Involvement of multiple sodium ions in intestinal d-glucose transport

Lower Rate of Water Absorption of an Oral Rehydration Solution with Partially Hydrolyzed Guar Gum in Conscious Rats

The purpose of the present study was to elucidate the rate of water absorption of an oral rehydration solution containing partially hydrolyzed guar gum (PHGG) in the small intestine, which is the main site of water absorption when water is drunk. Seven-week-old male SD rats were employed. We prepared four types of an aqueous solution, i.e., containing mineral and sugar, containing PHGG, containing mineral and sugar and PHGG, or containing no additives. After 24 h of food and 4 h of water deprivation, the aqueous solutions were infused into the stomach of conscious rats on their hands using a syringe with a stomach sonde. We sampled the stomach and the small intestine with contents 8 min after the infusions. Causal effects were calculated using a Bayesian network. PHGG increased the residual amount of water in the gastro-intestine, which depends negatively on the absorption of water in the small intestine/the flow rate to the small intestine. The absorption of water in the small intestine depended positively on the flow rate to the small intestine, which depended negatively on the free water in the solutions. PHGG decreased water absorption in the small intestine by decreasing the free water in the aqueous solutions.

Copper chloride inhibits brush border membrane enzymes, alters antioxidant and metabolic status and damages DNA in rat intestine: a dose-dependent study

Copper (Cu) is an extensively used heavy metal and an indispensible micronutrient for living beings. However, Cu is also toxic and exerts multiple adverse health effects when humans are exposed to high levels of this metal. We have examined the effect of single acute oral dose of copper chloride (CuCl₂) on parameters of oxidative stress, cellular metabolism, membrane and DNA damage in rat intestine. Adult male Wistar rats were divided into four groups and separately administered a single oral dose of 5, 15, 30 and 40 mg CuCl₂/kg body weight. Rats not administered CuCl₂ served as the control. Oral administration of CuCl₂ led to significant alterations in the activities of metabolic and membrane-bound enzymes; brush border enzymes were inhibited by 45-75% relative to the control set. Inhibition of antioxidant enzymes diminished the metal-reducing and free radical quenching ability of the cells. Oxidative damage caused cellular oxidation of thiols, proteins and lipids. Diphenylamine and comet assays showed that CuCl₂ treatment enhanced DNA damage while DNA-protein crosslinking was also increased in the intestinal cells. Examination of stained sections showed that CuCl₂ treatment led to marked histological changes in the intestine. All the changes seen were in a CuCl₂ dose-dependent manner with more prominent alterations at higher doses of CuCl₂. These results clearly show that oral administration of CuCl₂ results in oxidative damage to the intestine which can impair its digestive and absorptive functions.

Lymphocytic colitis associated with lansoprazole treatment

INTRODUCTION

There have been several reported cases of lansoprazole-associated collagenous colitis (CC) reported in the literature but only 1 reported case of lansoprazole-associated lymphocytic colitis (LC) in the literature. Both CC and LC are considered inflammatory bowel diseases, but they are distinctly classified based on the condition of the colon, which is typically confirmed through biopsy.

CASE SUMMARIES

A 52-year-old white male (Patient 1), with a height of 178 cm and weight of 75 kg, presented to Gazi University Hospital, Ankara, Turkey, with a 3-month history of abdominal pain and nonbloody, watery diarrhea. The patient reported receiving PO lansoprazole 30 mg/d to treat heartburn ~1 week prior to the onset of diarrhea. The patient's medical history revealed that he did not have any preexisting conditions, other than gastroesophageal reflux disease (GERD) for which lansoprazole was prescribed. The medical history report also revealed that the patient was not receiving any concomitant medications or treatments at the time. A colon biopsy confirmed LC. Additionally, a 43-year-old white female (Patient 2), with a height of 168 cm and weight of 61 kg, presented to the same facility with a 6-month history of nonbloody, watery diarrhea and mild lower abdominal cramping. The patient reported that initial onset began ~2 months after receiving a 10-day Helicobacter pylori eradication combination treatment regimen that included lansoprazole, amoxicillin, and clarithromycin, followed by lansoprazole monotherapy to treat GERD. The patient's medical history revealed no other concomitant medications were being adminstered at the time. A colon biopsy confirmed LC.

