Physiology of ghrelin and related peptides

Transcriptomic analysis of the HPG axis-related tissues reveals potential candidate genes and regulatory pathways associated with testicular size in Hu sheep

Testicular size is an excellent proxy for selecting high-fertility rams. The hypothalamus-pituitary-gonadal (HPG) axis plays an important role in regulating reproductive capacity in vertebrates, while key genes and regulatory pathways within the HPG axis associated with testicular size remain largely unknown in sheep. This study comprehensively compared the transcriptomic profiles in the hypothalamus, pituitary and testis of rams after sexual maturity between the large-testis group (LTG, testicular weight = 454.29 ± 54.24 g) and the small-testis group (STG, testicular weight = 77.29 ± 10.76 g). In total, 914, 795 and 10518 differentially expressed genes (DEGs) were identified in the hypothalamus, pituitary and testis between LTG and STG, respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed that these DEGs were mainly involved in the biological processes of reproduction, biological regulation, and development process. Notably, the neuroactive ligand-receptor interaction and cAMP signaling pathways, commonly enriched by the DEGs in the hypothalamus and pituitary between two groups, were considered as two key signal pathways regulating testicular development through the HPGs axis. Weighted gene co-expression network analysis (WGCNA) identified two modules that were significantly associated with testicular size, and 97 key genes were selected with high module membership (MM) and gene significance (GS) in these two modules. Finally, a protein-protein interaction (PPI) network was constructed, and ten genes with the highest degree were represented as hub genes, including FOS, NPY, SST, F2, AGT, NTS, OXT, EDN1, VIP and TAC1. Taken together, these results provide new insights into the molecular mechanism underlying the HPG axis regulating testicular size of Hu sheep.

An insight into the multifunctional role of ghrelin and structure activity relationship studies of ghrelin receptor ligands with clinical trials

Ghrelin is a multifunctional gastrointestinal acylated peptide, primarily synthesized in the stomach and regulates the secretion of growth hormone and energy homeostasis. It plays a central role in modulating the diverse biological, physiological and pathological functions in vertebrates. The synthesis of ghrelin receptor ligands after the finding of growth hormone secretagogue developed from Met-enkephalin led to reveal the endogenous ligand ghrelin and the receptors. Subsequently, many peptides, small molecules and peptidomimetics focusing on the ghrelin receptor, GHS-R1a, were derived. In this review, the key features of ghrelin's structure, forms, its bio-physiological functions, pathological roles and therapeutic potential have been highlighted. A few peptidomimetics and pseudo peptide synthetic perspectives have also been discussed to make ghrelin receptor ligands, clinical trials and their success.

Effects of poor maternal nutrition during gestation on ewe and offspring plasma concentrations of leptin and ghrelin

