Contribution of solvent drag through intercellular junctions to absorption of nutrients by the small intestine of the rat

Mechanisms and sites of mannitol permeability of small and large intestine in the rat

Mannitol is commonly used as an intestinal permeability probe, yet the mechanisms of its penetration of the intestinal barrier are not entirely clear. Therefore, we studied mannitol's permeability of different segments of the intestine and studied the kinetics and influence of intraluminal factors on mannitol permeability in vivo in perfused intestinal segments of rats. There was linear relationship between permeability rate of mannitol and its luminal concentration (y = 7.2x + 1.7; r = 0.98), indicating that passive diffusion is involved in mannitol's permeability. Increased luminal fluid osmolarity from 0.3 to 0.6 osmol/liter resulted in decreased net water flux with a corresponding decrease in mannitol permeability in both jejunum and colon (P < 0.01), indicating the prominent influence of solvent drag on net mannitol permeability. The relationship between mannitol permeability and water absorption at different osmolarities was linear in the jejunum and colon. At luminal osmolarity of 0.3 osmol/liter, 34.6% of mannitol permeability was mediated by passive diffusion and 65.4% was mediated by solvent drag in the jejunum. Mannitol permeability was much more dependent on solvent drag in the colon (88.9%) than in the small intestine (65.4%). The net permeability rate of mannitol was similar in the jejunum and ileum but was much higher in the colon (P < 0.01). Addition of chenodeoxycholate (5 mM) to the perfusate resulted in a significant decrease in absorption of water (P < 0.01) with a corresponding decrease in mannitol permeability (P < 0.01). These studies indicate that mannitol permeability of the intestinal barrier is mediated by passive diffusion and solvent drag, with the latter accounting for a greater fraction of the total permeability.

Intestinal absorption and excretion of octapeptides composed of D amino acids

Octapeptides synthesized from D amino acids were absorbed from the intestine and excreted in urine of normal rats drinking 5% glucose/1% creatinine containing the 125I-labeled peptides at 0.1-25 mg/dl. The rats ingested fluid at the rate of about 20 ml/hr and produced urine at 15 ml/hr for several hours during the nocturnal feeding period. Sixty-one +/- 4% of the ingested creatinine and 50 +/- 3% of a lipid-insoluble D octapeptide (EASASYSA, 784 Da) were excreted intact in the urine. The steady-state molar rate of absorption-excretion of creatinine equaled or exceeded the maximum rate of carrier-mediated intestinal transport of glucose, suggesting that both the creatinine and the D octapeptide were transported paracellularly by solvent drag through absorptive cell junctions that were dilated by the glucose. More than 70% of the ingested glucose was also absorbed paracellularly. The results demonstrate that intact oligopeptides can be absorbed efficiently from the intestine when they are not hydrolyzed by membrane-bound peptidases of the brush border. The results also provide support for recent theories proposing that coupling of membrane digestion with paracellular solvent drag accounts for a major fraction of normal intestinal absorption of nutrients.

Intestinal absorption of (-)-carbovir in the rat

(-)-Carbovir (CBV) is a carbocyclic nucleoside analogue with in vitro activity against the human immunodeficiency virus. The sites and mechanism of absorption of (-)-CBV from the rat small intestine were studied in the anesthetized male Sprague-Dawley rat. (-)-CBV was perfused through either duodenal, jejunal, or ileal segments at three concentration levels ranging from 1 to 500 micrograms/mL. The fraction remaining to be absorbed at steady-state and the absorptive clearance were calculated for each experiment. The effect of solvent drag on the absorptive clearance was also investigated. Two-way ANOVA for the absorptive clearance per unit length was not significant for either (-)-CBV concentration or site of perfusion. The fraction remaining to be absorbed at steady-state was found to be 0.804 +/- 0.091 (n = 30). A strong correlation was found between the absorptive clearance and the net water absorptive flux. The mechanism of (-)-CBV absorption across the rat small intestine apparently consists of both passive diffusion and convection.

The intestinal uptake of "enzymatically-stable" peptide drugs in rats as influenced by D-glucose in situ

In previous in situ and in vivo rat perfusion studies, the intestinal absorption of several low molecular weight drugs was increased by the presence of luminal D-glucose. The intent of this study was to determine the potential of this fed-state effect to improve the intestinal uptake of poorly permeable, small peptide and peptide-like drugs. Jejunal wall permeabilities (Pw*) of di-(D-kyotorphin), tri-(cephradine), hexa-(growth hormone releasing peptide, GHRP-6) and octa-(octreotide, a somatostatin analogue) peptides and corresponding net water fluxes were determined in rats using an in situ single-pass perfusion technique. Glucose was shown to enhance the uptake of the smaller (di- and tri-) peptides but not the larger peptides despite the fact that glucose elicited a significant net water absorption with each of the four peptide drugs. It is concluded that glucose enhances jejunal permeabilities of smaller peptides by solvent drag and the enhancement is limited in situ by peptide molecular size. The studies with nonmetabolizable 3-O-methylglucose suggest that the augmentation of the proton gradient across the transmucosal membrane by glucose contributes to the carrier-mediated transport observed with the smaller peptides.

A mechanism for isotonic fluid flow through the tight junctions of Necturus gallbladder epithelium

During isotonic fluid flow, Necturus gallbladder epithelium mediates net fluxes of paracellular probes by a convective process. We show here that the paracellular system is modeled by permeation through three populations of channels: (i) convective parallel-sided ones of width 7.7 nm (ii) small diffusive ones of radius approximately 0.6 nm, and (ii) large diffusive ones of radius exceeding 50 nm. The reflexion coefficient of the convective channels is very low and the calculated osmotic flow rate is close to zero when compared with the observed fluid absorptive rate of 2 x 10(-6) cm/sec. Analysis reveals that the convective channels behave as though closed to back-diffusion of probes; if this is due to solvent drag then very high fluid velocities are required, acting through minute areas. There are no transjunctional gradients that could drive the flow, and so the fluid must be propelled through the channel by components of the junction. We propose a mechanism based upon an active junctional peristalsis which allows discrimination on the basis of molecular size, in which the channels are always occluded at some point and so back-diffusion cannot occur. There is no local gradient of salt distal to the junctions and therefore the osmotic permeability of the membranes is irrelevant. High fluid velocities are not required, and the flow can occur over a substantial fraction of the junction. The mechanism must involve motile and contractile elements associated with the junction for which there is already considerable evidence.