DISCUSSION

A search of the literature using the MEDLINE database and all relevant English-language articles with key words lansoprazole and lymphocytic colitis, found that there were several cases of lansoprazole-associated CC reported and 1 reported case of lansoprazole-associated LC. Histologic findings from laboratory tests and colon biopsies confirmed diagnoses of LC in both patients in this case report. Patient 1 presented with diarrhea and cramping, which the patient reported had been ongoing for ~3 months, following lansoprazole administration. However, after lansoprazole was discontinued, the symptoms completely resolved within 7 days. Patient 2 presented with diarrhea and cramping, which had been occurring for ~6 months. That patient reported that initial onset commenced ~2 months after a 10-day H pylori eradication combination treatment regimen that included lansoprazole, amoxicillin, and clarithromycin, followed by lansoprazole monotherapy to treat GERD. However, after sulfasalazine (3 g/d) was prescribed for 2 months immediately upon diagnosis of LC, there was little improvement in the effort to control the diarrhea in this patient. After omeprazole 20 mg/d was substituted for lansoprazole, the patient's diarrhea ceased. Follow-up sigmoidoscopy 2 months later revealed normal mucosa and complete normalization of histologic findings. The patient remains diarrhea-free while on omeprazole. A causality assessment using the Naranjo adverse reaction algorithm produced scores of 6 for both patients, suggesting that LC was probably associated with lansoprazole treatment.

CONCLUSIONS

Here we report 2 cases of LC in patients probably associated with the administration of lansoprazole treatment. Complete remission occurred after lansoprazole was discontinued.

Effect of glutamine on water and sodium absorption in human jejunum at baseline and during PGE1-induced secretion

Glutamine, a major fuel for enterocytes, stimulates water and sodium absorption in animal models of secretory diarrhea, but data in humans are still limited. The aim of this study was to investigate the effect of glutamine on jejunal absorption during hypersecretion in humans. In six healthy adults, the effects of glutamine on jejunal absorption were assessed with a triple-lumen tube on two occasions, at baseline and during PGE1-induced hypersecretion (0.1 μg·kg−1·min−1) in a random order. Isoosmolar solutions containing polyethylene glycol 4000 as nonabsorbable marker were infused in the jejunum at 10 ml/min over 1-h periods: saline (sodium chloride 308 mmol/l), glucose-mannitol 45:45 mM, glucose 90 mM, alanine-glucose 45:45 mM, glutamine-glucose 45:45 mM, and glutamine 90 mM. Net absorptive and secretory fluxes were measured at steady state. At baseline, glutamine- and alanine-containing solutions induced a threefold increase of water and sodium absorption ( P < 0.05); 90 mM glutamine stimulated water absorption more than 90 mM glucose (3.6 ± 0.6 vs. 1.9 ± 0.3 ml·min−1·30 cm−1, P < 0.05). PGE1-induced hypersecretion was reduced ( P < 0.05) by solutions of alanine-glucose, glutamine-glucose, and glutamine 90 mM ( P < 0.05) and reversed to absorption by alanine-glucose and glutamine-glucose. Glutamine and alanine absorption was nearly complete and was not influenced by PGE1. In conclusion, glutamine stimulates water and electrolyte absorption in human jejunum, even during experimental hypersecretion. In addition to the metabolic effects of glutamine, these results support the evaluation of glutamine-containing solutions for the rehydration and the nutritional support of patients with secretory diarrhea.

Diarrhea associated with lansoprazole

Lansoprazole is a proton pump inhibitor widely prescribed for gastroesophageal reflux and benign peptic ulcer disease. According to the manufacturer's package insert (TAP Pharmaceuticals, Lake Forest, IL, USA), the most common side-effects are diarrhea, headache and abdominal pain, which occur in approximately 3% of patients and are reversible with drug discontinuation. An unusual case of microscopic colitis is reported in a previously asymptomatic patient who developed new-onset diarrhea after initiation of lansoprazole. The case is reviewed and possible mechanisms of diarrhea secondary to proton pump inhibitors are discussed.

