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/motilin-related peptide is a potent prokinetic to reverse gastric postoperative ileus in rat

A novel peptide called ghrelin or motilin-related-peptide (MTLRP) was found in the stomach of various mammals. We studied its effect on the motor function of the rat gastrointestinal tract. In normal, conscious unoperated animals, ghrelin/MTLRP (5 or 20 microg/kg iv) significantly accelerated the gastric emptying of a methylcellulose liquid solution (gastric residue after 15 min: 57 +/- 7, 42 +/- 11, 17 +/- 4, and 9 +/- 3% of the ingested meal with doses of 0, 1, 5, and 20 microg/kg iv, respectively) Transit of the methylcellulose liquid solution was also accelerated by ghrelin/MTLRP in the small intestine but not in the colon. Des-[Gln(14)]ghrelin, also found in the mammalian stomach, was as potent as ghrelin in emptying the stomach (gastric residue after 15 min: 12 +/- 3% at a dose of 20 microg/kg iv). In rats in which postoperative gastrointestinal ileus had been experimentally induced, ghrelin/MTLRP (20 microg/kg iv) reversed the delayed gastric evacuation (gastric residue after 15 min: 28 +/- 7% of the ingested meal vs. 82 +/- 9% with saline). In comparison, the gastric ileus was not modified by high doses of motilin (77 +/- 7%) or erythromycin (82 +/- 6%) and was only partially improved by calcitonin gene-related peptide (CGRP) 8-37 antagonist (59 +/- 7%). Ghrelin/MTLRP, therefore, accelerates the gastric emptying and small intestinal transit of a liquid meal and is a strong prokinetic agent capable of reversing the postoperative gastric ileus in rat.     

Effect of peripherally administered ghrelin on gastric emptying and acid secretion in the rat

Ghrelin is a gut peptide that is secreted from the stomach and stimulates food intake. There are ghrelin receptors throughout the gut and intracerebroventricular ghrelin has been shown to increase gastric acid secretion. The aim of the present study was to examine the effects of peripherally administered ghrelin on gastric emptying of a non-nutrient and nutrient liquid, as well as, basal and pentagastrin-stimulated gastric acid secretion in awake rats. In addition, gastric contractility was studied in vitro. Rats equipped with a gastric fistula were subjected to an intravenous infusion of ghrelin (10-500 pmol kg(-1) min(-1)) during saline or pentagastrin (90 pmol kg(-1) min(-1)) infusion. After administration of polyethylene glycol (PEG) 4000 with 51Cr as radioactive marker, or a liquid nutrient with (51)Cr, gastric retention was measured after a 20-min infusion of ghrelin (500 pmol kg(-1) min(-1)). In vitro isometric contractions of segments of rat gastric fundus were studied (10(-9) to 10(-6) M). Ghrelin had no effect on basal acid secretion, but at 500 pmol kg(-1) min(-1) ghrelin significantly decreased pentagastrin-stimulated acid secretion. Ghrelin had no effect on gastric emptying of the nutrient liquid, but significantly increased gastric emptying of the non-nutrient liquid. Ghrelin contracted fundus muscle strips dose-dependently (pD2 of 6.93+/-0.7). Ghrelin IV decreased plasma orexin A concentrations and increased plasma somatostatin concentrations. Plasma gastrin concentrations were unchanged during ghrelin infusion. Thus, ghrelin seems to not only effect food intake but also gastric motor and secretory function indicating a multifunctional role for ghrelin in energy homeostasis.     

