Induced GH Secretion Research Articles

The Complex World of Regulation of Pituitary Growth Hormone Secretion: The Role of Ghrelin, Klotho, and Nesfatins in It.

The classic concept of how pituitary GH is regulated by somatostatin and GHRH has changed in recent years, following the discovery of peripheral hormones involved in the regulation of energy homeostasis and mineral homeostasis. These hormones are ghrelin, nesfatins, and klotho. Ghrelin is an orexigenic hormone, released primarily by the gastric mucosa, although it is widely expressed in many different tissues, including the central nervous system and the pituitary. To be active, ghrelin must bind to an n-octanoyl group (n = 8, generally) on serine 3, forming acyl ghrelin which can then bind and activate a G-protein-coupled receptor leading to phospholipase C activation that induces the formation of inositol 1,4,5-triphosphate and diacylglycerol that produce an increase in cytosolic calcium that allows the release of GH. In addition to its direct action on somatotrophs, ghrelin co-localizes with GHRH in several neurons, facilitating its release by inhibiting somatostatin, and acts synergistically with GHRH stimulating the synthesis and secretion of pituitary GH. Gastric ghrelin production declines with age, as does GH. Klotho is an anti-aging agent, produced mainly in the kidneys, whose soluble circulating form directly induces GH secretion through the activation of ERK1/2 and inhibits the inhibitory effect that IGF-I exerts on GH. Children and adults with untreated GH-deficiency show reduced plasma levels of klotho, but treatment with GH restores them to normal values. Deletions or mutations of the Klotho gene affect GH production. Nesfatins 1 and 2 are satiety hormones, they inhibit food intake. They have been found in GH3 cell cultures where they significantly reduce the expression of gh mRNA and that of pituitary-specific positive transcription factor 1, consequently acting as inhibitors of GH production. This is a consequence of the down-regulation of the cAMP/PKA/CREB signaling pathway. Interestingly, nesfatins eliminate the strong positive effect that ghrelin has on GH synthesis and secretion. Throughout this review, we will attempt to broadly analyze the role of these hormones in the complex world of GH regulation, a world in which these hormones already play a very important role.

MON-464 Non-Clinical Profiling of ONO-5788, a Novel Oral Small Molecule Somatostatin Receptor Type-2 (SST2) Agonist, to Support Studies in Humans

Somatostatin analogues (SSAs) are indicated for use in acromegaly and gastroenteropancreatic neuroendocrine tumors (GEP NETs). However, SSAs are administered as injections which can be a challenge for patients and healthcare providers. ONO-5788 is a novel low molecular weight orally administered molecule with potent and selective agonistic effects at the human SST2 subtype. ONO-5788 could offer the advantages of a more convenient route of administration for patients with acromegaly and other indications and also a preferential safety profile compared to injections. In order to support clinical evaluation of ONO-5788, a series of non-clinical experiments were undertaken. Findings: In vitro, ONO-5788 and its active metabolite, ONO-ST1-641, has a greater agonistic effect on human SST2 compared to other somatostatin receptor subtypes, with EC50 values of 0.11 and 0.016 nmol/L, respectively. There is no difference in mean EC50 values between human SST2 and rat SST2. ONO-5788 and ONO-ST1-641 significantly inhibits growth hormone-releasing hormone (GHRH)-induced GH secretion from primary cultured rat pituitary cells. In vivo studies in rats confirmed that a single oral dose of ONO-5788 resulted in systemic exposure of ONO-5788 and ONO-ST1-641 that increased in a dose dependent manner. Orally administrated ONO-5788 significantly inhibited both GHRH-induced GH secretion and basal GH secretion at all the doses tested (≥0.1 and ≥0.03 mg/kg, respectively). ONO-5788 and ONO-ST1-641 administrated intravenously significantly inhibited GHRH-induced GH secretion with effect on maximum inhibition of GH secretion similar to octreotide (ONO-5788 98.0%, ONO-ST1-641 98.6%, octreotide 98.7%, respectively). Plasma concentrations of ONO-5788 and ONO-ST1-641 required to supress serum GH level to a clinically meaningful degree in acromegaly patients were estimated to be 21.3 and 1.29 ng/mL. In experiments evaluating the effects on insulin and glucagon secretion in anaesthetised rats, ONO-5788, ONO-ST1-641, octreotide and pasireotide significantly inhibited glucagon secretion, but ONO-5788 and ONO-ST1-641 had no significant impact on insulin secretion at doses up to 10-fold higher than the dose required for either of these compounds to almost maximally inhibit GH secretion. In contrast, pasireotide and octreotide showed a significant inhibition of insulin secretion at equivalent doses of both ONO-5788 and ONO-ST1-641. A standard battery of preclinical safety and toxicology studies provided adequate evidence of safety to support commencement of a Phase I study in healthy volunteers to measure the safety, pharmacokinetics, and pharmacodynamics of ONO-5788. Conclusion: Orally administered ONO-5788 demonstrates potency as a SST2 agonist and has the potential to be a new treatment option for patients with acromegaly and NET.

