The effect of somatostatin and enkephalin on ion transport in the intestine

Roles of gastrointestinal polypeptides in intestinal barrier regulation

The intestinal barrier is a dynamic entity that is organized as a multilayer system and includes various intracellular and extracellular elements. The gut barrier functions in a coordinated manner to impede the passage of antigens, toxins, and microbiome components and simultaneously preserves the balanced development of the epithelial barrier and the immune system and the acquisition of tolerance to dietary antigens and intestinal pathogens.Numerous scientific studies have shown a significant association between gut barrier damage and gastrointestinal and extraintestinal diseases such as inflammatory bowel disease, celiac disease and hepatic fibrosis. Various internal and external factors regulate the intestinal barrier. Gastrointestinal peptides originate from enteroendocrine cells in the luminal digestive tract and are critical gut barrier regulators. Recent studies have demonstrated that gastrointestinal peptides have a therapeutic effect on digestive tract diseases, enhancing epithelial barrier activity and restoring the gut barrier. This review demonstrates the roles and mechanisms of gastrointestinal polypeptides, especially somatostatin (SST) and vasoactive intestinal peptide (VIP), in intestinal barrier regulation.

Effect of sorbin on electrolyte transport in rat and human intestine

Stimulating water absorption in the colon represents an important target to reduce stool output in secretory diarrhea. Recently, a 153-amino-acid peptide was isolated from porcine upper small intestine and purified, taking into account the increase of water absorption in guinea pig gallbladder. Accordingly, this peptide was named sorbin. The aim of the present study was to determine if the COOH-terminal heptapeptide of sorbin (C7-sorbin) participates in the regulation of electrolyte transport in the colon. Different regions (from duodenum to colon) of stripped intestinal mucosa from rats or humans were mounted in Ussing chambers to measure the changes in short-circuit current (DeltaIsc) and net 22Na and 36Cl fluxes (JNanet and JClnet) after serosal exposure of 10(-7) to 10(-3) M C7-sorbin. In fasted rat intestine, C7-sorbin (10(-4) M) induced an immediate reduction in Isc in the distal ileum and proximal and distal colon but not in the duodenum and jejunum. In the colon, Isc reduction and JNanet and JClnet stimulation were dose dependent (EC50 = 2 x 10(-5) M). At 10(-3) M, maximal effect was observed (DeltaIsc = -1.14 +/- 0.05, DeltaJNanet = +4.97 +/- 1.38, and DeltaJClnet = +9.25 +/- 1.44 microeq. h-1. cm-2). C7-sorbin (10(-3) M) inhibited the increase in Isc induced by a series of 10 secretory agents such as secretin, vasoactive intestinal peptide, PGE2, and serotonin. In HT-29-Cl19A cells, C7-sorbin induced an increase in Isc, with a maximal effect at 10(-3) M (DeltaIsc = 0.29 +/- 0.10 microeq. h-1. cm-2). In human intestine, a dose-dependent decrease in Isc was observed in right and sigmoid colons in basal and stimulated conditions (EC50 congruent with 10(-5) M; at 10(-4) M, DeltaIsc = -2.66 +/- 0.17 microeq. h-1. cm-2) but not in the jejunum. The results indicate that C7-sorbin stimulated NaCl neutral absorption and inhibited electrogenic Cl- in rat and human intestinal epithelia. In addition, the antisecretory effect was essentially observed in the distal part of both rat and human intestine and the magnitude of the proabsorptive effect was directly related to the magnitude of the previously induced secretion.

Tonic activity of submucosal neurons influences basal ion transport

The influence of tonically active submucosal neurons on basal ion transport was studied using sheets of guinea pig ileum set up in flux chambers. Tetrodotoxin evoked an immediate and sustained decrease in short-circuit current that was sustained for 60 minutes compared with control tissues in which basal currents gradually decreased over time. Time-dependent changes in basal short-circuit currents in tissues treated with atropine were not significantly different from control tissues. The decrease in short-circuit current after tetrodotoxin resulted from a greater increase in net chloride absorption than sodium absorption. Changes in net sodium and chloride transport were due to an increase in the mucosal-to-serosal fluxes of these ions. The results suggest that tonic activity of submucosal neurons limits the absorptive capacity of the guinea pig ileum.

Opioids and opioid receptors in peripheral tissues

Opioid peptides belonging to the enkephalin, beta-endorphin or dynorphin family, acting on specific opiate receptors may be found in peripheral tissues. Enkephalins have a widespread peripheral distribution, while beta-endorphin and dynorphin may be found locally in the enteric nervous system. The peptides of the various families are formed from specific precursor molecules. Apart from the enteric nervous system, opioids are also found in the adrenal medulla as well as in several autonomic ganglia. There is some evidence of three different classes of opioid receptors in peripheral tissues, i.e. mu-, delta- and kappa-receptors. These receptors are not only found on enteric nervous and mucosa cells but also on various cells in the immune system where opioid peptides seem to have important actions and appear to link the neuroendocrine and immune systems to control immunological functions. The physiological as well as the pathophysiological role of opioid peptides in the periphery is gradually being elucidated and, based on such knowledge, new therapeutic implications in gastrointestinal or immune diseases may be developed.

