Sugar uptake into brush border vesicles from dog kidney. II. Kinetics

Kinetic characterization of Na+/D-mannose cotransport in dog kidney: comparison with Na+/D-glucose cotransport

Brush-border membrane vesicles (BBMV) prepared from whole dog kidney cortex, or separately from outer cortex (OC) and outer medulla (OM), were used to study the kinetics and inhibition specificity of Na(+)-dependent D-mannose cotransport. In BBMV from whole cortex the measured parameters for Na+/D-mannose uptake were Km = 0.07 +/- 0.01 mM and Vmax = 4.19 +/- 0.24 nmol/mg protein per min (n = 36). In OC BBMV the Km for Na+/D-mannose was 0.04 mM, Vmax = 3.41 nmol/mg per min. In OM the Km was 0.06 +/- 0.02 mM Vmax = 0.18 nmol/mg per min. Thus only about 5% of Na+/D-mannose activity occurs in OM. Both mannoheptulose (Ki = 5.6 mM) and methyl alpha-D-mannoside (Ki = 0.05 mM) are competitive inhibitors of Na+/D-mannose uptake, but at comparable concentrations have little effect on Na+/D-glucose uptake. Phlorizin is a noncompetitive inhibitor of Na+/D-mannose uptake (Ki = 4.45 microM) but a more potent and competitive inhibitor (Ki = 0.58 microM) of Na+/D-glucose uptake. Phloretin (Ki = 104 microM) is a noncompetitive inhibitor of Na+/D-mannose uptake in BBMV. We conclude that Na+/D-mannose uptake is mediated by a unique high-affinity carrier located in the OC presumably at the luminal surface of the proximal convoluted tubule, with strong specificity requirements for sugars with mannose-like structures (i.e., axial C-2 hydroxyl group). Phlorizin is an inhibitor of both Na+/D-mannose and Na+/D-glucose cotransporters but is approx. 10 times less potent for the Na+/D-mannose system and also has a different mode of inhibition (i.e., noncompetitive vs. competitive). The different phlorizin inhibitory mechanisms on the Na+/D-glucose and Na+/D-mannose cotransporters may be mediated by distinct hydrophobic and sugar binding sites that characterize phlorizin-carrier interaction.

Identification of two unique polypeptides from dog kidney outer cortex and outer medulla that exhibit different Na+/D-glucose cotransport functional properties

The cloned Na+/D-glucose cotransporter SGLT1 and an additional recently isolated human kidney cDNA Hu14/K15 belong to a family of similar cotransport proteins including the Na(+)-dependent nucleoside and Na(+)-dependent myo-inositol carrier SMIT1. For the present study we used two different polyclonal antibodies raised against the amino acid sequence 402-420 (Ab-E) and 565-574 (Ab-P) of SGLT1 to probe brush-border membrane fractions from different regions (outer cortex-->outer medulla) of dog kidney. In Western blots both Ab-E and Ab-P react specifically (peptide blockable) with two distinct bands migrating on SDS-PAGE under reducing conditions at 75.5 kDa and 72.5 kDa. The higher molecular mass polypeptide is greatly enriched (13:1) in outer cortex and diminishes progressively towards outer medulla, whereas the lower molecular mass band is barely detectable in outer cortex but is enriched in outer medulla (4:1). Brush-border membrane vesicles (BBMV) prepared from the same outer cortical and outer medullary regions that were probed with Ab-E and Ab-P exhibit strikingly different Na+/D-glucose functional transport behavior. The Na+/D-glucose cotransport activity in outer cortical BBMV is a low-affinity system with Km = 5.98 +/- 1.01 mM, Vmax = 13.05 +/- 0.55 nmol/mg protein per min, and with 1:1 Na+:D-glucose stoichiometry. Outer medulla BBMV exhibit high-affinity Km = 0.27 +/- 0.03 mM Vmax = 0.97 +/- 0.04 nmol/mg protein per min and 2:1 Na+:D-glucose stoichiometry. Comparison of SGLT1, Hu14/K15, SNST1 and SMIT indicates that Ab-E could cross react with all four, but Ab-P would recognize SGLT1, Hu14/K15, SNST1 but not SMIT. Also SNST1 is not expressed in outer cortex. Based on currently available sequence data, and its marked enrichment in outer cortex, the 75.5 kDa band is a likely candidate protein responsible for low-affinity and 1:1 Na+:D-glucose stoichiometric Na+/D-glucose cotransport activity (Hu14/K15) while the minor 72.5 kDa band in outer cortex is probably SGLT1. In outer medulla, the predominant band recognized by both Ab-E and Ab-P is the 72.5 kDa protein and this could be either SGLT1 or SNST1.

