Sustained elevation of pulsatile growth hormone (GH) secretion and insulin-like growth factor I (IGF-I), IGF-binding protein-3 (IGFBP-3), and IGFBP-5 concentrations during 30-day continuous subcutaneous infusion of GH-releasing peptide-2 in older men and women

Assessment of anterior pituitary reserve capacity based on growth hormone response to growth hormone-releasing peptide-2 test in the elderly

OBJECTIVE

The growth hormone (GH)-releasing peptide-2 (GHRP-2) test is relatively safe among endocrine stimulation tests for the elderly. We investigated whether anterior pituitary function in elderly patients could be assessed on the basis of GH response to the GHRP-2 test.

DESIGN

Sixty-five elderly patients aged 65 years and older with non-functioning pituitary neuroendocrine tumor (PitNET) who underwent pituitary surgery and preoperative endocrine stimulation tests were classified into the "GH normal group" and "GH deficiency group" based on GH response to the GHRP-2 test. The baseline characteristics and anterior pituitary function were compared between the groups.

RESULTS

Thirty-two patients were assigned to the GH normal group and 33 to the GH deficiency group. The cortisol and adrenocorticotropic hormone (ACTH) results in the corticotropin-releasing hormone test were significantly higher in the GH normal group than in the GH deficiency group (p < 0.001). The relationship between the cortisol and ACTH results and the GH response revealed significant correlations (p < 0.001). In addition, receiver operating characteristic curve analysis identified that the optimal cut-off point for a peak GH level in the correlation between adrenocortical function and GH response to the GHRP-2 test was 8.08 ng/mL (specificity 0.868, sensitivity 0.852).

CONCLUSION

The present study indicated that adrenocortical function was significantly correlated with GH response to the GHRP-2 test in elderly patients before pituitary surgery. For elderly patients with non-functioning PitNET, GH response to the GHRP-2 test may support in diagnosing adrenocortical insufficiency.

Interleukin-2 Transiently Inhibits Pulsatile Growth Hormone Secretion in Young but not Older Healthy Men

CONTEXT

Interleukin-2 (IL-2), a proinflammatory cytokine, has been used to treat malignancies. Increased cortisol and adrenocorticotropin (ACTH) were noted, but growth hormone (GH) secretion was not investigated in detail.

OBJECTIVE

We quantified GH secretion after a single subcutaneous injection of IL-2 in 17 young and 18 older healthy men in relation to dose, age, and body composition.

METHODS

This was a placebo-controlled, blinded, prospectively randomized, crossover study. At 20:00 hours IL-2 (3 or 6 million units/m2) or saline was injected subcutaneously. Lights were off between 23:00 and 07:00 hours. Blood was sampled at 10-minute intervals for 24 hours. Outcome measures included convolution analysis of GH secretion.

RESULTS

GH profiles were pulsatile under both experimental conditions and lower in older than young volunteers. Since the effect of IL-2 might be time limited, GH analyses were performed on the complete 24-hour series and the 6 hours after IL-2 administration. Total and pulsatile 24-hour GH secretion decreased nonsignificantly. Pulsatile secretion fell over the first 6 hours after IL-2 (P = .03), with visceral fat as a covariate (P = .003), but not age (P = .10). Plots of cumulative 2-hour bins of GH pulse mass showed a distinction by treatment and age groups: A temporary GH decrease of 32% and 28% occurred in the first 2-hour bins after midnight (P = .02 and .04) in young participants, whereas in older individuals no differences were present at any time point.

CONCLUSION

This study demonstrates that IL-2 temporarily diminishes GH secretion in young, but not older, men.

Dynamic Interactions Between LH and Testosterone in Healthy Community-Dwelling Men: Impact of Age and Body Composition

BACKGROUND

Aging is associated with diminished testosterone (Te) secretion, which may be attributed to Leydig cell dysfunction, decreased pituitary stimulation, and altered Te feedback.

OBJECTIVE

To study all regulatory nodes-gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH) and Leydig cell-in the same cohort of healthy men.

STUDY DESIGN

This was a placebo-controlled, blinded, prospectively randomized cross-over study in 40 men, age range 19 to 73 years, and body mass index (BMI) range 20 to 34.3 kg/m2. A submaximal dose of the GnRH antagonist ganirelix was used to assess outflow of GnRH, by calculating the difference between LH output during the control arm and ganirelix arm. Ketoconazole (a steroidogenic inhibitor) was used to estimate feedback, by the difference in LH output during the ketoconazole and control arm. High-dose ganirelix and repeated LH infusions were used to measure testicular responsivity. Blood sampling was performed at 10-minute intervals.

RESULTS

There were age-related, but not body composition-related decreases in estimated GnRH secretion, the feedback strength of Te on LH, and Leydig cell responsivity to LH, accompanied by changes in approximate entropy. Bioavailable Te levels were negatively related to both age and computed tomography (CT)-estimated abdominal visceral mass (AVF), without interaction between these variables. The LH response to a submaximal dose of GnRH was independent of age and AVF.

CONCLUSION

Advancing age is associated with (1) attenuated bioavailable Te secretion caused by diminished GnRH outflow and not by decreased GnRH responsivity of the gonadotrope, (2) diminished testicular responsivity to infused LH pulses, and (3) partial compensation by diminished Te feedback on central gonadotropic regulation.

The Safety and Efficacy of Growth Hormone Secretagogues

INTRODUCTION

Growth hormone (GH) increases lean body mass, decreases fat mass, increases exercise tolerance and maximum oxygen uptake, enhances muscle strength, and improves linear growth. Long-term studies of GH administration offer conflicting results on its safety, which has led to strict Food and Drug Administration criteria for GH use. The potential drawbacks of exogenous GH use are believed to be due in part to impaired regulatory feedback.

