Ion transport in rabbit ileal mucosa. I. Na and Cl fluxes and short-circuit current

Severe defects in absorptive ion transport in distal colons of mice that lack ClC-2 channels

BACKGROUND & AIMS

The fluid secretion model predicts that intestinal obstruction disorders can be alleviated by promoting epithelial Cl(-) secretion. The adenosine 3',5'-cyclic monophosphate (cAMP)-activated anion channel CFTR mediates Cl(-)-dependent fluid secretion in the intestine. Although the role of the ClC-2 channel has not been determined in the intestine, this voltage-gated Cl(-) channel might compensate for the secretory defects observed in patients with cystic fibrosis and other chronic constipation disorders. We investigated whether mice that lack ClC-2 channels (Clcn2(-/-)) have defects in intestinal ion transport.

METHODS

Immunolocalization and immunoblot analyses were used to determine the cellular localization and the amount of ClC-2 expressed in mouse early distal colon (EDC) and late distal colon (LDC). Colon sheets from wild-type and Clcn2(-/-) littermates were mounted in Ussing chambers to determine transepithelial bioelectrical parameters and Na(+), K(+), and Cl(-) fluxes.

RESULTS

Expression of ClC-2 was higher in the basolateral membrane of surface cells in the EDC compared with the LDC, with little expression in crypts. Neither cAMP nor Ca(2+)-induced secretion of Cl(-) was affected in the EDC or LDC of Clcn2(-/-) mice, whereas the amiloride-sensitive short-circuit current was increased approximately 3-fold in Clcn2(-/-) EDC compared with control littermates. Conversely, electroneutral Na(+), K(+), and Cl(-) absorption was dramatically reduced in colons of Clcn2(-/-) mice.

CONCLUSIONS

Basolateral ClC-2 channels are required for colonic electroneutral absorption of NaCl and KCl. The increase in the amiloride-sensitive short-circuit current in Clcn2(-/-) mice revealed a compensatory mechanism that is activated in the colons of mice that lack the ClC-2 channel.

Calcium-calmodulin-dependent Activation of Adenylate Cyclase in Prostaglandin-induced Electrically-monitored Intestinal Secretion in the Rat

The calcium-calmodulin antagonist 5-iodo-C8-W7 inhibited the PGE2-induced stimulation of cAMP production by isolated enterocytes from rat small intestine. It also reduced the secretory response of intestinal sheets to PGE2, measured as a rise in short-circuit current. It did not however, inhibit the electrical responses to forskolin and dibutyryl cAMP, nor to acetylcholine, a secretagogue whose effect is not mediated by cAMP. It is concluded that the receptor-mediated activation of adenylate cyclase and the subsequent secretory response are dependent upon calcium-calmodulin.

Zinc inhibits calcium-mediated and nitric oxide-mediated ion secretion in human enterocytes

Zn(2+) is effective in the treatment of acute diarrhea, but its mechanisms are not completely understood. We previously demonstrated that Zn(2+) inhibits the secretory effect of cyclic adenosine monophosphate but not of cyclic guanosine monophosphate in human enterocytes. The aim of the present study was to investigate whether Zn(2+) inhibits intestinal ion secretion mediated by the Ca(2+) or nitric oxide pathways. To investigate ion transport we evaluated the effect of Zn(2+) (35 microM) on electrical parameters of human intestinal epithelial cell monolayers (Caco2 cells) mounted in Ussing chambers and exposed to ligands that selectively increased intracellular Ca(2+) (carbachol 10(-6)M) or nitric oxide (interferon-gamma 300 UI/ml) concentrations. We also measured intracellular Ca(2+) and nitric oxide concentrations. Zn(2+) significantly reduced ion secretion elicited by carbachol (-87%) or by interferon-gamma (-100%), and inhibited the increase of intracellular Ca(2+) and nitric oxide concentrations. These data indicate that Zn(2+) inhibits ion secretion elicited by Ca(2+) and nitric oxide by directly interacting with the enterocyte. They also suggest that Zn(2+) interferes with three of the four main intracellular pathways of intestinal ion secretion that are involved in acute diarrhea.

Functional coupling of the downregulated in adenoma Cl-/base exchanger DRA and the apical Na+/H+ exchangers NHE2 and NHE3

Non-nutrient-dependent salt absorption across the brush-border membrane of intestinal epithelial cells is primarily mediated by coupled apical Na(+)/H(+) (aNHE) and anion exchange transport, with the latter suspected of being mediated by DRA (downregulated in adenoma; SLC26A3) that is defective in congenital chloridorrhea. To investigate DRA in greater detail and determine whether DRA and NHE activities can be coupled, we measured (22)Na(+) and (36)Cl(-) uptake in Caco2BBE colon cells infected with the tet-off-inducible DRA transgene. Under basal conditions, DRA activity was low in normal and infected Caco2BBE cells in the presence of tetracycline, whereas NHE activities could be easily detected. When apical NHE activity was increased by transfection or serum-induced expression of the aNHE isoforms NHE2 and NHE3, increased (36)Cl(-) uptake was observed. Inhibition of DRA activity by niflumic acid was greater than that by DIDS as well as by the NHE inhibitor dimethylamiloride and the carbonic anhydrase inhibitor methazolamide. DRA activity was largely aNHE-dependent, whereas a component of DRA-independent aNHE uptake continued to be observed. Coupled aNHE and DRA activities were inhibited by increased cellular cAMP and calcium and were associated with synaptotagmin I-dependent, clathrin-mediated endocytosis. In summary, these data support the role of DRA in electroneutral NaCl absorption involving functional coupling of Cl(-)/base exchange and apical NHE.

Growth hormone regulates intestinal ion transport through a modulation of the constitutive nitric oxide synthase-nitric oxide-cAMP pathway

AIM: Growth hormone (GH) directly interacts with the enterocyte stimulating ion absorption and reducing ion secretion induced by agonists of cAMP. Since nitric oxide (NO) is involved in the regulation of transepithelial ion transport and acts as a second messenger for GH hemodynamic effects, we tested the hypothesis that NO may be involved in the resulting effects of GH on intestinal ion transport.

METHODS: Electrical parameters reflecting trans-epithelial ion transport were measured in Caco-2 cell monolayers mounted in Ussing chambers and exposed to GH and cholera toxin (CT) alone or in combination, in the presence or absence of the NO synthase (NOS) inhibitor, Nω-nitro-L-arginine methyl ester (L-NAME). Similar experiments were conducted to determine cAMP and nitrite/nitrate concentrations. NOS expression was assayed by Western blot analysis.

RESULTS: L-NAME causes total abrogation of absorptive and anti-secretory effects by GH on intestinal ion transport. In addition, L-NAME was able to inhibit the GH-effects on intracellular cAMP concentration under basal conditions and in response to CT. GH induced a Ca²⁺-dependent increase of nitrites/nitrates production, indicating the involvement of the constitutive rather than the inducible NOS isoform, which was directly confirmed by Western blot analysis.

CONCLUSION: These results suggest that the GH effects on intestinal ion transport, either under basal conditions or in the presence of cAMP-stimulated ion secretion, are mediated at an intracellular level by the activity of cNOS.

The immune-enhancing enteral agents arginine and glutamine differentially modulate gut barrier function following mesenteric ischemia/reperfusion

BACKGROUND

Immune-enhancing enteral diets have been shown to improve patient outcome. One contributing mechanism may be via maintenance of gut barrier function. While recent data has shown that glutamine is beneficial, arginine may be harmful. We therefore hypothesized that the immune-enhancing agents, glutamine and arginine, differentially modulate gut barrier function.

METHODS

At laparotomy, rats had jejunal sacs filled with 10 mmol/L glutamine, arginine, fructose, or magnesium sulfate (osmotic control) followed by 60 minutes of superior mesenteric artery occlusion and 2 hours of reperfusion. Jejunum was harvested for histology, deconvolution microscopy, F:G actin, ATP, and permeability measurements.

RESULTS

Glutamine and fructose minimized mucosal injury compared with controls and arginine. Deconvolution microscopy confirmed that glutamine and fructose preserved the actin cytoskeleton but there was disruption by arginine which correlated with F:G actin ratios and tissue ATP levels. Permeability was enhanced by arginine compared with the other groups.

CONCLUSION

Arginine resulted in worsened mucosal injury, disruption of the actin cytoskeleton, decreased tissue ATP and enhanced permeability compared with glutamine which appeared protective. The immune-enhancing agent arginine results in breakdown of gut barrier function which may have important implications for critically injured patients.