Glucagon-like peptide 1 (GLP-1) suppresses ghrelin levels in humans via increased insulin secretion

INTRODUCTION: Ghrelin is an orexigenic peptide predominantly secreted by the stomach. Ghrelin plasma levels rise before meal ingestion and sharply decline afterwards, but the mechanisms controlling ghrelin secretion are largely unknown. Since meal ingestion also elicits the secretion of the incretin hormone glucagon-like peptide 1 (GLP-1), we examined whether exogenous GLP-1 administration reduces ghrelin secretion in humans. PATIENTS AND METHODS: 14 healthy male volunteers were given intravenous infusions of GLP-1(1.2 pmol x kg(-1) min(-1)) or placebo over 390 min. After 30 min, a solid test meal was served. Venous blood was drawn frequently for the determination of glucose, insulin, C-peptide, GLP-1 and ghrelin. RESULTS: During the infusion of exogenous GLP-1 and placebo, GLP-1 plasma concentrations reached steady-state levels of 139+/-15 pmol/l and 12+/-2 pmol/l, respectively (p<0.0001). During placebo infusion, ghrelin levels were significantly reduced in the immediate postprandial period (p<0.001), and rose again afterwards. GLP-1 administration prevented the initial postprandial decline in ghrelin levels, possibly as a result of delayed gastric emptying, and significantly reduced ghrelin levels 150 and 360 min after meal ingestion (p<0.05). The patterns of ghrelin concentrations in the experiments with GLP-1 and placebo administration were inversely related to the respective plasma levels of insulin and C-peptide. CONCLUSIONS: GLP-1 reduces the rise in ghrelin levels in the late postprandial period at supraphysiological plasma levels. Most likely, these effects are indirectly mediated through its insulinotropic action. The GLP-1-induced suppression of ghrelin secretion might be involved in its anorexic effects.     

Maintenance of a normal meal-induced decrease in plasma ghrelin levels in children with Prader-Willi syndrome.

Ghrelin is a 28-amino acid peptide recently identified in the stomach as the endogenous ligand for the growth hormone secretagogue receptor (GHS-R1a). Ghrelin is a potent stimulator of GH secretion. It was recently shown that circulating ghrelin levels in humans rise shortly before and fall shortly after every meal, and that ghrelin administration increases voluntary food intake. The hypothesis that ghrelin hypersecretion might contribute to genetic obesity has never been investigated. In this context, Prader-Willi syndrome is the most common form of human syndromic obesity. As ghrelin affects appetite as well as GH secretion and both are abnormal in PWS, it has been surmised that these alterations might be due to ghrelin dysregulation. The aim of the study was to investigate whether ghrelin is suppressed by the meals differently in PWS children than in PWS adults. Overnight circulating fasting ghrelin levels and ghrelin levels 120 min after breakfast were assayed in 7 PWS children (10.2 +/- 1.7 yr), 7 subjects with morbid obesity (10.3 +/- 1.3 yr), and 5 normal controls (8.4 +/- 1.4 yr). Because of the data spread, no statistical difference was observed in fasting ghrelin levels between PWS and control children (p = NS); anyway, fasting ghrelin levels were significantly lower in obese children than in the other groups (p < 0.05 vs. control and PWS children). Ghrelin levels were slightly suppressed by the meal in control subjects (mean fasting ghrelin: 160.2 +/- 82 pg/ml; after the meal, 141.2 +/- 57 pg/ml, p = NS); the meal failed to suppress ghrelin levels in obese children (mean fasting ghrelin: 126.4 +/- 8.5 pg/ml; after the meal, 119.1 +/- 8.3 pg/ml, p = NS). Interestingly, the meal markedly suppressed ghrelin levels in PWS children (mean fasting ghrelin: 229.5 +/- 70.4 pg/ml; after the meal, 155.8 +/- 34.2 pg/ml, p < 0.01). In conclusion, since a lack of decrease in circulating ghrelin induced by the meal was previously reported in PWS adults, the finding of a meal-induced decrease in ghrelin levels in our population of young PWS would imply that the regulation of the ghrelin system involved in the orexigenic effects of the peptide is operative during childhood, although it progressively deteriorates and is absent in adulthood when hyperphagia and obesity progressively worsen.     