Poor maternal nutrition during gestation can negatively affect offspring growth, development, and health. Leptin and ghrelin, key hormones in energy homeostasis and appetite control, may mediate these changes. We hypothesized that restricted- and over-feeding during gestation would alter plasma concentrations of leptin and ghrelin in ewes and offspring. Pregnant ewes (n = 37) were fed 1 of 3 diets starting on d 30 ± 0.02 of gestation until necropsy at d 135 of gestation or parturition: restricted- [RES; 60% National Research Council (NRC) requirements for total digestible nutrients, n = 13], control- (CON; 100% NRC, n = 11), or over-fed (OVER; 140% NRC, n = 13). Blood samples were collected from pregnant ewes at days 20, 30, 44, 72, 100, 128, and 142 of gestation. Offspring blood samples were collected within 24 h after birth (n = 21 CON, 25 RES, 23 OVER). Plasma leptin and ghrelin concentrations were determined by RIA. Ewe data were analyzed using the MIXED procedure in SAS with ewe as the repeated subject. Offspring data were analyzed using the MIXED procedure. Correlations between BW and leptin and ghrelin concentrations were identified using PROC CORR. At d 100, RES (5.39 ± 2.58 ng/mL) had decreased leptin concentrations compared with OVER (14.97 ± 2.48 ng/mL; P = 0.008) and at d 128, RES (6.39 ± 2.50 ng/mL) also had decreased leptin concentrations compared with OVER (13.61 ± 2.47 ng/mL; P = 0.04). At d 142, RES (0.26 ± 0.04 ng/mL) had increased ghrelin concentrations compared with CON (0.15 ± 0.04 ng/mL; P = 0.04). Leptin and ghrelin concentrations were also altered between days of gestation within a dietary treatment. In CON ewes, plasma concentrations of leptin were increased at d 30 (19.28 ± 7.43 ng/mL) compared with d 44 (5.20 ± 3.10 ng/mL; P = 0.03), and the plasma concentrations of ghrelin at d 128 (0.20 ± 0.03 ng/mL) were increased compared with d 30 (0.16 ± 0.03 ng/mL; P = 0.01) and d 100 (0.17 ± 0.03 ng/mL; P = 0.04). Maternal diet did not alter plasma ghrelin or leptin concentrations in the offspring (P > 0.50). There were no strong, significant correlations between ewe BW and leptin (r < 0.33; P > 0.06) or ghrelin (r > -0.47; P > 0.001) concentrations or lamb BW and leptin or ghrelin concentrations (r > -0.32, P > 0.06). Maternal alterations in circulating leptin and ghrelin may program changes in energy balance that could result in increased adiposity in adult offspring. Alterations in energy homeostasis may be a mechanism behind the long-lasting changes in growth, body composition, development, and metabolism in the offspring of poorly nourished ewes.

miR-709 inhibits GHRP6 induced GH synthesis by targeting PRKCA in pituitary

Pituitary growth hormone (GH) plays an essential role in processes of organism growth and metabolism. MicroRNA (miRNA) could also participate in diverse biological processes. However, the role of miRNA in the regulation of pituitary GH during the growth process remains unclear. In this study, we firstly confirmed that the second highly expressed pituitary miRNA (miR-709) significantly inhibited the GH synthesis and suppressed the viability of GH3 cells. The bioinformatics analysis and dual luciferase report system were used to ascertain the PRKCA is the direct target gene of miR-709, which is the coding gene of PKCα. Then the transcription and translation levels of Prkca were obvious reduced by the over-expression of miR-709 in GH3 cells, followed by the inhibition of the transcription factor (CREB1) of Gh1 gene and the ERK1/2 signaling pathway or the possible cross-talk signaling pathway (cAMP/PKA signaling pathway) detected by western blot, suggesting that ERK1/2 maybe an important factor involved in the GH3 cell viability mediated by PKCα. At last, GHRP6 increased PKCα and GH expression but reduced miR-709 expression in vitro and vivo assays, and this conclusion was further confirmed by the result of GHRP6 attenuated the inhibition of miR-709 on GH expression. These findings will provide new molecular mechanism on the regulation of pituitary GH.

Identification and Characterization of a New Series of Ghrelin O-Acyl Transferase Inhibitors

Ghrelin O-acyl transferase (GOAT; MBOAT4) catalyzes O-acylation at serine-3 of des-acyl ghrelin. Acyl ghrelin is secreted by stomach X/A-like cells and plays a role in appetite and metabolism. Therefore, GOAT has been expected to be a novel antiobesity target because it is responsible for acyl ghrelin production. Here, we report homogeneous time-resolved fluorescence (HTRF) and enzyme-linked immunosorbent assay (ELISA) methods utilizing human GOAT-expressing microsomes as a novel high-throughput assay system for the discovery of hit compounds and optimization of lead compounds. Hit compounds exemplified by compound A (2-[(2,4-dichlorobenzyl)sulfanyl]-1,3-benzoxazole-5-carboxylic acid) were identified by high-throughput screening using the HTRF assay and confirmed to have GOAT inhibitory activity using the ELISA. Based on the hit compound information, the novel lead compound (compound B, (4-chloro-6-{[2-methyl-6-(trifluoromethyl)pyridin-3-yl]methoxy}-1-benzothiophen-3-yl)acetic acid) was synthesized and exhibited potent GOAT inhibition with oral bioavailability. Both the hit compound and lead compound showed octanoyl-CoA competitive inhibitory activity. Moreover, these two compounds decreased acyl ghrelin production in the stomach of mice after their oral administration. These novel findings demonstrate that GOAT is a druggable target, and its inhibitors are promising antiobesity drugs.

