Serotonin-induced short-circuit current in pig jejunum

Luminal 5-HT4 receptors-A successful target for prokinetic actions

The prokinetic effects of 5-HT₄ receptor (5-HT₄ R) agonists have been utilized clinically for almost three decades to relieve symptoms of constipation. Surprisingly, the mechanism(s) of action of these compounds is still being debated. Recent studies highlight luminal 5-HT₄ Rs as an alternative and effective target for these prokinetic agents. These include the study by Shokrollahi et al (2019, Neurogastroenterol Motil, e13598) published in the current issue of Neurogastroenterology and Motility, who found that activation of mucosal 5-HT₄ Rs by intraluminal prucalopride, significantly enhanced propulsive motor patterns in rabbit colon. The authors highlight the idea that development of agonists targeting luminal 5-HT₄ Rs in the colonic mucosa might be more effective and safer in achieving prokinetic effects on intestinal motility. The purpose of this mini-review is to discuss the evidence for luminal 5-HT₄ Rs as an emerging target for prokinetic agents in facilitating propulsive motor patterns in the colon.

The role of serotonin in intestinal luminal sensing and secretion

This mini-review addresses the role of the neuroendocrine substance serotonin (5-hydroxytryptamine, 5-HT) in intestinal luminal sensing and secretion. Intestinal sensory neurones are activated by several mechanisms, in particular following stimulation of 'specialized' cells in the mucosa. These specialized cells are the enteroendocrine cells, which contain a wide variety of neuroendocrine transmitters. One of these enteroendocrine cells is the enterochromaffin (EC) cell, which is present throughout the intestines and contains large amounts of serotonin, predominantly in the duodenum in humans. The EC cells act as mucosal sensory transducers and secrete serotonin in response to various physiological and pathological luminal stimuli. Following release, serotonin participates in several mucosal protecting processes, one being secretion. Serotonin stimulates active ion, mucus and fluid secretion. Epithelial 5-HT(2) receptors and neuronal 5-HT(1P), 5-HT(3) and 5-HT(4) receptors mediate the secretory effect of serotonin. A transmembrane serotonin transporter terminates epithelial serotonergic signalling. The transient receptor potential ion channel family is important for processing intestinal luminal sensory signalling. Accumulating evidence suggests a significant interaction between serotonin and one of the transient receptor potential ion channels, the capsaicin-sensitive transient receptor potential vanilloid type 1. Accordingly, EC cells, serotonergic receptors and transporter(s), and transient receptor potential vanilloid type-1 ion channels are all explored as pharmacological targets for treatment of some intestinal functional disorders.

Porcine alveolar epithelial cells in primary culture: morphological, bioelectrical and immunocytochemical characterization

PURPOSE

The purpose of this study was to establish a primary culture of porcine lung epithelial cells as an alternative to the currently existing cell cultures from other species, such as e.g., rat or human. Primary porcine lung epithelial cells were isolated, cultivated and analyzed at distinct time points after isolation.

MATERIALS AND METHODS

The main part of the work focused on the morphology of the cells and the detection of alveolar epithelial cell markers by using electron microscopy, immunofluorescence microscopy and immunoblotting. Regarding a later use for in vitro pulmonary drug absorption studies the barrier properties of the cell monolayer were evaluated by monitoring bioelectrical parameters and by marker transport.

RESULTS

Epithelial cells isolated from porcine lung grew to confluent monolayers with typical intercellular junctions within a few days. Maximum transepithelial resistance of about 2,000 Omega cm2 was achieved and demonstrated the formation of a tight epithelial barrier. Permeability data of sodium fluorescein recommended a minimal transepithelial resistance of 600 Omega cm2 for transport studies. The cell population changed from a heterogeneous morphology and marker distribution (caveolin-1, pro-SP-C, surface sugars) towards a monolayer consisting of two cell types resembling type I and type II pneumocytes.

CONCLUSIONS

The porcine alveolar epithelial primary cell culture holds promise for drug transport studies, because it shares major hallmarks of the mammalian alveolar epithelium and it is easily available and scaled up for drug screening.