Effect of D-glucose on intestinal permeability and its passive absorption in human small intestine in vivo

BACKGROUND

Based on studies in animals, it has been proposed that carrier-mediated D-glucose absorption markedly enhances passive permeability of the jejunal mucosa, allowing the majority of D-glucose absorption to proceed passively. In this study, we evaluated this hypothesis in the human jejunum in vivo.

METHODS

Using the constant perfusion, nonabsorbable marker technique, permeability of jejunal mucosa was assessed by measuring the ratio of diffusion rates of urea/L-xylose and mannitol/L-xylose. Passive D-glucose absorption was quantitated using L-glucose and mannitol as probes for D-glucose.

RESULTS

Addition of D-glucose to perfused solutions did not change the diffusion ratios, indicating that D-glucose has no effect on the size of channels for passive diffusion across the jejunal mucosa. The fraction of total D-glucose absorption that could be attributed to a passive mechanism averaged 5%. In the human ileum in vivo, we detected no evidence of passive D-glucose absorption.

CONCLUSIONS

Carrier-mediated D-glucose absorption does not increase passive permeability of human jejunal mucosa to solutes with molecular radii between 2.6 and 4.0 A. The amount of D-glucose absorbed passively from the human jejunum is trivial compared with the overwhelmingly dominant mechanism, carrier-mediated transport. Our results do not support the concept that sodium-dependent nutrient transport increases tight junction permeability.

Intestinal epithelial function: the case for immunophysiological regulation. Implications for disease (2)

Substantial amounts of data have been reported showing a role for immunomodulation of epithelial function (particularly ion secretion and permeability) using animal models of anaphylactic reactions. In part one of this review we outlined the main immune cell types and mediators/cytokines that are currently known to influence epithelial physiology either directly, or indirectly via an intermediate cell type. Here we will expand on the significance of these studies and show how antigenic activation of the mucosal immune system can evoke changes in epithelial function that may be beneficial to the host by mediating loss/inactivation of the antigen. However, a continued and inappropriate immune stimulation can lead to pathophysiological reactions and disease. Thus, we will present data on immune regulation of epithelial function with direct applicability to understanding the mechanism underlying human intestinal inflammatory and secretory disease. Finally, we highlight key strategic points in the cascade of immune events that can control epithelial function and thus may be of relevance in the formulation of new therapeutic approaches to intestinal inflammation.

Transcellular fluid secretion induced by cholera toxin and vasoactive intestinal polypeptide in the small intestine of the rat

The permeation of intravenously administered 51Cr-EDTA and [14C]mannitol to the perfused intestinal lumen was measured in anaesthetized rats together with the net intestinal fluid. Net fluid secretion was induced by cholera toxin or by intravenous infusion of vasoactive intestinal polypeptide (VIP). The plasma clearance of Cr-EDTA and mannitol was 0.9 +/- 0.1 and 1.4 +/- 0.2 microliters min-1 g-1 intestine during the control period prior to the secretion and the net fluid absorption was about 7 +/- 5 microliters min-1 g-1. Cholera toxin induced a net fluid secretion of about 30 +/- 7 microliters min-1 g-1 but the clearance did not rise but decreased significantly. The findings for VIP-induced secretion were similar. No indication of solvent drag was seen. Thus it is concluded that the fluid was secreted in channels which were smaller than the probes and we propose that the secreted fluid entered the intestinal lumen through the epithelial cells and not by the paracellular route. The decreased permeation of Cr-EDTA and mannitol from plasma to lumen during volume secretion suggest that there was a decreased mucosal permeability during the secretion. The decrease in permeability was consistent with a decrease in pore size. One explanation of the data is that the pore radius contracted from about 35 to 15 A during cholera if we assume a homogenous pore population. However, the data indicated that there was not a uniform size of the pore.(ABSTRACT TRUNCATED AT 250 WORDS)

Nutrient-induced changes in the permeability of the rat jejunal mucosa

We examined the site of action of nutrients that enhance mucosal permeability by use of D-glucose as an archetype of nutrients of this class. We tested the hypothesis that D-glucose enhances mucosal permeability by either acting outside the intestinal lumen after absorption or acting inside the intestinal lumen to cause mediator release from either endocrine or nerve tissue. The rate of absorption of L-[14C]glucose, a passively absorbed molecule, from the lumen was used as an index of the permeability of the mucosa of a perfused segment. L-Glucose was absorbed more rapidly in the presence of D-glucose than in the presence of an equimolar concentration of mannitol. However, the permeability of the jejunal mucosa was unaffected by elevated blood glucose levels during intravenous infusion of D-glucose. The mucosal permeability was also unaffected by exposure of an adjacent segment to D-glucose, a result suggesting that D-glucose does not alter mucosal permeability by inducing the release of a blood-borne mediator from enteroendocrine cells. Finally, the effect of D-glucose on mucosal permeability could not be blocked by hexamethonium or tetrodotoxin, a result suggesting that the intestinal nerves do not mediate this phenomenon. Lidocaine significantly increased the rate of L-glucose absorption when D-glucose was present in the lumen but had no effect on L-glucose absorption under basal conditions. Our findings indicate that D-glucose must be in contact with the apical membranes of enterocytes to alter mucosal permeability. This suggestion is consistent with the hypothesis that the modulation of mucosal permeability results from the activation of sodium-dependent cotransport systems.