The Impact of Ghrelin in Metabolic Diseases: An Immune Perspective

Obesity and insulin resistance have reached epidemic proportions. Obesogenic conditions are associated with increased risk for the development of other comorbidities and obesity-related diseases. In metabolic disorders, there is chronic low-grade inflammation induced by the activation of immune cells, especially in metabolic relevant organs such as white adipose tissue (WAT). These immune cells are regulated by environmental and systemic cues. Ghrelin is a peptide secreted mainly by X/A-like gastric cells and acts through the growth hormone secretagogue receptor (GHS-R). This receptor is broadly expressed in the central nervous system (CNS) and in several cell types, including immune cells. Studies show that ghrelin induces an orexigenic state, and there is increasing evidence implicating an immunoregulatory role for ghrelin. Ghrelin mainly acts on the innate and adaptive immune systems to suppress inflammation and induce an anti-inflammatory profile. In this review, we discuss the immunoregulatory roles of ghrelin, the mechanisms by which ghrelin acts and potential pharmacological applications for ghrelin in the treatment of obesity-associated inflammatory diseases, such as type 2 diabetes (T2D).

Fundectomy-Evoked Osteopenia in Pigs Is Mediated by the Gastric-Hypothalamic-Pituitary Axis

The aim of the study was to determine the effects of gastric impairment in pigs on the axial and peripheral skeletal system properties and to test the hypothesis that fundectomy-evoked osteopenia is related to disturbed gastric-hypothalamic-pituitary axis function. Forty-day-old male piglets were subjected to experimental fundectomy (FX group, n = 6) to induce osteopenia, while sham operation was performed in the controls (SHO group, n = 6). At the age of 8 months, serum samples were collected, and the animals were sacrificed to obtain lumbar vertebrae (L1–L6) and right humerus for analysis. Using quantitative computed tomography (QCT) and dual-energy x-ray absorptiometry (DEXA) methods, bone mineral density and bone mineral content of the vertebrae and humerus were measured. The compression and three-point bending tests were applied to determine mechanical properties of lumbar vertebrae and humerus, respectively. Furthermore, geometric properties of humerus were assessed. Serum concentrations of ghrelin, growth hormone (GH), insulin-like growth factor-1 (IGF-1), and selected macro- and microelements were also determined. Performed fundectomy decreased body weight in pigs by 66% compared with pair-fed sham operated pigs ( P < 0.0001). Bone weight, bone mineral density, and bone mineral content of the lumbar vertebrae and humerus were significantly decreased in the fundectomized pigs ( P < 0.01). Mechanical parameters of the lumbar spine and humerus were decreased after the fundectomy, as well. Serum concentrations of ghrelin, GH, and IGF-1 were lowered by 74.4%, 90.6%, and 54.6% in the fundectomized pigs, respectively (all P < 0.001). Moreover, the serum concentrations of calcium, magnesium, iron and copper in the fundectomized animals were significantly decreased by 15.5%, 45.3%, 26.7%, and 26.2%, respectively ( P ≤ 0.05). In conclusion, the results obtained showed that both the disturbed gastric-hypothalamic-pituitary axis function and impaired mineral metabolism are associated with development of postfundectomy osteopenia of axial and peripheral skeleton in pigs.

