Net Chloride Absorption Research Articles

Keratin 8 knockdown leads to loss of the chloride transporter DRA in the colon.

Keratins (K) are intermediate filament proteins important in protection from stress. The roles of keratins in the intestine are not clear, but K8 knockout (K8(-/-)) mice develop a Th2-type colonic inflammation, epithelial hyperproliferation, and mild diarrhea caused by a keratin level-dependent decrease in short-circuit current and net sodium and chloride absorption in the distal colon. The lack of K8 leads to mistargeting or altered levels of membrane proteins in colonocytes; however, the main transporter responsible for the keratin-related ion transport defect is unknown. We here analyzed protein and mRNA levels of candidate ion transporters CFTR, PAT-1, NHE-3, and DRA in ileum, cecum, and proximal and distal colon. Although no differences were observed for CFTR, PAT-1, or NHE-3, DRA mRNA levels were decreased by three- to fourfold and DRA protein was almost entirely lost in K8(-/-) cecum and proximal and distal colon compared with K8(+/+), whereas the levels in ileum were normal. In K8(+/-) mice, DRA mRNA levels were unaltered, while decreased DRA protein levels were detected in the proximal colon. Immunofluorescence staining confirmed the loss of DRA in K8(-/-) distal colon, while K8(+/-) displayed a similar but more patchy apical DRA distribution compared with K8(+/+) DRA was similarly decreased when K8 was knocked down in Caco-2 cells, confirming that K8 levels modulate DRA levels in an inflammation-independent manner. Taken together, the loss of DRA in the K8(-/-) mouse colon and cecum explains the dramatic chloride transport defect and diarrheal phenotype after K8 inactivation and identifies K8 as a novel regulator of DRA.

Involvement of the cytoskeleton in the effect of PGE 2 on ion transport in the rat distal colon

This work aimed at studying the effect of PGE 2 on water and chloride absorption from the rat distal colon and at investigating the involvement of the cytoskeleton in the modulation of colonic transporters. PGE 2 increased significantly net water and chloride absorption. It increased also the activity of the Na +K +-ATPase and the expression of the Na +K +2Cl − cotransporter. The increase in pump activity was ascribed to its phosphorylation by PKA or PKC when activated upon binding of PGE 2 to its receptors, and was deemed responsible for the increase in Cl − absorption. Cytochalasin B (CytoB), a disrupter of microfilaments, decreased net water and chloride absorption in presence or absence of PGE 2. Furthermore it down-regulated both pump and cotransporter, and lowered Na +K +-ATPase activity. It was suggested that an intact actin cytoskeleton is required for the basal and the PGE 2-elicited trafficking of both transporters. On the other hand, colchicine, an inhibitor of microtubule polymerization, had no effect on the absorption of water and chloride but abrogated the stimulatory effect of PGE 2. Colchicine exerted a similar effect to that of cytochlasin on the expression of both pump and cotransporter in presence or absence of PGE 2 except for the basal activity of the pump which was not altered by microtubule disruption. It was concluded that both microfilament and microtubular networks are involved in the basal and PGE 2-elicited increase in colonic ion absorption.

PGE 2 reduces net water and chloride absorption from the rat colon by targeting the Na +/H + exchanger and the Na +K +2Cl − cotransporter

An effect of PGE 2 on water and chloride absorption was already established in a previous work. This study is an attempt to find the mechanism of action of the prostaglandin by investigating the involvement of three major transporters namely the Na +–K +ATPase, the Na +/H + exchanger and the Na +K +2Cl − cotransporter. Rats were injected with PGE 2 and 15 min later, the colon was perfused in situ with Krebs Ringer buffer, and net water and chloride absorption were determined. When the involvement of the cotransporter and/or the exchanger was investigated, animals were injected with, respectively, furosemide and amiloride 10 min before PGE 2. Superficial and crypt colonocytes were then isolated and the protein expression of the Na +–K + ATPase and the Na +K +2Cl − was determined by western blot analysis. The effect of PGE 2 on the pump activity in presence or absence of the transporters’ inhibitors was also studied. PGE 2 decreased net water and chloride absorption from the colon, increased the Na +–K + ATPase activity in superficial cells and reduced it in crypt cells. The prostaglandin was found to stimulate secretion in superficial cells by targeting the Na +K +2Cl − symporter, and reduce absorption in crypt cells by targeting the Na +/H + antiporter. Changes in the activity of the pump are secondary to changes in the activity of the other transporters.

