Tachykinins stimulate release of peptide hormones (glucagon-like peptide-1) and paracrine (somatostatin) and neurotransmitter (vasoactive intestinal polypeptide) from porcine ileum through NK-1 receptors

Neural regulation of glucagon-like peptide-1 secretion in pigs

Glucagon-like peptide (GLP)-1 is secreted rapidly from the intestine postprandially. We therefore investigated its possible neural regulation. With the use of isolated perfused porcine ileum, GLP-1 secretion was measured in response to electrical stimulation of the mixed, perivascular nerve supply and infusions of neuroactive agents alone and in combination with different blocking agents. Electrical nerve stimulation inhibited GLP-1 secretion, an effect abolished by phentolamine. Norepinephrine inhibited secretion, and phentolamine abolished this effect. GLP-1 secretion was stimulated by isoproterenol (abolished by propranolol). Acetylcholine stimulated GLP-1 secretion, and atropine blocked this effect. Dimethylphenylpiperazine stimulated GLP-1 secretion. In chloralose-anesthetized pigs, however, electrical stimulation of the vagal trunks at the level of the diaphragm had no effect on GLP-1 or GLP-2 and weak effects on glucose-dependent insulinotropic peptide and somatostatin secretion, although this elicited a marked atropine-resistant release of the neuropeptide vasoactive intestinal polypeptide to the portal circulation. Thus GLP-1 secretion is inhibited by the sympathetic nerves to the gut and may be stimulated by intrinsic cholinergic nerves, whereas the extrinsic vagal supply has no effect.

Glucagon-like peptide-1 secretion is influenced by perfusate glucose concentration and by a feedback mechanism involving somatostatin in isolated perfused porcine ileum

Glucagon-like peptide-1 (GLP-1) is released from intestinal L-cells in response to ingestion of meals. The mechanisms regulating its secretion are not clear, but local somatostatin (SS) restrains GLP-1 secretion. We investigated feedback and substrate regulation of GLP-1 and SS secretion, using isolated perfused porcine ileum (n=17). Effluents were measured for GLP-1 and SS. Perfusion pressure and motility were recorded. Investigated parameters included spontaneous fluctuations, changes in perfusate glucose concentrations (3.5, 5, 11 mM) and addition of insulin (1 nM). We also investigated the effect of proglucagon products, glucagon (10 nM), GLP-1 and GLP-2 (0.1, 1, and 10 nM) on GLP-1 and SS secretion, as well as on glucagon-like peptide-2 (GLP-2), peptide YY (PYY) and GIP secretion, all possible product of L-cells or neighbour cells. Perfusate glucose concentration dose-dependently stimulated GLP-1 secretion (p=0.011). Insulin had no effect. Glucagon weakly stimulated GIP secretion. GLP-1 stimulated SS secretion and motor activity, but inhibited GLP-2, GIP and PYY secretion and perfusion pressure. GLP-2 weakly stimulated SS secretion. We conclude (a) that GLP-1 secretion is influenced by perfusate glucose concentration and (b) that L-cell secretion is feedback regulated by GLP-1 itself, probably via paracrine SS activity.

Intraduodenal capsaicin inhibits gastric migrating motor complex via an extrinsic neural reflex in conscious dogs

The aim was to study the effect of intraduodenal capsaicin on interdigestive gastric contractions. Mongrel dogs were equipped with strain-gauge force transducers to measure gastroduodenal motility. The effects of intraduodenal capsaicin with or without pharmacological antagonists on spontaneous and motilin-induced interdigestive gastric contractions and on plasma motilin were studied in dogs with intact stomachs. The effect of intraduodenal capsaicin on gastric contractions was also studied in vagally denervated gastric (Heidenhain) pouch and vagally innervated antral pouch. Intraduodenal capsaicin inhibited spontaneous and motilin-induced gastric contractions. The spontaneous peak in plasma motilin was inhibited by intraduodenal capsaicin. The effect of intraduodenal capsaicin on motilin-induced gastric contractions was not affected by blockade of nitric oxide synthase, or by beta-adrenoceptor antagonist. Administration of alpha-adrenergic blocker inhibited basal interdigestive gastric motility. Intraduodenal capsaicin had no effect on contractions in the Heidenhain pouch but inhibited those in vagally innervated antral pouch. Duodenal afferent fibres stimulated by capsaicin inhibit gastric contractions via a nitric oxide-independent extrinsic neural reflex.