Accelerated escape from GH autonegative feedback in midpuberty in males: evidence for time-delimited GH-induced somatostatinergic outflow in adolescent boys

Lactate activates the somatotropic axis in rats

Under physical activity a wide variety of cellular metabolic products and hormones are altered in the blood stream, including lactate, a metabolite of pyruvate reduction, and growth hormone (GH). Although a positive correlation between lactate and GH seems to exist during exercise, the role of lactate as a mediator of GH production has never been investigated. Thus, the aim of this study was to investigate whether lactate could activate the somatotropic axis and stimulate GH synthesis/release, contributing to the enhanced somatotropic activity described in exercise conditions. Male adult Wistar rats were acutely treated with sodium lactate [15 or 150μmols, i.p.] at the beginning of the active period (Zeitgeber time 13-14), and euthanized by decapitation 30, 60 and 120min after the injections. Serum GH concentration were determined using ELISA and Gh and Igf-1 mRNA expressions were quantified by qPCR. Serum GH concentration and Gh mRNA expression were increased 30min after lactate injections for both treatments. However, [15μmols] of lactate injection kept GH serum concentration chronically high throughout the experimental period. Igf-1 mRNA expression was increased only 60min after challenge with [15μmols] of lactate, time point which corresponded to 30min after the serum GH peak. The present results led us to conclude that lactate mediates activation of the somatotropic axis, therefore emphasizing its possible role on GH synthesis/release, and further indicating that it could play a part on the increased GH secretion observed in exercise conditions.

Gender, sex-steroid, and secretagogue-selective recovery from growth hormone-induced feedback in older women and men

CONTEXT

GH negatively regulates its own secretion. How gender, sex steroids, and secretagogues modulate GH autofeedback is not known.

HYPOTHESIS/OBJECTIVE

Supplementation with sex steroids and/or a peptidyl secretagogue will enhance the escape of GH from autoinhibition, thus framing a mechanism for amplifying pulsatile GH secretion.

SUBJECTS AND SETTING

Ten healthy postmenopausal women and 10 comparably aged men participated at the Clinical-Translational Science Unit.

DESIGN/INTERVENTIONS

Randomly ordered, double-blind, prospective crossover treatment with placebo vs. testosterone (men) or placebo vs. estradiol (women). Autofeedback was imposed by an iv pulse of GH. Recovery of feedback inhibition was quantified during constant infusion of saline, GHRH, or GH-releasing peptide-2 (three peptide categories).

OUTCOMES/RESULTS

During negative feedback, total (integrated) GH recovery depended upon gender (P = 0.017), sex hormone (P < 0.001), and peptide category (P < 0.001). Mechanistic analysis revealed that feedback-suppressed nadir GH concentrations were determined by sex-steroid treatment (P = 0.018) but not by gender (P = 0.444). Peak GH escape was controlled by both treatment (P = 0.004) and gender (P = 0.003). Nadir GH and peak GH during feedback were enhanced by GHRH or GHRP-2 (P < 0.001 for both). Gender × peptide (P = 0.012 for nadir GH), treatment × peptide (P < 0.001 total and peak GH), and gender × treatment (P = 0.017 nadir GH) regulated GH recovery interactively.

CONCLUSION

Gender, sex-steroid supplementation, and secretagogue type confer distinct feedback-rescuing effects, introducing a new level of complexity in the control of pulsatile GH regulation.

Model-projected mechanistic bases for sex differences in growth hormone regulation in humans

Models of physiological systems facilitate rational experimental design, inference, and prediction. A recent construct of regulated growth hormone (GH) secretion interlinks the actions of GH-releasing hormone (GHRH), somatostatin (SRIF), and GH secretagogues (GHS) with GH feedback in the rat (Farhy LS, Veldhuis JD. Am J Physiol Regul Integr Comp Physiol 288: R1649–R1663, 2005). In contrast, no comparable formalism exists to explicate GH dynamics in any other species. The present analyses explore whether a unifying model structure can represent species- and sex-defined distinctions in the human and rodent. The consensus principle that GHRH and GHS synergize in vivo but not in vitro was explicable by assuming that GHS 1) evokes GHRH release from the brain, 2) opposes inhibition by SRIF both in the hypothalamus and on the pituitary gland, and 3) stimulates pituitary GH release directly and additively with GHRH. The gender-selective principle that GH pulses are larger and more irregular in women than men was conferrable by way of 4) higher GHRH potency and 5) greater GHS efficacy. The overall construct predicts GHRH/GHS synergy in the human only in the presence of SRIF when the brain-pituitary nexus is intact, larger and more irregular GH pulses in women, and observed gender differences in feedback by GH and the single and paired actions of GHRH, GHS, and SRIF. The proposed model platform should enhance the framing and interpretation of novel clinical hypotheses and create a basis for interspecies generalization of GH-axis regulation.