Regional small-intestinal permeability in vitro to different-sized dextrans and proteins in the rat

Molecular weight-dependent passage over different regions of the rat small intestine, using different-sized proteins/peptides and fluorescein isothiocyanate-dextrans in the 1- to 70-kDa range, was studied in vitro in modified Ussing chambers. The mucosal to serosal passage was inversely related to the molecular weight. After 120 min the passage in the proximal region usually dominated, but the nonapeptide (mercaptopropionic acid1, D-arginine8)-vasopressin differed by showing a consistently higher passage in the distal region. The similar apparent permeation coefficients obtained for the two macromolecular categories of corresponding molecular weight implied that the non-degradable dextrans could be used as permeability markers reflecting the passage per se of intact proteins. Furthermore, the results indicated two different transmucosal pathways, one of low permeability for molecules > 30 kDa, in which the molecular weight was of minor importance for the passage, and another more permeable one in the 1- to 30-kDa range, in which the passage was highly influenced by the molecular weight.

The effect of L-leucine on the absorption of levodopa, studied by regional jejunal perfusion in man

1. A new method for perfusing a 10 cm segment of jejunum in humans has been used in seven subjects to study the effect of the amino acid L-leucine (40 mM) on the intestinal absorption of levodopa (2.5 mM). The tube contains six channels and has two inflatable balloons, which enable a perfusion of a closed and defined segment of the proximal small intestine. 2. L-leucine decreased the intestinal absorption of levodopa from 40 +/- 19 to 21 +/- 15% but was without effect on the absorption of antipyrine, benserazide and D-glucose. 3. We confirm that levodopa is absorbed by the active transport system normally responsible for the absorption of large neutral amino acids (LNAA) in humans. Oral absorption by passive diffusion, probably by the paracellular route, might also occur for levodopa in the proximal part of the small intestine.

1H NMR spectroscopy of colon tumors and normal mucosal biopsies; elevated taurine levels and reduced polyethyleneglycol absorption in tumors may have diagnostic significance

Twenty-three pairs of normal mucosa and colonic adenocarcinoma biopsy specimens have been examined in this pilot study by 1H NMR spectroscopy at 9.4 T to determine whether it was possible to find spectral malignancy markers. The 3.2 ppm (trimethylamine-containing compounds)/0.9 ppm (methyl of fatty acids) resonance intensity ratio in water suppressed spectra, proposed by other authors as a malignancy marker, results in our hands, using resonance areas, in partial overlap between tumor and mucosa values, which reduces its diagnostic value. Furthermore, we have found that submucosa contamination could mask the normal mucosa pattern and artifactually decrease the 3.2/0.9 ppm, ratio value by increasing the 0.9 ppm resonance due to the known triglyceride content of normal submucosa. On the other hand, we have observed in the Hahn spin-echo spectra of intact biopsies resonances arising from taurine and exogenous polyethyleneglycol (PEG). Their assignment and quantification has been carried out in perchloric acid extracts of the tissue biopsies. The taurine (3.4 ppm)/creatine (3.0 ppm) area ratio produced an excellent discrimination between normal mucosa and tumour groups while the PEG (3.7 ppm)/creatine (3.0 ppm) area ratio presented a large overlap, although it was clearly higher in the mucosae than in the tumors for paired samples. These two NMR observable parameters are in our hands highly discriminating and are accordingly proposed as malignancy markers in tissue biopsies although their possible utility for in vivo studies remains to be demonstrated.

Uptake and transport of macromolecules by the intestine: possible role in clinical disorders (an update)

The intestine is exposed to a wide variety of macromolecules. Because macromolecules are antigenic, mechanisms have evolved in the gastrointestinal tract to regulate their absorption. Macromolecular uptake can be beneficial in delivering essential factors for growth and in sampling the antigenic milieu of the gastrointestinal tract. Specific transport mechanisms exist to execute this physiological absorption. However, inappropriate and uncontrolled antigen transport may occur in disease states or as a prelude to disease states in the gastrointestinal tract. Such transport may result in immune responses that are harmful. This review examines physiological transport of macromolecules through epithelia and through M cells. It also considers uncontrolled transport and its relation to disease states. The review concludes with an examination of the interrelationship between antigen transport and an altered immune system in the establishment of gastrointestinal disease.

Absorption of glucose polymers from rice in oral rehydration solutions by rat small intestine

BACKGROUND

This study aims to determine the effect of replacing glucose in oral rehydration solution (ORS) with starch hydrolysates from rice on absorption in the small intestine and levels of glucose in portal venous blood and on disaccharidase levels and morphometric measurements in intestines of rats.

METHODS

ORS containing standard composition of salts and 2% glucose (WHO ORS) or 2%, 5%, or 10% starch hydrolysates were infused into duodena of 60 Sprague-Dawley rats (250-350 g). Portal venous blood glucose levels were determined at 0, 30, 60, 90, and 120 minutes.

RESULTS

Significantly larger areas under the curve of glucose absorption (AUCs) were produced by ORS containing 10% unfractionated starch hydrolysates (123.2 +/- 3.8), 2%, 5%, and 10% starch hydrolysates with long-chain ( > 9 molecules) glucose polymers (109.5 +/- 10.6, 109.3 +/- 7.4, and 115.3 +/- 7.1, respectively), and 5% and 10% starch hydrolysates with short-chain (2-9 molecules) glucose polymers (124.4 +/- 6.1 and 128.1 +/- 6.8). ORS with 2% and 5% unfractionated starch hydrolysates and 2% short-chain glucose polymers produced AUCs comparable with those of WHO ORS (96.48 +/- 5.7). Disaccharidase levels and morphometric measurements were not significantly different.

CONCLUSIONS

Starch hydrolysates from rice containing glucose polymers can be used in ORS in higher concentrations than glucose to provide higher caloric density without increased osmolality.