Intestinal transport of beta-thioglycosides by Na+/glucose cotransporter

Intestinal metabolism and transport of p-nitrophenyl beta-D-thioglucoside (p-NPbetaSglc) and p-nitrophenyl beta-D-thiogalactoside (p-NPbetaSgal) by the Na+/glucose cotransporter were studied in excised small intestine of the rat. p-NPbetaSglc and p-NPbetaSgal were stable enough on the mucosal side to be transported to the serosal side. Transport of p-NPbetaSglc was inhibited in the presence of phloridzin (a Na+/glucose cotransporter inhibitor), glucose, or 3-O-methylglucose (3-OMG). p-NPbetaSglc transport was dependent on Na+ concentration in a sigmoidal manner. The activation energy for transport was 19.4 kcal mol(-1). The distribution of transport activity of p-NPbetaSglc in each region of the small intestine correlated well with that of 3-OMG. These results indicate that p-NPbetaSglc is transported by the Na+/glucose cotransporter in small intestine. The order of turnover rate for glycoside transport by Na+/glucose cotransporter was 3-OMG > p-nitrophenyl beta-O-glucoside > p-NPbetaSglc > p-NPbetaSgal, indicating that the presence of a galactose moiety and a sulphur between the monosaccharide moiety and aglycone decreases the turnover rate of the Na+/glucose cotransporter in the transport of glycosides. In an inhibition study using stable p-NPbetaSglc as a Na+/glucose cotransporter-transportable marker glucoside, it was also shown that the Na+/glucose cotransporter recognized several types of glycosides. In conclusion, displacement of the oxygen at carbon C-1 of glucose by sulphur in thioglycosides decreases the turnover rate of the Na+/glucose cotransporter, but thioglycosides are stable in the small intestine and are transported by the Na+/glucose cotransporter.

Enhancement of intestinal water absorption and sodium transport by glycerol in rats

Glycerol (Gly) is a hydrophilic, absorbable, and energy-rich solute that could make water absorption more efficient. We investigated the use of Gly in a high-energy beverage containing corn syrup (CS) by using a small intestine perfusion procedure in the rat, an approach shown earlier to provide good preclinical information. The effectiveness of several formulations with Gly and CS was compared with commercial products and to experimental formulas where Gly substituted for glucose (Glc). The CS-Gly combination was more effective than preparations on the market containing sucrose and Glc-fructose syrups (G-P and G-L, respectively) in maintaining a net water absorption balance in the test jejunal segment [CS-Gly = 0.21 +/- 0.226, G-L = -1.516 +/- 0.467, and G-P = -0.299 +/- 0.106 (SE) microliter.min-1.cm-1 (P = 0.0113)] and in reducing sodium release into the lumen [CS-Gly = -133.2 +/- 16.2, G-L = -226.7 +/- 25.2, and G-P = -245.6 +/- 23.4 nmol.min-1.cm-1 (P = 0.0022)]. In other preparations, at equal CS concentrations (60 and 80 g/l, respectively), Gly clearly improved net water absorption over a comparable Glc-containing product [CS60-Gly = 0.422 +/- 0.136 and CS80-Gly = 0.666 +/- 0.378 vs. CS60-Glc = -0.282 +/- 0.200 and CS80-Glc = -1.046 +/- 0.480 microliters.min-1.cm-1 (P = 0.0019)]. On the basis of the data of this rat intestine perfusion model, Gly could be a useful ingredient in energy-rich beverages and might enhance fluid absorption in humans.

Function and presumed molecular structure of Na(+)-D-glucose cotransport systems

Functional characterization of Na(+)-D-glucose cotransport in intestine and kidney indicates the existence of heterogeneous Na(+)-D-glucose cotransport systems. Target size analysis of the transporting unit and model analysis of substrate binding have been performed and proteins have been cloned which mediate (SGLT1) and modulate (RS1) the expression of Na(+)-D-glucose cotransport. The experiments support the hypothesis that functional Na(+)-D-glucose cotransport systems in mammals are composed of two SGLT1-type subunits and may contain one or two RS1-type proteins. SGLT1 contains up to twelve membrane-spanning alpha-helices, whereas RS1 is a hydrophilic extracellular protein which is anchored in the brush-border membrane by a hydrophobic alpha-helix at the C-terminus. SGLT1 alone is able to translocate glucose together with sodium; however, RS1 increases the Vmax of transport expressed by SGLT1. In addition, the biphasic glucose dependence of transport, which is typical for kidney and has been often observed in intestine, was only obtained after coexpression of SGLT1 and RS1.

Effect of zinc on L-threonine transport across the jejunum of rabbit

Zinc is an essential trace element for life. Many metalloenzymes involved in the metabolism of carbohydrates, lipids, protein, and nucleic acids require zinc for their functions. The aim of this study was to characterize how zinc acts on the intestinal amino acid absorption in rabbit. Results obtained show that zinc inhibits both L-threonine accumulation in the jejunum tissue, and mucosal-to-serosal transepithelial flux of this amino acid in a dose-dependent way. The inhibition does not increase by a 10-min previous intestinal exposure of the mucosa to the heavy metal, and is not reversed by washing the intestinal tissue with saline solution or 10mM EDTA, but is appreciably reversed with 10mM dithioerythritol. Zinc seems not to modify amino acid diffusion across the intestinal epithelium. The inhibition of intestinal amino acid transport by zinc seems to be of a competitive type, and appears to be a result of impairment of the active transport that is altered by its binding to proteins (prevailing to thiol groups) of the brush-border membrane of enterocytes.

