Regulation of ion transport in porcine distal colon: effects of putative neurotransmitters

E. coli infection disrupts the epithelial barrier and activates intrinsic neurosecretory reflexes in the pig colon

This study aims to assess the barrier integrity and possible activation of enteric neural pathways associated with secretion and motility in the pig colon induced by an enterotoxigenic Escherichia coli (ETEC) challenge. 50 Danbred male piglets were used for this study. 16 were challenged with an oral dose of the ETEC strain F4+ 1.5 × 10⁹ colony-forming unit. Colonic samples were studied 4- and 9-days post-challenge using both a muscle bath and Ussing chamber. Colonic mast cells were stained with methylene blue. In control animals, electrical field stimulation induced neurosecretory responses that were abolished by tetrodotoxin (10^-6M) and reduced by the combination of atropine (10^-4M) and α-chymotrypsin (10U/mL). Exogenous addition of carbachol, vasoactive intestinal peptide, forskolin, 5-HT, nicotine, and histamine produced epithelial Cl^- secretion. At day 4 post-challenge, ETEC increased the colonic permeability. The basal electrogenic ion transport remained increased until day 9 post-challenge and was decreased by tetrodotoxin (10^-6M), atropine (10^-4M), hexamethonium (10^-5M), and ondansetron (10^-5M). In the muscle, electrical field stimulation produced frequency-dependent contractile responses that were abolished with tetrodotoxin (10^-6M) and atropine (10^-6M). Electrical field stimulation and carbachol responses were not altered in ETEC animals in comparison with control animals at day 9 post-challenge. An increase in mast cells, stained with methylene blue, was observed in the mucosa and submucosa but not in the muscle layer of ETEC-infected animals on day 9 post-challenge. ETEC increased the response of intrinsic secretory reflexes and produced an impairment of the colonic barrier that was restored on day 9 post-challenge but did not modify neuromuscular function.

Short- and Long-Term Effects of Cocaine on Enteric Neuronal Functions

Cocaine is one of the most consumed illegal drugs among (young) adults in the European Union and it exerts various acute and chronic negative effects on psychical and physical health. The central mechanism through which cocaine initially leads to improved performance, followed by addictive behavior, has already been intensively studied and includes effects on the homeostasis of the neurotransmitters dopamine, partly mediated via nicotinic acetylcholine receptors, and serotonin. However, effects on the peripheral nervous system, including the enteric nervous system, are much less understood, though a correlation between cocaine consumption and gastrointestinal symptoms has been reported. The aim of the present study was to gain more information on the effects of cocaine on enteric neuronal functions and the underlying mechanisms. For this purpose, functional experiments using an organ bath, Ussing chamber and neuroimaging techniques were conducted on gastrointestinal tissues from guinea pigs. Key results obtained are that cocaine (1) exhibits a stimulating, non-neuronal effect on gastric antrum motility, (2) acutely (but not chronically) diminishes responses of primary cultured enteric neurons to nicotinic and serotonergic stimulation and (3) reversibly attenuates neuronal-mediated intestinal mucosal secretion. It can be concluded that cocaine, among its central effects, also alters enteric neuronal functions, providing potential explanations for the coexistence of cocaine abuse and gastrointestinal complaints.

Adrenergic activation of electrogenic K+ secretion in guinea pig distal colonic epithelium: involvement of beta1- and beta2-adrenergic receptors

