Conductance Flux Research Articles

Modulation of distal colonic epithelial barrier function by dietary fibre in normal rats

BACKGROUNDDietary fibre influences the turnover and differentiation of the colonic epithelium, but its effects on barrier function are unknown.AIMSTo determine whether altering the type and amount of fibre in the...

The effects of nutrient limitation on the response of Plantago major to ozone.

Most natural and semi-natural communities are limited by major nutrients such as nitrogen (N) or phosphorus (P), but most experiments on the effects of ozone on wild plants have used nutrient-rich composts or soil. In order to investigate interactions between ozone and low nutrient supply, two artificially selected lines (ozone-resistant and ozone-sensitive) of two populations Plantago major ssp. major L. were grown on a sandy loam, with (HN) and without (LN) addition of fertilizer. The soil was from a semi-natural grassland that has never been fertilized. Plants were exposed to either charcoal/Purafil®-filtered air (CF=<5 nl O3 l-1 ) or 70 nl O3 l-1 7 h d-1 from the seedling stage to seed production. Poor growth (c. 25% of that in HN) of the low-nutrient plants, and leaf concentrations of N and P showed that the LN plants were severely nutrient-limited. In addition to affecting the total dry mass of the plants, the nutrient supply altered seed production, reproductive effort (number of seeds per total mass) and root-to-shoot allocation. Exposure to ozone had significant effects on physiology, growth, and seed production that varied with population, selection line, time, and plant development. There also were significant interactions between ozone effects and nutrient regime. In the Lullington Heath population, ozone reduced plant dry weight at 4 wk only in the LN treatment, and in the sensitive line of the Bush population, seed production was reduced by ozone only in LN. Therefore, contrary to what was expected, in the present experiment, plants given the LN treatment were often more sensitive to ozone than those grown under the high-nutrient regime. This increase in sensitivity was despite the fact that the LN treatment reduced stomatal conductance and ozone flux. It is concluded that there are potentially important interactions between ozone and low nutrient supply that need further investigation, particularly under field conditions.

Synergistic effect of hydrochloric acid and bile acids on the pars esophageal mucosa of the porcine stomach

Abstract Objectives To determine effects of finely ground diet and food deprivation on pH and bile acid concentration in the proximal portion of the porcine stomach and effects of bile acids and pH on the pars esophageal mucosa in vitro. Animals Sixteen 15- to 30-kg pigs. Procedures Gastric content samples obtained from pigs fed a finely ground pelleted or coarsely ground meal diet were assayed for gastric pH and bile acids. Stratified squamous epithelium was studied in an Ussing chamber, and histologically. Electrical conductance and transmucosal mannitol fluxes (as indices of tissue permeability) were determined at pH 4.0, 2.0, and 1.5 and in response to treatment with 0, 1, 2, or 3 mM taurodeoxycholate or glycocholate. Results Pigs fed the finely ground feed had significantly (P = 0.01 ) lower proximal stomach pH than did pigs fed the coarse meal. Proximal stomach bile acids concentration was significantly (P = 0.04) higher in pigs fed the finely ground diet. The H+ and bile acids concentration increased with time after feeding. In vitro exposure of the stratified mucosa to high H+ (pH &lt; 4.0) and bile salt concentration (≥ 1.0 mM) resulted in significant (P &lt; 0.05) dose-dependent increase in tissue conductance and mannitol fluxes, whereas low pH or bile acids alone had little effect. Conclusions High H+ and bile acids concentration in the stomach of pigs fed finely ground diets or subjected to feed deprivation may contribute to ulceration of the pars esophageal tissue. Bile acids act synergistically and in dose-dependent manner, with low pH causing damage to the stratified squamous epithelium in vitro. (Am J Vet Res 1998;59:1170-1176)

Interleukin 10 prevents cytokine-induced disruption of T84 monolayer barrier integrity and limits chloride secretion