A novel GH secretagogue, A233, exhibits enhanced growth activity and innate immune system stimulation in teleosts fish

In teleosts fish, secretion of GH is regulated by several hypothalamic factors that are influenced by the physiological state of the animal. There is an interaction between immune and endocrine systems through hormones and cytokines. GH in fish is involved in many physiological processes that are not overtly growth related, such as saltwater osmoregulation, antifreeze synthesis, and the regulation of sexual maturation and immune functions. This study was conducted to characterize a decapeptide compound A233 (GKFDLSPEHQ) designed by molecular modeling to evaluate its function as a GH secretagogue (GHS). In pituitary cell culture, the peptide A233 induces GH secretion and it is also able to increase superoxide production in tilapia head-kidney leukocyte cultures. This effect is blocked by preincubation with the GHS receptor antagonist [d-Lys(3)]-GHRP6. Immunoneutralization of GH by addition of anti-tilapia GH monoclonal antibody blocked the stimulatory effect of A233 on superoxide production. These experiments propose a GH-mediated mechanism for the action of A233. The in vivo biological action of the decapeptide was also demonstrated for growth stimulation in goldfish and tilapia larvae (P<0.001). Superoxide dismutase levels, antiprotease activity, and lectin titer were enhanced in tilapia larvae treated with this novel molecule. The decapeptide A233 designed by molecular modeling is able to function as a GHS in teleosts and enhance parameters of the innate immune system in the fish larvae.

Exercising in the cold inhibits growth hormone secretion by reducing the rise in core body temperature

Objective Ambient temperature alters exercise induced GH secretion. It is unknown whether temperature affects GH secretion at exercise intensities above the anaerobic threshold when other factors may override the relationship seen at lower intensities. Methods Design Cross-over study of ambient temperature on exercise induced GH in swimmers and rowers. Setting St Thomas Hospital, London. Subjects Ten healthy men (age 21.7 ± 0.8 yrs). Five swimmers and five rowers. Intervention Forty-minute exercise test at 105% of anaerobic threshold at room temperature (RT) and at 4 °C. Measurements Cutaneous and core body temperature. Serum GH concentration. Results Cutaneous body temperature increased during exercise at RT but decreased in the cold. Although core temperature rose in both settings, the rise was greater at RT ( p = 0.021). GH increased at both temperatures but the onset was delayed by the cold. Peak GH tended to be higher at RT (17.4 ± 3.6 μg/L vs. 9.5 ± 1.5 μg/L, p = 0.07). Total GH secretion was greater at RT (353.3 ± 99.1 μg min/L) than 4 °C (128.3 ± 21.0 μg min/L), p = 0.038. Change in core temperature correlated with log peak GH ( r = 0.66, p = 0.039) and log incremental GH ( r = 0.67, p = 0.032) when exercising at 4 °C. There was no difference between swimmers and rowers. Conclusions Exercise at 4 °C reduces GH secretion during exercise at intensities above the anaerobic threshold. A change in core body temperature may be one mechanism by which exercise induces GH secretion. The difference in GH between swimmers and rowers during their respective events relates to the conditions under which they compete.

