Chloride ion transport into pig jejunal brush-border membrane vesicles

Electrophysiological characterization of chloride secretion across the jejunum and colon of pigs as affected by age and weaning

Hypersecretion of chloride can cause diarrhea, a disease frequently occurring in young pigs, particularly around weaning. We investigated the contribution of different channels to intestinal Cl(-) secretion as influenced by age and weaning. Jejunal and colonic epithelia from 4-month-old pigs and 4-week-old piglets were incubated in Ussing chambers and stimulated by carbachol and forskolin. Changes in short-circuit currents were taken as measure of electrogenic net Cl(-) secretion. DIDS or NPPB served to inhibit Ca-activated Cl(-)-channels and outwardly rectifying Cl(-)-channels (ORCC) or cystic fibrosis transmembrane regulator (CFTR), respectively. Depolarizing the basolateral membrane allowed to examine the influence of K(+)-channels on Cl(-) secretion. Forskolin-stimulated Cl(-) secretion was mediated by CFTR. ORCC were not involved. Carbachol-induced Cl(-) secretion could be ascribed to an enhanced driving force due to the opening of K(+)-channels, whereas Ca-dependent Cl(-) channels seemed not to be involved. In jejunum, piglets showed higher Cl(-) secretion than pigs. Two days after weaning forskolin induced an I (sc) overshoot and a faster increase in G (t). In colon, Cl(-) secretion was neither influenced by age nor by weaning. The data suggest a disposition of porcine jejunum for a higher Cl(-) secretion in young and freshly weaned piglets, which might be a natural defense mechanism as well as a predisposing factor for diarrhea.

Cloning a chloride conductance mediator from the apical membrane of porcine ileal enterocytes

Attempts to attribute ileal brush-border chloride conductance to specific proteins were pursued by screening a porcine intestinal cDNA library. A 0.94-kb clone was identified on expression screening with a monoclonal antibody that inhibited enterocyte brush-border chloride conductance. Further screening approaches led to the isolation of a 3.1-kb full-length sequence called pCLCA1, consistent with the identification of a 2.9-kb transcript through Northern analysis. This sequence had significant homology to the CLCA gene family of calcium-regulated chloride channels, especially to hCLCA1. However, a strong A-kinase consensus phosphorylation site in a predicted cytoplasmic loop of the protein was a notable difference from the hCLCA1 gene product. Several porcine exocrine epithelial tissues, including ileum, trachea, and the major salivary glands express pCLCA1 mRNA. In situ hybridization studies localized the expression of pCLCA1 mRNA to the crypt and villus epithelia of porcine ileum, whereas tracheal expression was observed in both surface epithelium and submucosal glands. In situ expression of pCLCA1 in mouse 3T3 cells induces an ionomycin-dependent chloride conductance activity in these cells.

Conductive pathways for chloride and oxalate in rabbit ileal brush-border membrane vesicles

To evaluate the possibility that an apical membrane conductive pathway for oxalate is present in the rabbit distal ileum, we studied oxalate ([14C]oxalate) and chloride (36Cl) uptake into brush-border membrane vesicles enriched 15- to 18-fold in sucrase activity. Voltage-sensitive pathways for oxalate and chloride were identified by the stimulation of uptake provided by an inwardly directed potassium diffusion potential in the presence of valinomycin. Additionally, outwardly directed oxalate (or chloride) gradients stimulated [14C]oxalate (or 36Cl) uptake to a greater degree in the absence of valinomycin (when intracellular and extracellular potassium are equal) than in the presence of valinomycin. Voltage-dependent anion uptake was poorly saturable: apparent affinity constants were 141 +/- 17 and 126 +/- 8 mM for chloride and oxalate, respectively. Activation energies for the voltage-dependent uptake processes were low: 4.7 and 6.3 kcal/mol for chloride and oxalate, respectively. Sensitivity profiles of voltage-dependent chloride and oxalate uptake to anion transport inhibitors were similar. We conclude that an anion conductance is present in the apical membranes of ileal enterocytes and that this conductance is a candidate pathway for oxalate efflux from the enterocyte during transepithelial oxalate secretion.

