Jj. Orrego et al., Semiquantification of hypothalamic GH-releasing hormone output in women: Evidence for sexual dimorphism in the mechanism of the somatopause, J CLIN END, 86(11), 2001, pp. 5485-5490
The neuroendocrine mechanisms underlying the decline of GH with aging (soma
topause) are uncertain. We recently found that the age-dependent diminution
of the hypothalamic GH-releasing hormone (GHRH) output contributes to the
somatopause in men. As the regulatory mechanisms of GH secretion are sexual
ly dimorphic, we assessed the suppressibility of spontaneous and GHRH-stimu
lated GH I secretion by graded doses of a specific competitive GHRH recepto
r antagonist in nine young (20-27 yr old) and eight elderly (65-77 yr old)
healthy nonobese women to semiquantify hypothalamic GHRH output. Nocturnal
mean GH was lower in elderly women (2.2 +/- 0.4 vs. 0.9 +/- 0.2 mug/liter;
P = 0.01). Graded boluses of GHRH-44 resulted in similar GH responses in bo
th populations (P = 0.28). Graded infusions of GHRH antagonist inhibited in
a dose-dependent manner the GH responses to GHRH in both groups (P = 0.000
1-0.04). The dose-inhibition curve for the lowest GHRH bolus dose was shift
ed to the left compared with the highest one (P = 0.04). However, the dose-
inhibition curves for spontaneous GH secretion were not different in young
and elderly women (P = 0.50). Thus, the female somatopause is not associate
d with a measurable decrease in hypothalamic GHRH output. When the dose-inh
ibition curves for young men and young women were compared, the latter was
shifted to the left (P = 0.009), suggesting that the somatotropic,system in
women operates with less GHRH. We conclude that the contribution of endoge
nous GHRH to the maintenance of GH secretion and the neuroendocrine mechani
sms of somatopause in humans are sexually dimorphic.