Effect of conjugated dihydroxy bile salts on electrolyte transport in rat colon

In vitro models to measure effects on intestinal deconjugation and transport of mixtures of bile acids

Bile acid metabolism and transport are critical to maintain bile acid homeostasis and host health. In this study, it was investigated if effects on intestinal bile acid deconjugation and transport can be quantified in vitro model systems using mixtures of bile acids instead of studying individual bile acids. To this end deconjugation of mixtures of selected bile acids in anaerobic rat or human fecal incubations and the effect of the antibiotic tobramycin on these reactions was studied. In addition, the effect of tobramycin on the transport of the bile acids in isolation or in a mixture across Caco-2 cell layers was characterized. The results demonstrate that both the reduction of bile acid deconjugation and transport by tobramycin can be adequately detected in vitro systems using a mixture of bile acids, thus eliminating the need to characterize the effects for each bile acid in separate experiments. Subtle differences between the experiments with single or combined bile acids point at mutual competitive interactions and indicate that the use of bile acid mixtures is preferred over use of single bile acid given that also in vivo bile acids occurs in mixtures.

Contributions of bile acids to gastrointestinal physiology as receptor agonists and modifiers of ion channels

Bile acids (BAs) are known to be important regulators of intestinal motility and epithelial fluid and electrolyte transport. Over the past two decades, significant advances in identifying and characterizing the receptors, transporters, and ion channels targeted by BAs have led to exciting new insights into the molecular mechanisms involved in these processes. Our appreciation of BAs, their receptors, and BA-modulated ion channels as potential targets for the development of new approaches to treat intestinal motility and transport disorders is increasing. In the current review, we aim to summarize recent advances in our knowledge of the different BA receptors and BA-modulated ion channels present in the gastrointestinal system. We discuss how they regulate motility and epithelial transport, their roles in pathogenesis, and their therapeutic potential in a range of gastrointestinal diseases.

Guts and Gall: Bile Acids in Regulation of Intestinal Epithelial Function in Health and Disease

Epithelial cells line the entire surface of the gastrointestinal tract and its accessory organs where they primarily function in transporting digestive enzymes, nutrients, electrolytes, and fluid to and from the luminal contents. At the same time, epithelial cells are responsible for forming a physical and biochemical barrier that prevents the entry into the body of harmful agents, such as bacteria and their toxins. Dysregulation of epithelial transport and barrier function is associated with the pathogenesis of a number of conditions throughout the intestine, such as inflammatory bowel disease, chronic diarrhea, pancreatitis, reflux esophagitis, and cancer. Driven by discovery of specific receptors on intestinal epithelial cells, new insights into mechanisms that control their synthesis and enterohepatic circulation, and a growing appreciation of their roles as bioactive bacterial metabolites, bile acids are currently receiving a great deal of interest as critical regulators of epithelial function in health and disease. This review aims to summarize recent advances in this field and to highlight how bile acids are now emerging as exciting new targets for disease intervention.

Involvement of butyrate in electrogenic K+ secretion in rat rectal colon

Short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, are synthesized from dietary carbohydrates by colonic bacterial fermentation. These SCFAs supply energy, suppress cancer, and affect ion transport. However, their roles in ion transport and regulation in the intracellular environment remain unknown. In order to elucidate the roles of SCFAs, we measured short-circuit currents (I_SC) and performed RT-PCR and immunohistochemical analyses of ion transporters in rat rectal colon. The application of 30 mM butyrate shifted I_SC in a negative direction, but did not attenuate the activity of epithelial Na⁺ channels (ENaC). The application of bumetanide, a Na⁺-K⁺-2Cl⁻ cotransporter inhibitor, to the basolateral side reduced the negative I_SC shift induced by butyrate. The application of XE991, a KCNQ-type K⁺ channel inhibitor, to the apical side decreased the I_SC shift induced by butyrate in a dose-dependent manner. The I_SC shift was independent of HCO₃⁻ and insensitive to ibuprofen, an SMCT1 inhibitor. The mucosa from rat rectal colon expressed mRNAs of H⁺-coupled monocarboxylate transporters (MCT1, MCT4, and MCT5, also referred to as SLC16A1, SLC16A3, and SLC16A4, respectively). RT-PCR and immunofluorescence analyses demonstrated that KCNQ2 and KCNQ4 localized to the apical membrane of surface cells in rat rectal colon. These results indicate that butyrate, which may be transported by H⁺-coupled monocarboxylate transporters, activates K⁺ secretion through KCNQ-type K⁺ channels on the apical membrane in rat rectal colon. KCNQ-type K⁺ channels may play a role in intestinal secretion and defense mechanisms in the gastrointestinal tract.

The Yin and Yang of bile acid action on tight junctions in a model colonic epithelium

Gastrointestinal epithelial barrier loss due to tight junction (TJ) dysfunction and bile acid‐induced diarrhea are common in patients with inflammatory diseases. Although excess colonic bile acids are known to alter mucosal permeability, few studies have compared the effects of specific bile acids on TJ function. We report that the primary bile acid, chenodeoxycholic acid (CDCA), and its 7α‐dehydroxylated derivative, lithocholic acid (LCA) have opposite effects on epithelial integrity in human colonic T84 cells. CDCA decreased transepithelial barrier resistance (pore) and increased paracellular 10 kDa dextran permeability (leak), effects that were enhanced by proinflammatory cytokines (PiC [ng/mL]: TNFα[10] + IL‐1ß[10] + IFNγ[30]). CDCA reversed the cation selectivity of the monolayer and decreased intercellular adhesion. In contrast, LCA alone did not alter any of these parameters, but attenuated the effects of CDCA ± PiC on paracellular permeability. CDCA, but not PiC, decreased occludin and not claudin‐2 protein expression; CDCA also decreased occludin localization. LCA ± CDCA had no effects on occludin or claudin expression/localization. While PiC and CDCA increased IL‐8 production, LCA reduced both basal and PiC ± CDCA‐induced IL‐8 production. TNFα + IL1ß increased IFNγ, which was enhanced by CDCA and attenuated by LCA. CDCA±PiC increased production of reactive oxygen species (ROS) that was attenuated by LCA. Finally, scavenging ROS attenuated CDCA's leak, but not pore actions, and LCA enhanced this effect. Thus, in T84 cells, CDCA plays a role in the inflammatory response causing barrier dysfunction, while LCA restores barrier integrity. Understanding the interplay of LCA, CDCA, and PiC could lead to innovative therapeutic strategies for inflammatory and diarrheal diseases.