Enterotoxigenicity of mature 45-kilodalton and processed 35-kilodalton forms of hemagglutinin protease purified from a cholera toxin gene-negative Vibrio cholerae non-O1, non-O139 strain

Cholera toxin gene-negative Vibrio cholerae non-O1, non-O139 strain PL-21 is the etiologic agent of cholera-like syndrome. Hemagglutinin protease (HAP) is one of the major secretory proteins of PL-21. The mature 45-kDa and processed 35-kDa forms of HAP were purified in the presence and absence of EDTA from culture supernatants of PL-21. Enterotoxigenicities of both forms of HAP were tested in rabbit ileal loop (RIL), Ussing chamber, and tissue culture assays. The 35-kDa HAP showed hemorrhagic fluid response in a dose-dependent manner in the RIL assay. Histopathological examination of 20 microg of purified protease-treated rabbit ileum showed the presence of erythrocytes and neutrophils in the upper part of the villous lamina propria. Treatment with 40 microg of protease resulted in gross damage of the villous epithelium with inflammation, hemorrhage, and necrosis. The 35-kDa form of HAP, when added to the lumenal surface of rat ileum loaded in an Ussing chamber, showed a decrease in the intestinal short-circuit current and a cell rounding effect on HeLa cells. The mature 45-kDa form of HAP showed an increase in intestinal short-circuit current in an Ussing chamber and a cell distending effect on HeLa cells. These results show that HAP may play a role in the pathogenesis of PL-21.

In vitro effects of deoxynivalenol on electrical properties of intestinal mucosa of laying hens

Deoxynivalenol (DON) is common in European cereal grains, and of all the trichothecenes, poses the greatest problems to animal health. The present study investigated the effects of DON on electrophysiological parameters in laying hens' jejunum mounted in Ussing chambers. In vitro studies were performed to measure the effects of different luminal concentrations of DON (0.5, 1, 5, and 10 microg/mL) on the transmural potential difference, electrical tissue resistance, and electrogenic ion flux rates (short-circuit current, Isc) across the isolated gut mucosa. Deoxynivalenol did not alter (P > 0.05) the transmural potential difference. Resistance was higher (P < 0.05) in the tissues exposed to DON compared with basal values. Deoxynivalenol caused a dose-dependent decrease in Isc (P < 0.05). To investigate the mechanism of action of DON, amiloride (a specific inhibitor for Na+ transport) was added after incubation of the tissue with DON. Amiloride did not decrease (P > 0.05) Isc under these conditions. This may indicate that DON inhibited the Na+ transport before addition of amiloride, which did not then show further inhibitory effects. The addition of D-glucose (5 mmol/L) on the luminal side of the isolated mucosa increased (P < 0.05) Isc, and this effect was reversed by phlorizin (a specific inhibitor of sodium/glucose transporter 1), indicating that the glucose-induced Isc increase may be due to Na+-D-glucose cotransport. In our study, DON decreased (P < 0.05) the glucose-induced Isc in a similar way to phlorizin. The remarkable similarity between the effects of phlorizin and DON on electrical properties seemed to be consistent with their common ability to inhibit Na+-D-glucose cotransport. In conclusion, DON decreased the Isc via inhibition of Na+ transport. The effect on intestinal electrical properties was similar to that of phlorizin after addition of glucose, suggesting that DON may inhibit Na+-D-glucose cotransport. The inhibition of Na+ transport and Na+-D-glucose cotransport are important mechanisms of DON toxicity in the intestine of laying hens.

Escherichia coli Heat Stable (STa) Enterotoxin and the Upper Small Intestine: Lack of Evidence in Vivo for Net Fluid Secretion

Heat stable (STa) enterotoxin from E. coli reduced fluid absorption in vivo in the perfused jejunum of the anaesthetized rat in Krebs-phosphate buffer containing lactate and glucose (nutrient buffer), in glucose saline and in glucose free saline. Bicarbonate ion enhanced fluid absorption of 98 +/- 7 (6) microl/cm/h was very significantly (P < 0.0001) reduced by STa to 19 +/- 4 (6) microl/cm/h, but net secretion was not found. When impermeant MES substituted for bicarbonate ion, net fluid absorption of 29 +/- 3 (6) microl/cm/h was less (P < 0.01) than the values for phosphate buffer and bicarbonate buffer. With STa in MES buffer, fluid absorption of 3 +/- 2 (6) microl/cm/h was less than (P < 0.001) that in the absence of STa and not significantly different from zero net fluid absorption. E. coli STa did not cause net fluid secretion in vivo under any of the above circumstances. Neither bumetanide nor NPPB when co-perfused with STa restored the rate of fluid absorption. In experiments with zero sodium ion-containing perfusates, STa further reduced fluid absorption modestly by 20 microl/cm/h. Perfusion of ethyl-isopropyl-amiloride (EIPA) with STa in zero sodium ion buffers prevented the small increment in fluid entry into the lumen caused by STa, indicating that the STa effect was attributable to residual sodium ion and fluid uptake that zero sodium-ion perfusates did not eradicate. These experiments, using a technique that directly measures mass transport of fluid into and out of the in vivo proximal jejunum, do not support the concept that E. coli STa acts by stimulating a secretory response.

Amendments to the theory underlying Ussing chamber data of chloride ion secretion after bacterial enterotoxin exposure

Bacterial enterotoxins may cause life-threatening diarrhoeal fluid loss in part because they stimulate enterocytes to secrete fluid into the small intestine as well as preventing normal fluid uptake. Abnormal chloride ion secretion is believed to provide the osmotic driving force for the inappropriate fluid movement. Evidence for enhanced chloride secretion consists of isotopic flux measurements in Ussing chambers, the standard apparatus for permeation studies. Flux from the lumen of the intestine is assumed to be determined solely by absorptive processes and flux towards the lumen solely by secretory processes. Bacterial enterotoxin increased flux towards the lumen is taken as an evidence of enhanced secretion. Examination of the flux equation solutions shows that the existing theoretical treatment of the Ussing chamber consists of the super-imposition of two contradictory unidirectional models. In contrast, the present analysis shows that a measured 'unidirectional' flux contains information both about absorptive and secretory processes, regardless of which flux is measured. Reciprocity is predicted for the fluxes, as decreases in the absorptive processes will cause increases in apparent secretory flux. Data from the literature show that mucosal-to-serosal chloride ion flux in rabbit ileum after exposure to secretagogues correlates inversely and highly significantly (r=0.74, n=17, p<0.001) with increases in serosal-to-mucosal chloride ion flux. As a category of evidence, flux data do not provide conclusive evidence of enhanced chloride secretion after exposure to enterotoxins, since an apparently enhanced serosal-to-mucosal flux would also be noted after inhibition of the mucosal-to-serosal flux. As interruption of absorptive processes can be misinterpreted as enhanced secretion in the Ussing chamber, this is a serious deficiency in the evidence for direct enterotoxin enhancement of the intestinal chloride ion channel as a basis for diarrhoeal disease.

Electroneutral sodium absorption and electrogenic anion secretion across murine small intestine are regulated in parallel

Electrolyte transport processes of small intestinal epithelia maintain a balance between hydration of the luminal contents and systemic fluid homeostasis. Under basal conditions, electroneutral Na(+) absorption mediated by Na(+)/H(+) exchanger 3 (NHE3) predominates; under stimulated conditions, increased anion secretion mediated by CFTR occurs concurrently with inhibition of Na(+) absorption. Homeostatic adjustments to diseases that chronically affect the activity of one transporter (e.g., cystic fibrosis) may include adaptations in the opposing transport process to prevent enterosystemic fluid imbalance. To test this hypothesis, we measured electrogenic anion secretion (indexed by the short-circuit current) across NHE3-null [NHE3(-)] murine small intestine and electroneutral Na(+) absorption (by radioisotopic flux analysis) across small intestine of mice with gene-targeted disruptions of the anion secretory pathway, i.e., CFTR-null [CFTR(-)] or Na(+)-K(+)-2Cl(-) cotransporter-null [NKCC1(-)]. Protein expression of NHE3 and CFTR in the intestinal epithelia was measured by immunoblotting. In NHE3(-), compared with wild-type small intestine, maximal and bumetanide-sensitive anion secretion following cAMP stimulation was significantly reduced, and there was a corresponding decrease in CFTR protein expression. In CFTR(-) and NKCC1(-) intestine, Na(+) absorption was significantly reduced compared with wild-type. NHE3 protein expression was decreased in the CFTR(-) intestine but was unchanged in the NKCC1(-) intestine, indicating that factors independent of expression also downregulate NHE3 activity. Together, these data support the concept that absorptive and secretory processes determining NaCl and water movement across the intestinal epithelium are regulated in parallel to maintain balance between the systemic fluid volume and hydration of the luminal contents.

