GROWTH-HORMONE RESPONSES TO GABA(B) RECEPTOR STIMULATION THROUGHOUT THE MENSTRUAL-CYCLE OF HEALTHY FEMALES

Growth hormone (GH) responses to the GABA(B) agonist baclofen (10 mg) were assessed in six normally cycling, healthy women at three differen t sex steroid phases of the menstrual cycle: early follicular, pre-ovu latory and luteal. The design was placebo-controlled, balanced and sin gle-blind with a total of six tests being carried out on each subject in two consecutive menstrual cycles. GH responses to baclofen increase d incrementally from early-, through mid- to late-cycle: 10.95 +/- 0.9 5 mu g/l, 24.53 +/- 4.73 mu g/l, 31.36 +/- 3.37 mu g/l respectively (p < 0.04). Responses between early- and mid-cycle and early- and late-c ycle were significantly different (p < 0.05). There was a direct relat ionship between baclofen/GH responses and both plasma oestradiol (E(2) ) concentrations (p = 0.05) and plasma progesterone concentrations (p = 0.02). Responses to placebo did not vary. E(2) has been demonstrated to exert a priming influence on GH responses to many pharmacological and physiological stimuli. An E(2)-induced priming effect on GH secret ion may underlie these results. Progesterone may augment this effect. Other possible influences are a sex steroid effect on somatostatin or GABA neurotransmission.