BACKGROUND & AIMS: The proinflammatory cytokine interferon gamma (IFN- gamma) disrupts epithelial barrier integrity and attenuates secretagogue-induced chloride secretion. This study tested the efficacy of the anti-inflammatory cytokine interleukin 10 (IL-10) in maintaining epithelial barrier and chloride secretory function in the presence of IFN-gamma. METHODS: T84 epithelial cell monolayers were treated with IL- 10, IFN-gamma, or IFN-gamma plus IL-10. Monolayer barrier integrity was assessed by measurements of electrical conductance, unidirectional mannitol and inulin fluxes, and tight junctional charge selectivity in Ussing chambers. Short-circuit current (Isc) was measured in response to carbachol and forskolin stimulation. RESULTS: IL-10 attenuated the IFN-gamma-induced increase in electrical conductance and totally prevented the IFN-gamma-induced increase in mannitol and inulin fluxes. IL-10 did not prevent the IFN-gamma-induced abolishment of tight junctional charge selectivity but did attenuate the total increase in sodium and chloride permeability. IFN-gamma and IL-10 both separately reduced peak forskolin and carbachol-stimulated Isc. IL-10 pretreatment further enhanced the IFN-gamma-induced reduction in secretagogue- induced Isc. CONCLUSIONS: In T84 epithelial monolayers, IL-10 maintains the size, but not the charge, selectivity of the epithelial tight junction in the presence of IFN-gamma. In addition, both IL-10 and IFN- gamma limit carbachol and forskolin-induced increase in Isc. (Gastroenterology 1997 Jul;113(1):151-9)

Apical effect of diosmectite on damage to the intestinal barrier induced by basal tumour necrosis factor-alpha.

In many digestive diseases the intestinal barrier is weakened by the release of proinflammatory cytokines, including tumour necrosis factor-alpha (TNF alpha). To investigate the protective effect of apical diosmectite on the intestinal dysfunction induced by the proinflammatory cytokine TNF alpha. Filter grown monolayers of the intestinal cell line HT29-19A were incubated for 48 hours in basal medium containing 10 ng/ml TNF alpha and 5 U/ml interferon-gamma (IFN gamma). Next, 1, 10, or 100 mg/ml diosmectite was placed in the apical medium for one hour. Intestinal function was then assessed in Ussing chambers by measuring ionic conductance (G) and apicobasal fluxes of 14C-mannitol (Jman), and intact horseradish peroxidase. In control intestinal monolayers, diosmectite did not significantly modify G, Jman, or intact horseradish peroxidase. After incubation with TNF alpha and IFN gamma, intestinal function altered, as shown by the increases compared with control values for G (22.8 (3.7) v (9.6 (0.5) mS/cm2), Jman (33.8 (7.5) v 7.56 (0.67) micrograms/h x cm2), and intact horseradish peroxidase (1.95 (1.12) v 0.14 (0.04) micrograms/h x cm2). G and Jman were closely correlated, suggesting that the increase in permeability was paracellular. Treatment with diosmectite restored al the variables to control values. Basal TNF alpha disrupts the intestinal barrier through the tight junctions, and apical diosmectite counteracts this disruption.

Characterization of the interactions of a polycationic, amphiphilic, terminally branched oligopeptide with lipid A and lipopolysaccharide from the deep rough mutant of Salmonella minnesota

The lipid A and lipopolysaccharide (LPS) binding and neutralizing activities of a synthetic, polycationic, amphiphilic peptide were studied. The branched peptide, designed as a functional analog of polymyxin B, has a six residue hydrophobic sequence, bearing at its N-terminus a penultimate lysine residue whose α- and E-amino groups are coupled to two terminal lysine residues. In fluorescence spectroscopic studies designed to examine relative affinities of binding to the toxin, neutralization of surface charge and fluidization of the acyl domains, the peptide was active, closely resembling the effects of polymyxin B and its nonapeptide derivative; however, the synthetic peptide does not induce phase transitions in LPS aggregates as do polymyxin B and polymyxin B nonapeptide. The peptide was also comparable with polymyxin B in its ability to inhibit LPS-mediated IL-1 and IL-6 release from human peripheral blood mononuclear cells. The synthetic compound is devoid of antibacterial activities and did not induce conductance fluxes in LPS-containing asymmetric planar membranes. These results strengthen the premise that basicity and amphiphilicity are necessary and sufficient physical properties that ascribe endotoxin binding and neutralizing activities, and further suggest that antibacterial/membrane perturbant and LPS neutralizing activities are dissociable, which may be of value in designing LPS-sequestering agents of low toxicity.