Active bicarbonate-dependent secretion evoked by 5-hydroxytryptamine in porcine ileal mucosa is mediated by opioid-sensitive enteric neurons

5-Hydroxytryptamine (5-HT) mediates intestinal hypersecretion associated with infection and inflammation. We tested the hypothesis that 5-HT-induced anion secretion is mediated by an opioid-sensitive enteric neural circuit. 5-HT, at a contraluminal concentration of 10 microM, increased short-circuit current by 58 +/- 7 microA/cm(2) in sheets of porcine ileal mucosa with attached inner submucosal plexus. Responses to 5-HT were inhibited by saxitoxin or indomethacin, and reduced in tissues bathed in Cl(-)- or HCO(3)(-)-deficient media. 5-HT action was attenuated by saxitoxin in tissues bathed in Cl(-)-free media, but not HCO(3)-free media. The delta-opioid receptor agonist [D-Pen(2,5)]enkephalin (0.1 microM) blunted the 5-HT change in short-circuit current by a mechanism sensitive to the delta-opioid receptor antagonist naltrindole. The inhibitory actions of [D-Pen(2,5)]enkephalin and saxitoxin were not additive. These results suggest that 5-HT stimulates HCO(3)(-)-dependent ion transport through a mechanism involving prostanoids and an enteric neural pathway modulated by opioids.

Effects of cyclooxygenase and lipoxygenase inhibition on basal- and serotonin-induced ion transport in rat colon

The purpose of this study was to determine the effect of a selective cyclooxygenase (COX)-2 inhibitor as compared to non-selective COX and lipoxygenase (LOX) inhibitors in rat colon. Basal- and serotonin (5-hydroxytryptamine, 5-HT)-induced electrogenic ion transport (short circuit current, SCC), prostaglandin E2 (PGE2) release and histological characteristics were measured. Muscle-stripped mucosal sheets of the proximal and distal segment of rat colon were investigated by employing the Ussing chamber technique, radioimmunoassays for PGE2 and light microscopy examinations for control of tissue integrity. 5-HT and PGE2 both induced a concentration-dependent increase in SCC by activation of multiple receptors. The response to 5-HT was bumetanide-sensitive. Neither the non-selective COX inhibitor piroxicam, nor the selective COX-2 inhibitor SC-'236, altered basal- SCC or 5-HT-induced SCC. Indomethacin reduced both basal- and 5-HT-induced SCC in both segments. Nordihydroguaiaretic acid reduced the 5-HT-induced increase in SCC, but did not change basal SCC. 5-HT-induced a concentration-dependent release of PGE2. Only high concentrations of piroxicam and indomethacin reduced basal PGE2 release and 5-HT-induced PGE2 release. Histological examination of the specimens demonstrated only minor changes following mounting in chambers. There were no apparent differences in the morphology following treatment with COX or LOX inhibitors. These results suggest that in rat colon only the COX-1 enzyme is expressed under basal conditions. Furthermore, data suggest neither the COX-1 nor the COX-2 enzyme to be of major importance for 5-HT-induced ion transport in rat colon in vitro. In conclusion, this study supports 5-HT as a mediator of chloride secretion by activating several receptor subtypes and the LOX enzyme, releasing mediators such as leucotrienes.

Effects of serotonin on the physiology of the rabbit small intestine

Serotonin has been shown to alter the intestinal transport of ions and intestinal motility. These effects may interfere with each other, modulating the whole physiology of the intestine. We have previously shown that serotonin also alters the transport of nutrients. Thus, the aims of the present work were to determine the possible interference between the secretagogue effect of serotonin and the mechanism by which serotonin inhibits the absorption of nutrients, and to study the effect of serotonin on the digestive activity of nutrients of the brush border membrane jejunum enterocyte in the rabbit. The results show that the secretagogue effect of serotonin neither affects the inhibitory effect of serotonin on the intestinal absorption of the nutrients, nor affects the activity of Na+/K+-ATPase. The activity of sucrase and aminopeptidase N was also not affected by serotonin in the rabbit jejunum. Finally, we also studied different parameters of the motility in the rabbit small intestine. Serotonin seemed to stimulate the motility of the rabbit small intestine by increasing integrated mechanical activity and tone of muscle fibers in duodenum, jejunum, and ileum. In conclusion, serotonin might alter or modulate the whole intestinal physiology.

Key words: serotonin, small intestine, motility, absorption, rabbit.

