Effect of endothelin 1 on ion transport in isolated rat colon

Endothelin-3 stimulates survival of goblet cells in organotypic cultures of fetal human colonic epithelium

Cells within the normal human colonic epithelium undergo a dynamic cycle of growth, differentiation, and death. The organotypic culture system of human fetal colonic epithelial cells seeded on top of collagen gels with embedded colonic fibroblasts allowed prolonged culture of the colonic epithelial cells (Kalabis J, Patterson MJ, Enders GM, Marian B, Iozzo RV, Rogler G, Gimotty PA, Herlyn M. FASEB J 17: 1115-1117, 2003). Herein, we have evaluated the role of endothelin-3 (ET3) and both cognate endothelin receptors (ETRA, ETRB) for human colonic epithelial cell growth and survival. ET3 was produced continuously by the fibroblasts as a result of adenovirus-mediated gene transfer. The presence and function of the endothelin receptors (ETRs) in epithelial cells was evaluated by [(3)H]thymidine incorporation using primary epithelial cells in monoculture and by immunohistochemistry on human fetal and adult paraffin-embedded tissues. In organotypic culture, ET3 increased the number of goblet cells but not of enteroendocrine cells. The increase in goblet cells was caused by prolonged cell survival and differentiation. The inhibition of both ETRA and ETRB significantly decreased the number of goblet cells and proliferation in epithelial cells, whereas the number of enteroendocrine cells remained unchanged. ET3 induced activation of IkappaB and MAPK in the epithelial cells, suggesting that these signaling pathways mediate its proproliferation and prosurvival activities. Our results demonstrate that ET3 is involved in regulating human colonic epithelial cell proliferation and survival, particularly for goblet cells, and may be an important component of colonic homeostasis.

Pharmacological characterization of endothelin receptors-mediated contraction in the mouse isolated proximal and distal colon

The study investigated the role of endothelin (ET) and the ET receptor subtypes ET(A) and ET(B) in mediating longitudinal contraction in the mouse proximal and distal colon. Cumulative concentration-response curves to a range of ET agonists (ET-1, ET-2, ET-3, (Ala(1,3,11,13)) ET and IRL 1620) were established by administering concentrations ranging from 0.01 nM to 0.3 microM. Concentration-response curves to ET-1, which exhibits a high affinity for both ET(A) and ET(B) receptor subtypes, were also established in the presence of the ET(A) antagonist BMS 182874 and the ET(B) antagonist IRL1038. The addition of the selective ET(A) receptor antagonist BMS 182874 caused a rightward shift of the concentration-response curve to ET-1 in both sections of the colon. The ET(B) receptor antagonist IRL1038 (0.3-1 microM) did not significantly effect the response to ET-1 in the proximal colon but caused a significant decrease in response towards higher concentrations ranges (>or=3 nM) in the distal colon. A comparison of the concentration-response curves to ET-1, ET-2 and ET-3 showed a rank order of potency ET-1>or=ET-2>>ET-3 in the proximal colon and ET-1>or=ET-2>or=ET-3 in the distal colon. The selective ET(B) receptor agonists, (Ala(1,3,11,13)) ET and IRL 1620 did not produce any response in the proximal sections of the colon but produced a smaller contraction in the distal segments. The data indicate that ET can contract the proximal tissues of the mouse colon predominantly via ET(A) receptors and in the distal tissues via ET(A) and ET(B) receptors.

Effects of endothelin-3 on intestinal ion transport

We investigated the effects of endothelin 3 (ET-3) on electrolyte transport in rat small intestine using a voltage clamp technique in Ussing's chamber. ET-3 diminished potential difference (PD) and short circuit current (Isc). ET-3 did not affect PD or Isc in low Na(+) and/or D-glucose-free medium. Phloridzine (an inhibitor of sodium-glucose cotransporter [SGLT1]) pretreatment abolished the effect of ET-3 on Isc. Methylene blue (a soluble guanylate cyclase inhibitor) or N-nitro-L-arginine methyl ester (a NOS inhibitor) pretreatment delayed the effect of ET-3 on PD and Isc. ET-3 enhanced NOS activity on enterocytes and systemic NO production. Then, ET-3 could inhibit SGLT1 with the participation of NO.