AIM

To review the literature on GH secretagogues (GHSs), which include GH-releasing peptides and the orally available small-molecule drug ibutamoren mesylate.

METHODS

Review of clinical studies on the safety and efficacy of GHSs in human subjects.

MAIN OUTCOME MEASURE

Report on the physiologic changes from GHS use in human subjects including its safety profile.

RESULTS

GHSs promote pulsatile release of GH that is subject to negative feedback and can prevent supra-therapeutic levels of GH and their sequelae. To date, few long-term, rigorously controlled studies have examined the efficacy and safety of GHSs, although GHSs might improve growth velocity in children, stimulate appetite, improve lean mass in wasting states and in obese individuals, decrease bone turnover, increase fat-free mass, and improve sleep. Available studies indicate that GHSs are well tolerated, with some concern for increases in blood glucose because of decreases in insulin sensitivity.

CONCLUSION

Further work is needed to better understand the long-term impact of GHSs on human anatomy and physiology and more specifically in the context of a diversity of clinical scenarios. Furthermore, the safety of these compounds with long-term use, including evaluation of cancer incidence and mortality, is needed. Sigalos JT, Pastuszak AW. The Safety and Efficacy of Growth Hormone Secretagogues. Sex Med Rev 2018;6:45-53.

GH-Releasing Hormone Promotes Survival and Prevents TNF-α-Induced Apoptosis and Atrophy in C2C12 Myotubes

Skeletal muscle atrophy is a consequence of different chronic diseases, including cancer, heart failure, and diabetes, and also occurs in aging and genetic myopathies. It results from an imbalance between anabolic and catabolic processes, and inflammatory cytokines, such as TNF-α, have been found elevated in muscle atrophy and implicated in its pathogenesis. GHRH, in addition to stimulating GH secretion from the pituitary, exerts survival and antiapoptotic effects in different cell types. Moreover, we and others have recently shown that GHRH displays antiapoptotic effects in isolated cardiac myocytes and protects the isolated heart from ischemia/reperfusion injury and myocardial infarction in vivo. On these bases, we investigated the effects of GHRH on survival and apoptosis of TNF-α-treated C2C12 myotubes along with the underlying mechanisms. GHRH increased myotube survival and prevented TNF-α-induced apoptosis through GHRH receptor-mediated mechanisms. These effects involved activation of phosphoinositide 3-kinase/Akt pathway and inactivation of glycogen synthase kinase-3β, whereas mammalian target of rapamycin was unaffected. GHRH also increased the expression of myosin heavy chain and the myogenic transcription factor myogenin, which were both reduced by the cytokine. Furthermore, GHRH inhibited TNF-α-induced expression of nuclear factor-κB, calpain, and muscle ring finger1, which are all involved in muscle protein degradation. In summary, these results indicate that GHRH exerts survival and antiapoptotic effects in skeletal muscle cells through the activation of anabolic pathways and the inhibition of proteolytic routes. Overall, our findings suggest a novel therapeutic role for GHRH in the treatment of muscle atrophy-associated diseases.

Impact of Adiposity and Fat Distribution on the Dynamics of Adrenocorticotropin and Cortisol Rhythms

Obesity impacts many hormonal systems, including pituitary hormones, as well as insulin and leptin. In this review we discuss articles which investigate the influence of obesity on the hypothalamic-pituitary-adrenal (HPA) axis. Different techniques have been used to assess the function of the HPA-axis in obesity, including measuring fasting and/or late evening levels of adrenocorticotropic hormone (ACTH) and (free) cortisol in plasma and saliva, studying feedback with dexamethasone or cortisol, and evaluating responsiveness of the system to corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) or ACTH 1-29. In addition, more elaborate studies investigated 24-h secretion patterns, analyzed with deconvolution techniques to quantitate pulsatile secretion rates of cortisol and less often ACTH. Other investigators used timed infusions of labeled cortisol for the estimation of the 24-h secretion rate, clearance rate and distribution volume. Many studies relied on the 24-h urinary excretion of free cortisol, but for quantitation of the 24-h secretion, measurement of all cortisol-derived metabolites is required. Several studies have applied modern liquid chromatography-tandem-mass spectrometry techniques to measure these metabolites. The picture emerging from all these studies is that, first, ACTH secretion is amplified, likely via enhanced forward drive; and, second, serum cortisol levels are normal or even low, associated with a normal 24-h cortisol secretion per liter distribution volume determined by deconvolution, but enhanced when based on the increased total distribution volume associated with obesity. Increased cortisol secretion was also established by isotope dilution studies and reports based on the measurement of all urinary cortisol metabolites. The responsiveness of the adrenal gland to ACTH is diminished. The studies do not address quantitative aspects of cortisol-cortisone metabolism on individual organs, including liver, central and peripheral fat, intestine, skin, and muscle.

Changes in pituitary function with ageing and implications for patient care

The pituitary gland has a role in puberty, reproduction, stress-adaptive responses, sodium and water balance, uterine contractions, lactation, thyroid function, growth, body composition and skin pigmentation. Ageing is marked by initially subtle erosion of physiological signalling mechanisms, resulting in lower incremental secretory-burst amplitude, more disorderly patterns of pituitary hormone release and blunted 24 h rhythmic secretion. Almost all pituitary hormones are altered by ageing in humans, often in a manner dependent on sex, body composition, stress, comorbidity, intercurrent illness, medication use, physical frailty, caloric intake, immune status, level of exercise, and neurocognitive decline. The aim of this article is to critically discuss the mechanisms mediating clinical facets of changes in the hypothalamic-pituitary axis during ageing, and the extent to which confounding factors operate to obscure ageing-related effects.