Adrenergic stimulation of electrogenic K+ secretion in isolated mucosa from guinea pig distal colon required activation of two beta-adrenergic receptor subtypes (beta-AdrR). Addition of epinephrine (epi) or norepinephrine (norepi) to the bathing solution of mucosae in Ussing chambers increased short-circuit current (Isc) and transepithelial conductance (Gt), consistent with this cation secretion. A beta-adrenergic classification was supported by propranolol antagonism of this secretory response and the lack of effect by the alpha-AdrR antagonists BE2254 (alpha1-AdrR) and yohimbine (alpha2-AdrR). Subtype-selective antagonists CGP20712A (beta1-AdrR), ICI-118551 (beta2-AdrR), and SR59320A (beta3-AdrR) were relatively ineffective at inhibiting the epi-stimulated Isc response. In combination, CGP20712A and ICI-118551 inhibited the response, which supported a synergistic action by beta1-AdrR and beta2-AdrR. Expression of mRNA for both beta1-AdrR and beta2-AdrR was indicated by RT-PCR of RNA from colonic epithelial cells. Protein expression was indicated by immunoblot showing bands at molecular weights consistent with monomers and oligomers. Immunoreactivity (ir) for beta1-AdrR and beta2-AdrR was prominent in basolateral membranes of columnar epithelial cells in the crypts of Lieberkühn as well as intercrypt surface epithelium. Cells in the pericryptal sheath also had beta1-AdrR(ir) but did not have discernable beta2-AdrR(ir). The adrenergic sensitivity of K+ secretion measured by Isc and Gt was relatively low as indicated by EC(50)s of 41 +/- 7 nM for epi and 50 +/- 14 nM for norepi. Adrenergic activation of electrogenic K+ secretion required the involvement of both beta1-AdrR and beta2-AdrR, occurring with an agonist sensitivity reduced compared with reported values for either receptor subtype.

Autonomic neurotransmitters modulate immunoglobulin A secretion in porcine colonic mucosa

Secretory immunoglobulin A (sIgA) plays a crucial role in mucosal surface defense. We tested the hypothesis that colonic sIgA secretion is under enteric neural control. Immunohistochemistry of the porcine distal colonic mucosa revealed presumptive cholinergic and adrenergic nerve fibers apposed to secretory component (SC)-positive crypt epithelial cells and neighboring IgA(+) plasmacytes. The cholinomimetic drug carbamylcholine elicited rapid, atropine-sensitive IgA secretion into the luminal fluid bathing mucosal explants mounted in Ussing chambers. The adrenergic receptor agonist norepinephrine also increased IgA secretion, an action inhibited by phentolamine. These effects were independent of agonist-induced anion secretion. In Western blots of luminal fluid, both agonists increased the density of protein bands co-immunoreactive for IgA and SC. Mucosal exposure to enterohemorrhagic Escherichia coli did not affect IgA secretion, and carbamylcholine treatment did not affect mucosal adherence of this enteropathogen. Acetylcholine and norepinephrine, acting respectively through muscarinic cholinergic and alpha-adrenergic receptors in the colonic mucosa, stimulate sIgA secretion and may enhance mucosal defense in vivo.

Cholinergic regulation of epithelial ion transport in the mammalian intestine

Acetylcholine (ACh) is critical in controlling epithelial ion transport and hence water movements for gut hydration. Here we review the mechanism of cholinergic control of epithelial ion transport across the mammalian intestine. The cholinergic nervous system affects basal ion flux and can evoke increased active ion transport events. Most studies rely on measuring increases in short-circuit current (ISC = active ion transport) evoked by adding ACh or cholinomimetics to intestinal tissue mounted in Ussing chambers. Despite subtle species and gut regional differences, most data indicate that, under normal circumstances, the effect of ACh on intestinal ion transport is mainly an increase in Cl- secretion due to interaction with epithelial M3 muscarinic ACh receptors (mAChRs) and, to a lesser extent, neuronal M1 mAChRs; however, AChR pharmacology has been plagued by a lack of good receptor subtype-selective compounds. Mice lacking M3 mAChRs display intact cholinergically-mediated intestinal ion transport, suggesting a possible compensatory mechanism. Inflamed tissues often display perturbations in the enteric cholinergic system and reduced intestinal ion transport responses to cholinomimetics. The mechanism(s) underlying this hyporesponsiveness are not fully defined. Inflammation-evoked loss of mAChR-mediated control of epithelial ion transport in the mouse reveals a role for neuronal nicotinic AChRs, representing a hitherto unappreciated braking system to limit ACh-evoked Cl- secretion. We suggest that: i) pharmacological analyses should be supported by the use of more selective compounds and supplemented with molecular biology techniques targeting specific ACh receptors and signalling molecules, and ii) assessment of ion transport in normal tissue must be complemented with investigations of tissues from patients or animals with intestinal disease to reveal control mechanisms that may go undetected by focusing on healthy tissue only.

