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 different sex steroid phases of the menstrual cycle: early follicular, pre-ovulatory and luteal. The design was placebo-controlled, balanced and single-blind with a total of six tests being carried out on each subject in two consecutive menstrual cycles. GH responses to baclofen increased incrementally from early-, through mid- to late-cycle: 10.95 +/- 0.95 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-cycle were significantly different (p < 0.05). There was a direct relationship 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 secretion may underlie these results. Progesterone may augment this effect. Other possible influences are a sex steroid effect on somatostatin or GABA neurotransmission.