Fixed feeding potentiates interdigestive gastric motor activity in rats: importance of eating habits for maintaining interdigestive MMC

Endogenous ghrelin regulates the occurrence of interdigestive gastric phase III-like contractions in rats. However, the fasted motor pattern is not as regular and potent in humans and dogs. We hypothesize that eating habits play an important role in maintaining a regular interdigestive gastric contractions. We studied the effect of fixed-feeding regimen on interdigestive gastric contractions and plasma acyl ghrelin levels. The fixed-fed rats were trained to the assigned meal feeding regimen, once daily at 12:00 PM to 4:00 PM for 14 days. Free-fed rats were maintained with free access to food. As ghrelin regulates gastric emptying as well, solid gastric emptying was also studied in fixed-fed rats and free-fed rats. In free-fed rats, two of six rats did not show interdigestive gastric phase III-like contractions. In contrast, phase III-like contractions were observed in all rats 14 days after starting the fixed-feeding regimen. The maximal amplitude of phase III-like contractions significantly increased from 8.4 +/- 0.6 to 16.3 +/- 1.8 g (n = 6, P < 0.05) 14 days after the start of the fixed feeding. Fasted and postprandial plasma ghrelin levels were significantly increased after 14 days of fixed feeding. Solid gastric emptying was significantly accelerated in fixed-fed rats (72.1 +/- 4.2%) compared with that of free-fed rats (58.7 +/- 2.7%, n = 6, P < 0.05). Our present findings suggest that fixed feeding increases plasma ghrelin levels, potent interdigestive contractions, and acceleration of gastric emptying.     

Two new peptides to improve post-operative gastric ileus in dog

Peptides can influence gastrointestinal motility, and from data obtained earlier in rats, we hypothesized that MTL-RP/Ghrelin, as well as CGRP receptor antagonist 8-37, could improve gastric post-operative ileus in dog. Dogs submitted to laparotomy were perfused with or saline or CGRP 8-37 or MTL-RP/Ghrelin on days 1-4 post-operatively while gastric emptying was estimated by measuring the postprandial increase in plasma acetaminophen ingested with a meal. As expected, in saline-treated animals the gastric emptying function was impaired post-operatively. The total amount of acetaminophen emptied (AUC over 150 min) on post-operative days 1-4 reached respectively 31+/-5%, 65+/-8%, 60+/-8% and 62+/-8% of the normal emptying capacity. CGRP antagonist increased the total emptying of acetaminophen to 52+/-5% on day 1, 95+/-2% on day 2 and 103+/-3% (P<0.05) on day 3. The delayed emptying of acetaminophen seen post-operatively in saline-treated animals could be completely reversed by MTL-RP/Ghrelin (P<0.01) whether it was given at 100 microg/kg on day 2 (102+/-7% of the normal emptying capacity), 4 microg/kg on day 3 (106+/-7%) or 20 microg/kg on day 4 (132+/-8%). As found earlier in rodents, CGRP receptor antagonist 8-37 as well as MTL-RP/Ghrelin are potent prokinetics to improve post-operative gastric ileus in dog.     

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.     

Determination of gastric emptying in nonobese diabetic mice

Animal studies on diabetic gastroparesis are limited by inability to follow gastric emptying changes in the same mouse. The study aim was to validate a nonlethal gastric emptying method in nonobese diabetic (NOD) LtJ mice, a model of type 1 diabetes, and study sequential changes with age and early diabetic status. The reliability and responsiveness of a [(13)C]octanoic acid breath test in NOD LtJ mice was tested, and the test was used to measure solid gastric emptying in NOD LtJ mice and nonobese diabetes resistant (NOR) LtJ mice. The (13)C breath test produced results similar to postmortem recovery of a meal. Bethanechol accelerated gastric emptying [control: 92 +/- 9 min; bethanechol: 53 +/- 3 min, mean half emptying time (T(1/2)) +/- SE], and atropine slowed gastric emptying (control: 92 +/- 9 min; atropine: 184 +/- 31 min, mean T(1/2) +/- SE). Normal gastric emptying (T(1/2)) in nondiabetic NOD LtJ mice (8-12 wk) was 91 +/- 2 min. Aging had differing effects on gastric emptying in NOD LtJ and NOR LtJ mice. Onset of diabetes was accompanied by accelerated gastric emptying during weeks 1-2 of diabetes. Gastric emptying returned to normal by weeks 3-5 with no delay. The [(13)C]octanoic acid breath test accurately measures gastric emptying in NOD LtJ mice, is useful to study the time course of changes in gastric emptying in diabetic NOD LtJ mice, and is able to detect acceleration in gastric emptying early in diabetes. Opposing changes in gastric emptying between NOD LtJ and NOR LtJ mice suggest that NOR LtJ mice are not good controls for the study of gastric emptying in NOD LtJ mice.     