Endogenous endothelin in a rat model of acute colonic mucosal injury

BACKGROUND

Endothelin (ET) is involved in various biologic activities in non-vascular and vascular tissues. While ET has some significant effects on gastrointestinal functions, the possible role of endogenous ET in the host response to mucosal injury has not been well clarified.

METHODS

The present study describes an investigation of the effects of an endothelin A receptor antagonist, BQ-123, on lactate dehydrogenase (LDH), mucus and albumin flux into the perfusate in a rat model of acute colonic injury, induced by acetic acid perfusion. The present study also examined localization of ET in damaged rat colons by using immunohistochemistry.

RESULTS

A 4% acetic acid treatment induced mild mucosal damage of perfused rat colon and increased LDH as well as albumin and protein-bound hexose release into the perfusate. Pretreatment with BQ-123 significantly reduced LDH activity and protein-bound hexose concentration in the perfusate and delayed the reduction of albumin leakage from damaged mucosa. Vascular endothelial, neural and surface epithelial cells of the colon showed strong ET-like immunoreactivity. Mucosal damage markedly influenced ET expression by epithelial cells. Mild mucosal damage decreased the ET expression by surface epithelial cells while moderate mucosal damage induced a mosaic location of ET-positive epithelial cells in the crypt. Severe mucosal damage abolished the ET expression by epithelial cells.

CONCLUSIONS

Endothelin may play a role in the host response to acute mucosal damage. Mucosal ET production is significantly affected by mucosal injury.

The endothelin system in normal human colon

Endothelin (ET)-1 is a potent vasoconstrictor and mitogenic peptide that has a variety of biological effects in noncardiovascular tissues. The precise cellular distribution of the ET-1 system in the wall of the normal human colon was studied to identify the physiological role of ET in the gut. In situ hybridization revealed ET-converting enzyme-1 (ECE-1) mRNA in all vessels, the colon epithelium, and macrophages. Prepro-ET-1 (PPET-1) mRNA had a similar distribution except for a scattered signal in mucosal microvessels. ET(A) and ET(B) receptor mRNAs were mainly in the lamina propria, pericryptal myofibroblasts, microvessels, and mononuclear cells, with ET(A) mRNA more abundant than ET(B) mRNA. (125)I-ET-1 binding showed ET(B) along the crypts and in nerve fibers descending from the ganglionic plexus that contained PPET-1, ECE-1, and ET(B) transcripts, whereas glia contained ET(A) receptors. The finding of the entire ET system in the normal mucosa suggests its implication in some characteristic functions of the colon and its secretion as both a neuroactive and a vasoactive peptide.

Effects of atrial natriuretic peptide in the gut

The intestinal tract is a target organ for atrial natriuretic peptide (ANP), characterized by various biologic activities, immunoreactivity, as well as specific binding sites for ANP. A review of previous studies reveals that ANP is an important regulator of water and nutrient intake, which acts via multiple signaling pathways including activation of guanylyl cyclase to produce its biologic responses. As a regulator, the peptide locally controls hydrosaline balance and acute systemic effects. Therefore, ANP could also act as a local mediator or paracrine effector of intestinal function.