Mucosally-directed adrenergic nerves and sympathomimetic drugs enhance non-intimate adherence of Escherichia coli O157:H7 to porcine cecum and colon

The sympathetic neurotransmitter norepinephrine has been found to increase mucosal adherence of enterohemorrhagic Escherichia coli O157:H7 in explants of murine cecum and porcine distal colon. In the present study, we tested the hypothesis that norepinephrine augments the initial, loose adherence of this important pathogen to the intestinal mucosa. In mucosal sheets of porcine cecum or proximal, spiral and distal colon mounted in Ussing chambers, norepinephrine (10 microM, contraluminal addition) increased mucosal adherence of wild-type E. coli O157:H7 strain 85-170; in the cecal mucosa, this effect occurred within 30-90 min after bacterial inoculation. In addition, norepinephrine transiently increased short-circuit current in cecal and colonic mucosal sheets, a measure of active anion transport. Norepinephrine was effective in promoting cecal adherence of a non-O157 E. coli strain as well as E. coli O157:H7 eae or espA mutant strains that are incapable of intimate mucosal attachment. Nerve fibers immunoreactive for the norepinephrine synthetic enzyme dopamine beta-hydroxylase appeared in close proximity to the cecal epithelium, and the norepinephrine reuptake blocker cocaine, like norepinephrine and the selective alpha2-adrenoceptor agonist UK-14,304, increased E. coli O157:H7 adherence. These results suggest that norepinephrine, acting upon the large bowel mucosa, modulates early, non-intimate adherence of E. coli O157:H7 and probably other mucosa-associated bacteria. Sympathetic nerves innervating the cecocolonic mucosa may link acute stress exposure or psychostimulant abuse with an increased microbial colonization of the intestinal surface. This in turn may alter host susceptibility to enteric infections.

Barakol Extracted from Cassia siamea Stimulates Chloride Secretion in Rat Colon

Barakol is a purified extract of Cassia siamea, a plant that has been used as a laxative in traditional medicine. In this study, the effect of barakol on anion transport across the rat colon epithelium was investigated. Colonic epithelium was mounted in Ussing chambers and bathed with Ringer's solution. Addition of 1 mM barakol to the basolateral solution produced a slow increase in short-circuit current (Isc) in proximal colon and distal colon by 24.5 +/- 2.2 and 24.2 +/- 1.4 microA/cm(2), respectively. Barakol increased Isc in a concentration-dependent manner with an EC(50) value of 0.4 mM. The barakol-stimulated increase in Isc was inhibited by subsequent treatment with 500 microM diphenylamine-2-carboxylic acid or 400 microM glibenclamide added to the apical solution and 200 microM bumetanide added to the basolateral solution. Pretreatment of the tissues with 200 microM bumetanide, but not 10 microM amiloride, completely abolished the barakol-increased Isc. Ion substitution experiments showed an inhibition of barakol-stimulated Isc in chloride-free solution but not in bicarbonate-free solution. In addition, pretreatment of tissues with 10 microM tetrodotoxin or 10 microM indomethacin, but not 1 microM atropine or 10 microM hexamethonium, partially inhibited the Isc response by barakol. The present results demonstrated the stimulatory effect of barakol on the bumetanide-sensitive chloride secretion in rat colon. The effect of barakol was partly mediated by the stimulation of submucosal nerves and through the release of cyclooxygenase metabolites. These findings thus provide an explanation for the underlying mechanism of barakol as a secretagogue in mammalian colon.

