Sugar uptake into brush border vesicles from dog kidney. I. Specificity

Tubular reabsorption of myo-inositol vs. that of D-glucose in rat kidney in vivo et situ

Filtered myo-inositol, an important renal intracellular organic osmolyte, is almost completely reabsorbed. To examine tubule sites and specificity and, thus possible mechanism of this reabsorption, we microinfused myo-[(3)H]inositol or D-[(3)H]glucose into early proximal (EP), late proximal (LP), or early distal tubule sections of superficial nephrons and into long loops of Henle (LLH) of juxtamedullary nephrons and papillary vasa recta in rats in vivo et situ and determined urinary fractional recovery of the (3)H label compared with comicroinfused [(14)C]inulin. To determine the extent to which the proximal convoluted tubule (PCT) alone contributes to myo-inositol reabsorption, we also microperfused this tubule segment between EP and LP puncture sites. We examined specificity of reabsorptive carrier(s) by adding high concentrations of other polyols and monosaccharides to the infusate. The results show that >60% of the physiological glomerular load of myo-inositol can be reabsorbed in the PCT and >90% in the short loop of Henle (SLH) by a saturable, phloridzin-sensitive process. myo-Inositol can also be reabsorbed in the ascending limb of LLH and can move from papillary vasa recta blood into ipsilateral tubular structures. Essentially no reabsorption occurred in nephron segments beyond the SLH or in collecting ducts. Specificity studies indicate that reabsorption probably occurs via a luminal Na(+)-myo-inositol cotransporter.

Identification and characterization of high molecular-mass mucin-like glycoproteins in the plasma membrane of airway epithelial cells

A previous lectin binding study demonstrated the presence of high molecular-mass mucin-like glycoproteins (HMGP) on the surface of hamster tracheal surface epithelial (HTSE) secretory cells (Proc. Natl. Acad. Sci. USA 1987;84:9304). In the present study, we intended to isolate and characterize these HMGP from the plasma membrane of the primary HTSE cells and then to determine whether or not these membrane HMGP are Muc-1 mucins, a type of mucins originally discovered on the surface of some carcinomas. A subcellular fraction enriched with the plasma membrane was obtained using a sucrose density gradient centrifugation. This fraction contained high molecular-mass glycoconjugates which were excluded from Sepharose CL-4B gel. Biochemical characterization of these glycoconjugates revealed the following characteristics: (1) susceptibility to both pronase and mild alkaline treatments, but totally resistant to proteoglycan-digesting enzymes; (2) partitioning in the detergent phase of Triton X-114 and resistance to digestion by phosphatidylinositol phospholipase C or D; (3) a buoyant density of 1.5 g/ml based on CsCl density gradient centrifugation; (4) polydispersity in terms of both size and charge density; and (5) lack of immunoreactivity with an anti-Muc-1 mucin antibody. We conclude that the plasma membrane of HTSE cells at confluence contains HMGP, which seem to be the integral membrane proteins but different from Muc-1 mucins, and that these membrane HMGP appear to share some similarities with secreted mucins in terms of size and charge.

Glucose-induced excitation in molluscan central neurons producing insulin-related peptides

The light green cells (LGCs) are a group of identified central neurons in the pond snail, Lymnaea stagnalis, that produce a number of insulin-related peptides. Freshly dissociated LGCs are activated by physiological concentrations of extracellular glucose. The response to glucose consists of a slow depolarization, which, at concentrations of 1 mM or more, rapidly induces regular spiking activity. The response persists during prolonged application of glucose but is completely reversed upon washing. The threshold concentration is 0.2 mM; the maximal effect occurs at 5 mM. In LGCs in central nervous system preparations kept in organ culture for 16-24 h, glucose causes a similar depolarization, which may lead to spiking activity. In freshly isolated preparations, which have very inexcitable LGCs, no direct response to glucose was seen. The response is specific to the LGCs; no other central neurons in Lymnaea showed consistent responses. The glucose response is evoked by D-glucose and the non-metabolized analogue 2-deoxy-D-glucose, but not by related hexoses, including L-glucose, nor pentoses or disaccharides. The response is not affected by interfering with the glucose metabolism, nor is the response mimicked by the metabolite D-glyceraldehyde or by injection of glucose. This suggests that glucose metabolites are not involved in the response. The glucose response depends on the presence of extracellular Na+ and is blocked by phlorizin, which specifically inhibits Na(+)-coupled glucose transport. This suggests that the response is due to activation of an electrogenic Na(+)-coupled glucose transporter.

