The growth hormone secretory pattern: a response to neuroendocrine signals

Model-based evaluation of growth hormone secretion

A minimal-model framework is that growth hormone (GH) secretion is controlled by an ensemble of interlinked peptides, namely, GH-releasing hormone (GHRH), somatostatin (SS), and ghrelin. Clinical studies, laboratory experiments, rare sporadic mutations, targeted gene silencing, and biomathematical models establish that at least three signals regulate GH secretion. A clarion implication of the concept of integrative control is that no one peptidic effector operates alone or can be adequately studied alone. A major unanswered question is how pathophysiology disrupts the core regulatory ensemble, thereby forcing relative GH and IGF-1 deficiency or excess. However, salient technical hurdles exist, namely, the lack of reliable experimental strategies and the paucity of validated analytical tools to distinguish the interlinked roles of GHRH, SS, and ghrelin. To address these significant obstacles requires administering peptide secretagogues in distinct combinations akin to the classical insulin/glucose clamp and implementing an analytical formalism to parse the interactive roles of GHRH, SS, and ghrelin objectively.

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

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.

Gender, age, body mass index, and IGF-I individually and jointly determine distinct GH dynamics: analyses in one hundred healthy adults

BACKGROUND

GH secretion is quantifiable as mean, peak, and nadir GH concentrations; degree of irregularity (approximate entropy); and spikiness (brief staccato-like fluctuations).

HYPOTHESIS

Distinct GH dynamics reflect relatively distinct (combinations of) subject variables, such as gender, age, body mass index (BMI), and IGF-I concentrations.

LOCATION

The study took place at a clinical translational research unit.

SUBJECTS

Subjects included 100 healthy adults ages 20-77 yr (59 women and 41 men), BMI 18-42 kg/m(2), and IGF-I 9.2-38 nmol/liter.

MEASURES

Immunofluorometric GH assay was done on 10-min samples collected for 24 h.

RESULTS

Stepwise forward-selection multivariate regression analysis revealed that mean GH concentrations were simultaneously determined (overall r = 0.36; P < 0.001) by gender (higher in women, P < 0.001), BMI (negatively, P < 0.001), and IGF-I (positively, P < 0.001). Peak GH levels were influenced (r = 0.28) by both BMI (P < 0.001) and IGF-I (P = 0.001). Nadir GH values were jointly affected by gender (higher in women, P = 0.005) and BMI (negatively, P = 0.001). GH approximate entropy was triply defined (r = 0.29) by gender (greater irregularity in women, P < 0.001), age (P = 0.022), and BMI (P = 0.008) and dually (r = 0.25) by gender (P = 0.0001) and BMI (P = 0.017) if sex steroids were included. GH spikiness was determined (r = 0.29) by gender (higher in women, P = 0.0016) and BMI (positively, P = 0.0002).

CONCLUSION

In healthy adults, combinations of gender, age, BMI, and IGF-I specify distinct GH dynamics, thus requiring balanced representation of these variables in comparative GH studies.

Proteins related to lipoprotein profile were identified using a pharmaco-proteomic approach as markers for growth response to growth hormone (GH) treatment in short prepubertal children

Background

The broad range in growth observed in response to growth hormone (GH) treatment is mainly caused by individual variations in both GH secretion and GH sensitivity. Individual GH responsiveness can be estimated using evidence-based models that predict the response to GH treatment; however, these models can be improved. High-throughput proteomics techniques can be used to identify proteins that may potentially be used as variables in such models in order to improve their predictive ability. Previously we have reported that proteomic analyses can identify biomarkers that discriminate between short prepubertal children with idiopathic short stature (ISS) who show good or poor growth in response to GH treatment. In this study we used a pharmaco-proteomic approach to identify novel factors that correlate with the growth response to GH treatment in prepubertal children who are short due to GH deficiency or ISS. The study included 128 short prepubertal children receiving GH treatment, of whom 39 were GH-deficient and 89 had ISS. Serum protein expression profiles at study start and after 1 year of GH treatment were analyzed using SELDI-TOF. Cross-validated regression and random permutation analyses were performed to identify significant correlations between protein expression patterns and the 2-year growth response to GH treatment.