Metabolic and Nutritional Consequences of Urinary Diversion Using Intestinal Segments to Reconstruct the Urinary Tract

Intestinal segments in various forms have been used to reconstruct the urinary tract since the mid-1800s. Currently, many different forms of continent and incontinent diversion options exist. Incorporating bowel mucosa within the urinary tract leads to predictable metabolic and nutritional consequences. The use of ileum or colon can cause a hyperchloremic metabolic acidosis, vitamin B12 deficiency, osteoporosis, fat malabsorption, urinary calculi, and ammoniagenic encephalopathy. Due to metabolic and nutritional consequences associated with the use of jejunum and gastric segments, the use of these bowel segments is not recommended.

Reduction of epithelial secretion in male rat distal colonic mucosa by bile acid receptor TGR5 agonist, INT-777: role of submucosal neurons

BACKGROUND

Recent evidence from rat neuron-free mucosa study suggests that the membrane bile acid receptor TGR5 decreases colonic secretion under basal and stimulated conditions. As submucosal neurons are key players in secretory processes and highly express TGR5, we investigated their role in TGR5 agonist-induced inhibition of secretion and the pathways recruited.

METHODS

TGR5 expression and localization were assessed in rat proximal (pC) and distal (dC) colon by qPCR and immunohistochemistry with double labeling for cholinergic neurons in whole-mount preparations. The influence of a selective (INT-777) or weak (ursodeoxycholic acid, UDCA) TGR5 agonist on colonic secretion was assessed in Ussing chambers, in dC preparation removing seromuscular ± submucosal tissues, in the presence of different inhibitors of secretion pathways.

KEY RESULTS

TGR5 mRNA is expressed in full thickness dC and pC and immunoreactivity is located in colonocytes and pChAT-positive neurons. Addition of INT-777, and less potently UDCA, decreased colonic secretion in seromuscular stripped dC by -58.17± 2.6%. INT-777 effect on basal secretion was reduced in neuron-free and TTX-treated mucosal-submucosal preparations. Atropine, hexamethonium, indomethacin, and L-NAME all reduced significantly INT-777's inhibitory effect while the 5-HT₄ antagonist, RS-39604, and lidocaine abolished it. INT-777 inhibited stimulated colonic secretion induced by nicotine, but not cisapride, carbachol or PGE2.

CONCLUSIONS & INFERENCES

TGR5 activation inhibits basal and stimulated distal colonic secretion in rats by acting directly on epithelial cells and also inhibiting submucosal neurons. This could represent a counter-regulatory mechanism, at the submucosal level, of the known prosecretory effect of bile acids in the colon.

The Farnesoid X Receptor: Good for BAD

Diarrhea is a feature of several chronic intestinal disorders that are associated with increased delivery of bile acids into the colon. Although the prevalence of bile acid diarrhea is high, affecting approximately 1% of the adult population, current therapies often are unsatisfactory. By virtue of its capacity to inhibit colonic epithelial fluid secretion and to down-regulate hepatic bile acid synthesis through induction of the ileal fibroblast growth factor 19 release, the nuclear bile acid receptor, farnesoid X receptor, represents a promising target for the development of new therapeutic approaches. Here, we review our current understanding of the pathophysiology of bile acid diarrhea and the current evidence supporting a role for farnesoid X receptor agonists in treatment of the disease.

Chenodeoxycholic acid requires activation of EGFR, EPAC, and Ca2+ to stimulate CFTR-dependent Cl- secretion in human colonic T84 cells

Bile acids are known to initiate intricate signaling events in a variety of tissues, primarily in the liver and gastrointestinal tract. Of the known bile acids, only the 7α-dihydroxy species, deoxycholic acid and chenodeoxycholic acid (CDCA), and their conjugates, activate processes that stimulate epithelial Cl^- secretion. We have previously published that CDCA acts in a rapid manner to stimulate colonic ion secretion via protein kinase A (PKA)-mediated activation of the dominant Cl^- channel, the cystic fibrosis transmembrane conductance regulator (CFTR) (Ao M, Sarathy J, Domingue J, Alrefai WA, and Rao MC. Am J Physiol Cell Physiol 305: C447-C456, 2013); however, PKA signaling did not account for the entire CDCA response. Here we show that in human colonic T84 cells, CDCA's induction of CFTR activity, measured as changes in short-circuit current (I_sc), is dependent on epidermal growth factor receptor (EGFR) activation and does not involve the bile acid receptors TGR5 or farnesoid X receptor. CDCA activation of Cl^- secretion does not require Src, mitogen-activated protein kinases, or phosphoinositide 3-kinase downstream of EGFR but does require an increase in cytosolic Ca²⁺ In addition to PKA signaling, we found that the CDCA response requires the novel involvement of the exchange protein directly activated by cAMP (EPAC). EPAC is a known hub for cAMP and Ca²⁺ cross talk. Downstream of EPAC, CDCA activates Rap2, and changes in free cytosolic Ca²⁺ were dependent on both EPAC and EGFR activation. This study establishes the complexity of CDCA signaling in the colonic epithelium and shows the contribution of EGFR, EPAC, and Ca²⁺ in CDCA-induced activation of CFTR-dependent Cl^- secretion.

Bile acid diarrhoea and FGF19: new views on diagnosis, pathogenesis and therapy

Chronic diarrhoea induced by bile acids is common and the underlying mechanisms are linked to homeostatic regulation of hepatic bile acid synthesis by fibroblast growth factor 19 (FGF19). Increasing evidence, including that from several large case series using SeHCAT (selenium homocholic acid taurine) tests for diagnosis, indicates that bile acid diarrhoea (BAD) accounts for a sizeable proportion of patients who would otherwise be diagnosed with IBS. Studies of other approaches for diagnosis of BAD have shown increased bile acid synthesis, increased faecal levels of primary bile acids, dysbiosis and different urinary volatile organic compounds when compared with healthy controls or with other diseases. The role of the ileal hormone FGF19 in BAD has been strengthened: a prospective clinical study has confirmed low FGF19 levels in BAD, and so a test to measure these levels could be developed for diagnosis. In animal models, FGF19 depletion by antibodies produces severe diarrhoea. Bile acids affect colonic function through farnesoid X receptor (FXR) and TGR5 receptors. As well as these effects in the colon, FXR-dependent stimulation of ileal FGF19 production could be a logical mechanism to provide therapeutic benefit in BAD. Further studies of FGF19 in humans hold promise in providing novel treatments for this cause of chronic diarrhoea.