Cystine uptake by cultured cells originating from dog proximal tubule segments

Large numbers of kidney epithelial cells were cultured successfully from isolated dog proximal tubule segments. Cells in primary culture and in first passage retained the cystine-dibasic amino acid co-transporter system which is found in vivo and in freshly isolated proximal tubule segments. In contrast to other cultured cells, the cystine-glutamate anti-porter was absent in primary cultures. However, this anti-porter system seemed to be developing in cells in first passage. The intracellular ratio of cysteine:reduced glutathione (CSH:GSH) was maintained at 1:36 in both primary cultures and in low passage cells. Incubation of cells in primary culture for 5 min at 37 degrees C with 0.025 mM [35S]L-cystine resulted in incorporation of approximately 36 and 8.5% of the label into intracellular CSH and GSH, respectively. These cultured cells, therefore, seem to be an excellent model system for the eventual elucidation of a) the inticacies of cystine metabolism and b) regulation of 1) the cystine-dibasic amino acid co-transporter system and 2) the development of the cysteine-glutamate anti-porter system.

Techniques for isolation of brush-border and basolateral membrane vesicles from dog kidney cortex

Two methods are reported for renal membrane preparation from the dog kidney cortex. One method is a simultaneous preparation of brush-border (BBMV) and basolateral (BLMV) membranes. Using readily available laboratory equipment, differential centrifugation produced a supernatant which was treated with Mg2+. The Mg2+ treatment produced a pellet (crude BLMV) which was added to Percoll and centrifuged to produce purified BLMV. The supernatant after Mg2+ treatment eventually yielded pure BBMV after additional Mg2+ precipitations. The second method used an acidic medium in conjunction with divalent-cation precipitation to prepare BBMV. Whichever method was used, BBMV and BLMV showed appropriate enzyme and transport activities.

Dog and rat kidney proximal tubule cells in culture: responses to parathyroid hormone

We have described and compared culture systems for proximal tubule cell (PTC) preparations from dog and rat kidney. Cells were prepared from kidney cortex by enzyme digestion and purified on Percoll density gradient. The dog PTC and rat PTC differed in their growth characteristics in culture. Although the dog PTC tended to overgrow the contaminating fibroblastic cells, the rat PTC tended to be overgrown by the latter cells when cultured in medium containing 15% fetal calf serum (FCS). Cultures of rat PTC in serum-free medium or medium containing only 2% FCS yielded only epithelium-like cells exhibiting characteristics of cells that are proximal tubular in origin. These properties include protrusion of microvilli, high alkaline phosphatase activity, ability to transport sugar, and responsiveness to parathyroid hormone (PTH) in terms of cAMP production. The time course and dose-response curves of PTH-stimulated cAMP accumulation were studied in dog and rat PTC. The estimated half-maximal concentrations (Kact) for PTH in dog and rat PTC were 1.2 and 100 nM, respectively. Both values are within the range reported in the literature for the respective renal membrane preparations. In addition to our previously reported data on dog PTC, this study revealed the presence of PTH-inhibitable 25-hydroxyvitamin D3-24-hydroxylase in dog PTC. These two model cell culture systems should prove useful in studying PTH action in renal cells.