Active absorption of Na+ and Cl- across the gill epithelium of the shore crab Carcinus maenas: voltage-clamp and ion-flux studies

Mechanisms of active NaCl uptake across the posterior gills of the shore crab Carcinus maenas were examined using radiochemical and electrophysiological techniques. In order to measure short-circuit current (Isc), transepithelial conductance (Gte) and area-related unidirectional fluxes of Na+ and Cl-, single split gill lamellae (epithelium plus cuticle) of hyperregulating shore crabs were mounted in a modified Ussing chamber. The negative short-circuit current measured with haemolymph-like NaCl saline on both sides of the epithelium could be inhibited by application of basolateral ouabain (ouabain inhibitor constant KOua=56&plusmn;10 &micro;mol l-1), 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB; KNPPB=7.5&plusmn;2.5 mmol l-1) or Cs+ (10 mmol l-1). From the apical side, Isc was nearly completely blocked by Cs+ (10 mmol l-1) or Ba2+ (15 &micro;mol l-1), whereas apical addition of furosemide (1 mmol l-1) resulted in only a small current decrease. Cl- influxes were linearly related to negative Isc. The ratio between net influxes of Cl- and Na+ was found to be approximately 2:1. With a single membrane preparation, achieved by permeabilizing the basolateral membrane with amphotericin B, Cl- influxes which were driven by a concentration gradient were shown to depend on the presence of apical Na+ and K+. On the basis of these observations, we propose that active and electrogenic absorption of NaCl across the gill epithelium of hyperregulating shore crabs proceeds as in the thick ascending limb of Henle's loop in the mammalian nephron. Accordingly, branchial NaCl transport is mediated by apical K+ channels in cooperation with apical Na+/K+/2Cl- cotransporters and by the basolateral Na+/K+-ATPase and basolateral Cl- channels.

Ion Transport Phenomena across the Parallel Composite Membrane

Abstract The equations of ionic currents in the parallel composite membrane-NaCl system were derived from the phenomenological equations presented by Kedem and Katchalsky. The elements of the conductance matrix for the composite membrane system, gαβ, as well as those for the single membrane systems with cation-exchange or anion-exchange membranes, or , respectively, were determined by the measurements of membrane potential, membrane conductance, and salt flux. It was found that gαβc or gαβa for the constituent elements of the composite membrane system was different from or for the single membrane systems, respectively. These differences may be attributed to the situation that gαβc and gαβa represent the intra- and intermembrane transport phenomena, while and represent the former phenomena only. The composite membrane system is characterized by the large cross coefficient which is comparable in the magnitude to the straight coefficients, i.e., gNa·Cl ≈ gNa·Na ≈ gCl·Cl. Kedem and Katchalsky have predicted that the strong circulation of current is produced by the partial currents through the element membranes even when the total membrane current is absent. However, because of the large cross coefficient, the partial current through each element membrane due to the conjugate force is nearly canceled by that due to the nonconjugate force.

An Analysis of Ball's Empirical Model of Stomatal Conductance

We analysed Ball's empirical model of stomatal conductance (Ball, Woodrow and Berry, 1987). The original interpretation was found to be flawed, and a new one is proposed which views the model as a description of the relationship between CO2 flux rate and stomatal conductance, rather than as a model of stomatal conductance alone. It is shown that this model is useful for describing changes in intercellular CO2 concentration. The model was tested against data from experiments in which the responses of stomatal conductance and CO2 flux density to humidity and temperature were measured in leaves of Hedera helix. It was found that the responses to temperature and humidity are not treated in a satisfactory way in the model whereas the response of the model to other variables is realistic. An alternative to Ball's model is described and tested. It is concluded that Ball's model is a useful starting point for development of simulation models to be used as submodels in canopy and regional models. However, as any empirical model, it is of no use for defining causal relationships.

