Ghrelin action on gastrointestinal functions and appetite in rat and man

Abstract

To study the effect of ghrelin on the contractility of smooth muscle strips in vitro as well as the effects on fasting small bowel motility and acid secretion in vivo in rats and gastric emptying in rats and humans. To study the effect of peripherally administered ghrelin on plasma levels of glucose, insulin, glucagon, orexin A (OXA), somatostatin and gastrin in rats and ghrelin, cholecystokinin (CCK), glucagonlike peptide-1 (GLP-1), peptide YY (PYY) and motilin in humans. To study the association between ghrelin leptin and adiponectin, states of hunger and fullness and eating behavior traits as measured by psychometric tests in lean and obese women.

Methods: The distribution of ghrelin, the ghrelin receptor (GRLN) and the growth hormone secretagog uereceptor 1b (GHS-R 1b) was studied in the gut using reversed transcriptase PCR (RT-PCR) in humans. In rats, isometric contractions of segments of gut and jejunum were studied in organ baths. The migrating myoelectric complex (MMC) was studied during IV infusion of ghrelin. In separate experiments, the rats were pre-treated with atropine or vagotomy. Gastric acid secretion and emptying of a nutrient or nonnutrient 51Cr-labelled liquid meal were studied in rats subjected to infusion of ghrelin or saline. In humans, gastric emptying of a 99mTc-labelled omelette was studied scintigraphically during infusion of ghrelin or saline in healthy volunteers.

Simultaneously, appetite ratings were measured using visual analogue scales (VAS). Plasma concentrations of ghrelin, somatostatin, insulin, glucagon and gastrin were analysed with radioimmunoassay (RIA). In another set of experiments, gastric emptying was studied in GH-deficient patients before and six months after substitution therapy. The three factor eating questionnaire (TFEQ) was used to measure Restraint (R), Disinhibition (D) and Hunger (H) in obese and normal weight women. Immediately before, after and in the period between a fixed breakfast and lunch, blood samples were taken and subjective sensation of hunger and satiety using VAS were measured. Plasma concentrations of ghrelin, leptin and adiponectin were assessed with RIA.

Results: Expression of the ghrelin gene, the GRLN and the GHS-1b-genes were found in the antrum and the corpus of the stomach. Ghrelin caused concentration-dependent contractions of jejunum and the fundus of the stomach. Pre-treatment with atropine abolished this effect. Ghrelin dose-dependently shortened the MMC-cycle length and decreased the duration of phase Ill at the duodenal recording site at all doses. The effect was blocked by atropine or vagotomy. W ghrelin stimulated gastric emptying in man. Gastric emptying was not different before and after GH-substitution in the GH-deficient subjects. Infusion of ghrelin to awake rats increased gastric emptying of a non-nutrient liquid. Ghrelin significantly inhibited pentagastrin-stimulated acid secretion at the highest dose studied (the inhibition seen in lower doses did not reach significance). Hunger and desire to eat were significantly higher and fullness ratings lower during ghrelin infusion. Plasma concentrations of leptin, ghrelin and adiponectin were correlated with Disinhibition (D) scores of the TFEQ in women of varying BM]. There was also a correlation between fasting leptin and Hunger (H). Only GLP-1 correlated with states, as measured by VAS, hunger (r=-0.42) and desire to eat (r=-0.36).

Conclusion: Ghrelin potently stimulates gastric emptying and appetite in humans and motility in rats, whereas the pentagastrin-stimulated gastric acid secretion in rats is inhibited. The effect seen on motility seems to be mediated via the vagus nerve. Ghrelin, leptin and adiponectin correlate to the eating behavior traits Disinhibition and Hunger, whereas GLP-1 correlates to the state hunger. Therefore these peptides could be regarded as biomarkers for the psychological traits involved in food intake. Ghrelin can be seen as a peptide that prepares the digestive tract for food consumption.