Transport of sodium into apical membrane vesicles prepared from fetal sheep alveolar type II cells

A method is described for isolating apical plasma membrane vesicles from fetal alveolar type II cells. The procedure yields purified apical membranes which are enriched 24-fold with the brush-border enzyme marker, alkaline phosphatase. Contamination of this fraction by basolateral membranes and organelles is minimal. Evidence for transport of Na+ into an intravesicular space is demonstrated by: (1) time-dependent uptake of Na+ with release of accumulated Na+ by treatment with detergent; (2) a linear inverse correlation between Na+ uptake and medium osmolarity. In addition, Na+ uptake is shown to be anion dependent (SCN- greater than Cl- greater than gluconate-) and sensitive to amiloride inhibition at a concentration of 1 mM.

Sodium D-glucose cotransport in the gill of marine mussels: studies with intact tissue and brush-border membrane vesicles

Glucose transport was studied in marine mussels of the genus Mytilus. Initial observations, with intact animals and isolated gills, indicated that net uptake of glucose occurred in mussels by a carrier-mediated, Na+-sensitive process. Subsequent studies included use of brush-border membrane vesicles (BBMV) in order to characterize this transport in greater detail. The highest activity of Na+-dependent glucose transport was found in the brush-border membrane fractions used in this study, while basal-lateral membrane fractions contained the highest specific binding of ouabain. Glucose uptake into BBMV showed specificity for Na+, and concentrative glucose transport was observed in the presence of an inwardly directed Na+ gradient. There was a single saturable pathway for glucose uptake, with an apparent Kt of 3 microM in BBMV and 9 microM in intact gills. The kinetics of Na+ activation of glucose uptake were sigmoidal, with apparent Hill coefficients of 1.5 in BBMV and 1.2 in isolated gills, indicating that more than one Na+ may be involved in the transport of each glucose. Harmaline inhibited glucose transport in mussel BBMV with a Ki of 44 microM. The uptake of glucose was electrogenic and stimulated by an inside-negative membrane potential. The substrate specificity in intact gills and BBMV resembled that of Na+-glucose cotransporters in other systems; D-glucose and alpha-methyl glucopyranoside were the most effective inhibitors of Na+-glucose transport, D-galactose was intermediate in its inhibition, and there was little or no effect of L-glucose, D-fructose, 2-deoxy-glucose, or 3-O-methyl glucose. Phlorizin was an effective inhibitor of Na+-glucose uptake, with an apparent Ki of 154 nM in BBMV and 21 nM in intact gills. While the qualitative characteristics of glucose transport in the mussel gill were similar to those in other epithelia, the quantitative characteristics of this process reflect adaptation to the seawater environment of this animal.

Relationship of the Donnan potential to the transmembrane pH gradient in tracheal apical membrane vesicles

Experiments were performed to determine the factors which contribute to the transmembrane pH gradient (delta pH) and the potential gradient (delta psi) in apical plasma membrane vesicles isolated from bovine tracheal epithelium. As indicated by the accumulation of 14C-methylamine, the vesicles maintained a delta pH (inside acidic) which was dependent upon the external pH. The delta pH was also proportional to the ionic strength of the suspending medium, suggesting that the H+ distribution was dictated by a Donnan potential. Measurements of the distribution of 86Rb+ demonstrated an electrical potential gradient across the vesicle membrane, inside negative which was proportional to the medium ionic strength. delta pH changed in parallel with delta psi in response to a variety of imposed conditions. These results are compatible with the existence of a H+ conductance in the vesicle membrane. Thus the endogenous electrical and proton gradients may be manipulated and used as a general experimental tool to complement kinetic analysis in investigations of transport mechanisms using isolated vesicle preparations.