Glucose-induced ion secretion in rat jejunum: a mucosal reflex that requires integration by the myenteric plexus

We tested the hypothesis that the mucosa of rat jejunum is stimulated by intraluminal D-glucose, resulting in nerve-mediated ion secretion. We examined the D-glucose-induced secretory response in two ways. First, we measured the unidirectional fluxes of sodium and chloride ions, in vivo, during perfusion of segments of jejunum with solution that contained either D-glucose or mannitol. Second, we measured the net rate of absorption of D-glucose from sodium-free solution; this parameter is related to the rate of sodium ion secretion. We used the above two approaches in conjunction with techniques for destroying specific subsets of the intestinal nerves. Thus, we evaluated the subset of intestinal nerves that integrates the secretory response of the mucosa to D-glucose. Jejunal segments perfused with D-glucose solution exhibited significantly greater rates of sodium and chloride ion secretion than did segments perfused with mannitol. Intestinal segments in which the myenteric nerve plexus had been destroyed exhibited a significantly lower rate of sodium ion secretion in the presence of D-glucose than was seen in fully innervated segments. A role for the myenteric nerves in D-glucose-induced ion secretion was also indicated by experiments that involved absorption of D-glucose from sodium-free solution. It was concluded that exposure of the mucosa of rat jejunum to glucose increases the rate of secretion of both sodium and chloride ions. The myenteric nerve plexus is apparently involved in the integration of this mucosal reflex.

The movement of solutes and cells across tight junctions

The TJ is a highly dynamic rate-limiting barrier for passive transepithelial solute flow. It is not only physiologically regulated but is modulated in various disease states as well. Such modulations occur as a result of epithelial cell interactions with immune cells or immune cell products and thus epithelial barrier function appears to be regulated in disease states.

Regional jejunal perfusion, a new in vivo approach to study oral drug absorption in man

Recently a new in vivo approach in man, using a regional intestinal perfusion technique, has been developed. The perfusion tube consists of a multichannel tube with two inflatable balloons, which are placed 10 cm apart. The tube is introduced orally and the time required for insertion and positioning of the tube is approximately 1 hr. In the present study eight healthy subjects were perfused in the proximal jejunum on three separate occasions. The first two perfusion experiments used the same flow rate, 3 ml/min, and the third experiment used 6 ml/min. Phenazone (antipyrine) was chosen as the model drug. The recovery of PEG 4000 in the outlet intestinal perfusate was complete in experiments 1 and 2, but slightly lower (90%) when the higher flow rate was used. The mean (+/- SD) fraction of phenazone absorbed calculated from perfusion data was 51 +/- 12% (3 ml/min), 64 +/- 19% (3 ml/min), and 42 +/- 27% (6 ml/min) for the three experiments, respectively. The mean fraction absorbed estimated by deconvolution of the plasma data was 47 +/- 16%, 51 +/- 19%, and 38 +/- 26%, respectively. The effective permeability of phenazone was 5.3 +/- 2.5, 11 +/- 6.8, and 11 +/- 12 (x 10(4) cm/sec, respectively. We have shown that it was possible to establish a tight intestinal segment which behaved as a well-mixed compartment. The low perfusion rate of 3 ml/min was preferred, since it resulted in the lowest variability in absorption.(ABSTRACT TRUNCATED AT 250 WORDS)

Boosted mucosal immune responsiveness in the rat intestine by actively transported hexose

Anaphylaxis-mediated intestinal fluid secretion was measured in Trichinella spiralis- or ovalbumin-immunized rats challenged intraduodenally with T. spiralis somatic antigen (1 mg protein/0.5 mL saline) or ovalbumin (1 mg/0.5 mL saline), respectively. Intestinal fluid volume was measured 30 minutes after challenge as an index of net secretion. Challenge with the antigenic bolus containing 40 mmol/L D-glucose induced twice the fluid secretion as that induced by either antigen alone. L-Glucose was an ineffective substitute for D-glucose. The enhancement of secretion by D-glucose was dependent on prior immunologic sensitization, was diminished in the presence of phlorizin, and was mimicked by beta-methyl glucoside. Results indicate that the active transport of D-glucose augments the antigen-mediated fluid secretion, possibly by enhancing permeation of the intestinal epithelium to antigen, thereby providing greater access to the mucosal immune system.

Passage of aerosolized BSA and the nona-peptide dDAVP via the respiratory tract in young and adult rats

The passage of the protein marker, bovine serum albumin (BSA, MW = 67,000), and the nona-peptide, 1-deaminocysteine-8-D-arginine vasopressin (dDAVP, MW = 1067), from the respiratory tract into the blood when applied as an aerosol with a MMAD of 1.7 microns was studied in 14-, 30-, and 100-120-day-old (adult) healthy rats and in adult rats with lung injury. In blood serum of adult rats the levels of immunoreactive BSA reached its maximum 16-24 h after a 1-h aerosol exposure with a calculated total passage of 6.4 +/- 1.8% of the given dose. dDAVP serum levels measured by RIA peaked after 0.5-1 h, giving a total passage of 84.3 +/- 12.9%. With increasing exposure periods from 0.5 to 3 h, which thereby increased the lung burden, the serum levels of BSA and dDAVP increased linearly indicating passive transepithelial transport processes for both molecules. For the young rats, similar serum level-time curves were obtained like those of the adult, with similar total passages of BSA, 4.6 +/- 0.8% for the 14-day-old rats and 5.2 +/- 1.6% for the 30-day-old rats. For dDAVP the total passage was significantly lower in both the 14-day-old rats, 40.9 +/- 12.1%, and the 30-day-old rats, 16.7 +/- 6.1% (p less than .05), as compared to the adult rats. Acute lung inflammation induced in rats by intratracheal instillation of 5 mg ferritin/kg body wt prior to a 1-h marker aerosol exposure increased the passage of BSA (58.7 +/- 18.8%, p less than .05), while the dDAVP passage was less affected (99.2 +/- 25.2%, p greater than .05) as compared to the healthy adult rats. The results indicate that after aerosol exposure the total passage of dDAVP over the respiratory tract was higher than that of the macromolecule BSA, the passage appeared to increase with the maturity of the rats and by inflammatory changes in the lung tissue.