Adrenergic modulation of Escherichia coli O157:H7 adherence to the colonic mucosa

Enteric neurotransmitters can modulate the biodefensive functions of the intestinal mucosa, but their role in mucosal interactions with enteropathogens is not well defined. Here we tested the hypothesis that norepinephrine (NE) modulates interactions between enterohemorrhagic Escherichia coli O157:H7 (EHEC) and the colonic epithelium. Mucosal sheets from porcine distal colon were mounted in Ussing chambers. Drugs and an inoculum of either Shiga toxin-negative or -positive EHEC were added to the contraluminal and luminal bathing medium, respectively. After 90 min, adherent bacteria were quantified by an adherence assay and by immunohistochemical methods; short-circuit current (I(sc)) was measured continuously to assess changes in active ion transport. NE-treated tissues exhibited concentration-dependent increases in I(sc) and EHEC adherence. NE did not alter adherence of a rodent-adapted, noninfectious E. coli strain or two porcine-adapted non-O157 E. coli strains. The actions of NE on EHEC adherence but not I(sc) were prevented by the alpha-adrenergic antagonist yohimbine and the PKA activator Sp-8-bromoadenosine-3',5'-cyclic monophosphorothioate. Like NE, the PKA inhibitor Rp-8-bromoadenosine-3',5'-cyclic monophosphorothioate or indirectly acting sympathomimetic agents increased EHEC adherence. Nerve fibers immunoreactive for the NE-synthesizing enzymes tyrosine hydroxylase and dopamine beta-hydroxylase appeared to innervate the colonic epithelium. EHEC-like immunoreactivity on the colonic surface had the appearance of bacterial microcolonies and increased after NE treatment by a phentolamine-sensitive mechanism. Through interactions with alpha(2)-adrenergic receptors, NE appears to increase EHEC adherence to the colonic mucosa. Changes in sympathetic neural outflow may alter intestinal susceptibility to infection.

Catecholamines modulate Escherichia coli O157:H7 adherence to murine cecal mucosa

Enterohemorrhagic Escherichia coli O157:H7 (EHEC) is an important food-borne pathogen. While the molecular mechanisms governing E. coli O157:H7 pathogenesis have been intensively investigated, the role of host factors has received less attention. In this study, we tested the hypothesis that the enteric catecholamines norepinephrine (NE) and dopamine (DA) modulate interactions of the cecal mucosa with E. coli O157:H7. Full-thickness sheets of murine cecum were mounted in Ussing chambers and short circuit current and tissue electrical conductance were periodically determined to assess active transepithelial ion transport and ionic permeability, respectively. Neurochemicals and stationary-phase E. coli O157:H7 were exposed respectively to the contraluminal and luminal aspects of the mucosa. Epithelial adherence of E. coli O157:H7 was quantified by a bacterial adhesion assay after 90 min of luminal E. coli O157:H7 exposure. DA and NE increased E. coli O157:H7 adherence relative to untreated control tissues at 50% effective concentrations of 3.8 microM and 4.2 microM respectively. Pretreatment of tissues with either the alpha-adrenergic antagonist phentolamine or the beta-adrenergic antagonist propranolol prevented the action of NE. The effect of DA was prevented by the dopamine antagonist haloperidol. The drugs did not impair tissue viability or transepithelial conductance. The present findings suggest that enteric catecholamines modulate E. coli O157:H7 adherence to the cecal epithelium. Conditions associated with elevated catecholamine release, such as stress exposure, may influence host susceptibility to E. coli O157:H7 infection.