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)

Computational analysis of models for cotransport

The order of substrate interaction with a cotransport carrier is studied by numerically fitting theoretical models to empirical data for a Na+-D-glucose pathway. Our analysis is based on a least-squares minimization routine developed at CERN, and uses data derived from tracer flux of substrate under equilibrium exchange conditions. Random, Ordered mirror, and Ordered glide models are considered and the applicability of both Random and Glide systems to the experimental observations is demonstrated. A more detailed study of the Glide model provides an estimation of the relative values of the individual rate constants describing each kinetic step in the mechanism. We argue that parameterization of competing models can result in tests which will distinguish between them, even when these contain many unknowns in the proposed structure of the kinetic mechanism. The essence of this means of model discrimination is the ability to find a single set of parameter values that fits a variety of experimental observations. The ability to obtain numerical estimates of obtain numerical estimates of individual rate constants is also a useful tool in investigation of the kinetic fine structure of the carrier. For the data at hand we conclude that a complete parameterization of the Glide model cannot be attained, possibly due to a heterogeneity in the temperature at which the experiments have been performed. Three solutions to the Glide model are presented, each of which corresponds to a fit of the Glide model to a subset of the experimental data.

Characterization of an essential disulfide bond associated with the active site of the renal brush-border membrane D-glucose transporter

In a previous report (J. Biol. Chem. 258 (1983) 3565-3570) we have demonstrated that the disulfide-reducing agent dithiothreitol has two effects on the sodium-dependent outer cortical brush border membrane D-glucose transporter; the first results in a reversible increase in the affinity of the transporter for the non-transported competitive inhibitor phlorizin, while the second results in a partially reversible loss of phlorizin binding and glucose-transport activity. Evidence was presented that both of these effects are the result of the reduction of disulfide bonds on the transport molecule. In the present paper we extend our observations on the inactivation of the transporter by dithiothreitol. We provide evidence here (i) that the inactivation of the transporter by dithiothreitol is independent of the effect of the reducing agent on the affinity of the transporter, (ii) that this inactivation process is first-order in dithiothreitol and thus presumably due to the reduction of a single disulfide bond essential to the functioning of the transporter. (iii) that it is the reduction of this disulfide bond and not some subsequent conformational or other change in the transporter which results in its inactivation, (iv) that phlorizin and substrates of the transporter provide protection against inactivation by dithiothreitol and that the degree of protection provided correlates well with the known specificity and phlorizin-binding properties of the transporter, and (iv) that the reactivity of the transporter with dithiothreitol is pH-dependent, decreasing with increasing pH over the pH range 6.5-8.5. We conclude that this site of action of dithiothreitol is a single essential disulfide bond intimately associated with the glucose-binding site on the transport molecule.

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-.

Na+-dependent hexose transport in vesicles from cultured renal epithelial cell line

Apical membrane vesicles were prepared from cultured epithelia formed by LLC-PK1 cells using a calcium precipitation technique. alpha-Methylglucoside uptake into this vesicle preparation was markedly stimulated by sodium and inhibited by phlorizin. In addition, a transient "overshoot" of intravesicular alpha-methylglucoside concentration above its equilibrium value was observed under initial sodium gradient conditions. The specificity of this sodium-dependent hexose transporter closely resembled that found in the mammalian kidney brush border membrane, e.g., alpha-methylglucoside, D-glucose, and D-galactose apparently share the transporter while 2-deoxy-D-glucose, mannose, and fructose do not. Kinetic analysis of the sodium-dependent component of alpha-methylglucoside flux into LLC-PK1 apical membrane vesicles indicates the existence of single transporter with Km congruent to 2 mM and Vmax congruent to 3 nmol.min-1.mg protein-1. Measurement of alpha-methylglucoside uptake as a function of sodium concentration is consistent with a sodium:sugar stoichiometry of approximately 2:1.l There is a good correlation over time between the development of the concentrating capacity of the intact epithelium for alpha-methylglucoside and the transport properties of the vesicle preparation.