Transmucosal impedance of small intestine: correlation with transport of sugars and amino acids

Transmucosal impedances of isolated perfused segments of jejunum from mice and hamsters were measured at frequencies from 10-100,000 Hz in the presence and absence of sugars and amino acids. Na-coupled transport of organic substrates caused large decreases of transmucosal impedance, reflecting contraction of cytoskeletal proteins controlling permeability of tight junctions, functional surface of basolateral membranes, and width of extracellular pathways. The observed changes of impedance were closely correlated with molar rates of Na-coupled active transport rather than with molecular species. Thus amino acids and sugars having the same molar rates of active transport also have the same effects on transmucosal impedance. It is proposed that a nonspecific increase of intracellular osmotic pressure during active transport is the first step initiating cytoskeletal contraction. Cell volume regulatory responses, including increased basolateral K+ conductance and Ca2+ influx, may be subsequent steps leading to contraction of perijunctional actomyosin, formation of junctional dilatations, and exposure of lateral membranes. Enhancement of oxygen capacity of perfusion fluids (e.g., with fluorocarbon emulsion) is required to maintain viability of isolated intestinal epithelium; in plain oxygenated Ringer-HCO3 solution, the transmucosal impedance is abnormally low and cytoskeletal contractile responses to Na-coupled transport are attenuated. An electrical circuit analog is presented that simulates almost exactly the observed transmucosal impedances and provides quantitative evaluation of the effects of Na-coupled transport of sugars and amino acids on resistances of tight junctions, capacitance of basolateral membranes, and postjunctional resistances of lateral intercellular spaces and villus cores.

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.

Mastoparan activates apical chloride and potassium conductances, decreases cell volume, and increases permeability of cultured epithelial cell monolayers

Mastoparan is a tetradecapeptide. Mastoparan added to the apical surface of monolayers of Madin-Darby canine kidney (MDCK) epithelial cells, cultured on micropore filters, activated ion transport and increased the permeability of the paracellular pathway across the monolayers. In monolayers of similar MDCK cells in which the basolateral membrane was permeabilized with Staphylococcus aureus alpha toxin (Staph. alpha toxin), the effects of mastoparan on apical membrane ion conductances were dependent on the presence of guanosine triphosphate (GTP). Mastoparan and GTP increased apical membrane chloride conductance more than potassium conductance, with very little change in sodium conductance. In intact monolayers, addition of barium to the apical bath prevented mastoparan activation of ion transport and the increase in paracellular permeability. Increasing bath potassium to 130 mM also reduced ion transport and prevented the increase in paracellular permeability. We hypothesized that these observations could be linked by mastoparan activation of apical chloride and potassium conductances, with consequent decreases in cell volume and resultant increases in paracellular permeability. Addition of 270 mM mannitol to isosmotic media to decrease cell volume decreased MDCK monolayer transepithelial resistance. Addition of mastoparan to monolayers of MDCK cells grown on micropore filters decreased cell volume to the same extent as addition of 270 mM mannitol to isosmotic media. Addition of the potassium channel inhibitor, barium, prevented the decrease in cell volume in response to mastoparan. Mastoparan activates apical membrane chloride and potassium conductances in MDCK cells. The loss of these ions from the cells decreases cell volume, and the decrease in cell volume increases the permeability of the paracellular pathway.

Water handling in the rat jejunum: effects of acidification of the medium

The minute-by-minute net water movement (Jw) in the rat jejunum was studied in relation to the diffusive water (Pw) and mannitol (Ps) permeabilities with the following results. (a) Jw was a linear function of the applied hydrostatic and osmotic transepithelial gradients (hydrostatic permeability coefficient, Phydr = 0.052 +/- 0.011 cm s-1; osmotic permeability coefficient, Posm = 0.0069 +/- 0.0014 cm s-1. (b) A fraction of this absorptive Jw (transport-associated Jw, Jwt = 0.086 +/- 0.024 microliter min-1 cm-2) was independent of the presence of any osmotic, hydrostatic or chemical gradient. (c) In the absence of Na+, Jwt was not significantly different from zero and there was an increase in Phydr but no change in Posm. (d) In the presence of a hydrostatic gradient (10 cm H2O, mucosal side), acidification of the medium (95% CO2 bubbling, pH 6.2) simultaneously and reversibly increased Jw and decreased Pw. (e) When an osmotic gradient was present (40 mM polyethyleneglycol on the serosal side) a net increase in Jw was observed. CO2 bubbling in these conditions reversibly reduced Jw while increasing Ps. (f) These effects were not observed when the serosal or mucosal pH was reduced in the presence of a nonpermeant buffer (HEPES/TRIS; MES/TRIS). If we accept that Ps is a good marker of paracellular movements and that Pw mainly reflects transcellular water movements, we may conclude that acidification of the medium, in the presence of bicarbonate, modifies both paracellular and transcellular routes. The experimental evidence indicates that an increase in proton concentration opens the paracellular pathway and probably has a blocking effect on a transcellular route.

Variation in amino acid transport along the rabbit small intestine. Mutual jejunal carriers of leucine and lysine

The jejuno-ileal variation of amino and imino acid transport across the brush-border membrane of intact rabbit small intestine was studied. For the amino acids tested--beta-alanine, leucine, lysine, MeAIB, proline--and for D-glucose, the rates of transport at constant concentrations increase from very low values in the proximal jejunum to maximum values in the most distal 30 cm of the ileum. The apparent affinity constant for jejunal taurine transport is identical to that of the distal ileum, while the jejunal transport capacity is less than half. In the jejunum, as in the distal ileum, leucine and lysine share both sodium-dependent and sodium-independent carriers. Approx. 50% of the quantitative difference in transport capacity is accounted for by the absence of the beta-alanine carrier in the jejunum. These data indicate that the gradients of transport along the small intestine reflect gradients of transport capacities rather than affinities. In comparison with hamster, man and rat, the rabbit seems unique with respect to the location of transport maximum and the steepness of the gradient along the intestine.

