Vasoactive intestinal polypeptide actions on the guinea pig intestinal mucosa during neural stimulation

Glucagon-like peptide-1 modulates neurally evoked mucosal chloride secretion in guinea pig small intestine in vitro

Glucagon-like peptide-1 (GLP-1) acts at the G protein-coupled receptor, GLP-1R, to stimulate secretion of insulin and to inhibit secretion of glucagon and gastric acid. Involvement in mucosal secretory physiology has received negligible attention. We aimed to study involvement of GLP-1 in mucosal chloride secretion in the small intestine. Ussing chamber methods, in concert with transmural electrical field stimulation (EFS), were used to study actions on neurogenic chloride secretion. ELISA was used to study GLP-1R effects on neural release of acetylcholine (ACh). Intramural localization of GLP-1R was assessed with immunohistochemistry. Application of GLP-1 to serosal or mucosal sides of flat-sheet preparations in Ussing chambers did not change baseline short-circuit current (I(sc)), which served as a marker for chloride secretion. Transmural EFS evoked neurally mediated biphasic increases in I(sc) that had an initial spike-like rising phase followed by a sustained plateau-like phase. Blockade of the EFS-evoked responses by tetrodotoxin indicated that the responses were neurally mediated. Application of GLP-1 reduced the EFS-evoked biphasic responses in a concentration-dependent manner. The GLP-1 receptor antagonist exendin-(9-39) suppressed this action of GLP-1. The GLP-1 inhibitory action on EFS-evoked responses persisted in the presence of nicotinic or vasoactive intestinal peptide receptor antagonists but not in the presence of a muscarinic receptor antagonist. GLP-1 significantly reduced EFS-evoked ACh release. In the submucosal plexus, GLP-1R immunoreactivity (IR) was expressed by choline acetyltransferase-IR neurons, neuropeptide Y-IR neurons, somatostatin-IR neurons, and vasoactive intestinal peptide-IR neurons. Our results suggest that GLP-1R is expressed in guinea pig submucosal neurons and that its activation leads to a decrease in neurally evoked chloride secretion by suppressing release of ACh at neuroepithelial junctions in the enteric neural networks that control secretomotor functions.

Glucagon-like peptide-2 modulates neurally evoked mucosal chloride secretion in guinea pig small intestine in vitro

Glucagon-like peptide-2 (GLP-2) is an important neuroendocrine peptide in intestinal physiology. It influences digestion, absorption, epithelial growth, motility, and blood flow. We studied involvement of GLP-2 in intestinal mucosal secretory behavior. Submucosal-mucosal preparations from guinea pig ileum were mounted in Ussing chambers for measurement of short-circuit current (I(sc)) as a surrogate for chloride secretion. GLP-2 action on neuronal release of acetylcholine was determined with ELISA. Enteric neuronal expression of the GLP-2 receptor (GLP-2R) was studied with immunohistochemical methods. Application of GLP-2 (0.1-100 nM) to the serosal or mucosal side of the preparations evoked no change in baseline I(sc) and did not alter transepithelial ionic conductance. Transmural electrical field stimulation (EFS) evoked characteristic biphasic increases in I(sc), with an initially rapid rising phase followed by a sustained phase. Application of GLP-2 reduced the EFS-evoked biphasic responses in a concentration-dependent manner. The GLP-2R antagonist GLP-2-(3-33) significantly reversed suppression of the EFS-evoked responses by GLP-2. Tetrodotoxin, scopolamine, and hexamethonium, but not vasoactive intestinal peptide type 1 receptor (VPAC1) antagonist abolished or reduced to near zero the EFS-evoked responses. GLP-2 suppressed EFS-evoked acetylcholine release as measured by ELISA. Pretreatment with GLP-2-(3-33) offset this action of GLP-2. In the submucosal plexus, GLP-2R immunoreactivity (-IR) was expressed in choline acetyltransferase-IR neurons, somatostatin-IR neurons, neuropeptide Y-IR neurons, and vasoactive intestinal peptide-IR neurons. We conclude that submucosal neurons in the guinea pig ileum express GLP-2R. Activation of GLP-2R decreases neuronally evoked epithelial chloride secretion by suppressing acetylcholine release from secretomotor neurons.