PH-dependent transepithelial transport of cephalexin in rabbit intestinal mucosa

Previous studies have demonstrated carrier-mediated cephalexin (CPX) transport in rat jejunum under basal conditions. The purpose of this study was to compare fluxes of cephalexin (CPX) and benzylpenicillin (BZP) across rabbit intestine to those reported in rat intestine, to determine whether these fluxes exhibit regioselectivity and to investigate the pH dependence of these fluxes. Fluxes of CPX were examined in rabbit jejunal, ileal and distal colonie mucosa in Ussing chambers. Concurrently, transepithelial (TE) potential difference, TE electrical conductance and mannitol fluxes were determined to evaluate tissue viability and integrity. Mucosal (m)-to-serosal(s) fluxes of CPX were ~ 3-times the s-to-m fluxes (0.66 vs 0.18% h −1 3 cm −2, respectively) in both jejunum and ileum under short-circuit conditions. Distal colonic m-to-s and s-to-m fluxes were ~ 0.18% h −1 3 cm −2. Inhibition of Na +/K +-ATPase activity with ouabain or incubation with the Na +H + exchange inhibitor, amiloride, reduced by 70 and 40%, respectively, the m-to-s fluxes of CPX in the ileum. Effects of changes in luminal pH on TE transport of CPX and mannitol were studied in ileal and colonic mucosa. At a luminal pH of 5.5, ileal m-to-s transport of CPX was increased to 2.85 ± 0.12% h −1 3 cm −2 while mannitol flux and electrical properties were unchanged. In distal colon, pH 5.5 in the luminal bathing solution did not alter CPX or mannitol fluxes. pH dependent carrier-mediated transport was not exhibited by BZP. Thus CPX appears to be absorbed by a pH dependent, carrier-mediated mechanism confined to the small intestine whereas BZP absorption appears to occur by an entirely passive process.

Preparation and properties of thin film YBa 2Cu 3O X devices

Low-noise YBa 2Cu 3O x dc SQUIDs and flux transformers were fabricated by laser deposition. The Josephson junctions were of grain boundary type, formed by epitaxial thin filmes, covering Ar + ion milled step edge substrates. The 5 μm wide dc SQUIDs were patterned by a SiO inhibit technique. The SQUID hole was varied between 5μm ∗10μm and 50μm ∗50μm, representing SQUID inductances between 20 pH and 140 pH. The optimum transfer function δV/δΦ at the appropriate flux bias was greater than 100 μV/Φo at 77K for samples with 20 pH. The best value of the energy resolution for samples with 20 pH is ϵ n,w(f > 30 Hz) = 6 ∗10 −31 J/Hz in the white noise region and dc bias modulation. The energy sensitivity was measured as a function of the SQUID inductance. The coloured noise could be reduced by one order of magnitude applying an ac-bias to the SQUIDs obtaining an energy resolution for samples with 20 pH of 3 ∗10 −30J/Hz at 1 Hz. Noise contributions from conductance fluctuations or flux hopping could be seperated by measuring the equivalent flux noise as a function of the bias current. A preparation technique for thin-film input coils has been developed, in which the three epitaxial layers were patterned either by deposition through shadow masks or by Ar + ion milling. A 12 turn flux transformer with 25 μm wide lines carried a persistent current of about 1 mA for more than two hours. To fabricate a homogeneous multilayer sequence of YBa 2Cu 3O x-SrTiO 3-YBa 2Cu 3O x on a SrTiO 3-substrate, an off axis laser deposition geometry was established, providing smooth films without laser particulates.

Leaf water status and root system water flux of shortleaf pine (Pinus echinata Mill.) seedlings in relation to new root growth after transplanting.