Differential pulsatile secretagogue control of GH secretion in healthy men

Pulsatile growth hormone (GH) secretion putatively reflects integrated regulation by GH-releasing hormone (GHRH), somatostatin (SST), and GH-releasing peptide (GHRP). GHRH and SST secretion is itself pulsatile. However, how GHRH and SST pulses act along with GHRP to jointly determine pulsatile GH secretion is unclear. Moreover, how testosterone (T) modulates such interactions is unknown. These queries were assessed in a prospectively randomized, placebo-controlled double-blind cohort comprising 26 healthy older men randomized to testosterone (T) vs. placebo supplementation. Pulses of GHRH, SST, or saline were infused intravenously at 90-min intervals for 13 h, along with either continuous saline or ghrelin analog (GHRP-2). The train of pulses was followed by a triple stimulus (combined l-arginine, GHRH, and GHRP-2) to estimate near-maximal GH secretion over a final 3 h. Testosterone vs. placebo supplementation doubled pulsatile GH secretion during GHRH pulses combined with continuous saline (GHRH/saline) (P < 0.01). Pulsatile GH secretion correlated positively with T concentrations (270-1,170 ng/dl) in the 26 men during saline pulses/saline (P = 0.015, R(2) = 0.24), GHRH pulses/saline (P = 0.020, R(2) = 0.22), and combined GHRH pulses/GHRP-2 (P = 0.016, R(2) = 0.25) infusions. Basal nonpulsatile GH secretion correlated with T during saline pulses/GHRP-2 drive (P = 0.020, R(2) = 0.16). By regression analysis, pulsatile GH secretion varied negatively with body mass index (BMI) during saline/GHRP-2 infusion (P = 0.001, R(2) = 0.36), as well as after the triple stimulus preceded by GHRH/GHRP-2 (P = 0.013, R(2) = 0.23). Mean (10-h) GH concentrations under GHRP-2 were predicted jointly by estradiol (positively) and BMI (negatively) (P < 0.001, R(2) = 0.520). These data indicate that estradiol, T, and BMI control pulsatile secretagogue-specific GH-regulatory mechanisms in older men.

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.

Sex steroids, GHRH, somatostatin, IGF-I, and IGFBP-1 modulate ghrelin's dose-dependent drive of pulsatile GH secretion in healthy older men

CONTEXT

Ghrelin is a potent endogenous stimulator of GH secretion. However, clinical factors that regulate ghrelin dose-responsiveness are incompletely defined.

OBJECTIVE

The aim of the study was to test the multipathway hypothesis that testosterone (T) and estradiol, GHRH, and somatostatin (SS) jointly modulate ghrelin's action.

DESIGN/PARTICIPANTS/SETTING

Healthy older men (n = 21) participated in a double-blind, prospectively randomized, placebo (Pl)-controlled study in a Clinical Translational Research Center.

INTERVENTIONS

To create a range of sex-steroid milieus, men received leuprolide + Pl (n = 10) or leuprolide + T addback (n = 11). Sixteen to 21 d later, subjects received three separate randomly ordered overnight constant i.v. infusions of saline, GHRH, and SS. Interactions between the peptide clamp and ghrelin were tested by superimposed injections of four randomly ordered bolus i.v. doses of ghrelin (0.03, 0.135, 0.60, and 2.7 μg/kg). GH was measured every 10 min, and GH responses were assessed by nonlinear dose-response analysis. Linear associations were assessed by stepwise regression.

OUTCOME MEASURES/RESULTS

The descending numerical order of ghrelin efficacy (maximal GH secretory-burst mass; micrograms/liter) was 107 (GHRH + Pl), 104 (GHRH + T), 73 (saline + T), 73 (SS + T), 60 (saline + Pl), and 52 (SS + Pl) [means], wherein SS + T exceeded SS + Pl. GHRH and IGF binding protein-1 augmented, whereas IGF-I attenuated ghrelin potency. Age and IGF-I decreased ghrelin/GHRH synergy. Ghrelin sensitivity was independent of interventions.

CONCLUSIONS

These studies introduce composite regulatory effects of sex hormones, GHRH, SS, IGF binding protein-1, and IGF-I on ghrelin dose-responsiveness, suggesting multipathway modulation of GH-secretagogue action.

Distinct metabolic surrogates predict basal and rebound GH secretion after glucose ingestion in men

CONTEXT

GH secretion declines rapidly after glucose ingestion and then recovers to higher-than-baseline levels (rebound GH release).

HYPOTHESIS

Selective metabolic markers predict the magnitude of glucose-suppressed GH release and postglucose rebound-like GH secretion.

DESIGN

Prospectively randomized crossover study of GH secretion after glucose vs. water ingestion.

SETTING

The study was conducted at a clinical translational research center.

PARTICIPANTS

Sixty-nine healthy men aged 19-78 yr with a body mass index of 18-39 kg/m(2) participated in the study.

OUTCOMES

OUTCOMES included nadir vs. peak GH concentrations and basal vs. pulsatile GH secretion.

RESULTS

Mean nadir GH concentrations were determined positively by sex hormone binding globulin (SHBG) after glucose administration (R(2) = 0.088, P = 0.0077). Peak rebound GH concentrations were related positively to adiponectin and negatively to computed tomography-estimated abdominal visceral fat (AVF) (R(2) = 0.182, P = 0.00049) after glucose ingestion. Deconvolution analysis showed that SHBG specifically predicted basal (nonpulsatile) GH secretion after glucose exposure (R(2) = 0.153, P = 0.00052). In contrast, together exercise history and adiponectin (both positively) and AVF (negatively) predicted pulsatile GH escape after glucose suppression (R(2) = 0.206, P = 0.00043). Moreover, adiponectin uniquely determined the size (mass), and AVF the mode (duration), of GH secretory bursts after glucose exposure (both P < 0.006).