Enteral glutamine but not alanine maintains small bowel barrier function after ischemia/reperfusion injury in rats

We previously demonstrated that glucose and glutamine, solutes metabolized by the gut, replenish ATP and enhance gut function compared with alanine, a solute not metabolized by the gut, following mesenteric ischemia/reperfusion (I/R). The purpose of the present study was to determine if the nonmetabolizable solute alanine differentially modulates cytoskeletal organization and paracellular small intestinal permeability compared with the metabolizable solutes glucose and glutamine following mesenteric I/R. At laparotomy, rats had jejunal sacs filled with 10 mM glucose, glutamine, alanine, or magnesium sulfate (5 mm, osmotic control) followed by superior mesenteric artery clamping for 60 min and 30 min of reperfusion or sham laparotomy. Jejunum was harvested for evaluation by deconvolution microscopy, fluorescent measurement of F:G actin ratio, or mounted in an Ussing chamber for determination of intestinal permeability. Deconvolution microscopy revealed that the actin cytoskeleton was preserved by enteral glutamine, comparable to shams, but disrupted by enteral alanine. Glucose and controls resulted in comparable disruption, which was less than that with alanine. The F:G actin ratio was highest for glutamine and lowest for alanine; glucose was comparable to controls. Intestinal permeability was highest for alanine and lowest for glutamine, which was comparable to shams. Permeability following glucose and controls was higher than that following glutamine but lower than that following alanine. The nonmetabolizable solute alanine resulted in disruption of the actin cytoskeleton and enhanced intestinal permeability under conditions of mesenteric I/R. The metabolizable solute glutamine was protective under these conditions, whereas glucose exerted minimal effect on the integrity of the cytoskeleton and intestinal permeability. The individual components of enteral diets may differentially modulate intestinal barrier function, which could have important implications when administered to critically injured patients.

Comparison of two Chinese medical herbs, Huangbai and Qianniuzi, on influence of short circuit current across the rat intestinal epithelia

Huangbai (Phellodendron spec.) and Qianniuzi (Pharbitis spec.) are two traditional Chinese medical herbs used for anti-diarrheal and laxative agents, respectively. Ethanol and water extracts of these two herbs were prepared and effects of the extracts on ion transport of the rat intestinal epithelia were studied. For measuring changes of the short circuit current across the epithelia, the rat intestinal epithelia were mounted in the Ussing chamber and attached with voltage/current clamp. The intestinal epithelia were firstly activated by serosal administration of 5 microM forskolin. As current raised and being stable, extracts of these herbs were added, respectively, and changes in the short circuit current were recorded. Ethanol extract of Huangbai attenuated the current increment; on the contrary, ethanol extract of Qianniuzi augmented the current increment additionally. Water extracts of the two herbs showed minor effects on the current in comparison to ethanol extracts. The results provide evidences to reveal the pharmacological mechanism of the two Chinese medical herbs on the intestinal tissue.

Effect of Ethanol Extracts of Three Chinese Medicinal Plants with Laxative Properties on Ion Transport of the Rat Intestinal Epithelia

The effects of ethanol extracts of three Chinese medicinal plants Dahuang (Rheum palmatum L.), Badou (Croton tiglium L.), and Huomaren (Cannabis sativa L.), on ion transport of the rat intestinal epithelia were studied. Rat intestinal epithelia mounted in an Ussing chamber attached with voltage/current clamp were used for measuring changes of the short-circuit current across the epithelia. The intestinal epithelia were activated with current raised by serosal administration of forskolin 5 microM. Ethanol extracts of the three plants all augmented the current additively when each was added after forskolin. In subsequent experiments, ouabain and bumetanide were added prior to ethanol extracts of these medicinal plants to determine their effect on Na(+) and Cl(-) movement. The results suggest that ethanol extracts of the three medicinal plants may affect the Cl(-) movement more directly than Na(+) movement in the intestinal epithelial cells. The results provide evidence for the pharmacologic mechanism of the three Chinese medicinal plants on the intestinal tract.

Effect of Ethanol Extracts of Three Chinese Medicinal Plants With Anti-diarrheal Properties on Ion Transport of the Rat Intestinal Epithelia

Effects of ethanol extracts of three Chinese medicinal plants, namely, Qinpi (Fraxini cortex), Kushen (Sophora flavescens, AITON), and Huanglian (Coptis teeta, WALLICH), on ion transport of the rat intestinal epithelia were determined in this study. Rat intestinal epithelia mounted in an Ussing chamber attached to a voltage/current clamp were used for measuring changes in the short circuit current across the epithelia. Activation of the intestinal epithelia by serosal administration of 5 microM forskolin resulted in an increase in basal short circuit current. The ethanol extracts of each of the three plants partially reduced the current stimulated by forskolin. In the following experiments, ouabain and bumetanide were added prior to adding the ethanol extract of these plants for revealing their effect on Na(+) and Cl(-) movement. The results suggest that the ethanol extract of the Qinpi would affect Cl(-) transport. On the contrary, the ethanol extract of Kushen would affect Na(+) transport rather than Cl(-) movement. This study provides evidences that reveal the pharmacological mechanism of the Chinese plants with anti-diarrheal properties.

cAMP inhibition of murine intestinal Na/H exchange requires CFTR-mediated cell shrinkage of villus epithelium

BACKGROUND AND AIMS

Unlike the intestine of normal subjects, small-intestinal epithelia of cystic fibrosis patients and cystic fibrosis transmembrane conductance regulator protein-null (CFTR(-)) mice do not respond to stimulation of intracellular cyclic adenosine monophosphate with inhibition of electroneutral NaCl absorption. Because CFTR-mediated anion secretion has been associated with changes in crypt cell volume, we hypothesized that CFTR-mediated cell volume reduction in villus epithelium is required for intracellular cyclic adenosine monophosphate inhibition of Na(+)/H(+) exchanger (primarily Na(+)/H(+) exchanger 3) activity in the proximal small intestine.

METHODS

Transepithelial (22)Na flux across the jejuna of CFTR(+), CFTR(-), the basolateral membrane Na(+)/K(+)/2Cl(-) co-transporter protein NKCC1(+), and NKCC1(-) mice were correlated with changes in epithelial cell volume of the midvillus region.

RESULTS

Stimulation of intracellular cyclic adenosine monophosphate resulted in cessation of Na(+)/H(+) exchanger-mediated Na(+) absorption (J(ms)(NHE)) in CFTR(+) jejunum but had no effect on J(ms)(NHE) across CFTR(-) jejunum. Cell volume indices indicated an approximately 30% volume reduction of villus epithelial cells in CFTR(+) jejunum but no changes in CFTR(-) epithelium after intracellular cyclic adenosine monophosphate stimulation. In contrast, cell shrinkage induced by hypertonic medium inhibited J(ms)(NHE) in both CFTR(+) and CFTR(-) mice. Bumetanide treatment to inhibit Cl(-) secretion by blockade of the Na(+)/K(+)/2Cl(-) co-transporter, NKCC1, of stimulated CFTR(+) jejunum prevented maximal volume reduction of villus epithelium and recovered approximately 40% of J(ms)(NHE). Likewise, J(ms)(NHE) and cell volume were unaffected by intracellular cyclic adenosine monophosphate stimulation in NKCC1(-) jejuna.

CONCLUSIONS

These findings show a previously unrecognized role of functional CFTR expressed in villus epithelium: regulation of Na(+)/H(+) exchanger 3-mediated Na(+) absorption by alteration of epithelial cell volume.