Down's syndrome fibroblasts exhibit enhanced inositol uptake

The inositol metabolism of Down's syndrome (DS, trisomy 21) skin fibroblasts was examined. We report that DS cells accumulated [3H]inositol 2-3-fold faster than did other aneuploid or diploid controls. In contrast, trisomy 21 did not affect the uptake of choline, serine or glucose. Kinetic analysis demonstrated an increased maximal velocity of high-affinity, Na(+)-dependent, inositol transport, consistent with the expression of higher numbers of transporters by DS cells. Enhanced uptake was accompanied by a proportional increase in the incorporation of radiolabelled inositol into phospholipid. We suggest that an imbalance of inositol metabolism may contribute to plasma membrane abnormalities characteristic of DS cells.

Low-affinity transport of pyroglutamyl-histidine in renal brush-border membrane vesicles

These studies were performed to determine if a low-affinity carrier is present in the luminal membrane of proximal tubular cells for the transport of the dipeptide, pyroglutamyl-histidine (pGlu-His). We have previously described the existence of a specific, high-affinity, low-capacity [transport constant (Kt) = 9.3 X 10(-8) M, Vmax = 6.1 X 10(-12) mol.mg-1.min-1] carrier for pGlu-His in renal brush-border membrane vesicles. In the present study, we sought to demonstrate that multiple carriers exist for the transport of a single dipeptide by determining whether a low-affinity carrier also exists for the uptake of pGlu-His. Transport of pGlu-His into brush-border membrane vesicles was saturable over the concentration range of 10(-5)-10(-3) M, yielding a Kt of 6.3 X 10(-5) M and a Vmax of 2.2 X 10(-10) mol.mg-1.min-1. Uptake was inhibited by the dipeptides glycyl-proline, glycyl-sarcosine, and carnosine but not by the tripeptide pyroglutamyl-histidyl-prolinamide. We conclude that 1) pGlu-His is transported across the luminal membrane of the proximal tubule by multiple carriers and 2) the lower affinity carrier, unlike the higher affinity carrier, is nonspecific with respect to other dipeptides.

Isolated proximal tubular cells from rat kidney as an in vitro model for studies on nephrotoxicity. I. An improved method for preparation of proximal tubular cells and their functional characterization by alpha-methylglucose uptake

Rat renal proximal tubular cells were isolated by successive EGTA and collagenase perfusions and purified by filtration and isopycnic centrifugation. The method is rapid and provides a much higher fraction of proximal tubular cells (90-95%) than comparable methods. The yield of viable (97 +/- 3%) cells is high (30 X 10(6) cells/g kidney). The intracellular ATP was 16 +/- 2 nmol/mg protein and remained essentially constant for at least 3 hr. The cells were characterized for transport of organic ions and glucose. Glucose transport was studied by alpha-[14C]methylglucose uptake; apparent Km and Vmax values were 3.4 +/- 0.5 mM and 4.1 +/- 0.9 nmol/min.mg protein, respectively. This transport could almost be completely inhibited by phloridzin, indicating that the uptake is mediated by the brush border glucose carrier.