Ghrelin attenuates the growth of HO-8910 ovarian cancer cells through the ERK pathway

Ovarian cancer is one of the most common causes of death from gynecologic tumors and is an important public health issue. Ghrelin is a recently discovered bioactive peptide that acts as a natural endogenous ligand of the growth hormone secretagogue receptor (GHSR). Several studies have identified the protective effects of ghrelin on the mammalian reproductive system. However, little research has been done on the effects of ghrelin on ovarian cancer cells, and the underlying mechanisms of these effects. We sought to understand the potential involvement of mitogen-activated protein kinases (MAPKs) in ghrelin-mediated inhibition of growth of the ovarian line HO-8910. We applied different concentrations of ghrelin and an inhibitor of the ghrelin receptor (D-Lys3-GHRP-6) to HO-8910 cells and observed the growth rate of cells and changes in phosphorylation of the MAPKs ERK1/2, JNK and p38. We discovered that ghrelin-induced apoptosis of HO-8910 cells was though phosphorylated ERK1/2, and that this phosphorylation (as well as p90rsk phosphorylation) was mediated by the GHSR. The ERK1/2 pathway is known to play an essential part in the ghrelin-mediated apoptosis of HO-8910 cells. Hence, our study suggests that ghrelin inhibits the growth of HO-8910 cells primarily through the GHSR/ERK pathway.

Endocrine regulation of compensatory growth in fish

Compensatory growth (CG) is a period of accelerated growth that occurs following the alleviation of growth-stunting conditions during which an organism can make up for lost growth opportunity and potentially catch up in size with non-stunted cohorts. Fish show a particularly robust capacity for the response and have been the focus of numerous studies that demonstrate their ability to compensate for periods of fasting once food is made available again. CG is characterized by an elevated growth rate resulting from enhanced feed intake, mitogen production, and feed conversion efficiency. Because little is known about the underlying mechanisms that drive the response, this review describes the sequential endocrine adaptations that lead to CG; namely during the precedent catabolic phase (fasting) that taps endogenous energy reserves, and the following hyperanabolic phase (refeeding) when accelerated growth occurs. In order to elicit a CG response, endogenous energy reserves must first be moderately depleted, which alters endocrine profiles that enhance appetite and growth potential. During this catabolic phase, elevated ghrelin and growth hormone (GH) production increase appetite and protein-sparing lipolysis, while insulin-like growth factors (IGFs) are suppressed, primarily due to hepatic GH resistance. During refeeding, temporal hyperphagia provides an influx of energy and metabolic substrates that are then allocated to somatic growth by resumed IGF signaling. Under the right conditions, refeeding results in hyperanabolism and a steepened growth trajectory relative to constantly fed controls. The response wanes as energy reserves are re-accumulated and homeostasis is restored. We ascribe possible roles for select appetite and growth-regulatory hormones in the context of the prerequisite of these catabolic and hyperanabolic phases of the CG response in teleosts, with emphasis on GH, IGFs, cortisol, somatostatin, neuropeptide Y, ghrelin, and leptin.

Age-related changes in gene expression of the growth hormone secretagogue and growth hormone-releasing hormone receptors in Holstein-Friesian cattle