Differential Regulation of GHRH-Receptor and GHS-Receptor Expression by Long-Term In Vitro Treatment of Ovine Pituitary Cells with GHRP-2 and GHRH

GH secretion is regulated by GHRH and somatostatin via actions on their specific receptors in pituitary somatotropes. Ghrelin and synthetic analogs, GHRPs, also stimulate GH release via GHS-receptors (GHS-R). To examine the long-term effect of GHRH and/or GHRP on somatotropes, primary cultured ovine somatotropes were treated with GHRH (10(-9) and 10(-8) M) and GHRP-2 (10(-8) and 10(-7) M) for up to 2 d. After treatment, culture medium was collected for GH assay, and total RNA was extracted for RT-PCR analysis. To evaluate cell cultures used in this report, somatotrope-enriched pituitary cells were challenged by 6 h GHRH and dexamethasone (DEX) treatment. As expected, GHRH significantly decreased, whereas DEX increased, the levels of GHRHR mRNA. Combined low doses of GHRH (10(-9) M) and GHRP-2 (10(-8) M) treatment for 24 h increased accumulated GH secretion, significantly more than that induced by high doses of GHRH (10(-8) M) and GHRP-2 (10(-7) M). While levels of GHRH-R mRNA increased, GHS-R mRNA levels were decreased by low doses of GHRH and GHRP-2 for 24 h. High doses of GHRH and/or GHRP-2 for 2 d did not increase GH secretion in the second day of treatment and reduced the level of GHRH-R mRNA. High doses of GHRP-2 treatment decreased the levels of both GHRH-R and GHS-R mRNA. Low doses of GHRH and/or GHRP-2 for 2 d increased the level of GHS-R mRNA without changing GHRH-R mRNA levels. Such treatment also increased ghrelin- (10(-9) M) or ghrelin/GHRH (10(-9) M)-induced GH secretion. These results suggest that low doses of GHRP-2 and GHRH prime somatotropes for stimulation by GHRH and ghrelin.

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.

Genomic structure and characterization of the 5'-flanking region of the human ghrelin gene.

Ghrelin, an endogenous ligand for the GH secretagogue receptor, induces GH secretion, food intake, and positive energy balance. Although ghrelin exhibits a variety of hormonal actions, the mechanisms regulating ghrelin expression and secretion remain unclear. To understand regulation of human ghrelin gene expression, we examined the genomic structure of approximately 5,000 bp of the 5'-flanking region of the human ghrelin gene. We performed rapid amplification of cDNA ends to estimate transcriptional start sites, indicating that there are two transcriptional initiation sites within the human ghrelin gene. Both transcripts were equally expressed in the human stomach, whereas the longer transcript was mainly expressed in a human medullary thyroid carcinoma (TT) cell line. Functional analysis using promoter-reporter constructs containing the 5'-flanking region of the gene indicated that the sequence residing within the -349 to -193 region is necessary for human ghrelin promoter function in TT cells. Within this region existed several consensus sequences for a number of transactivating regulatory proteins, including an E-box site. Destruction of this site decreased to 40% of the promoter activity. The upstream region of the promoter has two additional putative E-box sites, and site-directed mutagenesis suggested that these are also involved in promoter activation. Electrophoretic mobility shift assays demonstrated that the upstream stimulatory factor specifically bound to these E-box elements. These results suggest a potential role for upstream stimulatory factor transcription factors in the regulation of human ghrelin expression.

Photic stimulation inhibits growth hormone secretion in rats: a hypothalamic mechanism for transient entrainment.

It is well established that photic cues are used by the suprachiasmatic nucleus (SCN) to entrain circadian rhythms to the light/dark cycle, but the role of photic stimuli in the regulation of ultradian neuroendocrine rhythms is ill defined. In relation to the rhythms of GH secretion, recent studies have shown that nocturnal photic stimulation induces gene expression not only in the SCN but also in periventricular (PeN) somatostatin (SRIF) neurons. We have, therefore, investigated the effect of nocturnal photic stimulation on spontaneous and induced GH secretion in conscious rats. Nocturnal photic stimulation (lights on at 2400 h for 1 h) suppressed spontaneous GH secretion in male and female rats and reduced the GH response to SRIF withdrawal and iv injection of GH-releasing factor. A similar trough in GH secretion was also observed during the first hour of the normal light phase (0600 h). Using immunohistochemical analysis, we have also shown that expression of the transcription factor, Egr-1, is induced at the commencement of the light phase in the SCN, PeN, and medial preoptic nucleus. This effect is abolished by maintaining rats in the dark during this period. These data, together with our previous demonstration that 50% of SRIF-positive neurons in the PeN coexpress Egr-1 after photic stimulation, suggest that activation of SRIF neurons in the PeN may entrain the episodes of GH secretion to the dark/light interface. However, the absence of synchrony in GH pulses between animals by the second half of the light period suggests that this entrainment is transient.