Intestinal NaCl transport in NHE2 and NHE3 knockout mice

Sodium/proton exchangers [Na(+)/H(+) (NHEs)] play an important role in salt and water absorption from the intestinal tract. To investigate the contribution of the apical membrane NHEs, NHE2 and NHE3, to electroneutral NaCl absorption, we measured radioisotopic Na(+) and Cl(-) flux across isolated jejuna from wild-type [NHE(+)], NHE2 knockout [NHE2(-)], and NHE3 knockout [NHE3(-)] mice. Under basal conditions, NHE(+) and NHE2(-) jejuna had similar rates of net Na(+) (approximately 6 microeq/cm(2) x h) and Cl(-) (approximately 3 microeq/cm(2) x h) absorption. In contrast, NHE3(-) jejuna had reduced net Na(+) absorption (approximately 2 microeq/cm(2) x h) but absorbed Cl(-) at rates similar to NHE(+) and NHE2(-) jejuna. Treatment with 100 microM 5-(N-ethyl-N-isopropyl) amiloride (EIPA) completely inhibited net Na(+) and Cl(-) absorption in all genotypes. Studies of the Na(+) absorptive flux (J) indicated that J in NHE(+) jejunum was not sensitive to 1 microM EIPA, whereas J in NHE3(-) jejunum was equally sensitive to 1 and 100 microM EIPA. Treatment with forskolin/IBMX to increase intracellular cAMP (cAMP(i)) abolished net NaCl absorption and stimulated electrogenic Cl(-) secretion in all three genotypes. Quantitative RT-PCR of epithelia from NHE2(-) and NHE3(-) jejuna did not reveal differences in mRNA expression of NHE3 and NHE2, respectively, when compared with jejunal epithelia from NHE(+) siblings. We conclude that 1) NHE3 is the dominant NHE involved in small intestinal Na(+) absorption; 2) an amiloride-sensitive Na(+) transporter partially compensates for Na(+) absorption in NHE3(-) jejunum; 3) cAMP(i) stimulation abolishes net Na(+) absorption in NHE(+), NHE2(-), and NHE3(-) jejunum; and 4) electroneutral Cl(-) absorption is not directly dependent on either NHE2 or NHE3.

Evidence for absorption of ionic calcium and soluble calcium complexes by the duodenum and cecum in the rat

The absorption of dietary calcium (Ca) may in part be determined by the formation in the intestinal lumen of soluble Ca complexes and insoluble Ca salts. This study was undertaken to test the assumption that ionic Ca concentration (Ca2+) is the only species of Ca that is available for absorption. Bidirectional steady-state Ca fluxes were measured in vitro under short-circuit conditions across segments of the proximal duodenum and the cecum in the presence and absence of varying concentrations of soluble Ca citrate complexes. The presence of 5.0 mmol/L medium citrate reduced medium Ca2+ and cecal Ca mucosal-to-serosal fluxes (Jms) (29 +/- 18 versus 108 +/- 7 nmol Ca/cm2/h, P <.001), but did not reduce duodenal Ca Jms (31 +/- 5 versus 23 +/- 9, P not significant). Duodenal Ca Jms increased 106% as medium Ca citrate complex increased to 1.018 mmol/L and Ca2+ remained constant; cecal Jms increased by 48% under the same conditions. The formation of soluble Ca organic anion complexes with lactate, malate, and fumarate reduced medium Ca2+ and cecal Ca Jms decreased with the reduction of medium Ca2+. The results of this study indicate that Ca2+ is the form of Ca most readily absorbed by the small intestine and the colon. Soluble Ca citrate complexes are absorbed by the duodenum and, to a much lesser extent, by the cecum. The reduction of Ca Jms by citrate is caused by the reduction of medium Ca2+ through formation of Ca citrate complexes and not caused by a direct interaction of the anion with the intestinal epithelium.

Enteropathogenic Escherichia coli strain RDEC-1 produces a novel electrogenic factor active on rabbit ileum in vitro

BACKGROUND

Attaching and effacing Escherichia coli demonstrate marked species specificity in inducing diarrhea, although its mechanism remains largely unclear. The purpose of this study was to investigate the existence of a soluble, species-specific factor that induces diarrhea in an in vitro model.

METHODS

Stripped rabbit ileum was mounted in Ussing chambers, and changes in potential difference and short-circuit current were monitored after the addition of bacterial culture supernatant.

RESULTS

The culture supernatant from rabbit-specific strain RDEC-1, but not from human-specific enteropathogenic Escherichia coli strain E2348/69, induced an increase in potential difference and short-circuit current in rabbit ileum mounted in Ussing chambers. This electrical signal was related to chloride ion secretion, was absent in colonic tissue, and was retained in the 30 to 100-KDa fraction of the supernatant. Preliminary experiments failed to show an involvement of calcium or cyclic nucleotides as intracellular messengers. RDEC-1 cured of a 42-MDa plasmid lost the enterotoxicity whereas conjugation of the plasmid into the negative E. coli recipient HB101 resulted in the expression of toxicity.

CONCLUSIONS

The authors describe a novel, species-specific factor that helps to explain RDEC-1 diarrhea, which may be relevant to the pathogenesis of enteropathogenic Escherichia coli infection.

Absence of intestinal secretion on supernatants from macrophages stimulated with Clostridium difficile toxin B on rabbit ileum

Several studies have documented the involvement of both Clostridium difficile, toxins, A and B in the pathogenesis of antibiotic-associated diarrhea. Recently, we demonstrated that IL-1 beta is the intestinal secretory factor released by macrophages stimulated with toxin A. The aim of this study was to evaluate the importance of macrophages stimulated with toxin B on rabbit ileal ion transport. The changes in ion transport were analyzed by studying the short-circuit current of the rabbit ileal mucosa mounted in Ussing chambers. The supernatants of macrophages treated with toxin B (3.6 x 10(-7) M) had no effect on the ion transport (change in short-circuit current =28.0+/-9.2 vs. control=26.8+/-3.6 microA cm(-2)). Supernatants of macrophages stimulated with toxin A (3.2 x 10(-7) M), our positive control, induced a significant change in ileal ion transport (delta I(sc)=55.2+/-5.7 mA cm(-2)). It was also observed that, like toxin A, toxin B stimulated macrophages to produce TNF-alpha (555.0+/-37.9 pg/ml vs. control=182.0+/-39.8 pg/ml; p<0.05). Nevertheless, in contrast to toxin A, toxin B did not stimulate IL-1 beta synthesis (28.0+/-7.5 pg/ml vs. control=40. 0+/-14.4 pg/ml; p>0.05). We conclude that the supernatants of macrophages stimulated with toxin B are not able to stimulate ion transport and that both toxins stimulate the genesis of TNF-alpha, but only toxin A induces the synthesis of IL-1 beta, which, we have earlier reported, causes an electrogenic intestinal response in rabbit ileum.

Electrical properties of the intestinal mucosa of the chicken and the effects of luminal glucose

Transmural potential difference (PD), short-circuit current (Isc), and electrical resistance (R) were measured in the isolated mucosa of the duodenum, jejunum, ileum, proximal cecum, and rectum in order to characterize the electrical properties of the chicken small and large intestine. The chicken intestine was classified into three categories, regarding its electrical characteristics: 1) the duodenum, with four to five times higher R than the other segments and the lowest PD; 2) the group formed by the jejunum, the ileum, and the proximal cecum, with high PD and low R; 3) the rectum, with low PD and low R. In all segments, the addition of D-glucose into the luminal side stimulates Isc, and this effect can be reversed by phloridzin, indicating that the glucose-induced Isc increase is due to Na+-D-glucose co-transport. The effect of glucose is maximal in the rectum, with a fivefold Isc increase, suggesting that this segment may have an important role in the absorption of Na+ as well as of nutrients co-transported with Na+.

Growth hormone stimulates, through tyrosine kinase, ion transport and proliferation in human intestinal cells

BACKGROUND

Growth hormone (GH) stimulates intestinal growth and differentiation and promotes water and ion absorption in the rat intestine. Epidermal growth factor has similar effects, which involve tyrosine kinase activity. The effects of growth hormone on ion transport and cell growth and the role of tyrosine kinase in these effects were examined in a human-derived intestinal cell line (Caco-2).

METHODS

For transport study, electrical parameters were measured in human intestinal Caco-2 cell monolayers mounted in Ussing chambers. Cell growth was monitored by counting and 3H-thymidine incorporation in the presence and absence of growth hormone. The role of tyrosine kinase was investigated by using its specific inhibitor genistein.

RESULTS

The addition of growth hormone induced a rapid, Cl- -dependent, decrease in short-circuit current without affecting tissue conductance, which is consistent with an anion-absorptive effect. Incubation with growth hormone increased cell count by 85% and 3H-thymidine incorporation by 64% versus the count in control specimens. The absorptive and trophic effects of growth hormone were dose-dependent, and the maximum effective concentration was identical for each effect. Genistein blocked the growth hormone effect on ion transport and cell growth.

CONCLUSIONS

Growth hormone stimulates ion absorption and cell growth in human enterocytes. Both effects result from a direct growth hormone-enterocyte interaction, and both require tyrosine kinase activity. Growth hormone may have therapeutic potential in intestinal diseases characterized by epithelial atrophy and loss of water and electrolytes.