Water and solute absorption from hypotonic glucose-electrolyte solutions in human jejunum

While oral rehydration therapy with glucose-electrolyte solutions is highly effective, the optimal formulation has not yet been defined. Recent clinical studies suggest that stool volume, and thus water losses, may be reduced if glucose is replaced by a polymeric substrate which reduces osmolality. It is possible that the efficacy of glucose monomer based oral rehydration solutions (ORS) will also improve if osmolality is decreased. Using jejunal triple lumen perfusion in healthy adult volunteers net water and solute absorption were studied from three hypotonic solutions with different sodium concentrations (46, 60, 75 mmol/l) but identical glucose concentrations (90 mmol/l), thus allowing osmolality to rise (210, 240, and 270 mOsm/kg, respectively). Results from these solutions (ORS 45:210, ORS 60:240, and ORS 75:270) were compared with the World Health Organisation oral rehydration solution (WHO-ORS). Greatest water absorption was seen with ORS 60:240 (p less than 0.01). Sodium absorption from ORS 60:240 and WHO-ORS was similar and greater than sodium absorption from ORS 45:210 (p less than 0.05). Potassium and glucose absorption were greater from ORS 60:240 than from any of the other hypotonic solutions (p less than 0.05) and were equal to absorption from WHO-ORS). These results in a short segment of healthy human jejunum suggest that hypotonic ORS containing monomeric glucose may increase water absorption.

Proline uptake by monolayers of human intestinal absorptive (Caco-2) cells in vitro

Monolayers of the Caco-2 human intestinal cell line exhibit active and passive uptake systems for the imino acid L-proline. The active transport component is saturable and it is responsible for about two thirds of the observed flux over the nanomolar concentration range, at 37 degrees C and pH 7.4. In contrast to L-phenylalanine, specific L-proline uptake has a high degree of sodium dependency and the efficiency of the carrier system is significantly reduced when protein synthesis (cycloheximide), Na+/K(+)-ATPase (ouabain) or cellular metabolism (sodium azide) are inhibited. The expression of the L-proline carrier by Caco-2 cells was under some degree of nutritional control. Glucose deficiency, over the time scale of the experiment, had no effect. The temperature-dependence of the specific uptake process followed the Arrhenius model with an apparent activation energy of 93.5 kJ nmol-1. This pathway also displayed Michaelis-Menten concentration-dependence with a Ksdm of 5.28 mM and a maximal transport flux (Jsdmax) of 835 pmol min-1 (10(6) cells)-1. Although the passive component was unchanged, the pH of the donor phase exerted a profound effect on the active carrier component. Within the physiological pH range a local maximum efficiency was found at pH 7.4 but dramatic increases were noted as pH 5.0 was approached. In competition studies, with 100-fold excess of a second amino acid, strong inhibition of uptake was found with alpha-aminoisobutyric acid, L-alanine and L-serine whereas moderate inhibition was observed with glycine, D-proline and gamma-aminoisobutyric acid. Aromatic and branched amino acids showed weak (L-valine) or no interaction (L-phenylalanine, L-leucine) with the carrier system. These data indicate that the carrier system for the uptake of L-proline has many features in common with the A system for amino acid transport.

Influence of D-glucose-induced water absorption on rat jejunal uptake of two passively absorbed drugs

The intestinal absorption of D-glucose is coupled to transepithelial sodium transport and this process generates intestinal water absorption. In situ jejunal perfusions were performed in rats to determine the extent of water transport as a function of perfusion flow rate, perfusate osmolality, and D-glucose concentration. Jejunal perfusions of iso-osmolar D-glucose, at flow rates and concentrations representative of the fed state, increased the dimensionless membrane permeabilities of the analgesic acetaminophen from 0.6 to 1.4, and that of the corticosteroid prednisolone from 1.6 to 2.2. This increase is less important for the more hydrophobic prednisolone since its baseline permeability (1.6) is indicative of complete uptake from solution, while the lower baseline permeability (0.6) of the more hydrophilic acetaminophen represents incomplete membrane uptake. The results suggest that nutrient-induced water transport can enhance jejunal uptake of small hydrophilic solutes. This phenomenon may contribute to variability in the absorption of drugs in this physicochemical class during the fed state.

Gastrointestinal morphology and absorption of monosaccharides in fowls conditioned to different types and levels of dietary fibre

To test a possible influence of dietary fibre on intestinal sugar uptake, rates of absorption of 10 mM-D-[U-14C] glucose and 10 mM-D-[U-14C] xylose were measured in either jejunum or (distal) caecum, by in vivo lumen perfusion, in immature female fowls preconditioned to a standard diet containing (g/kg) either 0, 100, 200 or 400 added dried grass, 200 powdered cellulose, or 200 grass with a polysaccharidase enzyme supplement. When birds were killed after perfusion, dimensions of (unperfused) parts of their alimentary tracts were determined, and recoveries of 14C radioactivity in some body tissues were compared with measured activities absorbed. On average, absorption of glucose was 1.9 and 1.2 times faster than xylose in jejunum and caecum respectively, although these differences varied with dietary treatment and order of perfusion. Increasing grass in the diet caused significant changes in xylose absorption rate in both jejunum and caecum, but only when it was perfused before glucose. With any one sugar and intestinal segment, mean rates of absorption were correlated positively with corresponding mean rates of fluid loss from perfusate. Although their influence on sugar absorption was not well defined, the dietary fibre treatments had more pronounced effects on gross dimensions of parts of the alimentary tract and, hence, potentially on total rates of absorption. Compared with the basal diet, addition of 100, 200 or 400 g grass/kg or 200 g cellulose/kg caused significant increases in small intestine length while 200 g grass/kg with supplementary enzyme did not, and combined caecal length increased with the 400 g grass/kg and the supplementary enzyme treatments. Absorbed 14C activity was recovered in plasma after jejunal perfusions but not caecal ones, whereas it was recovered in liver and in the flushed perfused segment after both types of perfusion. Since there was overlap in absorption rates between jejunum and caecum, this result suggests that the liver may be able to distinguish and treat differently compounds absorbed in the two regions.