Cholinergic inhibition of electrogenic sodium absorption in the guinea pig distal colon

Submucosal cholinergic and noncholinergic neurons in intestines have been shown to be involved in regulating epithelial transport functions, particularly stimulating Cl(-) secretion. This study investigates the role of submucosal cholinergic neurons in regulating electrogenic Na(+) absorption in distal colon. Amiloride-sensitive short-circuit current (I(sc)) and (22)Na(+) flux were measured in mucosal and mucosal-submucosal preparations mounted in Ussing chambers. In the mucosal preparation, carbachol (CCh) added to the serosal side inhibited amiloride-sensitive I(sc) and amiloride-sensitive (22)Na(+) absorption. The inhibitory effect of CCh was observed at approximately 0.1 microM, and maximum inhibition of approximately 70% was attained at approximately 30 microM (IC(50) = approximately 1 microM). CCh-induced inhibition of amiloride-sensitive I(sc) was almost totally abolished by 10 microM atropine. Treatment of the tissue with ionomycin markedly reduced amiloride-sensitive I(sc), but a subsequent addition of CCh further decreased it. Also, CCh still had an inhibitory effect, although significantly attenuated, after the tissue had been incubated with a low-Ca(2+) solution containing ionomycin and BAPTA-AM. Applying electrical field stimulation to submucosal neurons in the mucosal-submucosal preparation resulted in inhibition of amiloride-sensitive I(sc), approximately 33% of this inhibition being atropine sensitive. Physostigmine inhibited amiloride-sensitive I(sc), this effect being abolished by atropine. In conclusion, submucosal cholinergic and noncholinergic neurons were involved in inhibiting electrogenic Na(+) absorption in colon. This inhibition by cholinergic neurons was mediated by muscarinic receptor activation.

Morphological and neurochemical identification of enteric neurones with mucosal projections in the human small intestine

Data on the axonal projections of enteric neurones in the human intestine are still scarce. The present study aimed to identify the morphology and neurochemical coding of enteric neurones in the human small intestine, which are involved in the innervation of the mucosa. The lipophilic neuronal tracer DiI was applied to one mucosal villus of small intestinal resection specimens. The tissue was kept in organotypic culture and subsequently processed for immunohistochemistry. Neurones labelled from the mucosa were located in all ganglionated nerve networks, including the myenteric plexus. In all plexuses, at least five neurochemical types of neurones could be observed, i.e. SOM-IR neurones, SP-IR neurones, SOM/SP-IR neurones, VIP-IR neurones and neurones lacking immunoreactivity for any of these markers. Most of the DiI-labelled neurones were multidendritic; a minority of neurones could be identified as Dogiel type II cells, suggesting the existence of a subgroup of primary afferent neurones in the DiI-filled cell population. The ratio of labelled multidendritic neurones (assumed to be secretomotor) to labelled Dogiel type II neurones (assumed to be primary afferent) in the myenteric plexus is higher in large mammals (pig and human) than in small mammals (guinea pig). This might point to the existence of a different topographical distribution of subsets of primary afferent neurones and/or topographically distinct intrinsic mucosal reflex circuits in large mammals, including humans.

Effect of age on vasoactive intestinal polypeptide-induced short-circuit current in porcine jejunum

Vasoactive intestinal polypeptide is a transmitter at the neuroepithelial junction of the small intestine in cholera toxin-induced secretion. We investigated whether the secretory effect in vitro of vasoactive intestinal polypeptide in porcine jejunum was changed with age. Stripped tissue preparations from three age groups, neonatal (7-11 days), young (6-8 weeks) and adult (13-15 weeks) pigs, were mounted in Ussing chambers and short-circuited. Vasoactive intestinal polypeptide produced concentration dependent increases in short-circuit current in all three age groups with EC50 values (in nM) of 14.5 +/- 1.9, 16.2 +/- 2.0 and 147 +/- 0 in neonatal, young and adult pigs, respectively. The peak increases in short-circuit current in adult pigs were significantly decreased compared with the other two age groups. To evaluate the secretory capacity, theophylline was added to tissue preparations in which baseline short-circuit current again was established. Theophylline caused a significantly lesser increase in short-circuit current in adult pigs (25.4 +/- 2.0 microA.cm-2) than neonatal (57.1 +/- 3.6 microA.cm-2) and young pigs (63.1 +/- 2.9 microA.cm-2). In conclusion, vasoactive intestinal polypeptide showed a marked decrease in the secretory response with age in porcine jejunum, at least partly caused by a reduced secretory capacity of the enterocytes.