CONCLUSION

Glucose ingestion provides a clinical model for investigating complementary metabolic surrogates that determine suppression and recovery of basal and pulsatile GH secretion in healthy men.

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.

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.

Integrating GHS into the Ghrelin System

Oligopeptide derivatives of metenkephalin were found to stimulate growth-hormone (GH) release directly by pituitary somatotrope cells in vitro in 1977. Members of this class of peptides and nonpeptidyl mimetics are referred to as GH secretagogues (GHSs). A specific guanosine triphosphatate-binding protein-associated heptahelical transmembrane receptor for GHS was cloned in 1996. An endogenous ligand for the GHS receptor, acylghrelin, was identified in 1999. Expression of ghrelin and homonymous receptor occurs in the brain, pituitary gland, stomach, endothelium/vascular smooth muscle, pancreas, placenta, intestine, heart, bone, and other tissues. Principal actions of this peptidergic system include stimulation of GH release via combined hypothalamopituitary mechanisms, orexigenesis (appetitive enhancement), insulinostasis (inhibition of insulin secretion), cardiovascular effects (decreased mean arterial pressure and vasodilation), stimulation of gastric motility and acid secretion, adipogenesis with repression of fat oxidation, and antiapoptosis (antagonism of endothelial, neuronal, and cardiomyocyte death). The array of known and proposed interactions of ghrelin with key metabolic signals makes ghrelin and its receptor prime targets for drug development.

Pituitary-hormone secretion by thyrotropinomas

Hormone secretion by somatotropinomas, corticotropinomas and prolactinomas exhibits increased pulse frequency, basal and pulsatile secretion, accompanied by greater disorderliness. Increased concentrations of growth hormone (GH) or prolactin (PRL) are observed in about 30% of thyrotropinomas leading to acromegaly or disturbed sexual functions beyond thyrotropin (TSH)-induced hyperthyroidism. Regulation of non-TSH pituitary hormones in this context is not well understood. We there therefore evaluated TSH, GH and PRL secretion in 6 patients with up-to-date analytical and mathematical tools by 24-h blood sampling at 10-min intervals in a clinical research laboratory. The profiles were analyzed with a new deconvolution method, approximate entropy, cross-approximate entropy, cross-correlation and cosinor regression. TSH burst frequency and basal and pulsatile secretion were increased in patients compared with controls. TSH secretion patterns in patients were more irregular, but the diurnal rhythm was preserved at a higher mean with a 2.5 h phase delay. Although only one patient had clinical acromegaly, GH secretion and IGF-I levels were increased in two other patients and all three had a significant cross-correlation between the GH and TSH. PRL secretion was increased in one patient, but all patients had a significant cross-correlation with TSH and showed decreased PRL regularity. Cross-ApEn synchrony between TSH and GH did not differ between patients and controls, but TSH and PRL synchrony was reduced in patients. We conclude that TSH secretion by thyrotropinomas shares many characteristics of other pituitary hormone-secreting adenomas. In addition, abnormalities in GH and PRL secretion exist ranging from decreased (joint) regularity to overt hypersecretion, although not always clinically obvious, suggesting tumoral transformation of thyrotrope lineage cells.

Testosterone supplementation in older men restrains insulin-like growth factor's dose-dependent feedback inhibition of pulsatile growth hormone secretion

BACKGROUND

Pulsatile GH secretion declines in older men. The causal mechanisms are unknown. Candidates include deficient feedforward (stimulation) by endogenous secretagogues and excessive feedback (inhibition) by GH or IGF-I due to age and/or relative hypoandrogenemia.

HYPOTHESIS

Testosterone (T) supplementation in healthy older men will restrain negative feedback by systemic concentrations of IGF-I.

SUBJECTS

Twenty-four healthy men (ages, 50 to 75 yr; body mass index, 24 to 30 kg/m(2)) participated in the study.

METHODS

We performed a prospectively randomized, double-blind, placebo-controlled assessment of the impact of pharmacological T supplementation on GH responses to randomly ordered separate-day injections of recombinant human IGF-I doses of 0, 1.0, 1.5, and 2.0 mg/m(2).

ANALYSIS

Deconvolution and approximate entropy analyses of pulsatile, basal, and entropic (pattern-sensitive) modes of GH secretion were conducted.

RESULTS

Recombinant human IGF-I injections 1) elevated mean and peak serum IGF-I concentrations dose-dependently (both P < 0.001); 2) suppressed pulsatile GH secretion (P = 0.003), burst mass (P = 0.025), burst number (P = 0.005), interpulse variability (P = 0.032), and basal GH secretion (P = 0.009); and 3) increased secretory pattern regularity (P = 0.020). T administration did not alter experimentally controlled IGF-I concentrations, but it elevated mean GH concentrations (P = 0.015) and stimulated pulsatile GH secretion (frequency P = 0.037, mass per burst P = 0.038). Compared with placebo, T attenuated exogenous IGF-I's inhibition of GH secretory-burst mass (P < 0.038) without restoring pulse number, basal secretion, or pattern regularity.

CONCLUSION

The capability of systemic T to mute IGF-I feedback on pulsatile GH secretion suggests a novel mechanism for augmenting GH production.

Motivations and methods for analyzing pulsatile hormone secretion

Endocrine glands communicate with remote target cells via a mixture of continuous and intermittent signal exchange. Continuous signaling allows slowly varying control, whereas intermittency permits large rapid adjustments. The control systems that mediate such homeostatic corrections operate in a species-, gender-, age-, and context-selective fashion. Significant progress has been made in understanding mechanisms of adaptive interglandular signaling in vivo. Principal goals are to understand the physiological origins, significance, and mechanisms of pulsatile hormone secretion. Key analytical issues are: 1) to quantify the number, size, shape, and uniformity of pulses, nonpulsatile (basal) secretion, and elimination kinetics; 2) to evaluate regulation of the axis as a whole; and 3) to reconstruct dose-response interactions without disrupting hormone connections. This review will focus on the motivations driving and the methodologies used for such analyses.

