Sugar uptake by intestinal basolateral membrane vesicles

Transcellular Barriers to Glucose Delivery in the Body

Glucose is the universal fuel of most mammalian cells, and it is largely replenished through dietary intake. Glucose availability to tissues is paramount for the maintenance of homeostatic energetics and, hence, supply should match demand by the consuming organs. In its journey through the body, glucose encounters cellular barriers for transit at the levels of the absorbing intestinal epithelial wall, the renal epithelium mediating glucose reabsorption, and the tight capillary endothelia (especially in the brain). Glucose transiting through these cellular barriers must escape degradation to ensure optimal glucose delivery to the bloodstream or tissues. The liver, which stores glycogen and generates glucose de novo, must similarly be able to release it intact to the circulation. We present the most up-to-date knowledge on glucose handling by the gut, liver, brain endothelium, and kidney, and discuss underlying molecular mechanisms and open questions. Diseases associated with defects in glucose delivery and homeostasis are also briefly addressed. We propose that the universal problem of sparing glucose from catabolism in favor of translocation across the barriers posed by epithelia and endothelia is resolved through common mechanisms involving glucose transfer to the endoplasmic reticulum, from where glucose exits the cells via unconventional cellular mechanisms.

Fluorinated carbohydrates for 18F-positron emission tomography (PET)

Carbohydrate diversity is foundational in the molecular literacy that regulates cellular function and communication. Consequently, delineating and leveraging this structure-function interplay continues to be a core research objective in the development of candidates for biomedical diagnostics. A totemic example is the ubiquity of 2-deoxy-2-[18F]-fluoro-D-glucose (2-[18F]-FDG) as a radiotracer for positron emission tomography (PET), in which metabolic trapping is harnessed. Building on this clinical success, more complex sugars with unique selectivities are gaining momentum in molecular recognition and personalised medicine: this reflects the opportunities that carbohydrate-specific targeting affords in a broader sense. In this Tutorial Review, key milestones in the development of 2-[18F]-FDG and related glycan-based radiotracers for PET are described, with their diagnostic functions, to assist in navigating this rapidly expanding field of interdisciplinary research.

SGLT2 and cancer

Glycolysis plays a central role in tumor metabolism and growth, and this is reflected in a high rate of glucose uptake. It is commonly assumed that the upregulation of the facilitated glucose transporter GLUT1 meets the tumor’s demand for sugar. This underlies the success in using 2FDG PET imaging in the clinic to identify and stage many tumors. However, 2FDG is not a substrate for a second class of glucose transporters, the sodium-dependent glucose cotransporters, SGLTs, and so 2FDG PET may not provide a complete picture. A specific new radiotracer to detect SGLT activity has been introduced, Me4FDG, and this provides an opportunity to explore the potential role of SGLTs in supporting tumor glycolysis. In this brief review, I highlight the development of Me4FDG and our preliminary studies of Me4FDG PET in cancer patients. We find that the renal isoform, SGLT2, is expressed in pancreatic and prostate tumors and glioblastomas, and Me4FDG PET introduces a new method to image tumors. As SGLT2 drugs are successful in treating type 2 diabetes mellitus, they may also provide a new therapy.

Integrative physiology of transcellular and paracellular intestinal absorption

Glucose absorption by the small intestine has been studied for nearly a century. Despite extensive knowledge about the identity, functioning and regulation of the relevant transporters, there has been and there remains controversy about how these transporters work in concert to determine the overall epithelial absorption of key nutrients (e.g. sugars, amino acids) over a wide range of dietary and/or luminal concentrations. Our broader, integrative understanding of intestinal absorption requires more than the reductionist dissection of all the components and their elaboration at molecular and genetic levels. This Commentary emphasizes the integration of discrete molecular players and processes (including paracellular absorption) that, in combination, determine the overall epithelial absorption of key nutrients (e.g. sugars, amino acids) and putative anti-nutrients (water-soluble toxins), and the integration of that absorption with other downstream processes related to metabolic demands. It identifies historic key advances, controversies and future research ideas, as well as important perspectives that arise through comparative as well as biomedical physiological research.

Alligators and Crocodiles Have High Paracellular Absorption of Nutrients, But Differ in Digestive Morphology and Physiology

Much of what is known about crocodilian nutrition and growth has come from animals propagated in captivity, but captive animals from the families Crocodilidae and Alligatoridae respond differently to similar diets. Since there are few comparative studies of crocodilian digestive physiology to help explain these differences, we investigated young Alligator mississippiensis and Crocodylus porosus in terms of (1) gross and microscopic morphology of the intestine, (2) activity of the membrane-bound digestive enzymes aminopeptidase-N, maltase, and sucrase, and (3) nutrient absorption by carrier-mediated and paracellular pathways. We also measured gut morphology of animals over a larger range of body sizes. The two species showed different allometry of length and mass of the gut, with A. mississippiensis having a steeper increase in intestinal mass with body size, and C. porosus having a steeper increase in intestinal length with body size. Both species showed similar patterns of magnification of the intestinal surface area, with decreasing magnification from the proximal to distal ends of the intestine. Although A. mississippiensis had significantly greater surface-area magnification overall, a compensating significant difference in gut length between species meant that total surface area of the intestine was not significantly different from that of C. porosus. The species differed in enzyme activities, with A. mississippiensis having significantly greater ability to digest carbohydrates relative to protein than did C. porosus. These differences in enzyme activity may help explain the differences in performance between the crocodilian families when on artificial diets. Both A. mississippiensis and C. porosus showed high absorption of 3-O methyl d-glucose (absorbed via both carrier-mediated and paracellular transport), as expected. Both species also showed surprisingly high levels of l-glucose-uptake (absorbed paracellularly), with fractional absorptions as high as those previously seen only in small birds and bats. Analyses of absorption rates suggested a relatively high proportional contribution of paracellular (i.e., non-mediated) uptake to total uptake of nutrients in both species. Because we measured juveniles, and most paracellular studies to date have been on adults, it is unclear whether high paracellular absorption is generally high within crocodilians or whether these high values are specific to juveniles.

The second sodium pump: from the function to the gene

Transepithelial Na(+) transport is mediated by passive Na(+) entry across the luminal membrane and exit through the basolateral membrane by two active mechanisms: the Na(+)/K(+) pump and the second sodium pump. These processes are associated with the ouabain-sensitive Na(+)/K(+)-ATPase and the ouabain-insensitive, furosemide-inhibitable Na(+)-ATPase, respectively. Over the last 40 years, the second sodium pump has not been successfully associated with any particular membrane protein. Recently, however, purification and cloning of intestinal α-subunit of the Na(+)-ATPase from guinea pig allowed us to define it as a unique biochemical and molecular entity. The Na(+)- and Na(+)/K(+)-ATPase genes are at the same locus, atp1a1, but have independent promoters and some different exons. Herein, we spotlight the functional characteristics of the second sodium pump, and the associated Na(+)-ATPase, in the context of its role in transepithelial transport and its response to a variety of physiological and pathophysiological conditions. Identification of the Na(+)-ATPase gene (atna) allowed us, using a bioinformatics approach, to explore the tertiary structure of the protein in relation to other P-type ATPases and to predict regulatory sites in the promoter region. Potential regulatory sites linked to inflammation and cellular stress were identified in the atna gene. In addition, a human atna ortholog was recognized. Finally, experimental data obtained using spontaneously hypertensive rats suggest that the Na(+)-ATPase could play a role in the pathogenesis of essential hypertension. Thus, the participation of the second sodium pump in transepithelial Na(+) transport and cellular Na(+) homeostasis leads us to reconsider its role in health and disease.

