Properties of the passive conductance pathway across in vitro rat jejunum

Axial heterogeneity of superficial proximal tubule paracellular transport in mice

A considerable amount of NaCl reabsorption in proximal tubules (PTs) occurs via the paracellular transport regulated by the tight junction proteins claudins (Cldns). However, the paracellular transport properties in mouse superficial PTs remain unclear. We characterized these properties in superficial PT S1-S3 segments from mice expressing [wild-type (WT, WTS1-WTS3)] or lacking [knockout (KO, KOS1-KOS3)] claudin-2. We isolated and perfused segments with symmetrical solutions in the presence of bath ouabain and measured the diffusion potential upon changing the salt composition of the lumen or bath. Based on the diffusion potential corrected for the liquid junction potential (dVT), we calculated the paracellular Na+ over Cl- permeability (PNa/PCl) ratio. The PNa/PCl values upon reducing luminal NaCl averaged 1.27, 1.04, and 0.85 in WTS1, WTS2, and WTS3 and 0.34, 0.55, and 0.80 in KOS1, KOS2, and KOS3, respectively. The dVT values exhibited a symmetrical response to bidirectional NaCl concentration gradients in WTS1-WTS3 and KOS1-KOS3. WTS1 and WTS3 were monovalent cation-selective, with WTS1 demonstrating stronger cation selectivity. The order of permeabilities relative to Cl- was K+ > Rb+ > Na+ > Li+, whereas both KOS1 and KOS3 exhibited monovalent cation selectivity loss and, consequently, enhanced anion selectivity, especially in KOS1. Protamine addition to the lumen and bath similarly decreased PNa/PCl values upon reduced luminal NaCl in the order of WTS1 > WTS3 > KOS3 > KOS1. Therefore, this study presents evidence of axial heterogeneity in paracellular transport across superficial PTs in mice.NEW & NOTEWORTHY Research on isolated perfused S2 segments of proximal tubules in mice, both expressing and lacking claudin-2, indicates that claudin-2 forms leaky monovalent cation-selective paracellular channels within the tight junctions of proximal tubules. This study characterized the paracellular transport properties in isolated and perfused superficial proximal tubule S1-S3 segments in both groups of mice. The findings demonstrate, for the first time, functional heterogeneity in the paracellular pathway along the axis of the superficial proximal tubules.

Amino Acid Transport Across the Mammalian Intestine

The small intestine mediates the absorption of amino acids after ingestion of protein and sustains the supply of amino acids to all tissues. The small intestine is an important contributor to plasma amino acid homeostasis, while amino acid transport in the large intestine is more relevant for bacterial metabolites and fluid secretion. A number of rare inherited disorders have contributed to the identification of amino acid transporters in epithelial cells of the small intestine, in particular cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, and dicarboxylic aminoaciduria. These are most readily detected by analysis of urine amino acids, but typically also affect intestinal transport. The genes underlying these disorders have all been identified. The remaining transporters were identified through molecular cloning techniques to the extent that a comprehensive portrait of functional cooperation among transporters of intestinal epithelial cells is now available for both the basolateral and apical membranes. Mouse models of most intestinal transporters illustrate their contribution to amino acid homeostasis and systemic physiology. Intestinal amino acid transport activities can vary between species, but these can now be explained as differences of amino acid transporter distribution along the intestine. © 2019 American Physiological Society. Compr Physiol 9:343-373, 2019.

Claudin-2-deficient mice are defective in the leaky and cation-selective paracellular permeability properties of renal proximal tubules

Claudin-2 is highly expressed in tight junctions of mouse renal proximal tubules, which possess a leaky epithelium whose unique permeability properties underlie their high rate of NaCl reabsorption. To investigate the role of claudin-2 in paracellular NaCl transport in this nephron segment, we generated knockout mice lacking claudin-2 (Cldn2(-/-)). The Cldn2(-/-) mice displayed normal appearance, activity, growth, and behavior. Light microscopy revealed no gross histological abnormalities in the Cldn2(-/-) kidney. Ultrathin section and freeze-fracture replica electron microscopy revealed that, similar to those of wild types, the proximal tubules of Cldn2(-/-) mice were characterized by poorly developed tight junctions with one or two continuous tight junction strands. In contrast, studies in isolated, perfused S2 segments of proximal tubules showed that net transepithelial reabsorption of Na(+), Cl(-), and water was significantly decreased in Cldn2(-/-) mice and that there was an increase in paracellular shunt resistance without affecting the apical or basolateral membrane resistances. Moreover, deletion of claudin-2 caused a loss of cation (Na(+)) selectivity and therefore relative anion (Cl(-)) selectivity in the proximal tubule paracellular pathway. With free access to water and food, fractional Na(+) and Cl(-) excretions in Cldn2(-/-) mice were similar to those in wild types, but both were greater in Cldn2(-/-) mice after i.v. administration of 2% NaCl. We conclude that claudin-2 constitutes leaky and cation (Na(+))-selective paracellular channels within tight junctions of mouse proximal tubules.