Effect of combined immunosuppressive drug therapy on small intestinal nutrient transport in the rat

OBJECTIVE

Prevention of rejection and preservation of graft function remain as obstacles to clinical small intestinal transplantation (SIT). This study evaluated the effects of combined immunosuppressive agents (FK506, Rapamycin, and Mycophenolate Mofetil) on intestinal function and animal well being.

METHODS

Screening for additive toxicity was done in experiment one (D1, n = 10); doses were: FK506 0.3 mg/kg/d, Rapamycin 2 mg/kg/d, and Mycophenolate Mofetil 20 mg/kg/d, orally once daily. Control animals (C1, n = 10) received equivalent vehicle. In the second phase of the experiment, the effect of an additional parenteral treatment phase was investigated, with drug treated animals (D2, n = 6) received FK506 0.3 mg/kg, Rapamycin 1 mg/kg, and Mycophenolate Mofetil 10 mg/kg sq q12h for 1 week followed by FK506 3 mg/kg, Rapamycin 1 mg/kg, and Mycophenolate Mofetil 10 mg/kg p.o. q12h for 4 weeks. Control animals (C2, n = 6) received equivalent vehicle. Parameters followed were weight gain, nutrient absorption, drug levels and nutrient transport in vitro.

RESULTS

Controls grew normally, while weight gain was significantly reduced in drug treated animals: This was paralleled by a reduction in dietary fat absorption. Drug levels were low to therapeutic for all drugs in both experiments; FK506 appeared to affect Rapamycin and Mycophenolate Mofetil metabolism, increasing levels of both as FK506 doses increased. Nutrient transport was either not effected (D1) or increased (D2).

CONCLUSIONS

We conclude that low dose combination immunosuppressive therapy inhibits weight gain, without affecting absorption of dietary energy, or adversely affecting glucose transport. We postulate a systemic metabolic cause, which requires additional investigation at the cellular level; additional studies are also required to determine if the additive immunosuppression outweigh the side effects for SIT.

Intestinal epithelial hyperpermeability. Mechanisms and relevance to disease

Pathologic increases in intestinal permeability to hydrophilic macromolecules has been identified in a number of clinical conditions. The significance of gut barrier dysfunction as a clinical issue remains to be delineated, although it seems likely that alterations in intestinal epithelial permeability play a causative role in a number of conditions ranging from inflammatory bowel disease to the development of complications after cardiopulmonary bypass. It is unlikely that any one mechanism can account for all cases of intestinal hyperpermeability. Rather, it is more probable that myriad factors or combinations of factors, including mesenteric ischemia and cytokine-induced phenomena, lead to alterations in permeability in different clinical entities. Nevertheless, from a purely mechanistic standpoint, some common themes, notably the role of ATP depletion, increases in [Ca2+]i, and cytoskeletal derangements in enterocytes, have emerged as being particularly important.

Acute effects of bile acids on the pancreatic duct epithelium in vitro

BACKGROUND

Acute pancreatitis is associated with passage of gallstones, although the mechanism(s) linking the two processes remains undefined. Bile reflux into the pancreatic duct could play a role but the experimental conditions often employed to induce pancreatitis rarely develop clinically. Here we examined whether low concentrations of bile affect ductal electrophysiology as an indirect measure of ductal epithelial integrity and function in vitro.

METHODS

The main duct was dissected out of freshly harvested bovine pancreata, cut into 1- x 2-cm sections, placed in tissue culture for 48-72 h, then placed in Ussing chambers. Changes in tissue resistance (Rt) and short-circuit current (Isc) were monitored. The responses to forskolin and bile (taurodeoxycholic acid, TDCA) were examined separately and together.

RESULTS

Forskolin (10 microM) produced a decrease in the Isc without a significant change in Rt, suggesting a secretory response, followed by a return to baseline. TDCA caused a similarly reversible decrease in the Isc at low doses, but a persistent drop at higher concentrations. A concurrent drop in Rt was noted at all TDCA concentrations, the duration of which correlated with dosage and degree of histological damage. Prior exposure to low (0.5 mM) doses of TDCA significantly blunted the response to subsequent forskolin challenge.

CONCLUSIONS

Acute exposure to TDCA in vitro causes epithelial damage at levels lower than those normally used to induce experimental pancreatitis. At the lower concentrations, Rt returns to baseline rapidly, suggesting recovery (restitution) from epithelial damage but with a persistent loss of the response to forskolin. Reflux of minute amounts of bile into the pancreatic duct could play a significant role in the pathogenesis of gallstone pancreatitis by uncoupling the normal stimulus-secretion apparatus of the ductal system and breaking down the epithelial barrier.

Production of Vibrio cholerae accessory cholera enterotoxin (Ace) in the yeast Pichia pastoris

Accessory cholera enterotoxin (Ace) is a recently identified toxin of Vibrio cholerae. Preliminary studies using crude toxin extracts in animal models indicate that Ace increases transcellular ion transport, which is proposed to contribute to diarrhea in cholera. The lack of purified toxin has hindered elucidation of the mechanism of action of Ace. In this study, ace was cloned and was expressed in and secreted by the methylotrophic yeast Pichia pastoris. Secreted toxin constituted 50% of the total supernatant protein from Pichia pastoris. Presumed monomer and dimer forms with molecular masses of 9 and 18 kDa, respectively, were observed. The 18-kDa form predominated. Biological activity was assayed by studying ion fluxes across epithelial membranes in Ussing chambers. Among the characteristics of Ace was the unusual property of staining with silver but not Coomassie blue stain. To our knowledge this is the first report of a biologically active bacterial toxin produced with the P. pastoris system. The purified protein may now be used in studies of the mechanism of action of Ace in physiologic systems.

Nutritional and intestinal effects of the novel immunosuppressive agents: deoxyspergualin, rapamycin, and mycophenolate mofetil

OBJECTIVES

Transplantation of the small intestine would be an attractive therapeutic option for treatment of short bowel syndrome if effective, nontoxic immunosuppressive agents could be developed. This study examines the effect of three newly developed immuno-suppressive agents: rapamycin, deoxyspergualin, and mycophenolate mofetil, on the nutritional status and intestinal function of normal juvenile rats.

DESIGN & METHODS

Rapamycin (2 mg/kg every second day), deoxyspergualin (2 mg/kg every second day) and mycophenolate mofetil (MM) (25 mg/kg every second day) were injected subcutaneously for six weeks.

RESULTS

Rapamycin and deoxyspergualin caused significant reductions in weight gain without impairing feed intake. Both drugs caused small decreases in fat absorption; treatment with DSG induced an increase in permeability to 99Tc-DTPA. However, the permeability to other markers, such as mannitol and lactulose, was decreased in the rapamycin and mycophenolate mofetil-treated animals. Intestinal function in vitro was quantified using glucose flux (absorption). In the rapamycin group, there was a significant decrease in ileal uptake of glucose, with the net flux (absorption) being zero; there was an associated loss of villous size histologically. In the deoxyspergualin-treated groups, there was a decrease in the jejunal glucose flux. In the mycophenolate mofetil-treated animals, there was a decrease in jejunal with a compensatory increase in ileal glucose absorption. There were minor variations in intestinal morphology, but these were not consistent.

CONCLUSIONS

Rapamycin and deoxyspergualin in these doses cause a significant reduction in weight gain in healthy juvenile animals, and all the drugs caused changes in the active transport characteristics of the intestine. Accordingly, the use of these drugs for intestinal transplantation should be evaluated carefully for their nutritional impact.

Comparative effects of growth hormone on water and ion transport in rat jejunum, ileum, and colon

Specific growth hormone (GH) receptors are located along the entire rat intestine. We have recently shown that GH induces water and ion absorption in the rat ileum. This raises the possibility that GH regulates water and ion transport throughout the intestine. To test this, we have evaluated the effects of GH administration on jejunal, ileal, and colonic water and ion transport, by the in vivo rat perfused intestine, and in vitro, in corresponding segments of intestine mounted in Ussing chambers. In vivo, GH increased water absorption by 250%, 180%, and 80% over baseline in the jejunum, ileum, and colon, respectively. The effect had similar kinetics in the three intestinal regions. In vitro, serosal GH administration induced a decrease in short-circuit current, consistent with an absorptive effect. The effect showed a proximal to distal decreasing pattern. These findings suggest that GH plays a role in the body fluid homeostatic control, promoting water and ion absorption.