Absorptive clearance of carbamazepine and selected metabolites in rabbit intestine

The intestinal permeability of carbamazepine, an antiepileptic drug, was examined as a function of intestinal site (duodenojejunum vs colon). A "through-and-through" in situ intestinal perfusion technique was adopted using the rabbit as an animal model. Coperfusion of the 10,11-epoxide and the 10,11-transdihydrodiol metabolites along with carbamazepine allowed for an examination of the effect of lipophilicity on intestinal permeability when molecular weight differences are negligible. Our results showed that carbamazepine is absorbed from rabbit duodenojejunum as well as the colon, which may explain the prolonged absorption behavior observed in humans. Also, the absorptive clearance of compounds having similar molecular weights is dependent not only on the lipophilicity but also on the extent of solvent drag during the course of the perfusion.

Assessment of water and solute absorption from experimental hypotonic and established oral rehydration solutions in secreting rat intestine

Water and solute absorption from three experimental hypotonic oral rehydration solutions (HYPO-ORS; sodium 45, 60 and 75 mmol/L, glucose 90 mmol/L), the World Health Organization recommended ORS (WHO-ORS; sodium 90 mmol/L, glucose 111 mmol/L), and the British National Formulary recommended ORS (BNF-ORS; sodium 35 mmol/L, glucose 200 mmol/L), have been assessed by perfusion studies in cholera toxin-induced secreting rat intestine. Net water absorption was greatest from the most hypotonic solution (HYPO-45; P less than 0.05). UK-ORS prevented net water secretion and WHO-ORS promoted moderate net water absorption. Net sodium secretion was seen with all solutions but was least from WHO-ORS and greatest with BNF-ORS (P less than 0.01). Glucose absorption was similar from BNF-ORS, WHO-ORS and HYPO-45 and in each case was greater than glucose absorption from HYPO-60 and HYPO-75 (P less than 0.05). These results suggest that net water and sodium absorption from ORS may be enhanced if osmolality is reduced by decreasing the glucose content.

Increased glucose transfer in the rat jejunum after dietary potassium loading: effect of amiloride

The glucose transfer across the jejunum was measured in Wistar rats under a high potassium diet (HKD). In 12 of 27 HKD animals the transfer coefficient for D-glucose was not significantly higher than in control ones, (7.38 +/- 0.88).10(-5) s-1. In the other 15 a clear increase in glucose transfer was observed, (23.31 +/- 2.50).10(-5) s-1. The D-glucose transfer in the first group (n = 12) was, as in the case of the control rats, insensitive to amiloride section (10(-4) M), while D-glucose transfer became sensitive to amiloride in the second group (mean inhibition 94 +/- 8%, n = 14). A smaller but significant increase in L-glucose and sucrose transfers was also observed when the D-glucose movement was increased. No differences in short-circuit current, transepithelial potential, resistance and mucosa to serosa Na+ fluxes were observed between control and HKD rats and no effects of amiloride (10(-4) M) on these parameters were observed either in control or in HKD animals. [3H]Glucose uptake as also performed in brush-border vesicles prepared from rat jejunum, under control and HKD conditions. The specific and Na(+)-dependent 'overshoot' in D-glucose concentration, in vesicles prepared from HKD rats, became sensitive to amiloride action (10(-5) M). It is concluded that, besides the cellular adaptation induced in the distal portion of the nephron and large intestine, dietary potassium loading induces important modifications in glucose transfer in the rat jejunum.

Effect of polyethylene glycol 400 on the intestinal permeability of carbamazepine in the rabbit

Because of the limited solubility of carbamazepine, aqueous solutions are usually prepared using glycols as cosolvents. This research focuses on the effect of varying the composition of polyethylene glycol 400 (PEG-400) in aqueous solutions in rabbit intestinal permeability of carbamazepine in the duodenojejunum and the ascending colon using an in situ perfusion technique. In both segments the intestinal permeability varied inversely with the percentage of PEG-400, when the concentration of carbamazepine in the perfusing solution was maintained constant. The decreased permeability may be explained by a reduction in the thermodynamic activity of carbamazepine with increased concentrations of PEG-400, as well as by reverse solvent drag because of the hyperosmolarity of the perfusing solutions.

An oligopeptide permeates intestinal tight junctions at glucose-elicited dilatations. Implications for oligopeptide absorption

Turnover of the Na(+)-glucose cotransporter in the apical membrane of intestinal absorptive cells elicits alterations in tight-junction structure including the appearance of intrajunctional dilatations. Paralleling these structural responses, epithelial permeability to ions and nutrient-sized solutes increases. However, it is not known how these observed permeability changes specifically relate to the structural alterations elicited by glucose. Using a hemeconjugated peptide tracer (MP-11; mol wt, approximately 1900), the present study shows that the glucose-elicited tight-junction dilatations are specific anatomical sites of junctional permeation. This peptide tracer penetrates tight junctions selectively at sites of dilatations and is detected focally within the paracellular space. This same tracer does not penetrate junctions when glucose is not present. A heme-conjugated macromolecule (horseradish peroxidase; mol wt, approximately 40,000) is excluded by both glucose-exposed and glucose-unexposed tissues. The results of this study show a paracellular pathway for small peptides that is regulated during Na(+)-glucose-activated absorption. It is speculated that the paracellular pathway may contribute to the meal-related oligopeptide absorption that is known to occur and has previously been wholly attributed to the transcellular route.

Supraphysiologic L-tryptophan elicits cytoskeletal and macromolecular permeability alterations in hamster small intestinal epithelium in vitro

We have previously shown that Na(+)-coupled transport of glucose and amino acids across the apical membrane of intestinal absorptive cells is accompanied by alterations in cytoskeletal structure and altered sieving of small hydrophilic solutes by tight junctions. Here we report that in response to the essential amino acid L-tryptophan at lumenal concentrations likely to be supraphysiological (1 mM or greater), these responses are so exaggerated as to induce disruption of tight junctions and transepithelial macromolecular leaks. Since these effects of L-tryptophan are energy and Na+ dependent and occur with mucosal but not serosal exposure to L-tryptophan, it appears they are triggered by activation of a Na(+)-nutrient cotransporter in the apical membrane of absorptive cells rather than by the presence of an unidentified trace contaminant. Our findings suggest the possibility that dietary supplementation by L-tryptophan may result in loss of the intestinal epithelial barrier to dietary antigens. We speculate that such a response to supraphysiologic tryptophan may contribute, in part, to the recently recognized curious tryptophan-induced eosinophilia myalgia syndrome.