Evaluation of Oral Rehydration Solution by Whole‐Gut Perfusion in Rats: Effect of Osmolarity, Sodium Concentration and Resistant Starch

Reduced osmolarity oral rehydration solution (ORS) improved small bowel absorption of fluid and electrolytes in segmental perfusion in experimental animals; this was borne out in clinical practice. Adding amylase-resistant starch (RS) to ORS is expected to increase colonic fluid absorption. This study used combined small and large bowel perfusion to evaluate combinations of reduced osmolarity and starch in ORS. Single-pass steady-state perfusions of the whole gut at 30 mL/h, using the nonabsorbable marker C-polyethylene glycol 4000, were performed in Wistar rats after exposure to cholera toxin or Escherichia coli heat-stable enterotoxin (STa). Steady state was established within 90 minutes after commencing perfusion. Net secretion of water, sodium and chloride induced by cholera toxin was partially reversed by standard glucose-ORS (G-ORS). Substituting glucose in G-ORS with RS (RS-ORS) substantially increased net water absorption (P < 0.001) as did reduced osmolarity ORS (RO-ORS) (P < 0.001); addition of RS to RO-ORS further increased water absorption (P < 0.001). In STa-treated intestine, RO-ORS and RS-ORS significantly improved water absorption compared to G-ORS (P < 0.005). RO- and RS-RO-ORS did not significantly augment net electrolyte absorption compared with G-ORS. RS-ORS was associated with highest net absorption of sodium and chloride compared with all other groups. RS increased net water (and sodium) absorption from isosmolar and reduced osmolar ORS consistent with increased absorption by the colon. RS in reduced osmolar ORS may have advantages to reduce severity of diarrhea and prevent hyponatremia in severe diarrhea and may be applicable to diarrhea of different etiologies.

Effect of processed and fermented soyabeans on net absorption in enterotoxigenic Escherichia coli-infected piglet small intestine

Infectious diarrhoea is a major problem in both children and piglets. Infection of enterotoxigenic Escherichia coli (ETEC) results in fluid secretion and electrolyte losses in the small intestine. In the present study the effect of processed and fermented soyabean products on net absorption during ETEC infection was investigated. Soyabean was processed into an autoclaved, a cooked and a mould-fermented (tempeh) product. The soyabean products were pre-digested and the effect of the products on net absorption in the small intestine of piglets was studied. Pairs of small-intestinal segments, one non-infected and the other ETEC-infected, were perfused simultaneously with the different products during 8 h. Net absorption of fluid, DM, Na, chloride, K and total solutes was determined. Net fluid absorption was highest for cooked soyabean followed by autoclaved soyabean and tempeh as a result of the osmolality of these products. In ETEC-infected segments, cooked soyabean and tempeh showed minor fluid losses (27 (SE 23) and 43 (SE 20) microl/cm(2), respectively) compared with the saline control (260 (SE 23) microl/cm(2)). Tempeh resulted in a high uptake of solutes. Processed soyabean products, particularly cooked soyabean and tempeh, are beneficial in maintaining fluid balance during ETEC infection. Additionally, tempeh showed high DM and total solute absorption. Therefore, particularly, tempeh may be beneficial in the case of post-weaning diarrhoea in piglets and possibly in children as well.