Characterization of newly established bovine intestinal epithelial cell line

Membranous epithelial cells (M cells) of the follicle-associated epithelium in Peyer's patches have a high capacity for transcytosis of several viruses and microorganisms. Here, we report that we have successfully established a bovine intestinal epithelial cell line (BIE cells) and developed an in vitro M cell model. BIE cells have a cobblestone morphology and microvilli-like structures, and strongly express cell-to-cell junctional proteins and cytokeratin, which is a specific intermediate filament protein of epithelial cells. After co-culture with murine intestinal lymphocytes or treatment with supernatant from bovine PBMC cultured with IL-2, BIE cells acquired the ability of transcytosis. Therefore, BIE cells have typical characteristics of bovine intestinal epithelial cells and also have the ability to differentiate into an M cell like linage. In addition, our results indicate that contact between immune cells and epithelial cells may not be absolutely required for the differentiation of M cells. We think that BIE cells will be useful for studying the transport mechanisms of various pathogens and also the evaluation of drug delivery via M cells.

Cultured monolayers of the dog jejunum with the structural and functional properties resembling the normal epithelium

The development of a culture of the normal mammalian jejunum motivated this work. Isolated crypt cells of the dog jejunum were induced to form primary cultures on Snapwell filters. Up to seven subcultures were studied under the electron microscope and in Ussing chambers. Epithelial markers were identified by RT-PCR, Western blot, and immunofluorescent staining. Confluent monolayers exhibit a dense apical brush border, basolateral membrane infoldings, desmosomes, and tight junctions expressing zonula occludens-1, occludin-1, and claudin-3 and -4. In OptiMEM medium fortified with epidermal growth factor, hydrocortisone, and insulin, monolayer transepithelial voltage was -6.8 mV (apical side), transepithelial resistance was 1,050 Omega.cm(2), and short-circuit current (I(sc)) was 8.1 microA/cm(2). Transcellular and paracellular resistances were estimated as 14.8 and 1.1 kOmega.cm(2), respectively. Serosal ouabain reduced voltage and current toward zero, as did apical amiloride. The presence of mRNA of alpha-epithelial Na(+) channel (ENaC) was confirmed. Na-d-glucose cotransport was identified with an antibody to Na(+)-glucose cotransporter (SGLT) 1. The unidirectional mucosa-to-serosa Na(+) flux (19 nmol.min(-1).cm(-2)) was two times as large as the reverse flux, and net transepithelial Na(+) flux was nearly double the amiloride-sensitive I(sc). In plain Ringer solution, the amiloride-sensitive I(sc) went toward zero. Under these conditions plus mucosal amiloride, serosal dibutyryl-cAMP elicited a Cl(-)-dependent I(sc) consistent with the stimulation of transepithelial Cl(-) secretion. In conclusion, primary cultures and subcultures of the normal mammalian jejunum form polarized epithelial monolayers with 1) the properties of a leaky epithelium, 2) claudins specific to the jejunal tight junction, 3) transepithelial Na(+) absorption mediated in part by SGLT1 and ENaC, and 4) electrogenic Cl(-) secretion activated by cAMP.