Biology of human sodium glucose transporters

There are two classes of glucose transporters involved in glucose homeostasis in the body, the facilitated transporters or uniporters (GLUTs) and the active transporters or symporters (SGLTs). The energy for active glucose transport is provided by the sodium gradient across the cell membrane, the Na(+) glucose cotransport hypothesis first proposed in 1960 by Crane. Since the cloning of SGLT1 in 1987, there have been advances in the genetics, molecular biology, biochemistry, biophysics, and structure of SGLTs. There are 12 members of the human SGLT (SLC5) gene family, including cotransporters for sugars, anions, vitamins, and short-chain fatty acids. Here we give a personal review of these advances. The SGLTs belong to a structural class of membrane proteins from unrelated gene families of antiporters and Na(+) and H(+) symporters. This class shares a common atomic architecture and a common transport mechanism. SGLTs also function as water and urea channels, glucose sensors, and coupled-water and urea transporters. We also discuss the physiology and pathophysiology of SGLTs, e.g., glucose galactose malabsorption and familial renal glycosuria, and briefly report on targeting of SGLTs for new therapies for diabetes.

Functional expression of SGLTs in rat brain

This work provides evidence of previously unrecognized uptake of glucose via sodium-coupled glucose transporters (SGLTs) in specific regions of the brain. The current understanding of functional glucose utilization in brain is largely based on studies using positron emission tomography (PET) with the glucose tracer 2-deoxy-2-[F-18]fluoro-D-glucose (2-FDG). However, 2-FDG is only a good substrate for facilitated-glucose transporters (GLUTs), not for SGLTs. Thus, glucose accumulation measured by 2-FDG omits the role of SGLTs. We designed and synthesized two high-affinity tracers: one, α-methyl-4-[F-18]fluoro-4-deoxy-D-glucopyranoside (Me-4FDG), is a highly specific SGLT substrate and not transported by GLUTs; the other one, 4-[F-18]fluoro-4-deoxy-D-glucose (4-FDG), is transported by both SGLTs and GLUTs and will pass through the blood brain barrier (BBB). In vitro Me-4FDG autoradiography was used to map the distribution of uptake by functional SGLTs in brain slices with a comparable result from in vitro 4-FDG autoradiography. Immunohistochemical assays showed that uptake was consistent with the distribution of SGLT protein. Ex vivo 4-FDG autoradiography showed that SGLTs in these areas are functionally active in the normal in vivo brain. The results establish that SGLTs are a normal part of the physiology of specific areas of the brain, including hippocampus, amygdala, hypothalamus, and cerebral cortices. 4-FDG PET imaging also established that this BBB-permeable SGLT tracer now offers a functional imaging approach in humans to assess regulation of SGLT activity in health and disease.

Absence of Na+/sugar cotransport activity in Barrett’s metaplasia

AIM: To evaluate the presence of Na⁺-dependent, active, sugar transport in Barrett's epithelia as an intestinal biomarker, based on the well-documented, morphological intestinal phenotype of Barrett's esophagus (BE).

METHODS: We examined uptake of the nonmeta-bolizable glucose analogue, alpha-methyl-D-glucoside (AMG), a substrate for the entire sodium glucose cotransporter (SGLT) family of transport proteins. During upper endoscopy, patients with BE or with uncomplicated gastroesophageal reflux disease (GERD) allowed for duodenal, gastric fundic, and esophageal mucosal biopsies to be taken. Biopsies were incubated in bicarbonate-buffered saline (KRB) containing 0.1 mmol/L ¹⁴C-AMG for 60 min at 20°C. Characterized by abundant SGLT, duodenum served as a positive control while gastric fundus and normal esophagus, known to lack SGLT, served as negative controls.

RESULTS: Duodenal biopsies accumulated 249.84 ± 35.49 (SEM) picomoles AMG/&mgr;g DNA (n = 12), gastric fundus biopsies 36.20 ± 6.62 (n = 12), normal esophagus 12.10 ± 0.59 (n = 3) and Barrett’s metaplasia 29.79 ± 5.77 (n = 8). There was a statistical difference (P < 0.01) between biopsies from duodenum and each other biopsy site but there was no statistically significant difference between normal esophagus and BE biopsies. 0.5 mmol/L phlorizin (PZ) inhibited AMG uptake into duodenal mucosa by over 89%, but had no significant effect on AMG uptake into gastric fundus, normal esophagus, or Barrett’s tissue. In the absence of Na⁺ (all Na⁺ salts replaced by Li⁺ salts), AMG uptake in duodenum was decreased by over 90%, while uptake into gastric, esophageal or Barrett’s tissue was statistically unaffected.

CONCLUSION: Despite the intestinal enterocyte phenotype of BE, Na⁺-dependent, sugar transport activity is not present in these cells.

Imino sugars are potent agonists of the human glucose sensor SGLT3

Imino sugars are used to treat type 2 diabetes mellitus [miglitol (Glyset)] and lysosomal storage disorders [miglustat (Zavesca)] based on the inhibition of alpha-glucosidases and glucosyltransferases. In this substrate specificity study, we examined the interactions of imino sugars with a novel human glucose sensor, sodium/glucose cotransporter type 3 (hSGLT3), using expression in Xenopus laevis oocytes and electrophysiology. The results for hSGLT3 are compared with those for alpha-glucosidases and human SGLT type 1 (hSGLT1), a well characterized sodium/glucose cotransporter of the SGLT family. In general, substrates have lower apparent affinities (K0.5) for hSGLT3 than hSGLT1 (D-glucose, alpha-methyl-D-glucose, 1-deoxy-D-glucose, and 4-deoxy-4-fluoro-D-glucose exhibit K0.5 values of 19, 21, 43, and 17 mM, respectively, for hSGLT3, and 0.5, 0.7, 10, and 0.07 mM, respectively, for hSGLT1). However, specificity of hSGLT3 binding is greater (D-galactose and 4-deoxy-4-fluoro-D-galactose are not hSGLT3 substrates, but have hSGLT1 K0.5 values of 0.6 and 1.3 mM). An important deviation from this trend is potent hSGLT3 activation by the imino sugars 1-deoxynojirimycin (DNJ), N-hydroxylethyl-1-deoxynojirimycin (miglitol), N-butyl-1-deoxynojirimycin (miglustat), N-ethyl-1-deoxynojirimycin, and 1-deoxynojirimycin-1-sulfonic acid, with K0.5 values of 0.5 to 9 microM. The diastereomer 1-deoxygalactonojirimycin activates hSGT3 with a K0.5 value of 11 mM, a 3000-fold less potent interaction than is observed for DNJ (4 microM). These imino sugar binding characteristics are similar to those for alpha-glucosidases, but there are no interactions with hSGLT1. This work provides insights into hSGLT3 and -1 substrate binding interactions, establishes a pharmacological profile to study endogenous hSGLT3, and may have important ramifications for the clinical application of imino sugars.

Prior exercise enhances passive absorption of intraduodenal glucose

The purpose of this study was to assess whether a prior bout of exercise enhances passive gut glucose absorption. Mongrel dogs had sampling catheters, infusion catheters, and a portal vein flow probe implanted 17 days before an experiment. Protocols consisted of either 150 min of exercise (n = 8) or rest (n = 7) followed by basal (-30 to 0 min) and a primed (150 mg/kg) intraduodenal glucose infusion [8.0 mg x kg-1x min-1, time (t) = 0-90 min] periods. 3-O-[3H]methylglucose (absorbed actively, facilitatively, and passively) and l-[14C]glucose (absorbed passively) were injected into the duodenum at t = 20 and 80 min. Phloridzin, an inhibitor of the active sodium glucose cotransporter-1 (SGLT-1), was infused (0.1 mg x kg-1 x min-1) into the duodenum from t = 60-90 min with a peripheral venous isoglycemic clamp. Duodenal, arterial, and portal vein samples were taken every 10 min during the glucose infusion, as well as every minute after each tracer bolus injection. Net gut glucose output in exercised dogs increased compared with that in the sedentary group (5.34 +/- 0.47 and 4.02 +/- 0.53 mg x kg-1x min-1). Passive gut glucose absorption increased approximately 100% after exercise (0.93 +/- 0.06 and 0.45 +/- 0.07 mg x kg-1 x min-1). Transport-mediated glucose absorption increased by approximately 20%, but the change was not significant. The infusion of phloridzin eliminated the appearance of both glucose tracers in sedentary and exercised dogs, suggesting that passive transport required SGLT-1-mediated glucose uptake. This study shows 1). that prior exercise enhances passive absorption of intraduodenal glucose into the portal vein and 2). that basal and the added passive gut glucose absorption after exercise is dependent on initial transport of glucose via SGLT-1.

