Tripartite neuroendocrine activation of the human growth hormone (GH) axisin women by continuous 24-hour GH-releasing peptide infusion: Pulsatile, entropic, and nyctohemeral mechanisms

Despite the discovery of potent GH-releasing peptides (GHRPs) more than 15 yr ago and the recent cloning of human, rat, and pig GHRP receptors in the hypothalamus and pituitary gland, the neuroregulatory mechanisms of action of GHRP agonists on the human hypothalamosomatotroph unit are not well deli neated. To gain such clinical insights, we evaluated the ultradian (pulsati le), entropic (pattern orderliness), and nyctohemeral GH secretory response s during continuous 24-h iv infusion of saline vs. the most potent clinical ly available hexapeptide, GHRP-2 (1 mu g/kg.h) in estrogen-unreplaced (mean serum estradiol, 12 +/- 2.4 pg/mL) postmenopausal women (n = 7) in a paire d, randomized design. Blood was sampled every 10 min for 24 h during infusi ons and was assayed by ultrasensitive GH chemiluminescence assay. Pulsatile GH secretion was quantitated by deconvolution analysis, orderliness of GH release patterns by the approximate entropy statistic, and 24-h GH rhythmic ity by cosinor analysis. Statistical analysis revealed that GHRP-2 elicited a 7.7-fold increase in (24-h) mean serum (+/-SEM) GH concentrations, viz f rom 0.32 +/- 0.042 (saline) to 2.4 +/- 0.34 mu g/L (GHRP-2; P = 0.0006). Th is occurred via markedly stimulated pulsatile GH release, namely a 7.1-fold augmentation of GH secretory burst mass: 0.87 +/- 0.18 (control) v. 6.3 +/ 1.3 mu g/L (GHRP-2; P = 0.0038). Enhanced GH pulse mass reflected a commen surate 10-fold (P = 0.023) rise in GH secretory burst amplitude [maximal GH secretory rate (micrograms per L/min) attained within a secretory pulse] w ith no prolongation in event duration. GH burst frequency, interpulse inter val and calculated GH half-life were all invariant of GHRP-2 treatment. Con currently, as detected in the ultrasensitive GH assay, GHRP-2 augmented dec onvolution-estimated interpulse (basal) GH secretion by 4.5-fold (P = 0.025 ). The approximate entropy of 24-h serum GH concentration profiles rose sig nificantly during GHRP-2 infusion; i.e, from 0.592 +/- 0.073 (saline) to 0. 824 +/- 0.074 (GHRP-2; P = 0.0011), signifying more irregular or disorderly GH release patterns during secretagogue stimulation. Cosinor analysis of 2 4-h GH rhythms disclosed a significantly earlier (daytime) acrophase at 213 8 h (+/-140 min) during GHRP-2 stimulation vs. 0457 h (+/-42 min) during sa line infusion (P = 0.013). Concomitantly, the cosinor amplitude rose 6-fold (P = 0.018), and the mesor (cosine mean) rose 5-fold (P = 0.003). Fasting (0800 h) plasma insulin like growth factor (IGF-I) concentrations rose by - 11 +/- 12 mu g/L, during saline infusion and by 102 +/- 18 mu g/L during GH RP-2 infusion (P = 0.0036). GHRP-2 infusion did not modify (24-h pooled) se rum LPI, FSH, or TSH concentrations and minimally increased serum (pooled) daily PRL (6.8 +/- 0.83 vs. 12 +/- 1.2 mu g/L; P < 0.05) and cortisol (5.3 +/- 0.59 to 7.0 +/- 0.74; P < 0.05) concentrations. In summary, 24-h constant iv GHRP-2 infusion in the gonadoprival female neu rophysiologically activates the GH-IGF-I axis by potentiating GH secretory burst mass and amplitude by 7- to 10-fold and augmenting the basal (nonpuls atile) GH secretion by 4.5-fold. GHRP-2 action is highly selective, as it d oes not alter GH secretory burst frequency, interpulse interval, event dura tion, or GH half-life. GHRP-2 effectively elevates IGF-I concentrations, un leashes greater disorderliness of GH release patterns, and heightens the 24 -h rhythmicity of GK secretion. These tripartite features of GHRP-2's actio n in estrogen-withdrawn (postmenopausal) women also characterize normal hum an puberty and/or sex steroid regulation of the GH-IGF-I axis. However, how or whether GHRP-2 interacts further with sex hormone modulation of GPI neu rosecretory control in older women and men is not yet known.