Neurogenic secretion mediated by the purinergic P2Y1 receptor in guinea-pig small intestine

We tested the hypothesis that ATP is an enteric neurotransmitter that acts at P2Y1 excitatory purinergic receptors on intestinal secretomotor neurons to evoke neurogenic mucosal secretion in the guinea pig. Ussing chamber methods for studying neurogenic intestinal secretion were used to test the hypothesis. Application of ATP evoked concentration-dependent increases in short circuit current (Isc) indicative of stimulation of electrolyte secretion. MRS2179, a selective P2Y1 purinergic receptor antagonist, suppressed the ATP-evoked responses in a concentration-dependent manner with an IC50 of 0.9+/-0.1 microM. Tetrodotoxin or a selective vasoactive intestinal peptide (VPAC1) receptor antagonist suppressed or abolished the ATP-evoked responses. A selective VPAC1 receptor antagonist also suppressed Isc responses evoked by electrical field stimulation of the secretomotor neurons. Secretory responses to ATP were not suppressed by scopolamine, piroxicam nor selective adenosine receptor antagonists. Region-specific differences in responses to ATP corresponded to regional differences in the expression of mRNA transcripts for the P2Y1 receptor. Post-receptor signal transduction for the P2Y1-evoked responses involved stimulation of phospholipase C and an IP3/Ca2+-calmodulin/protein kinase C signaling cascade. Our evidence suggests that ATP is released as a neurotransmitter to stimulate neurogenic mucosal secretion by binding to P2Y1 receptors expressed by VIP-ergic secretomotor neurons.

Mediation of neurogenic ion transport by acetylcholine, prostanoids and 5-hydroxytryptamine in porcine ileum

Enteric neural activity modulates active transepithelial ion transport in the intestine. We investigated the neural circuits mediating neurogenic secretion in mucosal explants from porcine ileum. Transmural electrical stimulation increased short-circuit current, a measure of active ion transport, by 35+/-2 microA/cm2. The neuronal Na+ channel blocker saxitoxin, the muscarinic cholinergic receptor antagonist atropine, the 5-hydroxytryptamine3 receptor antagonist tropisetron, and the cyclooxygenase inhibitor indomethacin inhibited this response. In addition, tropisetron inhibited the atropine-resistant portion of the response, and both atropine and indomethacin attenuated the saxitoxin-resistant component. Neurogenic secretion in porcine ileum appears to be mediated by tryptaminergic and prostanoid-sensitive cholinergic pathways.

Chemical coding of the human gastrointestinal nervous system: cholinergic, VIPergic, and catecholaminergic phenotypes

The aim of this investigation was to identify the proportional neurochemical codes of enteric neurons and to determine the specific terminal fields of chemically defined nerve fibers in all parts of the human gastrointestinal (GI) tract. For this purpose, antibodies against the vesicular monoamine transporters (VMAT1/2), the vesicular acetylcholine transporter (VAChT), tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), serotonin (5-HT), vasoactive intestinal peptide (VIP), and protein gene product 9.5 (PGP 9.5) were used. For in situ hybridization (35)S-labeled VMAT1, VMAT2, and VAChT riboprobes were used. In all regions of the human GI tract, 50-70% of the neurons were cholinergic, as judged by staining for VAChT. The human gut unlike the rodent gut exhibits a cholinergic innervation, which is characterized by an extensive overlap with VIPergic innervation. Neurons containing VMAT2 constituted 14-20% of all intrinsic neurons in the upper GI tract, and there was an equal number of TH-positive neurons. In contrast, DBH was absent from intrinsic neurons. Cholinergic and monoaminergic phenotypes proved to be completely distinct phenotypes. In conclusion, the chemical coding of human enteric neurons reveals some similarities with that of other mammalian species, but also significant differences. VIP is a cholinergic cotransmitter in the intrinsic innervation of the human gut. The substantial overlap between VMAT2 and TH in enteric neurons indicates that the intrinsic catecholaminergic innervation is a stable component of the human GI tract throughout life. The absence of DBH from intrinsic catecholaminergic neurons indicates that these neurons have a dopaminergic phenotype.

Vasoactive intestinal peptide is a neuropeptide mediator of the secretory response to serotonin in rat

BACKGROUND

The chloride secretory response to serotonin (5-HT) has nonneural and neural mechanisms, the latter mediated through a 5-HT(3) receptor. We hypothesized that 5-HT(3)-induced C1(-) secretion is partially mediated by VIP as a neurosecretory transmitter. Therefore it should be inhibited by a VIP receptor antagonist, VIP 6-28. Furthermore, exogenous VIP should induce secretion in the presence of tetrodotoxin (TTX).