Involvement of active transport systems in the mobilization of cadmium by dithiocarbamates in vivo

Previously, we reported that the action of cadmium (Cd) complexing dithiocarbamates, such as N-benzyl-D-glucamine dithiocarbamate (BGD) and N-p-hydroxymethylbenzyl-D-glucamine dithiocarbamate (HBGD), in removing Cd from the kidney involves a probenecid-sensitive organic anion transport system. However, other mechanisms responsible for Cd mobilizing effects of BGD and HBGD are still unclear. Therefore, in the present study we examined the effects of phloretin (an inhibitor of plasma membrane glucose carrier), phloridzin (an inhibitor of Na(+)-dependent active hexose transport) and alpha-aminoisobutyric acid (AIB, an inhibitor of amino acid transport) on the excretion and distribution of the chelating agents and Cd in mice. Phloretin pretreatment markedly decreased the biliary and urinary excretions of BGD and HBGD. Phloridzin pretreatment also decreased the biliary and urinary excretions of HBGD, but had no effect on the BGD. AIB pretreatment had no effect on the excretions of either BGD or HBGD. Phloretin pretreatment increased the hepatic and renal contents of BGD and HBGD. Contrary to this, phloridzin pretreatment decreased the hepatic content of BGD and hepatic and renal contents of HBGD, while AIB pretreatment decreased the renal contents of BGD and HBGD. The mobilizing effects of BGD and HBGD on the hepatic and renal Cd was also investigated using Cd-exposed mice. Phloretin or phloridzin pretreatment decreased the mobilizing effect of BGD and HBGD on the hepatic Cd, but had no effect on the renal Cd. These results suggest that BGD and HBGD are taken up into the liver and kidney by phloridzin- and phloretin-sensitive transport system, respectively; that Cd-BGD and Cd-HBGD complexes formed in the hepatic cells are secreted to the bile by phloridzin- and phloretin-sensitive transport systems; and that free BGD and HBGD secreted from these organ to the bile and urine might have occurred, at least in part, by different mechanisms.

Kinetic analysis of L-glutamine transport into porcine jejunal enterocyte brush-border membrane vesicles

L-Glutamine transport into porcine jejunal enterocyte brush border membrane vesicles was studied. Uptake was mediated by a Na(+)-dependent and a Na(+)-independent pathway as well as by diffusion. The initial rates of glutamine uptake over a range of concentrations is both Na(+)-gradient and Na(+)-free conditions were analyzed and kinetic parameters were obtained. Na(+)-dependent glutamine transport had a K(m) of 0.77 +/- 0.16 mM and a Jmax of 70.7 +/- 5.8 pmol mg protein-1 s-1; Na(+)-independent glutamine transport had a K(m) of 3.55 +/- 0.78 mM and a Jmax of 55.1 +/- 6.6 pmol mg protein-1 s-1. The non-saturable component measured with HgCl2-poisoned brush border membrane vesicles in the Na(+)-free condition contained passive diffusion and non-specific membrane binding and was defined to be apparent glutamine diffusion and the glutamine permeability coefficient (Kdiff) was estimated to be Kdiff = 3.78 +/- 0.06 pmol 1 mg protein-1 mmol-1 s-1. Results of inhibition experiments showed that Na(+)-dependent glutamine uptake occurred primarily through the brush border system-B degree transporters, whereas Na(+)-independent glutamine uptake occurred via the system-L transporters. Furthermore, the kinetics of L-leucine and L-cysteine inhibition of L-glutamine uptake demonstrated that neutral amino acids sharing the same brush border transporters can effectively inhibit each other in their transport.

Functional expression of P-glycoprotein in the hepatic canalicular membrane of developing rats

P-glycoprotein (P-gp), the multidrug resistance gene product, is expressed in a normal liver exclusively on the canalicular membrane of the hepatocyte. The objective of this study was to examine the effect of age on the P-gp transport system using canalicular membrane (cLPM) vesicles isolated from the liver of developing (22 days old) and adult rats. No differences in protein yield, intravesicular volumes, and enrichments of cLPM enzymes or enzymes representing contamination of subcellular organelles were found for vesicles isolated from both groups, demonstrating the isolation of similar cLPM vesicle preparations. The transport of daunomycin (DNM), a P-gp substrate, was used to study age-related functional differences in P-gp. DNM uptake in the presence of ATP was greater than uptake in the absence of ATP in both young and adult cLPM vesicles, showing that P-gp is functional in both groups. In young and adult groups only ATP was a potent stimulator of transport when compared with ATP degradation products and a nonhydrolyzable ATP analogue. Although ATP-dependent uptake tended to be greater in the adult compared to the young, there was no statistically significant difference in DNM kinetics (Vmax, km, gamma) between groups. Canalicular membrane from the young rats showed decreased fluidity, as assessed by the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene; however there was no significant difference between groups. Examination of P-gp expression using the monoclonal antibody C219 revealed similar levels of expression in the young as in the adult. Our results suggest that P-gp in the bile canaliculus of developing rats is functional with similar levels of function and expression as observed in the adult.

Transport of citrate across the brush border and basolateral membrane of rat small intestine

It was the aim of the present study to investigate the transport of tricarboxylates (citrate, tricarballylate) across the basolateral membrane (BLM) of the small intestine. Experiments were performed using BLM vesicles isolated from the jejunum of rats. For comparison, some experiments with brush border membrane (BBM) vesicles were also performed. Finally, transfer of citrate and tricarballylate across the intestinal wall was investigated using sacs of everted small intestine. Uptake of citrate by BBM vesicles occurs by a Na+ gradient-driven transport mechanism specific for tri- and dicarboxylates. The partially protonated forms of citrate seem to be much better transported than the completely dissociated form, since lowering the extravesicular pH from 7.8 to 5.6 resulted in a marked stimulation of Na(+)-dependent citrate uptake. In contrast to citrate uptake across the BBM, uptake of citrate across the BLM was neither influenced by Na+ nor by pH changes. Neither structurally related tri- and dicarboxylates (tricarballylate, succinate) nor other organic and inorganic anions (e.g. lactate, p-aminohippurate, sulfate, chloride, bicarbonate) significantly influenced citrate uptake by BLM vesicles under cis-conditions. Uptake of citrate as a function of the extravesicular substrate concentration was linear over a concentration range from 0.1 to 10 mmol/l. Thus, citrate uptake under these conditions seems to be Na(+)-independent and not to be mediated by a carrier. However, preloading the BLMV with citrate clearly trans-stimulated the uptake of citrate and tricarballylate, respectively. Furthermore, citrate significantly inhibited tricarballylate uptake into BLMV preloaded with citrate. These results indicate uptake of tricarboxylates across the BLM by an exchange mechanism. Using sacs of everted small intestine, no transfer of intact citrate against a concentration gradient occurred, but some evidence for metabolization of citrate within the intestinal wall was obtained. In contrast, the non-metabolizable tricarboxylate tricarballylate was significantly accumulated in the serosal compartment of everted intestinal sacs.

