Ghrelin stimulates gastric emptying but is without effect on acid secretion and gastric endocrine cells

Interactions of gastrointestinal peptides: ghrelin and its anorexigenic antagonists

Food intake behaviour and energy homeostasis are strongly regulated by a complex system of humoral factors and nerval structures constituting the brain-gut-axis. To date the only known peripherally produced and centrally acting peptide that stimulates food intake is ghrelin, which is mainly synthesized in the stomach. Recent data indicate that the orexigenic effect of ghrelin might be influenced by other gastrointestinal peptides such as cholecystokinin (CCK), bombesin, desacyl ghrelin, peptide YY (PYY), as well as glucagon-like peptide (GLP). Therefore, we will review on the interactions of ghrelin with several gastrointestinal factors known to be involved in appetite regulation in order to elucidate the interdependency of peripheral orexigenic and anorexigenic peptides in the control of appetite.

Ghrelin stimulates gastric acid secretion by rat gastric mucosal cells in vitro

AIM: To investigate the effects of ghrelin on gastric exocrine secretion.

METHODS: Gastric mucosa was obtained from weaned rats. Gastric mucosal cells were dispersed from freshly obtained gastric mucosa and divided into two groups: treatment group and control group. The treatment group was incubated with fresh culture medium containing different concentrations of ghrelin (1 × 10^-4, 1×10^-3, 1 × 10^-2 and 1 × 10^-1 μmol/L), while the control group was incubated with fresh culture medium without ghrelin. Four hours after incubation, culture supernatants were collected, and cells were harvested. Cell viability was measured by methyl thiazolyl tetrazolium (MTT) assay. Pepsin activity and H⁺-K⁺-ATPase mRNA expression and activity were evaluated.

RESULTS: No significant difference was noted in cell viability between the control group and treatment group. Ghrelin at a concentration of 1 × 10^-3 μmol/L significantly augmented pepsin activity and H⁺-K⁺-ATPase mRNA expression (both P < 0.05). Ghrelin at a concentration of 1 × 10^-4 and 1 × 10^-3 μmol/L significantly increased H⁺-K⁺-ATPase activity in gastric mucosal cells (both P < 0.05).

CONCLUSION: Ghrelin can significantly stimulate the secretion of both pepsin and gastric acid by gastric mucosal cells in vitro.

Gastric ghrelin in relation to gender, stomach topography and Helicobacter pylori in dyspeptic patients

AIM: To investigate the level of gastric ghrelin in stomach mucosa of dyspeptic patients in relation to Helicobacter pylori (H pylori) infection, bacterial cytotoxicity, topography and gender.

METHODS: The study comprised 40 premenopausal women (19 H pylori positive) and 48 men (17 H pylori positive) with functional dyspepsia. All gastric biopsy specimens revealed normal mucosa or non-atrophic gastritis. Gastric ghrelin concentration was determined by Enzyme linked immunosorbent assay. The cagA and vacA strains of bacterial DNA were identified by multiplex polymerase chain reaction.

RESULTS: In general, infection with H pylori caused an increase in gastric ghrelin level regardless of gender and stomach topography. Significantly more hormone was present in both, non-infected and H pylori positive female samples, as compared to males. The distribution of bacterial strains showed cagA(+) vacA s1m1 and cagA(-) vacA s2m2 genotypes as the most common infections in the studied population. A tendency to higher ghrelin levels was observed in less cytotoxic (cagA negative) strain-containing specimens from the antrum and corpus of both gender groups (without statistical significance).

CONCLUSION: An increase in gastric ghrelin levels at the stage of non-atrophic gastritis in H pylori positive patients, especially in those infected with cagA(-) strains, can exert a gastroprotective effect.

Ghrelin levels in children with congenital heart disease

BACKGROUND

Ghrelin is a novel growth hormone-releasing peptide that causes a positive energy balance by stimulating food intake and inducing adiposity and has effects on growth. Many children with congenital heart disease (CHD) present with growth retardation and malnutrition owing to multifactorial reasons.

AIM

To evaluate the circulating level of ghrelin in Egyptian children with congenital cyanotic and acyanotic heart disease and its relation to anthropometric measurements.

MATERIALS AND METHODS

The study included 40 patients with cyanotic and acyanotic CHD (18 cyanotic and 22 acyanotic) and 18 age- and sex-matched healthy control children. All children were subjected to measurement of height, weight, body mass index (BMI) and serum ghrelin was measured using ELISA technique.

RESULTS

Weight, height and BMI were significantly lower in cyanotic and acyanotic patients compared to the control group (p = 0.0001). Serum ghrelin levels were significantly higher in children with cyanotic and acyanotic CHD in comparison to the controls (p = 0.0001). There was a significant negative correlation between ghrelin and BMI in the three groups (r = -0.534, p = 0.023; r = -0.558, p = 0.007; r = -0.608, p = 0.007 respectively for cyanotic, acyanotic and the control groups).

CONCLUSION

Circulating ghrelin level was elevated in children with congenital cyanotic and acyanotic heart disease, and was associated with a decrease in BMI. This elevation in ghrelin level may represent malnutrition and growth retardation in those patients as obvious by anthropometric measures too. This may suggest that ghrelin may have an important role as a compensatory mechanism in the regulation of the metabolic balance in them.

Role of obesity in Barrett's esophagus and cancer

The incidence of esophageal adenocarcinoma (EAC) has increased dramatically in the western world, and there also appears to have been an increase in the incidence of Barrett's esophagus and gastroesophageal reflux disease in recent years. The contemporaneous increase in obesity has focused interest on whether obesity is a risk factor for EAC and its precursors. This article reviews current evidence for the role that overweight/obesity and body fat distribution have in development of the esophagitis metaplasia-dysplasia-adenocarcinoma sequence. Particular attention is paid to the stage at which adiposity may act to influence the risk of EAC, because this determines the importance of weight control and weight loss at each stage in the disease spectrum for the prevention of EAC.

Low gastric acid and high plasma gastrin in high-anxiety Wistar Kyoto rats

OBJECTIVE

Wistar Kyoto (WKY) rats are more susceptible to stress-evoked ulcerations than Sprague-Dawley (SPD) rats. We have already demonstrated that gastrin cells are more active and ghrelin cells less active in WKY rats than in SPD rats. The purpose of this study was to compare endocrine cell activity and gastric acid output in WKY and SPD rats.

MATERIAL AND METHODS

Gastric acid output was determined in conscious rats with gastric fistula. Plasma gastrin and ghrelin levels were measured after an overnight fast. Acid secretagogues (gastrin, histamine and carbachol) were given by continuous subcutaneous infusion.

RESULTS

The volume of gastric juice, and the acidity and acid output were all significantly lower (p <0.05) in fasted WKY rats than in fasted SPD rats. Gastrin evoked a 4-fold (p <0.01) and 3-fold (p <0.05) increase in gastric acid output in SPD rats and WKY rats, respectively. Histamine raised the acid output 1.6-fold in SPD rats (p=0.06) and 3-fold in WKY rats (p <0.05), while carbachol failed to affect the acid output (weak increase, p >0.05). Fasting plasma ghrelin levels were 2-fold higher in SPD rats than in WKY rats (p <0.01) while fasting gastrin levels were 10-fold higher in WKY rats than in SPD rats (p <0.05). Neither the parietal-cell density nor the oxyntic mucosal thickness differed between the two strains.

CONCLUSIONS

The results of the present study suggest that a high gastrin cell activity in WKY rats is secondary to a low gastric acidity. Whether the high gastrin cell activity is linked to susceptibility to stress ulcer in WKY rats warrants further investigation.

Direct effects of nutrients, acetylcholine, CCK, and insulin on ghrelin release from the isolated stomachs of rats

Ghrelin is a powerful orexigenic peptide predominantly secreted by the stomach. Blood concentration of ghrelin increases before meals and fall postprandial. Its regulation appears to be influenced by the type of macronutrient ingested, the vagus nerve stimulation and by other post-meal stimulated hormonal factors. However, the direct role of nutrients (amino acids or lipids), neuronal (vagal neurotransmitter acetylcholine) and satiety-inducing factor such as CCK are not known. To study this we applied amino acids, lipids, acetylcholine and CCK via vascular perfusion to the isolated stomachs and found that amino acids significantly reduced ghrelin release from the isolated stomach by approximately approximately 30% vs. the control while lipids (10% intralipid) had no affect. Acetylcholine (1 microM) increased ghrelin release from the stomach by approximately 37% whereas insulin (10nM) decreased it by approximately 30% vs. the control. Interestingly, CCK (100 nM) potently increased ghrelin release by approximately 200% vs. the control. Therefore it appears that ghrelin secretion from the stomach is under direct influence of amino acids, neurotransmitter acetylcholine and hormones such as insulin and CCK.