PGE2 exerts dose-dependent opposite effects on net water and chloride absorption from the rat colon

This work investigated the effect of different doses of PGE2 on net water and Cl − absorption from the rat colon, using an in situ perfusion technique. PGE2 exerted opposite effects at different concentrations. Net water and Cl − absorption was significantly reduced at low doses with a minimum at 0.4 μg/100 g BW, and significantly elevated at high doses with an observed maximal effect at 21 μg/100 g BW. At low doses, PGE2 increased in superficial cells, the activity of the Na +–K + ATPase and the protein expression of the Na +K +2Cl − cotransporter, but reduced them in crypt cells. Thus, the reduction in net water and Cl − absorption was ascribed to an increase in secretion by surface cells that masked absorptive processes. At high doses, PGE2 increased significantly the activity of the Na +–K + ATPase in superficial cells only, and was without any effect on the protein expression of the cotransporter and the pump in both surface and crypt cells. The observed increase in net water and Cl − absorption was attributed in this case to an increase in absorptive processes with no effect on secretion.

Saccharomyces boulardii Inhibits Water and Electrolytes Changes Induced by Castor Oil in the Rat Colon

The biotherapeutic agent Saccharomyces boulardii has been shown to inhibit castor oil-induced diarrhoea in rats. The present study investigated the mechanism(s) of this antidiarrhoeal effect in terms of water and electrolyte (sodium, potassium and chloride) changes using two rat models. A single oral dose of S. boulardii of up to 12 x 10(10) CFU/kg of viable cells did not inhibit castor oil-induced fluid secretion in the enteropooling model. However, the yeast dose dependently reduced castor oil induced fluid secretion into the colon, with a significant protection at 12 x 10(10) CFU/kg. In this model, castor oil reversed net sodium and chloride absorption into net secretion, and increased net potassium secretion into the lumen. Single pre-treatment with S. boulardii at 4 and 12 x 10(10) CFU/kg dose dependently decreased these electrolyte changes. In conclusion, S. boulardii possesses potent anti-secretory properties versus water and electrolyte secretion induced by castor oil in the rat colon.

TNF-α down-regulates the Na +–K + ATPase and the Na +–K +–2Cl −cotransporter in the rat colon via PGE 2

TNF-α is believed to play a pivotal role in the pathogenesis of inflammatory bowel diseases which have diarrhea as one of their symptoms. This work studies the effect of the cytokine on electrolyte and water movements in the rat distal colon using an intestinal perfusion technique and attempts to determine its underlying mechanism of action. TNF-α inhibited net water and chloride absorption, down-regulated in both surface and crypt colonocytes the Na +–K +–2Cl − cotransporter, and reduced the protein expression and activity of the Na +–K + ATPase. Indomethacin up-regulated the pump and the cotransporter in surface cells but not in crypt cells, and in its presence, TNF-α could not exert its effect, suggesting an involvement of PGE 2 in the cytokine action. The effect of TNF-α on the pump and symporter was studied also in cultured Caco-2 cells in isolation of the effect of other cells and tissues, to test whether the cytokine acts directly on intestinal cells. In these cells, TNF-α and PGE 2 had a similar effect on the pump expression and activity as that observed in crypt cells but were without any effect on the Na +–K +–2Cl − cotransporter. It was concluded that the effect of the cytokine on colonocytes is mediated via PGE 2. By inhibiting the Na +–K + ATPase, it reduces the Na + gradient needed for NaCl absorption, and by down-regulating the expression of the Na +–K +–2Cl − symporter, it reduces basolateral Cl − entry and luminal Cl − secretion. The inhibitory effect on absorption is more significant than the inhibitory effect on secretion resulting in a decrease in net electrolyte uptake and consequently in more water retention in the lumen.

Angiotensin II inhibits NaCl absorption in the rat medullary thick ascending limb.

