Glucagon-like peptide-1 relaxes gastric antrum through nitric oxide in mice

Abstract

Glucagon-like-peptide-1 (GLP-1) is a proglucagon-derived peptide expressed in the intestinal enteroendocrine-L cells and released after meal ingestion. GLP-1 reduces postprandial glycemia not only by its hormonal effects, but also by its inhibitory effects on gastrointestinal motility. Recently, we showed that GLP-1 acts in the enteric nervous system of mouse intestine. Therefore our working hypothesis was that GLP-1 may have also a direct influence on the gastric mechanical activity since the major part of experimental studies about its involvement in the regulation of gastric motility have been conducted in in vivo conditions. The purposes of this study were (i) to examine exogenous GLP-1 effects on mouse gastric mechanical activity using isolated whole stomach; (ii) to clarify the regional activity of GLP-1 using circular muscular strips from gastric fundus or antrum; (iii) to analyze the mechanism of action underlying the observed effects; (iv) to verify regional differences of GLP-1 receptors (GLP-1R) expression by RT-PCR. In the whole stomach GLP-1 caused concentration-dependent relaxation significantly anatagonized by exendin (9–39), an antagonist of GLP-1R and abolished by tetrodotoxin (TTX) or N_ω-nitro-l-arginine methyl ester (l-NAME), inhibitor of nitric oxide (NO) synthase. GLP-1 was without any effect in fundic strips, but it induced concentration-dependent relaxation in carbachol-precontracted antral strips. The effect was abolished by TTX or l-NAME. RT-PCR analysis revealed a higher expression of GLP-1R mRNA in antrum than in fundus. These results suggest that exogenous GLP-1 is able to reduce mouse gastric motility by acting peripherally in the antral region, through neural NO release.

Introduction

Glucagon-like peptide-1 (GLP-1) is a gastrointestinal regulatory peptide secreted from the mucosal endocrine L cells in response to the nutrient ingestion [31]. GLP-1 acts mainly as an incretin enhancing glucose-stimulated insulin secretion and inducing insulin gene expression [13]. It also stimulates somatostatin release and inhibits glucagon secretion [36]. These properties provide the rationale for its potential use as a therapeutic agent in the treatment of diabetes [5], [19], [36]. In addition, GLP-1 acts as an anorexigen peptide decreasing hunger feelings. In fact, GLP-1 inhibits food intake upon intracerebroventricular injection in animal models and after intravenous infusion in normal weight and obese humans [14], [18], [22], [40].

GLP-1 reduces postprandial glycemia not only by its hormonal effects (i.e., reduced glucagon and increased insulin release) but also by its effects on gastrointestinal motility [10]. In fact, GLP-1, which is one of the mediators of the ileal brake [15], reduces gut motility [20], [25], [34], [38], retards gastric emptying of liquid and solid meals, inhibits antro-pyloro-duodenal motility [4], [20], [28], [30], [37], [38], [41], [42] and increases gastric accommodation [2], [3], [11]. However, the GLP-1 action mechanism on the gastrointestinal motility is unclear yet.

The peptide actions are initiated by activation of specific G protein-coupled receptors (GLP-1 receptor: GLP-1R) [19]. GLP-1R has been identified in several regions of central nervous system that control feeding behavior as brainstem and hypothalamus and in the nodose ganglion of the vagus nerve [7], [27]. In rodents and humans GLP-1R is also expressed in pancreatic islets, brain, heart, kidney, small and large intestine and in the stomach [7], [8], [12], [21], [27], [39].

The distribution of the GLP-1R in the central nervous system [16] and in the nodose ganglion [27] together with functional evidence suggest that GLP-1 effects on gastrointestinal motility are exerted through the interaction with centers in the brain or afferent neural pathways [5], [6], [20], [21], [30], [37]. However, recently we have shown that GLP-1 can act through a peripheral mechanism by binding GLP-1R present in murine colonic and duodenal myenteric neurons [1].

Therefore, it is likely to hypothesize that GLP-1 may have also a direct influence on the gastric mechanical activity since the major part of experimental studies about its involvement in the regulation of gastric motility have been conducted in in vivo conditions [17], [20], [29], [38], [42].

The present study was undertaken to examine the effects of exogenous GLP-1 on mouse gastric spontaneous mechanical activity and to analyze the mechanism of action responsible for the observed effects. The muscle function of whole stomach was examined in vitro where the influence of external factor is removed, but the muscle performs in a manner analogues to its in vivo capacity [26]. Furthermore, to clarify the regional activity of GLP-1, the effects of the peptide were tested on circular muscular strips from gastric fundus or antrum and by RT-PCR was investigated the possible regional differences in the expression of GLP-1R in mouse stomach.