Growth hormone (GH) release from the anterior pituitary gland is predominantly regulated by the two antagonistic hypothalamic peptides, growth hormone-releasing hormone (GHRH) and somatostatin. Appraising endogenous GHRH action is thus made difficult by the confounding effects of (variable) hypothalamic somatostatin inhibitory tone. Accordingly, to evaluate endogenous GHRH actions, we used a clinical model of presumptively acute endogenous somatostatin withdrawal with concomitant GHRH release. To this end, we administered in randomized order placebo or the indirect cholinergic agonist, pyridostigmine, for 48 h to 13 healthy men of varying ages (29-77 years) and body mass indices (21-47 kg/m2). We sampled blood at 10-min intervals for 48h during both placebo and pyridostigmine (60 mg orally every 6 h) administration, and used an ultrasensitive GH chemiluminescence assay (sensitivity 0.002-0.005 μg/l) to capture GH pulse profiles. Multiparameter deconvolution analysis was applied to quantitate the number, amplitude, mass, and duration of significant underlying GH secretory bursts, and simultaneously estimate the GH half-life and concurrent basal GH secretion. Approximate entropy was utilized as a novel regularity statistic to quantify the relative orderliness of the hormone release process. All measures of GH secretion/half-life and orderliness were statistically invariant across the two consecutive 24-h placebo sessions. In contrast, pyridostigmine treatment significantly increased the mean serum GH concentration from 0.23 ± 0.054 μg/l during placebo to 0.45 ± 0.072 μg/l during the first day of treatment (P < 0.01). There was also a significant rise in the calculated 24-h pulsatile GH production rate from 8.9 ± 1.7 μg/l/day on placebo to 27 ± 5.6 μg/l/day during active drug treatment (P < 0.01). Pyridostigmine significantly and selectively amplified GH secretory burst mass to 1.5±0.35 μg/l compared with 0.74 ± 0.19 μg/l on placebo (P < 0.01). This was attributable to stimulation of GH secretory burst amplitude (maximal rate of GH secretion attained within the release episode) with no prolongation of estimated burst duration. Basal GH secretion and approximate entropy were not altered by pyridostigmine. However, age was strongly related to more disorderly GH release during both days of pyridostigmine treatment (r=+0.79, P=0.0013). During the second 24-h of continued pyridostigmine treatment, most GH secretory parameters decreased by 15-50%, but in several instances remained significantly elevated above placebo. Body mass index, but not age, was a significantly negative correlate of the pyridostigmine-stimulated increase in GH secretion (r = -0.65, P = 0.017). In summary, assuming that somatostatin is withdrawn and (rebound) GHRH release is stimulated via pyridostigmine administration, we infer that relatively unopposed GHRH action principally controls GH secretory burst mass and amplitude, rather than apparent GH secretory pulse duration, the basal GH secretion rate, or the serial regularity/orderliness of the GH release process in the human. Moreover, we infer that increasing age is accompanied by greater disorderliness of somatostatin-withdrawn GHRH, and hence rebound GH, release. The strongly negative correlation between pyridostigmine-stimulated GH secretion and body mass index (but not age) further indicates that increased relative adiposity may result in decreased effective (somatostatin-withdrawn) endogenous GHRH stimulus strength.
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