Influence of sildenafil on gastric sensorimotor function in humans

After a meal, the proximal stomach relaxes probably through the activation of nitrergic neurons in the gastric wall. Nitric oxide-induced smooth muscle relaxation involves activation of soluble guanylate cyclase, with cGMP production, which is then degradated by phosphodiesterase-5 (PDE-5). The aim of this study was to investigate the effect of sildenafil, a selective PDE-5 inhibitor, on fasting and postprandial proximal gastric volume and on gastric emptying rates in humans. A gastric barostat was used to study gastric compliance and perception to isobaric distension in healthy subjects before and after placebo (n = 13) or sildenafil, 50 mg (n = 15). In 10 healthy subjects, two gastric barostat studies were performed in randomized order to study the effect of placebo or sildenafil on postprandial gastric relaxation. Similarly, solid and liquid gastric emptying rates were studied in 12 healthy subjects. Sildenafil significantly increased fasting intragastric volume (141 +/- 15 vs. 163 +/- 15 ml, P < 0.05) and volumes of first perception. Sildenafil induced a higher and prolonged gastric relaxation either at 30 min (357 +/- 38 vs. 253 +/- 42 ml, P < 0.05) or 60 min (348 +/- 49 vs. 247 +/- 38 ml, P < 0.05) after the meal. Sildenafil did not alter solid half-emptying time but significantly delayed liquid emptying (43 +/- 4 vs. 56 +/- 4 min, P < 0.01). In conclusion, sildenafil significantly increases postprandial gastric volume and slows liquid emptying rate, confirming that meal-induced accommodation in humans involves the activation of a nitrergic pathway. The effect of sildenafil on gastric fundus suggests a therapeutic potential for phosphodiesterase inhibitors in patients with impaired gastric accommodation.     

LPS inhibits fasted plasma ghrelin levels in rats: role of IL-1 and PGs and functional implications

LPS injected intraperitoneally decreases fasted plasma levels of ghrelin at 3 h postinjection in rats. We characterized the inhibitory action of LPS on plasma ghrelin and whether exogenous ghrelin restores LPS-induced suppression of food intake and gastric emptying in fasted rats. Plasma ghrelin and insulin and blood glucose were measured after intraperitoneal injection of LPS, intravenous injection of IL-1beta and urocortin 1, and in response to LPS under conditions of blockade of IL-1 or CRF receptors by subcutaneous injection of IL-1 receptor antagonist (IL-1Ra) or astressin B, respectively, and prostaglandin (PG) synthesis by intraperitoneal indomethacin. Food intake and gastric emptying were measured after intravenous injection of ghrelin at 5 h postintraperitoneal LPS injection. LPS inhibited the elevated fasted plasma ghrelin levels by 47.6 +/- 4.9%, 58.9 +/- 3.3%, 74.4 +/- 2.7%, and 48.9 +/- 8.7% at 2, 3, 5, and 7 h postinjection, respectively, and values returned to preinjection levels at 24 h. Insulin levels were negatively correlated to those of ghrelin, whereas there was no significant correlation between glucose and ghrelin. IL-1Ra and indomethacin prevented the first 3-h decline in ghrelin levels induced by LPS, whereas astressin B did not. IL-1beta inhibited plasma ghrelin levels, whereas urocortin 1 had no influence. Ghrelin injected intravenously prevented an LPS-induced 87% reduction of gastric emptying and 61% reduction of food intake. These data showed that IL-1 and PG pathways are part of the early mechanisms by which LPS suppresses fasted plasma ghrelin and that exogenous ghrelin can normalize LPS-induced-altered digestive functions.     