Growth hormone secretion from the anterior pituitary gland is controlled by interactions between three hormone receptors, between GHRH and GHRH receptor (GHRH-R), between ghrelin and growth hormone secretagogue receptor (GHS-R1a), and between somatostatin and somatostatin receptors in the hypothalamus and anterior pituitary gland. Ghrelin-GHS-R1a is involved in many important functions, including GH secretion and appetite. We investigated age-related changes in the expressions of GHS-R1a, GHS-R1b (the truncated-type receptor), and GHRH-R mRNAs by real-time reverse transcription-PCR using 16 tissues, leukocytes, oocytes, and cumulus cells in Holstein-Friesian cattle. The tissue samples were divided into three age classes: 1) 19 to 26 d of age (preweaning calves), 2) 2 mo to 6.5 mo of age (postweaning calves), and 3) 3.2 to 8.1 yr of age (cows). The GHS-R1a mRNA was highly (P < 0.05) expressed in the arcuate nucleus, pituitary gland, and liver compared with that of the other tissues in all age classes. Expression of GHS-R 1a mRNA in the arcuate nucleus of postweaning calves was > 10-fold greater (P < 0.01) than those of preweaning calves and cows, and its expression level was the greatest (P < 0.01) in all tissues examined in age group 2. GHS-R1a and GHRH-R mRNA expressions in the pituitary gland of preweaning calves tended to be greater (P < 0.20 and P < 0.17, respectively) than those of postweaning calves and cows. GHS-R1b mRNA expression was detected in all tissues examined, and abundance was greater (P < 0.05) in the pancreas, pituitary gland, spleen, arcuate nucleus, adipose tissue, and leukocyte compared with that of the other tissues examined in age group 3. Interestingly, a relatively large animal-to-animal variation was observed in pancreas GHS-R 1b mRNA expression. The GHRH-R mRNA was markedly increased (P < 0.01) in the pituitary gland in all age groups compared with that of the other tissues. GHRH-R mRNA abundance in the arcuate nucleus, pituitary gland, liver, spleen, adipose tissue, and heart of preweaning calves tended to be greater than those of postweaning calves and cows. The GHRH-R mRNA was not detected in the mammary gland and adipose tissue of nonlactating cows.

Intracerebroventricular and intravenous administration of growth hormone secretagogue L-692,585, somatostatin, neuropeptide Y and galanin in pig: dose-dependent effects on growth hormone secretion

Central regulation of growth hormone (GH) secretion by the GH secretagogue, L-692,585 (585), was determined in Yorkshire barrows (40-45kg BW) with intracerebroventricular (icv) stainless steel cannulas placed by stereotaxic coordinates and indwelling external jugular vein (iv) cannulas for injecting 585 or saline during 3h serial blood sampling. Dose-dependent effects of 585 were determined by icv injections of saline vehicle, 3, 10, and 30microg/kg BW by once daily increment. A switchback study of iv and icv 585 treatment determined central and peripheral regulation of GH secretion by the secretagogue at 30microg/kg BW. When administered icv, 585 increased GH concentration in a dose-dependent manner, with a return to baseline by 60min. GH secretion was attenuated by increased numbers of icv 585 injections (p<0.05); however, it was not affected by increased numbers of iv 585 injections. Icv administration of somatostatin (SRIF) decreased (p<0.05) GH secretion compared with saline-treated controls, and decreased (p<0.05) peak GH response when given in combination with 585 as compared with 585 alone. Porcine galanin (pGAL) modestly increased (p<0.05) GH levels compared with saline controls, but when given icv in combination with 585 peak GH response was lower (p<0.05) compared with 585 alone. Porcine neuropeptide Y (pNPY) administered icv was without effect on GH levels compared with saline controls and decreased (p<0.05) peak GH response when given in combination with 585 as compared with 585 alone. The pharmacological actions by icv administration indicate that the GH secretagogue and neuropeptides act at the level of both porcine pituitary and hypothalamus.

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.

Role for des-acyl ghrelin in the responsiveness of plasma hormones and metabolites to ghrelin in Holstein steers

Gastric-derived peptide hormone ghrelin is known for its potent growth hormone (GH) stimulatory effects. The acyl-modification on N-terminal Ser(3) residue is reported to be important to stimulate the ghrelin receptor, GH secretagogue-receptor type1a (GHS-R1a). However, major portion of circulating ghrelin lacks in acylation, and some biological properties of des-acyl ghrelin have been reported in monogastric animals. In the present study, the responsiveness of plasma hormones and metabolites to ghrelin in steers was characterized, and role for des-acyl ghrelin in these changes was investigated. The repeated intravenous administrations of bovine ghrelin (1.0 microg/kg BW) every 2h for 8h to Holstein steers significantly increased the plasma acylated ghrelin, total ghrelin, GH, insulin and NEFA levels. The GH responses in peak values and area under the curves (AUCs) were attenuated by repeated injections of ghrelin, however, the responses of plasma total ghrelin were similar. Plasma insulin AUC decreased after fourth injection of ghrelin while plasma NEFA AUCs gradually increased by repeated injections of ghrelin. Pretreatment of des-acyl ghrelin (10.0 microg/kg BW) 5 min prior to the single injection of ghrelin (1.0 microg/kg BW) did not affect the ghrelin-induced hormonal changes. Moreover, the responses of plasma GH to bovine and porcine ghrelin, which differ in C-terminal amino acid residues, were similar in calves. These data show that (1) GH release was attenuated by repeated administration of ghrelin, (2) ghrelin regulates glucose and fatty acid metabolism probably via different pathway, and (3) des-acyl ghrelin is unlikely the antagonist for ghrelin to induce endocrine effects in Holstein steers.