Tacrolimus (FK506)--its effects on intestinal glucose transport

Tacrolimus (FK506) is at present the mainstay of immunosuppression for small intestinal transplantation. This study investigates the effects of chronic treatment with varying dosages of tacrolimus on animal well-being, weight gain, intestinal permeability, and the active transport of nutrients as measured by in vitro studies quantifying glucose flux. The effect of acute treatment with high-dose tacrolimus on glucose flux was also investigated. In the chronic studies, juvenile male Lewis rats were given tacrolimus in a dosage of 0.1 mg/kg, 0.5 mg/kg, and 2 mg/kg q. second day by subcutaneous injection for five weeks. In the acute studies, animals were treated with 2 mg/kg given q. 24 hr [mult] 48 hr, 24 hr and 12 hr prior to sacrifice. In the acute treatment groups, tacrolimus caused no change in glucose flux. In the chronically treated animals, FK506 levels were within the clinically relevant range. Chronic treatment with 0.5 and 2 mg/kg caused a significant reduction in weight gain. These same groups of animals had a significant increase in intestinal permeability as measured by absorption of 99Te-DTPA. Glucose flux was affected in all chronically treated groups, with net flux increasing in the jejunum and decreasing in the ileum. These findings show that chronic treatment with low-dose tacrolimus is well tolerated, but in higher doses there are significant effects in intestinal permeability and nutrient uptake, and animal weight gain. We suggest that these changes are due to alterations in intestinal permeability that do not appear to be mediated by an acute drug effect and more likely represent chronic changes, possibly from alterations in gene expression. These findings suggest that further studies regarding the effects of tacrolimus on nutrient transport, intestinal permeability, and the known immunologically related functions of tacrolimus should be done.

Regulation of L-valine absorption by opioids interacting with mu-receptors in rabbit ileum

In intact tissue, [D-Ala2,MePhe4,Gly-ol5]enkephalin (10(-5) M; mu-ligand), diminished short-circuit current (Isc) and increased water, Na+ and Cl- net fluxes in vitro under open circuit conditions; it also inhibited L-valine absorption and L-valine-dependent variations of short-circuit current (delta Isc,val). Naloxone (10(-6) M) antagonized these effects. In the absence of the muscularis and myenteric plexus this enkephalin or morphine (mu-ligand) reduced Isc and delta Isc,val. These enkephalin effects occurred at different times. Different concentrations of enkephalin were tested for their effects on delta Isc,val. [D-Ala2,D-Leu5]enkephalin (mainly a delta-ligand) significantly decreased Isc but not delta Isc,val. The reduction of L-valine absorption does not depend on the effects on basal ion transport. Interaction of opioids with mu-receptors located in the submucosal plexus and/or in the epithelial cell accounts for this reduction. This enkephalin effect seems to be at least partially under the control of the myenteric plexus.

Evidence for apical Na+/H+ exchanger in bovine main pancreatic duct

The finding of a high PCO2 in basally secreted pancreatic juice of man and dog raises the hypothesis of proton secretion from ductal epithelial cells presumably through a Na+/H+ exchanger. To test this possibility, H+ luminal secretion and Na+ movements were measured in vitro on samples of bovine pancreatic ducts mounted in Ussing-type chambers. The rate of luminal acidification measured by the pH stat method, using bicarbonate-free media gassed with 100% O2, reached 2.75 muEq/cm2/hr. Proton secretion was blocked in the presence of 1 nM amiloride or in the absence of Na+ (replaced by choline) in the mucosal solution. Study of transepithelial 22Na fluxes in short-circuited tissue, bathed on both sides by control Ringer solution, gassed by 95% O2-5% CO2 demonstrated a net sodium transport from the mucosal to the interstitial side of the duct (net 22Na flux = 3.23 +/- 0.8 muEq/cm2/hr). This net sodium transport was electroneutral and blocked by mucosal amiloride (0.5-1 mM/liter) or by interstitial ouabain (1 mM/liter). These results are consistent with the existence of a Na+/H+ exchanger on the luminal side of the bovine main pancreatic duct.

Calcium-dependent intestinal chloride secretion by Vibrio parahaemolyticus thermostable direct hemolysin in a rabbit model

BACKGROUND & AIMS

Vibrio parahaemolyticus is a major agent of seafood gastroenteritis that induces intestinal secretion in the rabbit through its thermostable direct hemolysin. The aim of this study was to characterize the enterotoxicity of purified hemolysin in vitro.

METHODS

Rabbit ileum was mounted in Ussing chambers, and changes in potential difference and short-circuit current were monitored after addition of hemolysin. Intracellular calcium concentrations in the nontumoral rat crypt-derived cell line IEC-6 were measured using microspectrofluorometry.

RESULTS

In Ussing chamber experiments, mucosal toxin addition up to 50 hemolytic units per milliliter induced a proportional increase of the electrical parameters in normal but not Cl(-)-free Ringer's solution. The response to the toxin was not additive to that of calcium ionophore A23187 and was eliminated by preloading the tissue with 1-2-bis(o-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA), a calcium buffer. In IEC-6 cells, a 10-fold increase in intracellular calcium level was found after addition of hemolysin. Such an increase was totally quenched by BAPTA. Finally, preincubation with trisialoganglioside GT1b, but not monosialoganglioside GM1, eliminated toxin-induced increases in potential difference and short-circuit current.

CONCLUSIONS

These data support the hypothesis that the thermostable direct hemolysin induces intestinal chloride secretion using GT1b as a putative receptor and Ca2+ as a second messenger.

Bicarbonate conductance and pH regulatory capability of cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is an epithelial Cl- channel regulated by protein kinase A. The most common mutation in cystic fibrosis (CF), deletion of Phe-508 (delta F508-CFTR), reduces Cl- secretion, but the fatal consequences of CF have been difficult to rationalize solely in terms of this defect. The aim of this study was to determine the role of CFTR in HCO3- transport across cell membranes. HCO3- permeability was assessed from measurements of intracellular pH [pHi; from spectrofluorimetry of the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5-(and -6)carboxyfluorescein] and of channel activity (patch clamp; cell attached and isolated, inside-out patches) on NIH 3T3 fibroblasts and C127 mammary epithelial cells transfected with wild-type CFTR (WT-CFTR) or delta F508-CFTR, and also on mock-transfected cells. When WT-CFTR-transfected cells were acidified (pulsed with NH4Cl) and incubated in Na(+)-free (N-methyl-D-glucamine substitution) solutions (to block Na(+)-dependent pHi regulatory mechanisms), pHi remained acidic (pH approximately 6.5) until the cells were treated with 20 microM forskolin (increases cellular [cAMP]); pHi then increased toward (but not completely to) control level (pHi 7.2) at a rate of 0.055 pH unit/min. Forskolin had no effect on rate of pHi recovery in delta F508 and mock-transfected cells. This Na(+)-independent, forskolin-dependent pHi recovery was not observed in HCO3-/CO2-free medium. Forskolin-treated WT-CFTR-transfected (but not delta F508-CFTR or mock-transfected) cells in Cl(-)-containing, HCO3(-)-free solutions showed Cl- channels with a linear I/V relationship and a conductance of 10.4 +/- 0.5 pS in symmetrical 150 mM Cl-. When channels were incubated with different [Cl-] and [HCO3-] on the inside and outside, the Cl-/HCO3- permeability ratio (determined from reversal potentials of I/V curves) was 3.8 +/- 1.0 (mean +/- SEM; n = 9); the ratio of conductances was 3.9 +/- 0.5 (at 150 mM Cl- and 127 mM HCO3-. We conclude that in acidified cells the WT-CFTR functions as a base loader by allowing a cAMP-dependent influx of HCO3- through channels that conduct HCO3- about one-quarter as efficiently as it conducts Cl-. Under physiological conditions, the electrochemical gradients for both Cl- and HCO3- are directed outward, so CFTR likely contributes to the epithelial secretion of both ions. HCO3- secretion may be important for controlling pH of the luminal, but probably not the cytoplasmic, fluid in CFTR-containing epithelia. In CF, a decreased secretion of HCO3- may lead to decreased pH of the luminal fluid.

Localization of bicarbonate transport along the crypt-villus axis in rabbit ileum

BACKGROUND/AIMS

HCO3- can be absorbed as well as secreted in the rabbit ileum. With 25 mmol/L HCO3- on the serosal side only, a serosa-to-mucosa flux (Jsm) is found; with 25 mmol/L on the mucosal side only, epinephrine elicits a mucosa-to-serosa flux (Jms). This study aimed to localize these two processes along the crypt-villus axis.