Distinctive inhibitory mechanisms of age and relative visceral adiposity on growth hormone secretion in pre- and postmenopausal women studied under a hypogonadal clamp

BACKGROUND

Aging, body composition, and sex steroids jointly determine GH production. However, the actions of any given factor are confounded by the effects of the other two.

HYPOTHESIS

Age and abdominal visceral fat (AVF) mass govern GH secretion via individually distinctive mechanisms, which can be unmasked by short-term sex steroid deprivation.

DESIGN/SUBJECTS

In a university setting, healthy pre- and postmenopausal volunteers underwent GnRH agonist-induced down-regulation for 6 wk to deplete ovarian sex steroids. GH secretion was evaluated by frequent blood sampling, saline vs. dual secretagogue infusions, an irregularity statistic, variable waveform deconvolution analysis, and a simplified feedback model. Computerized tomography was used to estimate AVF mass. OUTCOMES/MEASURES: In the sex steroid-deficient milieu, postmenopausal compared with premenopausal women exhibited 1) lower concentrations of IGF-I (P = 0.028) and GH (P < 0.05); 2) reduced pulsatile, but elevated basal, GH secretion (P < 0.05); 3) more irregular GH patterns (P = 0.027); 4) an attenuated GH response to simultaneous GHRH/GH-releasing peptide-2 stimulation (P < 0.01); and 5) more rapid onset of GH release within secretory bursts (P < 0.01). In contrast, AVF negatively forecast GH responses to L-arginine/GH-releasing peptide-2 (r2= 0.45; P < 0.001) and L-arginine/GHRH (r2= 0.57; P = 0.007). From these marked contrasts, model-based analyses predicted distinguishable mechanisms by which aging and AVF alter pulsatile GH production.

CONCLUSION

Under limited confounding by sex steroids, age and body composition modulate GH secretion via highly selective peptidyl pathways in healthy women.

Deterministic construct of amplifying actions of ghrelin on pulsatile growth hormone secretion

Ghrelin is a native ligand for the growth hormone secretagogue (GHS) receptor that stimulates pulsatile GH secretion markedly. At present, no formal construct exists to unify ensemble effects of ghrelin, GH-releasing hormone (GHRH), somatostatin (SRIF), and GH feedback. To model such interactions, we have assumed that ghrelin can stimulate pituitary GH secretion directly, antagonize inhibition of pituitary GH release by SRIF, oppose suppression of GHRH neurons in the arcuate nucleus (ArC) by SRIF, and induce GHRH secretion from ArC. The dynamics of such connectivity yield self-renewable GH pulse patterns mirroring those in the adult male and female rat and explicate the following key experimental observations. 1) Constant GHS infusion stimulates pulsatile GH secretion. 2) GHS and GHRH display synergy in vivo. 3) A systemic pulse of GHS stimulates GH secretion in the female rat at any time and in the male more during a spontaneous peak than during a trough. 4) Transgenetic silencing of the neuronal GHS receptor blunts GH pulses in the female. 5) Intracerebroventricular administration of GHS induces GH secretion. The minimal construct of GHS-GHRH-SRIF-GH interactions should aid in integrating physiological data, testing regulatory hypotheses, and forecasting innovative experiments.

Gender modulates sequential suppression and recovery of pulsatile growth hormone secretion by physiological feedback signals in young adults

The basic mechanisms that drive the renewal of GH pulses in the human are not understood. Recent ensemble models predict that pulse regeneration requires quenching of an ongoing GH pulse by somatostatin outflow and evocation of a new burst by rebound GHRH release. We reasoned that related principles might explain why women consistently maintain higher-amplitude GH secretory bursts than men. Accordingly, the present study tests the hypothesis that gender modulates the successive dynamics of GH feedback and escape in the morning fasting, when GH pulses are larger in women. To this end, we infused single iv pulses of recombinant human (rh) GH (0, 1, and 3 microg/kg) in eight young men and six women on separate randomly ordered mornings fasting and quantitated serial inhibition and recovery of GH secretion by frequent sampling, immunochemiluminometry, a deconvolution procedure, and regularity analysis. Statistical contrasts revealed gender-comparable peak concentrations and kinetics of rhGH. However, women differed from men by way of: (1) 3.5- and 4.0-fold less feedback suppression of GH secretory-burst mass; (2) more irregular patterns of GH release during negative feedback; and (3) 12-and 14-fold greater postnadir rebound-like GH secretion after rhGH pulses. Mechanistic analyses based on a minimal feedback construct predicted that women generate higher endogenous secretagogue stimulation per unit somatostatin outflow than men. In summary, negative feedback induced by near-physiological GH pulses unmasks prominent gender-related contrasts in hypothalamo-pituitary autoregulation in young adults. A frugal but sufficient explanation of the ensemble outcomes is that women sustain greater hypothalamo-pituitary agonist input than men.