Bile acids: short and long term effects in the intestine

Bile acids have secretory, motility and antimicrobial effects in the intestine. In patients with bile acid malabsorption the amount of primary bile acids in the colon is increased compared to healthy controls. Deoxycholic acid is affecting the intestinal smooth muscle activity. Chenodeoxycholic acid has the highest potency to affect intestinal secretion. Litocholic acid has little effect in the lumen of intestine compared to both deoxycholic acid and chenodeoxycholic acid. There is no firm evidence that clinically relevant concentrations of bile acids induce colon cancer. Alterations in bile acid metabolism may be involved in the pathophysiology of constipation.

Staphylococcus aureus delta toxin as an enterotoxin

The classical enterotoxins are known primarily for their ability to cause emesis and diarrhoea in cases of staphylococcal food poisoning but they also exhibit other biological activities. The seven antigenic types of toxin have molecular weights in the range 25 000-35 000. All types induce emesis in man and monkey and are of comparable potency. The enterotoxins seem to induce emesis by stimulating neural receptors in the intestine rather than acting on the medulla directly. The mechanism whereby diarrhoea is produced is unclear. Another product of Staphylococcus aureus which meets the more recent definition of an enterotoxin is the delta toxin. This toxin is an amphipathic peptide having an Mr of 2977 and possessing the ability to interact with a variety of hydrophobic substances. It is cytotoxic, can increase vascular permeability in guinea-pig skin, and can increase cellular cyclic AMP levels in guinea-pig ileum. In the ileum delta toxin also inhibits water absorption, apparently by increasing the bidirectional movement of Na+ and Cl- across the mucosa. This response does not appear to be mediated by cyclic AMP since the changes in ion fluxes precede the increases in cellular cyclic AMP levels. In high doses delta toxin also elicits a positive response in the neonatal mouse after intragastric inoculation.

Three distinct mechanisms of HCO3- secretion in rat distal colon

HCO(3)(-) secretion has long been recognized in the mammalian colon, but it has not been well characterized. Although most studies of colonic HCO(3)(-) secretion have revealed evidence of lumen Cl(-) dependence, suggesting a role for apical membrane Cl(-)/HCO(3)(-) exchange, direct examination of HCO(3)(-) secretion in isolated crypt from rat distal colon did not identify Cl(-)-dependent HCO(3)(-) secretion but did reveal cAMP-induced, Cl(-)-independent HCO(3)(-) secretion. Studies were therefore initiated to determine the characteristics of HCO(3)(-) secretion in isolated colonic mucosa to identify HCO(3)(-) secretion in both surface and crypt cells. HCO(3)(-) secretion was measured in rat distal colonic mucosa stripped of muscular and serosal layers by using a pH stat technique. Basal HCO(3)(-) secretion (5.6 +/- 0.03 microeq.h(-1).cm(-2)) was abolished by removal of either lumen Cl(-) or bath HCO(3)(-); this Cl(-)-dependent HCO(3)(-) secretion was also inhibited by 100 microM DIDS (0.5 +/- 0.03 microeq.h(-1).cm(-2)) but not by 5-nitro-3-(3-phenylpropyl-amino)benzoic acid (NPPB), a Cl(-) channel blocker. 8-Bromo-cAMP induced Cl(-)-independent HCO(3)(-) secretion (and also inhibited Cl(-)-dependent HCO(3)(-) secretion), which was inhibited by NPPB and by glibenclamide, a CFTR blocker, but not by DIDS. Isobutyrate, a poorly metabolized short-chain fatty acid (SCFA), also induced a Cl(-)-independent, DIDS-insensitive, saturable HCO(3)(-) secretion that was not inhibited by NPPB. Three distinct HCO(3)(-) secretory mechanisms were identified: 1) Cl(-)-dependent secretion associated with apical membrane Cl(-)/HCO(3)(-) exchange, 2) cAMP-induced secretion that was a result of an apical membrane anion channel, and 3) SCFA-dependent secretion associated with an apical membrane SCFA/HCO(3)(-) exchange.

Keratins modulate colonocyte electrolyte transport via protein mistargeting

The function of intestinal keratins is unknown, although keratin 8 (K8)–null mice develop colitis, hyperplasia, diarrhea, and mistarget jejunal apical markers. We quantified the diarrhea in K8-null stool and examined its physiologic basis. Isolated crypt-units from K8-null and wild-type mice have similar viability. K8-null distal colon has normal tight junction permeability and paracellular transport but shows decreased short circuit current and net Na absorption associated with net Cl secretion, blunted intracellular Cl/HCO₃-dependent pH regulation, hyperproliferation and enlarged goblet cells, partial loss of the membrane-proximal markers H,K-ATPase-β and F-actin, increased and redistributed basolateral anion exchanger AE1/2 protein, and redistributed Na-transporter ENaC-γ. Diarrhea and protein mistargeting are observed 1–2 d after birth while hyperproliferation/inflammation occurs later. The AE1/2 changes and altered intracellular pH regulation likely account, at least in part, for the ion transport defects and hyperproliferation. Therefore, colonic keratins have a novel function in regulating electrolyte transport, likely by targeting ion transporters to their cellular compartments.

Apical NHE isoforms differentially regulate butyrate-stimulated Na absorption in rat distal colon

Bicarbonate and butyrate stimulate electroneutral Na absorption via apical membrane Na-H exchange (NHE) in rat distal colon. cAMP downregulates NHE-3 isoform and inhibits HCO3-dependent, but not butyrate-dependent, Na absorption. This study sought to determine whether 1) the apical membrane NHE-2 and NHE-3 isoforms differentially mediated HCO3- and butyrate-dependent Na absorption, and 2) cAMP had different effects on NHE-2 and NHE-3 isoforms. The effect of specific inhibitors of NHE-2 and NHE-3 isoforms (50 microM HOE 694 and 2 microM S3226, respectively) on unidirectional 22Na transepithelial fluxes performed across isolated mucosa from rat distal colon under voltage-clamp conditions was examined. HCO3 stimulation of Na absorption was inhibited by EIPA, a nonspecific inhibitor of all NHE isoforms, by S3226 and dibutyryl cAMP but not by HOE 694. In contrast, butyrate stimulation of Na absorption was not altered by dibutyryl cAMP and was not inhibited by HOE 694 in the absence of dibutyryl cAMP, but in the presence of dibutyryl cAMP was HOE694 sensitive. In contrast, S3226 inhibited butyrate-stimulated Na absorption in the absence of dibutyryl cAMP, but not in its presence. We conclude that 1) HCO3-stimulated Na absorption is mediated solely by NHE-3 isoform, whereas butyrate-stimulated Na absorption is mediated by either NHE-3 or NHE-2 isoform, and 2) dibutyryl cAMP selectively inhibits NHE-3 isoform but stimulates NHE-2 isoform. Dibutyryl cAMP does not inhibit butyrate-stimulated Na absorption as a result of its differential effects on NHE-2 and NHE-3 isoforms.