Modulation of chloride, potassium and bicarbonate transport by muscarinic receptors in a human adenocarcinoma cell line

1. Short-circuit current (I(SC)) responses to carbachol (CCh) were investigated in Colony 1 epithelia, a subpopulation of the HCA-7 adenocarcinoma cell line. In Krebs-Henseleit (KH) buffer, CCh responses consisted of three I(SC) components: an unusual rapid decrease (the 10 s spike) followed by an upward spike at 30 s and a slower transient increase (the 2 min peak). This response was not potentiated by forskolin; rather, CCh inhibited cyclic AMP-stimulated I(SC). 2. In HCO3- free buffer, the decrease in forskolin-elevated I(SC) after CCh was reduced, although the interactions between CCh and forskolin remained at best additive rather than synergistic. When Cl- anions were replaced by gluconate, both Ca2+- and cyclic AMP-mediated electrogenic responses were significantly inhibited. 3. Basolateral Ba2+ (1-10 mM) and 293B (10 microM) selectively inhibited forskolin stimulation of I(SC), without altering the effects of CCh. Under Ba2+- or 293B-treated conditions, CCh responses were potentiated by pretreatment with forskolin. 4. Basolateral charybdotoxin (50 nM) significantly increased the size of the 10 s spike of CCh responses in both KH and HCO3- free medium, without affecting the 2 min peak. The enhanced 10 s spike was inhibited by prior addition of 5 mM apical Ba2+. Charybdotoxin did not affect forskolin responses. 5. In epithelial layers prestimulated with forskolin, the muscarinic antagonists atropine and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, both at 100 nM) abolished subsequent 10 microM CCh responses. Following addition of p-fluoro hexahydro-sila-difenidol (pF-HHSiD, 10 microM) or pirenzepine (1 microM), qualitative changes in the CCh response time-profile also indicated a rightward shift of the agonist concentration-response curve; however, 1 microM gallamine had no effect. These results suggest that a single M3-like receptor subtype mediates the secretory response to CCh. 6. It is concluded that CCh and forskolin activate discrete populations of basolateral K+ channels gated by either Ca2+ or cyclic AMP, but that the Cl- permeability of the apical membrane may limit their combined effects on electrogenic Cl- secretion. In addition, CCh activates a Ba2+-sensitive apical K+ conductance leading to electrogenic K+ transport. Both agents may also modulate HCO3- secretion through a mechanism at least partially dependent on carbonic anhydrase.

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.

Structural organization and neuropeptide distribution in the mammalian enteric nervous system, with special attention to those components involved in mucosal reflexes

Gastrointestinal events such as peristalsis and secretion/absorption processes are influenced by the enteric nervous system, which is capable of acting largely independently from other parts of the nervous system. Several approaches have been used to further our understanding of the underlying mechanisms of specific enteric microcircuits. Apart from pharmacological and physiological studies, the deciphering of the chemical coding of distinct morphological and functional enteric neuron classes, together with a detailed analysis of their projections by the application of immunocytochemistry, of tracing, and of denervation techniques, have substantially contributed to our knowledge. In view of existing interspecies and regional differences, it is of major importance to expand our knowledge of the enteric nervous system in mammals other than the guinea-pig, the most commonly used experimental animal in this research area. This will increase our chances of finding a valid model, from which well-founded extrapolations can be made regarding the precise function of distinct enteric neuron types regulating motility and ion transport in the human gastrointestinal tract.