Nasal drug delivery--evaluation of an in vitro model using porcine nasal mucosa

An in vitro model for permeation studies using porcine nasal mucosa was developed and evaluated. The viability and integrity of the mucosa were determined by electrophysiological measurements, permeation studies involving 14C-mannitol and D-(2-3H) glucose, histological studies and a biochemical assay. Enzymatic activity in the mucosa was determined by serosal addition of ouabain. Three different types of porcine nasal mucosa (cavity mucosa, natural septum mucosa and dermatomed septum mucosa) were examined. The results showed that cavity mucosa was the most suitable; this type remained viable for up to 8 h after removal. Lower limits for electrophysiological data were defined in order to establish criteria for tissue viability. This in vitro method using porcine nasal mucosa appears potentially valuable as a tool for further permeation and mechanistic studies within nasal drug delivery research.

5-Hydroxytryptamine-induced secretion by rat jejunum in-vitro involves several 5-hydroxytryptamine receptor subtypes

The receptors contributing to 5-hydroxytryptamine (5-HT)-induced anion secretion by rat jejunum have been investigated by testing the effects of selective agonists and antagonists in-vitro using both intact and stripped intestinal sheets. In both intact and stripped jejunum 5-HT and 5-methoxytryptamine, an agonist that lacks affinity for 5-HT3 receptors, induced concentration-dependent increases in the short-circuit current (SCC), although 5-methoxytryptamine induced a smaller maximum response. In intact sheets 1-phenylbiguanide, a selective 5-HT3 agonist, induced a response that was similar in magnitude to that of 5-methoxytryptamine, but in stripped preparations it had little effect. Tetrodotoxin inhibited the response of intact jejunum to 5-HT (by 86%) and 5-methoxytryptamine (by 85%) and abolished the response to 1-phenylbiguanide. In stripped sheets inhibition of the 5-HT response by tetrodotoxin was reduced to 27%. Desensitization to 1-phenylbiguanide reduced the response to 5-HT in intact but not stripped sheets whereas, in contrast, desensitization to 5-methoxytryptamine inhibited the 5-HT response in stripped sheets but was without effect in intact sheets. Mianserin, a 5-HT1, 5-HT2 and 5-HT3 antagonist, and renzapride, a 5-HT1 and 5-HT3 antagonist, both reduced the maximum response to 5-HT, but 5-HTP-DP, a 5-HT1 antagonist, was without effect. The 5-HT3 antagonist granisetron reduced the response to 5-HT in intact, but not in stripped sheets. Tropisetron, a 5-HT3 and 5-HT4 antagonist, inhibited the response to 5-methoxytryptamine in both preparations, but did not alter the response to 5-HT. It is concluded that 5-HT-induced jejunal secretion involves more than one 5-HT receptor subtype, with both neural and non-neural mechanisms contributing to the response.

Regulation of ion transport in the porcine intestinal tract by enteric neurotransmitters and hormones

In the present paper, the mechanisms underlying the neural and hormonal regulation of mucosal ion transport in the pig intestinal tract are reviewed. The active transport of NaCl by isolated sheets of porcine intestinal mucosa is modulated by cholinergic and non-cholinergic neurons of undetermined neurochemical identity that lie in the submucosa. The application of electrical field stimulation to mucosa-submucosa preparations from porcine jejunum, ileum, or colon produces rapid elevations in short-circuit current which are inhibited by tetrodotoxin or omega-conotoxin GVIA, blockers of neuronal Na+ and Ca2+ channels, respectively. In porcine ileum, these elevations in current are mimicked in large part by cholinergic agonists and have been attributed to anion secretion. The majority of classical neurotransmitters and gut peptides that have been examined to date increase active transepithelial anion secretion through interactions with G protein-coupled receptors associated with submucosal neurons or situated on the basolateral membranes of epithelial cells. A small number of neuropeptides interact with neuronal receptors to augment NaCl absorption or decrease anion secretion. Noradrenergic control of intestinal transport differs in the porcine small and large intestines, and displays considerable inter-species variability in its cellular underpinnings. Transport regulation by bombesin-like peptides may be mediated by receptors distributed in both the apical and basolateral membrane domains of epithelial cells in porcine colon. The transport process affected by these peptides may be linked to epithelial growth and differentiation. The pig intestinal tract appears to be a useful biological model for resolving the cellular mechanisms by which gut neurotransmitters and hormones act in regulating transepithelial ion fluxes. Its general relevance to human intestinal function is discussed.

Regional differences in the effect of mucosal glucose and amino acids on ion transport in normal and cholera toxin-stimulated porcine small intestine

BACKGROUND

This study explores regional differences in the response to mucosal D-glucose and L-amino acids in both normal intestine and intestine stimulated with cholera toxin.