Influence of ghrelin on the gastric accommodation reflex and on meal-induced satiety in man

Ghrelin increases gastric tone in the fasting state and enhances gastric emptying in gastroparesis. The aims of the study were to evaluate the effect of ghrelin on postprandial gastric tone and on meal-induced satiety in health. Ten healthy volunteers underwent a barostat study on two occasions. After determination of intra-abdominal pressure (minimal distending pressure, MDP), isobaric volume measurement was performed for 90 min at MDP + 2 mmHg. After 20 min, ghrelin (40 microg) or saline was administered i.v. over 30 min in a double-blind-randomized cross-over design, followed 10 min later by a liquid meal (200 mL, 300 kcal). Stepwise isobaric distentions (+2 mmHg per 2 min) were performed 60 min after the meal. Data (mean +/- SEM) were compared using paired Student's t-test and ANOVA. Separately, a satiety drinking test (15 mL min(-1) until satiety score 5) was performed on 10 subjects twice, after treatment with placebo or ghrelin. Ghrelin infusion significantly inhibited gastric accommodation (mean volume increase adjusted means 108.0 +/- 50 vs 23.0 +/- 49 mL, P = 0.03, ANCOVA with the premeal postinfusion volume as covariate) and reduced postprandial gastric volumes (197.2 +/- 24.6 vs 353.5 +/- 50.0 mL, P = 0.01). Pressures inducing perception or discomfort during postprandial gastric distentions were not altered. During satiety testing, ghrelin did not alter nutrient volume ingested till maximal satiety (637.5 +/- 70.9 vs 637.5 +/- 56.2 mL, ns). Ghrelin administered during the meal significantly inhibits gastric accommodation in health, but this is not associated with early satiation.

Serum high-molecular weight adiponectin is related to early postprandial glycemic increases and gastric emptying in patients with type 2 diabetes mellitus

BACKGROUND

Postprandial hyperglycemia and hyperlipidemia are frequently associated with type 2 diabetes mellitus. The aim of the present study is to investigate the clinical determinants of postprandial glycemia and lipemia, especially serum high-molecular weight adiponectin.

METHODS

Twenty-seven diabetic patients treated with diet alone and 13 healthy volunteers took liquid test meal containing 53 g carbohydrate and 47 g lipid, dosed with nonradioactive isotope (13)C-acetate. Venous blood and breath samples were obtained for 180 min after the meal. Gastric emptying was evaluated by peak excretion time of (13)CO(2) in the breath samples. Delayed gastric emptying was defined as peak excretion time > 2.5 h (mean + 2 SD in the healthy volunteers).

RESULTS

Diabetic patients showed delayed insulin secretion, postprandial hyperglycemia and hyperlipidemia compared with control. Postprandial glycemic increases significantly correlated with enhanced gastric emptying. Serum high-molecular weight adiponectin correlated with postprandial glycemic increases at 30 and 60 min after meal (r = 0.42, p < 0.05; r = 0.37, p < 0.05, respectively). Serum high-molecular weight adiponectin also correlated with gastric emptying (versus peak excretion time r = - 0.58, p < 0.05). In addition, diabetic patients with delayed gastric emptying showed the suppressed postprandial glycemia with lower serum high-molecular weight adiponectin than those with normal gastric emptying. On the other hand, postprandial increases in serum triglyceride were not related to serum high-molecular weight adiponectin or gastric emptying, but significantly related to liver function test (serum transaminases) and body mass index.

CONCLUSIONS

Early postprandial glycemic increases were related to elevated serum high-molecular weight adiponectin, which might be associated with enhanced gastric emptying.

The potential of ghrelin as a prokinetic

Ghrelin is a 28 amino-acid peptide discovered in 1999. It is synthesized mainly in endocrine cells of the gastric mucosa and secreted into the blood. Physiological contributions of the peptide in human include probably regulation of growth hormone and of appetite. Ghrelin can also influence the gastrointestinal system, mostly by stimulating the contractility of the proximal gut. Whether ghrelin is a significant regulator of the digestive physiology remains to be shown. Whether ghrelin agonist agents could be useful prokinetic drugs in human medicine is currently under exploration.

House musk shrew (Suncus murinus, order: Insectivora) as a new model animal for motilin study

Although many studies have demonstrated the action of motilin on migrating motor complex by using human subjects and relatively large animals, the precise physiological mechanisms of motilin remain obscure. One reason for the lack of progress in this research field is that large animals are generally not suitable for molecular-level study. To overcome this problem, in this study, we focused on the house musk shrew (Suncus murinus, order: Insectivora, suncus named as laboratory strain) as a small model animal, and we present here the results of motilin gene cloning and its availability for motilin study. The motilin gene has a high homology sequence with that of other mammals, including humans. Suncus motilin is predicted to exist as a 117-residue prepropeptide that undergoes proteolytic cleavage to form a 22-amino-acid mature peptide. The results of RT-PCR showed that motilin mRNA is highly expressed in the upper small intestine, and low levels of expression were found in many tissues. Morphological analysis revealed that suncus motilin-producing cells were present in the upper small intestinal mucosal layer but not in the myenteric plexus. Administration of suncus motilin to prepared muscle strips of rabbit duodenum showed almost the same contractile effect as that of human motilin. Moreover, suncus stomach preparations clearly responded to suncus or human motilin stimulation. To our knowledge, this is the first report that physiological active motilin was determined in small laboratory animals, and the results of this study suggest that suncus is a suitable model animal for studying the motilin-ghrelin family.

Pharmacotherapy of gastroparesis

Gastroparesis is a relatively common and often disabling condition that is characterized by a broad range of clinical presentation ranging from dyspeptic symptoms to nausea, vomiting, abdominal pain, malnutrition, frequent hospitalizations and incapacitation. The treatment of gastroparetic symptoms can be challenging to the gastroenterologist and the intensity of therapy varies with the physician's knowledge. Hence the determination that a patient is refractory to 'standard medical therapy' is an assessment that is subspeciality-based and could differ around the world depending on medications available. In this article, we review the use of available prokinetics, antiemetic agents, the approach for analgesia in the context of gastroparesis, and also discuss potential and evolving pharmacotherapies. The progress has been relatively limited as far as availability of new medications for gastroparesis is concerned; however, active research in developing newer prokinetics holds great promise for the future of management of this challenging entity.

Ghrelin infusion in humans induces acute insulin resistance and lipolysis independent of growth hormone signaling

OBJECTIVE

Ghrelin is a gut-derived peptide and an endogenous ligand for the growth hormone (GH) secretagogue receptor. Exogenous ghrelin stimulates the release of GH (potently) and adrenocorticotropic hormone (ACTH) (moderately). Ghrelin is also orexigenic, but its impact on substrate metabolism is controversial. We aimed to study direct effects of ghrelin on substrate metabolism and insulin sensitivity in human subjects.

RESEARCH DESIGN AND METHODS

Six healthy men underwent ghrelin (5 pmol . kg(-1) . min(-1)) and saline infusions in a double-blind, cross-over study to study GH signaling proteins in muscle. To circumvent effects of endogenous GH and ACTH, we performed a similar study in eight hypopituitary adults but replaced with GH and hydrocortisone. The methods included a hyperinsulinemic-euglycemic clamp, muscle biopsies, microdialysis, and indirect calorimetry.

RESULTS

In healthy subjects, ghrelin-induced GH secretion translated into acute GH receptor signaling in muscle. In the absence of GH and cortisol secretion, ghrelin acutely decreased peripheral, but not hepatic, insulin sensitivity together with stimulation of lipolysis. These effects occurred without detectable suppression of AMP-activated protein kinase phosphorylation (an alleged second messenger for ghrelin) in skeletal muscle.

CONCLUSIONS

Ghrelin infusion acutely induces lipolysis and insulin resistance independently of GH and cortisol. We hypothesize that the metabolic effects of ghrelin provide a means to partition glucose to glucose-dependent tissues during conditions of energy shortage.

Bravo capsule system optimizes intragastric pH monitoring over prolonged time: Effects of ghrelin on gastric acid and hormone secretion in the rat

AIM: To evaluate measurements of intragastric pH with the Bravo capsule system over a prolonged time.