Results

At start of treatment we identified a combination of seven protein peaks that correlated with the 2-year growth response in the GH-deficient group (R² = 0.73). After 1 year of treatment, a combination of four peaks in the GH-deficient group (R² = 0.64), eight peaks in the ISS group R² = 0.47) and eight peaks in the total study group correlated with the 2-year growth response R² = 0.38).

The peaks identified corresponded to apolipoproteins A-I, A-II, C-I, C-III, transthyretin and serum amyloid A 4, which are all part of the high-density lipoprotein.

Conclusion

Using a proteomic approach we identified biomarkers related to the lipoprotein profile that could be used to predict growth response to GH treatment in prepubertal children who are short as a result of GH-deficiency or who have ISS.

These results support our previous findings that apolipoproteins and transthyretin may have a role in GH sensitivity.

Production of nitric oxide by murine peritoneal macrophages in vitro on treatment with prolactin and growth hormone: Involvement of protein tyrosine kinases, Ca++, and MAP kinase signal transduction pathways

Prolactin (PRL) and growth hormone (GH) (somatotropin) have been known to possess immunomodulatory properties. In the present studies we have investigated the production of nitric oxide (NO) and TNF-alpha by murine peritoneal macrophages in vitro on treatment with PRL and GH and the signal transduction mechanism involved. It is observed that significantly enhanced production of NO is induced in macrophages on treatment with PRL and GH. It is further observed that protein tyrosine kinases, MAP kinases and Ca(++) channeling are involved in NO production by macrophages on in vitro treatment with PRL and GH. GH and PRL induced nitric oxide did not have any effect on the expression and production of TNF-alpha. PRL or GH induced TNF-alpha production by murine macrophages was insensitive in the presence of competitive inhibitor of NOS, L-NMMA. Similarly, there is no autocrine or paracrine effect of TNF-alpha on GH or PRL induced NO production and iNOS expression.

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.

Transgenic mice overexpressing des-acyl ghrelin show small phenotype

Ghrelin, a 28-amino acid acylated peptide, displays strong GH-releasing activity in concert with GHRH. The fatty acid modification of ghrelin is essential for the actions, and des-acyl ghrelin, which lacks the modification, has been assumed to be devoid of biological effects. Some recent reports, however, indicate that des-acyl ghrelin has effects on cell proliferation and survival. In the present study, we generated two lines of transgenic mice bearing the preproghrelin gene under the control of chicken beta-actin promoter. Transgenic mice overexpressed des-acyl ghrelin in a wide variety of tissues, and plasma des-acyl ghrelin levels reached 10- and 44-fold of those in control mice. They exhibited lower body weights and shorter nose-to-anus lengths, compared with control mice. The serum GH levels tended to be lower, and the serum IGF-I levels were significantly lower in both male and female transgenic mice than control mice. The responses of GH to administered GHRH were normal, whereas those to administered ghrelin were reduced, especially in female transgenic mice, compared with control mice. These data suggest that overexpressed des-acyl ghrelin may modulate the GH-IGF-I axis and result in small phenotype in transgenic mice.

Putative GH pulse renewal: periventricular somatostatinergic control of an arcuate-nuclear somatostatin and GH-releasing hormone oscillator

Growth hormone (GH) pulsatility requires periventricular-nuclear somatostatin(SRIF(PeV)), arcuate-nuclear (ArC) GH-releasing hormone (GHRH), and systemic GH autofeedback. However, no current formalism interlinks these regulatory loci in a manner that generates self-renewable GH dynamics. The latter must include in the adult rat 1) infrequent volleys of high-amplitude GH peaks in the male, 2) frequent discrete low-amplitude GH pulses in the female, 3) disruption of the male pattern by severing SRIF(PeV) outflow to ArC, 4) stimulation of GHRH and GH secretion by central nervous system delivery of SRIF, 5) inhibition of GH release by central exposure to GHRH, and 6) a reboundlike burst of GHRH secretion induced by stopping peripheral infusion of SRIF. The present study validates by computer-assisted simulations a simplified ensemble formulation that predicts each of the foregoing six outcomes, wherein 1) blood-borne GH stimulates SRIF(PeV) secretion after a long time latency, 2) SRIF(PeV) inhibits both pituitary GH and ArC GHRH release, 3) ArC GHRH and SRIF(ArC) oscillate reciprocally with brief time delay, and 4) SRIF(PeV) represses and disinhibits the putative GHRH-SRIF(ArC) oscillator. According to the present analytic construction, time-delayed feedforward and feedback signaling among SRIF(PeV), ArC GHRH, and SRIF(ArC) could endow the complex physiological patterns of GH secretion in the male and female.