Water relations and root growth of shortleaf pine (Pinus echinata Mill.) were studied four weeks after seedlings from a half-sib family had been transplanted to one of three regimes of soil water availability at a root zone temperature of either 15 or 20 degrees C. About one-third of the variation in new root growth was explained by the root zone environment. The interaction between root zone temperature and soil water availability accounted for 10% of the variation in new root growth. In the most favorable root environment, new roots averaged 620 mm(2) of projected surface area. Leaf water potential increased exponentially with new root projected surface area, becoming constant at about 300 mm(2). Leaf conductance and root system water flux increased linearly with new root growth.

The seasonal course of physiological processes in Juniperus occidentalis

The seasonal course of carbon dioxide assimilation, leaf conductance, transpiration, leaf temperature, xylem pressure potentials, growth, foliage macro- and micronutrient element concentrations, and water- and nitrogen-use efficiencies were measured in juvenile, small-adult, and large-adult western juniper ( Juniperus occidentalis) growing under ambient field conditions in eastern Oregon to explore the timing of, and seasonal limitations on, those processes. Highest daily total assimilation occurred during July and August; over the 15 month period juveniles had significantly higher daily total assimilation and transpiration per gram foliage than small adults or large adults. As soil water became limiting, total daily assimilation of juveniles and small adults declined to levels similar to those of large adults. Juveniles had the highest leaf conductance flux densities in all except three measurement periods, when leaf conductances in all three morphotypes were equally low. Juveniles also had the tightest stomatal control over water loss; however, during periods of low soil water, juveniles had the most negative xylem pressure potentials. Soil drought affected leaf conductance independent of plant water potentials or vapor pressure deficits. Juveniles had the capacity for higher carbon dioxide assimilation over a broader range of stomatal resistances and xylem pressure potentials than did small or large adults. Foliage nitrogen concentrations were usually highest in small adults and large adults. High rates of transpiration may be required to acquire an adequate supply of nitrogen, but nitrogen did not appear to be allocated preferentially to photosynthetic compounds. Potential photosynthetic nitrogen-use efficiences of J. occidentalis were very low compared with other growth forms. Branchlet elongation was greatest in June and July. Juveniles were more responsive to the changing environment than were small adults; large adults had the least seasonal variation in measured proceses.

Electrogenic anion absorption in rabbit distal colon.

The mechanisms of anion transport in the rabbit distal colon were investigated in vitro under short-circuit conditions by examining the effects of transport inhibitors (the stilbene derivatives SITS and DIDS) under a variety of conditions. These agents consistently inhibited Jm-sCl: SITS (10(-3) M) reduced both unidirectional chloride fluxes to the same degree and did not alter JnetCl. In contrast, 10(-4) M DIDS had no effect on Js-mCl and had a significant chloride antiabsorptive effect. DIDS had no effect on either tissue cyclic AMP levels or on basal flux of potassium. The effects of SITS and the cyclic AMP-related secretagogue theophylline on Isc were independent. Additionally, there was no significant alteration of intracellular potential difference or apical membrane fractional resistance elicited by SITS during microelectrode impalement of colonic surface epithelial cells. These results suggest a complex mechanism of anion transport in the distal colon, with a component of electrogenic anion absorption inhibited by the stilbenes. The subsequent changes in current, conductance, and chloride fluxes are dependent upon additional, independent anion transport processes. These pharmacologic agents exhibit an antiabsorptive effect, rather than a stimulation of electrogenic chloride secretion.