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)

2-Deoxy-D-glucose uptake by rat granular pneumocytes in primary culture

Uptake of 2-deoxy-D-glucose (DG) was investigated with rat granular pneumocytes isolated in primary culture. Cells attached to flasks were incubated in Minimal Essential Medium usually containing 5 mM DG in place of glucose. Uptake of DG increased progressively with time of incubation and approached a plateau value of 35-40 mumol/10(6) cells at 60 min. Uptake increased as a function of external DG concentration with half-maximal uptake at approximately 2.0 mM DG. DG uptake was inhibited by the presence of glucose, alpha-methylglucoside, phlorizin, ouabain, or sodium-free medium. After 60 min incubation, approximately 20% of total intracellular DG was in the free form, and the calculated mean intracellular concentration of free DG (n = 4) was approximately twice the external concentration. Phosphatase activity was indicated by increase in free DG and efflux from cells after removal of external DG. In comparison with pneumocytes, uptake of DG by alveolar macrophages showed different kinetics, and intracellular free DG did not exceed the extracellular concentration. These findings indicate that type II cells take up DG by a sodium-dependent, carrier-mediated transport process that results in accumulation of free sugar against a concentration gradient.

Electrophysiological analysis of rat renal sugar and amino acid transport. II. Dependence on various transport parameters and inhibitors

Transepithelial and cellular electrical potential changes were measured in response to luminal perfusion of D-glucose and related substrates in micropuncture experiments on rat kidney in vivo. By studying the dependence of the potential response on various experimental parameters, some insight was obtained into the mechanism of Na+ coupled glucose absorption. The experiments confirm the driving forces for glucose absorption in the living cell to be: a) the Na concentration gradient, b) the electrical potential gradient and c) the glucose concentration gradient across the brush-border membrane. Furthermore they describe the substrate specificity of the cotransport mechanism and the mechanism of inhibition of D-glucose transport by various inhibitors, such as phlorizin, harmaline and ouabain. The latter experiments suggest that the active Na+ pump in the peritubular cell membrane, which establishes the Na+ ion gradient and the electrical potential gradient across the brushborder, contributes a measurable partial conductance to the overall electrical conductance of the peritubular cell membrane.

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.

Transport of glucose analogues in rat lung

Uptake of nonmetabolizable glucose analogues was investigated in isolated rat lungs ventilated with 5% CO2 in air and perfused with Krebs-Ringer bicarbonate medium. In some experiments, [5-3H]glucose, methyl(alpha-D-[U-14C]gluco)pyranoside (alpha-MG), 3-O-methyl-D-[U-14C]glucose (3-O-MG), or various inhibitors were added to the initial perfusate to determine glucose utilization by the rate of 3H2O production or to characterize the uptake of glucose analogues. [1,2-3H]polyethylene glycol (PEG) was used as an indicator of the extracellular water space and gave a mean value of 0.35 ml/g tissue; calculated mean intracellular H2O space was 0.48 ml/g tissue. Glucose utilization was 56.4 +/- 6.6 (mean +/- SE, n = 6) mumol . g dry wt-1 . h-1 and decreased by 61% with 3 mM phlorizin. After 1-2 h perfusion, intracellular alpha-MG concentration was 1.4-1.9 times the extracellular concentration. The mean tissue-to-medium ratio (T/M) for alpha-MG decreased by more than 30% in the presence of glucose (5.0 mM), phlorizin (0.5 mM), ouabain (0.5 mM), or the absence of external Na+. Intracellular 3-O-MG concentration did not exceed extracellular concentration during 2 h of perfusion and the mean T/M did not change with any of the inhibitors studied. The results indicate that the nonmetabolizable glucose analogue alpha-MG is accumulated against a concentration gradient by an active Na+-dependent transport process, whereas 3-O-MG is apparently taken up by a different mechanism.

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.