Allosterism and Na(+)-D-glucose cotransport kinetics in rabbit jejunal vesicles: compatibility with mixed positive and negative cooperativities in a homo- dimeric or tetrameric structure and experimental evidence for only one transport protein involved

We first present two simple dimeric models of cotransport that may account for all of the kinetics of Na(+)-D-glucose cotransport published so far in the small intestine. Both the sigmoidicity in the Na+ activation of transport (positive cooperativity) and the upward deviations from linearity in the Eadie-Hofstee plots relative to glucose concentrations (negative cooperativity) can be rationalized within the concept of allosteric kinetic mechanisms corresponding to either of two models involving sequential or mixed concerted and sequential conformational changes. Such models also allow for 2 Na+: 1 S and 1 Na+: 1 S stoichiometries of cotransport at low and high substrate concentrations, respectively, and for partial inhibition by inhibitors or substrate analogues. Moreover, it is shown that the dimeric models may present physiological advantages over the seemingly admitted hypothesis of two different cotransporters in that tissue. We next address the reevaluation of Na(+)-D-glucose cotransport kinetics in rabbit intestinal brush border membrane vesicles using stable membrane preparations, a dynamic approach with the Fast Sampling Rapid Filtration Apparatus (FSRFA), and both nonlinear regression and statistical analyses. Under different conditions of temperatures, Na+ concentrations, and membrane potentials clamped using two different techniques, we demonstrate that our data can be fully accounted for by the presence of only one carrier in rabbit jejunal brush border membranes since transport kinetics relative to glucose concentrations satisfy simple Michaelis-Menten kinetics. Although supporting a monomeric structure of the cotransporter, such a conclusion would conflict with previous kinetic data and more recent studies implying a polymeric structure of the carrier protein. We thus consider a number of alternatives trying to reconcile the observation of Michaelis-Menten kinetics with allosteric mechanisms of cotransport associated with both positive and negative cooperativities for Na+ and glucose binding, respectively. Such models, implying energy storage and release steps through conformational changes associated with ligand binding to an allosteric protein, provide a rational hypothesis to understand the long-time debated question of energy transduction from the Na+ electrochemical gradient to the transporter.

Blood and urine responses to ingesting fluids of various salt and glucose concentrations

Several hours before returning to Earth, Space Shuttle astronauts consume fluid and salt tablets equivalent to a liter of 0.9% saline as a countermeasure to postflight orthostatic intolerance. This countermeasure is not completely successful. Therefore, in search of a countermeasure that would protect against orthostatic intolerance better and for a longer duration, the authors compared the blood and urine responses of five men (21-41 yr) after they drank 1 L of 0.9% saline to their responses after drinking five other solutions: distilled water, 1% glucose, 0.74% saline with 1% glucose, 0.9% saline with 1% glucose, and 1.07% saline. Each subject ingested a different solution on 6 different days and remained seated for the ensuing 4 hours. Heart rate, blood pressures, and urine variables were measured before ingestion of the fluids and every 30 minutes thereafter; blood samples were drawn before, immediately after, and every 60 minutes after ingestion. Change in plasma volume, which was estimated from hemoglobin and hematocrit, was considered the most critical variable. Data for all solutions were compared by analysis of variance. Since plasma volume was increased most after ingestion of 1.07% saline, all variables (at 2 hours, at 3 hours and at 4 hours) were compared between 1.07% saline and 0.9% saline, the current countermeasure. Plasma volume was increased more after 1.07% saline than after 0.9% saline, and this difference was most significant at 4 hours after ingestion (P = .056). Diuresis occurred promptly after ingestion of the two saline-free solutions, water and 1% glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

The transport of electrolytes in the gut and the use of oral rehydration solutions

This presentation summarizes the recent advances in knowledge of the physiologic and pathophysiologic process underlying the transport of electrolytes across the small intestine and formulate a rationale for the use of oral electrolyte solutions in diarrheal diseases.