Regulation of colonic ion transport by GRP. I. GRP stimulates transepithelial K and Na secretion

Regulation of electrolyte transport across porcine distal colon epithelium by gastrin-releasing peptide (GRP) was examined using mucosal sheets mounted in Ussing chambers. Serosal GRP produced a biphasic response consisting of a transient increase in short-circuit current (ISC) followed by a long-lasting decrease. Indomethacin and tetrodotoxin inhibited the ISC increase without affecting the secondary decrease. Addition of GRP to the mucosal solution produced a decrease in ISC similar to that observed with serosal treatment, but no transient increase in ISC was observed. GRP and bombesin (50% effective concentrations of 26 and 30 nM, respectively) were more effective than neuromedin B in decreasing the ISC, and the GRP receptor antagonist [D-Phe(6)]bombesin(6-13)-O-methyl produced a sixfold dextral shift in the GRP concentration-response relationship. The GRP-stimulated decrease was reduced in the absence of Cl and by serosal bumetanide. Flux measurements showed that GRP increased Rb and Na secretion while having no effect on transepithelial Cl transport. Phosphoinositide turnover was increased by GRP, suggesting that the ion transport changes may be mediated by intracellular Ca concentration. The results of this study demonstrate that GRP stimulates K and Na secretion across the porcine distal colon epithelium and that these processes are dependent, in part, on a bumetanide-sensitive transport pathway located in the basolateral membrane.

Regulation of colonic ion transport by GRP. II. GRP modulates the epithelial response to PGE2

The purpose of this study was to examine the potential modulatory effects of gastrin-releasing peptide (GRP) on prostaglandin (PG) E2-stimulated electrolyte transport across the distal colon epithelium. In an earlier study, PGE2 was shown to reduce net Cl absorption without altering the serosal-to-mucosal unidirectional Cl flux in porcine distal colon (19). In the present study, tissues were pretreated with serosal or mucosal GRP and subsequently stimulated with PGE2. The resulting increase in short-circuit current (ISC) was 152% (serosal GRP) and 49% (mucosal GRP) greater than control PGE2 responses alone. Serosal, but not mucosal, GRP also enhanced the ISC response to vasoactive intestinal peptide. On the basis of flux measurements, the combined effects of serosal GRP and PGE2 resulted in the activation of a transcellular pathway for Cl secretion, which was not activated by either mediator alone. The time course of the PGE2 response was also affected by GRP. Serosal GRP shortened the time to maximum ISC by 35%, whereas mucosal peptide lengthened the time to maximum ISC by 68% These results suggest that GRP acts as a modulator of PG action on electrolyte transport in the distal colon.

Stimulation of bicarbonate secretion by alpha- and beta-adrenoceptor agonists in rat caecum in vitro

This study examines the effects of adrenergic drugs on bicarbonate secretion by the rat caecum in vitro. Noradrenaline, phenylephrine but not clonidine, stimulated secretion in a concentration-related manner. Noradrenaline responses were antagonised by alprenolol (20 microM) but not phentolamine (10 microM) whilst phenylephrine was antagonised by phentolamine (10 microM), prazosin (5 microM) but not yohimbine (5 microM), alprenolol or tetrodotoxin (1 microM). Replacement of mucosal Cl- abolished the phenylephrine response. Combined stimulation with maximum concentrations of phenylephrine and isoprenaline gave a response which was not greater than that to either agonist alone but it did involve both alpha- and beta-adrenoceptors as judged from the effects of alprenolol and phentolamine either alone or combined. Submaximum concentrations of the two agonists did show additive responses. The results show that alpha 1- but not alpha 2-adrenoceptor agonists stimulate bicarbonate secretion and may act on the same transport mechanism as beta-adrenoceptor agonists. Noradrenaline stimulates via beta-adrenoceptors.