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.     

Constant intravenous ghrelin infusion in healthy young men: clinical pharmacokinetics and metabolic effects

Ghrelin levels fluctuate rapidly and dynamically with surges before meal times and postprandial troughs, and ghrelin increases appetite and food intake. Circulating ghrelin correlates negatively with body mass index (BMI), but obese individuals have a reduced postprandial decrease in ghrelin levels. Whether this reflects changes in secretion or clearance of ghrelin is uncertain. We therefore studied the pharmacokinetics of ghrelin in relation to anthropometric and biochemical measures. We also studied the effects of ghrelin on hormones and metabolites. In fasting humans, we used a constant infusion rate of ghrelin lasting 180 min at 5 pmol.kg body wt(-1).min(-1) in a randomized, double-blind, placebo-controlled crossover study. Serum ghrelin (s-ghrelin; total levels) was distributed and eliminated according to a two-compartment model. s-Ghrelin initial half-life was 24 +/- 2 min and terminal half-life 146 +/- 36 min, respectively. Mean residence time (MRT) of ghrelin was 93 +/- 16 min. MRT correlated positively with both BMI (r = 0.51, P < 0.001) and high-density cholesterol (HDL) levels (r = 0.75, P < 0.001). Serum insulin levels remained constant during ghrelin infusion, whereas plasma glucose increased 0.3 +/- 0.1 mmol/l (P < 0.01) and free fatty acid levels more than doubled (to 1.03 +/- 0.08 mmol/l, P < 0.001), translating into a significant reduction of insulin sensitivity (P < 0.001). In conclusion, 1) we describe novel pharmacokinetics of ghrelin that are useful when tailoring ghrelin infusion rates in clinical experiments, 2) BMI and HDL correlate positively with MRT of infused ghrelin, and 3) supraphysiological ghrelin levels impair insulin sensitivity.     

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

Ghrelin, a recently discovered peptide hormone, is produced by endocrine cells in the stomach, the so-called A-like cells. Ghrelin binds to the growth hormone (GH) secretagogue receptor and releases GH. It is claimed to be orexigenic and to control gastric acid secretion and gastric motility. In this study, we examined the effects of ghrelin, des-Gln¹⁴-ghrelin, des-octanoyl ghrelin, ghrelin-18, -10 and -5 (and motilin) on gastric emptying in mice and on gastric acid secretion in chronic fistula rats and pylorus-ligated rats. We also examined whether ghrelin affected the activity of the predominant gastric endocrine cell populations, G cells, ECL cells and D cells. Ghrelin and des-Gln¹⁴-ghrelin stimulated gastric emptying in a dose-dependent manner while des-octanoyl ghrelin and motilin were without effect. The C-terminally truncated ghrelin fragments were effective but much less potent than ghrelin itself. Ghrelin, des-Gln¹⁴-ghrelin and des-octanoyl ghrelin neither stimulated nor inhibited gastric acid secretion, and ghrelin, finally, did not affect secretion from either G cells, ECL cells or D cells.     

Central nervous system action of TRH to stimulate gastric emptying in rats

The effects of intracisternal injection of TRH on gastric emptying of a liquid meal was investigated in 24 h fasted rats using the phenol red method. Intracisternal injection of TRH, RX 77368, or [N-Val2]-TRH, an analog devoid of TSH-releasing activity, 5 min prior to a meal, stimulated gastric emptying measured 20 min later. TRH action was dose dependent (1-100 ng), and rapid in onset. The calculated time for emptying half of the meal was decreased from 16 +/- 3 min (control group) to 4 +/- 1 min (TRH 30 ng). The stable analog, RX 77368, unlike TRH, stimulated gastric emptying when the meal was given 60 min after peptide injection. Intravenous injection of atropine (2.5 micrograms) inhibited and that of carbachol (1 microgram) stimulated gastric emptying whereas i.v. injection of TRH (0.1-1 microgram) had no effect. Vagotomy but not adrenalectomy reversed the increase in gastric emptying induced by intracisternal TRH. Atropine blocked the stimulatory effect of TRH and carbachol. These results demonstrate that TRH acts within the brain to stimulate gastric emptying through vagus-dependent and cholinergic pathways whereas alterations of adrenal and pituitary-thyroid secretion do not play an important role.     