A model of human small intestinal absorptive cells. 1. Transport barrier

The Caco-2 cell culture model of human small intestinal absorptive cells was used to investigate transepithelial transport. Transport of permeability markers such as mannitol demonstrated that Caco-2 monolayers became less permeable with increasing age in culture. Cells were routinely used for transport studies between day 18 and day 32. A transport index was determined for each compound by calculating the ratio of transport of the molecules under investigation to transport of an internal standard such as the permeability marker mannitol. Comparison of transport rates at 4 and 37 degrees C was a simple approach for differentiating primary transport mechanisms (passive paracellular, passive transcellular, or transporter-mediated) but must be coupled with additional experimental manipulations for definitive determination of transport pathways. Compounds predicted to undergo predominantly paracellular transport (mannitol, FITC, PEG-900, and PEG-4000), transporter-mediated transcellular transport (glucose, biotin, spermidine, or alanine), or lipophilic transcellular transport (alprenolol, propranolol, clonidine, or diazepam) showed differential effects of temperature on rates of transport as well as the transport index.

Absorption of hexose and pentose sugars in vivo in perfused intestinal segments in the fowl

1. Rates of absorption of two hexose (D-glucose and D-galactose) and two pentose (D-xylose and D-arabinose) sugars were measured by in vivo perfusion, in jejunum, ileum and (distal) caecum, in immature hens conditioned to either a standard (ST) or "high fibre" (ST + 20% grass) diet. 2. Each bird was tested in one intestinal segment with all four (U-14C-labelled, 10 mM) sugars, with either the hexoses preceding the pentoses or vice versa. 3. With all treatments, absorption rates of the hexoses were alike, as were those of the pentoses. Hexose absorption was twice as fast as pentose absorption in jejunum and ileum with both dietary pretreatments, whereas in caecum hexose and pentose rates were similarly high, except when pentose (and its associated fluid transfer) was apparently inhibited by prior hexose absorption with the ST diet. 4. With the ST diet, hexose absorption (per unit length and dry weight) was faster in caecum than in jejunum and ileum, and pentose absorption was also fastest in caecum when all pentose data from testing after hexose were excluded. 5. With the ST/grass diet, hexose absorption was faster in jejunum than in ileum and caecum when expressed per unit length, and pentose absorption was fastest in caecum on a dry weight basis. 6. Hexose absorption was faster in jejunum and slower in caecum with the ST/grass pretreatment than with ST. However, the dietary comparison was not conclusive because it involved birds form (two) different hatches (of similar age and weight) tested at different times.

Polyethylene glycol 400 penetration of the colonic epithelial barrier of the rat

Permeability changes of polyethylene glycol 400 have been seen in patients with inflammatory bowel diseases. Because the colon can be involved in inflammatory bowel disease, the mechanisms, kinetics, and influence of intraluminal factors on polyethylene glycol 400 permeation of perfused colonic segments of rats were studied. The absorption rate of polyethylene glycol 400 was linearly related to its luminal concentration (r = 0.94), suggesting that passive diffusion is a significant mechanism involved in polyethylene glycol 400 absorption. Changing the perfusate pH from 6.0 to 7.5 did not affect water absorption or polyethylene glycol 400 permeation. Increasing luminal osmolarity significantly decreased water and polyethylene glycol 400 absorption (P less than 0.01). The relationship between polyethylene glycol 400 and water absorption at different luminal osmolarities was linear (r = 0.97). At luminal osmolarity of 0.3 osm/L, 14.3% of polyethylene glycol 400 absorption was mediated by passive diffusion and 85.7% was mediated by convection. The solvent drag reflection coefficient for polyethylene glycol 400 in the colon was 0.03. Taurocholic acid (10 mmol/L) and chenodeoxycholic acid (5 mmol/L) decreased polyethylene glycol 400 and water absorption (P less than 0.01). Addition of 1 micrograms/mL of 16,16-dimethyl prostaglandin E2, 2 mmol/L of dibutyryladenosine-3',5'-cyclic monophosphate, or 10 mmol/L of aminophylline significantly decreased water and polyethylene glycol 400 absorption (P less than 0.01). These studies demonstrate that polyethylene glycol 400 permeation of the colon is mediated by both passive diffusion and solvent drag. Convective absorption is the major mechanism of polyethylene glycol 400 permeation of the colon. Polyethylene glycol 400 permeation is modified by bile acids, prostaglandins, and cyclic nucleotides through changes in water flux.

Transport of a large neutral amino acid (phenylalanine) in a human intestinal epithelial cell line: Caco-2

We have characterized the transcellular transport of a large neutral amino acid (LNAA) in Caco-2 cell monolayers. The apical (AP) to basolateral (BL) transport of phenylalanine (Phe) was approximately 10-times faster than BL-to-AP transport. The evidence for the carrier-mediated AP-to-BL transport of Phe include: (a) temperature dependence and saturability, (b) Phe transport was not affected by a reverse gradient, (c) the activation energy for transport was 12.0 kcal/mol, and (d) an excess amount of unlabeled Phe caused a 75% reduction in transport rate and a delay (lag time) in the appearance of Phe in the BL side. The Vm and Km for Phe transport were 572.4 pmol.mg protein-1.min-1 and 0.56 mM, respectively. Phe transport was decreased in the absence of glucose and in the presence of sodium azide or ouabain. The carrier interacted with LNAAs and with cationic amino acids but not with small neutral or anionic amino acids.