Identification of somatostatin receptors controlling growth hormone and thyrotropin secretion in the chicken using receptor subtype-specific agonists.

Somatostatin (SRIH) functions as an endocrine mediator in processes such as growth, immune resistance and reproduction. Five SRIH receptors (sstr1-5) have been identified in mammals, where they are expressed in both the brain and peripheral tIssues. To study the specific function of each receptor subtype, specific agonists (ag1-5) have been synthesized. The high degree of homology between mammalian and avian SRIH receptors suggests that these agonists might also be used in chickens. In this paper we describe two in vitro protocols (static incubation and perifusion system) to identify the SRIH receptors controlling the secretion of GH and TSH from the chicken pituitary. We found that basal GH or TSH secretion were never affected when SRIH or an agonist (1 microM) were added. SRIH diminished the GH as well as the TSH response to TSH-releasing hormone (TRH; 100 nM) in both systems. Our results have indicated that the SRIH actions at the level of the pituitary are regulated through specific receptor subtypes. In both the static and flow incubations, ag2 lowered the GH response to TRH, whereas stimulated TSH release was diminished by both ag2 and ag5. Ag3 and ag4 tended to increase rather than decrease the responsiveness of both pituitary cell types to TRH in perifusion studies. Our data have indicated that SRIH inhibits chicken pituitary function through sstr2 and sstr5. Only sstr2 seems to be involved in the control of chicken GH release, whereas both sstr2 and sstr5 inhibit induced GH secretion in mammals. The possible stimulatory action of ag3 and ag4 may point towards a species-specific function of sstr3 and sstr4.

Is GHRH receptor essential to GHRP-2-induced GH secretion in primary cultured rat pituitary cells?

It is still controversial in rat whether the stimulation of GH secretion by GH-releasing peptides (GHRP) requires both GHRP receptor (GHRP-R) and GH-releasing hormone receptor (GHRH-R). To clarify this issue, we have postulated that inhibition of GHS-R or GHRH-R gene transcription should block GHRP-2-induced GH secretion. Rat pituitary cells were incubated for 3 days in the presence or absence of antisense 18-mer phosphorothiate oligonucleotides (ONs) complementary to the codon region of GHS-R or GHRH-R mRNAs. A significant decrease in GHRH-R and GHS-R mRNA levels was found in corresponding antisense-treated cells compared with the control cells treated with sense ON. Treatment with antisense GHS-R ON reduced (but not abolished) GHRP-2-induced GH secretion although GHRH-induced GH secretion was not altered. GHRH-stimulated GH secretion was totally abolished by the treatment with antisense GHRH-R ON, whereas GHRP-2 induced GH secretion was not affected. Treatment of cells with both GHS-R and GHRH-2 ONs however completely inhibited GHRH and GHRP-2-stimulated GH secretion. These results suggest that GHRH-R is vital for GHRH-induced GH secretion but only partially involved in GHRP-2-stimulated GH secretion under the condition of down-regulation of GHS-R gene transcription.

Blockade of the Growth Hormone (GH) Receptor Unmasks Rapid GH-Releasing Peptide-6-Mediated Tissue-Specific Insulin Resistance