Ghrelin induces growth hormone secretion via a nitric oxide/cGMP signalling pathway

The presence of ghrelin and its receptor, growth hormone (GH) secretagogue receptor, in the hypothalamus and pituitary, and its ability to stimulate GH release in vivo and in vitro, strongly support a significant role for this peptide in the control of somatotroph function. We previously demonstrated that ghrelin elicits GH secretion directly in somatotrophs by activating two major signalling cascades, which involve inositol phosphate and cAMP. In as much as nitric oxide (NO) and its mediator cGMP have been recently shown to contribute substantially to the response of somatotrophs to key regulatory hormones, including GH-releasing hormone, somatostatin and leptin, we investigated the possible role of this signalling pathway in ghrelin-induced GH release in vitro. Accordingly, cultures of pituitary cells from prepuberal female pigs were challenged with ghrelin (10(-8) m, 30 min) in the absence or presence of activators or blockers of key steps of the NO synthase (NOS)/NO/guanylate cyclase (GC)/cGMP route and GH secretion was measured. Two distinct activators of the NO route, S-nitroso-N-acetylpenicillamine (SNAP) (5 x 10(-4) m) and L-arginine methyl ester hydrochloride (L-AME) (10(-3) m), comparably stimulated GH secretion when applied alone. The presence of L-AME enhanced ghrelin-stimulated GH secretion, whereas SNAP did not alter its effect. Conversely, two different NOS/NO pathway inhibitors, N(w)-nitro-L-arginine methyl ester hydrochloride (10(-5) m) or haemoglobin (20 microg/ml), similarly blocked ghrelin-induced (but not basal) GH release, thus indicating that NO contributes critically to ghrelin action in somatotrophs. Moreover, incubation with a permeable cGMP analogue, 8-Br-cGMP (10(-8) m) stimulated GH secretion, but did not modify the stimulatory action of ghrelin, suggesting that cGMP could mediate the action of NO. Indeed, inhibition of GC by 10 microm LY-53,583 did not alter basal GH secretion but abolished the GH-releasing action of ghrelin. Taken together, our results provide novel evidence indicating that ghrelin requires activation of the NOS/NO route, and its subsequent GC/cGMP signal transduction pathway, as necessary steps to induce GH secretion from somatotrophs.

Differential involvement of nitric oxide signaling in dopamine and PACAP stimulation of growth hormone release in goldfish

Previous studies in goldfish pituitary cells have shown that nitric oxide synthase (NOS)/nitric oxide (NO) signaling is involved in mediating the growth hormone (GH) release response to gonadotropin-releasing hormones. In this study, the involvement of this signaling pathway in mediating the action of two cAMP-mobilizing neuroendocrine stimulators of GH release, pituitary adenylate cyclase-activating polypeptide (PACAP) and dopamine (DA), was investigated in cell column perifusion experiments with primary cultures of dispersed pituitary cells. GH responses to PACAP were unaffected by three NOS inhibitors, aminoguanidine hemisulfate, 1400W and 7-nitroindazole (7-Ni). PACAP-stimulated GH release was also not reduced by two NO scavengers, rutin hydrate and PTIO, but NO-donor sodium nitroprusside (SNP)-elicited GH release was additive to the GH response to PACAP. In contrast, DA-induced GH secretion was reduced by 7-Ni, rutin hydrate and PTIO while not being additive to the GH response induced by SNP. These results indicate that although both PACAP and DA stimulation of acute GH release involve activation of adenylate cyclase/cAMP, DA- but not PACAP-signaling also utilizes the NOS/NO second messenger system.