Monoclonal antibodies that bind the renal Na+/glucose symport system. 1. Identification

Phlorizin is a specific, high-affinity ligand that binds the active site of the Na+/glucose symporter by a Na+-dependent mechanism but is not itself transported across the membrane. We have isolated a panel of monoclonal antibodies that influence high-affinity, Na+-dependent phlorizin binding to pig renal brush border membranes. Antibodies were derived after immunization of mice either with highly purified renal brush border membranes or with apical membranes purified from LLC-PK1, a cell line of pig renal proximal tubule origin. Antibody 11A3D6, an IgG2b, reproducibly stimulated Na+-dependent phlorizin binding whereas antibody 18H10B12, an IgM, strongly inhibited specific binding. These effects were maximal after 30-min incubation and exhibited saturation at increased antibody concentrations. Antibodies did not affect Na+-dependent sugar uptake in vesicles but significantly prevented transport inhibition by bound phlorizin. Antibodies recognized a 75-kDa antigen identified by Western blot analysis of brush border membranes, and a 75-kDa membrane protein could be immunoprecipitated by 18H10B12. These properties, taken together with results in the following paper [Wu, J.-S.R., & Lever, J.E. (1987) Biochemistry (following paper in this issue)], provide compelling evidence that the 75-kDa antigen recognized by these antibodies is a component of the renal Na+/glucose symporter.

Current-voltage relations of sodium-coupled sugar transport across the apical membrane of Necturus small intestine

The current-voltage (I-V) relations of the rheogenic Na-sugar cotransport mechanism at the apical membrane of Necturus small intestine were determined from the relations between the electrical potential difference across the apical membrane, psi mc, and that across the entire epithelium, psi ms, when the latter was varied over the range +/- 200 mV, under steady conditions in the presence of galactose and after the current across the apical membrane carried by the cotransporter, ImSNa, is blocked by the addition of phloridzin to the mucosal solution. ImSNa was found to be strongly dependent upon psi mc over the range -50 mV less than psi mc less than EmSNa where EmSNa is the "zero current" or "reversal" potential. Over the range of values of psi mc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so that ImSNa = gmSNa (EmSNa - psi mc) where gmSNa is the contribution of this mechanism to the conductance of the apical membrane and is "near constant." In several instances ImSNa "saturated" at large hyperpolarizing or depolarizing values of psi mc. The values of EmSNa determined in the presence of 1, 5, and 15 mM galactose strongly suggest that if the Na-galactose cotransporters are kinetically homogeneous, the stoichiometry of this coupled process is unity. Finally, the shapes of the observed I-V relations are consistent with the predictions of a simple kinetic model which conforms with current notions regarding the mechanico-kinetic properties of this cotransport process.

The potential dependence of the intestinal Na+-dependent sugar transporter

The unidirectional influx of the lipophilic cation tetraphenylphosphonium (TPP+) into isolated intestinal epithelial cells exhibits a marked dependence on the membrane potential (delta psi) maintained by or experimentally imposed on these cells. By taking advantage of this fact, we have described a "crossover" procedure that allows the relative permeability of a cation and anion pair to be determined. Measurements of such relative permeabilities permits diffusion potentials of defined magnitude to be imposed across the plasma membrane of ATP-depleted cells. This in turn allows description of the relationship between [14C]TPP+ influx and delta psi. We have determined that the flux-potential relationship is that predicted by the Goldman flux equation. Using this relationship as a calibration tool for delta psi, we then determined the quantitative relationship between membrane potential and the Na+-dependent influx of an actively transported sugar, alpha-methylglucoside (alpha-MG). The influx of [14C]alpha-MG also shows an exponential dependence on delta psi although it is more sharply potential dependent than that shown by TPP+. The specific relationship is consistent with that expected for a system with 2:1 Na+ stoichiometry which obeys the potential dependence predicted by Eyring rate theory with a single energy barrier occurring near the midpoint of the membrane. Over the range of potentials from +33 to -61 mV, we find no evidence for a minimum or threshold potential necessary to support transport and no evidence for an optimal potential that can maximize sugar transport. The data raise the possibility for using either [14C]TPP+ or [14C]alpha-MG influx as the basis for a new noninvasive procedure for measurement of delta psi.

Effects of 1 alpha,25-dihydroxycholecalciferol on sodium-ion translocation across chick intestinal brush-border membrane