The relationship between renal metabolism and proximal tubule transport during ontogeny

The proximal tubules of newborn and adult animals reabsorb a similar fraction of the filtered load of Na+ and H2O (65%-70%). In tubules from adult animals, transcellular, active Na+ reabsorption accounts for one-third of the total, while two-thirds occur passively through the paracellular pathway, driven by hydrostatic and oncotic forces (one-third) and by cell-generated effective osmotic and ionic gradients (one-third). Since two-thirds of the Na+ is reabsorbed passively and does not require energy, the mature proximal tubule has a high Na+/O2 molar ratio (48 Eq of Na+/mol of O2). Measurements of ouabain-sensitive oxygen consumption in suspensions of proximal tubules indicate that in newborn, aerobic metabolism can support about 50% of the net Na+ transport rate compared with the 33% in tubules from adult animals. Independent confirmation of the direct and proportional relationship between active Na+ transport and ouabain-sensitive O2 consumption exists for the adult but not for the newborn. However, measurements of epithelial conductances and of transepithelial hydrostatic and oncotic pressure differences indicate that passive paracellular fluxes can account for the remaining 50% of the proximal Na+ reabsorption in newborn. The high permeability of the proximal tubules of newborn animals to small molecular weight solutes suggests that cell-generated osmotic and ionic transepithelial gradients are minimal in the tubules of newborn animals. Yet in the newborn, the osmolality of the end proximal tubule fluid was found to exceed that in plasma. This indicates that osmotic gradients due to differences in reflection coefficients for preferentially reabsorbed solutes and Cl- do exist across the proximal tubules of the newborn and suggests that these gradients may contribute to Na+ and H2O reabsorption. If this is indeed the case, then the contribution of active and of hydrostatic and oncotic pressure-driven flows to the overall reabsorption of Na+ and fluid has been overestimated. Resolution of this discrepancy requires measurements of the reflection coefficients for HCO3- and Cl- in the proximal tubule of the newborn. The metabolic processes by which energy is supplied to renal proximal cells during development are also incompletely characterized. There is evidence that maturation of aerobic metabolism, Krebs cycle enzymes activity, and of the mitochondrial membrane surface area precede the development of net reabsorptive transport (Na+, H2O, HCO3, glucose). By contrast, maturation of Na(+)-K(+)-ATPase activity at the basolateral cell membrane follows that in reabsorptive transport and does not limit its development.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Carrier-mediated transport of urate by chicken (Gallus domesticus) renal brush-border membrane vesicles

1. Urate is transported by an anion exchange system in chicken renal brush-border membrane vesicles. An outward chloride gradient stimulates an overshoot in urate uptake. 2. The anion exchanger is similar to those described for the kidneys of rats and dogs except that the chicken exchanger has a higher specificity for urate. 3. p-Aminohippurate has only a weak affinity, if any, for the urate exchanger. 4. There were no apparent qualitative differences in urate transport between brush-border membrane vesicles from genetically normouricemic and hyperuricemic chickens.

Primary active sodium transport, oxygen consumption, and ATP: coupling and regulation

Several metabolic aspects of primary active transport have been explored in this communication. One emphasized theme entailed the need to investigate the properties of the mitochondria and the active transport systems within the intact cell. Several methodological and conceptual approaches were described that permitted such an analysis. The answers provided were sometimes qualitative or quantitative. Qualitative information was provided regarding the cytosolic signal linking active transport with respiration, suggesting that the cytosolic ADP concentration was an important element in that link. The intact renal cell was found to work normally at 50 to 60% of its maximal respiratory capacity, indicating that sufficient reserve capacity was present for increased metabolic demands. Several examples were described in which a combination of qO2 measurements and/or optical techniques were used to differentiate between effects of agents which act primarily on transport or metabolic events. Finally, the control of transport by metabolism was discussed, primarily emphasizing the role of ATP and Pi. One of the overall conclusions from these studies is that, in general, the mitochondria and the transport systems seem to display similar properties in the intact cell as they do in isolated form. However, uncertainties concerning the cellular microenvironment surrounding the mitochondria and the plasma membrane transporters have produced some interesting surprises concerning their function in the intact cell. More quantitative information on the energy compartmentation of the renal cell would be helpful to clarify numerous aspects of metabolic function.