Combined administration of ghrelin and GHRH synergistically stimulates GH release in Holstein preweaning calves

Ghrelin is a gut peptide which participates in growth regulation through its somatotropic, lipogenic and orexigenic effects. Synergism of ghrelin and growth hormone-releasing hormone (GHRH) on growth hormone (GH) secretion has been reported in humans and rats, but not in domestic animals in vivo. In this study, effects of a combination of ghrelin and GHRH on plasma GH and other metabolic parameters, and changes in plasma active and total ghrelin levels were studied in Holstein bull calves before and after weaning. Six calves were intravenously injected with vehicle (0.1% BSA-saline), ghrelin (1 microg/kg BW), GHRH (0.25 microg/kg BW) or a combination of ghrelin plus GHRH at the age of 5 weeks and 10 weeks (weaning at 6 weeks of age). Ghrelin stimulated GH release with similar potency as GHRH and their combined administration synergistically stimulated GH release in preweaning calves. After weaning, GH responses to ghrelin and GHRH became greater compared with the values of preweaning calves, but a synergistic effect of ghrelin and GHRH was not observed. The GH areas under the concentration curves for 2h post-injection were greater in weaned than in preweaning calves (P<0.05) if ghrelin or GHRH were injected alone, but were similar if ghrelin and GHRH were injected together. Basal plasma active and total ghrelin levels did not change around weaning, but transiently increased after ghrelin injection. Basal plasma insulin, glucose and non-esterified fatty acid levels were reduced after weaning, but no changes by treatments were observed. In conclusion, ghrelin and GHRH synergistically stimulated GH release in preweaning calves, but this effect was lost after weaning.

Evidence that ghrelin is as potent as growth hormone (GH)-releasing hormone (GHRH) in releasing GH from primary pituitary cell cultures of a nonhuman primate (Papio anubis), acting through intracellular signaling pathways distinct from GHRH

Ghrelin is more effective than GHRH in stimulating GH release in normal adult humans and monkeys in vivo. This robust effect of ghrelin has been largely attributed to regulation of hypothalamic input, whereas the direct effect of ghrelin on pituitary GH release has been minimized by the observation that ghrelin has only a modest impact on GH release, compared with GHRH, in cultures prepared from human fetal pituitaries and GH-producing adenomas, as well as pituitaries from nonprimate species. However, comparable in vitro studies have not been performed to test the direct effect of ghrelin on normal adult primates. Therefore, in the present study, primary pituitary cell cultures from female baboons (Papio anubis) were used as a model system to test the direct effects of ghrelin on primate somatotrope function. In this model, both ghrelin and GHRH increased GH release in a dose-dependent fashion. Surprisingly, at maximal concentrations (10 nM), both ghrelin and GHRH elicited a robust increase in GH release (4 and 24 h, respectively), and both up-regulated GH secretagogue-receptor and GHRH-receptor mRNA levels (24 h). Combined treatment with ghrelin and GHRH resulted in an additive effect on GH release, suggesting that distinct intracellular signaling pathways are activated by each ligand, as confirmed by the use of specific inhibitors of intracellular signaling. Together, these results present the first evidence that a direct effect of ghrelin on somatotrope function may play a major role in stimulating GH release in primates.

Chronobiology in the endocrine system

Biological signaling occurs in a complex web with participation and interaction of the central nervous system, the autonomous nervous system, the endocrine glands, peripheral endocrine tissues including the intestinal tract and adipose tissue, and the immune system. All of these show an intricate time structure with rhythms and pulsatile variations in multiple frequencies. Circadian (about 24-hour) and circannual (about 1-year) rhythms are kept in step with the cyclic environmental surrounding by the timing and length of the daily light span. Rhythmicity of many endocrine variables is essential for their efficacy and, even in some instances, for the qualitative nature of their effects. Indeed, the continuous administration of certain hormones and their synthetic analogues may show substantially different effects than expected. In the design of drug-delivery systems and treatment schedules involving directly or indirectly the endocrine system, consideration of the human time organization is essential. A large amount of information on the endocrine time structure has accumulated, some of which is discussed in this review.