Effect of bile salts on colonic mucus secretion in isolated vascularly perfused rat colon

Mainly composed of mucins, mucus secreted by goblet cells in the intestinal epithelium is critically involved in the protection of the gastrointestinal mucosa. The hypothesis that bile and some bile salts can induce mucus secretion was tested in the isolated perfused rat colon. Mucus release was evaluated using enzyme-linked immunosorbent assays and supported by histological analysis. Luminal administration of bile extract (1%) provoked mucus secretion in the rat colon. Deoxycholate (0.5-10 mM) induced a dose-dependent increase in rat colonic mucus release. Chenodeoxycholate (10 mM) and hyodeoxycholate (10 mM) also evoked mucus discharge, whereas 10 mM cholate, 10 mM ursodeoxycholate, or Tween-20 did not release mucus. Taurine-conjugated bile salts (deoxycholate, hyodeoxycholate, and chenodeoxycholate) were less potent mucus secretagogues than the corresponding unconjugated forms. The deoxycholate-induced mucus discharge was not altered by pharmacological blockers (tetrodotoxin, atropine), indomethacin, mast cell stabilizers (ketotifen, doxantrazole), H1 histamine receptor antagonist (pyrilamine), or 5-HT receptor antagonists (ketanserin, ondansetron, SDZ 205-557). Our findings suggest that some bile salts, especially in the unconjugated form, may provoke colonic mucus secretion, probably through a direct action on mucus-secreting cells.

Metabolic complications of urinary diversions: an overview

Patients with urinary diversions present unique challenges to internists who have an important role in their long-term management. Advances in surgical techniques over the past 30 years have given rise to a number of urinary diversion procedures that use various intestinal segments. In its normal function, the intestine absorbs water and solutes. When placed in contact with the urinary stream, the intestine can create numerous metabolic abnormalities. These include bone disease, hepatobiliary disease, infection, malignancy, neurologic complications, nutritional deficiencies, and a number of electrolyte and acid-base disorders. An overview of these metabolic abnormalities and their causes is provided, as well as recommendations for screening and management of patients.

Tauroursodeoxycholate increases rat liver ursodeoxycholate levels and limits lithocholate formation better than ursodeoxycholate

BACKGROUND & AIMS

To explain the greater hepatoprotective effect of tauroursodeoxycholic acid vs. ursodeoxycholic acid, the absorption, hepatic enrichment, and biotransformation of these bile acids (250 mg/day) were compared in rats.

METHODS

Bile acids were determined in intestinal contents, feces, urine, plasma, and liver by gas chromatography-mass spectrometry.

RESULTS

The concentration of ursodeoxycholate in the liver of animals administered tauroursodeoxycholic acid (175 +/- 29 nmol/g) was greater (P < 0.05) than in animals administered ursodeoxycholic acid (79 +/- 19 nmol/g). Hepatic lithocholate was substantially higher after ursodeoxycholic acid administration (21 +/- 10 nmol/g) than after tauroursodeoxycholic acid administration (12 +/- 1 nmol/g). A concomitant reduction in the proportion of hydrophobic bile acids occurred that was greatest during tauroursodeoxycholic acid administration. In the intestinal tract, the mass of ursodeoxycholate and its specific metabolites was greater in rats administered tauroursodeoxycholic acid (27.2 mg) than those administered ursodeoxycholic acid (13.2 mg). In feces, the proportion of lithocholate was 21.9% +/- 4.9% and 5.4% +/- 4.0% after ursodeoxycholic acid and tauroursodeoxycholic acid administration, respectively.

CONCLUSIONS

Compared with ursodeoxycholic acid, tauroursodeoxycholic acid induces a greater decrease in the percent composition of more hydrophobic bile acids within the pool, limits lithocholate formation, and increases hepatic ursodeoxycholate concentration. These differences are explained by increased hepatic extraction and reduced intestinal biotransformation and not by enhanced absorption of the amidated species.

Mast cells and histamine contribute to bile acid-stimulated secretion in the mouse colon

Certain dihydroxy bile acids cause secretory diarrhea when present in the colonic lumen at inappropriately high concentrations. However, the mechanism underlying the secretagogue activity has not been fully elucidated. Experiments were performed to test whether mast cells and one of their major mediators, histamine, might contribute to the secretory effect. Chenodeoxycholic acid, a secretory bile acid, and ursodeoxycholic acid, a nonsecretory, hydrophilic bile acid, were compared for their ability to induce chloride secretion across segments of mouse colon mounted in Ussing chambers. Chenodeoxycholic acid, but not ursodeoxycholic acid, induced dose-dependent, biphasic chloride secretion that was greater after serosal than mucosal addition and was greater in distal versus proximal colonic segments. The secretory effect of chenodeoxycholic acid was inhibited by H1 histamine receptor antagonists and modified by the cyclooxygenase inhibitor indomethacin. However, it was unaffected by an H2 histamine receptor antagonist or by atropine. Secretory effects of chenodeoxycholic acid were diminished in magnitude and delayed in colonic tissues from mice with a genetic deficiency of tissue mast cells. Concentrations of chenodeoxycholic acid inducing secretion also released histamine from tissue segments. These data indicate that mast cells and histamine-mediated processes contribute significantly to the secretory effects of dihydroxy bile acids in the murine colon.

Inhibitory effect of sodium ursodeoxycholate on basal and stimulated short-circuit current across the isolated toad skin

The effect of sodium ursodeoxycholate (U) on short-circuit current (SCC), an index of basal and stimulated net ion transport across isolated skins of Bufo arenarum toads, was tested. U inhibited basal SCC when added to the epidermal side of the skins. The inhibitory effect was reversible after rinsing the preparation during 60 min. U also inhibited the natriferic response to oxytocin, db-cAMP and theophylline by 82%, 49% and 47%, respectively. Inhibition of SCC by exposure to U was reversed by the polyene antibiotic nystatin. In turn, SCC induced by nystatin in the amiloride-treated skin was insensitive to U and blocked by ouabain, a Na+, K(+)-ATPase inhibitor. These results strongly suggest that the effect of U is exerted at the apical membrane of sodium transporting cells, and rule out the existence of an additional site of inhibitory action of U.