Joint pituitary-hypothalamic and intrahypothalamic autofeedback construct of pulsatile growth hormone secretion

Growth hormone (GH) secretion is vividly pulsatile in all mammalian species studied. In a simplified model, self-renewable GH pulsatility can be reproduced by assuming individual, reversible, time-delayed, and threshold-sensitive hypothalamic outflow of GH-releasing hormone (GHRH) and GH release-inhibiting hormone (somatostatin; SRIF). However, this basic concept fails to explicate an array of new experimental observations. Accordingly, here we formulate and implement a novel fourfold ensemble construct, wherein 1) systemic GH pulses stimulate long-latency, concentration-dependent secretion of periventricular-nuclear SRIF, thereby initially quenching and then releasing multiphasic GH volleys (recurrent every 3-3.5 h); 2) SRIF delivered to the anterior pituitary gland competitively antagonizes exocytotic release, but not synthesis, of GH during intervolley intervals; 3) arcuate-nucleus GHRH pulses drive the synthesis and accumulation of GH in saturable somatotrope stores; and 4) a purely intrahypothalamic mechanism sustains high-frequency GH pulses (intervals of 30-60 min) within a volley, assuming short-latency reciprocal coupling between GHRH and SRIF neurons (stimulatory direction) and SRIF and GHRH neurons (inhibitory direction). This two-oscillator formulation explicates (but does not prove) 1) the GHRH-sensitizing action of prior SRIF exposure; 2) a three-site (intrahypothalamic, hypothalamo-pituitary, and somatotrope GH store dependent) mechanism driving rebound-like GH secretion after SRIF withdrawal in the male; 3) an obligatory role for pituitary GH stores in representing rebound GH release in the female; 4) greater irregularity of SRIF than GH release profiles; and 5) a basis for the paradoxical GH-inhibiting action of centrally delivered GHRH.

Neuroendocrine facets of human puberty

The unfolding of pubertal growth and maturation entails multisystem collaboration. Most notably, the outflow of gonadotropins and growth hormone (GH) proceeds both independently and jointly. The current update highlights this unique dependency in the human.

Hypothalamic growth hormone secretagogue receptor regulates growth hormone secretion, feeding, and adiposity

Growth hormone secretagogues (GHSs) stimulate GH secretion and food intake. GHS receptor (GHS-R) mRNA has been identified mainly in the arcuate nucleus (Arc) and ventromedial nucleus of the hypothalamus and in the pituitary. Ghrelin, an endogenous ligand for GHS-R, has recently been purified from rat stomach. Although ghrelin is also expressed in the hypothalamus, the physiological significance of the ghrelin/GHS-R system is still unknown. We have created transgenic (Tg) rats expressing an antisense GHS-R mRNA under the control of the promoter for tyrosine hydroxylase (TH), thus selectively attenuating GHS-R protein expression in the Arc. Tg rats had lower body weight and less adipose tissue than did control rats. Daily food intake was reduced, and the stimulatory effect of GHS treatment on feeding was abolished in Tg rats. GH secretion and plasma insulin-like growth factor-I levels were reduced in female Tg rats. These results suggest that GHS-R in the Arc is involved in the regulation of GH secretion, food intake, and adiposity.