Transport of cycasin by the intestinal Na+/glucose cotransporter

The medicinal and food use of seed from the cycad plant (Cycas spp.), which contains the neurotoxin cycasin, is a proposed etiological factor for amyotrophic lateral sclerosis/Parkinsonism dementia complex (ALS/PDC), a prototypical neurodegenerative disease found in the western Pacific. Cycasin, the beta-D-glucoside of methylazoxymethanol might enter neurons and other cells via a glucose transporter. Since the intestinal brush-border Na+/glucose cotransporter plays a major role in the absorption of monosaccharides, the following studies were conducted to determine if cycasin, the beta-D-glucoside of methylazoxymethanol, is a substrate for the transporter. We measured the ability of cycasin to (i) inhibit Na+/glucose uptake into rabbit intestinal brush-border membrane vesicles, and (ii) to generate current by the cloned Na+/glucose cotransporter (SGLT1) expressed in Xenopus laevis oocytes. The results show that cycasin inhibits Na(+)-dependent sugar transport in the vesicles, and cycasin generates phlorizin-sensitive currents in oocytes. We conclude that cycasin is a substrate for the intestinal brush-border Na+/glucose cotransporter, albeit with a lower affinity than D-glucose. This suggests that cycasin may be absorbed from the gut lumen by the cotransporter, and as a result either cycasin or the aglycone is presented to the blood-brain barrier for uptake into the brain.

GLUT2 is the transporter for fructose across the rat intestinal basolateral membrane

BACKGROUND

The exact roles of disaccharidases and GLUT5 in the brush border membrane and GLUT2 in the basolateral membrane in the absorption of fructose across the intestine have not been fully determined. This paper describes characterization of fructose transport across the jejunal basolateral membrane using isolated membrane vesicles.

METHODS

Transport of fructose was measured using rapid filtration of vesicles. Luminal perfusion in vivo with glucose and fructose before vesicle preparation was used to assess modulation of GLUT2 activity. Western blotting measured the abundance of GLUT2 in the membrane.

RESULTS

The maximal rate of transport for fructose was 1100 pmol/mg protein/s and the Michaelis constant was 16 mmol/L. Fructose and glucose could completely inhibit the transport of each other. Perfusion of the intestinal lumen with fructose or glucose saline for 4 hours produced a fourfold increase in maximal fructose transport.

CONCLUSIONS

These data indicate that the one transport protein, GLUT2, is responsible for moving both fructose and glucose out of the enterocyte across the basolateral membrane under basal conditions. The activity of this, or a closely related carrier, is rapidly upregulated by the presence of hexoses in the intestinal lumen, explaining the potentiation of fructose absorption by luminal glucose and obviating any need to involve apical disaccharidases.

Altered proteoglycan synthesis via the false acceptor pathway can be dissociated from beta-D-xyloside inhibition of proliferation

beta-D-Xylosides have been used to perturb proteoglycan (PG) synthesis to elucidate the function of PGs in a number of cellular processes, including proliferation, migration, and differentiation. This study was designed to examine whether specific xylosides affect the proliferation of several different cell types and, if so, whether this effect is dependent on altered PG synthesis via the false acceptor pathway. Both methylumbelliferyl beta-D-xylopyranoside and p-nitrophenyl beta-D-xylopyranoside (PNP beta-xyloside) inhibit cell proliferation and modulate PG synthesis; however, the alpha form of PNP xyloside which does not perturb PG synthesis inhibits the proliferation of cultured cells on a molar basis equally as well as the beta form. Conversely, beta-methyl xylopyranoside stimulates the synthesis of free glycosaminoglycan chains equally as well as PNP beta-xyloside and yet has no measurable effect on cell proliferation at comparable doses, indicating that cells can grow normally while experiencing disruption of their proteoglycan metabolism. At doses ranging from 0.5 to 5 mM, PNP beta-xyloside arrests cells in the G1 phase of the cell cycle at the same time point as serum starvation. It also delays the exist of cycling cells from the S phase. This treatment is not cytotoxic and is rapidly reversed by the replacement of PNP beta-xyloside containing medium with control medium. Dimethyl sulfoxide, the most commonly used solvent for beta-xyloside in proteoglycan studies, potentiates the inhibitory effect of PNP beta-xyloside on cell proliferation. These results indicate that the perturbation of PG synthesis via the false acceptor pathway can be uncoupled from control of cell proliferation.

Glucose transporters do not serve as water channels in renal and intestinal epithelia

Glucose carriers have been shown to serve as water channels in macrophages and in oocytes injected with messenger ribonucleic acid (mRNA) encoding the glucose carrier protein (Fischbarg et al. The contribution, therefore, of glucose carriers to osmotic water permeability (Pf) in renal and intestinal epithelial cells was investigated. Pf of brush border membrane vesicles (BBMVs) and of basolateral membrane vesicles (BLMVs) was studied using stopped-flow spectrophotometry. Osmotic shrinkage of renal vesicles exhibited fast and slow components at 4 degrees C and 37 degrees C. The fast component could be inhibited by HgCl2 or dimethylsulphoxide (DMSO) at these temperatures, whereas the slow component was inhibited only at 4 degrees C. Osmotic shrinkage of intestinal BBMVs and BLMVs was homogeneous at 4 degrees C and 37 degrees C and was slightly inhibitable by HgCl2 or DMSO at 4 degrees C but not 37 degrees C. In both tissues, vesicle uptake of glucose was sensitive to HgCl2, but not to DMSO. Phlorizin and phloretin inhibited D-glucose uptake in BBMVs and BLMVs respectively, but had no significant effect on Pf. In membrane vesicles of kidney origin, Pf was tenfold higher than in membranes from intestine. This difference was not reflected by the phlorizin- and phloretin-sensitive D-glucose uptakes. Our study concludes that glucose transporters do not serve as water channels in kidney or intestine. Although membrane proteins contribute slightly to Pf at 4 degrees C, this contribution is insignificant at 37 degrees C. A membrane protein serving specifically as a water channel could only be demonstrated in renal cortical membranes.

Intestinal and renal Na+/glucose cotransporters share common structures

Polyclonal antibodies were raised to peptides selected from three different regions of the cloned rabbit intestinal Na+/glucose cotransporter. Western blot analysis was used to identify the fully mature protein in intestinal and renal brush borders. Two of the antibodies specifically identified a approximately 70-kDa protein band in rabbit intestinal brush borders but did not specifically immunoreact with membranes that do not have Na(+)-dependent glucose transport activity. The immunoreactive proteins had an apparent isoelectric point between pH 4.7 and 6.8. The antibodies also specifically recognized a similar-sized protein in human and seven other mammalian intestinal brush borders. Similar protein bands were identified in four mammalian renal brush-border membranes, indicating shared epitopes between intestinal and renal cotransport proteins. In some species, e.g., lamb and pig, the epitope for one antibody was missing in both intestinal and renal brush borders, suggesting that this epitope is not essential for function. These results suggest that 1) the cloned intestinal Na+/glucose cotransporter is that identified in earlier biochemical studies, 2) there is close structural similarity between intestinal and renal cotransporters, and 3) the structure of these proteins has been conserved during evolution.