METHODS: A Bravo capsule was placed inside the rat gastric body and pH was studied for periods up to five consecutive days. For comparison, a gastric fistula model was used. Effects of ghrelin and esomeprazole, with or without pentagastrin, on gastric pH were studied. In addition, effects of esomeprazole on plasma ghrelin, gastrin and somatostatin were analyzed.

RESULTS: All rats recovered after surgery. The average 24-h pH during free feeding was 2.3 ± 0.1 (n = 20) with a variation of 18% ± 6% over 5 d. Ghrelin, 2400 pmol/kg, t.i.d. increased pH from 1.7 ± 0.1 to 3.1 ± 0.3 (P < 0.01) as recorded with the Bravo system. After esomeprazole (1 mg/kg, 3 mg/kg and 5 mg/kg) there was a dose-dependent pH increase of maximally 3.4 ± 0.1, with day-to-day variation over the entire period of 8% ± 3%. The fistula and pH studies generated similar results. Acid inhibition with esomeprazole increased plasma ghrelin from 10 ± 2 pmol/L to 65 ± 26 pmol/L (P < 0.001), and somatostatin from 10 ± 2 pmol/L to 67 ± 18 pmol/L (P < 0.001).

CONCLUSION: pH measurements with the Bravo capsule are reliable, and comparable to those of the gastric fistula model. The Bravo system optimizes accurate intragastric pH monitoring over prolonged periods and allows both short- and long-term evaluation of effects of drugs and hormones.

Vagotomy and accompanying pyloroplasty down-regulates ghrelin mRNA but does not affect ghrelin secretion

In this study, we have examined how the lack of vagus activity affects the long-term secretion of total and active ghrelin. We subjected mice to sham-operation, pyloroplasty or vagotomy and pyloroplasty. The study lasted for 2 weeks, during which body weight development and daily food intake was monitored. At the end of the study, the mice were sacrificed, and serum and fundus were collected. Measurements of total and active serum ghrelin revealed no difference between the surgical groups and sham-operated mice, despite the fact that fundic ghrelin mRNA was down-regulated. The results presented here suggest that the vagus activity is not required for the long-term secretion of neither total nor active ghrelin in mice. They also suggest that fundic ghrelin mRNA expression is affected by pyloroplasty and vagotomy but that this effect does not translate into changes in ghrelin levels in the circulation.

Prokinetic effects of a ghrelin receptor agonist GHRP-6 in diabetic mice

AIM: To investigate the effects of a ghrelin receptor agonist GHRP-6 on delayed gastrointestinal transit in alloxan-induced diabetic mice.

METHODS: A diabetic mouse model was established by intraperitoneal injection with alloxan. Mice were randomized into two main groups: normal mice and diabetic mice treated with GHRP-6 at doses of 0, 20, 50, 100 and 200 μg/kg ip. Gastric emptying (GE), intestinal transit (IT), and colonic transit (CT) were studied in mice after they had a phenol red meal following injection of GHRP-6. Based on the most effective GHRP-6 dosage, atropine was given at 1 mg/kg for 15 min before the GHRP-6 injection for each measurement. The mice in each group were sacrificed 20 min later and the percentages of GE, IT, and CT were calculated.

RESULTS: Percentages of GE, IT, and CT were significantly decreased in diabetic mice as compared to control mice. In the diabetic mice, GHRP-6 improved both GE and IT, but not CT. The most effective dose of GHRP-6 was 200 μg/kg and atropine blocked the prokinetic effects of GHRP-6 on GE and IT.

CONCLUSION: GHRP-6 accelerates delayed GE and IT, but has no effect on CT in diabetic mice. GHRP-6 may exert its prokinetic effects via the cholinergic pathway in the enteric nervous system, and therefore, has therapeutic potential for diabetic patients with delayed upper gastrointestinal transit.

Ghrelin, des-acyl ghrelin and obestatin: three pieces of the same puzzle

The major active product of ghrelin gene is a 28-amino acid peptide acylated at the serine 3 position with an octanoyl group, called simply ghrelin. Ghrelin has a multiplicity of physiological functions, affecting GH release, food intake, energy and glucose homeostasis, gastrointestinal, cardiovascular, pulmonary and immune function, cell proliferation and differentiation and bone physiology. Nevertheless, recent developments have shown that ghrelin gene can generate various bioactive molecules besides ghrelin, mainly des-acyl ghrelin and obestatin, obtained from alternative splicing or from extensive post-translational modification. Although their receptors have not yet been identified, they have already proven to be active, having intriguingly subtle but opposite physiological actions to ghrelin. This suggests the existence of a novel endocrine system with multiple effector elements which not only may have opposite actions but may regulate the action of each other. In this review, we summarize the steps which lead to the production of the different ghrelin gene products and examine the most significant differences between them in terms of structure and actions.

Leptin and ghrelin in relation to Helicobacter pylori status in adult males

CONTEXT

Leptin and ghrelin, hormones involved in human energy homeostasis, are both produced in the stomach.

OBJECTIVE

We sought to determine whether the presence of Helicobacter pylori affects gastric and systemic levels of leptin and ghrelin.

DESIGN, SETTING, AND PATIENTS

We consecutively enrolled 256 patients referred for upper endoscopy at a Veterans Affairs outpatient endoscopy center.

OUTCOMES

We obtained fasting serum, fundic and antral biopsies, and gastric juice. Based on histological, biochemical, and serological assays, patients were categorized as H. pylori+ or H. pylori-. Leptin and total ghrelin levels in serum, gastric biopsies, and gastric juice were determined by specific ELISAs.

RESULTS

Of the 256 subjects, 120 were H. pylori+ and 96 were H. pylori-; 40 patients of indeterminate status were excluded. Serum and fundic leptin levels correlated with body mass index in the H. pylori+ (r = 0.35; P < 0.0001 and r = 0.35; P < 0.0001, respectively) and H. pylori- (r = 0.65; P < 0.0001 and r = 0.41; P < 0.0001, respectively) groups, but H. pylori+ subjects had significantly lower serum leptin levels [median 2.2 ng/ml (interquartile range 0.9-4.6) vs. 4.0 ng/ml (1.7-7.2); P = 0.0003]. Serum ghrelin levels were similar in the H. pylori+ and H. pylori- groups [median 1651 pg/ml (interquartile range 845-2247) vs. 1629 pg/ml (992-2886); P = 0.23]. H. pylori status did not significantly affect gastric biopsy leptin and ghrelin levels. Ghrelin levels in gastric juice varied over 4 log(10) (<80-776,000 pg/ml) and correlated with gastric juice pH in the H. pylori+ group (r = 0.68; P < 0.0001).

CONCLUSIONS

These findings provide evidence that H. pylori status affects leptin and ghrelin homeostasis, presumably via intragastric interactions.

Primary structure, tissue distribution, and biological activity of chicken motilin receptor

Motilin is a peptide hormone involved in gastrointestinal motility. GPR38, initially cloned as an orphan receptor, is now considered a specific receptor for motilin. Previously, molecular characterization of the motilin receptor had only been performed in mammalian and fish species. In this study, we cloned cDNA for chicken motilin receptor from the duodenum and characterized its primary structure, tissue distribution, and biological activity. The cDNA encoded 349 amino acids showing significant overall sequence identity to mammalian motilin receptors. Chicken motilin increased intracellular Ca2+ concentration in human embryonic kidney (HEK) 293 cells transiently expressing the recombinant chicken motilin receptor. Comparison of the cDNA sequence with the genomic sequence of chicken motilin receptor revealed that the chicken motilin receptor gene consists of two exons separated by an intron. Real-time PCR analysis showed that chicken motilin receptor mRNA is expressed in a wide range of tissues in 21-day-old chickens, with markedly high levels in the proventriculus, duodenum, and oviduct. The expression levels of the mRNA in the proventriculus and duodenum were highest just before hatching and rapidly decreased during post-hatch development. These results suggest that chicken motilin receptor is largely involved in gastrointestinal functions at pre- and post-hatch periods through an intracellular signaling pathway accompanied by an increase in Ca2+ levels.

Ghrelin improves delayed gastrointestinal transit in alloxan-induced diabetic mice

AIM: To investigate the effects of ghrelin on delayed gastrointestinal transit in alloxan-induced diabetic mice.