NaCl reabsorption in the medullary thick ascending limb of Henle (MTALH) contributes to NaCl balance and is also responsible for the creation of medullary interstitial hypertonicity. Despite the presence of angiotensin II subtype 1 (AT(1)) receptors in both the luminal and the basolateral plasma membranes of MTALH cells, no information is available on the effect of angiotensin II on NaCl reabsorption in MTALH and, furthermore, on angiotensin II-dependent medullary interstitial osmolality. MTALHs from male Sprague-Dawley rats were isolated and microperfused in vitro; transepithelial net chloride absorption (J(Cl)) as well as transepithelial voltage (V(te)) were measured. Luminal or peritubular 10(-11) and 10(-10) M angiotensin II had no effect on J(Cl) or V(te). However, 10(-8) M luminal or peritubular angiotensin II reversibly decreased both J(Cl) and V(te). The effect of both luminal and peritubular angiotensin II was prevented by the presence of losartan (10(-6) M). By contrast, PD-23319, an AT(2)-receptor antagonist, did not alter the inhibitory effect of 10(-8) M angiotensin II. Finally, no additive effect of luminal and peritubular angiotensin II was observed. We conclude that both luminal and peritubular angiotensin II inhibit NaCl absorption in the MTALH via AT(1) receptors. Because of intrarenal angiotensin II synthesis, angiotensin II concentration in medullary tubular and interstitial fluids may be similar in vivo to the concentration that displays an inhibitory effect on NaCl reabsorption under the present experimental conditions.

Birth and Prematurity Influence Intestinal Function in the Newborn Pig

Developmental changes in intestinal function occur in the perinatal period of many species. We investigated the hypothesis that gestational age at delivery and the mode of delivery influence intestinal function. Newborn pigs (106–108 or 113–115 days gestation, term = 115 ± 2 days) were either delivered by caesarean section or born vaginally following induction of parturition with a prostaglandin F 2 α analogue. The pigs were killed at birth and used for measurements of intestinal ion transport in vitro (using Ussing chambers) or killed at 2 days of age, after being fed porcine colostrum to follow the absorption of intact proteins into plasma. The results indicate that premature birth is associated with increased paracellular permeability to ions. The uptake and net absorption of chloride were higher in the term, vaginally-delivered pigs than in the remaining pigs. Among the newborn pigs, the preterm caesarean-delivered pigs exhibited the lowest chloride secretion in response to the secretagogue, theophylline. The latter pigs also absorbed the lowest amounts of immunoglobulin G and albumin from colostrum. In conclusion, gestational age at delivery and the mode of delivery have significant effects on intestinal transport of ions and intact proteins. However, the observed variation in the magnitude and the direction of responses indicate that (a) prematurity and birth influence the transport of ions and intact proteins through independent regulatory mechanisms and (b) the absorption pathways for ions and intact proteins in the neonatal pig intestine are not closely associated.

Segmental Difference of Water and Electrolyte Transport in Rat Colon in vivo

The segmental difference of water and electrolyte transport in the rat colon was studied in vivo. The proximal and distal colon segments were perfused separately but simultaneously at a constant rate with physiological solution, and net movements of water, sodium and chloride were determined. The effects of osmolality and sodium concentration of perfusate were assessed. The effect of a sodium channel blocker on the net transport of water and electrolytes was also studied in each colon segment. The net absorption of water, sodium and chloride correlated with the sodium concentration and osmolality of the perfusion solution in both colon segments and were dominant in the distal colon segment in each condition, compared with that in the proximal colon segment. The concentrations of three electrolytes in the collected fluid were almost the same as those of the perfusion solutions in both segments and these results indicated that water was transported isotonically through the colon lumen. Benzamil, a specific sodium ion channel blocker, inhibited net water and sodium absorption by 58.8% and 63.1% in the proximal colon segment and by 52.0% and 43.6% in the distal colon segment, respectively. These results suggest the existence of an electrogenic sodium transport mechanism and a paracellular pathway in normal (i.e., not treated with corticosteroids or sodium-depleted food) rats which has not been detected in in vitro studies with both apical and basolateral membrane vesicles.