METHODS

Proximal, mid and distal small intestines from 6- to 8-week-old pigs were bathed in Ussing chambers with a buffer containing 15 mM serosal glucose, and the effect of adding a cocktail giving luminal chamber concentrations of 15 mM D-glucose and 20 mM of each L-alanine, L-proline, L-lysine, L-phenylalanine, and L-glutamine on transmucosal Na+ and Cl- transport was measured.

RESULTS

In all segments of both normal and cholera toxin-treated intestine, electrogenic Na+ and electroneutral NaCl absorption were promoted. No significant differences in the net increase of Na+ and Cl- absorption between normal and cholera toxin-stimulated intestine were present. Under both conditions no segmental differences were present in the stimulated Cl- absorption, describing identical capacity for stimulated electroneutral NaCl absorption. In contrast the electrogenic Na+ absorption was, compared to the proximal part, doubled in the mid and distal parts under both conditions.

CONCLUSIONS

We conclude that mucosal D-glucose and L-amino acids stimulate electroneutral NaCl and electrogenic Na+ absorption to the same degree in normal and cholera toxin-treated small intestine. There is no segmental difference in stimulated electroneutral NaCl absorption, while electrogenic Na+ absorption is highest in mid and distal parts.

Effect of 5-HT2 and 5-HT3 receptor antagonists on cholera toxin-induced fluid hypersecretion in the pig jejunum

5-Hydroxytryptamine is a mediator in cholera toxin-induced hypersecretion in the small intestine. The aim of this study was to determine the effect of the 5-hydroxytryptamine receptor antagonists ketanserin, granisetron, ondansetron and tropisetron on cholera toxin-induced hypersecretion in the pig jejunum. Hypersecretion was induced by cholera toxin in ligated jejunal loops. The antagonists were administered subcutaneously at a dose of 100 micrograms/kg. Furthermore, the effect of intraluminally instilled ondansetron was studied. None of the antagonists altered basal absorption or caused fluid hypersecretion. Cholera toxin caused a dose-dependent electrolyte and fluid hypersecretion. The apparent maximal effect, 6.8 +/- 0.4 mg fluid x mg dry loop-1, was reduced by ondansetron, granisetron and tropisetron by about 40%, 30%, and 20%, respectively, whereas ketanserin had no effect. Intraluminal ondansetron reduced the effect of cholera toxin by about 50%. These results demonstrate that 5-hydroxytryptamine3 antagonists administered subcutaneously reduce the cholera toxin-induced hypersecretion in the pig jejunum. Finally, the results support species differences with respect to the antagonistic effect of the tested drugs in cholera toxin-induced hypersecretion.

Serotonin-induced changes in L-leucine transport across rabbit jejunum

The aim of the present study has been to determine the effect of serotonin (5-HT) on the absorption of L-leucine across the rabbit jejunum. The results show that serotonin significantly diminishes the uptake and steady-state tissue accumulation and the mucosal to serosal flux of L-leucine. This effect does not change with previous intestinal exposure of the mucosa to the 5-HT. Serotonin does not seem to modify amino acid simple diffusion across the intestinal epithelium. The effect on the amino acid uptake is due to an inhibition of the Na(+)-dependent system of transport, mainly through a reduction of the apparent Vmax. Moreover, this hormone, added to the incubation solution, does not affect the L-leucine uptake across brush border membrane vesicles. In presence of trifluoroperazine, TMB-8 and staurosporine, the serotonin effect disappears. These results suggest an effect mediated by intracellular processes related to protein kinase C which inhibit the intestinal absorption of L-leucine.

Experimental studies of intestinal ion and water transport

A major advance in transport physiology was H. H. Ussing's development of the voltage-clamp method, and later the Koefoed-Johnsen-Ussing model for Na+ transport. In the same decade, J. C. Skou identified the Na(+)-K(+)-ATPase, which maintains the Na+ and K+ gradients that drive most epithelial transport processes. With this foundation, Danish scientists have pursued the mechanism of ion transport and the resulting solute-linked water flow. Recent contributions have been on isosmotic transport, suggesting solute recycling, and KCl-water cotransport in the basolateral epithelial cell membrane. Efficient small intestinal nutrient absorption is dependent on coupling to the Na+ gradient. Cotransport of Na+ and glucose is quantitatively the most important absorptive mechanism in the small intestine, as illustrated by the success of oral rehydration solutions in diarrhoea. The majority of amino acids are likewise transported by Na+ dependent carriers, but recent experiments have identified a concomitant Cl- dependency for some. Regulation of intestinal secretion, both under normal digestive processes, and in response to enterotoxins, has turned out to be very complex. It involves local and central neuronal regulation through an array of neurotransmitters and local actions of gastrointestinal hormones. Major effectors are the submucosal neurons and the main transmitters serotonin, vasoactive intestinal peptide, acetylcholine, substance P, and neurotensin. Development of antisecretagogues is impeded by the existence of several receptor subtypes and significant species differences. The Na+ and water-conserving properties of the large intestine have been shown to be regulated by adrenocortical hormones, with aldosterone as a potent stimulator of colonic Na+ absorption. A major colonic function is the symbiosis with the anaerobic bacterial population. The fermentation of carbohydrate to short-chain fatty acids, which can be absorbed, supplements small intestinal digestive function.