The mechanism of maleic acid nephropathy: investigations using brush border membrane vesicles

Brush border membrane vesicles from dog kidney proximal tubule have been used to investigate the pathogenesis of glucosuria resulting from maleic acid nephropathy. Direct exposure of brush border vesicles to 9 mM maleic acid for up to 36 minutes has no effect of the Na+-dependent D-glucose uptake mechanism. Brush border membrane vesicles were then prepared from dogs in which glucosuria had first been induced by prior administration of maleic acid (250 mg/Kg body weight). The timed uptake of D- and L-glucose was measured in the presence in the presence of both Na+ chemical and electropotential driving forces and compared to uptake in vesicles prepared from matched control kidneys from the same animal. Maleic acid treated and control vesicles were also assayed for the presence of the Na+-dependent high affinity phlorizin receptor. Our results indicate that there is no alteration in behavior of the Na+-dependent transport system for glucose at the brush border membrane in the model Fanconi state produced by maleic acid. Moreover, there is no change in the coupling of sugar transport either to the Na+ chemical or the electrical potential gradient across the brush border membrane. These studies imply that further efforts to define the pathogenesis of maleic acid nephropathy are probably best carried out using intact tissue or cells because the molecular defect is at the level of the Na+ pump and/or at some intracellular site affecting production of ATP.

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.

Sugar transport in the LLC-PK1 renal epithelial cell line: similarity to mammalian kidney and the influence of cell density

Cells of confluent cultures of the established pig renal epithelial line, LLC-PK1, accumulate alpha-methyl-D-glucoside against a concentration gradient. This transport system is strongly inhibited by phlorizin and 6-deoxy-D-glucose, moderately inhibited by phloretin, and only weakly inhibited by 3-0-methyl-D-glucose, paralleling the situation in mammalian kidney. The time courses for the uptake of alpha-methyl-D-glucoside and for the carrier-mediated but passive uptake of 3-0-methyl-D-glucose are identical to those seen in mammalian kidney. Subconfluent cultures of LLC-PK1 cells are unable to accumulate alpha-methyl-D-glucoside, and their transport of this glucose analog is less sensitive to phlorizin inhibition than is the transport system in confluent cultures. Transmission electron micrographs show that cells from subconfluent cultures lack the microvillous surface seen in cells from confluent cultures. Cell density is thus a factor in the occurrence of structural and functional differentiated properties related to transport in these cells.

Participation of the ring oxygen in sugar interaction with transporters at renal tubular surfaces

The pulse-injection indicator-dilution technique in vivo has been used to study the interaction of 5-thio-D-glucose and methyl-beta-D-thiogalactopyranoside with renal tubular surfaces in dog kidney. (i) 5-Thio-D-glucose and methyl-beta-D-thiogalactopyranoside have nor antiluminal interaction. (ii) 37 +/- 5% of 5-thio-D-glucose is extracted at the luminal surface relative to simultaneously filtered creatinine. (iii) Luminal extraction of 5-thio-D-glucose is blocked by preloading with D-glucose and phlorizin. (iv) Methyl-beta-D-thiogalactopyranoside in contrast to D-galactose has not luminal interaction. It is concluded that 5-thio-D-glucose shares the glucose transporter at the luminal surface of the proximal tubule. The data also suggest that the ring oxygen participates in the interaction of pyranosides with luminal and antiluminal membrane carriers. At the luminal surface, its absence is quantitatively important while at the antiluminal surface it is apparently essential for the sugar-transporter interaction.

Stereospecific D-glucose transport in mixed membrane and plasma membrane vesicles derived from cultured chick embryo fibroblasts