METHODS: A diabetic mouse model was established by intraperitoneal injection with alloxan. Mice were randomized into two main groups: normal mice group and diabetic mice group treated with ghrelin at doses of 0, 20, 50, 100 and 200 &mgr;g/kg ip. Gastric emptying (GE), intestinal transit (IT), and colonic transit (CT) were studied in mice after they had a phenol red meal following injection of ghrelin. Based on the most effective ghrelin dosage, atropine was given at 1 mg/kg 15 min before the ghrelin injection for each measurement. The mice in each group were sacrificed 20 min later and their stomachs, intestines, and colons were harvested immediately. The amount of phenol red was measured. Percentages of GE, IT, and CT were calculated.

RESULTS: Percentages of GE, IT, and CT were significantly decreased in diabetic mice as compared to control mice (22.9 ± 1.4 vs 28.1 ± 1.3, 33.5 ± 1.2 vs 43.2 ± 1.9, 29.5 ± 1.9 vs 36.3 ± 1.6, P < 0.05). In the diabetic mice, ghrelin improved both GE and IT, but not CT. The most effective dose of ghrelin was 100 &mgr;g/kg and atropine blocked the prokinetic effects of ghrelin on GE and IT.

CONCLUSION: Ghrelin accelerates delayed GE and IT but has no effect on CT in diabetic mice. Ghrelin may exert its prokinetic effects via the cholinergic pathway in the enteric nervous system, and therefore has therapeutic potential for diabetic patients with delayed upper gastrointestinal transit.

Does ghrelin explain accelerated gastric emptying in the early stages of diabetes mellitus?

During the early stages of diabetes, gastric emptying is often accelerated, rather than delayed. The mechanism of accelerated gastric emptying in diabetes has not been fully studied. A recent study showed that plasma ghrelin levels were elevated in diabetes. As postprandial antropyloric coordination plays an important role in mediating solid gastric emptying, we hypothesize that the elevated plasma ghrelin levels increase postprandial antropyloric coordination to accelerate emptying in the early stages of diabetes. To test this hypothesis, rats were made diabetic by streptozotocin (STZ; 50 mg/kg) injection, and, 2 wk later, pre- and postprandial plasma ghrelin levels, antropyloric coordination, and solid gastric emptying were determined. In control rats, plasma ghrelin levels were immediately reduced after feeding. In contrast, plasma ghrelin levels remained within the fasted levels in STZ rats after feeding. In STZ rats, gastric emptying was significantly accelerated (77.4 +/- 3.2%, n = 6), compared with that of control rats (58.8 +/- 2.5%, n = 6, P < 0.05). Treatments with anti-ghrelin antibodies attenuated accelerated gastric emptying in STZ rats (50.1 +/- 3.5%, n = 6, P < 0.05), while having little effect in vehicle control rats. The incidence of postprandial antropyloric coordination was significantly increased in STZ rats, compared with that of control rats (P < 0.05). Treatments with anti-ghrelin antibodies suppressed this enhanced antropyloric coordination in STZ rats. Our study suggests that elevated endogenous ghrelin enhances antropyloric coordination, which accelerates gastric emptying in the early stages of diabetes.

Suppression of intestinal mucosal apoptosis by ghrelin in fasting rats

Ghrelin is mainly produced in the stomach and has several physiologic functions. The aim of this study was to investigate whether ghrelin regulates apoptosis in the small intestinal mucosa of fasting rats. Intestinal mucosal apoptosis was evaluated as the percentage of fragmented DNA, villus height, and terminal deoxynucleotidyl transferase-mediated dUDP-biotin nick end-labeling (TUNEL) staining and by Western blot analysis of caspase-3 in 48-hr fasting rats. Crypt cell proliferation was evaluated by counting the number of 5-bromo-2-deoxyuridine (BrdU) positive cells. Ghrelin was administered intraperitoneally at dosages of 2.5, 25, and 250 microg/kg per 48 hrs by continuous infusion via an Alzet micro-osmotic pump or injections at 12-hr intervals. Ghrelin was also infused in rats that underwent truncal vagotomy. The lowest dosage of ghrelin (2.5 microg/kg per 48 hrs) was administered into the third cerebroventricle. Ghrelin treatment attenuated the percentage of fragmented DNA in the small intestinal mucosa in 48-hr fasting rats in a dose-dependent manner. Continuous infusion of ghrelin and injections of ghrelin at 12-hr intervals suppressed intestinal apoptosis almost equally. This effect on apoptosis was not attenuated by truncal vagotomy. Cerebroventricular infusion of ghrelin also attenuated intestinal apoptosis. The antiapoptotic effect of ghrelin was confirmed by decreased TUNEL staining, recovery of the villus height, and decreased expression of caspase-3. BrdU uptake indicated that ghrelin enhanced cell proliferation in the intestinal crypt. Taken together, these data indicate that ghrelin enhanced intestinal growth with the suppression of small intestinal mucosal apoptosis in 48-hr fasting rats, suggesting that ghrelin controls intestinal function through the regulation of intestinal apoptosis.

Gastric motor effects of ghrelin and growth hormone releasing peptide 6 in diabetic mice with gastroparesis

AIM: To investigate the potential therapeutic significance of ghrelin and growth hormone releasing peptide 6 (GHRP-6) in diabetic mice with gastric motility disorders.

METHODS: A diabetic mouse model was established by intraperitoneal (ip) injection of alloxan. Diabetic mice were injected ip with ghrelin or GHRP-6 (20-200 &mgr;g/kg), and the effects on gastric emptying were measured after intragastric application of phenol red. The effect of atropine, N^G-nitro-L-arginine methyl ester hydrochloride (L-NAME) or D-Lys³-GHRP-6 (a growth hormone secretagogue receptor (GHS-R) antagonist) on the gastroprokinetic effect of ghrelin or GHRP-6 (100 &mgr;g/kg) was also investigated. The effects of ghrelin or GHRP-6 (0.01-10 &mgr;mol/L) on spontaneous or carbachol-induced contractile amplitude were also investigated in vitro, in gastric fundic circular strips taken from diabetic mice. The presence of growth hormone secretagogue receptor 1a transcripts in the fundic strips of diabetic mice was detected by reverse transcriptase polymerase chain reaction (RT-PCR).

RESULTS: We established a diabetic mouse model with delayed gastric emptying. Ghrelin and GHRP-6 accelerated gastric emptying in diabetic mice with gastroparesis. In the presence of atropine or L-NAME, which delayed gastric emptying, ghrelin and GHRP-6 (100 &mgr;g/kg) failed to accelerate gastric emptying. D-Lys³-GHRP-6 also delayed gastric emptying induced by the GHS-R agonist. Ghrelin and GHRP-6 increased the carbachol-induced contractile amplitude in gastric fundic strips taken from diabetic mice. RT-PCR confirmed the presence of GHS-R mRNA in the strip preparations.

CONCLUSION: Ghrelin and GHRP-6 increase gastric emptying in diabetic mice with gastroparesis, perhaps by activating peripheral cholinergic pathways in the enteric nervous system.

Possible entrainment of ghrelin to habitual meal patterns in humans

Ghrelin is reportedly a meal-initiation signal based on observations that concentrations increase before meals coincident with rising hunger. However, evidence that ghrelin peaks vary with feeding schedules suggests that it rises in anticipation of an expected meal, rather than eliciting feeding. To explore the entrainment of ghrelin profiles, this study investigated the association between varying habitual meal patterns and plasma ghrelin concentrations. Lean and obese adults following either a short intermeal interval (SII) pattern, with 2.5-3.5 h between their habitual breakfast and lunch times, or a long intermeal interval (LII) pattern, with 5.5-6.5 h between these eating occasions, participated. Food intake and appetite were recorded for 2 baseline days. On the subsequent test day, blood samples were collected over 8 h while participants ate a breakfast and lunch matched to their customary meals and pattern. Appetite ratings were obtained and ghrelin, insulin, glucose, and leptin concentrations were measured. Peak ghrelin concentrations differed significantly by group and occurred prior to each group's respective lunch time. Ghrelin concentrations directly correlated with subjective hunger. This association was stronger when hunger preceded ghrelin, a pattern inconsistent with ghrelin causing the hunger rise. Ghrelin concentrations were inversely correlated with insulin, and peak insulin concentrations preceded nadir ghrelin concentrations postprandially. Ghrelin concentrations periprandially, and over the entire test session, did not differ by meal group, likely because of similar intakes between groups. These data demonstrate that the timing of ghrelin peaks is related to habitual meal patterns and may rise in anticipation of eating rather than eliciting feeding.