Effects of noradrenaline and somatostatin on basal and stimulated mucosal ion transport in the guinea-pig small intestine

Noradrenaline (NA) and somatostatin (SOM) stimulate intestinal water and ion absorption and are found in mucosal nerve fibres and nerve terminals in submucous ganglia of the guinea-pig small intestine. As the main projection of submucous neurons is to the mucosa, NA and SOM might alter mucosal transport either by a direct effect on the epithelium or indirectly, by affecting submucous neurons. In this study these two possible sites of action of NA and SOM have been investigated in mucosa-submucosa preparations of guinea-pig ileum. In addition, the actions of NA and SOM on the secretory responses caused by stimulation of different populations of submucous neurons have been studied. The stimulants of secretion used were a nicotinic agonist, 1,1-dimethyl-4-phenylpiperazinium (DMPP, 10(-5) M), 5-hydroxytryptamine (5-HT, 10(-7) M) and electrical field stimulation (EFS), which activate cholinergic, noncholinergic and mixed populations of submucous secretomotor neurons, respectively. Segments of intestine were dissected free of external muscle and myenteric plexus and mounted in Ussing chambers. Short-circuit current (Isc) was measured as an indication of net active ion transport across the tissue. NA (greater than or equal to 10(-8) M) and SOM (greater than 10(-10) M) each caused a decrease in Isc, indicating a net increase in ion absorption. The NA response was abolished and the magnitude of the SOM response was reduced to 20% by tetrodotoxin (10(-7) M). DMPP, 5-HT and EFS each stimulated nerves that increased Isc and each of these responses was significantly diminished by NA and SOM.(ABSTRACT TRUNCATED AT 250 WORDS)

Separation of enkephalin-degrading enzymes from longitudinal muscle layer of bovine small intestine. Enzyme inhibition by arphamenine A

The enkephalin-degrading enzymes, such as aminopeptidase, dipeptidyl aminopeptidase, dipeptidyl carboxypeptidase and carboxypeptidase, were purified partially by DEAE-cellulose column chromatography, using longitudinal muscle from bovine small intestine. These enzyme were inhibited by EDTA and o-phenanthroline. Several protease inhibitors of microbial origin, and synthetic compounds, were tested for their abilities to inhibit these enkephalin-degrading enzymes. Among them, arphamenine A, a new potent inhibitor for aminopeptidase B, was shown to be a useful compound in inhibiting all of the enkephalin-degrading enzymes in small intestine.

Interaction of Leu-enkephalin with isolated enterocytes from guinea pig: binding to specific receptors and stimulation of cAMP accumulation

The specific binding of Leu-enkephalin and the stimulatory effect of the peptide on cAMP accumulation have been assessed in isolated enterocytes of guinea pig. The binding was reversible as well as time and temperature dependent. Two classes of binding sites could be defined: a class with a relatively high affinity (Kd = 0.7 microM) that represented 1% of total binding capacity, and another class with low affinity (Kd = 55.5 microM). The stimulation of cAMP accumulation was also shown to depend on time and temperature and was potentiated by a phosphodiesterase inhibitor. Half-maximal stimulation of cAMP accumulation was observed at 119 microM and maximal stimulation (27-fold basal level) at 300 microM Leu-enkephalin. Both steps of the interaction were not modified by Na+ but exhibited a high specificity since modification in the structure of Leu-enkephalin resulted in an important loss of binding affinity and stimulatory activity.

Bradykinin-stimulated electrolyte secretion in rabbit and guinea pig intestine. Involvement of arachidonic acid metabolites

Bradykinin (BK) increases short-circuit current (Isc) when added to the serosal side of rabbit or guinea pig ileum or rabbit colon. Significant effects on Isc are seen at concentrations as low as 10(-10) M. Anion substitution experiments and unidirectional 36Cl flux measurements indicate that this effect of BK on Isc is due to Cl secretion. The effect of BK on Isc can be partially blocked (60-70% inhibition) by cyclooxygenase inhibitors (indomethacin and/or naproxen) and completely blocked by the phospholipase inhibitor, mepacrine. The combined cyclooxygenase/lipoxygenase inhibitors BW 755 and eicosa-5,8,11,14-tetraynoic acid (ETYA) also completely block the effect of BK on Isc but the slow-reacting substance of anaphylaxis (SRS-A) antagonist FPL 55712 has no effect. None of the above inhibitors diminish the effect on Isc of other exogenously added secretory stimuli such as vasoactive intestinal peptide (VIP), theophylline, or prostaglandin E2 (PGE2). Prior desensitization of rabbit ileum to PGE2 blocks the effect on Isc of BK but not those of VIP or theophylline. Conversely, prior desensitization of rabbit ileum to BK greatly reduces the effect of PGE2 on Isc. BK also stimulates the synthesis of PGE2 in rabbit ileal and colonic mucosa and this effect can be blocked by prior addition of either indomethacin or mepacrine. These effects of BK are similar to those of exogenously added arachidonic acid (AA). AA also stimulates Cl secretion and increases PGE2 synthesis and its effect on Isc can be inhibited by prior desensitization to PGE2 or by prior addition of indomethacin. The above results indicate that BK stimulates active Cl secretion in both small and large intestine and suggest that this effect is due to the intracellular release of AA. Although the prostaglandins appear to be the major products of AA metabolism contributing to the secretory response, lipoxygenase products may also play a role.