Interleukin 10 modulates ion transport in rat small intestine

BACKGROUND & AIMS: It has been proposed that the anti-inflammatory cytokine interleukin (IL)-10 might be an effective therapeutic agent in the treatment of inflammatory bowel disease. This study examined the effects of human recombinant IL-10 on ileal sodium and chloride transport in Sprague-Dawley rats. METHODS: Unidirectional fluxes of sodium and chloride and tissue electrical parameters were measured under voltage-clamped conditions in ussing chambers. Intracellular levels of adenosine 3',5'-cyclic monophosphate (cAMP) were measured in isolated enterocytes. RESULTS: Jejunal and ileal tissue responded to serosal addition of IL-10 with a transient decrease in short-circuit current reflecting an IL-10-induced increase in net sodium and chloride absorption because of an increase in mucosal to serosal ion movement. The IL-10-induced absorption was not prevented by tetrodotoxin but did show tachyphylaxis. IL-10 reversed, or markedly attenuated, forskolin- and carbachol-induced net chloride secretion. The effects of IL-10 on net secretion were accompanied by a reduction in forskolin-stimulated cAMP levels and a decrease in basal cAMP levels. An additional effect of IL-10 was its induction of bicarbonate secretion only in the presence of secretagogues. CONCLUSIONS: This study shows that IL-10 enhances intestinal electroneutral sodium and chloride absorption, inhibits stimulated chloride secretion, and under some secretory conditions stimulates bicarbonate secretion. (Gastroenterology 1996 Oct;111(4):936-44)

Effect of supplementary feeding during the sucking period on net absorption from the small intestine of weaned pigs

An intestinal perfusion technique was used to measure the effects of supplementary feeding (experiment 1) and temporary weaning (experiment 2) during the sucking period on the net absorption of fluid, sodium, chloride and potassium from the small intestine of pigs after weaning. The technique was also applied to control pigs which did not receive supplementary feed and were not temporarily weaned during the sucking period. Paired intestinal segments were prepared at five sites along the small intestine in each of 80 pigs in experiment 1 and 36 pigs in experiment 2. The cranial segment of each pair was infected with enterotoxigenic Escherichia coli ( etec). In both experiments, four days after weaning, the net absorption of fluid, sodium and chloride in the uninfected segments was in general significantly less in all the pigs than at weaning or seven, 11 and 14 days after weaning and it was significantly greater in the pigs given supplementary feed than in the other pigs. Net absorption values in the infected segments of the control pigs were less four days after weaning than on the day of weaning. No differences were found between pigs that were temporarily weaned or not and those that were not fed during the sucking period. Supplementary feeding during the sucking period partially prevented the decrease of net absorption usually observed after weaning in uninfected and in etec-infected segments of intestine.

Effect of cholera toxin on glucose absorption and net movements of water and electrolytes in the intestinal loop of sheep.

This study was designed to evaluate the effect of cholera toxin on glucose absorption and net movement of water and electrolytes in the jejunal loop of sheep. Intraluminal perfusion was performed at the rate of 1 ml/min with isotonic 10 mM glucose solution. Osmolality was adjusted by adding NaCl, and the outflow solution was collected every 10 min. After a 30 min control period, cholera toxin was applied intraluminally for 30 min at doses of 30, 60, and 120 micrograms/loop. In the control period, water, sodium and chloride were absorbed, while potassium and bicarbonate were secreted. Cholera toxin reversed the net absorption of water, sodium and chloride to net secretions, and this secretory response to cholera toxin was dose-dependent. Bicarbonate secretion was stimulated dose-dependently by cholera toxin. Potassium secretion was also increased at all doses, though this response was not dose-dependent. The net glucose absorption was decreased dose-dependently by cholera toxin. In conclusion, these results indicate that cholera toxin stimulates water and electrolyte secretion, and inhibits glucose absorption in the jejunal loop of sheep.

Colonic secretory effect in response to enteral feeding in humans.