Acute effects of ghrelin administration on glucose and lipid metabolism

CONTEXT

Ghrelin infusion increases plasma glucose and nonesterified fatty acids, but it is uncertain whether this is secondary to the concomitant release of GH.

OBJECTIVE

Our objective was to study direct effects of ghrelin on substrate metabolism.

DESIGN

This was a randomized, single-blind, placebo-controlled two-period crossover study.

SETTING

The study was performed in a university clinical research laboratory.

PARTICIPANTS

Eight healthy men aged 27.2 +/- 0.9 yr with a body mass index of 23.4 +/- 0.5 kg/m(2) were included in the study.

INTERVENTION

Subjects received infusion of ghrelin (5 pmol x kg(-1) x min(-1)) or placebo for 5 h together with a pancreatic clamp (somatostatin 330 microg x h(-1), insulin 0.1 mU x kg(-1) x min(-1), GH 2 ng x kg(-1) x min(-1), and glucagon 0.5 ng.kg(-1) x min(-1)). A hyperinsulinemic (0.6 mU x kg(-1) x min(-1)) euglycemic clamp was performed during the final 2 h of each infusion.

RESULTS

Basal and insulin-stimulated glucose disposal decreased with ghrelin [basal: 1.9 +/- 0.1 (ghrelin) vs. 2.3 +/- 0.1 mg x kg(-1) x min(-1), P = 0.03; clamp: 3.9 +/- 0.6 (ghrelin) vs. 6.1 +/- 0.5 mg x kg(-1) x min(-1), P = 0.02], whereas endogenous glucose production was similar. Glucose infusion rate during the clamp was reduced by ghrelin [4.0 +/- 0.7 (ghrelin) vs. 6.9 +/- 0.9 mg.kg(-1) x min(-1); P = 0.007], whereas nonesterified fatty acid flux increased [131 +/- 26 (ghrelin) vs. 69 +/- 5 micromol/min; P = 0.048] in the basal period. Regional lipolysis (skeletal muscle, sc fat) increased insignificantly with ghrelin infusion. Energy expenditure during the clamp decreased after ghrelin infusion [1539 +/- 28 (ghrelin) vs. 1608 +/- 32 kcal/24 h; P = 0.048], but the respiratory quotient did not differ. Minor but significant elevations in serum levels of GH and cortisol were observed after ghrelin infusion.

CONCLUSIONS

Administration of exogenous ghrelin causes insulin resistance in muscle and stimulates lipolysis; these effects are likely to be direct, although a small contribution of GH and cortisol cannot be excluded.

Plasma obestatin is lower at fasting and not suppressed by insulin in insulin-resistant humans

Obestatin, a recently discovered 23-amino acid peptide, is involved in the regulation of appetite and body weight in antagonistic fashion to ghrelin, both deriving from a common precursor peptide. Ghrelin was shown to be associated with insulin resistance, which may also affect obestatin. We investigated the association between insulin resistance and plasma concentrations of obestatin and ghrelin in nondiabetic individuals with high (IS; n = 18, 13 females and 5 males, age 47 +/- 2 yr, BMI = 25.5 +/- 0.9 kg/m(2)) and low (IR; n = 18, 12 females and 6 males, age 45 +/- 2 yr, P = 0.49, BMI = 27.5 +/- 1.1 kg/m(2), P = 0.17) insulin-stimulated glucose disposal (M), measured by 2-h hyperinsulinemic (40 mU.min(-1).m(-2)) isoglycemic clamp tests. M(100-120 min) was higher in IS (10.7 +/- 0.7) than in IR (4.4 +/- 0.2 mg.min(-1).kg(-1), P < 10(-9)), whereas insulin-dependent suppression of free fatty acids (FFA) in plasma was reduced in IR (71 +/- 6% vs. IS: 82 +/- 5%, P < 0.02). In both groups, plasma ghrelin concentrations were comparable at fasting and similarly reduced by 24-28% during insulin infusion. IR had lower fasting plasma obestatin levels (383 +/- 26 pg/ml vs. IS: 469 +/- 23 pg/ml, P < 0.02). Clamp insulin infusion reduced plasma obestatin to approximately 81% of basal values in IS (P < 0.00002), but not in IR. Fasting plasma obestatin was correlated positively with M (r = 0.34, P = 0.04), HDL cholesterol (r = 0.45, P = 0.01), and plasma ghrelin concentrations (r = 0.80, P < 0.000001) and negatively with measures of adiposity, plasma FFA during clamp (r = -0.42, P < 0.01), and systolic blood pressure (r = -0.33, P < 0.05). In conclusion, fasting plasma concentrations of obestatin, but not of ghrelin, are reduced in insulin resistance and are positively associated with whole body insulin sensitivity in nondiabetic humans. Furthermore, plasma obestatin is reduced by insulin in insulin-sensitive but not in insulin-resistant persons.

Plasma acylated ghrelin levels are higher in patients with chronic atrophic gastritis

OBJECTIVES

Ghrelin is mainly produced by the endocrine cells of the gastric oxyntic mucosa. For this reason we decided to investigate the modification of the circulating levels not only of total but also of acylated ghrelin in a series of patients with chronic atrophic gastritis.

DESIGN

Twenty-five patients with chronic atrophic gastritis and 25 healthy subjects were studied. In all 50 subjects gastrin and total and acylated ghrelin levels were evaluated. All patients underwent endoscopy with multiple biopsies, and the possibility of Helicobacter pylori infection was investigated.

RESULTS

Significantly higher acylated ghrelin levels (82.8 +/- 61.3 vs. 35.1 +/- 17.1 pmol/l), acylated/total ghrelin ratio (0.422 +/- 0.202 vs. 0.152 +/- 0.085) and gastrin levels (1071 +/- 816 vs. 66 +/- 22 ng/l) were observed in the 25 patients with chronic atrophy than in the healthy subjects. Otherwise, no significant relationships were found when total ghrelin was correlated with the presence of atrophy, or with gastrin levels. In the healthy subjects, but not in the patients, acylated and total ghrelin levels were significantly higher in female than in male patients.

CONCLUSIONS

The increase in acylated ghrelin levels and in the acylated/total ghrelin ratio in patients with atrophy of the body and fundus can be explained by hypothesizing an increase in the acylating process in the presence of gastric atrophy. It suggests that there may be a compensatory increase in plasma active ghrelin concentration in response to gastric atrophy, a condition which causes a loss of ghrelin-producing cells and an increase in gastric pH.

Proghrelin-derived peptides influence the secretion of insulin, glucagon, pancreatic polypeptide and somatostatin: a study on isolated islets from mouse and rat pancreas

Proghrelin, the precursor of the orexigenic and adipogenic peptide hormone ghrelin, is synthetized in endocrine (A-like) cells in the gastric mucosa. During its cellular processing, proghrelin gives rise to the 28-amino acid peptide desacyl ghrelin, which after octanoylation becomes active acyl ghrelin, and to the 23-amino acid peptide obestatin, claimed to be a physiological opponent of acyl ghrelin. This study examines the effects of the proghrelin products, alone and in combinations, on the secretion of insulin, glucagon, pancreatic polypeptide (PP) and somatostatin from isolated islets of mice and rats. Surprisingly, acyl ghrelin and obestatin had almost identical effects in that they stimulated the secretion of glucagon and inhibited that of PP and somatostatin from both mouse and rat islets. Obestatin inhibited insulin secretion more effectively than acyl ghrelin. In mouse islets, acyl ghrelin inhibited insulin secretion at low doses and stimulated at high. In rat islets, acyl ghrelin inhibited insulin secretion in a dose-dependent manner but the IC(50) for the acyl ghrelin-induced inhibition of insulin release was 7.5 x 10(-8) M, while the EC(50) and IC(50) values, with respect to stimulation of glucagon release and to inhibition of PP and somatostatin release, were in the 3 x 10(-12)-15 x 10(-12) M range. The corresponding EC(50) and IC(50) values for obestatin ranged from 5 x 10(-12) to 20 x 10(-12) M. Desacyl ghrelin per se did not affect islet hormone secretion. However, at a ten times higher concentration than acyl ghrelin (corresponding to the ratio of the two peptides in circulation), desacyl ghrelin abolished the effects of acyl ghrelin but not those of obestatin. Acyl ghrelin and obestatin affected the secretion of glucagon, PP and somatostatin at physiologically relevant concentrations; with obestatin this was the case also for insulin secretion. The combination of obestatin, acyl ghrelin and desacyl ghrelin in concentrations and proportions similar to those found in plasma resulted in effects that were indistinguishable from those induced by obestatin alone. From the data it seems that the effects of endogenous, circulating acyl ghrelin may be overshadowed by obestatin or blunted by desacyl ghrelin.