By utilizing isolated brush-border vesicles, Na+ transport across the luminal membrane of chick small intestine was found to be a composite of (i) a saturable (Km 10mM-Na+) amiloride-sensitive Na+/H+ antiport and (ii) a potential-sensitive conductive pathway. No evidence was obtained for the existence of a Na+/Cl- symport system. With the exception of the duodenum, luminal Na+ transfer in the entire small intestine was subject to regulation by vitamin D. Repletion of vitamin D-deficient chicks with 1 alpha,25-dihydroxycholecalciferol [1 alpha,25(OH)2D3] significantly decreased net Na+ uptake by isolated membrane vesicles (by approximately 30%). The sterol suppresses the conductive pathway (25-45% inhibition) as well as the Na+/H+ antiport system. Kinetic analysis of the latter revealed that 1 alpha,25(OH)2D3 altered Vmax (from 12.9 to 4.8 nmol of Na+/20s per mg of protein), but did not change Km. Diminution of Na+ transfer, entailing an increase in the electrochemical transmembrane Na+ gradient, provides an explanation of the simultaneously observed stimulatory action of 1 alpha,25(OH)2D3 on Na+-gradient-driven solute transport in chick small intestine. Indirect evidence was obtained that the luminal plasma membrane of chick small intestine displays a definite H+ permeability that is positively affected by 1 alpha,25(OH)2D3.

Kinetic analysis of mechanism of intestinal Na+-dependent sugar transport

Zero-trans kinetics of Na+-sugar cotransport were investigated. Sugar influx was measured at various sodium and sugar concentrations in K+-loaded cells treated with rotenone and valinomycin. Sugar influx follows Michaelis-Menten kinetics as a function of sugar concentration but not as a function of Na+ concentration. Nine models with 1:1 or 2:1 sodium:sugar stoichiometry were considered. The flux equations for these models were solved assuming steady-state distribution of carrier forms and that translocation across the membrane is rate limiting. Classical enzyme kinetic methods and a least-squares fit of flux equations to the experimental data were used to assess the fit of the different models. Four models can be discarded on this basis. Of the remaining models, we discard two on the basis of the trans sodium dependence and the coupling stoichiometry [G. A. Kimmich and J. Randles, Am. J. Physiol. 247 (Cell Physiol. 16): C74-C82, 1984]. The remaining models are terter ordered mechanisms with sodium debinding first at the trans side. If transfer across the membrane is rate limiting, the binding order can be determined to be sodium:sugar:sodium.

Sodium-sugar coupling stoichiometry in chick intestinal cells

Sodium-dependent sugar transport systems involve the function of membrane components that couple the transmembrane flow of Na+ to the concomitant flow of certain sugar molecules. The coupling stoichiometry between Na+ and sugar fluxes via these systems must be measured under conditions in which the membrane potential does not change due to the induction of transport or during the interval of flux measurement. This can be accomplished by utilizing gradients of highly permeant ions (NO-3 and K+ plus valinomycin) to create diffusion potentials of sufficient magnitude that the sugar-induced Na+ flux does not introduce an appreciable change in the imposed potential. Under these conditions, the coupling stoichiometry for chicken intestinal cells proves to be 2 Na+:1 sugar as reported earlier for studies performed in the absence of a membrane potential. When control of the potential is not maintained, a coupling ratio of 1:1 is observed. The stoichiometry does not change as a function of Na+ concentration, which suggests that carrier forms with only one Na+ bound do not contribute to the carrier-mediated Na+ or sugar fluxes. When no potential is present, the stoichiometry is modified by the level of intracellular Na+ and sugar in a manner indicative of a transport mechanism in which Na+ must dissociate from the "loaded" carrier at the inward facing membrane surface before the sugar molecule dissociates.

Ion permeability of rabbit intestinal brush border membrane vesicles

The ion permeability of rabbit jejunal brush border membrane vesicles was studied by measuring unidirectional fluxes with radioactive tracers and bi-ionic diffusion potentials with the potential-sensitive fluorescent dye, diS-C3-(5). Tracer measurements provide estimates of the absolute magnitudes of permeability coefficients, while fluorescence measurements provide estimates of relative and absolute ion permeabilities. The magnitudes of the permeability coefficients for Na+, K+, Rb+, and Br- were approximately 5 nanoliters/(mg protein X sec) or 10(-5) cm/sec as determined by radioactive tracer measurements. The apparent selectivity sequence, relative to Na+, as determined by bi-ionic potential measurements was: F-, isethionate, gluconate, choline (less than 0.1) less than Na+(1.0) less than Cl-(1.5) = NO-3(1.5) less than Br-(2.3) less than K+(2.4) less than Rb+(2.5) less than Cs+(2.6) less than Li+(3.9) less than NH+4(12) less than I-(40). The origin of this selectivity sequence and its relationship to the ion permeability of the brush border membrane in the intact epithelium are discussed.