Unequal autonegative feedback by GH models the sexual dimorphism in GH secretory dynamics

Growth hormone (GH) secretion, controlled principally by a GH-releasing hormone (GHRH) and GH release-inhibiting hormone [somatostatin (SRIF)] displays vivid sexual dimorphism in many species. We hypothesized that relatively small differences within a dynamic core GH network driven by regulatory interactions among GH, GHRH, and SRIF explain the gender contrast. To investigate this notion, we implemented a minimal biomathematical model based on two coupled oscillators: time-delayed reciprocal interactions between GH and GHRH, which endow high-frequency (40-60 min) GH oscillations, and time-lagged bidirectional GH-SRIF interactions, which mediate low-frequency (occurring every 3.3 h) GH volleys. We show that this basic formulation, sufficient to explain GH dynamics in the male rat [Farhy LS, Straume M, Johnson ML, Kovatchev BP, and Veldhuis JD. Am J Physiol Regulatory Integrative Comp Physiol 281: R38-R51, 2001], emulates the female pattern of GH release, if autofeedback of GH on SRIF is relaxed. Relief of GH-stimulated SRIF release damps the slower volleylike oscillator, allowing emergence of the underlying high-frequency oscillations that are sustained by the GH-GHRH interactions. Concurrently, increasing variability of basal somatostatin outflow introduces quantifiable, sex-specific disorderliness of the release process typical of female GH dynamics. Accordingly, modulation of GH autofeedback on SRIF within the interactive GH-GHRH-SRIF ensemble and heightened basal SRIF variability are sufficient to transform the well-ordered, 3.3-h-interval, multiphasic, volleylike male GH pattern into a femalelike profile with irregular pulses of higher frequency.

A construct of interactive feedback control of the GH axis in the male

Growth hormone (GH) secretion is controlled by GH-releasing hormone (GHRH), the GH release-inhibiting hormone somatostatin (SRIF), and autofeedback connections. The ensemble network produces sexually dimorphic patterns of GH secretion. In an effort to formalize this system, we implemented a deterministically based autonomous feedback-driven construct of five principal dose-responsive regulatory interactions: GHRH drive of GH pituitary release, competitive inhibition of GH release by SRIF, GH autofeedback via SRIF with a time delay, delayed GH autonegative feedback on GHRH, and SRIF inhibition of GHRH secretion. This formulation engenders a malelike pattern of successive GH volleys due jointly to positive time-delayed feedback of GH on SRIF and negative feedback of SRIF on GH and GHRH. The multipeak volley is explicated as arising from a reciprocal interaction between GH and GHRH during periods of low SRIF secretion. The applicability of this formalism to neuroendocrine control is explored by initial parameter sensitivity analysis and is illustrated for selected feedback-dependent experimental paradigms. The present construct is not overparameterized and does not require an ad hoc pulse generator to achieve pulsatile GH output. Further evolution of interactive constructs could aid in exploring more complex feedback postulates that confer the vivid sexual dimorphism of female GH profiles.

Neurophysiological regulation and target-tissue impact of the pulsatile mode of growth hormone secretion in the human

Neuroendocrine axes function as an ensemble of regulatory loci which communicate and maintain homeostasis via time-delayed blood-borne signals. The growth hormone (GH)-insulin-like growth factor I (IGF-I) feedback axis sustains a vividly pulsatile mode of interglandular signalling. Pulsatility is driven jointly by hypothalamic GH-releasing hormone (GHRH) and GH-releasing peptide (GHRP), and modulated by somatostatinergic restraint. Paradoxically, intermittent somatostatin inputs also facilitate somatotrope-cell responses to recurrent secretagogue stimuli, thereby amplifying pulsatile GH secretion. A concurrent low basal (8-12% of normal total) rate of GH release is controlled positively by GHRH and GHRP and negatively by somatostatin. Sex-steroid hormones (such as oestradiol and aromatizable androgen) and normal female and male puberty augment GH secretory-burst mass 1.8- to 3.5-fold, whereas ageing, relative obesity, physical inactivity, hypogonadism, and hypopituitarism mute the amplitude/mass of pulsatile GH output. An abrupt rise in circulating GH concentration stimulates rapid internalization of the GH receptor in peripheral target tissues, and evokes second-messenger nuclear signalling via the STAT 5b pathway. Discrete GH peaks stimulate linear (skeletal) growth and drive muscle IGF-I gene expression more effectually than basal (time-invariant) GH exposure. A brief pulse of GH can saturate the plasma GH-binding protein system and achieve prolonged plasma GH concentrations by convolution with peripheral distribution and clearance mechanisms. A single burst of GH secretion also feeds back after a short latency on central nervous system (CNS) regulatory centres via specific brain GH receptors to activate somatostatinergic and reciprocally subdue GHRH outflow. This autoregulatory loop probably contributes to the time-dependent physiologically pulsatile dynamics of the GH axis. More slowly varying systemic IGF-I concentrations may also damp GH secretory pulse amplitude by delayed negative-feedback actions. According to this simplified construct, GH pulsatility emerges due to time-ordered multivalent interfaces among GHRH/GHRP feedforward and somatostatin, GH and IGF-I feedback signals. Resultant GH pulses trigger tissue-specific gene expression, thereby promoting skeletal and muscular growth, metabolic and body compositional adaptations, and CNS reactions that jointly maintain health and homeostasis.