Abnormal passive chloride absorption in cystic fibrosis jejunum functionally opposes the classic chloride secretory defect

Due to genetic defects in apical membrane chloride channels, the cystic fibrosis (CF) intestine does not secrete chloride normally. Depressed chloride secretion leaves CF intestinal absorptive processes unopposed, which results in net fluid hyperabsorption, dehydration of intestinal contents, and a propensity to inspissated intestinal obstruction. This theory is based primarily on in vitro studies of jejunal mucosa. To determine if CF patients actually hyperabsorb fluid in vivo, we measured electrolyte and water absorption during steady-state perfusion of the jejunum. As expected, chloride secretion was abnormally low in CF, but surprisingly, there was no net hyperabsorption of sodium or water during perfusion of a balanced electrolyte solution. This suggested that fluid absorption processes are reduced in CF jejunum, and further studies revealed that this was due to a marked depression of passive chloride absorption. Although Na+-glucose cotransport was normal in the CF jejunum, absence of passive chloride absorption completely blocked glucose-stimulated net sodium absorption and reduced glucose-stimulated water absorption 66%. This chloride absorptive abnormality acts in physiological opposition to the classic chloride secretory defect in the CF intestine. By increasing the fluidity of intraluminal contents, absence of passive chloride absorption may reduce the incidence and severity of intestinal disease in patients with CF.

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+.

Physiological regulation of transepithelial impedance in the intestinal mucosa of rats and hamsters

Isolated intestinal segments from rats or hamsters were recirculated with balanced salt solutions containing fluorocarbon emulsion to provide 6 vpc oxygen. The lumen contained an axial Ag-AgCl electrode, and the serosal surface was surrounded by a cylindrical shell of Ag-AgCl. Transmural impedances were measured at frequencies from 0.01-30 kHz before and after removal of the mucosal epithelium. The resistance of intercellular junctions, RJ, the distributed resistance of the lateral spaces, RL, and the distributed membrane capacitance, CM, were computed from the relations between frequency and impedance. Activation of Na-coupled solute transport by addition of glucose, 3-0-methyl glucose, alanine or leucine caused two- to threefold decreases of transepithelial impedance. Typical changes induced by glucose in hamster small intestine were RJ 30----13 omega, RL 23----10 omega, and CM 8----20 microF (per cm length of segment). Half maximal response occurred at a glucose concentration of 2-3 mM. The area per unit path length of the junctions (Ap/delta chi = specific resistance divided by RJ) in glucose activated epithelium was 3.7 cm in hamster midgut and 6.8 cm in rat. These values are close to the 4.3 cm estimated independently from coefficients of solvent drag and hydrodynamic conductance in glucose-activated rat intestine in vivo. The transepithelial impedance response to Na-coupled solute transport was reversibly dependent upon oxygen tension. It is proposed that activation of Na-coupled solute transport triggers contraction of circumferential actomyosin fibers in the terminal web of the microvillar cytoskeletal system, thereby pulling apart junctions and allowing paracellular absorption of nutrients by solvent drag as described in the previous accompanying paper. Anatomical evidence in support of this hypothesis is presented in the following second accompanying paper.

Permselectivity for cations over anions in the upper portion of descending limbs of Henle's loop of long-loop nephron isolated from hamsters

The permselectivity of the upper portion of long descending limb of Henle (LDLu) was investigated with electrophysiological methods in the isolated perfused tubule preparation of hamster kidney. The diffusion potential (Vt) was determined in three different protocols. In protocol 1, the tubules were initially perfused with modified Krebs Ringer's solution on both sides of the epithelium. Then the bath solution was exchanged consecutively with another solution in which 50 mmol/l NaCl replaced by 50 mmol/l KCl, RbCl, NH4Cl, CsCl, LiCl, NaBr, NaNO3, NaI, Na acetate or 75 mmol/l NaCl replaced by mannitol. The permeabilities for these ions relative to chloride were calculated by Goldman's constant field equation. The segment was found to be cation selective, with all cations being 5-9 times more permeable than all anions. The sequence of permeability was K+ greater than Rb+ greater than Li+ greater than NH+4 = Cs+ greater than or equal to Na+ much greater than Cl- greater than or equal to Br- greater than or equal to NO3- greater than or equal to I- greater than Acetate-. In protocol 2, pure 150 mM NaCl was used for the basal solution to avoid interference by other ions. The bathing solution was exchanged by other solutions which contained 150 mmol/l KCl, NH4Cl, CsCl, RbCl, LiCl, NaI, NaBr, NaNO3, Na acetate or 75 mmol/l NaCl with mannitol. Thus simple biionic substitution was performed. Again, the segment was found to be cation selective, with all cations being 4-10 times more permeable than all anions.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport of neutral and cationic amino acids across the brush-border membrane of the rabbit ileum