Recent Developments in Ghrelin Receptor Ligands

The 28-amino acid peptide ghrelin is a neuroendocrine hormone synthesized primarily in the stomach. It stimulates growth hormone secretion and appetite, thus promoting food intake and body-weight gain. The pharmacological properties of this peptide are mediated by the growth hormone secretagogue receptor type 1a (GHS-R1a). Given its wide spectrum of biological activities, it is evident that the discovery of ghrelin and its receptor has opened up many perspectives in the fields of neuroendocrine and metabolic research and has had an influence on such fields of internal medicine as gastroenterology, oncology, and cardiology. It is therefore increasingly likely that synthetic, peptidyl, and nonpeptidyl GHS-R1a ligands, acting as agonists, partial agonists, antagonists, or inverse agonists, could have both clinical and therapeutic potential. This review summarizes the various types of GHS-R1a ligands that have been described in the literature and discusses the recent progress made in this research area.

Ghrelin accelerates gastric emptying via early manifestation of antro-pyloric coordination in conscious rats

Ghrelin is known to enhance gastric motility and accelerate gastric emptying of liquid and solid food in rats. As solid gastric emptying is regulated by the coordinated motor pattern between the antrum and pylorus (antro-pyloric coordination), we studied the correlation between solid gastric emptying and antro-pyloric coordination in response to ghrelin. Rats were given 1.5 g of solid food after a 24-h fasting. Immediately after the ingestion, ghrelin (0.4-8.0 microg/kg) or saline was administered by intraperitoneal (i.p.) injection. Ninety minutes after the feeding, rats were euthanized and gastric content was removed to calculate gastric emptying. To evaluate the antro-pyloric coordination, strain gauge transducers were sutured on the antrum and pylorus. The incidence of postprandial antro-pyloric coordination was compared between ghrelin-and saline-injected rats. In saline-injected rats, gastric emptying was 58.3+/-3.7% (n=6). Ghrelin (4.0-8.0 microg/kg), accelerated gastric emptying. Maximum effect was obtained by ghrelin (4.0 microg/kg), which significantly accelerated gastric emptying to 77.4+/-3.7% (n=6, p<0.05). The number of antro-pyloric coordination 20-40 min after feeding was significantly increased in ghrelin-injected rats, compared to that of saline-injected rats (n=4, p<0.05). It is suggested that enhanced antro-pyloric coordination play an important role in accelerated solid gastric emptying induced by ghrelin.

Ghrelin and motilin are cosecreted from a prominent endocrine cell population in the small intestine

CONTEXT

Ghrelin is a novel hormone produced mainly in the gastric body. Hitherto, mapping studies of ghrelin cells covering the entire gastrointestinal (GI) tract in humans have been lacking. Furthermore, the phenotype of extragastric ghrelin cells is not known.

OBJECTIVE

The objective of the study was to perform a detailed mapping with specimens from all parts of the GI tract, and colocalization studies to phenotype ghrelin cells along the tract. In addition, mapping of ghrelin cells was performed in porcine GI tract, and the plasma profiles of ghrelin and motilin in blood from the porcine intestine were measured.

DESIGN

Biopsies from patients were obtained during gastroscopy or surgery. Ghrelin cell density and phenotyping was assessed with immunocytochemistry, in situ hybridization, and immunogold electron microscopy. Plasma ghrelin and motilin levels were measured in pigs, fitted with cannulas in the mesenteric vein.

RESULTS

The upper small intestine is unexpectedly rich in ghrelin cells, and these cells contribute to circulating ghrelin. Ghrelin and motilin are coproduced in the same cells in the duodenum and jejunum of both species, and ghrelin and motilin are stored in all secretory granules of such cells in humans, indicating cosecretion. The plasma profiles of ghrelin and motilin in pig were parallel, and a correlation between ghrelin and motilin (r(2) = 0.22; P < 0.001) was evident in intestinal blood.

CONCLUSIONS

The upper small intestine is an important source of ghrelin. The likely cosecretion of intestinal ghrelin and motilin suggests concerted actions of the two hormones. These data may have implications for understanding gut motility and clinical implications for dysmotility and bariatric surgery.

Ghrelin affects gastrectomy-induced decrease in UCP1 and β3-AR mRNA expression in mice

In this study we investigated the effects of gastrectomy (Gx) and of the gastric hormone, ghrelin, on the expression of proteins in brown adipose tissue (BAT) that are thought to be involved in thermogenesis. Heat production in BAT is known to depend upon activation and increased expression of beta3-adrenergic receptors (beta3-AR) and the consequent up-regulation of uncoupling protein 1 (UCP1). Mice were subjected to Gx or sham operation. One week later they started to receive daily subcutaneous injections of either saline or ghrelin (12 nmol) for two or eight weeks. Neither Gx nor ghrelin affected daily food intake. Gx did not lower body weight gain (except during the first post-operative week) but Gx mice responded to eight weeks of ghrelin treatment with a greater body weight increase (37%, p<0.05) than saline-injected Gx mice; sham-operated mice did not respond to ghrelin. Gx resulted in a greatly reduced expression of both UCP1 and beta3-AR mRNA in BAT (50% reduction or more, p<0.01) compared to sham-operated mice. Eight weeks of ghrelin treatment raised the UCP1 as well as the beta3-AR mRNA expression in the Gx mice, whereas two weeks of ghrelin treatment decreased UCP1 and beta3-AR mRNA expression compared to Gx mice receiving saline. In fact, mRNA expression in Gx mice after treatment with ghrelin for eight weeks was similar to that in saline-treated sham-operated mice. Ghrelin did not affect UCP1 and beta3-AR mRNA in sham-operated mice neither two nor eight weeks after the operation. The results suggest 1) that signals from the stomach stimulate BAT UCP1 (and possibly thermogenesis) and 2) that ghrelin may contribute to the control of UCP1 expression.

Serum ghrelin levels and risk of subsequent adenocarcinoma of the esophagus

OBJECTIVE

Several large studies have shown a negative association between Helicobacter pylori (H. pylori) infection and esophageal adenocarcinoma. Diminution of gastric ghrelin secretion by H. pylori could protect against esophageal malignancy by decreasing appetite, food intake, and acid production, thereby decreasing weight and gastroesophageal reflux.

METHODS

We evaluated the association of ghrelin with esophageal adenocarcinoma using a population from a previous nested case-control study. Among 128,992 enrolled in a multiphasic health checkup (MHC) between 1964 and 1969, 52 patients developed esophageal adenocarcinoma by the year 2000. Three random controls from the MHC cohort were matched to each case by age, sex, race, and the date and site of their MHC. Serum samples collected at the MHC had been previously tested for IgG antibodies against H. pylori and the CagA protein. Serum ghrelin concentrations were determined by a commercial EIA on 52% of the initial subjects (31 cases and 79 controls).

RESULTS

A concentration of ghrelin greater than 3,200 pg/mL at MHC (fourth quartile) was associated with a lower risk of esophageal cancer (H. pylori and body mass index [BMI] adjusted OR=0.18 [CI 0.04-0.78]). This inverse association was seen only in overweight subjects (BMI>or=25, P value for interaction=0.09). The effects of H. pylori and ghrelin were independent.

CONCLUSION

Contrary to the original hypothesis, high rather than low serum ghrelin was associated with protection against esophageal adenocarcinoma but only among overweight subjects.

