased plasma gonadotropin levels. However, similar treatment to rats immunized against GH failed to suppress plasma LH and FSH levels. The basal testosterone levels were not changed by immunization against GH. These results demonstrate that induction of GH excess decreases PRL and LH secretion, whereas biological neutralization of endogenous GH increased circulating gonadotropin concentrations. Thus, GH modulates the hypothalamic-pituitary function in adult male rats.