Gene expression, localization, and pharmacological characterization of endothelin receptors in diabetic rat bladder dome

As there are significant amounts of functional endothelin receptors in the mammalian urinary tract, we examined the effect of experimental diabetes on the expression of endothelin receptors and their mRNAs in the rat bladder dome. The density of endothelin receptors in the rat bladder dome was higher (8 and 16 weeks following the onset of diabetes) than in age-matched controls. Insulin treatment, started 8 weeks after the induction of diabetes, partially reversed the endothelin receptor alterations. The pharmacological profile of the endothelin receptors in the bladder dome was similar in all groups and was consistent with the predominance of the endothelin ET(A) receptor subtype (ET(A):ET(B)=approximately 4:1). Autoradiographic studies demonstrated that the endothelin receptors were located in all tissue components of the bladder, including epithelial and muscular layers. Semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) data indicated that diabetes increased the expression level of gene transcripts for both endothelin receptor subtypes and that insulin treatment reversed the mRNA upregulation.

Tachykinins mediate changes in ion transport in porcine jejunum through release of prostaglandins and neurotransmitters

In the present study, we investigated the mediators involved in substance P (SP) and neurokinin A (NKA) induced ion transport. Stripped preparations of porcine jejunal tissue were mounted in Ussing-chambers and short-circuited. The cyclo-oxygenase inhibitor, piroxicam (10 microM) and the neuronal conduction blocker, tetrodotoxin (TTX) (0.1 microM) both significantly decreased the SP (0.1 microM) (66% and 36%, respectively) and NKA (1 microM) (64% and 31%, respectively) induced increase in short-circuit current (SCC). Pretreatment with both piroxicam and TTX totally abolished the SP and NKA response. SP (0.1 microM) caused a significant release of prostaglandin E2 (PGE2), whereas the release of PGE2 induced by NKA was not significant. Experiments were performed to clarify if vasoactive intestinal polypeptide (VIP) was mediating SP or NKA responses. VIP caused a TTX-insensitive and a concentration-dependent increase in SCC. Two VIP antagonists did not change the response to VIP (10 nM and 0.1 microM). Thus, these antagonists could not be used to further elucidate the role of VIP. We were unable to measure a significant release of VIP after SP or NKA treatment. These results indicate, that SP and NKA regulate ion transport in porcine jejunum, entirely through the release of prostaglandins and enteric neurotransmitters.

Endogenous nitric oxide inhibits endothelin-1-induced chloride secretion in guinea pig colon

Segments of guinea pig distal colon, stripped of the external muscle layers, were set up in flux chambers for measurement of short-circuit current (Isc) indicative of active, electrogenic ion transport. During neural blockade with tetrodotoxin, the nitric oxide scavenger, hemoglobin, and the nitric oxide synthase inhibitor, N omega-nitro-L-arginine (L-NNA), reduced Isc. The reduction in Isc in response to hemoglobin was reversed by L-arginine and blockers of chloride secretion, including bumetanide and diphenylamine-2-carboxylic acid, but not by the potassium channel blockers, barium and tetraethylammonium, nor by amiloride, an epithelial sodium channel blocker. The hemoglobin-induced reduction in Isc was not affected by blockade of prostaglandin synthesis with piroxicam. During neural blockade, the nitric oxide donors, sodium nitroprusside and NONOate, increased Isc which was abolished by piroxicam. Endothelin-1 (ET-1) also evoked an increase in Isc that was unaffected by amiloride and was inhibitable by bumetanide, chloride-free solutions, tetrodotoxin, piroxicam, and the ETA receptor antagonist, BQ123. The ETB receptor agonist, [Ala1,3,11,15]-endothelin-1, had no appreciable effect on Isc. Hemoglobin and L-NNA enhanced the ET-1-induced Isc response by about twofold without affecting prostaglandin E2 release or its secretory response. The results suggest that endogenous nitric oxide stimulates a low level of chloride secretion that is independent of prostaglandins, unlike nitric oxide donors which increase chloride secretion by releasing prostaglandins. In addition, endogenous nitric oxide suppresses ET-1-evoked chloride secretion by mechanisms that are unrelated to the release of prostaglandin E2 or its ability to stimulate epithelial cells. Endogenous nitric oxide may play an important role in modulating chloride secretion during ischemic challenge when endothelin levels are high.