Determinants of dual secretagogue drive of burst-like growth hormone secretion in premenopausal women studied under a selective estradiol clamp

The present study tests the hypothesis that estradiol (E(2)), compared with placebo (Pl), amplifies combined-secretagogue stimulation of GH secretion in premenopausal women studied at comparable IGF-I and testosterone concentrations. To this end, 13 women underwent GnRH agonist-induced gonadal down-regulation followed by graded transdermal addback of E(2) or Pl and randomly ordered iv infusions of saline or paired secretagogues on separate morning fasting. GH secretion was assessed by frequent blood sampling, immunochemiluminometry, and variable-waveform deconvolution analysis. Two-way ANOVA revealed that specific secretagogue combination (P < 0.001), E(2) status (P = 0.012), and their interaction (P = 0.038) jointly determined GH secretory-burst mass. Compared with Pl, the E(2)-clamped milieu elevated mean fasting GH concentrations (P = 0.032), the mass of GH secreted in bursts (P = 0.037), and maximal stimulation by paired l-arginine/GH-releasing peptide (GHRP)-2 (P = 0.028). E(2) also markedly accelerated the initial release of GH induced by GHRH/GHRP-2 (P < 0.001) and l-arginine/GHRH (P < 0.01). By linear regression analysis, E(2) concentrations positively forecast 41% of intersubject variability in GH secretion stimulated by combined l-arginine/GHRP-2 (P = 0.018), whereas abdominal visceral-fat mass negatively predicted 49% of that due to l-arginine/GHRH (P = 0.012). These data indicate that pulsatile GH secretion in young women studied at constant IGF-I and testosterone concentrations is dictated 3-fold jointly by secretagogue pair, E(2) availability, and intraabdominal adiposity. Moreover, the rapidity of GH release is controlled 2-fold jointly by E(2) and GHRH.

Dual secretagogue drive of burst-like growth hormone secretion in postmenopausal compared with premenopausal women studied under an experimental estradiol clamp

We show that in an experimentally enforced estradiol-predominant milieu, postmenopausal compared with premenopausal women maintain 1) decreased fasting GH and IGF-I concentrations, 2) reduced basal and pulsatile GH secretion, and 3) attenuated GH secretion after maximal stimulation by the paired secretagogues l-arginine/GH-releasing peptide (GHRP)-2, l-arginine/GHRH, and GHRP-2/GHRH. These foregoing outcomes are selective, because menopausal status did not determine mean GH secretory-burst frequency or peptide-induced waveform shortening. Abdominal visceral fat mass predicted up to 25% of the variability in fasting and stimulated GH secretion in the combined cohorts under fixed systemic estradiol availability. Accordingly, as much as three-fourths of interindividual differences in burst-like GH secretion among healthy pre- and postmenopausal women arise from age-related mechanisms independently of short-term systemic estrogen availability and relative intraabdominal adiposity.

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.

Human GH pulsatility: an ensemble property regulated by age and gender

Age and gender impact the full repertoire of neurohormone systems, including most prominently the somatotropic, gonadotropic and lactotropic axes. For example, daily GH production is approximately 2-fold higher in young women than men and varies by 20-fold by sexual developmental status and age. Deconvolution estimates of 24-h GH secretion rates exceed 1200 microg/m2 in adolescents and fall below 60 microg/m2 in aged individuals. The present overview highlights plausible factors driving such lifetime variations in GH availability, i.e., estrogen, aromatizable androgen, hypothalamic peptides and negative feedback by GH and IGF-I. In view of the daunting complexity of potential neuromodulatory signals, we underline the utility of conceptualizing a simplified three-peptide regulatory ensemble of GHRH, GHRP (ghrelin) and somatostatin. The foregoing signals act as individual and conjoint mediators of adaptive GH control. Regulation is enforced at 3-fold complementary time scales, which embrace pulsatile (burst-like), entropic (orderly) and 24-h rhythmic (nycthemeral) modes of GH release. This unifying platform offers a convergent perspective of multivalent control of GH outflow.

Growth, body composition and hormonal axes in children and adolescents

Growth and physical maturation are dynamic processes that encompass a broad range of cellular and somatic changes. Most investigators who study growth have focused on linear growth (change in height over time), but alterations in the relative body proportions, body composition, and the regional distribution of body fat (upper body vs lower body, axial vs appendicular, and sc vs deep visceral) are essential elements for growth and sexual maturation. In fact, cardiovascular risk assessment in the adult relies heavily on the regional distribution of body fat. The antecedents for the adult pattern of fat are clearly present in the adolescent, if not the younger child. Standards for each of these parameters have been developed for multiple ethnic and racial populations and aid materially in the identification of children with normal growth and physical development, variations within the broad normal (physiological) range, and those with clearly pathological growth patterns.