Conformational changes in the intestinal brush border sodium-glucose cotransporter labeled with fluorescein isothiocyanate

Fluorescein isothiocyanate (FITC) was used to label the rabbit intestinal brush border Na+-glucose carrier, identify the carrier protein on sodium dodecyl sulfate/polyacrylamide gel electrophoresis, and monitor the effect of ions and substrates on fluorescence quenching. Enriched brush border preparations were employed to study both glucose transport and FITC binding. FITC and a nonfluorescent analog (phenyl isothiocyanate, PITC) both inhibited Na+-dependent D-glucose transport irreversibly. Inhibition was blocked completely by the presence of Na+ and D-glucose during labeling. PITC was used to label nonspecific amino groups in the presence of glucose and Na+, and then the glucose carrier was labeled with FITC in the absence of substrates. Fluorescence of FITC bound to the carrier was quenched specifically with Na+ in a saturable fashion, and this indicates a Na+-dependent conformational change in the carrier. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of FITC-labeled membranes revealed specific labeling of a 71,000-dalton peptide. We conclude that Na+ induces a conformational shift in the 71,000-dalton glucose carrier, and this is quite consistent with the kinetics of Na+-dependent glucose transport in these membranes.

Demonstration of sodium-dependent, electrogenic substrate transport in rat small intestinal brush border membrane vesicles by a cyanine dye

The cyanine dye DiS-C2(5) was tested as an indicator for changes in membrane potential of subfractionated rat jejunal brush border membrane vesicles. The fluorescence of this dye increased with inside positive and decreased with inside negative potentials. The sensitivity to inside negative potentials was greater than to inside positive potentials. The addition of L-alanine, L-phenylalanine, L-methionine, D-galactose and D-glucose in the presence of sodium provoked a transient fluorescence increase indicating an inside positive membrane potential due to electrogenic, sodium-coupled transport. Besides the sodium-dependence, the dye reflected stereo-specificity and saturability of D-glucose transport. When D-glucose loaded vesicles were incubated in D-glucose-free medium, a decrease in fluorescence was observed indicating that D-glucose efflux is also electrogenic.

Kinetics of sodium D-glucose cotransport in bovine intestinal brush border vesicles

Brush border membrane vesicles ( BBMV ) purified from steer jejunum were used to study the kinetics of sodium D-glucose cotransport under voltage clamped, zero-trans conditions. When the initial rate of glucose transport ( Jgluc ) was measured over a wide range of glucose concentrations ([S] = 0.01-20 mM), curvature of the Woolf - Augustinsson -Hofstee plots was seen, compatible with a diffusional and one major, high capacity (maximal transport rate Jmax = 5.8-8.8 nmol/mg X min) saturable system. Further studies indicated that changes in cis [Na] altered the Kt, but not the Jmax, suggesting the presence of a rapid-equilibrium, ordered bireactant system with sodium adding first. Trans sodium inhibited Jgluc hyperbolically, KCl-valinomycin diffusion potentials, inner membrane face positive, lowered Jgluc , while potentials of the opposite polarity raise Jgluc . At low glucose concentrations ([S] less than 0.05 mM), a second, minor, high affinity transport system was indicated. Further evidence for this second saturable system was provided by sodium activation curves, which were hyperbolic when [S] = 0.5 mM, but were sigmoidal when [S] = 0.01 mM. Simultaneous fluxes of 22Na and [3H]glucose at 1 mM glucose and 30 mM NaCl yielded a cotransport-dependent flux ratio of 2:1 sodium/glucose, suggestive of 1:1 (Na/glucose) high capacity, low affinity system and a approximately 3:1 (Na/glucose) high affinity, low capacity system. Kinetic experiments with rabbit jejunal brush borders revealed two major Na-dependent saturable systems. Extravesicular (cis) Na changed the Kt, but not the Jmax of the major system.

Na+, Li+, and Cl- transport by brush border membranes from rabbit jejunum

Na+, Li+, K+, Rb+, Br-, Cl- and SO4(2-) transport were studied in brush border membrane vesicles isolated from rabbit jejunum. Li+ uptakes were measured by flameless atomic absorption spectroscopy, and all others were measured using isotopic flux and liquid scintillation counting. All uptakes were performed with a rapid filtration procedure. A method is presented for separating various components of ion uptake: 1) passive diffusion, 2) mediated transport and 3) binding. It was concluded that a Na+/H+ exchange mechanism exists in the jejunal brush border. The exchanger was inhibited with 300 microM amiloride or harmaline. The kinetic parameters for sodium transport by this mechanism depend on the pH of the intravesicular solution. The application of a pH gradient (pHin = 5.5, pHout = 7.5) causes an increase in Jmax (50 to 125 pmol/mg protein . sec) with no change in Kt (congruent to 4.5 nM). Competition experiments show that other monovalent cations, e.g. Li+ and NH4+, share the Na+/H+ exchanger. This was confirmed with direct measurements of Li+ uptakes. Saturable uptake mechanisms were also observed for K+, Rb+ and SO4(2-), but not for Br-. The Jmax for K+ and Rb+ are similar to the Jmax for Na+, suggesting that they may share a transporter. The SO4(2-) system appears to be a Na+/SO4(2-) cotransport system. There does not appear to be either a Cl-/OH- transport mechanism of the type observed in ileum or a specific Na+/Cl- symporter.

