Amino acids differ in their capacity to stimulate GLP-1 release from the perfused rat small intestine and stimulate secretion by different sensing mechanisms.

The aim of this study was to explore individual amino acid-stimulated GLP-1 responses and the underlying stimulatory mechanisms, as well as to identify the amino acid-sensing receptors involved in amino acid-stimulated GLP-1 release. Experiments were primarily based on isolated perfused rat small intestines, which have intact epithelial polarization allowing discrimination between luminal and basolateral mechanisms as well as quantitative studies of intestinal absorption and hormone secretion. Expression analysis of amino acid sensors on isolated murine GLP-1 secreting L-cells was assessed by qPCR. We found that l-valine powerfully stimulated GLP-1 secretion but only from the luminal side (2.9-fold increase). When administered from the vascular side, l-arginine and the aromatic amino acids stimulated GLP-1 secretion equally (2.6- to 2.9-fold increases). Expression analysis revealed that Casr expression was enriched in murine GLP-1 secreting L-cells, whereas Gpr35, Gprc6a, Gpr142, Gpr93 (Lpar5), and the umami taste receptor subunits Tas1r3 and Tas1r1 were not. Consistently, activation of GPR35, GPR93, GPR142, and the umami taste receptor with specific agonists or allosteric modulators did not increase GLP-1 secretion (P > 0.05 for all experiments), whereas vascular inhibition of CaSR reduced GLP-1 secretion in response to luminal infusion of mixed amino acids. In conclusion, amino acids differ in their capacity to stimulate GLP-1 secretion. Some amino acids stimulated secretion only from the intestinal lumen, whereas other amino acids exclusively stimulated secretion from the vascular side, indicating that amino acid-stimulated GLP-1 secretion involves both apical and basolateral (postabsorptive) sensing mechanisms. Sensing of absorbed amino acids involves CaSR activation as vascular inhibition of CaSR markedly diminished amino acid stimulated GLP-1 release.NEW & NOTEWORTHY Using isolated perfused rat small intestines, we show that amino acids differ in their mechanisms and capacity of stimulating GLP-1 release. Furthermore, we demonstrate that sensing by GPR142, GPR35, GPR93, and the umami taste receptor (Tas1R1/Tas1R3) are not involved in amino acid stimulated GLP-1 release. In contrast to previous studies, this experimental model allows discrimination between the luminal and the vascular side of the intestine, which is essential when studying mechanisms of amino acid-stimulated GLP-1 secretion.

In Vivo Inhibition of Dipeptidyl Peptidase 4 Allows Measurement of GLP-1 Secretion in Mice.

Dipeptidyl peptidase 4 (DPP-4) and neprilysin (NEP) rapidly degrade glucagon-like peptide 1 (GLP-1) in mice. Commercially available sandwich ELISA kits may not accurately detect the degradation products, leading to potentially misleading results. We aimed to stabilize GLP-1 in mice, allowing reliable measurement with sensitive commercially available ELISA kits. Nonanesthetized male C57Bl/6JRj mice were subjected to an oral glucose tolerance test (OGTT; 2 g/kg glucose), and plasma total and intact GLP-1 were measured (Mercodia and Alpco ELISA kits, respectively). No GLP-1 increases were seen in samples taken beyond 15 min after the glucose load. Samples taken at 5 and 10 min after the OGTT showed a minor increase in total, but not intact, GLP-1. We then administered saline (control), or a DPP-4 inhibitor (valine pyrrolidide or sitagliptin) with or without an NEP-inhibitor (sacubitril), 30 min before the OGTT. In the inhibitor groups only, intact GLP-1 increased significantly during the OGTT. After injecting male C57Bl/6JRj mice with a known dose of GLP-1(7-36)NH2, peak GLP-1 levels were barely detectable after saline but were 5- to 10-fold higher during sitagliptin and the combination of sitagliptin/sacubitril. The half-life of the GLP-1 plasma disappearance increased up to sevenfold during inhibitor treatment. We conclude that reliable measurement of GLP-1 secretion is not possible in mice in vivo with commercially available sandwich ELISA kits, unless degradation is prevented by inhibition of DPP-4 and perhaps NEP. The described approach allows improved estimates of GLP-1 secretion for future studies, although it is a limitation that these inhibitors additionally influence levels of insulin and glucagon.

GLP-1 - Incretin and pleiotropic hormone with pharmacotherapy potential. Increasing secretion of endogenous GLP-1 for diabetes and obesity therapy.