Ghrelin response to protein and carbohydrate meals in relation to food intake and glycerol levels in obese subjects

Obese subjects have lower basal and an attenuated decrease of postprandial plasma ghrelin following carbohydrate-rich meals, while the response to protein is unknown. Therefore, plasma ghrelin levels were examined after ingestion of satiating amounts of a protein- or carbohydrate-rich meal in relation to food and energy intake and hunger/satiety ratings in 30 obese subjects followed 240 min later by ad lib sandwiches. Food intake and hunger/satiety ratings were identical while energy intake was significantly greater after bread (861 +/- 62.7 vs. 441 +/- 50.4 kcal, p < 0.001). Second meal food and energy intake were not different. Ghrelin decreased after bread, but increased by 50 pg/ml (p < 0.001) after meat. The corresponding increase of insulin was 55 vs. 9 microU/ml (p < 0.001). Glycerol levels decreased significantly less after the protein meal compared to carbohydrates. After protein glycerol was significantly correlated to the rise of ghrelin but not insulin. These data demonstrate that, in obese subjects, protein has no different satiating effect than carbohydrate despite divergent ghrelin levels. Energy intake corresponds to energy density of the respective food items. Ghrelin response to both meals is qualitatively similar but quantitatively attenuated compared to normal weight subjects. The relationship between ghrelin and glycerol would support recent observations of a possible role of ghrelin in fat metabolism.     

Gastric inhibitory polypeptide does not inhibit gastric emptying in humans

The insulinotropic gut hormone gastric inhibitory polypeptide (GIP) has been demonstrated to inhibit gastric acid secretion and was proposed to possess "enterogastrone" activity. GIP effects on gastric emptying have not yet been studied. Fifteen healthy male volunteers (23.9 +/- 3.3 yr, body mass index 23.7 +/- 2.3 kg/m(2)) were studied with the intravenous infusion of GIP (2 pmol.kg(-1).min(-1)) or placebo, each administered to the volunteers on separate occasions from -30 to 360 min in the fasting state. At 0 min, a solid test meal (250 kcal containing [(13)C]sodium octanoate) was served. Gastric emptying was calculated from the (13)CO(2) exhalation rates in breath samples collected over 360 min. Venous blood was drawn in 30-min intervals for the determination of glucose, insulin, C-peptide, and GIP (total and intact). Statistical calculations were made by use of repeated-measures ANOVA and one-way ANOVA. During the infusion, GIP rose to steady-state concentrations of 159 +/- 15 pmol/l for total and 34 +/- 4 pmol/l for intact GIP (P < 0.0001). Meal ingestion further increased GIP concentrations in both groups, reaching peak levels of 265 +/- 20 and 82 +/- 9 pmol/l for total and 67 +/- 7 and 31 +/- 9 pmol/l for intact GIP during the administration of GIP and placebo, respectively (P < 0.0001). There were no differences in glucose, insulin, and C-peptide between the experiments with the infusion of GIP or placebo. Gastric half-emptying times were 120 +/- 9 and 120 +/- 18 min (P = 1.0, with GIP and placebo, respectively). The time pattern of gastric emptying was similar in the two groups (P = 0.98). Endogenous GIP secretion, as derived from the incremental area under the curve of plasma GIP concentrations in the placebo experiments, did not correlate to gastric half-emptying times (r(2) = 0.15, P = 0.15 for intact GIP; r(2) = 0.21, P = 0.086 for total GIP). We conclude that gastric emptying does not appear to be influenced by GIP. The secretion of GIP after meal ingestion is not suppressed by its exogenous administration. The lack of effect of GIP on gastric emptying underlines the differences between GIP and the second incretin glucagon-like peptide 1.     