METHODS

Excised ileal segments were exposed luminally to 2 mol/L Na2SO4 (hypertonic treatment) or to isotonic Ringer's solution for 15 minutes. Mucosa was then chamber-mounted, and measurements were made of Jsm or Jms and of short-circuit current (Isc) responses to glucose plus alanine and to either theophylline or epinephrine.

RESULTS

With HCO3-/CO2 added to the serosal side only, hypertonically treated tissues showed a 22% decline in Jsm; a 25% decline in Isc response to theophylline; and a 71% decline in Isc response to glucose plus alanine compared with control. With HCO3-/CO2 added to the mucosal side only, tissues showed 92% and 87% declines in Jms and Isc responses to epinephrine, respectively, and a 87% decline in Isc response to glucose plus alanine. Histological examination showed destruction of villus caused by hypertonic treatment but sparing of crypt cells.

CONCLUSIONS

Both HCO3- and Cl- are secreted mainly by crypt cells and absorbed mainly by villus cells.

Enterotoxic effect of stool supernatant of Cryptosporidium-infected calves on human jejunum

BACKGROUND/AIMS

The clinical pattern of cryptosporidial diarrhea suggests an enterotoxic mechanism. No evidence for this mechanism has been reported thus far. This study aimed to look for enterotoxic effect elaborated by Cryptosporidium.

METHODS

The effects on human intestinal transport of stool supernatant of diarrheal calves infected with Cryptosporidium parvum were examined. Aliquots of centrifuged and filtered stools were added to the mucosal or serosal side of human jejunum obtained from patients undergoing surgery and mounted in Ussing chambers. Electrical parameters were recorded. Stool supernatants of uninfected calves served as a control.

RESULTS

The mucosal addition of 2.5 mg protein of fecal supernatant from diarrheal calves induced a prompt and significant increase in short circuit current with no effects on tissue conductance. The serosal addition of this material and the addition of control supernatant to either side did not induce modifications of electrical parameters. The enterotoxic effect was dose-dependent and saturable. It was reversible by withdrawing the supernatant from the incubation medium. The electrical effect was chloride- and calcium-dependent and was sensitive to heating.

CONCLUSIONS

An enterotoxic activity is present in the stools of Cryptosporidium-infected calves. This activity may be responsible for secretory diarrhea in humans.

Bicarbonate secretion in rabbit ileum: electrogenicity, ion dependence, and effects of cyclic nucleotides

BACKGROUND

Ileal HCO3- secretion is not well understood. The aim of this study was to examine its Na+ and Cl- dependencies, electrogenicity, and responses to amiloride, 4-acetamido-4'-isothiocyano-stilbene-2,2'-disulfonate (SITS), and cyclic nucleotides.

METHODS

The serosa to mucosa HCO3- flux (Jsm) across rabbit ileal mucosa mounted between HCO(3-)-free mucosal solution and HCO(3-)-containing serosal solutions was determined by titration.

RESULTS

In SO4(2-)-containing Ringer's solution, Jsm varied with [Na+] in two phases, one with a high and one with a low affinity for Na+; amiloride inhibited the high- and SITS inhibited the low-affinity phase. Switching from SO4(2-)- to Cl(-)-containing Ringer's solution caused a SITS-inhibitable 42% increase in Jsm. Changes in Jsm were coupled 3:2 with changes in short-circuit current. Cyclic nucleotide effects on Jsm were as follows. In SO4(2-)-containing Ringer's solution at 141 (but not 80) mmol/L Na+, theophylline caused equal increases in Jsm and short-circuit current that equaled the combined effects of 8-Br-5'-cyclic guanosine monophosphate (cGMP) and 8-Br-5'-cyclic adenosine monophosphate (cAMP). Serosal SITS blocked these effects, but amiloride did not. In Cl(-)-containing Ringer's solution, theophylline and bumetanide together (but not separately) increased Jsm.

CONCLUSIONS

(1) Basolateral HCO3- entry occurs via Na+/H exchange and a SITS-inhibitable process (Na(+)-HCO3- cotransport?). (2) Most HCO3- exit across the brush border occurs by a Cl(-)-independent process and some by Cl-/HCO3- exchange. (3) At low cellular [Cl-], HCO3- can be secreted via anion channels activated by cAMP and cGMP. (4) Ileal HCO3- secretion is electrogenic.

Comparison of the effects of loperamide and loperamide oxide on absorptive processes in rat small intestine

Mucosal loperamide inhibited the absorption of glycine by everted sacs of rat small intestine over 10-, 30- or 60-min incubation periods, but loperamide oxide was without effect. In stripped intestinal sheets, loperamide inhibited the rise in short-circuit current associated with Na(+)-linked glucose absorption, but this effect was not observed with loperamide oxide. It is concluded that although there is evidence that loperamide oxide is converted to loperamide in the intestinal lumen, the rate at which free loperamide appears is not sufficient to inhibit absorptive processes.

Enteroaggregative Escherichia coli heat-stable enterotoxin 1 represents another subfamily of E. coli heat-stable toxin

Enteroaggregative Escherichia coli (EAggEC) are associated with persistent diarrhea in young children. Some of these organisms produce a low-molecular-weight, heat-stable, plasmid-encoded enterotoxin that has been named EAggEC heat-stable enterotoxin 1 (EAST1). We have cloned a 4.4-kb DNA fragment from the virulence plasmid of prototype EAggEC strain 17-2, which expresses enterotoxic activity as measured by electrogenic response in Ussing chambers mounted with rabbit ileal tissue. DNA-sequence analysis of this fragment identified an open reading frame (ORF) encoding a cysteine-rich polypeptide of 38 amino acids (M(r), 4100). Insertional and deletional mutations in this ORF resulted in loss of enterotoxic activity. The ORF was cloned into a T7 expression vector, and postinduction culture filtrates exhibited enterotoxic activity and increased ileal tissue cGMP levels. A synthetic peptide consisting of predicted amino acid residues 8-29 also showed enterotoxic activity. These data indicate that this ORF, named astA (EAggEC heat-stable enterotoxin), represents the EAST1 structural gene. EAST1 shows significant homology with the enterotoxic domain of heat-stable enterotoxin a (STa) of enterotoxigenic E. coli and with guanylin, a mammalian analog of STa. Unlike STa, which requires six cysteines and three disulfide linkages for full biological activity, both EAST1 and guanylin contain four cysteine residues. Based on the cGMP data and the sequence homology to STa and guanylin, it is predicted that EAST1 stimulates the particulate form of guanylate cyclase through the same receptor-binding region as STa and guanylin.

Increased intestinal vitamin D receptor in genetic hypercalciuric rats. A cause of intestinal calcium hyperabsorption

In humans, familial or idiopathic hypercalciuria (IH) is a common cause of hypercalciuria and predisposes to calcium oxalate nephrolithiasis. Intestinal calcium hyperabsorption is a constant feature of IH and may be due to either a vitamin D-independent process in the intestine, a primary overproduction of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], or a defect in renal tubular calcium reabsorption. Selective breeding of spontaneously hypercalciuric male and female Sprague-Dawley rats resulted in offspring with hypercalciuria, increased intestinal calcium absorption, and normal serum 1,25(OH)2D3 levels. The role of the vitamin D receptor (VDR) in the regulation of intestinal calcium absorption was explored in 10th generation male genetic IH rats and normocalciuric controls. Urine calcium excretion was greater in IH rats than controls (2.9 +/- 0.3 vs. 0.7 +/- 0.2 mg/24 h, P < 0.001). IH rat intestine contained twice the abundance of VDR compared with normocalciuric controls (536 +/- 73 vs. 243 +/- 42 nmol/mg protein, P < 0.001), with no difference in the affinity of the receptor for its ligand. Comparable migration of IH and normal intestinal VDR on Western blots and of intestinal VDR mRNA by Northern analysis suggests that the VDR in IH rat intestine is not due to large deletion or addition mutations of the wild-type VDR. IH rat intestine contained greater concentrations of vitamin D-dependent calbindin 9-kD protein. The present studies strongly suggest that increased intestinal VDR number and normal levels of circulating 1,25(OH)2D3 result in increased functional VDR-1,25(OH)2D3 complexes, which exert biological actions in enterocytes to increase intestinal calcium transport. Intestinal calcium hyperabsorption in the IH rat may be the first example of a genetic disorder resulting from a pathologic increase in VDR.