5-(N,N-Dimethyl)-amiloride to discriminate the Unidirectional electrolyte transports in rat small intestine and proximal colon in vivo

The effect of dimethyl-amiloride (DMA), a selective Na+/H+ exchange blocker, was studied on electrolyte net fluxes and unidirectional fluxes of Na and Cl at four levels of rat intestine in vivo in basal conditions. DMA was applied intraluminally at concentrations of 10(-4) and 10(-3) M in the model of ligated loops prepared from duodenum, proximal jejunum, distal ileum and ascending colon in fasted Sprague Dawley rats. Two iso-osmotic test solutions were used: (1) hypo-ionic: Na+ 80 mM and (2) iso-ionic: Na+ 148 mM, pH 8.2. 22Na was placed in the loop and 36Cl was given by intravenous route at the beginning of the experiment. Na+/H+ was calculated by two different means, one was based on pH variation following amiloride inhibition of Na influx, the other on the calculation of the passive Na transport. The quantitative evaluation shows that Na/H exchange largely contributes to the electroneutral absorption and luminal pH regulation. The exchanger activity decreases from duodenum, jejunum, ileum and colon where it is completed by K/H exchange to assure low colon luminal pH.

Mechanisms of zinc transport into pig small intestine brush-border membrane vesicles

1. The purpose of the present work was to examine certain membrane transport mechanisms likely to carry zinc across the brush-border membrane of pig small intestine, isolated in a vesicular form. 2. In initial velocity conditions, saturation kinetics revealed a great effect of pH on zinc transport: optimal conditions were observed with an intravesicular pH of around 6.6 with or without a H+ gradient; however, this did not allow us to conclude the existence of a neutral exchange between Zn2+ and H+ ions. 3. By measuring 36Cl uptakes, the presence of the Cl(-)-HCO3- or Cl(-)-OH-antiporter with typical 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) sensitivity was detected in vesicles; zinc did not alter this anionic exchange activity. A 65Zn time course, performed in conditions identical with those for 36Cl uptake, was DIDS insensitive and was greatly inhibited by an outward OH- gradient. This could argue against a transport of zinc as a complex with Cl- and HCO3- through the anion antiporter. 4. When external Cl- and HCO3- were replaced by SCN-, able to form a Zn(SCN)4(2-) complex, we observed a stimulating effect of outward HCO3- gradients on 65Zn uptake but neither DIDS nor diphenylamine-2-carboxylate (DPC) inhibited the transport in these conditions. This suggested that the intestinal anion antiporter was not a major route for zinc reabsorption. 5. The tripeptide Gly-Gly-His at low concentrations stimulated 65Zn uptake, then inhibited it in a dose-dependent manner either in the presence of an inward H+ gradient or in the presence of a membrane potential 'negative inside' or in both situations. These conditions are necessary for the active transport of the peptide and this strongly suggests that zinc can be transported as a [Gly-Gly-His-Zn] complex, utilizing the peptide carrier system.

A Cl- conductance sensitive to external pH in the apical membrane of rat duodenal enterocytes

1. The pH dependence of a chloride conductance in the apical membrane of rat duodenal enterocytes was examined. 2. A stepwise reduction of both internal and external pH from 7.4 to 6.8 resulted in a significant stimulation of 36Cl flux driven by an inside-positive membrane potential. 3. A stepwise reduction in pH had no significant effect upon other parameters such as the initial rate of D-[3H]glucose or voltage-independent 36Cl uptake, suggesting a specific effect upon the chloride conductance. 4. The pH-dependent stimulation of 36Cl uptake exhibited saturation kinetics, with an apparent Vmax (maximum velocity) of 5.5 nmol (mg protein)-1 (4 s)-1 and an apparent Km (Michaelis-Menten constant) of 88 nM H+ ions. 5. To determine the site of action of protons upon the conductance the effect of asymmetrically reducing either the internal or external pH was examined. 6. A step reduction of extracellular pH from 7.8 to 6.8 significantly stimulated the rate of 36Cl uptake. In contrast, a step reduction of internal pH from 7.8 to 6.8 was without effect upon the rate of 36Cl uptake. 7. These results suggest that the chloride conductance on the apical membrane of rat duodenal enterocytes is allosterically regulated by protons at an external site.