Diarrhoea complicating enteral feeding is a common clinical problem affecting up to 25% of patients. Its pathogenesis remains unknown. A new technique of human in vivo segmental colonic perfusion was used to investigate colonic water and electrolyte movement in response to enteral feeding. Four groups of studies were performed in which low and high load polymeric enteral diet infusions were undertaken, either intragastrically or intraduodenally (n = 6 each group). Net absorption of sodium, chloride, and water occurred during fasting throughout the colon in all groups. There was a significant net secretion of sodium, chloride, and water in the ascending colon during low load (sodium: -42 mmol/h; 95% confidence limits -52 to -19, Chloride: -18 mmol/h; -50 to +16, water: -174 ml/h; -348 to -30) and high load (sodium: -24 mmol/h; -60 to +8, chloride: -18 mmol/h; -31 to +16, water: -120 ml/h; -246 to +6) gastric feeding, and during high load duodenal feeding (sodium: -12 mmol/h; -22 to -6, chloride; -6 mmol/h; -16 to +3, water: -72 ml/h; -144 to -6). Net secretion persisted in the distal colon only during high load gastric feeding. In the other three groups there was a net absorption in the distal colon. This study identified a significant colonic secretory response to enteral feeding, which is related to the site and load of the diet infusion. This response may play an important part in the pathogenesis of enteral feeding related diarrhoea.

Effects of weaning and enterotoxigenic Escherichia coli on net absorption in the small intestine of pigs

The purpose of this study was to measure the net absorption of fluid, sodium, potassium and chloride in the small intestine of weaned pigs and of their unweaned littermates and to correlate these values with villus height and crypt depth. Five pairs of segments of the small intestine were prepared in each of 80 pigs; the cranial segment of each pair was injected with an enterotoxigenic strain of Escherichia coli and the caudal segment with a control solution. Net absorption was measured on the day of weaning and four, seven, 11 and 14 days after weaning. In unweaned pigs the net absorption of fluid, potassium and chloride did not vary with time. In weaned pigs the net absorption of fluid in the control segments was significantly less on days 4, 7 and 14 after weaning and of sodium and chloride on days 4 and 7 than in unweaned littermates. In infected segments of weaned pigs the net absorption of fluid was significantly less than in unweaned littermates on day 11 and 14, of sodium and potassium on day 11 and of chloride on days 4 and 11 after weaning. Net absorption was negatively correlated with villus height but only in the infected segments of weaned pigs; no other correlations were found. It was concluded that after weaning the net absorption of fluid and electrolytes in the small intestine of pigs is temporarily decreased, a condition that may initiate diarrhoea.

Alteration of colonic absorption by long-chain unsaturated fatty acids. Influence of hydroxylation and degree of unsaturation.

Unabsorbed dietary unsaturated fatty acids may cause diarrhea in patients with steatorrhea, but their ability to cause colonic fluid secretion is not known. The present study investigated the effect of several dietary long-chain unsaturated fatty acids on colonic absorption and morphology in the rat colon in vivo. The fatty acids tested induced concentration-dependent net water secretion. The ability of these fatty acids to induce net water secretion varied as follows: linolenic acid (18:3) > linoleic acid (18:2), ricinoleic acid (18:1 OH) > oleic acid (18:1), palmitoleic acid (16:1). Net absorption of sodium and chloride were decreased in fatty acid perfusions. Mucosal activity of sodium potassium adenosine triphosphate and adenyl cyclase were not significantly altered by fatty acids. Epithelial cell damage was noted and correlated with the ability of the fatty acid to induce fluid secretion. Unsaturated fatty acids induce epithelial cell damage and fluid secretion in the colon, their effect being related to the degree of unsaturation.

A long-term perfusion test to measure net absorption in the small intestine of weaned pigs

To study the effects of bacteria on net absorption of fluid and electrolytes in the small intestine of newly weaned pigs,- a more comprehensive and ethical alternative to the ligated loop test was developed. Five paired segments, located at 10, 25, 50, 75 and 95 per cent sites along the small intestine, were cannulated at both ends and solutions perfused continuously through the segments for 10 hours. Net absorption was determined by both a volume method and a method using a non-absorbable marker. Net absorption of fluid, sodium, potassium and chloride was significantly less in segments infected with an enterotoxigenic Escherichia coli than in control segments. This method was superior to the ligated loop test because (i) it was performed entirely under anaesthesia, (ii) the small intestine did not distend during a test, (iii) net absorption was determined per cm 2 and along the whole length of the small intestine. Net absorption determined by the nonabsorbable marker was significantly less than that determined by the volume method.