Thyrotropin secretion by thyrotropinomas is characterized by increased pulse frequency, delayed diurnal rhythm, enhanced basal secretion, spikiness, and disorderliness

CONTEXT

Hormone secretion by somatotropinomas, corticotropinomas, and prolactinomas exhibits increased pulsatility and basal secretion, accompanied by greater disorderliness.

OBJECTIVE

Our objective was to evaluate TSH secretion by thyrotropinomas with up-to-date analytical and mathematical tools.

DESIGN

Twenty-four hour blood samplings at 10-min intervals in a clinical research laboratory in five patients with a thyrotropinoma and 10 healthy age- and gender-matched controls were performed. The obtained serum TSH profiles were analyzed with a new deconvolution method, approximate entropy, Cosinor analysis, and by quantification of spikiness.

RESULTS

TSH burst frequency and basal secretion were increased in patients compared with controls. TSH secretion patterns in patients were more irregular than in controls, but the diurnal rhythm was preserved at a higher mean in all patients, although with a 2-h phase delay.

CONCLUSION

TSH secretion by thyrotropinomas shares many characteristics with other pituitary hormone-secreting adenomas.

Estrogen supplementation selectively enhances hypothalamo-pituitary sensitivity to ghrelin in postmenopausal women

CONTEXT

Sex-steroid hormones amplify pulsatile GH secretion by unknown mechanisms. Ghrelin is the most potent natural GH secretagogue discovered to date. A plausible unifying postulate is that estradiol (E(2)) enhances hypothalamo-pituitary sensitivity to ghrelin (a physiological effect). The hypothesis is relevant to understanding the basis of hyposomatotropism in aging and other relatively hypogonadal states.

OBJECTIVE

Our objective was to test the hypothesis that E(2) supplementation potentiates ghrelin's stimulation of pulsatile GH secretion.

SETTING

The study was conducted at an academic medical center.

SUBJECTS

Healthy postmenopausal women (n = 20) were included in the study.

INTERVENTIONS

Separate-day iv infusions of saline vs. five graded doses of ghrelin were performed in volunteers prospectively randomly assigned to receive (n = 8) or not receive (n = 12) transdermal E(2) for 21 d were performed.

MEASURES

GH secretion was estimated by deconvolution analysis and abdominal visceral fat mass determined by computerized axial tomography were calculated.

RESULTS

E(2) supplementation augmented ghrelin's stimulation of basal (nonpulsatile) GH secretion by 3.6-fold (P = 0.022), increased GH responses to low-dose ghrelin by 2.9-fold (P = 0.035), did not alter ghrelin efficacy, and elicited more regular patterns of acylated ghrelin concentrations during saline infusion (P = 0.033). Abdominal visceral fat negatively determined responses to ghrelin (R = -0.346; P < 0.005).

CONCLUSIONS

Transdermal E(2) supplementation potentiates GH secretion stimulated by physiological but not pharmacological concentrations of acylated ghrelin, and concomitantly regularizes patterns of bioactive ghrelin secretion in postmenopausal women. Accordingly, the estrogen milieu appears to control sensitivity of the hypothalamopituitary unit to acylated ghrelin.

Twenty-four hour continuous ghrelin infusion augments physiologically pulsatile, nycthemeral, and entropic (feedback-regulated) modes of growth hormone secretion

BACKGROUND

Ghrelin is a 28-amino acid acylated peptide that potentiates GHRH stimulation and opposes somatostatin inhibition acutely. Whether prolonged ghrelin administration can sustain physiological patterns of GH secretion remains unknown.

HYPOTHESIS

Continuous delivery of ghrelin will amplify physiological patterns of GH secretion over 24 h.

SUBJECTS

Men and women ages 29-69 yr, body mass indices 23-52 kg/m2, were included in the study.

LOCATION

The study was performed at an academic medical center.

METHODS

Twenty-four hour continuous sc infusion of saline vs. ghrelin (1 microg/kg.h) with frequent sampling was examined. Deconvolution and entropy analyses were performed.

OUTCOMES

IGF-I concentrations were determined. Basal, pulsatile, nycthemeral, and entropic measures of GH secretion were calculated.

RESULTS

Ghrelin infusion compared with saline infusion for 24 h elevated (median) acylated ghrelin, GH, and IGF-I concentrations by 8.1-fold (P < 0.001),11-fold (P < 0.001), and 1.4-fold (P = 0.002). GH secretory-burst mass and frequency increased by 6.6-fold (P = 0.004) and 1.7-fold (P < 0.001), respectively, resulting in a 12-fold increase in pulsatile GH secretion (P < 0.001). Interpulse variability decreased significantly (P = 0.046), whereas GH secretory-burst shape and half-life did not change. The amplitude of the nycthemeral GH rhythm increased by 3.4-fold (P < 0.001), and GH patterns became more irregular (higher approximate entropy P < 0.001). Combining GHRH with ghrelin was not an additive in driving GH secretion.

CONCLUSIONS

Continuous ghrelin infusion for 24 h elevates acylated ghrelin, GH and IGF-I concentrations, and stimulates pulsatile, nycthemeral, and entropic modes of GH secretion. The consistency of outcomes in a heterogeneous cohort of adults suggests potentially broad utility of this physiological secretagogue in hyposomatotropic states.