The transport of sugars and amino acids across the brush-border membrane of the distal rabbit ileum has been studied. The kinetics of the transport of glucose demonstrated that the data obtained with the present technique are less distorted by unstirred layers than those obtained with the same technique adapted to the use of magnetic stirring. The role of depolarization of the electrical potential difference across the brush-border membrane in mutual inhibition between different classes of amino acids was estimated by measurements of the effects of high concentrations of alanine and lysine on the transport of galactose. It was found that this role would be insignificant in the present study. By measurements of the transport of alanine, leucine and lysine and the inhibitory interactions between these amino acids the function of three transport systems has been delineated. The transport of lysine is resolved in a high- and a low-affinity contribution. At 140 mM sodium these transport systems may also function as respectively high- and low-affinity contributors to the transport of neutral amino acids. At 0 mM sodium the high-affinity system remains a high-affinity system for cationic and neutral amino acids with reduced capacity especially for the neutral amino acids. At 0 mM sodium the low-affinity system's affinity for lysine is reduced and it is inaccessible to neutral amino acids. In addition to the two systems for lysine transport the existence of a lysine-resistant, sodium-dependent, high-affinity system for the transport of neutral amino acids has been confirmed. It seems unlikely that the distal ileum is equipped with a low-affinity, sodium-independent system for the transport of neutral amino acids.

The involvement of calcium in the intestinal response to secretagogues in the rat

The involvement of Ca2+ in the regulation of intestinal secretion was investigated in stripped sheets of rat mid-intestine. Removal of serosal Ca2+ together with the addition of EGTA at concentrations of 0.5 and 1 mM inhibited the rise in short-circuit current (s.c.c.) induced by both acetylcholine and theophylline, a similar degree of inhibition being observed with both secretagogues. Ca2+-free serosal fluid with 0.5 mM-EGTA added reduced significantly the rises in s.c.c. induced by A23187, acetylcholine, 5-hydroxytryptamine, theophylline, dibutyryl cyclic AMP and prostaglandin E2, but not the increased s.c.c. associated with glucose absorption. The Ca2+ channel blocker verapamil produced similar results. The calmodulin antagonist trifluoperazine inhibited secretagogue action while its sulphoxide derivative was without effect at the same concentration. The intracellular Ca2+ antagonist TMB-8 reduced the increased s.c.c. observed with acetylcholine and dibutyryl cyclic AMP. The net Cl- secretion, but not the decreased mucosal-to-serosal Na+ flux, induced by acetylcholine was abolished in Ca2+-free conditions. There was no consistent effect on the reduction in the residual ion flux caused by acetylcholine. Absence of Ca2+ converted the stimulation of Cl- secretion induced by dibutyryl cyclic AMP observed under control conditions to an enhancement of net Na+ and Cl- absorption. It is concluded that intestinal secretagogues, whether they act through cyclic AMP or not, require both internal and external sources of Ca2+ if they are to produce their full effects. Moreover, it appears that the nature of the response to dibutyryl cyclic AMP depends on the prevailing Ca2+ concentration.

Lysine transport in the guinea-pig small intestine

Interactions between cationic and neutral amino acids in transport across the brush-border membrane, Jmc, of the small intestine have been examined using preparations from the distal rabbit ileum and the rat and guinea-pig mid-small intestine. (1) In the guinea pig, the dependence of Jmc Lys on the concentration of lysine is best described in terms of two saturable transport mechanism in addition to free diffusion. (2) It is shown that the discrepancy between cis-effects of low concentrations of neutral amino acids on the Jmc of cationic amino acids, cis-stimulation in the guinea pig contra cis-inhibition in the rabbit and rat, represents species differences. In the guinea pig, imposing sodium-free conditions turns cis-stimulation into cis-inhibition. (3) It is demonstrated that in rat and guinea pig, leucine is transported both by the transport system(s) for cationic amino acids and by transport system(s) which cannot be inhibited by cationic amino acids.