Effects of insulin, leptin, and glucagon on ghrelin secretion from isolated perfused rat stomach

Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor, was originally purified from the rat stomach. Although ghrelin has been recognized as an important regulator of energy metabolism, the regulation of the ghrelin secretion is largely unknown. Here, we examined the direct effects of insulin, leptin, and glucagon on the release of ghrelin from the isolated rat stomach. The isolated pancreas-spleen-duodenum deprived preparation of rat stomach was used. After a baseline control infusion into the left gastric artery, insulin, leptin, or glucagon were infused for 15 min at concentrations of 0.1, 1, and 10 nM. The levels of immunoreactive ghrelin in the venous effluents were measured with a radioimmunoassay. Insulin and leptin inhibited ghrelin secretion dose-dependently (total amount of ghrelin release: insulin at 1 nM, 73.5+/-7.3% of the control infusion; leptin at 1 nM, 81.8+/-2.5% of the control infusion; n=5, P<0.05), while glucagon increased it dose-dependently (total amount of ghrelin released at 10 nM was 143.9+/-19.3% of the control infusion; n=5, P<0.01). These results indicate that the ghrelin responses observed in vivo could be due to direct effects of multiple hormonal signals on the stomach.     

The influence of insulin on circulating ghrelin

Ghrelin is a novel peptide that acts on the growth hormone (GH) secretagogue receptor in the pituitary and hypothalamus. It may function as a third physiological regulator of GH secretion, along with GH-releasing hormone and somatostatin. In addition to the action of ghrelin on the GH axis, it appears to have a role in the determination of energy homeostasis. Although feeding suppresses ghrelin production and fasting stimulates ghrelin release, the underlying mechanisms controlling this process remain unclear. The purpose of this study was to test the hypotheses, by use of a stepped hyperinsulinemic eu- hypo- hyperglycemic glucose clamp, that either hyperinsulinemia or hypoglycemia may influence ghrelin production. Having been stable in the period before the clamp, ghrelin levels rapidly fell in response to insulin infusion during euglycemia (baseline ghrelin 207 +/- 12 vs. 169 +/- 10 fmol/ml at t = 30 min, P < 0.001). Ghrelin remained suppressed during subsequent periods of hypoglycemia (mean glucose 53 +/- 2 mg/dl) and hyperglycemia (mean glucose 163 +/- 6 mg/dl). Despite suppression of ghrelin, GH showed a significant rise during hypoglycemia (baseline 4.1 +/- 1.3 vs. 28.2 +/- 3.9 microg/l at t = 120 min, P < 0.001). Our data suggest that insulin may suppress circulating ghrelin independently of glucose, although glucose may have an additional effect. We conclude that the GH response seen during hypoglycemia is not regulated by circulating ghrelin.     

Exercise-induced suppression of acylated ghrelin in humans.

Ghrelin is an orexigenic hormone secreted from endocrine cells in the stomach and other tissues. Acylation of ghrelin is essential for appetite regulation. Vigorous exercise induces appetite suppression, but this does not appear to be related to suppressed concentrations of total ghrelin. This study examined the effect of exercise and feeding on plasma acylated ghrelin and appetite. Nine male subjects aged 19-25 yr participated in two, 9-h trials (exercise and control) in a random crossover design. Trials began at 0800 in the morning after an overnight fast. In the exercise trial, subjects ran for 60 min at 72% of maximum oxygen uptake between 0800 and 0900. After this, they rested for 8 h and consumed a test meal at 1100. In the control trial, subjects rested for 9 h and consumed a test meal at 1100. Area under the curve values for plasma acylated ghrelin concentration (assessed from venous blood samples) were lower over the first 3 h and the full 9 h of the exercise trial compared with the control trial: 317+/-135 vs. 510+/-186 pg.ml(-1).3 h and 917+/-342 vs. 1,401+/-521 pg.ml(-1).9 h (means+/-SE) respectively (P<0.05). Area under the curve values for hunger (assessed using a visual scale) were lower over the first 3 h of the exercise trial compared with the control trial (P=0.013). These findings demonstrate that plasma acylated ghrelin concentration and hunger are suppressed during running.