Jejunum is preferable for construction of a Bianchi bowel-lengthening procedure in swine short bowel

This study compared the efficacy of a Bianchi bowel-lengthening procedure performed in residual ileum and jejunum of a 75% short bowel model. Eighteen female piglets underwent a 75% mid small bowel resection. After a 6-week period, animal weights were similar and pigs were randomly assigned to one of three treatment groups: (1) a control group receiving no further therapy; (2) a group receiving a Bianchi procedure in the residual jejunal segment; and (3) a group receiving a Bianchi procedure in the residual ileal segment. All were followed for a further 12 weeks. Jejunal Bianchi-treated short bowel animals demonstrated a greater final weight gain (78.8 +/- 4.9 kg) compared with nontreated short bowel (63.0 +/- 6.6 kg) and ileal Bianchi-treated short bowel groups (69.3 +/- 6.9 kg) in addition to a larger jejunal diameter. The increased weight gain in the jejunal Bianchi-treated group was not a consequence of initial bowel length, food intake, changes in bowel length, digestibility of nitrogen or fat, or nutritional status. Furthermore, kinetic constants for D-glucose absorption following 18 weeks of short-bowel syndrome demonstrated a lowered glucose maximal transport rate (Vmax) in animals with nontreated short bowel compared with sham-operated controls. Additionally, jejunal and ileal glucose Vmax was further lowered in the presence of a Bianchi procedure. We conclude that: (1) during short-bowel syndrome, body weight gain was significantly higher in animals when the Bianchi procedure was performed in jejunum; (2) the short-bowel syndrome decreased intestinal glucose absorption; and (3) the Bianchi procedure itself further impaired glucose transport.

Endotoxin-induced alterations in rat colonic water and electrolyte transport

This study examines the effects of endotoxin on intestinal water and electrolyte transport in adult male rats. Endotoxin (1.55 mg/kg, intravenously) reduced in vivo colonic saline absorption 61% in 1 hour. In vitro unidirectional and net 22Na and 36Cl fluxes showed that endotoxin significantly decreased net colonic 22Na absorption compared with control colons (0.3 +/- 1.7 vs. 4.8 +/- 1.1 microEq/h x cm2). Although endotoxin had no significant effect on basal short circuit current (Isc) and conductance, 3H-inulin flux studies suggested an increase in colonic permeability. Isc responses to the 5'-cyclic adenosine monophosphate (cAMP)-dependent secretagogues prostaglandin E2 (1 mumol/L) and vasoactive intestinal peptide (0.1 mumol/L) were diminished by 80% and 50%, respectively. However, cytosolic cAMP-dependent protein kinase activity under basal and stimulated (6 mumol/L 8-bromo-cAMP) conditions was not altered by endotoxin treatment. The Isc responses to 10 mumol/L bethanechol, a Ca(2+)-dependent agonist, were not effected by endotoxin treatment. It was concluded that endotoxin significantly affects colonic transport function and may contribute to the development of diarrhea in inflammatory bowel diseases.

Enterotoxigenicity of Vibrio parahaemolyticus with and without genes encoding thermostable direct hemolysin

Vibrio parahaemolyticus produces a thermostable direct hemolysin (TDH) that has been implicated in the pathogenesis of diarrheal disease caused by this organism. However, previous studies attempting to demonstrate the contribution of the hemolysin to virulence have been inconclusive. We investigated this putative virulence factor by using an isogenic TDH-negative (TDH-) strain constructed by specifically inactivating the two copies of the tdh gene encoding TDH. The enterotoxigenicities of the parent strain (AQ3815) and the mutant strain were tested by adding sterile culture supernatants to rabbit ileal tissue mounted in Ussing chambers. The culture filtrate of the parent strain produced a significant increase in short-circuit current (Isc), compared with the change induced by the TDH- mutant. The capacity of the culture filtrate of AQ3815 to increase the Isc was reduced by neutralization with anti-TDH serum, and the return of the cloned tdh gene to the TDH- mutant restored the ability to increase the Isc. These results were corroborated by rabbit ileal loop assays in which AQ3815 caused fluid accumulation but the TDH- mutant did not. No microscopic damage was seen in mucosal tissues exposed to the culture filtrate of either strain. These results indicate that TDH has an enterotoxigenic effect on rabbit small intestine and could be responsible for the watery diarrhea seen with V. parahaemolyticus.

Pathophysiology of small intestinal malabsorption in gerbils infected with Giardia lamblia

Mongolian gerbils were infected with a human pathogenic Giardia lamblia strain and compared with sham-treated control animals 6 days after inoculation. Infection resulted in crypt hyperplasia associated with an increased enterocyte migration rate. Villus height was decreased in the duodenum, unchanged in the jejunum, and increased in the ileum of infected animals. Epithelial microvilli were markedly shortened, and brush border surface area decreased in the jejunum and ileum of infected animals. Thymidine kinase activity was increased in isolated duodenal villus enterocytes but did not differ in the jejunum and ileum. In vitro and in vivo experiments showed that the infection resulted in decreased jejunal glucose-stimulated electrolyte, water, and 3-O-methyl-D-glucose absorption, whereas in the ileum in vitro electrolyte and 3-O-methyl-D-glucose absorption was similar in infected and control animals. Thus, in the jejunum infection causes electrolyte, solute, and fluid malabsorption associated with decreased brush border surface area. The results indicate that the diarrhea associated with giardiasis is caused by malabsorption rather than active secretion.

Evidence for an electrically silent, neurogenic fluid secretion in the rat jejunum in vivo

The aim of this study was to evaluate the mechanism behind neurogenic fluid secretion in the rat jejunum. In vitro, short-circuit current (SCC) and potential difference (PD) were measured with the conventional Ussing technique. In vivo, electric parameters and net fluid transport (NFT) were simultaneously recorded with two different techniques. In separate in vivo experiments alkaline secretion (As) was estimated. In vitro, the chloride channel blocking substance 5-nitro-2(-3-phenylpropyl-amino)benzoic acid (NPPB) and the loop diuretic substance furosemide (F) inhibited SCC, whereas the carbonic anhydrase inhibitor acetazolamide (Ace) lacked effect. Noradrenaline inhibited SCC and this effect was antagonized by NPPB and F. In vivo, cholera toxin induced a parallel increase in PD/SCC and fluid secretion. Conversely, mesenteric nerve stimulation (MNS) or administration of the nicotinic antagonist hexamethonium (Hx), concomitantly inhibited PD/SCC and fluid secretion. However, there was a poor correlation between the magnitudes of these effects. F inhibited SCC in vivo and also the SCC-effect of MNS. However, F had no effect on fluid secretion in vivo, nor on the NFT-effect of MNS. Jejunal As was stimulated by cholera toxin and MNS significantly inhibited As. The present results challenge the current view on the role of electrogenic chloride secretion in intestinal secretion. Alternative mechanisms are tentatively discussed.

Mucosal glutamine utilization after small-bowel transplantation: an electrophysiologic study

A short course of FK 506 after small bowel transplantation averts rejection in the rat and achieves indefinite survival of the recipient whose nutritional status is dependent on the function of the intestinal graft. Ex vivo electrophysiologic studies using the Ussing Cell were conducted to delineate functional competence of the graft by evaluating mucosal ion transport and glutamine utilization. Orthotopic small-bowel transplantation was performed in Lewis (LEW) rats as recipients of either Brown-Norway (BN) allografts or LEW syngeneic grafts. Allograft recipients received FK 506 either as a short course (2 mg/kg on Day 0-4 after transplantation) or continuously (2 mg/kg Day 0-4, then 0.5 mg/kg weekly). Ileal mucosa was harvested from small bowel grafts 9 and 60 days after transplantation and mounted in the Ussing Cell containing Hanks' balanced salt solution with/without L-glutamine (20 mM). Transmembrane potential difference (PD), which represents mucosal active ion transport, and mucosal resistance, an index of membrane integrity, were recorded. Nine days after transplantation, mucosal PD was the same in the ileum from syngeneic grafts, allografts treated with FK 506 and normal LEW and BN rats, and the addition of glutamine increased PD equally in all groups. In comparison, PD was markedly decreased in allografts undergoing rejection, and the glutamine response was blunted. Sixty days after transplantation, mucosal PD was reduced in allografts treated with a short course of FK 506, but normal in allografts receiving continuous immunosuppression with FK 506 and in syngeneic grafts. A decrease of mucosal resistance was not a feature of rejection nor a sequel of limited FK 506 therapy. Our data indicate that allograft rejection results in a significant decrease in mucosal PD and a poor response to glutamine. Control of rejection by FK 506 preserves normal electrophysiologic responses of the allograft mucosa.