Glycyl-L-proline transport in rabbit enterocyte basolateral-membrane vesicles

The properties of a peptide-transport system in rabbit enterocyte basolateral membrane were examined with glycyl-L-proline as the substrate. Basolateral-membrane vesicles prepared from rabbit proximal intestine were characterized in terms of both purity and orientation. Marker-enzyme assays show that the basolateral-membrane marker, ouabain-sensitive K(+)-activated phosphatase, is enriched 17-fold with respect to the initial homogenate. The activities of enzymes used as markers for other membranes and organelles are low, and contamination of the final membrane fraction with these is minimal. The use of immunoblotting techniques further confirms the absence of brush-border-membrane contamination. Proteins in the basolateral-membrane vesicle preparation gave no cross-reaction with antibodies against the 140 kDa antigen and the Na+/glucose-symport protein, markers specific to the brush-border membrane of the enterocyte. Conversely, antibodies raised against the classical basolateral-membrane marker, the RLA class I histocompatibility complex, reacted strongly with a 43 kDa basolateral-membrane protein. The orientation of the basolateral-membrane vesicles was shown to be predominantly inside-out on determination by two independent criteria. The uptake of [1-14C]glycyl-L-proline by these vesicles is stimulated by the presence of an inwardly directed pH gradient, and this stimulation can be abolished by the proton ionophores carbonyl cyanide p-trichloromethoxyphenylhydrazone (CCCP) and tetrachlorotrifluoromethylbenzimidazole (TTFB). Transport is also inhibited by HgCl2, thimerosal, Na+ and other glycyl dipeptides.

Characterization of a Na+/glucose cotransporter cloned from rabbit small intestine

The Na+/glucose cotransporter from rabbit intestinal brush border membranes has been cloned, sequenced, and expressed in Xenopus oocytes. Injection of cloned RNA into oocytes increased Na+/sugar cotransport by three orders of magnitude. In this study, we have compared and contrasted the transport properties of this cloned protein expressed in Xenopus oocytes with the native transporter present in rabbit intestinal brush borders. Initial rates of 14C-alpha-methyl-D-glucopyranoside uptake into brush border membrane vesicles and Xenopus oocytes were measured as a function of the external sodium, sugar, and phlorizin concentrations. Sugar uptake into oocytes and brush borders was Na+-dependent (Hill coefficient 1.5 and 1.7), phlorizin inhibitable (Ki 6 and 9 microM), and saturable (alpha-methyl-D-glucopyranoside Km 110 and 570 microM). The sugar specificity was examined by competition experiments, and in both cases the selectivity was D-glucose greater than alpha-methyl-D-glucopyranoside greater than D-galactose greater than 3-O-methyl-D-glucoside. In view of the close similarity between the properties of the cloned protein expressed in oocytes and the native brush border transporter, we conclude that we have cloned the classical Na+/glucose cotransporter.

Cytochalasin B binding to rabbit proximal tubular basolateral membranes

Cytochalasin B binds to the Na+-independent D-glucose transporter in non-renal tissues. We have shown previously that the Na+-independent D-glucose transporter of the rabbit renal proximal tubular cell is localized exclusively in the basolateral membrane. To determine whether cytochalasin B binds to this renal transporter we measured binding of [3H]cytochalasin B to proximal tubular basolateral membranes isolated from rabbit kidneys. A steady state of binding is reached by 15 minutes at 20 degrees C over a concentration range of 0.01 to 50 microM. Non-linear regression analysis of cytochalasin B binding from 0.01 to 20 microM plotted according to Scatchard reveals two classes of binding sites with Kd 5.88 x 10(-8) M, Bmax 16.1 pmol/mg protein; and Kd 5.62 x 10(-5) M, Bmax 2816 pmol/mg protein. [3H]cytochalasin B (0.1 microM) binding to basolateral membranes is a reversible process; it is displacable by excess unlabeled cytochalasin B with a time course similar to binding of [3H]cytochalasin B. Binding of [3H]cytochalasin B is inhibited by 500 mM D-glucose (21%), 2-deoxy-D-glucose (57%) and 3-O-methyl-D-glucose (64%), but not by L-glucose. [3H]cytochalasin B binding is reduced 71% by 0.1 mM phloretin, but only 26% by 0.1 mM phlorizin. Such substrate specificity and inhibitor sensitivity are similar to those previously demonstrated in non-renal tissues by others as well as in rabbit renal proximal tubular basolateral membranes by us. Our data suggest that cytochalasin B binds to the Na+-independent D-glucose transporter or a component of the transporter in the renal proximal tubular basolateral membrane.

Cytochalasin B-sensitive, sodium ion-dependent glucose transport in intestinal microvillous membrane

It was found that sodium ion-dependent glucose uptake by microvillous membrane (MVM) vesicles was partially inhibited by cytochalasin B with a half-maximum inhibition at ca. 10 microM. The MVM was photolabeled with [3]cytochalasin B. The Kd value and the maximum number of binding sites for cytochalasin B were ca. 8 microM and 70 pmol/mg protein, respectively. SDS-PAGE of the photolabeled MVM revealed 2 binding components. One was 86 K in Mr and the other 42 K. The binding of cytochalasin B to the 86 K component was affected neither by cytochalasin E nor by the presence of 0.5 M NaCl, but was depressed in the presence of 2-deoxy-D-glucose or phlorizin, which had no effect on the labeling of the 42 K component. These and other data suggested that the 86 K component might be responsible for a cytochalasin B-sensitive glucose transport in intestinal epithelial MVM.

Sugar transport in isolated rat kidney papillary collecting duct cells

D-Glucose is an important substrate of energy metabolism and osmolyte synthesis in the renal papillary collecting duct. In order to characterize the cellular entry of D-glucose in this tubular segment, collecting duct cells were isolated from rat kidney papilla and the rate of D-glucose uptake was measured indirectly by monitoring the D-glucose-dependent O2 uptake in the presence of the uncoupler CCCP. D-Glucose uptake was found to be sodium-independent and not sensitive to phlorizin even at a concentration of 10(-3) M. Uptake was, however, completely inhibited by 10(-5) M cytochalasin B and 10(-4) M phloretin. The apparent Ki for cytochalasin B was 1.5 x 10(-6) M and for phloretin 2.0 x 10(-5) M. Studies on the substrate specificity revealed that at 1 mM D-mannose is taken up and metabolized to the same extent as D-glucose. A 50-fold higher concentration of 2-deoxy-D-glucose and 2-amino-2-deoxy-D-glucose inhibited D-glucose uptake completely whereas alpha-methyl-D-glucoside, D-allose, and D-galactose were without effect. Under conditions where D-glucose utilization was maximally stimulated an apparent Km of 1.2 mM and a Vmax of 1 mmol D-glucose/g protein.hour was found for D-glucose uptake. These results indicate that the D-glucose uptake into papillary collecting duct cells is probably mediated by a transport system similar to the one found in basal-lateral membranes of polarized renal, intestinal, and liver cells as well as in nonpolarized fat cells and erythrocytes.

Na+/H+ exchange mechanism in the basolateral membrane of the rat enterocyte

Basolateral membrane vesicles from rat jejunal enterocytes, especially purified of brush-border contamination, were used for Na+ uptake. The basolateral membrane vesicles are osmotically active and under our experimental conditions Na+ binding is much lower than transport. An outwardly directed proton gradient stimulates Na+ uptake at both 5 microM and 5 mM concentrations. The proton gradient effect can be inhibited completely by 2 mM amiloride and partially by either FCCP or NH4Cl (NH3 diffusion). Membrane potential effects can be excluded by having valinomycin plus K+ on both sides of the vesicles. These results suggest that there is an Na+/H+ exchanger in the basolateral membrane of rat enterocytes.