Endothelin-1 is synthesized and inhibits cyclic adenosine monophosphate- dependent anion secretion by an autocrine/paracrine mechanism in gallbladder epithelial cells

Ion and fluid transport across the biliary epithelium contributes to bile secretion. Since endothelin (ET)-1 affects ion transport activities and is released by human gallbladder- derived biliary epithelial cells in primary culture, we examined the expression of ET peptides and ET receptors and the influence of ET-1 on ion transport in this epithelium ex vivo. In freshly isolated gallbladder epithelial cells, preproET-1, -2, and -3 mRNAs were detected by reverse transcription PCR and ET-1 isopeptide was identified by chromatography. The cells also displayed ET receptor mRNAs and high-affinity binding sites for ET-1, mostly of the ETB type. Electrogenic anion secretion across intact gallbladder mucosa was stimulated by forskolin, secretin, and exogenous ATP, as assessed by short-circuit current (Isc) increases in Ussing-type chambers. ET-1 inhibited forskolin- and secretin-induced changes in Isc, without affecting baseline Isc or ATP-induced changes. Accordingly, ET-1 significantly reduced the accumulation of intracellular cAMP elicited by forskolin and secretin in the epithelial cells, and this effect was abolished by pertussis toxin. This is the first evidence that ET-1 is synthesized and inhibits, via a Gi protein-coupled receptor, cAMP-dependent anion secretion in human gallbladder epithelium, indicating a role in the control of bile secretion by an autocrine/paracrine mechanism.

Endothelin-1 potently stimulates chloride secretion and inhibits Na(+)-glucose absorption in human intestine in vitro

1. Serosally added synthetic endothelin-1 (ET-1) increased short-circuit current (Isc) across isolated muscle-stripped human colonic mucosa in vitro. Bumetanide inhibited Isc responses, indicating that ET-1 stimulates electrogenic Cl- secretion. 2. In isolated human jejunal mucosa, ET-1 exhibited a concentration-dependent dual action. At low concentrations it induced rapid increases in Isc and these were inhibited by bumetanide. At a higher concentration (0.1 microM), ET-1 provoked a drastic and progressive decrease in Isc below the baseline value. 3. Pretreatment with phlorizin or omission of glucose from the Krebs-Ringer solution at the apical (luminal) side of the jejunal mucosa prevented the decreases in Isc evoked by ET-1, suggesting that the peptide inhibits the glucose-coupled electrogenic Na+ absorption. Indeed, flux experiments with D-[14C]glucose demonstrated that ET-1 decreases jejunal glucose absorption by approximately 80% within 30 min. 4. Electron microprobe analyses of cryosections of human jejunum showed that ET-1 (0.1 microM) evokes a significant decrease in intracellular Na+ concentrations of villus (not crypt) epithelial cells, suggesting that the peptide attenuates apical Na(+)-glucose entry by reducing the activity of the Na(+)-glucose cotransporter, SGLT1. 5. In the presence of tetrodotoxin (TTX), ET-1-induced Cl- secretion was significantly reduced, in both human jejunal and colonic mucosa. However, the inhibitory effect on jejunal Na(+)-glucose absorption was not affected by TTX. 6. ET-1 increases electrogenic Cl- secretion across human intestinal mucosa in vitro. This effect is mediated in part via the activation of enteric nerves. Responses of the human jejunal mucosa to high ET-1 concentrations exhibit a second component, namely the rapid inhibition of electrogenic Na(+)-glucose absorption, which might be mediated by an inhibition of the transport activity of SGLT1. This effect is independent from neuronal mediators. Our results suggest different cellular action sites for ET-1 in human small and large intestine.

