Estimation of growth hormone secretion rate: impact of kinetic assumptions intrinsic to the analytical approach

The GH/IGF-1 Axis in Obesity: Physiological and Pathological Aspects

The cluster of cardiovascular risk factors-abdominal obesity, dyslipidaemia, insulin resistance and hypertension-has been recognized as the core of the metabolic syndrome. Adults with severe growth hormone (GH) deficiency have, to a large extent, features of the metabolic syndrome, and there is a strong inverse association between visceral fat accumulation and blunted GH secretion in adults. Hyposomatotropism in abdominal obesity has therefore been suggested to be of importance for its metabolic consequences. However, the underlying pathophysiological mechanisms are poorly understood. Prevalence of the metabolic syndrome is steadily increasing worldwide. Overnutrition and sedentary habits are the stigmata of modern society that predispose genetically susceptible individuals to develop central obesity and other features of the metabolic syndrome including glucose intolerance, hypertension and dyslipidemia. Although there are still no unified definitions of the syndrome, it is clear that this condition is associated with an increased risk for development of cardiovascular disease (CVD) and diabetes mellitus (DM). In this review, we discuss current evidence regarding alterations in the GH-IGF- 1 axis in abdominal obesity and its possible impact on other features of the metabolic syndrome.

Pubertal alterations in growth and body composition: IX. Altered spontaneous secretion and metabolic clearance of growth hormone in overweight youth

Deconvolution analysis was used to determine 12-hour spontaneous nocturnal growth hormone (GH) secretion and GH half-life in lean (body mass index, <85th percentile; n = 39) and overweight (body mass index, > or =85th percentile; n = 18) youth. The integrated GH concentration, GH burst mass, and half-life were lower (P < .05) in overweight than in lean youth. For each unit increase in percentage of body fat, integrated serum GH concentrations, secretory burst mass, and half-life declined by 83.6 microg/L per minute (r = -0.39, P < .01), 0.22 microg/L (r = -0.28, P < .05), and 0.2 minute (r = -0.38, P < .01), respectively. The effect of overweight on GH secretion was independent of pubertal status. Hierarchical regression models tested the hypothesis that altered GH secretion in youth is more related to total adiposity than abdominal visceral fat. When age, sex, fat-free mass, testosterone, and estradiol were held constant, the sequential addition of abdominal visceral fat did not increase R2 for any GH secretion variable. Sequential addition of percentage of body fat increased R2 (P < .05) for integrated GH concentration, total secretory rate, secretory burst mass, and pulsatile production rate. We conclude that serum GH concentrations are reduced in overweight youth primarily because of reduced GH burst mass with no change in the number of secretory events and secondarily to reduced GH half-life. Based on the model that GH-releasing hormone predominantly increases GH pulse amplitude whereas somatostatin primarily controls GH pulse frequency, these results suggest that overweight in youth diminishes GH-releasing hormone stimulation resulting in truncated GH bursts but does not alter the number of somatostatin withdrawal intervals so that GH burst frequency is conserved.

Sex-steroid modulation of growth hormone (GH) secretory control: three-peptide ensemble regulation under dual feedback restraint by GH and IGF-I

Technical, genetic, and clinical developments have unveiled a burgeoning array of novel effectors of GH secretion. The present appraisal of central neuroregulatory components of the somatotropic axis highlights a simplifying concept of ensemble control by the final common peptides, GH-releasing hormone (GHRH), GH-releasing peptide(s) (GHRP, ghrelin), and somatostatin. These potent signals act individually, antagonistically, and synergistically to direct pulsatile GH secretion. GHRH, GHRP/ghrelin, and somatostatin further adapt to autonegative feedback by GH and IGF-I. Estradiol modulates the impact of each of the primary peptidyl inputs; viz.: (i) enhances submaximally effective feedforward by discrete pulses of (injected) recombinant human GHRH-1,44-amide (as defined by increased agonistic potency and pituitary sensitivity); (ii) potentiates the submaximally stimulatory effects of GHRP-2, a hexapeptidyl mimetic of ghrelin; (iii) blunts dose-dependent inhibition of fasting GH secretion by somatostatin- 14; and (iv) relieves rhGH-enforced negative feedback on GHRP-2 (but not on basal, exercise, or GHRH)-stimulated GH secretion. The foregoing estrogenic activities collectively augment GH secretory burst mass by amplifying feedforward (via both GHRH and GHRP) and attenuating feedback (imposed by somatostatin and GH). Whether testosterone fully mimics the foregoing mechanistic actions of estradiol is not known. In conclusion, the present conceptual platform of tri-peptide-directed feedforward and GH/IGF-I-mediated feedback should aid in unraveling some of the complex regulatory dynamics targeted by sex-steroid hormones.