Glucose-dependent respiration in suspensions of rabbit cortical tubules

The effects of glucose on cellular respiration were examined in suspensions of rabbit cortical tubules. When glucose was removed from the bathing fluid, oxygen consumption (QO2) decreased from 18.6 +/- 0.8 to 15.7 +/- 0.5 nmol O2/mg protein X min (P less than 0.01). The transported but nonmetabolized analogue of glucose, alpha-methyl-D-glucoside (alpha MG), was found to support QO2 to the same extent as glucose. These observations were also evident in the presence of butyrate, a readily oxidized substrate of the renal cortex. Additional studies with nystatin and ouabain indicated that glucose-related changes in QO2 were the result of changes in Na, K-ATPase associated respiration. The effect of glucose was localized to the luminal membrane since phlorizin (10(-5) M), a specific inhibitor of luminal glucose-sodium cotransport, also significantly reduced QO2 by 10 +/- 1%. Phlorizin inhibition of QO2 was also evident in the presence of alpha MG but was abolished when glucose was removed from the bathing medium. Finally, measurement of NADH fluorescence showed that addition of glucose (5 mM) to a tubule suspension causes an oxidation of NAD. These data are all consistent with glucose acting to increase respiration by stimulating sodium entry at the luminal membrane (via glucose-sodium cotransport) followed by increased sodium pump activity and its associated increase in mitochondrial respiration.

Transport of L-cysteine by rat renal brush border membrane vesicles

Brush border membranes were isolated from rat renal cortex by a divalent cation precipitation method. L-35S-cysteine uptake into the vesicles was measured by a rapid filtration method. Only minimal binding of the amino acid to the vesicles was observed. Sodium stimulates L-cysteine uptake specifically. Anion replacement experiments, experiments in the presence of potassium/valinomycin-induced diffusion potential as well as experiments with a potential-sensitive fluorescent dye document an electrogenic sodium-dependent uptake mechanism for L-cysteine. Tracer replacement experiments as well as the fluorescence experiments indicate a preferential transport of L-cysteine. Transport of L-cysteine is inhibited by L-alanine and L-phenylalanine but not by L-glutamic acid and the L-basic amino acids. Initial, linear influx kinetics provide evidence for the existence of two transport sites. The results suggest (a) sodium-dependent mechanism(s) for L-cysteine shared by other neutral amino acids.

Stoichiometric studies of the renal outer cortical brush border membrane D-glucose transporter

The stoichiometric properties of the renal outer cortical brush-border membrane D-glucose transporter are studied. Experiments which establish the glucose/sodium, glucose/phlorizin and phlorizin/sodium stoichiometries are reported. Three independent method of determining the substrate/activator (glucose/sodium) stoichiometry for coupled transport systems are presented and discussed. One of these, the "Static Head Method," is introduced here for the first time. This type of experiment appears to be more generally applicable than the usual procedure of directly measuring the coupled fluxes of substrate and activator to determine stoichiometric coupling ratios. The results presented in this paper demonstrate that the glucose/sodium/phlorizin stoichiometry of the renal outer cortical brush-border membrane D-glucose transport system is 1:1:1.

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)

Further studies of proximal tubular brush border membrane D-glucose transport heterogeneity

The properties of two sodium-dependent D-glucose transporters previously identified in renal proximal tubule brush border membrane (BBM) vesicles are studied. The low-affinity system, found in BBM vesicles from the outer cortex (early proximal tubule), is shown to be associated with the high-affinity phlorizin binding site typically found in renal BBM preparations. The high-affinity system, found in BBM vesicles from the outer medulla (late proximal tubule), is almost two orders of magnitude less sensitive to inhibition by phlorizin and is apparently not associated with high-affinity phlorizin binding. The sodium/glucose stoichiometry of the outer medullary transporter is found to be 2:1 by two independent methods. Previous measurements have established that the stoichiometry of the outer cortical system is 1:1. It is suggested that this arrangement of transporters in series along the proximal tubule enables the kidney to reabsorb glucose from the urine in an energy-efficient fashion. The bulk of the glucose load is reabsorbed early in the proximal tubule at an energetic cost of one Na+ per glucose molecule. Then in the late proximal tubule a larger coupling ratio and hence a larger driving force is employed to reabsorb the last traces of glucose from the urine.