Aging and hormones of the hypothalamo-pituitary axis: gonadotropic axis in men and somatotropic axes in men and women

Neuroendocrinology of the aging (male) gonadal and (male and female) somatotropic axes will be reviewed. A companion chapter discusses reproductive hormonal changes in aging women. Both the gonadal and growth-hormone/insulin-like growth factor (GH/IGF-I) axes function as ensembles. The ensembles comprise tripartite interactions among the brain (hypothalamus), anterior pituitary gland (gonadotrope and somatotrope cells) and target organs (testis, liver, muscle, fat and brain). Compelling evidence indicates that combined hypothalamic and gonadal adaptations operate in the reproductive axis of older men, and multiple hypothalamic adaptations prevail in the GH axis of elderly men and women. Evolving investigative methods allow more precise parsing of the particular mechanisms that subserve such age-related changes, and suggest novel interventional strategies to evaluate the physiological impact of the dynamic alterations discerned in aging individuals.

GHRP-2, a GHS-R agonist, directly acts on myocytes to attenuate the dexamethasone-induced expressions of muscle-specific ubiquitin ligases, Atrogin-1 and MuRF1

Recent reports suggest that Atrogin-1 and MuRF1, E3 ubiquitin ligases, play a pivotal role in muscle atrophy. In the present study, effect of Growth Hormone Releasing Peptide-2 (GHRP-2), a GH secretagogue receptor (GHS-R) agonist, on the expressions of Atrogin-1 and MuRF1 in vivo rat muscles was examined. Dexamethasone administration increased Atrogin-1 mRNA level in rat soleus muscle. The increased mRNA level of Atrogin-1 was significantly attenuated by GHRP-2. In addition, GHRP-2 decreased MuRF1 mRNA level irrespective of the presence of dexamethasone. Although IGF-I is a well-known protective factor for muscle atrophy, GHRP-2 did not influence plasma IGF-I levels and IGF-I mRNA levels in muscles. To clarify a direct effect of GHRP-2, differentiated C2C12 myocytes were used. Ten micrometer dexamethasone increased both Atrogin-1 and MuRF1 mRNA levels in C2C12 cells. GHRP-2 attenuated dexamethasone-induced expression of them dose-dependently and decreased the basal level of MuRF1 mRNA. The suppressive effect on the expressions of Atrogin-1 and MuRF1 by GHRP-2 was blocked by [D-Lys(3)]-GHRP-6, a GHS-R1a blocker, suggesting the effect of GHRP-2 was mediated through GHS-R1a. Taken together, GHRP-2 directly attenuates Atrogin-1 and MuRF1 mRNA levels through ghrelin receptors in myocytes.

Tripartite control of growth hormone secretion in women during controlled estradiol repletion

CONTEXT

Studies of how aging attenuates GH secretion are confounded by differences in sex-steroid milieus, abdominal visceral fat mass (AVF), and IGF-I concentrations and limited in interpretability by the use of pharmacological doses of secretagogues.

HYPOTHESIS

In a controlled estrogenic milieu, near-physiological secretagogue drive will unmask distinct influences of age, AVF, and IGF-I on GH secretion.

LOCATION

The study was conducted at an academic medical center.

SUBJECTS

Subjects included 10 healthy pre- (PRE) and 10 postmenopausal (POST) women.

PROCEDURE

In a defined estradiol (E(2)) milieu, we compared GH secretion after submaximal stimulation with GH-releasing peptide (GHRP)-2 (ghrelin analog), GHRH, and l-arginine (an inhibitor of somatostatin outflow).

ANALYSIS

We related GH responses to age stratum (dichotomous variable) and AVF and IGF-I concentrations (continuous variables).

RESULTS

In the face of comparable concentrations of E(2), testosterone, and SHBG: 1) age (P < 0.001) and secretagogue type (P < 0.001) independently determined GH secretion; 2) GH responses in POST subjects were only 26-33% of those in PRE (P < or = 0.002) across all secretagogues; 3) POST women lost the PRE order of secretagogue potency (GHRP-2 > GHRH = l-arginine); and 4) in the combined cohorts, higher AVF predicted reduced l-arginine-stimulated GH secretion (R(2) = 0.46, P = 0.0013), whereas higher IGF-I concentrations forecast increased GHRP-2 and GHRH drive (R(2) > or = 0.52, P < or = 0.013).

CONCLUSION

A paradigm of near-physiological secretagogue drive in an E(2)-clamped milieu unmasks tripartite deficits in peptide-signaling pathways in healthy POST, compared with PRE, women. Post hoc analyses indicate that both greater visceral adiposity and lower IGF-I concentrations mark this triple regulatory defect.

Selective alteration at the growth-hormone- releasing-hormone nerve terminals during aging in GHRH-green fluorescent protein mice

Growth hormone (GH) secretion decreases spontaneously during lifespan, and the resulting GH deficiency participates in aging-related morbidity. This deficiency appears to involve a defect in the activity of hypothalamic GH-releasing hormone (GHRH) neurons. Here, we investigated this hypothesis, as well as the underlying mechanisms, in identified GHRH neurons from adult ( approximately 13 weeks old) and aged ( approximately 100 weeks old) transgenic GHRH-green fluorescent protein mice, using morphological, biochemical and electrophysiological methods. Surprisingly, the spontaneous action potential frequency was similar in adult and aged GHRH neurons studied in brain slices. This was explained by a lack of change in the intrinsic excitability, and simultaneous increases in both stimulatory glutamatergic- and inhibitory GABAergic-synaptic currents of aged GHRH neurons. Aging did not decrease GHRH and enhanced green fluorescent protein contents, GHRH neuronal number or GHRH-fibre distribution, but we found a striking enlargement of GHRH-positive axons, suggesting neuropeptide accumulation. Unlike in adults, autophagic vacuoles were evident in aged GHRH-axonal profiles using electron microscopy. Thus, GHRH neurons are involved in aging of the GH axis. Aging had a subtle effect at the nerve terminal level in GHRH neurons, contrasting with the view that neuronal aging is accompanied by more widespread damage.