Microvillus inclusion disease: In vitro jejunal electrolyte transport

Microvillus inclusion disease is an inherited intestinal brush border membrane defect that causes severe fluid and electrolyte malabsorption. In an infant with microvillus inclusion disease (confirmed by electron microscopic evaluation of rectal, jejunal, and gallbladder mucosae), basal stool output was massive (greater than 125 mL . kg-1 . day-1) and was not altered by treatment with clonidine or octreotide. A proximal jejunostomy with mucous fistula was placed, allowing separation of proximal from distal tract outputs (60 mL . kg-1 . day-1 and 100 mL . kg-1 . day-1, respectively). A 10-cm jejunal segment was excised during surgery and mounted in Ussing chambers for determination of transepithelial Na+ and Cl fluxes. Compared with intestine of normal infants, this infant's epithelium showed transmural conductance and unidirectional ion fluxes that were only 30% of normal. With respect to both Na+ and Cl, the excised jejunum was in a net secretory state. Theophylline (5 mmol/L) increased net Cl secretion slightly. In response to mucosal D-glucose (30 mmol/L), jejunal mucosal-to-serosal Na+ flux doubled. In the infant, glucose-electrolyte solution administered intrajejunally did not significantly change stool output, suggesting that all of the solution (40 mL/kg) was absorbed. Subtotal enterocolectomy, in theory, could have decreased purging by 66% in this infant with microvillus inclusion disease, but diarrhea would still have been significant.

Acid-induced increase in electrical conductance of guinea pig duodenal mucosa in vitro: Temporary protection by combined effects of bicarbonate and prostaglandin E2

Electrical conductance as a sensitive indicator of acid damage has been investigated in guinea pig duodenal mucosa using Ussing-chamber techniques. Reductions of luminal pH from 7.4 to 3.0, 2.3, or 2.0 caused concentration-dependent, progressive increases in conductance, accompanied (pH 2.0) by a continuous increase in hydrogen permeation as determined by pH-stat titration. Increases in conductance and hydrogen flux were related to base-line conductance, with higher values conditioning for a sooner onset and/or more marked elevation. Conductance increases were prevented by timely back titration. Recently, it has been shown that serosal HCO3 reduces conductance by actions dependent on prostaglandins and serosal Na and sensitive to loop diuretics. Here, serosal HCO3 delayed the onset of acid-induced conductance increase by approximately 8 minutes, an effect reduced by omission of serosal Na and during exposure to serosal furosemide (10(-3) mol/L). In the presence of serosal indomethacin (10(-4) mol/L) and HCO3, prostaglandin E2 (10(-6) mol/L serosal bath) delayed the conductance increase. Because HCO3 secretion is negligible in this model, these results indicate effects of HCO3/prostaglandin E2 beyond mere buffering of invading hydrogen. These results are consistent with intracellular actions that tighten the paracellular pathway against acid and thus provide temporary protection from acid injury. In agreement with this view, HCO3 also limited conductance increases after luminal alkalinization by a furosemide-sensitive action.

Errors in the calculation of evaporation and leaf conductance in steady-state porometry: the importance of accurate measurement of leaf temperature

Leaf temperatures of sugar maple leaves (Acersaccharum Marsh.) were measured over three growth seasons using in situ copper–constantan thermocouples with wire and junction diameters of 70 and 130 μm, respectively. These thermocouples were coated with a thin layer of adhesive and attached to the abaxial leaf surface over a length of 4 to 5 cm of wire. On sunny days leaf temperatures usually rose 5 to 15 °C above the air temperature. When leaf conductances and evaporative flux were measured with a Li-Cor steady-state porometer under the same conditions, similar leaf temperatures were rarely measured by the Li-Cor leaf thermocouple. We argue that the in situ thermocouple is more likely to measure the actual leaf temperature than the Li-Cor thermocouple. Consequently, the values for leaf conductance and evaporative flux computed by the Li-Cor microprocessor can be wrong by as much as a factor of 2. Errors of similar magnitude may apply to other published measurements for forest species but are not as large in many crop species.