Transepithelial transport in cell culture: D-glucose transport by a pig kidney cell line (LLC-PK1)

The pig kidney cell line LLC-PK1 cultured on a collagen coated membrane filter formed a continuous sheet of oriented asymmetrical epithelial cells joined by occluding junctions. A transepithelial electrical potential (PD) and short-circuit current (SCC) were dependent on the presence of Na and sugar in the apical bathing solution. In the presence of 5.5 mM D-glucose, a PD of 2.8 mV. apical surface negative a SCC of 13 microA cm-2 and transepithelial resistance of 211 ohm.cm2 were recorded. The SCC was promptly reduced by the addition of phlorizin to the apical bath but unaffected when placed in the basolateral bath. The effect on SCC of various sugars was compared by the concentrations required for half-maximal SCC: 0.13 mM beta-methyl-D-glucoside, 0.28 mM D-glucose, 0.65 mM alpha-methyl-D-glucoside, 0.77 mM 6-deoxy-D-glucose, 4.8 mM D-galactose, and 29 mM 3-O-methyl-glucose. When [Na] was reduced, the concentration of D-glucose required for half-maximal SCC increase. Isotopically labeled 3H and 14C D-glucose were used to simultaneously determine bidirectional fluxes; a resultant net apical-to-basolateral transport was present and abolished by phlorizin. The transported isotope cochromatographed with labeled D-glucose, indicating negligible metabolism of transported glucose. The pig kidney cell line, LLC-PK1, provides a cell culture model for the investigation of mechanisms of transepithelial glucose transport.

Interaction of phlorizin and sodium with the renal brush-border membrane D-glucose transporter: stoichiometry and order of binding

The order and stoichiometry of the binding of phlorizin and sodium to the renal brush-border membrane D-glucose transporter are studied. The experimental results are consistent with a random-binding scheme in which the ratio of phlorizin- to sodium-binding sites is one-to-one. When the kinetics of phlorizin binding are measured as a function of increasing sodium concentration no significant variation is found in the apparent number of binding sites; however, the apparent binding constant for phlorizin decreases rapidly from approximately 16 microM at [Na] = 0 to 0.1 microM at [Na] = 100 mM and approaches 0.05 microM as [Na] leads to infinity. The experimental data are fit to a random carrier-type model of the coupled transport of sodium and D-glucose. A complete parameterization of the phlorizin binding properties of this model under sodium equilibrium conditions is given.

Chloride uptake by brush border membrane vesicles isolated from rabbit renal cortex. Coupling to proton gradients and K+ diffusion potentials

Brush border membrane vesicles were isolated from rabbit renal cortex by Mg(++)-precipitation and differential centrifugation. (36)Cl(-) and [(3)H]glucose uptakes were simultaneously determined by a rapid filtration technique. Lysis of the vesicles with distilled water abolished 90-95% of the radioactivity on the filters, suggesting that nearly all of the (36)Cl(-) and [(3)H]glucose counts represented uptake into an osmotically reactive intravesicular space. Inwardly directed K(+) gradients plus valinomycin stimulated (36)Cl(-) uptake, demonstrating a conductive pathway for chloride uptake into brush-border membrane vesicles. (36)Cl(-) uptake could also be stimulated by inwardly directed proton gradients (pH(outside) < pH(inside)). This effect was seen in the absence of sodium, as well as in the presence of valinomycin when the vesicles had equal K(+) concentrations inside and out. An "overshoot" phenomenon was observed when external (36)Cl(-) was 2 mM and the external pH was lowered from 7.5 to 6.0 or to 4.5. The effect of the proton gradient was presumed to be different from the conductive mechanism because (a) the stimulation of (36)Cl(-) uptake by inwardly directed K(+) diffusion potentials was additive to the proton gradient effect, and (b) competition studies revealed statistically significant effects of thiocyanate on the conductive pathway, but not on the proton-driven pathway.HCl cotransport or anion exchange are electrically neutral mechanisms which could couple (36)Cl(-) uptake to inwardly-directed proton gradients in a brush border membrane vesicle. If both electrically neutral and conductive path ways for chloride transport are present in the luminal membrane of the proximal tubule, then the mechanism as well as the direction of net chloride transport will be influenced by the nature of the accompanying cation transport process.