Kinetics of sodium-dependent solute transport by rabbit renal and jejunal brush-border vesicles using a fluorescent dye

The kinetics of Na-coupled solute transport by renal and jejunal brush-border vesicles in the rabbit were examined using the potential-sensitive fluorescent dye diS-C3-(5). All organic solutes known to be transported across these membranes by Na-coupled mechanisms increase the fluorescence of the dye in the presence of Na, but not K. An increase in fluorescence (delta F) corresponds to a depolarization of the electrical potential difference (5-60 mV) across the brush-border membrane in the intact cell. delta F was independent of the valency of the transported solute. The fluorescence response was saturable, and for twelve solutes the Kf, i.e. the concentration of the substrate generating 50% of the maximal response, agreed quite closely with the Kt values reported from tracer studies. For six solutes increasing the Na concentration decreased Kf, and this agrees with the effect of Na on the kinetics of succinate transport in renal vesicles. We conclude that D-glucose, neutral amino acids and imino acids are co-transported with Na across both renal and jejunal brush-border membranes, and that carboxylic acids, beta-amino acids, and dibasic amino acids are co-transported with Na across the renal, but not jejunal, membranes.

The small-intestinal Na+, D-glucose cotransporter: an asymmetric gated channel (or pore) responsive to delta psi

At delta psi approximately equal to 0, D-glucose influx into, and efflux out of, membrane vesicles from small-intestinal brush borders are affected by trans Na+ and trans D-glucose to different extents. D-glucose influx and efflux respond to delta psi (negative at the trans side) to different extents. The small-intestinal Na+, D-glucose cotransporter is thus functionally asymmetric. This is not unexpected, in view of the structural asymmetry previously found. The characteristics of the delta psi-dependence of transinhibition by D-glucose are compatible with the mobile part of the cotransporter bearing a negative charge of at least 1 (in the substrate-free form). They are not compatible with its mobile part being electrically neutral. Pertinent equations are given in the Appendix. Partial Cleland's kinetic analysis and other criteria rule out (Iso) Ping Pong mechanisms and makes likely a Preferred Ordered mechanism, with Na+out binding to the cotransporter prior to the sugarout. A likely model is proposed aimed at providing a mechanism of flux coupling and active accumulation.

Transepithelial transport in cell culture: stoichiometry of Na/phlorizin binding and Na/D-glucose cotransport. A two-step, two sodium model of binding and translocation

The renal cell line LLC-PK1 cultured on a membrane filter forms a functional epithelial tissue. This homogeneous cell population exhibits rheogenic Na-dependent D-glucose coupled transport. The short-circuit current (Isc) was accounted for by net apical-to-basolateral D-glucose coupled Na flux, which was 0.53 +/- 0.09(8) mueq cm-2hr-1, and Isc, 0.50 +/- 0.50(8) mueq cm-2hr-1. A linear plot of concurrent net Na vs. net D-glucose apical-to-basolateral fluxes a gave a regression coefficient of 2.08. As support for a 2:1 transepithelial stoichiometry, sodium was added in the presence of D-glucose and the response of Isc analyzed by a Hill plot. A slope of 2.08 +/- 0.06(5) was obtained confirming a requirement of 2 Na for 1 D-glucose coupled transport. A Hill plot of Isc increase to added D-glucose in the presence of Na gave a slope of 1.02 +/- 0.02(5). A direct determination of the initial rates of Na and D-glucose translocation across the apical membrane using phlorizin, a nontransported glycoside competitive inhibitor to identify the specific coupled uptake, gave a stoichiometry of 2.2. A coupling ratio of 2 for Na, D-glucose uptake, doubles the potential energy available for Na-gradient coupled D-glucose transport. In contrast to coupled uptake, the stoichiometry for Na-dependent-phlorizin binding was 1.1 +/- 0.1(8) from Hill plot analyses of Na-dependent-phlorizin binding as a function of [Na].(ABSTRACT TRUNCATED AT 250 WORDS)