Water-stress effects on the carbon exchange rates of three upland cotton ( Gossypium hirsutum) cultivars in the field

Net carbon exchange rates (net photosynthetic rate, P n) of the upland cotton ( Gossypium hirsutum L.) cvs Acala GC510, Jaen, and Paymaster 792 were studied in the field at the leaf and canopy levels under three different irrigation regimes. No significant differences were found among cultivars in net leaf P n per unit area as related to leaf conductance and photosynthetic photon flux density ( Q p ∗ ). Maximum leaf P n of about 50 μmol CO 2 m −2s −1 were measured at 2.0 mmol photons m −1 s −1 without full light saturation. Maximum canopy P n per unit ground area was similar to maximum leaf P n, and could be estimated from profiles of the latter. Canopy P n appeared to be related to canopy size, and increased with leaf area index of 3.5 to 4. The three cultivars exhibited similar patterns of midday canopy P n for most of the season. Water stress cause canpy P n to level off earlier in the season and to decline more markedly near the end. Canopy P n showed an hysteretic morning vs. afternoon response to Q p ∗ that seemed to become more obvious with increasing water stress and canopy age. There were marked differences in leaf nitrogen content among water treatments as canopies aged, partially accounting for the observed differences in P n late in the season.

Castor-oil induced diarrhoea in ponies: a model for acute colitis.

A reproducible, reversible model of colitis induced in ponies by administering castor oil (2.5 ml/kg bodyweight [bwt] per os) was characterised by abdominal pain, fever, watery diarrhoea, dehydration, hypovolaemia, toxaemia, leucopenia, decreased serum Cl, Na and K levels and metabolic acidosis. The signs were most severe between 24 and 48 h post induction, stabilisation was frequently observed after 72 h, although diarrhoea could persist beyond 96 h. Morphological and in vitro transport studies (right ventral colon) were conducted on tissues from animals destroyed at 24, 48 and 72 h. In the caecum and colon, surface epithelial disruption and exfoliation from the basement membrane occurred between 24 and 48 h. Early signs of recovery were evident by replenishment of denuded areas with columnar epithelium at 72 h. The crypt epithelium was unaffected throughout the intestinal tract. In vitro transport studies were consistent with the morphological findings. Decreased Na-Cl absorption and normal Cl secretion indicated an impaired surface epithelium coincident with an undamaged cryptal epithelium. Increased mucosal permeability was demonstrated by high ionic conductance and large unidirectional isotopic fluxes. Tissue conductance improved during in vitro incubation suggesting epithelial repair after removal of castor oil. Changes in the population and proportion of bacteria in the faeces as diarrhoea ensued were confirmed at necropsy with a predominance of E. coli and Enterobacter/Klebsiella sp in the large bowel. The experimental induction of castor oil colitis showed many similarities to intestinal endotoxaemia and the secretory type diarrhoea encountered in naturally occurring acute colitis syndromes in horses. The model could prove applicable in studying the pathophysiological mechanisms precipitating such life-threatening disorders.

Characteristics of the paracellular pathway of rabbit cortical collecting duct.

The nature of the paracellular pathway of the rabbit cortical collecting duct (CCD) was examined under conditions designed to eliminate all cellular ion transport. Transcellular conductive pathways were blocked by addition of amiloride and Ba2+ to the perfusate. Cl self-exchange was eliminated by removing Cl from the bath solution, and HCO3 transport was eliminated by omitting HCO3 and CO2 from the solutions. The residual transepithelial conductance (GT) and radioisotopic tracer flux under these conditions probably occur via the paracellular pathway. The GT measured in nontransporting CCD bathed in NaCl solutions was 1.1-1.2 mS/cm2. When Na or Cl was replaced by a less mobile ion, the GT decreased by an amount commensurate with the decrease in solution conductivity. The Na-to-Cl permeability ratio determined by NaCl dilution voltages ranged from 0.55 to 0.82. An independent estimate of paracellular selectivity was obtained by comparing the lumen-to-bath tracer rate coefficients for Na (kNa) and Cl (kCl). The ratio kNa:kCl was 0.75. These observations suggest that the paracellular pathway displays a Na:Cl permselectivity not substantially different from the ratio of their mobilities in water (0.65). Additional experiments demonstrated that the summed partial ionic conductances of Na and Cl calculated from the tracer fluxes were in close agreement with the measured GT. We conclude that the paracellular conductance of CCD is nonselective in character and approximately 1-2 mS/cm2 in magnitude.