Mucosal protection through active intestinal secretion: neural and paracrine modulation by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is widely distributed within enteroendocrine cells and neurons of the digestive tract. It stimulates active anion secretion in the intestinal epithelium, an effect which promotes the dilution and elimination of luminal pathogens. The intestinal secretory effects of 5-HT appear to be mediated in part by epithelial 5-HT2 like receptors that are linked to phosphatidylinositol turnover. In addition, 5-HT appears to interact with 5-HT3 or 5-HT4 receptors on submucosal neurons to elicit neuronal depolarization and an associated increase in intestinal secretion. Although the precise cellular mechanisms by which 5-HT modulates mucosal ion transport are incompletely understood, it is clear that 5-HT plays an important role as a intestinal secretagogue in certain diarrheal states, in laxation, and in intestinal hypersensitivity reactions.

Tropisetron and octreotide reduce serotonin-induced fluid hypersecretion in pig jejunum

In vitro data suggest that the serotonin receptor subtype 4 (5-HT4) mediate part of the serotonin (5-HT)-induced intestinal secretion. This study elucidates the involvement of the intestinal 5-HT4 receptor subtype and the anti-diarrhoeal therapeutic potentials of tropisetron and octreotide in 5-HT-induced intestinal hypersecretion in vivo. The effects of intraluminal 5-HT, 5-methoxytryptamine, and tropisetron (ICS 205-930), and subcutaneous octreotide (SMS 201-995) on fluid hypersecretion (accumulation) was studied in tied-off loops in pig jejunum. 5-HT, 5-methoxytryptamine (5-HT4 agonist), and tropisetron (5-HT3/5-HT4 antagonist) all induced a dose-dependent hypersecretion. Low doses of tropisetron reduced, while high doses of tropisetron enhanced the 5-HT and 5-methoxytryptamine responses. Taking into account the hypersecretory effect by itself, tropisetron seemed to completely block the hypersecretory effects of 5-HT and 5-methoxytryptamine. Finally, octreotide reduced the hypersecretory effect of 5-HT, maximally by 30%. These results suggest the involvement of the intestinal 5-HT4 receptor subtype in 5-HT-induced hypersecretion in pig jejunum in vivo. Furthermore, this study demonstrates a potential therapeutic value for octreotide in 5-HT-related diarrhoeagenic disorders in the pig.

5-Hydroxytryptamine2 and 5-hydroxytryptamine3 receptors mediate serotonin-induced short-circuit current in pig jejunum

The purpose of this study was to determine the effect of methysergide, ketanserin, granisetron, cisapride, and renzapride on serotonin (5-hydroxytryptamine-evoked short-circuit current in muscle and myenteric plexus-stripped pig jejunum using the Ussing chamber technique. Ketanserin, granisetron, cisapride, and renzapride all reduced the 5-hydroxytryptamine-induced increase in short-circuit current by about 50%. Combination of ketanserin and granisetron only reduced the 5-hydroxytryptamine-induced peak increase in short-circuit current by 25%. Cisapride caused a small concentration-dependent increase in short-circuit current. Atropine and hexamethonium both almost completely suppressed the cisapride-induced peak increase in short-circuit current. Atropine and hexamethonium both almost completely suppressed the cisapride-induced peak increase in short-circuit current. Ketanserin, granisetron, methysergide, and renzapride did not alter the basal short-circuit current. These results suggest that 5-hydroxytryptamine elicits an increase in short-circuit current by activating epithelial and submucosal 5-hydroxytryptamine2 and 5-hydroxytryptamine3 receptor subtypes. Furthermore, the short-circuit current-increasing effect of cisapride, is due to activation of at least muscarinic and nicotinic receptors.