Inhibition of ileal brush-border chloride conductance by specific antibody

Antibody raised in mice was used in attempting to identify proteins responsible for the conductive chloride transport that can be measured in porcine ileal brush border membrane vesicles. Ileal brush-border membrane vesicle protein from pig was separated into five different molecular mass fractions by preparative SDS polyacrylamide disc gel electrophoresis. Separated protein fractions were used to immunize mice. Antibody was screened for reactivity with antigen by Western blotting, and for effects on conductive chloride transport in ileal brush border membrane vesicles. Immunization with brush-border protein from fraction I proteins (> 110 kDa) produced polyclonal antisera which specifically inhibited the conductive component of chloride uptake by ileal brush border vesicle preparations. Western blotting of the antigen showed the presence of several protein species of molecular mass > 100 kDa that were recognized by immune serum. Spleen cells from a mouse producing antiserum that inhibited conductive chloride transport were fused with a myeloma cell line. The resulting hybridoma colonies produced antibody that reacted with at least seven distinct protein bands by Western blot assay and inhibited chloride conductance in brush-border membrane vesicles.

Effect of covalent modification on the binding of cholera toxin B subunit to ileal brush border surfaces

A competitive binding assay has been developed to determine how modifications to the B subunit of cholera toxin affect the binding affinity of the subunit for an ileal brush border membrane surface. The Ricinus communis120 agglutinin (RCA120) specifically binds to terminal beta-D-galactosyl residues such as those found in oligosaccharide side chains of glycoproteins and ganglioside GM1. Conditions were designed to produce binding competition between the B subunit of cholera toxin and the RCA120 agglutinin. Displacement of RCA120 from brush border surfaces was proportional to the concentration of B subunit added. This assay was used to study the effect of modification of B subunit on competitive binding affinity for the ileal brush border surface. The B subunit of cholera toxin was modified by coupling an average of five sulfhydryl groups to each B subunit molecule and by reaction of the SH-modified B subunit with liposomes containing a surface maleimide group attached to phosphatidylethanolamine. SH-modified B subunit was approximately 200-fold more effective than native B subunit in displacing lectin from brush border surfaces in the competitive binding assay. The enhanced binding activity was retained on covalent attachment of the modified B subunit to the liposome surface. We conclude that the B subunit of cholera toxin may be a useful targeting agent for directing liposomes to cell surfaces that contain a ganglioside GM1 ligand.

Effect of sulfhydryl compounds on ATP-stimulated H+ transport and Cl- uptake in rabbit renal cortical endosomes

The vacuolar H+ ATPase is inhibited by N-ethylmaleimide (NEM), a sulfhydryl compound, suggesting the involvement of a sulfhydryl group in this transport process. We have examined the effects of several sulfhydryl-containing compounds on the vacuolar H+. ATPase of rabbit renal cortical endosomes. A number of such compounds were effective inhibitors of endosomal H+ transport at 10(-5)-10(-6) M, including NEM, mersalyl, aldrithiol, 5,5' dithiobis (2-nitrobenzoic acid), p-chloromercuribenzoic acid (PCMB) and p-chloromercuriphenyl sulfonic acid (PCMBS). NEM, mersalyl, aldrithiol and PCMBS had no effect on pH-gradient dissipation, whereas PCMB decreased the pH gradient faster than control. In the absence of ATP, PCMB (10(-4) M) stimulated endosomal 36Cl- uptake, particularly in the presence of an inside-alkaline pH gradient (pHin = 7.6/pHout = 5.5). This result was not an effect of PCMB on the Cl(-)-conductive pathway. The less permeable PCMBS did not stimulate 36Cl- uptake. The effects of PCMB were concentration dependent and were prevented by dithioerithritol. ATP-dependent 36Cl- uptake was decreased by addition of PCMB. Finally, PCMB had no effect on 45Ca2+ uptake. These results support the presence of two functionally important sulfhydryl groups in this endosomal preparation. One such group is involved with ATP-driven H+ transport and must be located on the cytoplasmic surface of the endosomal membrane. The second sulfhydryl group must reside on the internal surface of the endosomal membrane and relates to a PCMB-activated Cl-/OH- exchanger that is functional both in the presence and absence of ATP. This endosomal transporter is similar to the PCMB-activated Cl-/OH- exchanger recently described in rabbit renal brush-border membranes.