Selective hyperresponsiveness to ovalbumin-induced epithelial transport in inflamed guinea pig distal colon

This study examined whether a mild inflammatory state induced by acetic acid would alter ovalbumin-induced motor and secretory responses of the actively sensitized colon. Short-circuit current (Isc), a measure of active transepithelial ion transport, and longitudinal contractility were measured, respectively, in mucosa-submucosa or smooth muscle sheets from guinea pig distal colon. Ovalbumin produced similar, concentration-dependent increases in Isc in noninflamed and inflamed colonic mucosa. Chlorpheniramine, an H1-histamine antagonist, produced a concentration-related decrease in antigen efficacy that was greater in noninflamed mucosa than in inflamed tissues. Lipoxygenase inhibitors (R840 and A64077) were equally effective in decreasing ovalbumin-induced secretion in both inflamed and noninflamed tissues. Ovalbumin also produced longitudinal muscle contractions that were of similar magnitude in inflamed and noninflamed strips. Moreover, chlorpheniramine and lipoxygenase inhibitors inhibited contractile responses in muscle from both inflamed and noninflamed colons. These results suggest that inflammation produces hyperresponsiveness in the colonic mucosa but not in the underlying longitudinal smooth muscle.

Hormonal and pharmacologic regulation of sodium absorption in rabbit cecum in vitro

The rabbit cecum is a moderately tight epithelium with amiloride-resistant but phenamil-sensitive electrogenic Na absorption. We performed flux and electrical studies under short-circuit conditions in vitro to further characterize the mechanisms of ion transport in cecum in normal and animals pretreated with methylprednisolone (MP) and deoxycorticosterone acetate (DOCA). MP treatment increased Na absorption and decreased tissue conductance. In contrast, DOCA increased Isc but did not significantly alter Na or Cl fluxes. Amiloride analogs with primary specificity for Na channel and Na/H exchanger both inhibited Isc and Na absorption. Ethacrynic acid, but not bumetanide, inhibited Isc. Nystatin and amphotericin B increased Isc. We conclude that: (1) Steroids have a differential effect on cecal ion transport; methylprednisolone increases Na absorption, but DOCA does not. (2) The response to amiloride analogs is different from other electrogenic transport systems, suggesting a distinct mechanism of Na transport in cecum. (3) The effect of ethacrynic acid was unexpected, suggesting an inhibitory response on an alternate transport system. (4) The effects of polyene antibiotics are similar to those found in other tight epithelia. Electrogenic Na absorption in rabbit cecum represents a distinct transport system, significantly different from Na absorptive mechanisms in other segments of the gut.

Cholinergic neurons and muscarinic receptors regulate anion secretion in pig distal jejunum

The neurotransmitter acetylcholine is thought to modulate epithelial ion transport in the mammalian small intestine. In this study, the non-selective acetylcholine agonist, carbachol, produced rapid changes in short-circuit current (Isc), an electrical measure of active anion secretion, across isolated sheets of the distal jejunal mucosa-submucosa from swine. The potency of carbachol in elevating Isc was decreased 26-fold in the presence of 10 nM atropine, but remained unchanged by 1-100 microM hexamethonium or 0.1 microM tetrodotoxin. The acetylcholine antagonists produced little or no decrease in spontaneous Isc, whereas tetrodotoxin decreased Isc by 54 microA/cm2. [3H]Quinuclidinyl benzilate (QNB), a muscarinic acetylcholine receptor blocker, bound to a single species of sites within the mucosa-submucosa with a KD = 38 pM and Bmax = 94 fmol/mg protein. Selective muscarinic acetylcholine receptor blockers competed with [3H]QNB for this site with a rank order of affinity indicative of an interaction with a M3-muscarinic acetylcholine receptor. Specific [3H]QNB binding sites were autoradiographically localized in the jejunal wall to the epithelium, submucosa and muscularis propria. Transmural electrical stimulation (3-600 pulses/10 s, 0.5 ms, 60 V) of the mucosa-submucosa produced increases in Isc which were dependent upon the number of impulses delivered and did not undergo tachyphylaxis upon repeated stimulation. Responses to electrical transmural stimulation were inhibited by atropine and hexamethonium, as well as the respective neuronal Na+ and Ca2+ channel blockers tetrodotoxin and omega-conotoxin GVIA, suggesting that electrical transmural stimulation depolarizes submucosal cholinergic neurons which terminate on ion-transporting epithelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)