EFFECTS OF AGE AND ENDOGENOUSLY SECRETED HUMAN GROWTH-HORMONE ON THE REGULATION OF GONADOTROPIN-SECRETION IN FEMALE AND MALE TRANSGENIC MICE EXPRESSING THE HUMAN GROWTH-HORMONE GENE

In aging rats and humans, GH secretion is reduced. In transgenic mice bearing the human (h) GH gene, hGH is secreted during the entire lifespan. To evaluate the effects of endogenously secreted hGH on age-related changes in hypothalamic-pituitary function, the following two experiments were conducted in young (2.5-4 months of age) and old (11-14 months of age) female and male transgenic mice expressing the hGH gene and their normal siblings. In Exp I, young and old female transgenic mice and their normal siblings were ovariectomized. On days 8 and 9 after ovariectomy, mice were injected (sc) with oil or primed with 0.5 mug estradiol benzoate (EB) in oil, 24 h later treated with 10 mug EB/100 g BW, and a day later bled for the determination of FSH, LH, PRL, and hGH levels by RIAs. In Exp II, young and old male transgenic mice and their normal littermates were castrated and injected with either peanut oil or testosterone propionate (TP; 1 mug/g BW) in oil. Blood samples were obtained 18-20 h after oil or TP injection. Plasma FSH, LH, PRL, and hGH levels were measured by RIAs. hGH was present in the circulation of young and old transgenic mice, but not in normal siblings. Circulating FSH and LH levels were significantly lower in ovariectomized young and old transgenic mice than in similarly treated young and old normal siblings. The suppressive effect of EB on LH secretion was reduced in ovary-ablated young and old transgenic mice. The PRL response to EB treatment was increased (P < 0.005) in old ovariectomized transgenic mice. In the male, the castration-induced increase in plasma LH levels was higher (P < 0.05) in young transgenic mice. Administration of TP failed to suppress the absolute plasma LH levels in castrated young and old transgenic mice. However, the percent decrease in circulating LH levels was lower in young and old transgenic mice. The castration-induced increase in FSH secretion was reduced (P < 0.005) in aged transgenic mice. The effects of TP on plasma FSH levels were similar to its effects on LH secretion. Gonad-ablated young and old transgenic mice of both sexes are hypoprolactinemic. These observations demonstrate that endogenously secreted hGH modulates gonadotropin and PRL secretion in young and old mice bearing the hGH gene and indicate that the changes in the hypothalamic-pituitary system of mice expressing the hGH gene are similar to those observed in aging human subjects. Although hGH secretion causes premature death at about 16 months of age, it attenuates further derangement of neuroendocrine function in aging, possibly due to the GH effects of hGH.