Secretion of ghrelin from rat stomach ghrelin cells in response to local microinfusion of candidate messenger compounds: a microdialysis study

Ghrelin is produced by A-like cells (ghrelin cells) in the mucosa of the acid-producing part of the stomach. The mobilization of ghrelin is stimulated by nutritional deficiency and suppressed by nutritional abundance. In an attempt to identify neurotransmitters and regulatory peptides that may contribute to the physiological, nutrient-related regulation of ghrelin secretion, we challenged the ghrelin cells in situ with a wide variety of candidate messengers, including known neurotransmitters (e.g. acetylcholine, catecholamines), candidate neurotransmitters (e.g. neuropeptides), local tissue hormones (e.g. serotonin, histamine, bradykinin, endothelin), circulating gut hormones (e.g. gastrin, CCK, GIP, neurotensin, PYY, secretin) and other circulating hormones/regulatory peptides (e.g. calcitonin, glucagon, insulin, PTH). Microdialysis probes were placed in the submucosa of the acid-producing part of the rat stomach. Three days later, the putative messenger compounds were administered via the microdialysis probe (reverse microdialysis) at a screening dose of 0.1 mmol l(-1) for regulatory peptides and 0.1 and 1 mmol l(-1) for amines and amino acids. The rats were awake during the experiments. The resulting microdialysate ghrelin concentration was monitored continuously for 3 h (radioimmunoassay), thereby revealing stimulators or inhibitors of ghrelin secretion. Dose-response curves were constructed for each candidate messenger that significantly (p<0.05) affected ghrelin mobilization at the screening dose. Peptides that showed a (non-significant) tendency to affect ghrelin release at the screening dose were also given at a dose of 0.3 or 1 mmol l(-1). Adrenaline, noradrenaline, endothelin and secretin stimulated ghrelin release, while somatostatin and GRP inhibited. Whether these agents act directly or indirectly on the ghrelin cells remains to be investigated. All other candidate messengers were without measurable effects, including acetylcholine, serotonin, histamine, GABA, aspartic acid, glutamic acid, glycine, VIP, PACAP, CGRP, substance P, NPY, PYY, PP, gastrin, CCK, GIP, insulin, glucagon, GLP and glucose.

Pharmacokinetics and pharmacodynamic effects of an oral ghrelin agonist in healthy subjects

CONTEXT

An oral formulation of EP01572, a peptidomimetic growth hormone secretagogue, was studied. An oral delivery system would be preferable in many of the possible therapeutic indications of ghrelin agonists such as EP01572.

OBJECTIVES

Our objective was to establish the pharmacological profile and the GH-releasing activity of increasing oral doses of EP01572 in healthy volunteers. In addition, the pharmacokinetics and pharmacological effects of EP01572 were investigated after intraduodenal (ID) administration.

SETTING

This study was a single-center escalating dose study with oral and ID applications.

SUBJECTS AND METHODS

In the first part, EP01572 was given orally to 36 male subjects; the treatment consisted of one oral dose of either EP01572 or placebo (0.005, 0.05, and 0.5 mg/kg body weight). Six subjects received two additional oral doses of EP01572: 0.125 and 0.25 mg/kg body weight. In the second part, the following treatments were performed in a randomized order: 1) administration of a bolus of saline (placebo) to the small intestine; 2) ID administration of a bolus of EP01572 at 0.2 mg/kg body weight; 3) ID perfusion of a bolus of EP01572 at 0.35 mg/kg body weight; and 4) ID perfusion of a bolus of EP01572 at 0.5 mg/kg body weight.

RESULTS

The oral and ID administration of EP01572 induced a rapid and dose-dependent increase in plasma drug concentrations and a potent GH release in healthy male volunteers.

CONCLUSIONS

This study showed that EP01572 was active with regard to stimulation of GH release in humans after oral and ID administration.

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.

Obestatin is not elevated or correlated with insulin in children with Prader-Willi syndrome

CONTEXT

Obestatin is a peptide hormone derived from the proteolytic cleavage of ghrelin preprohormone. In Prader-Willi syndrome (PWS), the levels of total ghrelin (TG) and acylated ghrelin (AG) are increased, and these hormones are regulated by insulin.

OBJECTIVE

Our objective was to analyze the changes in the obestatin levels after glucose loading and to characterize the correlations of obestatin with TG, AG, and insulin.

DESIGN

Plasma obestatin, TG, AG, and insulin levels were measured in PWS children (n = 15) and controls (n = 18) during an oral glucose tolerance test.

SETTING

All subjects were admitted to the Samsung Medical Center.

INTERVENTIONS

An oral glucose tolerance test was performed after an overnight fast.

MAIN OUTCOME MEASURES

The plasma levels of obestatin, TG, AG, and serum insulin were measured at 0, 30, 60, 90, and 120 min after glucose challenge, and areas under the curves (AUCs) were calculated.

RESULTS

No significant difference in AUC of the plasma obestatin was found between the PWS children and normal obese controls (P = 0.885), although AUC of AG (P = 0.002) and TG (P = 0.003) were increased in the PWS children. Moreover, There was a negative correlation between the AUC of AG and AUC of insulin both in PWS (r = -0.432; P = 0.049) and in controls (r = -0.507; P = 0.016). However, AUC of obestatin was not significantly correlated with AUC of insulin (in PWS, r = 0.168 and P = 0.275; in controls, r = -0.331 and P = 0.09).

CONCLUSIONS

Our results indicate that plasma obestatin is not elevated in PWS children and is not regulated by insulin both in PWS children and in obese controls.

Therapeutic potential of ghrelin in the treatment of heart failure

Ghrelin is a novel growth hormone (GH)-releasing peptide, isolated from the stomach, which has been identified as an endogenous ligand for the GH secretagogues receptor. The discovery of ghrelin indicates that the release of GH from the pituitary might be regulated, not only by hypothalamic GH-releasing hormone, but also by ghrelin derived from the stomach. Considering the haemodynamic and anabolic effects of GH, ghrelin may have beneficial effects on cardiac function and energy metabolism in heart failure through GH-dependent mechanisms. On the other hand, ghrelin has some GH-independent actions: ghrelin stimulates food intake and induces adiposity. Interestingly, ghrelin acts directly on the CNS to decrease sympathetic nerve activity. It also inhibits apoptosis of cardiomyocytes and endothelial cells. An experimental study has shown that repeated administration of ghrelin improves cardiac structure and function, and attenuates the development of cardiac cachexia in chronic heart failure (CHF). These results suggest that ghrelin has cardiovascular effects and regulates energy metabolism through GH-dependent and -independent mechanisms. Thus, administration of ghrelin may be a new therapeutic strategy for the treatment of severe CHF.

Altered gastrointestinal and metabolic function in the GPR39-obestatin receptor-knockout mouse

BACKGROUND & AIMS

The G-protein-coupled receptor GPR39 is a member of a family that includes the receptors for ghrelin and motilin. Recently the peptide obestatin was identified as a natural ligand for GPR39. The objective of this study was to gain insight into the biological function of the GPR39 receptor.

METHODS

GPR39(-/-) mice were generated and analyzed.

RESULTS

Endogenous GPR39 expression was detected in the brain (septum-amygdala) and the gastrointestinal system (parietal cells, enterocytes, neurons, and pancreas). Gastric emptying of a solid meal (measured by the (14)C octanoic breath test) in GPR39(-/-) mice was accelerated significantly with a gastric half-emptying time of 49.5 +/- 2.2 minutes compared with 86.9 +/- 8.4 minutes in GPR39(+/+) mice. A more effective expulsion of distally located pellets (30%-75% of length) was observed in the colon of GPR39(-/-) mice. Four hours after pylorus ligation, the volume of gastric secretion was increased significantly (GPR39(-/-): 638 +/- 336 microL; GPR39(+/+): 225 +/- 170 microL), but gastric acid secretion was unchanged. The mature body weight and body fat composition of GPR39(-/-) mice was significantly higher compared with GPR39(+/+) mice, but this was not related to hyperphagia because 24-hour food intake did not differ between both genotypes. In contrast, deficiency of the GPR39 receptor led to reduced hyperphagia after fasting. The cholesterol levels were increased significantly in the GPR39(-/-) mice.

CONCLUSIONS

Our data partially confirm and extend the described in vivo effects of obestatin and suggest that this peptide plays a functional role in the regulation of gastrointestinal and metabolic function through interaction with the GPR39 receptor.