Physiological, pathological and potential therapeutic roles of ghrelin

Ghrelin, a hormone that is produced mainly by the stomach, was identified originally as the endogenous ligand of the growth hormone secretagogue (GHS) receptor. Ghrelin might also be synthesized in other organs, where it might have autocrine or paracrine effects. GHS receptors are present in tissues other than the hypothalamus and pituitary, which indicates that ghrelin has other effects in addition to stimulating the release of growth hormone. Recently, it has been suggested that ghrelin might be involved in the pathogenesis of many diseases and be a therapeutic target in these diseases. Here, we provide an overview of the physiological effects of ghrelin and of its pathological and potential therapeutic roles.

Use of growth-hormone-releasing peptide-6 (GHRP-6) for the prevention of multiple organ failure

Novel therapies for the treatment of MOF (multiple organ failure) are required. In the present study, we examined the effect of synthetic GHRP-6 (growth hormone-releasing peptide-6) on cell migration and proliferation using rat intestinal epithelial (IEC-6) and human colonic cancer (HT29) cells as in vitro models of injury. In addition, we examined its efficacy when given alone and in combination with the potent protective factor EGF (epidermal growth factor) in an in vivo model of MOF (using two hepatic vessel ischaemia/reperfusion protocols; 45 min of ischaemia and 45 min of reperfusion or 90 min of ischaemia and 120 min of reperfusion). In vitro studies showed that GHRP-6 directly influenced gut epithelial function as its addition caused a 3-fold increase in the rate of cell migration of IEC-6 and HT29 cells (P<0.01), but did not increase proliferation ([3H]thymidine incorporation). In vivo studies showed that, compared with baseline values, ischaemia/reperfusion caused marked hepatic and intestinal damage (histological scoring), neutrophilic infiltration (myeloperoxidase assay; 5-fold increase) and lipid peroxidation (malondialdehyde assay; 4-fold increase). Pre-treatment with GHRP-6 (120 microg/kg of body weight, intraperitoneally) alone truncated these effects by 50-85% (all P<0.05) and an additional benefit was seen when GHRP-6 was used in combination with EGF (1 mg/kg of body weight, intraperitoneally). Lung and renal injuries were also reduced by these pre-treatments. In conclusion, administration of GHRP-6, given alone or in combination with EGF to enhance its effects, may provide a novel simple approach for the prevention and treatment of MOF and other injuries of the gastrointestinal tract. In view of these findings, further studies appear justified.

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.

Production, characterization and applications of mouse anti-grass carp (Ctenopharyngodon idellus) growth hormone monoclonal antibodies

Mouse anti-grass carp growth hormone (gcGH) monoclonal antibody (MAb) secretors were produced by PEG-mediated fusion of NS-1 myeloma cells and splenic B-lymphocytes of gcGH hyper-immunized mice. Positive secretors were screened by direct ELISA and cloned by limiting dilution. Three positive secretors, 21D3, 22G5 and 23B3, were obtained in a single fusion trial. Anti-gcGH MAbs were produced by growing hybridomas in the peritoneal cavity of pristane-primed mouse. The three MAbs were isotyped to be IgG2a, IgG2b and IgM, respectively. IgG MAbs were purified from ascitic fluid by Hitrap protein G column and IgM MAb was purified by gel filtration chromatography. The purified MAbs were highly specific and had moderate binding affinity. The MAbs were successfully used for the purification of native gcGH from mature grass carp pituitary extract by one-step immunoaffinity chromatography, for the quantification of gcGH by competitive sandwich ELISA, and for the probing of somatotropes in grass carp pituitary by immunohistochemistry.