Complex regulation of GH autofeedback under dual-peptide drive: studies under a pharmacological GH and sex steroid clamp

Abstract

To test the postulate that sex difference, sex steroids, and peptidyl secretagogues control GH autofeedback, 11 healthy postmenopausal women and 14 older men were each given 1) a single iv pulse of GH to enforce negative feedback and 2) continuous iv infusion of saline vs. combined GHRH/GHRP-2 to drive feedback escape during pharmacological estradiol (E(2); women) or testosterone (T; men) supplementation vs. placebo in a double-blind, prospectively randomized crossover design. By three-way ANCOVA, sex difference, sex hormone treatment, peptide stimulation, and placebo/saline responses (covariate) controlled total (integrated) GH recovery during feedback (each P < 0.001). Both sex steroid milieu (P = 0.019) and dual-peptide stimulation (P < 0.001) determined nadir (maximally feedback-suppressed) GH concentrations. E(2)/T exposure elevated nadir GH concentrations during saline infusion (P = 0.003), whereas dual-peptide infusion did so independently of T/E(2) and sex difference (P = 0.001). All three of sex difference (P = 0.001), sex steroid treatment (P = 0.005), and double-peptide stimulation (P < 0.001) augmented recovery of peak (maximally feedback-escaped) GH concentrations. Peak GH responses to dual-peptidyl agonists were greater in women than in men (P = 0.016). E(2)/T augmented peak GH recovery during saline infusion (P < 0.001). Approximate entropy analysis corroborated independent effects of sex steroid treatment (P = 0.012) and peptide infusion (P < 0.001) on GH regularity. In summary, sex difference, sex steroid supplementation, and combined peptide drive influence nadir, peak, and entropic measurements of GH release under controlled negative feedback. To the degree that the pharmacological sex steroid, GH, and dual-peptide clamps provide prephysiological regulatory insights, these outcomes suggest major determinants of pulsatile GH secretion in the feedback domain.

Fig. 1.

Mean (± SE) 8-h growth hormone (GH) concentration profiles obtained by sampling blood every 10 min from 0600 to 1400 fasting. A single pulse of GH (1 μg/kg) was injected intravenously at 0630 to enforce negative feedback. To evaluate the role of peptide secretagogues in feedback control, saline or combined GH-releasing hormone (GHRH)/GH-releasing peptide-2 (GHRP-2) (1 μg·kg⁻¹·h⁻¹) was infused continuously (0600–1400). Data are from 11 women and 14 men.

Fig. 2.

Total integrated GH escape during autonegative feedback along with saline or GHRH/GHRP-2 infusion in 11 women and 14 men. Subjects were pretreated with placebo (Pl), estradiol (E₂), or testosterone (T) (see methodology). Data are results of 3-way analysis of covariance (ANCOVA) to assess main and interactive effects of sex difference (women or men), infusion type (saline or combined GHRH/GHRP-2), and sex steroid treatment (E₂ or T vs. placebo) on feedback-suppressed 5.5-h integrated GH concentrations. Overall, P < 0.001 for the ANCOVA model, with covariate P < 0.001. Means with different letters differ significantly as main effects. Data are geometric means ± SE.

Fig. 3.

Three-way ANCOVA of the effects of sex difference, 2-peptide, and sex steroid stimulation on nadir (feedback-suppressed) GH concentrations. Main effect and interactive P values are given in each of the 3 parts of the figure. Top: sex difference × treatment interaction. Middle: sex difference × peptide interaction. Bottom: treatment × peptide interaction. Bars with different (unique) letters differ significantly. Data are the geometric means ± SE.

Fig. 4.

Three-way ANCOVA assessment of sex difference, sex steroid, and peptide infusion effects on peak GH concentrations attained during experimentally controlled negative feedback in healthy older women (n = 11) and men (n = 14). See Fig. 3 for data format.

Fig. 5.

GH approximate entropy (ApEn) as a model-free measurement of relative feedforward/feedback signal strength. Data are presented as described in Fig. 3.

Fig. 6.

Inverse relationship between body mass index and nadir GH concentrations monitored during experimental negative feedback in the placebo saline setting. Data are from 25 adults.

Fig. 7.

Linear regression of peak GH concentrations attained during feedback recovery on serum IGF-I concentrations. Data were obtained during E₂ supplementation in women (n = 11) and T supplementation in men (n = 14) along with dual-peptide infusion.