METHODS

Unstripped sheets of rat colon (n = 6) were mounted in Ussing chambers. The 5-HT(3) receptor agonist 2-Me-5-HT (10 microM) was added in the absence and presence of VIP 6-28 (30 microM). In companion studies VIP (1 microM) was added to tissue with or without TTX. Changes in short-circuit current (DeltaI(SC)) were recorded and repeat-measure ANOVA was used to analyze data.

RESULTS

Addition of 2-Me-5-HT induced a rise in DeltaI(SC) seen in controls at 1 to 5 min (3.2 +/- 1.5 to 12.3 +/- 3.7 microA/cm(2), P < 0.02). VIP 6-28 blunted DeltaI(SC) (1.2 +/- 0.4 to 3.7 +/- 1.3 microA/cm(2), P < 0.01). VIP caused DeltaI(SC) to increase above baseline in 15 min (4.7 +/- 2.6 to 10.4 +/- 3.0 microA/cm(2), P < 0.01). The addition of TTX prior to VIP did not alter DeltaI(SC).

CONCLUSION

Activation of the neural 5-HT(3) receptor by 2-Me-5-HT induces a secretory response in rat colon that is inhibited by a VIP receptor antagonist. Exogenous VIP mimics this response and is unaffected by TTX. VIP is a likely nonadrenergic, noncholinergic neurotransmitter in this pathway.

Expression of mRNA for the N-methyl-D-aspartate (NMDAR1) receptor and vasoactive intestinal polypeptide (VIP) co-exist in enteric neurons of the rat

Anatomical, physiological and pharmacological evidence suggests that vasoactive intestinal polypeptide neurons are important mediators of the descending inhibitory component of intestinal peristalsis. Pharmacological data also indicate that glutamate may modulate intestinal motility. Recently, we demonstrated that mRNA coding for the glutamate N-methyl-D-aspartate (NMDA) receptor is expressed by rat enteric neurons. In order to ascertain whether NMDA receptor message is expressed by vasoactive intestinal polypeptide (VIP) neurons, we employed a double in situ hybridization technique. We hybridized tissue sections from the stomach, ileum and descending colon of adult rats with an NMDAR1 oligoprobe and a VIP oligonucleotide. Enteric neurons expressing mRNA for both NMDA and VIP were found in the myenteric and submucosal ganglia at all of the sampling sites. These data suggest that the mechanism for glutamatergic excitation is present in VIP-containing enteric neurons.

Activation of ion transport by combined effects of ionomycin, forskolin and phorbol ester on cultured HT-29cl.19A human colonocytes

The differentiated clone 19A of the HT-29 human colon carcinoma cell line was used as a model to study the intracellular electrophysiological effects of interaction of the cAMP, the protein kinase C (PKC) and the Ca2+ pathways. (a) A synergistic effect between ionomycin and forskolin was observed. From intracellular responses it was concluded that the synergistic effect is caused by activation of an apical Cl- conductance by protein kinase A and a basolateral K+ conductance by Ca2+. (b) A transient synergistic effect of ionomycin and the phorbol ester phorbol dibutyrate (PDB) was found. The decrease of the response appeared to be due to PKC-dependent inactivation of the basolateral K+ conductance. The synergism is caused by PKC-dependent increase of the apical Cl- conductance and Ca(2+)-dependent increase of the basolateral K+ conductance. (c) The effects of carbachol and PDB were not fully additive presumably because of their convergence on PKC activation. (d) Forskolin and PDB, when added in this order, had a less than additive effect. Results of cell-attached patch-clamp studies, presented in the accompanying paper, showed a synergistic effect of forskolin and PDB on non-rectifying small-conductance Cl- channels. Assuming that these channels are involved in the transepithelial responses it is suggested that forskolin and PDB induce a modulatory, synergistic increase of the apical Cl- conductance when both pathways are activated simultaneously. (e) The HT-29cl.19A cells differ from T84 cells in that the latter did n ot respond with an increase of the short-circuit current to addition of phorbol ester. this may be due to a very low expression of PKA alpha.

The enteric nervous system modulates mammalian duodenal mucosal bicarbonate secretion

BACKGROUND

Interaction of the enteric nerves in regulating mammalian duodenal mucosal bicarbonate secretion is not well understood. The purpose of the present experiments was to evaluate the role of the enteric nervous system on bicarbonate secretion from rabbit duodenal mucosa in vitro.