Evaluation of self-similar features in time series of serum growth hormone and prolactin levels by fractal analysis: effect of delayed sleep and complexity of diurnal variation

We assayed the diurnal concentrations of growth hormone (GH) and prolactin (PRL) in 6 healthy male volunteers to evaluate the self-similar features in the time series of each hormone on the basis of fractal theory and to determine the fractal dimension as an index of the complexity of the diurnal variation. In addition, we assessed the effects of a 6-hour delay in the sleep period on the complexity of the diurnal variation of these hormones. There was a statistically significant fractal feature in the serum levels of GH both under the nocturnal-sleep and delayed-sleep conditions in all subjects. The time series of the serum PRL concentrations also showed a statistically significant fractal feature under the nocturnal-sleep and delayed-sleep conditions in all subjects. The fractal dimensions of the patterns of the GH or PRL levels were 1.879 and 1.929 or 1.754 and 1.785 under the nocturnal-sleep and delayed-sleep conditions, respectively. Two-way ANOVA revealed no significant difference in the fractal dimension between the two sleep conditions but did reveal a significant difference between the fractal dimensions of the GH and PRL levels. These results showed (1) that delayed sleep had no significant effect on the complexity of the diurnal pattern of these hormones, and (2) that the diurnal pattern of the GH levels was more complex than that of the PRL levels.

Electrical stimulation of the rat periventricular nucleus influences the activity of hypothalamic arcuate neurones

In rats, the release of growth hormone (GH) is inhibited during electrical stimulation of the periventricular nucleus but after the end of stimulation, there is a rebound 'hypersecretion' of GH. We examined the responses of arcuate neurones in pentobarbitone-anaesthetized male rats, following electrical stimulation of the periventricular nucleus to test the hypothesis that the effects of periventricular nucleus stimulation on GH secretion are mediated via effects upon GH-releasing hormone (GRF) neurones in the arcuate nucleus. The electrical activity of 2 groups of arcuate neurones were analysed before, during and after periventricular nucleus stimulation (10 Hz, 5 min, 0.5 mA biphasic, 0.5/1.0 ms): a) putative neurosecretory cells which were antidromically identified (AD) as projecting to the median eminence (n = 53) and b) non-neurosecretory cells, identified by their spontaneous 'bursting' pattern of activity (n = 29). During stimulation predominantly inhibitory responses were observed in both AD and bursting cell groups. Of the 39 AD cells which were spontaneously active, 25 were inhibited during the periventricular nucleus stimulation, and 10 of these showed a rebound hyperactivation following the end of stimulation. Fifteen bursting cells were inhibited during stimulation and 4 of these displayed a rebound hyperactivation following the end of stimulation. Additional evidence was sought for the identity of these cells by testing their response to electrical stimulation of the basolateral amygdala (which has previously been shown to increase plasma GH concentration without influencing the release of other pituitary hormones).(ABSTRACT TRUNCATED AT 250 WORDS)