The roles of GH and its receptor (GHR) in metabolic control are not yet fully understood. We studied the roles of GH and the GHR using the GHR antagonist pegvisomant for metabolic control of healthy nonobese men in fasting and nonfasting conditions. Ten healthy subjects were enrolled in a double blind, placebo-controlled study on the effects of pegvisomant on GHRH and GH-releasing peptide-6 (GHRP-6)-induced GH secretion before and after 3 days of fasting and under nonfasting conditions (n = 5). Under the condition of GHR blockade by pegvisomant in the nonfasting state, GHRP-6 (1 microg/kg) caused a increase in serum insulin (10.3 +/- 2.1 vs. 81.3 +/- 25.4 mU/L; P < 0.001) and glucose (4.2 +/- 0.3 vs. 6.0 +/- 0.6 mmol/L; P < 0.05) concentrations. In this group, a rapid decrease in serum free fatty acids levels was also observed. These changes were not observed under GHR blockade during fasting or in the absence of pegvisomant. We conclude that although these results were obtained from an acute study, and long-term administration of pegvisomant could render different results, blockade of the GHR in the nonfasting state induces tissue-specific changes in insulin sensitivity, resulting in an increase in glucose and insulin levels (indicating insulin resistance of liver/muscle), but probably also in an increase in lipogenesis (indicating normal insulin sensitivity of adipose tissue). These GHRP-6-mediated changes indicate that low GH bioactivity on the tissue level can induce changes in metabolic control, which are characterized by an increase in fat mass and a decrease in lean body mass. As a mechanism of these GHRP-6-mediated metabolic changes in the nonfasting state, direct nonpituitary-mediated GHRP-6 effects on the gastroentero-hepatic axis seem probable.

Modulatory effects of a synthetic steroid (tibolone) and estradiol on spontaneous and GH-RH-induced GH secretion in postmenopausal women

Objective: Since hormonal replacement therapy (HRT) affects plasma GH levels, the present study aimed to verify the effect of tibolone, a synthetic steroid, on modulating spontaneous and growth hormone releasing hormone (GH-RH) induced GH secretion. Methods: Postmenopausal women ( n = 30) were enrolled and randomly subdivided in three groups ( n = 10 each group): (1) treated with transdermal estradiol (50 μg) (Dermestrill, Rottapharm, Monza, Italy) biweekly; (2) treated with transdermal estradiol (100 μg) (Dermestrill, Rottapharm, Monza, Italy) biweekly; (3) treated with tibolone 2.5 mg/day (Livial, Organon Italia, Rome, Italy). Patients underwent a GH-RH test (1 μg/kg) and 15 of them underwent to a pulsatility study before and 5 weeks after treatment. Results: Mean(+S.E.M.) GH plasma levels increased in all patients after any type of HRT. GH response to GH-RH stimulation (expressed as maximal response to GH-RH or as delta value) was similar in the three groups while significant changes occurred in spontaneous pulsatile GH release. Tibolone and both dosages of transdermal estradiol significantly reduced GH pulse frequency and increased pulse amplitude. Conclusions: The reduced plasma GH levels observed during postmenopause are probably related to a reduced endogenous GH-RH and not to a reduced pituitary ability to respond to GH-RH. In addition tibolone, as well as transdermal estradiol, are effective in restoring the spontaneous GH episodic release.

Inhibition of GH releasing factor (GRF)-induced GH secretion by intraruminal infusion of volatile fatty acids (VFA) in sheep.

A VFA mixture solution containing acetate, propionate and butyrate (the molar ratio of acetate, propionate and n-butyrate = 61.7:24.3:14.0) was infused into the rumen at various rates (53.5, 107 and 214 mumol kg-1 min-1) over 6 h to examine the effects on basal and growth hormone-releasing factor (GRF, 0.25 micrograms kg-1)-induced increase in secretion of GH, insulin, glucagon and somatostatin (SRIF) in five castrated male sheep. Intraruminal infusion of the VFA mixture into the 18-h-fasted animals at the rates of 53.5, 107 and 214 mumol kg-1 min-1 finally raised the total intraruminal VFA concentration from 91.4 to 100.2 (P > 0.05), 175.9 (P < 0.05) and 234.5 (P < 0.05) mmol l-1, respectively. A preliminary experiment showed that an infusion rate of 107 mumol kg-1 min-1 mimics the postprandial increase in ruminal VFA. The basal plasma GH concentrations (2 to 4 h after the start of VFA infusion) and the area under the profiles for GH release in response to the intravenous GRF injection, which was done 4 h after the start of VFA infusion, were significantly decreased by the VFA infusion rates of 107 and 214 mumol kg-1 min-1. Furthermore, the VFA infusion noticeably increased basal plasma concentrations of insulin, but it scarcely changed the basal levels of glucagon, SRIF and glucose. From these results we conclude that an increase in the ruminal VFA concentration, even within the physiological range, would suppress GH secretion from the ovine anterior pituitary, and that the postprandial rise in the ruminal VFA concentration may be one of the factors normally suppressing GH secretion in sheep.