Serum bile acids in companion animal medicine

Quantification of total serum bile acids is used as a method for appraising liver function and perfusion in contemporary small animal practice. This article provides a historical perspective and a comprehensive review of bile acid physiology, laboratory methodologies for bile acid quantification, and normal values published for total serum bile acids measured using the spectrophotometric enzymatic method and for serum bile acids measured using validated radioimmunoassay procedures. The variables influencing the enterohepatic circulation of bile acids and, consequently, the fasting and postprandial serum bile acid concentrations are discussed with the intent of clarifying the application of test results to clinical patients. A brief discussion of the therapeutic use of dehydrocholate and ursodeoxycholic acid in clinical patients is provided.

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,I-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.

Prostaglandin- and theophylline-induced C1 secretion in rat distal colon is inhibited by microtubule inhibitors

The aim of the present study was to examine the possible role of microtubules in chloride secretion by distal rat colon stimulated by prostaglandin (PGE2) and theophylline. Distal colonic tissue from male rats was mounted in Ussing chambers, and short-circuit current (Isc) was measured to assess chloride secretion. Three microtubule inhibitors, colchicine, nocodazole, and taxol, all inhibited the stimulated Isc and reduced the 60-min integrated secretory response to PGE2 and theophylline (integral of Iscdt) by 39-52%, whereas the inactive colchicine analog lumicolchicine did not. Atropine and tetrodotoxin had no effect on stimulated chloride secretion. To confirm the source of Isc, unidirectional 22Na+ and 36Cl- fluxes were measured in tissues exposed to lumicolchicine (control) or colchicine. Control tissues absorbed both chloride [5.0 (1.1-8.6) (median and 95% confidence interval) mueq/cm2/hr] and sodium [2.8 (0.9-7.2) mueq/cm2/hr], and this net absorption was reduced by 96% and 79%, respectively, by treatment with PGE2 and theophylline due to an increase in serosal-to-mucosal chloride and sodium movement. Colchicine-treated tissues exhibited similar net basal chloride and sodium absorption that was reduced by 71% and 75%, respectively, by treatment with PGE2 and theophylline. Thus the PGE2- and theophylline-induced increase in chloride secretion was significantly reduced by colchicine (P < 0.05 by Wilcoxon rank-sum test), whereas colchicine had no effect on PGE2- and theophylline-induced changes in sodium fluxes. Furthermore, the colchicine-related changes in stimulated chloride secretion were numerically similar to colchicine-related changes in stimulated Isc.(ABSTRACT TRUNCATED AT 250 WORDS)

Mucosal ouabain and Na+ inhibit active Rb+(K+) absorption in normal and sodium-depleted rat distal colon

To determine the effect of mucosal sodium and mucosal ouabain on active Rb+(K+) absorption, unidirectional and net 86Rb+ fluxes were measured under voltage-clamp conditions in the distal colon of normal and sodium-depleted rats. The role of mucosal sodium (independent of serosal sodium) was evaluated in a model of Rb+(K+) absorption in which serosal ouabain markedly enhanced active Rb+(K+) absorption. In normal rats, mucosal sodium was a competitive inhibitor of Rb+(K+) absorption, and Rb+(K+) absorption consisted of a mucosal sodium-sensitive component and a mucosal sodium-insensitive component. Further, mucosal ouabain almost completely inhibited the mucosal sodium-insensitive component but did not affect the mucosal sodium-sensitive component. In sodium-depleted rats, both mucosal sodium-sensitive and mucosal sodium-insensitive fractions of Rb+(K+) absorption were also identified. Aldosterone markedly stimulated the mucosal sodium-sensitive component (1.68 +/- 0.15 vs. 0.60 +/- 0.10 muEq.h-1.cm-2) but not the sodium-insensitive component (0.88 +/- 0.09 vs. 0.64 +/- 0.06 muEq.h-1.cm-2) component of Rb+(K+) absorption; however, in contrast to normal animals, mucosal sodium in sodium-depleted animals was a noncompetitive inhibitor of Rb+(K+) absorption. The mucosal sodium-insensitive component of Rb+(K+) absorption in sodium-depleted animals was substantially inhibited by mucosal ouabain, but the mucosal sodium-sensitive component, unlike that in normal animals, was partially inhibited by mucosal ouabain. These studies indicate that the characteristics of the Rb+(K+) absorptive process in sodium-depleted animals differ significantly from those present in normal animals, suggesting that aldosterone induces an Rb+(K+) absorptive mechanism not present in normal animals.

Mechanism of electroneutral sodium chloride absorption in distal colon of the rat

This investigation was designed to establish the mechanism of sodium and chloride transport in the rat distal colon by determining ion fluxes across isolated mucosa under voltage clamp conditions. The net rates of sodium and chloride absorption in the distal colon of the rat were approximately equal (5.8 +/- 0.3 and 6.9 +/- 0.5 microEq/h X cm2, respectively) and significantly greater than the short circuit current (0.9 +/- 0.1 microEq/h X cm2). Net sodium absorption and net chloride absorption were markedly reduced by the removal of chloride and sodium, respectively, but were not affected by the absence of potassium from the mucosal bathing solution. Both net sodium absorption and net chloride absorption were also significantly inhibited by 1.0 mM amiloride and by 0.1 mM acetazolamide. In contrast, 0.1 mM amiloride and 1.0 mM furosemide did not inhibit either sodium or chloride absorption. These results confirm that electroneutral sodium chloride absorption is the predominant mechanism of sodium and chloride absorption and suggest that parallel ion (Na-H and Cl-HCO3) exchanges, rather than independent electrogenic sodium and chloride transport, coupled sodium-chloride cotransport, or coupled Na-K-2Cl cotransport, are most likely responsible for sodium chloride absorption in this epithelium.