Because of the beneficial actions of the hormone glucagon-like peptide-1 on glucose metabolism and appetite, food intake and eventually body weight, and because of the observation that the similar metabolic effects of gastric bypass surgery are associated with excessive secretion of GLP-1, attempts are now being made to stimulate the endogenous secretion of this hormone. By targeting the natural cellular origin of GLP-1 it is anticipated that also the physiological pathways of hormone action (which may include neural mechanisms) would be engaged, which might generate fewer side effects. In addition, release of other products of the responsible intestinal endocrine cells, the L-cells, namely the appetite inhibitory hormone, PYY 3-36, and the dual glucagon-GLP-1 co-agonist, oxyntomodulin, would also be promoted. Here, the normal mechanisms for stimulation of L-cell secretion are reviewed, and the potential of identified secretagogues is discussed. Paracrine regulation of L-cell secretion is also discussed and the potential of somatostatin receptor antagonists is emphasized. A therapeutic approach based on stimulation of endogenous secretion of GLP-1/PYY still seems both attractive and potentially feasible.

Ghrelin Does Not Directly Stimulate Secretion of Glucagon-like Peptide-1.

CONTEXT: The gastrointestinal hormone ghrelin stimulates growth hormone secretion and appetite, but recent studies indicate that ghrelin also stimulates the secretion of the appetite-inhibiting and insulinotropic hormone glucagon-like peptide-1 (GLP-1). OBJECTIVE: To investigate the putative effect of ghrelin on GLP-1 secretion in vivo and in vitro. SUBJECTS AND METHODS: A randomized placebo-controlled crossover study was performed in eight hypopituitary subjects. Ghrelin or saline was infused intravenously (1 pmol/min × kg) after collection of baseline sample (0 min), and blood was subsequently collected at time 30, 60, 90, and 120 minutes. Mouse small intestine was perfused (n = 6) and GLP-1 output from perfused mouse small intestine was investigated in response to vascular ghrelin administration in the presence and absence of a simultaneous luminal glucose stimulus. Ghrelin receptor expression was quantified in human (n = 11) and mouse L-cells (n = 3) by RNA sequencing and RT-qPCR, respectively. RESULTS: Ghrelin did not affect GLP-1 secretion in humans (area under the curve [AUC; 0-120 min]: ghrelin infusion = 1.37 ± 0.05 min × nmol vs. saline infusion = 1.40 ± 0.06 min × nmol [P = 0.63]), but induced peripheral insulin resistance. Likewise, ghrelin did not stimulate GLP-1 secretion from the perfused mouse small intestine model (mean outputs during baseline/ghrelin infusion = 19.3 ± 1.6/25.5 ± 2.0 fmol/min, n = 6, P = 0.16), whereas glucose-dependent insulinotropic polypeptide administration, used as a positive control, doubled GLP-1 secretion (P < 0.001). Intraluminal glucose increased GLP-1 secretion by 4-fold (P < 0.001), which was not potentiated by ghrelin. Finally, gene expression of the ghrelin receptor was undetectable in mouse L-cells and marginal in human L-cells. CONCLUSIONS: Ghrelin does not interact directly with the L-cell and does not directly affect GLP-1 secretion.

Short-chain fatty acids and regulation of pancreatic endocrine secretion in mice.

The intestinal microbiota has been demonstrated to influence host metabolism, and has been proposed to affect the development of obesity and type 2 diabetes (T2D), possibly through short-chain fatty acids (SCFAs) produced by fermentation of dietary fiber. There are some indications that SCFAs inhibit glucose-stimulated insulin secretion (GSIS) in rodents, but research on this subject is sparse. However, it has been reported that receptors for SCFAs, free fatty acid receptor 2 (FFAR2) and FFAR3 are expressed not only on gut endocrine cells secreting GLP-1 and PYY, but also on pancreatic islet cells. We hypothesized that SCFAs might influence the endocrine secretion from pancreatic islets similar to their effects on the enteroendocrine cells. We studied this using isolated perfused mouse pancreas which responded adequately to changes in glucose and to infusions of arginine. None of the SCFAs, acetate, propionate and butyrate, influenced glucagon secretion, whereas they had weak inhibitory effects on somatostatin and insulin secretion. Infusions of two specific agonists of FFAR2 and FFAR3, CFMB and Compound 4, respectively, did not influence the pancreatic secretion of insulin and glucagon, whereas both induced strong increases in the secretion of somatostatin. In conclusion, the small effects of acetate, propionate and butyrate we observed here may not be physiologically relevant, but the effects of CFMB and Compound 4 on somatostatin secretion suggest that it may be possible to manipulate pancreatic secretion pharmacologically with agonists of the FFAR2 and 3 receptors, a finding which deserves further investigation.