An orally active non-selective endothelin receptor antagonist, bosentan, markedly reduces injury in a rat model of colitis

Activation of endothelial cells by vasoactive mediators, such as endothelins, may be an early, strategically important step in the initiation of inflammation in the intestine. In view of recent evidence that inflammatory bowel disease is associated with elevated intestinal concentrations of endothelins and upregulated expression of endothelin receptors on vascular endothelium in intestine, endothelins may become therapeutic targets in inflammatory bowel disease. The recent availability of an orally active, mixed endothelin receptor antagonist, bosentan, allowed us to examine the role of endothelins in a rat model of colitis. Colitis was induced by intra-rectal administration of trinitrobenzene sulphonic acid. In each treatment group, rats were treated with bosentan (10-60 mg/kg p.o.) 24 and 2 h prior to (pre-dose) or 1 h after the induction (post-induction) of colitis and all animals were treated every 24 h thereafter for 5 days. On day 6, stool consistency and the presence of adhesions in the peritoneal cavity were accessed. Colonic tissue samples were removed for determination of macroscopic and microscopic tissue injury, and myeloperoxidase activity. Colitis was typified by tissue ulceration in the distal colon and a corresponding 35-fold increase in myeloperoxidase activity compared to non-inflamed controls. Daily treatment with bosentan dose-dependently reduced colonic damage and myeloperoxidase activity when bosentan was given prior to induction of colitis. In the pre-dose group, the greatest beneficial effect of bosentan was observed at 60 mg/kg; colonic damage and granulocyte infiltration were attenuated by > 80%. A partial therapeutic effect of bosentan was also observed at 60 mg/kg when the pre-treatment regimen was excluded. These findings demonstrate that an orally active, mixed endothelin receptor antagonist has marked protective and therapeutic effects in an animal model of colitis.

Peptide YY-induced alteration of colonic electrolyte transport in the rat

The effect of peptide YY (PYY) on active electrolyte transport in rat colon was studied under short-circuited conditions. PYY (10(-6) M) decreased the basal short-circuit current (Isc) in both the distal and proximal segments of the colon. The decrease in Isc induced by PYY in the distal colon was about 3 times larger than that in the proximal colon. The response to PYY was inhibited by diphenylamine 2-carboxylate, a specific blocker of the Cl-channel, but not by amiloride, a Na-channel blocker. Unidirectional flux measurements in the distal colon revealed that PYY increased the net Na and Cl absorption and decreased the serosal-to-mucosal Cl flux. PYY inhibited the neurally mediated secretory response to electrical field stimulation in a concentration-dependent manner. PYY was also shown to reduce the direct action of the cholinergic agonist bethanechol on the epithelium. These results suggest that PYY inhibits electrogenic Cl secretion and stimulates electroneutral NaCl absorption via both presynaptic and postsynaptic sites in the distal colon.

Endothelin-1 expression in myenteric neurons cultured from rat small intestine

Endothelin is a potent vasoactive peptide. More recently, endothelin-1 (ET-1) has been found in neural tissues such as spinal cord, brain and peripheral ganglion cells. Inagaki (Gastroenterology 101 (1991) 47) reported evidence of ET-1-like immunoreactivity in enteric neurons, but there are no reports of ET-1 peptide or mRNA expression specifically in myenteric neurons. Using a primary culture of myenteric neurons, we set out to evaluate ET-1 peptide and mRNA expression. Myenteric neurons were cultured using a dissection and enzyme dispersion technique. ET-1 reactivity was localized to neurons and ET-1 levels from cells and media were assayed by radioimmunoassay under a variety of media conditions or with depolarizing buffer or veratridine (75 microM). Prepro ET-1 mRNA expression was determined by Northern analysis of total RNA utilizing a rat ET-1 cDNA. ET-1 immunoreactivity was observed almost exclusively in myenteric neurons. Cells contained 0.78 pg/micrograms protein and did not vary with variations in media conditions. Basal release/secretion into media occurred but was not enhanced by depolarizing media or veratridine. High levels of ET-1 mRNA expression were identified. These results of high level constitutive expression of ET-1 linked with previous reports of ET-1 modulation of cholinergic intestinal smooth muscle contraction suggest a neuromodulatory role.