Effects of diclofenac sodium and disodium ethylenediaminetetraacetate on electrical parameters of the mucosal membrane and their relation to the permeability enhancing effects in the rat jejunum

The effects of diclofenac sodium and disodium ethylenediaminetetraacetate (EDTA) on electrical parameters of rat jejunal membrane were investigated, together with measurement of the mucosal-to-serosal flux of sulphanilic acid or L-phenylalanine. Both adjuvants increased the flux rate of sulphanilic acid to a similar extent when added to the mucosal solution at 10 mM, but there were apparent differences in their effects on the electrical parameters. The addition of EDTA induced the gradual reduction in the membrane resistance (Rm) by 6-8 ohm cm-2, while the effect of diclofenac on Rm was complicated and concentration-dependent. The short circuit current (Isc) was reduced rapidly to the level of 30-40 microA cm-2 by the addition of diclofenac, but was less affected by EDTA. The flux rate of L-phenylalanine was decreased extensively by diclofenac or the 10 mM concentration of EDTA, suggesting an inhibition of carrier-mediated transport systems in the membrane. Together with our preceding communication (Yamashita et al 1985, J. Pharm. Pharmacol. 37: 512-513), it became obvious that the sites of action of diclofenac and EDTA were different, the former directly interacting with the epithelial cell to alter the permeability and functions of the cell membrane, while the primary effect of EDTA could be at the intercellular junctions.

Effect of theophylline on ion transport in the lizard colon

1. The effect of theophylline on ion transport was examined using an in vitro short-circuited preparation of lizard colon. 2. Theophylline increased short circuit current (Isc) and transmural potential difference (PD). This increase caused by theophylline was accompanied by a small increase in transmural conductance (Gt). 3. Theophylline did not inhibit the absorption of Na+ but reversed Cl- absorption to secretion. This latter effect was due to an increase of the serosal-to-mucosal flux of Cl-. 4. Ion substitution experiments revealed that the effect of theophylline was Na+- and HCO3(-)-dependent and that these ions were required in the bathing solution. 5. These results with lizard colon are compared with those reported for mammalian colon and the mechanism of theophylline-induced Cl- secretion in these epithelia is discussed.

Differential effects of two bile salts on ion transport characteristics of teleost intestine

The effects of a dihydroxy and a trihydroxy bile salt on the Na+- and Cl(-)-absorbing, goby posterior intestine are quite different. Taurochenodeoxycholate, a dihydroxy bile salt, increases tissue permeability to Cl-, primarily by opening the paracellular shunt pathway. The trihydroxy bile salt taurocholate lacks these effects and may, in fact, decrease tissue permeability. In light of the general structural similarity of these two molecules, a detergent action is considered unlikely and, instead, a more specific (perhaps receptor-mediated) mechanism is suggested.

Effects of extracellular calcium and magnesium on bile-salt-stimulated amylase release from rat pancreatic acini

The effects of extracellular calcium and magnesium on bile-salt-induced amylase release from rat pancreatic acini have been studied. The amylase releases caused by from 0.25 mM to 1.0 mM taurodeoxycholate (TDC) and by taurochenodeoxycholate (TCDC) at a concentration of more than 0.75 mM were reduced by ethylenediaminetetraacetic acid (EDTA) and increased by verapamil. EDTA and verapamil had no significant effect on the taurocholate (TC) (1.0-5.0 mM)-stimulated amylase release. The inhibiting effect of EDTA began to appear already during the initial 5 min and was not parallel to any change of lactate dehydrogenase release. The TDC- and TCDC-stimulated amylase release was strongly dependent on the concentrations of extracellular calcium and was only weakly dependent on extracellular magnesium. The TC-induced amylase release was slightly increased only at high concentrations of calcium and magnesium. It is suggested that the mechanism of dihydroxy bile-salt-induced amylase release from rat pancreatic acini is different from that of trihydroxy bile salt. The dihydroxy bile-salt-stimulated amylase release is dependent on extracellular calcium but does not seem to be related to the uptake of calcium by the acini.

Ion transport in proximal colon of the rat. Sodium depletion stimulates neutral sodium chloride absorption

The model of sodium and chloride transport proposed for the colon is based on studies performed in the distal segment and tacitly assumes that ion transport is similar throughout the colon. In rat distal colon, neutral sodium-chloride absorption accounts for the major fraction of overall sodium absorption and aldosterone stimulates electrogenic, amiloride-sensitive sodium absorption. Since we have demonstrated qualitative differences in potassium transport in proximal and distal segments of rat colon, unidirectional 22Na and 36Cl fluxes were performed under short-circuit conditions across isolated proximal colon of control and sodium-depleted rats with secondary hyperaldosteronism. In the control group, net sodium absorption (JNanet) (7.4 +/- 0.5 mu eq/h . cm2) was greater than Isc (1.4 +/- 0.1 mu eq/h . cm2), and JClnet was 0 in Ringer solution. Residual flux (JR) was -5.2 +/- 0.5 mu eq/h . cm2 consistent with hydrogen ion secretion suggesting that neutral sodium absorption may represent sodium-hydrogen exchange. 1 mM mucosal amiloride, which inhibits sodium-hydrogen exchange in other epithelia, produced comparable decreases in JNanet and JR (4.1 +/- 0.6 and 3.2 +/- 0.6 mu eq/h . cm2, respectively) without a parallel fall in Isc. Sodium depletion stimulated JNanet, JClnet, and Isc by 7.0 +/- 1.4, 6.3 +/- 1.9, and 0.8 +/- 0.2 mu eq/h . cm2, respectively, and 1 mM amiloride markedly inhibited JNanet and JClnet by 6.0 +/- 1.1 and 4.0 +/- 1.6 mu eq/h . cm2, respectively, with only a minimal reduction in Isc.

CONCLUSIONS

the predominant neutral sodium-absorptive mechanism in proximal colon is sodium-hydrogen exchange. Sodium depletion stimulates electroneutral chloride-dependent sodium absorption (most likely as a result of increasing sodium-hydrogen and chloride-bicarbonate exchanges), not electrogenic chloride-independent sodium transport. The model of ion transport in the proximal colon is distinct from that of the distal colon.

Oxalate transport by anion exchange across rabbit ileal brush border

This study demonstrates the presence of oxalate transporters on the brush border membrane of rabbit ileum. We found that an inside alkaline (pH = 8.5 inside, 6.5 outside) pH gradient stimulated [14C]oxalate uptake 10-fold at 1 min with a fourfold accumulation above equilibrated uptake at 5 min. 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonate (disodium salt; DIDS) profoundly inhibited the pH-gradient stimulated oxalate uptake. Using an inwardly directed K+ gradient and valinomycin, we found no evidence for potential sensitive oxalate uptake. In contrast to Cl:HCO3 exchange, HCO3 did not stimulate oxalate uptake more than was seen with a pH gradient in the absence of HCO3. An outwardly directed Cl gradient (50 mM inside, 5 mM outside) stimulated oxalate uptake 10-fold at 1 min with a fivefold accumulation above equilibrated uptake. Cl-stimulated oxalate uptake was largely inhibited by DIDS. Addition of K+ and nigericin only slightly decreased the Cl gradient-stimulated oxalate uptake, which indicates that this stimulation was not primarily due to the Cl gradient generating an inside alkaline pH gradient via Cl:OH exchange. Further, an outwardly directed oxalate gradient stimulated 36Cl uptake. These results suggested that both oxalate:OH and oxalate:Cl exchange occur on the brush border membrane. To determine if one or both of these exchanges were on contaminating basolateral membrane, the vesicle preparation was further fractionated into a brush border and basolateral component using sucrose density gradient centrifugation. Both exchangers localized to the brush border component. A number of organic anions were examined (outwardly directed gradient) to determine if they could stimulate oxalate and Cl uptake. Only formate and oxaloacetate were found to stimulate oxalate and Cl uptake. An inwardly directed Na gradient only slightly stimulated oxalate uptake, which was inhibited by DIDS.