Response of the rat small-intestine epithelium to methotrexate

We studied jejunal epithelial structure and function in rats 24, 48, 96, and 192 hours after a single intravenous injection of methotrexate (MTX) 30 mg/kg. The acute effect of the drug on the gut at 24 and 48 hours was characterised, as expected, by reduced mitoses in crypts, shortened villi, and depressed activity of thymidine kinase (an enzyme normally confined to intestinal crypt cells). At 96 hours, when MTX was no longer detectable in serum, the intestine had entered a proliferative phase characterised by increased crypt mitoses, accelerated migration of enterocytes along villi, and the presence on villi of epithelial cells with the enzyme profile of crypt cells, decreased disaccharidase, alkaline phosphatase, and Na+-K+ATPase activities and increased thymidine kinase activity. Although the enzyme data suggested that enterocyte maturation was defective during this proliferative phase, glucose-stimulated Na+ transport, normally a function of fully differentiated villus cells, was normal at 96 hours. Measured both in Ussing chambers and in suspensions of enterocytes isolated from villi, Na+ transport responded normally to glucose at 96 hours, although the response had been significantly depressed at 24 hours. These findings cannot be attributed to MTX-induced malnutrition, as all comparisons included pair-fed controls. We conclude that, in the MTX-induced malnutrition, as all comparisons included pair-fed controls. We conclude that, in the small intestine under conditions of altered epithelial renewal, some components of enterocyte function may be affected more than others. Comparing the present experimental model with another intestinal disorder, acute viral enteritis, in which proliferative activity is excessive, it is clear that the nature of the original intestinal injury is a significant determinant of the pattern of enterocyte response.

Cellular and paracellular calcium transport in rat ileum: effects of dietary calcium

The mechanism of calcium transport and its modulation by dietary Ca restriction in rat ileum have been investigated employing an in vitro voltage-clamp technique. Ca fluxes directed from mucosa-to-serosa (J(Ca)ms) and serosa-to-mucosa (J(Ca)sm) exhibit components consistent with both cellular and paracellular pathways. The cellular Ca fluxes are both dependent on medium Na and are abolished at 10 degrees C. In addition, a cellular component of J(Ca)ms displays voltage dependence. A low-calcium diet, which induces the formation of 1,25-dihydroxycholecalciferol, causes a marked increase in both cellular J(Ca)ms and Ca influx from media to cells with little absolute effect on the paracellular pathway. This evidence is consistent with the existence of electrogenic Na-Ca exchange pumps at both brush-border and basolateral membranes, driven in part by the Na electrochemical gradient. Dietary Ca conditioning may control the direction of net Ca transport by modulation of the saturable influx process at the brush border.

Kinetics of the absorption of amino acids by the rat intestine in vivo

The kinetics of L-phenylalanine and L-lysine absorption by the rat small intestine in vivo have been studied by perfusing intestinal segments and monitoring simultaneously the uptake of the substrate into the intestinal tissue and its disappearance from the perfusate. The rate of phenylalanine disappearance is a linear function of the substrate concentration. Its uptake into the tissue is rapid and obeys saturation kinetics, but is not concentrative. Both tissue uptake and disappearance rate can be inhibited by leucine or methionine, but are not influenced by hydrophilic neutral or dibasic amino acids. Lysine disappearance from the perfusate and its uptake into the tissue both display saturation kinetics. Lysine transport is quantitatively smaller than that of phenylalanine. Both uptake and disappearance are inhibited by arginine and leucine, but are unaffected by other neutral amino acids or sugars. To analyse the kinetic results, integrated equations were developed to express the final concentration in the perfusate in terms of the original concentration. The disappearance rate was considered as a mixed process (saturable and non-saturable in parallel) in a one-compartment system, and the uptake by the tissue was treated as a two-compartment system in which the amino acid entered the cells by a mixed process but left them by a pure non-saturable mechanism. The results concerning disappearance from the lumen are compatible with the one-compartment model. Phenylalanine absorption can be described by a major non-saturable component and a minor saturable one, while lysine absorption occurs almost entirely by a saturable process. The two-compartment model does not adequately describe the tissue uptake results.

Maturation of jejunum and ileum in rats. Water and electrolyte transport during in vivo perfusion of hypertonic solutions

During osmotic diarrhea, loss of water and electrolytes appears to be greater in infants than in adults. In 2-, 3-, and 7-wk-old rats, we studied net transport of H(2)O, Na, and Cl, during in vivo perfusion of segments of the jejunum and ileum, from solutions with osmolalities of 300, 375, 500, or 700 mosmol/kg. In the jejunal segments, from the hypertonic solutions net transport of H(2)O, Na, and Cl was into the lumen and greater in the 2- than 7-wk-old rats. In the ileal segments, transport of water was into the lumen, transport of Na was minimal and variable, whereas transport of Cl was usually out the lumen. In 3-wk-old rats, transport rates were intermediate between those in 2- and 7-wk-old rats. The calculated filtration coefficient (microliters of H(2)O transported per hour per unit osmolality gradient-lumen-serum-per gram dry weight) of water suggested that the resistance to water flow did not increase with rise in luminal hypertonicity in the jejunum of the 2- and 3-wk-old rats, whereas in jejunum of the 7-wk-old rats and in ileum of rats in all three ages, the resistance to water flow increased with the rise in luminal osmolality. The differences in the transport rates and the resistance to water flow, between segments of the 2-, 3-, and 7-wk-old rats, suggested a maturational phenomenon that appears to continue beyond the 3rd wk of life and could have been due to differences in some physical property of the mucosal membrane.