Biotin transport in basolateral membrane vesicles of human intestine

The characteristics of the exit process of biotin from the enterocyte, i.e., transport across the basolateral membrane, was determined using an enriched basolateral membrane vesicle preparation of human intestine. Purity and suitability of basolateral membrane vesicles for transport studies was confirmed by enzymatic and functional criteria. Orientation of human basolateral membrane vesicles was determined by [3H]ouabain binding studies and was found to be 64% inside-out vesicles and the rest right-side-out vesicles and membrane sheets. Osmolarity studies indicated that the uptake of biotin by these vesicles represents transport into the intravesicular compartment, with little binding to membrane surfaces. The rate of biotin transport was linear for approximately 40 s but decreased thereafter. Transport of biotin was (a) Na+-independent, (b) saturable as a function of concentration, with an apparent KM of 1.1 microM and Vmax of 0.9 pmol/mg protein.15 s, (c) inhibited by structural analogues (desthiobiotin and biotin methyl ester) and related compounds (thioctic acid and thioctic amide), and (d) stimulated by inducing a positive intravesicular electrical potential. These studies are the first to demonstrate the existence of a carrier-mediated transport system for biotin in the basolateral membrane of human intestine.

Intestinal transfer of sodium [14C]taurocholate in streptozotocin-treated rats

The effect of streptozotocin (SZ) administration on sodium [14C]taurocholate (TC) transmural transfer was studied in the everted rat ileum. The excretion of fecal bile acids was also studied in living rats injected with that compound. The viability of the preparation used for the in vitro experiments was evaluated by light microscopy and by the rate of glucose uptake by tissue from the mucosal fluid. The results obtained showed that TC transfer to the serosal fluid was impaired after 24 h of SZ injection, as well as the active transport observed in control preparations. The amount of TC accumulated in the intestinal tissue was also diminished. In addition, total ATPase activity of tissue was decreased, and intracellular electrolyte concentration was altered. Therefore, a slower saturation of binding sites could be responsible for the effects of SZ on TC tissue accumulation, and a decreased ATPase activity for the impairment of the TC concentrative transport system. The results observed in vitro were supported by data in vivo because fecal bile acid excretion was significantly diminished in SZ-treated rats.

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.

Basolateral amino acid and glucose transport by the intestine of the teleost, Anguilla anguilla

1. D-glucose transport into BLMV was osmotically reactive, sodium independent, and inhibited by phloretin but not by phloridzin. 2. The survey of 6 L-amino acids identified three groups with respect to transfer across the basolateral cell border. Transport of proline and glutamate occurred by Na-dependent carriers and by apparent simple diffusion. Alanine, lysine and phenylalanine were transported by Na-independent carriers and apparent simple diffusion. Glycine transport was stimulated above apparent simple diffusion only by a simultaneous inwardly-directed Na gradient and outwardly-directed K gradient. 3. Only proline and glutamate demonstrated the ability to depolarize the membrane potential, consistent with Na-dependent rheogenic transport.

Characterization of the D-glucose/Na+ cotransport system in the intestinal brush-border membrane by using the specific substrate, methyl alpha-D-glucopyranoside

By using isolated membrane vesicles, we have investigated the tenet that D-glucose transport across the intestinal brush-border membrane involves at least two distinct, Na+-activated agencies (D-glucose transport systems S-1 and S-2), only one of which (S-1) can use methyl alpha-D-glucopyranoside (methyl alpha-glucoside) as a substrate. Our results with this glucose analogue show that: (a) As a function of time, methyl alpha-glucoside uptake exhibits a typical overshoot, similar to but smaller than that given by D-glucose with the same vesicle batch. (b) Nonlinear regression analysis of substrate-saturation curves reveals that, contrary to D-glucose, methyl alpha-glucoside transport involves a single transport system which we have identified as S-1. (c) Methyl alpha-glucoside exhibits an apparent affinity (defined as the reciprocal of Km) 4-times smaller than that of D-glucose for S-1 (Km(Dglucose) = 0.5 mM; Km(methyl alpha-glucoside) = 2 mM). However, methyl alpha-glucoside has a Vmax (230 pmol/mg protein per s) identical to that characterizing D-glucose transport by this system. (d) In the absence of Na+, methyl alpha-glucoside uptake is indistinguishable from simple diffusion, confirming that Na+ is an obligatory activator of S-1. (e) Phlorizin behaves as a fully competitive inhibitor of methyl alpha-glucoside transport (Ki = 18 microM), again indicating that S-1 is involved. (f) Neither phloretin nor cytochalasin B affects methyl alpha-glucoside uptake. We conclude that methyl alpha-glucoside is a substrate specific for S-1, which permits study of the properties of this system without interference by substrate fluxes taking place through any other channel.

A carrier-mediated transport for folate in basolateral membrane vesicles of rat small intestine

The mechanism of exit of folate from the enterocyte, i.e. transport across the basolateral membrane, is not known. In this study we examined, using basolateral membrane vesicles, the transport of folic acid across the basolateral membrane of rat intestine. Uptake of folic acid by these vesicles represents transport of the substrate into the intravesicular compartment and not binding to the membrane surface. The rate of folic acid transport was linear for the first 1 min of incubation but decreased thereafter, reaching equilibrium after 5 min of incubation. The transport of folic acid was: (1) saturable as a function of concentration with an apparent Km of 0.6 +/- 0.17 microM and Vmax. of 1.01 +/- 0.11 pmol/30 s per mg of protein; (2) inhibited in a competitive manner by the structural analogues 5-methyltetrahydrofolate and methotrexate (Ki = 2 and 1.4 microM, respectively); (4) electroneutral; (5) Na+-independent; (6) sensitive to the effect of the anion exchange inhibitor 4,4'-di-isothiocyanatostilbene-2,2'-disulphonic acid (DIDS). These data indicate the existence of a carrier-mediated transport system for folic acid in rat intestinal basolateral membrane and demonstrate that the transport process is electroneutral, Na+-independent and sensitive to the effect of anion exchange inhibition.

The Na+-independent D-glucose transporter in the enterocyte basolateral membrane: orientation and cytochalasin B binding characteristics

Phloridzin-insensitive, Na+-independent D-glucose uptake into isolated small intestinal epithelial cells was shown to be only partially inhibited by trypsin treatment (maximum 20%). In contrast, chymotrypsin almost completely abolished hexose transport. Basolateral membrane vesicles prepared from rat small intestine by a Percoll gradient procedure showed almost identical susceptibility to treatment by these proteolytic enzymes, indicating that the vesicles are predominantly oriented outside-out. These vesicles with a known orientation were employed to investigate the kinetics of transport in both directions across the membrane. Uptake data (i.e. movement into the cell) showed a Kt of 48 mM and a Vmax of 1.14 nmol glucose/mg membrane protein/sec. Efflux data (exit from the cell) showed a lower Kt of 23 mM and a Vmax of 0.20 nmol glucose/mg protein/sec. D-glucose uptake into these vesicles was found to be sodium independent and could be inhibited by cytochalasin B. The Ki for cytochalasin B as an inhibitor of glucose transport was 0.11 microM and the KD for binding to the carrier was 0.08 microM. D-glucose-sensitive sensitive binding of cytochalasin B to the membrane preparation was maximized with L- and D-glucose concentrations of 1.25 M. Scatchard plots of the binding data indicated that these membranes have a binding site density of 8.3 pmol/mg membrane protein. These results indicate that the Na+-independent glucose transporter in the intestinal basolateral membrane is functionally and chemically asymmetric. There is an outward-facing chymotrypsin-sensitive site, and the Kt for efflux from the cell is smaller than that for entry. These characteristics would tend to favor movement of glucose from the cell towards the bloodstream.