Differentiation of secretagogue drugs by chlorpromazine in rat intestine in vivo

The effect of chlorpromazine (CPZ) on passive epithelial permeability and net fluid movement induced by secretagogues was tested in the rat intestine in vivo. CPZ, in a dose of 20 mg/kg intramuscularly, did not alter colonic permeability either in control conditions or during increased permeability caused by deoxycholic acid (DOC) or bisacodyl. Fluid secretion induced by cholera toxin and theophylline was strongly reduced by CPZ. The effects of oxyphenisatin and bisacodyl were only slightly but significantly inhibited by CPZ, whereas the action of DOC was unaffected. It is concluded, that the increase of the epithelial permeability is the main reason for the augmented fluid secretion caused by DOC. Bisacodyl and oxyphenisatin seem to act partly via an increase in permeability and to some degree via an induction of an active secretory process.

The importance of the enteric nervous system for the bile-salt-induced secretion in the small intestine of the rat

We have investigated the possible involvement of the enteric nervous system in the intestinal secretion induced by sodium deoxycholate. Hexamethonium, lidocaine, and tetrodotoxin significantly inhibited the fluid secretion in extrinsically innervated and denervated rat jejunal segments. Atropine had no effect. We conclude that the sodium-deoxycholate-induced intestinal secretion is partly caused by the activation of local nervous reflexes.

Water and electrolyte absorption by the colon in tropical sprue

A defect in colonic absorption of electrolytes and water was demonstrated in patients with tropical sprue by perfusing the colon with normal saline containing a non-absorbable marker. Colonic water absorption correlated negatively with stool weight and was abnormal in patients with steatorrhoea. The possible mechanisms producing this defect are discussed. This defect may be related to colonocyte damage produced by unabsorbed unsaturated fatty acids in patients with steatorrhoea.

Kinins stimulate net chloride secretion by the rat colon

1 Short circuit current (SCC), transepithelial conductance and ion fluxes were measured across the isolated descending colon of the rat in response to bradykinin or kallidin. 2 Kinins added to the serosal bath caused immediate increases in SCC but were ineffective when added to the mucosal bath. Increases in SCC were accompanied by significant increases in transepithelial conductance. Threshold kinin concentration was 0.5nM and maximal increases were seen at 50-100 nM. 3 A rat glandular kallikrein (7nM) or mellitin (2 microM) also increased SCC if added to the serosal bath. 4 Responses to kinins were unaffected by mucosal amiloride (100 micron) but attenuated or blocked by serosal frusemide (100 microM), indomethacin (1 microM) or mepacrine (50 microM). 5 Replacement of chloride ion in the serosal bath by gluconate and sulphate ions abolished responses to kinins which reappeared after chloride re-addition. 6 Measurement of 36Cl, 22Na and 86Rb fluxes across the tissue showed that the kinin-induced increase in SCC resulted principally from increased net chloride secretion. Effects upon 22Na or 86Rb flux were minimal and made no contribution to the current responses observed in this tissue. 7 The results prove that kinins stimulate net chloride secretion in the rat colon, most probably via a prostaglandin-dependent pathway.

Antiabsorptive effects of 16,16 dimethyl prostaglandin E2 in isolated rat colon

Ulcerative colitis is distinguished by abundant prostaglandin E2 (PGE 2) in the stools and by severe diarrhea. To determine whether luminal PGE2 alters normal colonic absorption, NA+ and Cl-transport across isolated rat proximal colon were studied before and after 16,16 dimethyl PGE2 (dmPGE2) addition to flux chambers. Luminal administration of dmPGE2 significantly reduced the net mucosal to serosal fluxes of Na+ and Cl-. These antiabsorptive tive effects of dmPGE2 on NA+ and Cl- active transport were reflected by a reduced metabolic rate of colonic tissue slices incubated with dmPGE2. Addition of dmPGE2 significantly reduces oxidation of glucose by the colon. Structurally, dmPGE2 reduced the length of colonic mucosal microvilli, thereby decreasing absorptive surface area. These results suggest that PGE2 released into the colonic lumen of patients with ulcerative colitis exerts antiabsorptive effects on the colon and in this way contributes to the associated diarrhea.

Effects of secretagogues on the K+ permeability of mucosal and serosal borders of rabbit colonic mucosa

(1) K+ efflux rates from the mucosal and serosal surfaces of sheets of rabbit colonic mucosa have been determined by measuring net K+ loss into K+-free Ringer solution bathing each side of the tissue. (2) Initially, there is a high rate of K+ loss from the tissue, this falls to a lower steady-state rate after 20 min. Loss of K+ from the tissue into the serosal bath is 6-8 fold faster than loss to the mucosal bath. (3) A number of intestinal secretagogues, e.g. theophylline, cyclic AMP, carbachol, ionophore A23187, as well as the laxative bisacodyl, raise the K+ efflux rate across the mucosal border by 200-300%. In the case of K+ efflux induced by carbachol the effect is shown to be dependent on raised levels of intracellular Ca2+. Ca2+-calmodulin complex does not appear to be be involved in activation of K+ efflux across the mucosal border. (4) Amiloride does not block mucosal K+ efflux, but tetraethyl-ammonium does inhibit K+ efflux across the mucosal border, induced by either bisacodyl or raised intracellular Ca2+. (5) The results suggest that laxatives may increase the rate of K+ secretion into the colonic lumen by raising the K+ permeability of the mucosal border.

