Impact of Estradiol Supplementation on Dual Peptidyl Drive of GH Secretion in Postmenopausal Women

As an indirect probe of estrogen-regulated hypothalamic somatostatin restraint, the present study monitors the ability of short-term oral E2 supplementation to modulate GH secretion during combined continuous stimulation by recombinant human GHRH [GHRH-(1-44)-amide] and the potent and selective synthetic GH-releasing peptide, GHRP-2. According to a simplified tripeptidyl model of GH neuroregulation, the effects of estrogen in this dual secretagogue paradigm should mirror alterations in endogenous somatostatinergic signaling. To this end, seven healthy postmenopausal women underwent frequent (10-min) blood sampling for 24 h during simultaneous i.v. infusion of GHRH and GHRP-2 each at a rate of 1 microg/kg x h on d 10 of randomly ordered placebo or 17beta-estradiol (E2) (1 mg orally twice daily) replacement. Serum GH concentrations (n = 280/subject) were assayed by chemiluminescence. The resultant GH time series was evaluated by deconvolution analysis, the approximate entropy statistic, and cosine regression to quantitate pulsatile, entropic (feedback-sensitive), and 24-h rhythmic GH release, respectively. Statistical comparisons revealed that E2 repletion increased the mean (+/- SEM) serum E2 concentration to 222 +/- 26 pg/ml from 16 +/- 1.7 pg/ml during placebo (P < 0.001) and suppressed the serum LH by 48% (P = 0.0033), serum FSH by 64% (P < 0.001), and serum IGF-I by 44% (P = 0.021). Double peptidyl secretagogue stimulation elevated mean 24-h serum GH concentrations to 8.1 +/- 1.0 microg/liter (placebo) and 7.7 +/- 0.89 microg/liter (E2; P = NS) and evoked prominently pulsatile patterns of GH secretion. No primary measure of pulsatile or basal GH release was altered by the disparate sex steroid milieu, i.e. GH secretory burst amplitudes of 0.62 +/- 0.93 (placebo) and 0.72 +/- 0.16 (E2) microg/liter x min, GH pulse frequencies of 27 +/- 1.8 (placebo) and 23 +/- 1.9 (E2) events/24 h, GH half-lives of 12 +/- 0.74 (placebo) and 15 +/- 4.5 (E2) min, and basal (nonpulsatile) GH secretion 70 +/- 22 (placebo) and 57 +/- 18 (E2) ng/liter x min. The approximate entropy (ApEn) of serial GH release [1.297 +/- 0.061 (placebo) and 1.323 +/- 0.06 (E2)] and the mesor (cosine mean), amplitude, and acrophase (time of the maximum) of 24-h rhythmic GH secretion were likewise invariant of estrogen supplementation. Estimated statistical power exceeded 90% for detecting significant (P < 0.05) within-subject changes exceeding 30-50% in the mean serum GH concentration, GH ApEn, or GH mesor. In contrast, ApEn analysis of the evolution of successive GH secretory burst-mass values over 24 h disclosed that E2 replacement disrupts the serial regularity of pulsatile GH output (elevates the ApEn ratio) during combined GHRH/GHRP-2 stimulation (P = 0.004). In summary, short-term elevation of serum E2 concentrations in postmenopausal individuals into the midfollicular phase range observed in young women does not significantly alter 24-h basal, pulsatile, entropic, or nyctohemeral GH secretion monitored under continuous combined drive by GHRH and GHRP-2. As E2 repletion without enforced GHRH/GHRP-2 stimulation augments each of the foregoing regulated facets of GH release, we infer that one or both of the infused peptidyl secretagogues may itself participate in E2's short-term amplification of GH secretion in postmenopausal individuals. Estrogen's disruption of the orderliness of sequential GH pulse-mass values during fixed GHRH/GHRP-2 feedforward would be consistent with a subtle reduction in the release and/or actions of hypothalamic somatostatin or an (unexpected) direct pituitary action of the sex steroid. Whether comparable dynamics mediate the effects of endogenous estrogen on the GH axis in premenopausal women or pubertal girls is not known.