GH-deficient dw/dw rats and lit/lit mice show increased Fos expression in the hypothalamic arcuate nucleus following systemic injection of GH-releasing peptide-6.

In the rat, the synthetic GH secretagogue GH-releasing peptide (GHRP-6) acts centrally to activate a subpopulation of arcuate neurones as reflected by increased electrical activation and by the detection of Fos protein in cell nuclei. Since GHRP-6 also induces GH secretion via a direct action on the pituitary, we set out to determine whether the central actions of GHRP-6 are mediated by GH itself. First, we demonstrated that peripherally administered GHRP-6 induces Fos expression in the arcuate nucleus of GH-deficient animals (dw/dw rats and lit/lit mice). Secondly, in dw/dw rats, neither intracerebroventricular injection of 15 micrograms recombinant bovine GH nor 1 microgram recombinant human IGF-I resulted in an increase in the number of cells expressing Fos protein in the arcuate nucleus (or in any other hypothalamic structure studied). These results support our hypothesis that GHRP-6 has a central site and mechanism of action and provide evidence to suggest that the activation of arcuate neurones by GHRP-6 is not mediated by a central action of GH or IGF-I. Furthermore, since the lit/lit mouse pituitary does not release GH following GHRP-6 administration, our finding that the central actions of GHRP-6 remain intact in these animals suggests the possible existence of two subpopulations of putative GHRP-6 receptors.

Effect of galanin on basal and stimulated secretion of prolactin, gonadotropins, thyrotropin, adrenocorticotropin and cortisol in humans.

Galanin enhances both baseline and growth hormone-releasing hormone (GHRH)-induced GH secretion both in animals and in man. Although galanin has a clear influence on the secretion of other anterior pituitary hormones in animals, in man it increases prolactin (PRL) slightly but does not affect spontaneous thyrotropin (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH) or adrenocorticotropin (ACTH) secretion. The aim of our study was to verify the effect of galanin on basal and releasing hormone-stimulated release of gonadotropins, PRL, TSH, ACTH and cortisol secretion. As GH release has been shown to be inhibited by corticotropin-releasing hormone (CRH), we also studied the effect of CRH on galanin-stimulated GH increase. The effect of porcine galanin (15 micrograms/kg iv infused in 60 min) alone and in combination with thyrotropin-releasing hormone (TRH, 200 micrograms iv bolus), CRH (100 micrograms iv bolus) and gonadotropin-releasing hormone (GnRH, 100 micrograms iv bolus) on GH, PRL, TSH, ACTH, cortisol, FSH and LH secretion in seven normal young women (aged 25-30 years) was studied. Galanin infusion caused an increase in serum GH levels (p < 0.02) but failed to modify significantly the spontaneous PRL, LH, FSH, TSH, ACTH and cortisol secretion. The combined administration of TRH, GnRH and CRH caused a significant increase in PRL (p < 0.02), LH (p < 0.02), FSH (p < 0.02), TSH (p < 0.02), ACTH (p < 0.02) and cortisol (p < 0.05), but not in GH levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Blocked growth hormone-releasing peptide (GHRP-6)-induced GH secretion and absence of the synergic action of GHRP-6 plus GH-releasing hormone in patients with hypothalamopituitary disconnection: evidence that GHRP- 6 main action is exerted at the hypothalamic level

Blocked growth hormone-releasing peptide (GHRP-6)-induced GH secretion and absence of the synergic action of GHRP-6 plus GH-releasing hormone in patients with hypothalamopituitary disconnection: evidence that GHRP- 6 main action is exerted at the hypothalamic level

Blocked growth hormone-releasing peptide (GHRP-6)-induced GH secretion and absence of the synergic action of GHRP-6 plus GH-releasing hormone in patients with hypothalamopituitary disconnection: evidence that GHRP-6 main action is exerted at the hypothalamic level.