METHODS

Proximal duodenum from male New Zealand White rabbits was stripped of seromuscular layers, mounted in Ussing chambers, and studied under short-circuited conditions. Effects of electrical field stimulation, vasoactive intestinal polypeptide (VIP), carbachol, prostaglandin E2 (PGE2), dibutyryl-cyclic adenosine monophosphate (db-cAMP), and the neurotoxin tetrodotoxin (TTX) and muscarinic blockade by atropine were studied.

RESULTS

Electrical field stimulation significantly (P < 0.01) stimulated bicarbonate secretion, short-circuit current (Isc), and electrical potential difference (PD) that was sensitive to both TTX and atropine. VIP-stimulated bicarbonate secretion was significantly inhibited by TTX (-73%), yet Isc and PD remained unchanged. Atropine decreased VIP-induced bicarbonate secretion (-69%) and Isc (-43%). Carbachol-stimulated bicarbonate secretion, Isc, and PD were abolished by atropine, whereas TTX was without affect. Neither TTX nor atropine had a significant effect on PGE2 or db-cAMP-stimulated bicarbonate secretion.

CONCLUSIONS

These results suggest that (1) enteric nerve stimulation activates an acetylcholine receptor that in turn stimulates duodenal epithelial bicarbonate secretion; (2) VIP stimulates bicarbonate secretion, in large part, via the enteric nervous system; and (3) PGE2 and cAMP stimulate bicarbonate secretion independent of the enteric nervous system.

Effects of substance P and vasoactive intestinal polypeptide on contractile activity and epithelial transport in the ferret jejunum

Previous studies in the ferret demonstrated that vagal nerve stimulation induced an atropine-resistant water secretion. Substance P and vasoactive intestinal polypeptide are possible mediators of this secretory response. The objectives of this study were to investigate the in vivo effects of substance P and vasoactive intestinal polypeptide on the jejunal musculature and epithelium. Substance P caused an increase in jejunal motility, water secretion, and transmural potential difference. Cholinergic blockade did not affect the substance P-induced contractions, but did reduce the increase in transmural potential difference, suggesting an inhibition of water secretion. Vasoactive intestinal polypeptide abolished motor activity; however, it induced an increase in transmural potential difference that was atropine and tetrodotoxin resistant. By immunohistochemical methods, immunoreactive vasoactive intestinal polypeptide and immunoreactive substance P were localized to both nerve cell bodies and nerve fibers in the ferret intestine. Determination of intestinal concentrations of vasoactive intestinal polypeptide and substance P in the ferret showed concentrations of these two neuropeptides that were similar to those in human intestine and demonstrated much higher concentrations of these substances in the muscular layer than in the epithelial layer. Our data demonstrate that in the ferret substance P excites and vasoactive intestinal polypeptide inhibits jejunal motor activity. However, both peptides increase water secretion. Our results suggest that in response to vagal stimulation, neuronally released substance P or vasoactive intestinal polypeptide may participate in the atropine-resistant water secretion.

Enkephalin affects ion transport via the enteric nervous system in guinea-pig ileum

The endogenous opioid enkephalin drives ion transport towards absorption. To determine the site and mechanism of this effect, fractionated stripping of guinea-pig ileum was carried out. The muscularis propria, including myenteric plexus, was removed by partial stripping. The submucosa, including the submucosal plexus, plus the muscularis mucosae were removed by total stripping. For binding studies, epithelial cells were removed by the method of Weiser leaving the lamina propria mucosae with the mucosal plexus. Radio-receptor-assay with (3H)2-D-ala-5-D-leu-enkephalin revealed enkephalin binding sites in the submucosa plus muscularis mucosae (KD = 3.6 nmol l-1; Vmax = 7.3 fmol mg-1) and in the lamina propria mucosae (KD = 4.2 nmol l-1; Vmax = 5.1 fmol mg-1. The binding was stereospecific in both layers. No binding was detected on epithelial cells. In the Ussing chamber, partially stripped ileum exhibited spontaneous ISC which was abolished by addition of tetrodotoxin (TTX) or by total stripping indicating that this ISC was neuronally stimulated by the submucosal plexus. Electrogenic chloride secretion was identified as contributing to this ISC, since the TTX-sensitive part of ISC in the partially stripped ileum was lacking in Cl- and HCO3-free medium, reappeared after addition of Cl consistent with Michaelis-Menten kinetics (Km = 19 nmol l-1) and was reversed by serosal addition of bumetanide. In addition, enkephalin increased electroneutral NaCl-absorption as obtained by Na- and Cl-flux measurements. Enkephalin decreased this spontaneous neuronally stimulated electrogenic Cl-secretion in the partially stripped ileum, but had no effect in totally stripped ileum if ISC was stimulated at the cellular level by theophylline or PGE1. We conclude that ganglia located in the submucosal plexus regulate intestinal ion transport. Enkephalin acts by presynaptic inhibition via receptors on these neurons in the submucosa and/or via receptors on their neurites in the lamina propria mucosae.