Sodium chloride transport across the chicken coprodeum. Basic characteristics and dependence on sodium chloride intake

1. The transport characteristics of the chicken coprodeum have been examined in vitro using the isolated mucosa. The short-circuit current (I(sc)), the transepithelial electrical potential difference (p.d.), the unidirectional transmural fluxes (J(ms), J(sm)) of sodium and chloride measured in the short-circuited state, and the unidirectional influx of sodium and chloride across the brush border membrane measured under open-circuit conditions have been studied. The effect of the sodium chloride contents of the diet on these parameters have been investigated.2. The isolated mucosa depends functionally on the presence of glucose in the incubation media. This dependence reflects the need of glucose as a fuel. There is no indication of coupling between transport of sugars and sodium across the brush border membrane. For preparations from chickens on a low sodium diet a very high and stable I(sc) can quantitatively be accounted for by the net transport of sodium. Influx of sodium across the brush border membrane is not significantly different from the net flux of sodium. By feeding the chickens a high sodium diet the I(sc) is reduced by more than 95%, the net transport of sodium is abolished, and the transepithelial electrical conductance is reduced by more than 50%.3. Both unidirectional transepithelial fluxes of chloride, and the serosa to mucosa flux of sodium appear to proceed through a paracellular shunt.4. Under the conditions of the low sodium diet the paracellular pathway appears to be anion selective. Whereas, under the conditions of the high sodium regimen the paracellular route appears to be cation selective. After adaptation to a high sodium diet the influx of sodium across the brush border membrane is only moderately reduced. Consequently the decisive event in the adaptation must be localized elsewhere.

Alterations in electrophysiology of isolated amphibian small intestine produced by removing the muscle layers

Isolated segments of Amphiuma small intestine bathed in chloride or sulfate buffer generate a greater short-circuit current and a larger change in current in response to galactose when the serosal muscle layers are stripped from the mucosa. Intact (unstripped) segments are not apparently anoxic since stripped segments exposed to serosal N2 for 3 h display normal short-circuit currents but a reduced potential response to galactose, while the presence of muscle layers tends to reduce the short-circuit current but does not alter the potential response to galactose. Bullfrog small intestine also generates greater short-circuit current following removal of the muscle layers. The enhancing effect of stripping appears to be related to removal of a resistance to ion flow across the tissue.

Electrical properties and active solute transport in rat small intestine. II. Conductive properties of transepithelial routes

The transepithelial resistance, the cell membrane resistance and the ratio of resistances of the serosal (baso-lateral) to the mucosal (brush border) cell membrane were measured in rat duodenum, jejunum and ileum by means of microelectrode techniques. These measured values were not affected in the presence of actively transported solutes in the mucosal bathing fluid. Contribution of an electrical conductance through the extracellular shunt pathway to the total transepithelial conductance was quantitatively estimated using an electrically equivalent circuit analysis. These values estimated in respective tissues of small intestine were approx. 95% of the total transepithelial conductance, remaining unaffected by an active solute transport. From these data, the changes in emf's of the mucosal and serosal membrane induced by D-glucose or glycine were separately evaluated.

Electrical properties and active solute transport in rat small intestine. I. Potential profile changes associated with sugar and amino acid transports

Addition of D-glucose to the mucosal fluid resulted in a significant depolarization of the mucosal membrane potential (V-m) in rat duodenum, jejunum, and ileum accompanied by an increase in the transepithelial potential difference (PDt). On the other hand, L-glucose did not induce PDt and Vm changes. Glycine applied from the mucosal side also induced Vm-depolarization and PDt-increment in the ileum. Phlorizin added to the mucosal fluid or ouabain added to the serosal fluid inhibited the sugar-dependent changes in PDt and Vm. According to the analysis with an equivalent circuit model for the epithelium, it was concluded that an actively transported solute induced not only a depolarization of the mucosal (brush border) membrane but also a hyperpolarization of the serosal (baso-lateral) membrane of an epithelial cell, so that the origin of solute-induced PDt changes should be attributed to changes in emf's at both membranes. The hyperpolarization of the serosal membrane in the presence of an actively transported solute was attributed to a mechanism of serosal electrogenic sodium pump stimulated by the increase in the extrusion rate of Na+ co-transported into the cell with sugar or amino acid.