Characterisation of the effects of anthranilic and (indanyloxy) acetic acid derivatives on chloride transport in membrane vesicles

The effects of the Cl- channel blockers, NPPB, IAA94/95 and a number of related compounds on 36Cl- transport in membrane vesicles from bovine kidney cortex and rabbit ileum mucosa brush borders have been studied. These vesicles have been previously shown to be enriched in Cl- channel and Cl-/anion cotransport activity, respectively. Chloride transport was assayed in both types of vesicles by measuring the uptake of 36Cl- in response to an outwardly-directed Cl- concentration gradient. In kidney microsomes, a large proportion of the observed 36Cl- uptake was mediated by an electrogenic uniport and could be substantially reduced by clamping the membrane potential at zero mV using K+ and valinomycin. Chloride uptake was inhibited by both NPPB and IAA94/95 with apparent IC50 values of around 10 microM under optimal conditions (i.e., 4 min uptake at 4 degrees C). Under other conditions (e.g., 10 min uptake at 25 degrees C), where uptake had reached a steady-state level, much higher concentrations of inhibitor were required to cause inhibition. Therefore, previous differences in the reported potency of these compounds may, in part, have been due to the conditions under which Cl- uptake was measured. In addition, both NPPB and, to a lesser extent, IAA94/95 were found to have other effects on the vesicles, in that, when added at a concentration of 100 microM, they induced a leakage of pre-accumulated 36Cl-. This was probably caused by either dissipation of membrane potential or damage to the vesicle membranes. The sulphonic acid derivatives of NPPB and IAA94/95 (NPPB-S and ISA94/95, respectively) blocked 36Cl- uptake with around the same potency as NPPB and IAA94/95, but did not cause any non-specific Cl- leakage, when added at concentrations up to 100 microM. Inhibition of 36Cl- uptake by all four compounds was almost completely reversible. However, when vesicles were incubated with the inhibitors in the presence of an outward Cl- concentration gradient, or if vesicles were freeze/thawed in the presence of the compounds, inhibition could be only partially reversed. In rabbit brush border membrane vesicles, 36Cl- uptake was not reduced when the vesicles were voltage clamped using valinomycin and K+, and was therefore probably mediated by Cl-/Cl- exchange. However, despite the lack of effect of valinomycin, 36Cl- uptake was inhibited by both NPPB (approx. 80% inhibition at 100 microM) and, to a lesser extent, by IAA94/95 (approx. 30% inhibition at 100 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of Na+, Cl- and HCO3- transport by carbachol in pig distal jejunum

Acetylcholine and cholinomimetics such as carbachol are potent stimulants of epithelial Cl- secretion in the small and large intestines of several mammalian species. In this study, the effects of carbachol were characterized in vitro on active ion transport in sheets of submucosa-mucosa from the distal jejunum of swine. Carbachol (10 microM) produced an increase in serosa-positive short-circuit current (Isc) in this tissue after its serosal, but not luminal administration. The Na(+)-K(+)-Cl- transport blocker bumetanide (10 microM) produced a 50% decrease in the carbachol-induced Isc elevation after its serosal administration. Peak increases in Isc evoked by carbachol were significantly reduced by 60-85% in tissues bathed in media lacking Cl-, HCO3-, or both anions. The initial drug-induced increase in net charge transfer from serosa to lumen was dependent upon both HCO3- and Cl-, whereas sustained elevations in charge transfer were dependent upon extracellular Cl- only. Radiotracer flux analyses revealed that the drug decreased net Na+ absorption and increased Cl- secretion. In the absence of HCO3-, carbachol decreased Cl- absorption. The effects of carbachol on HCO3- transport were examined by pH-stat titration. The drug rapidly alkalinized the luminal medium immediately after its serosal administration. These results suggest that carbachol stimulates electrogenic anion secretion in the mucosa of the porcine distal jejunum. Furthermore, the ability of carbachol to inhibit spontaneous Na+ absorption is dependent upon extracellular HCO3-.