Vasoactive intestinal peptide interacts with alpha-adrenergic-, cholinergic-, and substance-P-mediated responses in rat parotid and submandibular glands

Secretory dose-response curves were obtained with both acetylcholine and phenylephrine treatment in rat parotid and submandibular glands. Vasoactive intestinal peptide (VIP), which produced relatively low volumes of protein-rich saliva in rat salivary glands, also enhanced acetylcholine-, phenylephrine-, and substance-P-mediated fluid and protein secretion when administered in combination with these agents. The specific mechanisms involved in the synergistic actions of VIP with substances such as acetylcholine, phenylephrine, and substance P, which are primarily linked to the production of fluid secretion in rat salivary glands, have yet to be determined.

Novel autonomic neurotransmitters and intestinal function

A wide variety of substances, including amines and peptides, have been detected within the complex neuronal pathways of the enteric nervous system using immunohistochemical techniques. In this article we have discussed some of the more recent data on the effects of these substances on intestinal activity. We have also commented on the many difficulties associated with ascribing neurotransmitter status to individual compounds. The technique of immunoblockade of neurogenic functional responses has been used in an attempt to identify some of the putative neurotransmitter substances. The search for selective antagonists continues.

Regulation of epithelial transport in the jejunal mucosa of the guinea pig by neurokinins

This study characterizes the actions of the neurokinins and calcitonin-gene related peptide (CGRP) on electrolyte transport across the mucosa of the guinea pig jejunum in vitro in a modified Ussing chamber. By following changes in short circuit current (Isc) induced by substance P (SP) and neurokinins A & B (NKA & NKB) in the presence and absence of tetrodotoxin (TTX) and atropine, it was established that two distinct neurokinin receptors are involved in the regulation of electrolyte transport. NKA preferentially activates a neuronal receptor since the actions of this neurokinin were inhibited by both TTX and atropine. SP, whose actions were reduced to a lesser extent by TTX and atropine, is considered to activate preferentially a receptor on the epithelial cells. The third neurokinin, NKB, appears to act non-selectively on both the neuronal and epithelial receptors. CGRP, which per se did not affect Isc, markedly potentiated the increases in Isc induced by SP and NKB, and thus acts synergistically with the epithelial neurokinin receptor. These results suggest that two distinct neurokinin receptors (the NK-1 and the NK-2) regulate epithelial transport in the jejunal mucosa of the guinea pig, and furthermore indicate that at least one of the peptides found in enteric nerves (i.e. CGRP) modulates the actions of neurokinins on epithelial cells.

Pancreatic cholera syndrome due to a vasoactive intestinal polypeptide-producing tumor: further insights into the pathophysiology

This case report describes a patient with pancreatic cholera caused by a vasoactive intestinal polypeptide-producing pancreatic tumor. The case presents several unusual characteristics of this disease. The primary tumor was a mucinous adenocarcinoma of the pancreas. The serum vasoactive intestinal polypeptide level of 2400 pmol/L is the highest reported. At this vasoactive intestinal polypeptide level, the somatostatin analogue SMS 201-995 at doses up to 2 mg/24 h did not control the 21 L/24 h stool output. Fecal incontinence due to a manometrically documented hypotonic internal anal sphincter occurred. Using surgically created stomas, the segmental gastrointestinal fluid and sodium losses were shown to be greatest from the jejunum, whereas potassium losses from the colon and small intestine were equal. The cellular mechanism for the small intestinal potassium secretion is not known.

Receptor regulation of ion transport in the intestinal epithelium

The active transport of ions by the intestinal epithelium is regulated by a number of enteric neurotransmitters, hormones and other substances. Our knowledge of the receptors mediating the actions of these substances is generally fragmentary. This review summarizes current knowledge on the location and functional characteristics of transmitter receptors regulating transport function in the small intestine, highlighting recent research on cholinergic and bradykinin receptors.