Expression of size-selected mRNA encoding the intestinal Na/glucose cotransporter in Xenopus laevis oocytes

The expression of the rabbit intestinal brushborder Na/glucose cotransporter has been studied in Xenopus oocytes. Poly(A)+ RNA isolated from the intestinal mucosa was injected into oocytes, and the expression of the transporter in the oocyte plasma membrane was assayed by measuring the Na-dependent phlorizin-sensitive uptake of methyl alpha-D-[14C]glucopyranoside (MeGlc). Expression of the glucose carrier was detected 3-7 days after mRNA injection, and the rate of glucose transport was proportional to the amount of mRNA injected. mRNA (50 ng) increased the maximum velocity (Vmax) of MeGlc uptake by as much as 10-fold over background. The total mRNA was fractionated by preparative agarose gel electrophoresis and each fraction was assayed for its ability to induce transport activity. The mRNA encoding the Na/glucose cotransporter was found in a single fraction of approximately 2.3 kilobases (kb), which contained 3% of the total mRNA. A similar mRNA fraction (2.0-2.6 kb) isolated from colon did not induce expression of this transporter. In vitro translation of the fractionated intestinal mRNA showed enhanced synthesis of two protein bands at 57 and 63 kDa. The mRNA encoding the cotransporter is smaller (2.3 kb) than that (2.6-2.9 kb) encoding the 55-kDa facilitated glucose carrier in human hepatoma cells and rat brain.

Effects of semistarvation on transintestinal D-glucose transport and D-glucose uptake in brush border and basolateral membranes of rat enterocytes

The present work shows that semistarvation (8-10 g of food for 10 days) increases net D-glucose, Na+ and water transport in the everted and perfused rat jejunum. A linear and positive correlation between cell sugar concentration and transport was found in control and semistarved rats, but the phenomenon was more relevant only in semistarved animals. Membrane vesicle experiments showed that semistarvation increases sugar overshoot only in brush border membrane vesicles, while this situation does not occur in basolateral membrane vesicles. The effect of partial food deprivation seems to enhance net sugar transport by increasing sugar entry across the apical membrane of enterocytes.

Taurocholate transport by rat intestinal basolateral membrane vesicles. Evidence for the presence of an anion exchange transport system

The transport of bile acid was studied in basolateral membrane vesicles isolated from rat small intestine. Taurocholate transport into an osmotically reactive intravesicular space was Na+ independent. The uptake of taurocholate in jejunal and ileal vesicles preloaded with sulfate was stimulated with respect to uptake in unpreloaded vesicles. Glycocholate inhibited the transstimulation of taurocholate uptake by sulfate. Sulfate and taurocholate uptake in ileal vesicles preloaded with bicarbonate was stimulated with respect to uptake in unpreloaded vesicles. Taurocholate inhibited the transstimulation of sulfate uptake by bicarbonate. When ileal vesicles were loaded with p-aminohippurate, an early transstimulation of taurocholate was found that exceeded equilibrium uptake, was insensitive to a K+ diffusion potential, and was cis-inhibited by taurocholate, glycocholate, pyruvate, p-aminohippurate, probenecid, chloride, sulfate, and bicarbonate. These data indicate the presence of an anion exchanger in intestinal basolateral membrane vesicles that may be involved in the exit of bile acids from the enterocyte.

Distribution of Ca2+-ATPase, ATP-dependent Ca2+-transport, calmodulin and vitamin D-dependent Ca2+-binding protein along the villus-crypt axis in rat duodenum

The migration of intestinal epithelial cells from the crypts to the tips of villi is associated with progressive cell differentiation. The changes in Ca2+-ATPase activity and ATP-dependent Ca2+-transport rates in basolateral membranes from rat duodenum were measured during migration along the crypt-villus axis. In addition, vitamin D-dependent calcium-binding protein and calmodulin content were measured in homogenates of six cell populations which were sequentially derived from villus tip to crypt base. Alkaline phosphatase activity was highest at the tip of the villus (fraction I) and decreased more than 20-fold towards the crypt base (fraction VI). (Na+ + K+)-ATPase activity also decreased along the villus-crypt axis but in a less pronounced manner than alkaline phosphatase. ATP-dependent Ca2+-transport in basolateral membranes was highest in fraction II (8.2 +/- 0.3 nmol Ca2+/min per mg protein) and decreased slightly towards the villus tip and base (fraction V). The youngest cells in the crypt had the lowest Ca2+-transport activity (0.9 +/- 0.1 nmol Ca2+/min per mg protein). The distribution of high-affinity Ca2+-ATPase activity in basolateral membranes correlated with the distribution of ATP-dependent Ca2+-transport. The activity of Na+/Ca2+ exchange was equal in villus and crypt basolateral membranes. Compared to the ATP-dependent Ca2+-transport system, the Na+/Ca2+ exchanger is of minor importance in villus cells but may play a more significant role in crypt cells. Calcium-binding protein decreased from mid-villus towards the villus base and was undetectable in crypt cells. Calmodulin levels were equal along the villus-crypt axis. It is concluded that vitamin D-dependent calcium absorption takes primarily place in villus cells of rat duodenum.

Contraluminal transport of hexoses in the proximal convolution of the rat kidney in situ

In order to study contraluminal hexose transport, concentration and time-dependent influx of 3H-2-deoxy-D-glucose from the interstitium into cortical tubular cells has been measured. The influx curves fit to a two parameter kinetics (Km 1.3 +/- 0.2 mmol/l, Jmax 0.67 +/- 0.16 pmol/s X cm) plus an additional diffusion term (with P = 6 X 10(-8) cm2/s) and a distribution ratio extracellular to intracellular amount of 2-deoxy-D-glucose of 1:0.6. Since the extracellular to intracellular free water space as estimated from morphological data was 1:2, one must conclude that glucose has only free access to 1/3 of the cell water. The intracellularly accessible space was augmented when the tubules were preperfused for 10 s with hypotonic saline. Thereby an increase of the compartment into which diffusion occurs was revealed and a final rupture of this intracellular compartment at 1/4 isotonic solutions was observed. Total replacement of ions in the peritubular perfusate by mannitol did not change 2-deoxy-D-glucose influx, indicating that it is Na+-independent. By adding isotonic concentrations of the respective sugars to the capillary perfusate, three degrees of inhibition of 2-deoxy-D-glucose influx could be revealed: strong inhibition by D-glucose, methyl-beta-D-glucoside, D-mannose, 3-O-methyl-D-glucose, 2-deoxy-D-galactose, methyl-beta-D-galactoside and 6-deoxy-D-glucose, moderate inhibition by D-galactose, L-glucose, L-mannose and D-fructose, no or borderline inhibition by methyl alpha-D-glucoside, 2-deoxy-methyl-alpha-D-galactoside, 1-thio-beta-D-glucose, 1-thio-beta-D-galactose, 5-thio-alpha-D-glucose, myo-inositol and mannitol.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose transport across the basal plasma membrane of human placental syncytiotrophoblast

Transfer of glucose from maternal to fetal circulations requires transport across both the microvillous (maternal-facing) and basal (fetal-facing) plasma membranes of the placental syncytium. We have previously reported transport properties of the microvillous membrane and we now report those of the basal membrane. Basal plasma membrane vesicles were prepared by selective sonication and density gradient centrifugation. Glucose or glucose analogues were rapidly transported across these membranes by facilitated diffusion. Transport was inhibited by cytochalasin B, phloretin and phloridzin. L-Glucose at 1 mM was transferred at only 1/700 of the rate of D-glucose, which indicated an insignificant nonspecific diffusion component. Transport was independent of sodium gradients, and kinetic studies under equilibrium-exchange conditions demonstrated a Km of 23 mM. Competition studies demonstrated that aldohexoses in the C-1 chair conformation were the preferred substrates. Placental steroids estriol and progesterone inhibited transport. In contrast to other polarized epithelia, the basal and microvillous membranes of the human placental syncytium possess transport systems with similar properties. Thus, the directionality and rate of transfer of glucose across the intact syncytium are likely to be direct functions of the materno-fetal concentration gradient and the total transport capacities of the two plasma membranes.