Actions of neuropeptide Y on basal, cyclic AMP-induced and neurally evoked ion transport in porcine distal jejunum

Neuropeptide Y (NPY) and its homolog, peptide YY, are present respectively in neurons and endocrine cells within the mammalian small intestine. In this study, we examined the actions of NPY on ion transport in the porcine distal jejunum mucosa-submucosa in vitro. Peptide YY and NPY were equieffective in producing rapid and sustained decreases in basal short-circuit current (Isc), a bioelectrical measure of active ion transport, eliciting half-maximal decreases at respective serosal concentrations of 0.8 and 30 nmol/l. NPY-induced changes in Isc were due to increased mucosa-to-serosa and net Cl fluxes and were not affected by the absence of extracellular HCO3 ions. NPY activity was correlated with the magnitude of the basal Isc and appeared to depend on the spontaneous production of eicosanoids. The peptide also decreased Isc stimulated by forskolin and 8-bromo-cyclic AMP, but the ionic bases for this effect were complex and differed from those determined under basal conditions. NPY attenuated increases in Isc produced by electrical stimulation of enteric neurons with an IC50 = 5 nmol/l. The actions of the peptide on basal and cyclic AMP-induced ion transport were abolished by the neuronal conduction blocker tetrodotoxin, but not by the opiate antagonist naloxone. The alpha-adrenoceptor blocker phentolamine diminished the effects of NPY on basal, but not cyclic AMP-induced Isc. These results indicate that NPY is capable of modulating NaCl transport in the porcine jejunal mucosa under several different conditions. Furthermore, the effects of the peptide are mediated in part through noradrenergic nerves as well as enteric neurons of unknown chemical identity.

Inhibiting conductive Cl uptake in membrane vesicles: specificity of alpha-phenylcinnamate

alpha-Phenylcinnamate has been investigated in comparison to other inhibitors of chloride ion transport into porcine jejunal brush-border membrane vesicles. The transport modes studied included uptake driven only by a chemical Cl gradient, Cl uptake dependent on a transmembrane potential, self-exchange of Cl with no chemical or potential gradient, and Cl uptake dependent on a chemical gradient for bicarbonate. Uptake driven by the chemical gradient for Cl was strongly inhibited by millimolar concentrations of diphenylamine-2-carboxylate, 5-nitro-2-(2-phenylethylamino)benzoate (NPEB), and to a lesser extent by 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate (SITS). Similar concentrations of alpha-phenylcinnamate did not reduce this mode of Cl uptake. Conductive Cl uptake driven by a potassium gradient was inhibited by approx. 50% at 2.5 mM alpha-phenylcinnamate. alpha-Phenylcinnamate was equally effective in reducing the initial rate of conductive chloride accumulation in vesicles with naturally opened Cl channels (conductance activation by cyclic AMP and Ca2+), or with Cl channels opened by exposure to tetramethylammonium (TMA) buffer. In comparison with diphenylamine-2-carboxylate, NPEB and SITS, alpha-phenylcinnamate had the least effect on Cl-HCO3 exchange at inhibitor concentrations which reduced conductance activity. Self-exchange rates of physiological concentrations of Cl were also relatively unaffected by low mM concentrations of alpha-phenylcinnamate. Kinetic analysis indicated that alpha-phenylcinnamate was an uncompetitive inhibitor, requiring the presence of the normal Cl ligand for binding to, and inhibition of, conductive Cl transport by pig intestinal brush-border vesicles.

Activation of chloride conductance in pig jejunal brush border vesicles

Requirements for the activation of Cl- conductance have been investigated in pig jejunal brush border vesicles. The stability of ATP as a substrate for protein kinase activity, the stability of the phosphoprotein product of protein kinase action, and the choice of buffer system used for vesicle preparation were studied as variables which affected the outcome of in vitro activation attempts. Arsenate was selected as the most effective agent in protecting ATP from hydrolysis by the phosphatase activity in this vesicle system. Brush border vesicle protein appeared to prevent the accumulation of phosphoprotein in a cAMP-dependent protein kinase reaction, and vesicle protein only had phosphate acceptor activity when KF was added as a presumptive inhibitor of phosphoprotein phosphatase. A Cl- conductance response to a potassium gradient and valinomycin was present in vesicles prepared in buffers containing tetramethylammonium. Cl- conductance activity was not increased in this system by the addition of ATP, dibutyryl cyclic AMP, and cyclic AMP-dependent protein kinase. There was no Cl conductance response to a potassium gradient in vesicles buffered with imidazolium-acetate. Incorporation of ATP, AsO4(3-), and F- into these nonconductive vesicles by homogenization, followed by addition of dibutyryl cAMP, produced substantial conductance activity. Maximal activation of Cl- conductance was obtained with vesicles prepared in imidazolium-acetate buffering, using precautions to stabilize ATP and phosphoprotein prior to conductance measurements.