Bidirectional sodium ion movements via the paracellular and transcellular routes across short-circuited rabbit ileum

1. It has been confirmed that the agent 2,3,6-triaminopyrimidine decreases Na+ conductance in the paracellular pathway of rabbit ileum. 2. Triaminopyrimidine has been used as a means of measuring transcellular bidirectional Na+ flux, and also, of assessing the contribution of the paracellular pathway to transepithlial Na+ flux. 3. Reduction of Ringer [Na+] to 25 mM or incubation with 0.1 mM ouabain reduces paracellular Na+ permeability. This effect may be due to lateral space collapse. Ringer galactose increases serosa to mucosa Na+ flux by a stimulating reflux through the tight junctions. A proportion of net Na+ flux in control tissues is due to asymmetry generated in the paracellular pathway. It is likely that this passive asymmetry results from an osmotic pressure gradient across the tight-junction. 4. Measurement of the tissue isotope specific activity ratio together with bidirectional transcellular Na+ fluxes allows calculation of the four unidirectional fluxes across the mucosal and serosal boundaries. Values obtained for Na+ entry (J12) and exit (J21) across the mucosal boundary are 7.97 alnd 7.13 mumol-cm(-2)-h(-1) respectively. Entry flux (J12) is a saturable function of Ringer [Na+]. The calculated Km is 295 mM and the V is 17.6 mumul-cm(-2)-h(-1). Na+ entry flux is insensitive to ouabain (0.1 mM). Ouabain results in elevation of exit (J21) flux of Na+ across the brush border. D-Galactose causes a saturable increase in Na+ flux (J12) across the mucosal boundary; the Km for this relationship is 1.2 mM and the V 2.17 mumol-cm(-2)-h(-1). The stoichiometry between sugar and Na+ entry is applixmately 1:1. In contrast to the effect of galactose on entry flux, no change in Na+ efflux across the mucosal boundary is observed when Ringer [galactose] is raised. This finding is dissonant with the prediction of the Na+ -gradient hypothesis. The calculated values of exit (J23) and entry (J32) Na+ fluxes across the serosal border are 16.74 and 15.90 mumol-cm(-2)-h(-1). 0.1 mM ouabain markedly reduces both these unidirectional fluxes. This result is consistent with a serosal location of the Na+-pump. Serosal Na+ exit flux J23 increases as a hyperbolic function of Ringer [galactose]. A small galactose-dependent decrease in entry (J32) is also observed. 0.1 mM ouabain abolishes these galactose-dependent changes. 5. The present findings together with those in the previous paper are discussed in relation to the convective-diffusion model for sugar transport.

Osmotically induced electrical changes in isolated bullfrog small intestine

1. Steady state values of cell water, intracellular Na+ and K+ concentrations, and the electrical parameters ETr, Em, and Isc in the mucosa of isolated bullfrog small intestine were determined following immersion in sodium sulfate Ringer solutions with identical ionic composition but different osmolalities. 2. Cell water and intracellular K+ concentration were inversely related to the osmolality of the bathing medium. During 1 h immersion, intracellular Na+ concentration was not significantly affected by an increase or decrease in external osmolality. 3. Replacement of a hypotonic or an approximately isotonic (normal) medium by a medium of greater osmolality caused statistically significant decreases in ETr, Isc and the (inside negative) magnitude of Em. Conversely, when a hypertonic or a normal medium was replaced by one of lower osmolality, significant increases in the magnitude of these parameters were observed. 4. An equivalent circuit model for the epithelial cell layer, in which the resistance of a relatively highly conducting extracellular shunt pathway is assumed to be the major determinant of the electrical responses of the small intestine to external osmolality, has been shown to account satisfactorily for the observed changes in ETr and Em. In terms of this model, the experimentally observed dependence of Isc on external osmolality requires that, even when both the mucosal and the serosal sides of the tissue are bathed by identical media, isolated bullfrog small intestine maintains a finite diffusion potential across the shunt pathway. This is consistent with current views concerning transepithelial ionic transfer mechanisms.