The basolateral membrane of rat enterocyte: its purification from brush border contamination

Basolateral membranes obtained by self-orienting Percoll-gradient centrifugation were treated with 5 mM CaCl2 to minimize the cross-contamination by brush border membranes. From marker enzyme-specific activities it was calculated that in this preparation the basolateral/brush border membrane ratio was 22.6. A low L-glucose permeability across basolateral membrane vesicles together with ATP-dependent sodium uptake was observed.

Substrate utilization in the isolated perfused cortical thick ascending limb of rabbit nephron

Isolated segments of cortical thick ascending limbs (cTAL) of rabbit kidney were perfused in vitro and the equivalent short circuit current (Isc) was measured. In a first series all substrates were removed on either side. Isc fell rapidly to 50 +/- 12% after 3 min and to 27 +/- 6% (n = 5) after 10 min. This indicates that in cTAL segments Isc is strictly dependent on the presence of substrates. In series two it was tested what substrates can be utilized by the cTAL segment, and from which epithelial side [bath (b) or lumen (1)] the substrates are taken up. From the 1-side only butyrate (10 mmol X 1(-1) sustained the Isc at 95 +/- 2% (n = 7). All other tested substrates (10 mmol X 1(-1): pyruvate, acetate, beta-OH-butyrate, D-glucose, and L-lactate lead to a marked decline in Isc. From the b-side several substrates (5--10 mmol X 1(-1) sustained the Isc: D-glucose, D-mannose, butyrate, beta-OH-butyrate, acetoacetate, L-lactate, acetate and pyruvate. Other compounds (1--10 mmol X 1(-1): citrate, alpha-ketoglutarate, succinate, glutamate, glutamine, propionate, caprylate and oleate did not sustain Isc. In the third series the mechanism of substrate utilization from the basolateral cell side was studied. It was shown that the Isc is a saturable function of the D-glucose, L-lactate, acetate, pyruvate or beta-OH-butyrate concentration with apparent Km's between 0.05--1.0 mmol X 1(-1). Several known inhibitors of sugar and of anion transport were tested at the bath side: phlorrhizin was without effect. Phloretin (500 mumol X 1(-1) inhibited Isc by 96%, yet its effect was not dependent on the presence of substrates on the b-side since inhibition occurred also if the b-perfusate contained no substrate and Isc was driven by luminal butyrate. Also SITS (5 mmol X 1(-1) exerted only a small inhibitory effect which was not specific since it was also observed with luminal butyrate. alpha-Cyano-m-OH-cinnamate (10 mmol X 1(-1) inhibited the Isc specifically when L-lactate was the bath substrate. Probenecid (1 mmol X 1(-1) had a similar yet less marked inhibitory effect. The D-glucose uptake from the b-side was specifically inhibited by cytochalasin B at 5 X 10(-6) mol X 1(-1). We conclude that the cTAL segment of the rabbit utilizes D-glucose and/or small anions such as pyruvate or L-lactate or acetate to energize salt reabsorption.(ABSTRACT TRUNCATED AT 400 WORDS)

Bicarbonate movement across basolateral membrane vesicles from rat jejunum

The self-orienting Percoll-gradient centrifugation was used to separate basolateral membranes (BLMs) from rat jejunal enterocytes. Bicarbonate uptake into osmotically active BLM vesicles was studied by a rapid filtration technique. The time course of [14C] -labelled bicarbonate uptake was followed for 30 min at 18 degrees C and at pH 8.2. Bicarbonate uptake was fast, not saturable and stimulated by countertransport. This work demonstrates that it is possible to determine bicarbonate flux across BLM vesicles at pH and temperature values close to usual experimental conditions. Together with simple diffusion, bicarbonate would cross the BLM of the enterocyte via a carrier-mediated diffusion process.

Bicarbonate uptake by basolateral membrane vesicles from rat jejunum

Basolateral membranes from rat jejunal enterocytes have been obtained by self-orienting Percoll-gradient centrifugation. Bicarbonate and L-glucose uptake into osmotically active basolateral membrane vesicles has been studied by a rapid filtration technique. In closed vessels and at pH 8.2 the uptake kinetics of both [14C]bicarbonate and L[3H]glucose have been followed for 30 min at 18 degrees C. Bicarbonate uptake seems to be fast and in efflux experiments SITS and DIDS effect is negligible. This work demonstrates that it is possible to determine bicarbonate flux across basolateral membrane vesicles at pH and temperature values close to usual experimental conditions.

Effect of phloretin on Na+-dependent D-glucose uptake by intestinal brush border membrane vesicles

In the present work we studied the effects of phloretin on Na+-dependent D-glucose uptake by brush border membrane vesicles isolated from rabbit small intestine. Phloretin had no inhibitory effect on Na+-dependent D-glucose uptake in the presence of equilibrated Na+, although it inhibited the process in the presence of a Na+ gradient. Phlorizin inhibited Na+-dependent D-glucose uptake both in the presence and in the absence of a Na+ gradient. Phloretin and phlorizin competed with each other for the inhibition of Na+-energized D-glucose uptake. These results indicate that phloretin has no direct interaction with the Na+-dependent D-glucose transporter per se, though phloretin and phlorizin may have a common step(s) in their inhibition mechanisms. Phloretin, but not phlorizin, was found to increase the equilibrium level of D-glucose uptake whether Na+ was present or not. The increase in the equilibrium level was due to phloretin-induced binding of D-glucose to the brush border membrane, but not due to an increase in the intravesicular space. It seems to occur by a mechanism different from that by which phloretin inhibits Na+-energized D-glucose uptake, because the equilibrium level of D-glucose uptake is increased by the ionized form of phloretin but Na+-energized D-glucose uptake is inhibited by the unionized form.

Isolation of transporting plasma membrane vesicles from bovine tracheal epithelium

A method is described for isolating plasma membrane vesicles from bovine tracheal epithelium. The procedure yields highly purified apical membranes which are enriched 19-fold in the marker enzyme, alkaline phosphatase. Contamination of this fraction by other organelles is minimal. Basolateral membranes isolated from the same preparation have a 4-fold enrichment of (Na+ + K+)-ATPase and a 2-fold reduction in alkaline phosphatase specific activity compared to the starting material. Assays of Na+ uptake by the apical membrane vesicles demonstrate their suitability for transport studies. Transport of Na+ into an intravesicular space was demonstrated by (1) a linear inverse correlation between Na+ uptake and medium osmolarity; (2) complete release of accumulated Na+ by treatment with detergent; and (3) a marked temperature-dependence of Na+ uptake rate. Other features of Na+ transport were (1) inhibition by amiloride; (2) insensitivity to furosemide; and (3) anion-dependence of uptake rate with the following selectivity:SCN- greater than Cl- greater than gluconate-.

Reconstitution of a partially purified Na+-independent D-glucose transport system from rat jejunal basolateral membranes

1. Basolateral membranes of rat small intestine were first solubilized in a 0.6% cholate buffer and then the insoluble fraction was reextracted with a 1.2 or 1.6% cholate buffer. 2. Proteoliposomes reconstituted from the 1.2 or 1.6% cholate-extracted membrane fraction demonstrated characteristic Na+-independent D-glucose transport of the native basolateral membrane vesicles: inhibitable by mercuric chloride and D-galactose. 3. To further purify this D-glucose transport system, the 1.6% cholate-extracted membrane fraction was chromatographed on either hydroxylapatite, concanavalin A, wheat-germ lectin or castor bean lectin-120 affinity gels. 4. Proteoliposomes reconstituted from the membrane proteins adsorbed on hydroxylapatite and subsequently passed through agarose-castor bean lectin-120 showed a 12-fold enrichment of Na+-independent D-glucose transport activity over that of the native membrane vesicles. 5. SDS-electrophoretic analysis showed that the protein composition of the hydroxylapatite-castor bean lectin-120 treated fraction was much simpler than that of both 1.6% cholate-extracted fraction and the native membrane vesicles.