GH-releasing peptide (GHRP-6; His-D Trp-Ala-Trp-D Phe-Lys-NH2) is a synthetic compound that releases GH in a specific and dose-related manner through mechanisms and a point of action that are mostly unknown but different from those of GHRH. In man, GHRP-6 is more efficacious than GHRH, and a striking synergistic action on GH release is observed when GHRP-6 and GHRH are administered simultaneously. Based on such a synergistic action, it has been hypothesized that GHRP-6 acts through a double mechanism by actions exerted both at the pituitary and hypothalamic levels. The aim of the present study was 2-fold: 1) to further characterize the mechanism of action and synergistic effects of GHRP-6; and 2) to study its action in patients with hypothalamopituitary disconnection. Twelve patients with different neuroendocrine pathologies leading to a state of hypothalamopituitary disconnection (functional stalk section) and 11 age- and sex-matched normal controls were studied. Each subject underwent 3 tests on separate occasions, being challenged with GHRH (100 micrograms, i.v.), GHRP-6 (90 micrograms, i.v.), or GHRH plus GHRP-6. GH was analyzed as the area under the curve (mean +/- SE, micrograms per L/120 min). In normal subjects GH secretion was 483.7 +/- 99.2 after GHRH, 1434.8 +/- 393.0 after GHRP-6, and 3771.5 +/- 399.6 after GHRH plus GHRP-6; the level of GH secreted after GHRH plus GHRP-6 treatment was significantly (P < 0.05) higher than after the arithmetic sum of GH levels after both compounds administered separately. In the group of patients with hypothalamopituitary disconnection, the level of GH secreted after GHRH was similar to that in controls (423.4 +/- 62.8); however, a complete blockade was observed after GHRP-6 (97.3 +/- 7.9), significantly (P < 0.05) lower than after GHRH as well as lower than the GHRP-6-induced GH release in control subjects (P < 0.01). After GHRH plus GHRP-6, the patients with hypothalamopituitary disconnection showed severely reduced secretion (745.3 +/- 67.6; P < 0.01 vs. controls), a value that was not significantly different from the arithmetic addition of levels produced by both compounds administered separately.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of growth hormone release after the combined administration of GHRH and GHRP-6 in patients with Cushing's syndrome.

In patients with Cushing's syndrome there is a blunted GH response to all types of stimuli. Although inferential data point towards a direct perturbation in the pituitary exerted by glucocorticoids, the basic mechanism is unknown. His-D-TRP-ALA-TRP-D-Phe-Lys-NH2 (GHRP-6) is a synthetic hexapeptide which releases GH by a direct pituitary effect through receptors other than GHRH receptors. Furthermore, the combined administration of GHRH and GHRP-6 is able to induce a large GH discharge even in some pathological states such as obesity, associated with GH blockade. To gain further insight into the disrupted mechanisms of GH secretion, Cushing's syndrome patients were challenged with either GHRH, GHRP-6 or GHRH together with GHRP-6. A group of normal subjects was included for control purposes. Three different tests were undertaken: (a) GHRH 100 micrograms i.v.; (b) GHRP-6 100 micrograms i.v. and (c) GHRH plus GHRP-6 100 micrograms i.v. of each; administered to each subject on different days, at least 4 days apart. Ten patients (8 women, 2 men) with untreated Cushing's syndrome, 9 Cushing's disease and 1 adrenal adenoma. Five healthy volunteers (3 women, 2 men) of similar ages served as a control group. Plasma GH levels were measured by immunoradiometric assay. The areas under the curve (AUC) of GH secretion (mean +/- SEM in mU/I/120 min) in the control subjects after each test were: GHRH, 1420 +/- 330; GHRP-6, 2278 +/- 290 and GHRH plus GHRP-6, 7332 +/- 592 (P < 0.05 vs each compound alone). The AUCs for Cushing's syndrome patients were: GHRH, 248 +/- 165; GHRP-6 530 +/- 170 and for GHRH plus GHRP-6, 870 +/- 258 (P < 0.05 vs GHRH alone). After the combined stimulus only one out of the ten patients with hypercortisolism showed a GH peak over 20 mU/I, while all the controls had a peak over 84 mU/I. GHRP-6 induced GH secretion as well as the GH discharge elicited by GHRH and GHRP-6 are considerably reduced in Cushing's syndrome patients. This suggests that the main impairment of GH secretion in that pathological state resides at pituitary level.