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.

Estradiol potentiates ghrelin-stimulated pulsatile growth hormone secretion in postmenopausal women

CONTEXT

Ghrelin and an estrogen-rich milieu individually amplify pulsatile GH secretion by increasing the amount of hormone released per burst. However, how these distinct agonists interact in controlling pulsatile GH output is not known.

OBJECTIVE

The objective of the study was to test the hypothesis that elevated estradiol (E(2)) concentrations potentiate hypothalamo-pituitary responses to a near-physiological ghrelin stimulus.

DESIGN

This was a double-blind, placebo-controlled, prospectively randomized, parallel-cohort study.

SETTING

The study was conducted at an academic medical center.

SUBJECTS

Twenty-one postmenopausal women participated in the study.

INTERVENTIONS

Eleven subjects received placebo (Pl) and 10 others E(2) transdermally in escalating doses over 3 wk to mimic late follicular-phase E(2) concentrations. Saline or a submaximally stimulatory amount of ghrelin (0.3 microg/kg) was infused iv on separate randomly ordered mornings fasting after 17-21 d of Pl or E(2) administration.

OUTCOMES

Outcomes included serum concentrations of E(2), ghrelin, GH, IGF-I, IGF binding protein (IGFBP)-1 and IGFBP-3, and the estimated mass and waveform of stimulated GH secretory bursts.

RESULTS

Administration of E(2) yielded late follicular-phase E(2) concentrations. Compared with Pl, E(2) did not alter ghrelin concentrations but reduced IGF-I and IGFBP-3 and elevated IGFBP-1 concentrations. Compared with saline, ghrelin infusion amplified pulsatile GH secretion by 7.1-fold (P < 0.01). The effect of E(2) alone was 2.0-fold placebo and that of combined ghrelin/E(2) 10.4-fold (P < 0.01). Ghrelin and E(2) accelerated initial GH release individually but nonadditively by more than 2-fold (P < 0.01).

CONCLUSIONS

Estrogen augments ghrelin's near-physiological stimulation of pulsatile GH secretion and mimics ghrelin's acceleration of initial GH release. Thus, we hypothesize that estrogen and a GH secretagogue act via independent as well as convergent mechanisms.

Somatotropic and gonadotropic axes linkages in infancy, childhood, and the puberty-adult transition

Integrative neuroendocrine control of the gonadotropic and somatotropic axes in childhood, puberty, and young adulthood proceeds via multiple convergent and divergent pathways in the human and experimental animal. Emerging ensemble concepts are required to embody independent, parallel, and interacting mechanisms that subserve physiological adaptations and pathological disruption of reproduction and growth. Significant advances in systems biology will be needed to address these challenges.

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.

Short-term testosterone supplementation does not activate GH and IGF-I production in postmenopausal women

BACKGROUND

Testosterone (Te), oestradiol, GH and IGF-I concentrations fall in postmenopausal women.

OBJECTIVE

To test the effect of increased Te availability on impoverished GH/IGF-I secretion in this context.

DESIGN/PATIENTS

Placebo (Pl) and a low vs. high dose of Te were administered transdermally for 7 days to each of 15 healthy older women (ages 48-81 years) in a randomized, double-blind crossover design.

MEASUREMENTS

Frequent blood sampling, GHRP-2 (Growth hormone releasing peptide-2) infusion, and deconvolution analysis were used to assess GH secretion.

RESULTS

Graded Te supplementation increased serum Te concentrations (nmol/l) from 0.87 +/- 0.06 [Pl] to 5 +/- 1 [low] and 7.3 +/- 1.6 [high] (P < 0.001), but did not affect: (i) pulsatile GH secretion (microg/l/12 h; conversion factor: 1 microg/l = 0.33 mU/l), 29 +/- 7.2, 32 +/- 5.6 and 27 +/- 4.9; (ii) GH regularity (approximate entropy), 0.52 +/- 0.05, 0.57 +/- 0.05 and 0.56 +/- 0.05; (iii) IGF-I concentrations (microg/l; conversion factor: 1 microg/l = 0.131 nmol/l), 126 +/- 13, 135 +/- 15 and 141 +/- 13; (iv) IGFBP-3 (mg/l), 3.7 +/- 0.2, 3.6 +/- 0.1 and 3.7 +/- 0.2; (v) IGFBP-1 (microg/l), 41 +/- 2.3, 44 +/- 3.8 and 44 +/- 3.2; and (vi) the mass of GH secreted (microg/l/3 h) after GHRP-2 injection, 123 +/- 27, 146 +/- 32 and 147 +/- 38.

CONCLUSION

Up to 8-fold elevation of Te concentrations for 1 week in postmenopausal women does not detectably amplify GH secretion or action, thus indicating that low androgen availability is not a proximate acute mediator of hyposomatotropism in postmenopausal individuals.