Ghrelin improves burn-induced delayed gastrointestinal transit in rats

Delayed gastrointestinal transit is common in patients with severe burn. Ghrelin is a potent prokinetic peptide. We aimed at testing the effect of ghrelin on burn-induced delayed gastrointestinal transit in rats. Gastric emptying (GE), intestinal transit (IT), and colonic transit (CT) studies were performed in male Sprague-Dawley rats. Rats were randomized into two main groups as follows: sham injury and ghrelin-treated burn injury with doses of 0, 2, 5, and 10 nmol/rat ip 6 h after burn. Sham/burn injury was induced under anesthesia. Rats received a phenol red meal 20 min following ghrelin injection. Based on the most effective ghrelin dose, 1 mg/kg sc atropine was given 30 min before the ghrelin in one group of rats for each study. The rats in each group were killed 30-90 min later; their stomachs, intestines, and colons were harvested immediately, and the amount of phenol red recovered was measured. Percentage of gastric emptying (GE%) and geometric center for IT and CT were calculated. We found 1) severe cutaneous burn injury significantly delayed GE, IT, and CT compared with sham injury (P < 0.05); 2) ghrelin normalized both GE and IT, but not the CT; 3) the most effective dose of ghrelin was 2 nmol/rat; and 4) atropine blocked the prokinetic effects of ghrelin on GE% and IT. In conclusion, ghrelin normalizes burn-induced delayed GE and IT but has no effect on CT in rats. The prokinetic effects of ghrelin are exerted via the cholinergic pathway. Ghrelin may have a therapeutic potential for burn patients with delayed upper gastrointestinal transit.

Ghrelin stimulates gastric emptying and hunger in normal-weight humans

CONTEXT

Ghrelin is produced primarily by enteroendocrine cells in the gastric mucosa and increases gastric emptying in patients with gastroparesis.

MAIN OBJECTIVE

The objective of the study was to evaluate the effect of ghrelin on gastric emptying, appetite, and postprandial hormone secretion in normal volunteers.

DESIGN

This was a randomized, double-blind, crossover study.

SUBJECTS

Subjects included normal human volunteers and patients with GH deficiency.

INTERVENTION

Intervention included saline or ghrelin (10 pmol/kg.min) infusion for 180 min after intake of a radioactively labeled omelette (310 kcal) or GH substitution in GH-deficient patients.

MAIN OUTCOME MEASURES

Measures consisted of gastric empty-ing parameters and postprandial plasma levels of ghrelin, cholecystokinin, glucagon-like peptide-1, peptide YY, and motilin.

RESULTS

The emptying rate was significantly faster for ghrelin (1.26 +/- 0.1% per minute), compared with saline (0.83% per minute) (P < 0.001). The lag phase (16.2 +/- 2.2 and 26.5 +/- 3.8 min) and half-emptying time (49.4 +/- 3.9 and 75.6 +/- 4.9 min) of solid gastric emptying were shorter during ghrelin infusion, compared with infusion of saline (P < 0.001). The postprandial peak in plasma concentration for cholecystokinin and glucagon-like peptide-1 occurred earlier and was higher during ghrelin infusion. There was no significant effect of ghrelin on plasma motilin or peptide YY. There was no difference in gastric emptying before and after GH substitution.

CONCLUSION

Our results demonstrate that ghrelin increases the gastric emptying rate in normal humans. The effect does not seem to be mediated via GH or motilin but may be mediated by the vagal nerve or directly on ghrelin receptors in the stomach. Ghrelin receptor agonists may have a role as prokinetic agents.

Expression of gastric ghrelin and H(+)-K(+)-ATPase mRNA in weanling piglets and effect of ghrelin on H(+)-K(+)-ATPase expression and activity in gastric mucosal cells in vitro

This study was designed to investigate the effect of ghrelin on gastric acid secretion in weaning piglets both in vivo and in vitro. Thirty newborn piglets were selected from six litters and on 28, 35 (weaning), 38, 42 and 45d of age, respectively, one piglet from each litter was killed and the mucosal tissue from gastric fundus was collected for detecting ghrelin mRNA as well as H(+)-K(+)-ATPase mRNA expression and activity. Primary cultures of gastric mucosal cells from 5-week-old weaning piglets were challenged with 3x10(-5), 3x10(-4), 3x10(-3), 3x10(-2) and 3x10(-1)nmol/ml h-ghrelin, respectively, for 4h in order to further clarify the effect of ghrelin on gastric H(+)-K(+)-ATPase mRNA expression and activity. Ghrelin mRNA expression in gastric fundus kept stable from 28d to 42d, followed by a sudden increase on 45d, exhibiting a peak that was significantly higher than any other age groups investigated. H(+)-K(+)-ATPase activity and mRNA expression showed similar trends of increase with slightly different timing. H(+)-K(+)-ATPase mRNA expression tended to increase on 42d, while H(+)-K(+)-ATPase activity started to rise from 35d, but neither of them reached significantly higher levels until 45d. In vitro, ghrelin significantly increased H(+)-K(+)-ATPase activity of gastric mucosal cells at 3x10(-4), 3x10(-3), and 3x10(-2)nmol/ml, but augmented H(+)-K(+)-ATPase mRNA expression only at 3x10(-4)nmol/ml. The results indicate that ghrelin mRNA expression is up-regulated 10 days after weaning in the gastric fundus of piglets, coinciding with the increase of H(+)-K(+)-ATPase mRNA expression and activity. Ghrelin acts on gastric mucosal cells to stimulate both mRNA expression and activity of H(+)-K(+)-ATPase in vitro.

Evidence that stimulation of ghrelin receptors in the spinal cord initiates propulsive activity in the colon of the rat

Previous studies have failed to reveal an effect of the gastrointestinal peptide hormone ghrelin on colonic motility. In the present work, ghrelin was applied into the lumbo-sacral spinal cord in the region of defecation control centres, and a synthetic ghrelin receptor agonist, CP464709, which crosses the blood-brain barrier, was applied intravenously or into the lumbo-sacral cord. Both ghrelin and CP464709 elicited propulsive contractions and emptying of the colon in anaesthetized rats. In conscious rats, subcutaneous CP464709 caused fecal expulsion. The sites of action and nerve pathways involved in the stimulation of the colon by ghrelin receptor activation were investigated in anaesthetized rats. Intrathecal application of CP464709 at L6-S1, but not application at ponto-medullary levels or to the thoracic spinal cord, elicited propulsive contractions. The stimulation evoked by intravenous CP464709 was prevented if the pelvic nerve outflows were severed, but not if the spinal cord was cut rostral to the defecation centre at L6-S3. The response was also blocked by hexamethonium. When ghrelin, applied intrathecally, was used to desensitize its receptors, the effect of intravenous CP464709 was blocked. CP464709 did not affect small intestine motility or the amplitudes of visceromotor reflexes caused by colorectal distension. It is concluded that activation of ghrelin receptors in the lumbo-sacral spinal cord triggers co-ordinated propulsive contractions that empty the colo-rectum. The pathways through which these responses are generated pass out of the spinal cord via the pelvic nerves and cause propulsive contractions through activation of enteric neurons.

Ghrelin and the short- and long-term regulation of appetite and body weight

Ghrelin, an acylated upper gastrointestinal peptide, is the only known orexigenic hormone. Considerable evidence implicates ghrelin in mealtime hunger and meal initiation. Circulating levels decrease with feeding and increase before meals, achieving concentrations sufficient to stimulate hunger and food intake. Preprandial ghrelin surges occur before every meal on various fixed feeding schedules and also among individuals initiating meals voluntarily without time- or food-related cues. Ghrelin injections stimulate food intake rapidly and transiently, primarily by increasing appetitive feeding behaviors and the number of meals. Preprandial ghrelin surges are probably triggered by sympathetic nervous output. Postprandial suppression is not mediated by nutrients in the stomach or duodenum, where most ghrelin is produced. Rather, it results from post-ingestive increases in lower intestinal osmolarity (information probably relayed to the foregut via enteric nervous signaling), as well as from insulin surges. Consequently, ingested lipids suppress ghrelin poorly compared with other macronutrients. Beyond a probable role in meal initiation, ghrelin also fulfills established criteria for an adiposity-related hormone involved in long-term body-weight regulation. Ghrelin levels circulate in relation to energy stores and manifest compensatory changes in response to body-weight alterations. Ghrelin crosses the blood-brain barrier and stimulates food intake by acting on several classical body-weight regulatory centers, including the hypothalamus, hindbrain, and mesolimbic reward system. Chronic ghrelin administration increases body weight via diverse, concerted actions on food intake, energy expenditure, and fuel utilization. Congenital ablation of the ghrelin or ghrelin-receptor gene causes resistance to diet-induced obesity, and pharmacologic ghrelin blockade reduces food intake and body weight. Ghrelin levels are high in Prader-Willi syndrome and low after gastric bypass surgery, possibly contributing to body-weight alterations in these settings. Extant evidence favors roles for ghrelin in both short-term meal initiation and long-term energy homeostasis, making it an attractive target for drugs to treat obesity and/or wasting disorders.