Effect of Escherichia coli heat-stable enterotoxin, cholera toxin and theophylline on ion transport in porcine colon

1. The effect of heat-stable enterotoxin (ST) of Escherichia coli, cholera toxin (CT), and theophylline (a phosphodiesterase inhibitor) on ion and water transport was studied with an in vivo isolated loop system of the pig colon.2. All three agents abolished net Na absorption as a result of a decrease in the lumen to blood Na flux alone. With all three agents, net Cl absorption was reduced, but not abolished, and net HCO(3) secretion was elicited. Luminal p(CO2) was reduced with CT and theophylline from that observed in normal Ringer alone.3. Theophylline resulted in a prompt and sustained increase in both cyclic adenosine monophosphate (cyclic AMP) and cyclic guanosine monophosphate (cyclic GMP) levels in colonic mucosa studied in vitro. ST selectively elevated cyclic GMP, whereas CT selectively elevated cyclic AMP. These responses paralleled the time course and magnitude of response of the transepithelial electrical potential difference (psi(LB)) measured in vivo.4. Ion replacement studies in the presence or absence of theophylline showed that in the absence of Na, Cl absorption was slightly reduced and HCO(3) secretion was elicited; no further additive effects of theophylline in the absence of luminal Na were observed. In the absence of luminal Cl, net Na absorption was abolished and HCO(3) was absorbed; theophylline resulted in significant net Na and HCO(3) secretion. Theophylline also increased psi(LB) in the absence of either luminal Na or Cl.5. Results suggest that in the presence of theophylline or enterotoxin, the coupled Na-H and Cl-HCO(3) exchange processes that are normally responsible for at least half of the net NaCl absorption by this tissue are interrupted. Active HCO(3) secretion is observed and Cl absorption under these conditions can be entirely explained as a consequence of psi(LB). Thus, these studies indicate that the colon may participate in the production of diarrhoea of enterotoxigenic origin. They also suggest an important functional role of cyclic nucleotides in controlling the acidity and volume of colonic contents.

Effect of prostaglandin on ion transport across isolated colonic mucosa

The effect of prostaglandins (PG) on colonic ion transport was investigated in in vitro experiments in the rat. Both PGE1 and PGA1 increased short-circuit current (Isc), potential difference, and mucosal cyclic AMP levels, but PGE1 was more potent than PGA1. 10(-4) M PGE1 inhibited active sodium transport across short-circuited colonic mucosa (6.1-1.3 microEq/hr/cm2). This effect, coupled with an increase in Isc, is consistent with prostaglandin stimulation of active anion secretion and with the recent suggestions that prostaglandins may be important intermediaries in the process by which several laxatives alter large-intestinal fluid and electrolyte movement.

Effect of D-alanine methionine enkephalin amide on ion transport in rabbit ileum

The presence of enkephalins in the intestine and the use of opiates to treat diarrheal diseases suggests that enkephalins may affect intestinal ion transport. Using isolated rabbit ileal mucosa, we found that leucine enkephalin, methionine enkephalin, and D Ala2-methionine enkephalin amide (D Ala2-Met E) decreased the short circuit current (Isc) and potential difference although the effect of D Ala2-Met E was more pronounced and prolonged. D Ala2-Met E increased net sodium (+1.27 +/- 0.5 mu eq/cm2h), and chloride absorption (+2.33 +/- 0.4), and increased tissue conductance by 37%. Although the effect of enkaphalin on ion transport is opposite that of cyclic AMP, D-Ala2-Met had no effect on basal or vasoactive intestinal polypeptide-stimulated cyclic AMP levels. The effect of D-Ala2-Met E on Isc was blocked by naloxone, suggesting the involvement of specific opiate receptors. Tetrodotoxin completely blocked the decrease in Isc induced by D-Ala2-Met E but not by epinephrine, inferring that enkephalins are preganglionic neurotransmitters. The effect of D-Ala2-Met E on Isc was not blocked by phentolamine, haloperidol, or pretreatment of animals with 6-hydroxydopamine, suggesting that enkephalin does not affect the Isc by stimulating the release of alpha-adrenergic or dopaminergic agonists. D-Ala2-Met E also decreased the Isc in the presence of carbachol and bethanechol, indicating that enkephalin does not inhibit the release of acetylcholine. Further, up to 10 mu M atropine had no effect on the Isc. These studies demonstrate that enkephalins stimulate intestinal ion transport and may do so by stimulating (or inhibiting) the release of a nonadrenergic, noncholinergic neurotransmitter.

Dose-related effects of chenodeoxycholic acid in the rabbit colon

The effects of bile acid (BA) concentration on fluid secretion, mucus secretion, and mucosal damage were investigated during dose-response studies in the rabbit colon with 1, 2.5, and 5 mM sodium chenodeoxycholate (NaCDC). 1 mM NaCDC resulted in mucus secretion followed by mucosal damage but no change in fluid transport was observed. At 2.5 mM concentration, mucus secretion and mucosal damage were evident within 1 hr of perfusion whereas fluid secretion developed in the second hour only. At 5 mM concentrations, all changes occurred simultaneously. The magnitude of changes increased with the concentration of BA perfused. These results are consistent with the hypothesis that mucus secretion with loss of its cytoprotective effect precedes, and thus may permit, the detergent effects of the di-alpha-hydroxy bile acid on the mucosa, resulting in mucosal damage. Both these effects precede changes in fluid and electrolyte transport.

Absorption by the colon during prolonged infusions in conscious, unrestrained rats

1. Colonic absorption was studied in conscious, unrestrained rats during prolonged infusions through implanted cannulae. During infusion of predominantly NaCl-containing solution at rates up to 0.7 ml h-1 per 100 g body wt., absorption of fluid and Na increased considerably without significant change of transmucosal p.d.; diarrhoea did not occur. 2. Exclusion of the distal colon by colostomy showed that the proximal part of the colon was chiefly responsible for the increased absorption and inspection during the infusion showed it to be much dilated. Removal of the caecum showed that it contributed considerably to absorption by the proximal part. The distal colon influenced Na concentration in the faeces but absorbed little volume while direct infusion into this region rapidly produced diarrhoea. 3. The addition of Na deoxycholate (5 mmol/l) to the infusion solution impaired absorption provoking diarrhoea with mucus loss; there was no evidence that fluid secretion was stimulated. Substitution of SO4 for Cl in the infused solution produced only small changes of transmucosal p.d. but considerably impaired absorption and diarrhoea resulted. 4. The findings indicate that the proximal colon and caecum possess a considerable potential for increasing fluid and Na absorption and suggest the possibility that a neutral Na-Cl coupled absorptive mechanism is stimulated by loading the colon with fluid and NaCl.