A 5-HT3 Receptor Agonist Induces Neurally Mediated Chloride Transport in Rat Distal Colon

Having previously demonstrated that serotonin (5HT)-induced chloride secretion in rat distal colon is mediated at both neural and nonneural receptors, we isolated the neural component of this response by adding the selective 5-HT3 receptor agonist, 2-methyl-5-hydroxytryptamine (2Me5HT), to in vitro sheets of rat distal colon with intact neural plexuses. Rats were sacrificed, and the distal colon excised, opened, cut into sections and mounted, all layers intact, as flat sheets in Ussing chambers under short-circuited conditions. 2Me5HT induced a prompt, significant (P < 0.01), concentration-dependent rise in short-circuit current (Isc; EC50 6.2 μM); 50 μM 2Me5HT decreased both net sodium and chloride absorption (-0.1 ± 0.5 and -2.1 ± 0.8 μEq/cm2 · hr, respectively); the difference (2.0 ± 0.8 μEq/cm2 · hr) in these changes was not statistically different from the rise in Isc (1.5 ± 0.3 μEq/cm2 · hr). Since the only significant change inunidirectional flux was the rise in electrogenic CI- secretion (P < 0.01), the ΔIsc induced by 2Me5HT may be used as a measure of electrogenic chloride secretion induced by the agonist. The rise in Isc induced by 2Me5HT was abolished by both 0.2 μM tetrodotoxin and 0.1 μM ICS 205-930 (a 5-HT3 antagonist) but was not inhibited by 1.0 M atropine, 100 μM hexamethonium, 10 μM phentolamine, 10 μM propranolol, 10 μM 5-HTP-DP (a 5-HT1p antagonist), or 0.1 μM ketanserin (a 5-HT2 antagonist). These results indicate that 2-methyl-5-HT is a highly selective agonist for neurally based 5-HT3 receptors in this model. Since 2Me5HT-stimulated secretion is inhibited by neither adrenergic nor cholinergic antagonists, it is likely that the secretomotor neurotransmitter for this transport change is a neuropeptide.

Effect of endothelin 1 on ion transport in isolated rat colon

Background: Endothelin may play a significant role in the regulation of gastrointestinal function because it has a variety of biological activities and because endothelin-like immunoreactivity as well as its specific binding sites have been found in the gastrointestinal tract. This study investigated the secretory effect and mechanism of action of endothelin 1 in mammalian large intestine. Methods: Distal colonic segments from Sprague-Dawley rats were stripped of their muscle layers and mounted in Ussing chambers. The effects of endothelin 1 on short-circuit current in rat colonic mucosa were studied in the absence or presence of specific inhibitors. Transmural unidirectional 22Na+ and 36Cl− fluxes and endothelin 1-induced prostacyclin release were also measured. Results: Serosal addition of endothelin 1 evoked a sustained increase in short-circuit current that was significantly reduced by tetrodotoxin or atropine, and virtually abolished by a selective endothelin A receptor antagonist (BQ-123), furosemide, piroxicam, d,l-verapamil, or removal of serosal calcium. Hexamethonium, amiloride, diphenhydramine, or a specific platelet-activating factor antagonist (CV-6209) did not influence the response to endothelin 1. Endothelin 1 significantly decreased net sodium and net chloride absorption and induced a marked increase in prostacyclin release from the serosal surface of stripped colonic mucosa. Conclusions: Endothelin 1 has a secretory effect in rat colon. Its action seems to be mediated by cyclo-oxygenase products and enteric nerves via the activation of an endothelin A receptor.