Influence of sex, age and adrenergic pathways on the growth hormone response to GHRP‐6

His-dTrp-Ala-Trp-dPhe-Lys-NH2 (GHRP-6) is a synthetic compound that releases GH in a dose-related and specific manner in several species including man. To further characterize the effects of GHRP-6 on GH secretion in normal human subjects, we assessed plasma GH levels following GHRP-6 administration in normal male adult subjects, normal female adult subjects at different stages of their menstrual cycle and in normal prepubertal male and female children. We also studied the influence of adrenergic pathways on GHRP-6 induced GH secretion in normal adult male subjects. In a group of eight volunteers the following tests were carried out: GHRP-6 alone (1 microgram/kg i.v. at 0 minutes); propranolol (40 mg p.o. at -30 minutes) plus GHRP-6; and prazosin (3 mg p.o. at -120 minutes) plus GHRP-6. Another group of eight volunteers were studied with GHRP-6 as above; clonidine alone (300 mg p.o. at -60 minutes); and clonidine plus GHRP-6. A group of nine women were studied with 1 microgram/kg i.v. of GHRP-6 at 0 minutes, at different stages of their menstrual cycle. Finally, 12 children were studied with GHRP-6 using the same dose and methods as above. Twenty-five normal adult subjects (16 male and nine female) and 12 normal prepubertal children (six male and six female) wer studied after giving informed consent. Plasma GH levels were measured by radioimmunoassay. No differences in GH responses to GHRP-6 were found between children and normal adult male or female subjects at different stages of their menstrual cycle. Administration of propranolol and clonidine did not modify the GH responses to GHRP-6 in male adults. In contrast, prazosin administration induced an increase in plasma GH levels that was statistically different from that of GHRP-6 alone (. < 0.05 between area under curve). GHRP-6 exerts a potent stimulatory effect on GH secretion in adults and children. Its effects, at least at the dose studied, are independent of sex and age. Noradrenergic pathways through alpha 2 adrenergic receptors are unlikely to influence this response.

Effect of arginine on the GHRH-stimulated GH secretion in patients with hyperthyroidism.

Patients with hyperthyroidism have reduced GH responses to pharmacological stimuli and reduced spontaneous nocturnal GH secretion. The stimulatory effect of arginine on GH secretion has been suggested to depend on a decrease in hypothalamic somatostatin tone. The aim of our study was to evaluate the effects of arginine on the GH-releasing hormone (GHRH)-stimulated GH secretion in patients with hyperthyroidism. Six hyperthyroid patients with recent diagnosis of Graves' disease [mean age +/- SEM, 39.2 +/- 1.4 years; body mass index (BMI) 22 +/- 0.4 kg/m2] and 6 healthy nonobese volunteers (4 males, 2 females; mean age +/- SEM, 35 +/- 3.5 years) underwent two experimental trials at no less than 7-day intervals: GHRH (100 micrograms, i.v.)-induced GH secretion was evaluated after 30 min i.v. infusion of saline (100 ml) or arginine (30 g) in 100 ml of saline. Hyperthyroid patients showed blunted GH peaks after GHRH (13.2 +/- 2.9 micrograms/l) as compared with normal subjects (23.8 +/- 3.9 micrograms/l, p < 0.05). GH peaks after GHRH were only slightly enhanced by arginine in hyperthyroid subjects (17.6 +/- 2.9 micrograms/l), whereas, in normal subjects, the enhancement was clear cut (36.6 +/- 4.4 micrograms/l; p < 0.05). GH values after arginine + GHRH were still lower in hyperthyroid patients with respect to normal subjects. Our data demonstrate that arginine enhances but does not normalize the GH response to GHRH in patients with hyperthyroidism when compared with normal subjects. We hypothesize that hyperthyroxinemia may decrease GH secretion, both increasing somatostatin tone and acting directly at the pituitary level.