Insulin-like growth factor II and binding proteins 1 and 3 from second trimester human amniotic fluid are associated with infant birth weight

The developing fetus begins to swallow amniotic fluid (AF) early in gestation, a process that results in ingestion of numerous growth factors. Our objectives were 2-fold: 1) to assess the concentration and distribution of insulin-like growth factor II (IGF II) and its binding proteins (BP) 1 and 3 in 2nd trimester amniotic fluid using ELISA, and 2) to establish whether concentrations of AF IGF II and its binding proteins IGF BP1 and 3, measured early in pregnancy, were associated with and predictive of infant birth weight. Birth weights were categorized using recently developed birth-weight-for-gestational-age percentiles for fetal growth in which infants < 10% were classified as SGA (small-for-gestational-age) and those > 90% as LGA (large-for-gestational-age). AF samples were collected after routine genetic testing (15.1 +/- 0.04 wk, range 12-20 wk) from 543 mother-infant pairs in Montreal, QC, Canada. Maternal and fetal characteristics were obtained from questionnaires and medical chart review. Multivariate regression analysis that controlled for maternal height, prepregnancy weight, smoking behavior, infant gender, gestational age, parity, as well as amniocentesis week showed that higher AF IGF BP1 was associated with lower birth weight (partial r2 = 0.0062). Regression analyses revealed that AF IGF BP3 was positively associated with birth weight within LGA and macrosomia subpopulations (partial r2 = 0.0283 and 0.0404, respectively). These results show that 2nd trimester AF IGF BP1, BP3, and IGF II may emerge as early indicators of fetal growth.

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.

Complementary secretagogue pairs unmask prominent gender-related contrasts in mechanisms of growth hormone pulse renewal in young adults

The present study examines the thesis that pulsatile GH secretion is controlled simultaneously by three principal signals; viz., GHRH, GH-releasing peptide (GHRP, ghrelin), and somatostatin (SS). According to this ensemble notion, no single regulatory peptide acts alone or can be interpreted in isolation. Therefore, to investigate gender-specific control of pulsatile GH secretion, we designed dual-effector stimulation paradigms in eight young men and six women as follows: 1) L-arginine/GHRH (to clamp low SS and high GHRH input); 2) L-arginine/GHRP-2 (to clamp low SS and high GHRP drive); 3) GHRH/GHRP-2 (to clamp high GHRH and high GHRP feedforward); vs. 4) saline (unclamped). Statistical comparisons revealed that: 1) fasting pulsatile GH secretion was 7.6-fold higher in women than men (P < 0.001); 2) L-arginine/GHRH and L-arginine/GHRP-2 evoked, respectively, 4.6- and 2.2-fold greater burst-like GH release in women than men (P < 0.001 and P = 0.015); and 3) GHRH/GHRP-2 elicited comparable GH secretion by gender. In the combined cohorts, estradiol concentrations positively predicted responses to L-arginine/GHRP-2 (r2= 0.49, P = 0.005), whereas testosterone negatively predicted those to L-arginine/GHRH (r2= 0.56, P = 0.002). Based upon a simplified biomathematical model of three-peptide control, the current outcomes suggest that women maintain greater GHRH potency, GHRP efficacy, and opposing SS outflow than men. This inference upholds recent clinical precedence and yields valid predictions of sex differences in self-renewable GH pulsatility.

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.

Growth hormone releasing peptide-2 (GHRP-2), like ghrelin, increases food intake in healthy men

GHRP-2 is a synthetic agonist of ghrelin, the newly-discovered gut peptide which binds to the growth hormone (GH) secretagogue receptor. Ghrelin has two major effects, stimulating both GH secretion and appetite/meal initiation. GHRP-2 has been extensively studied for its utility as a growth hormone secretagogue (GHS). Animal studies have shown its effect on food intake. However, whether GHRP-2 can also stimulate appetite in humans when administered acutely is not known. We subcutaneously infused 7 lean, healthy males with GHRP-2 (1 microg/kg/h) or saline for 270 minutes and then measured their intake of an ad libitum, buffet-style meal. Similar to what has been reported for ghrelin administration, our subjects ate 35.9 +/- 10.9% more when infused with GHRP-2 vs. saline, with every subject increasing their intake even when calculated per kg body weight (136.0 +/- 13.0 kJ/kg [32.5 +/- 3.1 kcal/kg] vs. 101.3 +/- 10.5 kJ/kg [24.2 +/- 2.5 kcal/kg], p = 0.008). The macronutrient composition of consumed food was not different between conditions. As expected, serum GH levels rose significantly during GHRP-2 infusion (AUC 5550 +/- 1090 microg/L/240 min vs. 412 +/- 161 microg/L/240 min, p = 0.003). These data are the first to demonstrate that GHRP-2, like ghrelin, increases food intake, suggesting that GHRP-2 is a valuable tool for investigating ghrelin effects on eating behavior in humans.

Effects of combined long-term treatment with a growth hormone-releasing hormone analogue and a growth hormone secretagogue in the growth hormone-releasing hormone knock out mouse

GH secretagogues (GHS) are synthetic ghrelin receptor agonists that stimulate GH secretion. It is not clear whether they act predominantly by stimulating the secretion of hypothalamic growth hormone-releasing hormone (GHRH), or directly on the somatotrope cells. In addition, it is not known whether combined treatment with GHRH and GHS has synergistic effects on growth. To address these questions, we used the GH-deficient GHRH knock out (GHRHKO) mouse model, which has severe somatotrope cell hypoplasia. We treated GHRHKO mice for 5 weeks (from week 1 to week 6 of age) with the GHRH analogue JI-38 alone, or in combination with a GHS (GHRP-2), and at the end of the treatment we examined their response to an acute stimulus with GHRP-2 or GHRP-2 plus JI-38. We used placebo-treated GHRHKO mice and animals heterozygous for the GHRHKO allele as controls. Animals treated with JI-38+GHRP-2 reached higher body length and weight than animals treated with JI-38 alone. All the animals receiving JI-38 (with or without GHRP-2) showed similar correction of somatotrope cell hypoplasia. None of the GHRHKO animals showed a serum GH response to the acute stimulation with GHRP-2 alone, while both treated groups responded to the combined test with JI-38 + GHRP-2. These data demonstrate that in GHRHKO mice, GHRP-2 has a growth-stimulating effect that augments the response induced by JI-38. In addition, the presence of GHRH seems necessary for the stimulation of GH secretion by GHRP-2.

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.