Mixed membrane vesicles prepared from cultured chick embryo fibroblasts possess a stereospecific D-glucose transport system, the properties of which are identical to those of the system in intact cells. Uptake of D-glucose proceeds without chemical alteration. The rate of stereospecific uptake of D-glucose into the mixed vesicles is 70% greater than that of the homogenate and uptake is directly proportional to membrane protein concentration. Stereospecific D-glucose uptake appears linear for 0.3 min, reaches a maximum at 2--5 min, and declines to zero by 5 h as L-glucose enters the vesicles. Uptake is osmotically sensitive and inhibited by cytochalasin B (Ki = 0.13 microM) and the structural analogues of D-glucose : D-mannose, 2-deoxy-D-glucose, 3-O-methyl-D-glucose, D-galactose and maltose, but not by sucrose of L-glucose. Uphill counterflow can be demonstrated and the apparent activation energy displays a transition from 47.7 kcal/mol below 11 degrees C to 18.1 kcal/mol above 11 degrees C. Stereospecific uptake rates of mixed vesicles prepared from Rous sarcoma virus-transformed cells are increased 30% over control values, and are increased 66% in vesicles derived from cells incubated for 24 h in glucose-free medium. Plasma membrane vesicles prepared from these cells by a dextran cushion centrifugation procedure display a 9-fold increase in the specific activity of stereospecific D-glucose uptake relative to the homogenate. Extraction of these membranes with dimethylmaleic anhydride (5 mg/mg protein) results in substantial or complete removal of major polypeptides of molecular weight 40 000, 55 000, 75 000, 78 000 and 200 000 with no loss in total uptake activity. Following extraction, major polypeptides of molecular weight 28 000, 33 000 and 68 000 remain in the membrane residue.

Effects of metabolic intermediates on sugar and amino acid uptake in rabbit renal tubules and brush border membranes

1. The effects of tricarboxylic acid cycle intermediates on the renal transport of alpha-methyl-D-glucoside and alpha-amino-isobutyric acid were examined using separated renal tubules of the rabbit. 2. The effect of citrate on alpha-methyl-D-glucoside and alpha-amino-isobutyric acid uptake was markedly biphasic with maximum stimulation of transport occurring at a citrate concentration of 0.64 mM. Biphasic effects were also apparent for L-malate, succinate, fumarate, alpha-ketoglutarate and oxaloacetate. 3. The route of uptake of alpha-methyl-D-glucoside into separated renal tubules is primarily across the brush border (luminal) membrane. 4. Tricarboxylic acid cycle intermediates produced significant stimulation of renal O2 consumption; however, the effects on O2 consumption were not biphasic suggesting that reduced stimulation of transport at high substrate concentration was not caused by a reduction in the supply of metabolic energy. 5. In purified renal cortical brush border membrane vesicles, citrate and alpha-ketoglutarate inhibited the uptake of alpha-methyl-D-glucoside and alpha-amino-isobutyric acid indicating that inhibition of their transport in respiring renal tubules by high concentrations of tricarboxylic acid cycle intermediates occurs via an effect at the membrane level.

Sugar uptake by intestinal basolateral membrane vesicles

A high yield of membrane vesicles was prepared from the basolateral surface of rat intestinal cells using an N2 cavitation bomb and density gradient centrifugation. The membranes were enriched 10-fold and were free of significatn contamination by brush border membranes and mitochondria. The rate of D-E114C]glucose and L-E13H]glucose uptake into the vesicle was measured using a rapid filtration technique. D-Glucose equilibrated within the vesicles with a half-time 1/25th that for L-glucose. The stereospecific uptake exhibited saturation kinetics with a Km of approx. 44 mM and a V of approx. 110 nmol . mg-1 min-1 at 10 degrees C. The activation energy for the process was 14 kcal . mol-1 below 15 degrees C and it approached 3 kcal . mol-1 above 22 degrees C. Carrier-mediated uptake was eliminated in the presence of 1 mM HgCl2 and 0.5 mM phloretin. The rate of transport was unaffected by the absence or presence of sodium concentration gradients. Competition studies demonstrated that all sugars with the D-glucose pyranose ring chair conformation shared the transport system, and that, with the possible exception of the -OH group at carbon No. 1, there were no specific requirements for an equatorial -OH group at any position in the pyranose ring. In the case of alpha-methyl-D-glucoside its inability to share the D-glucose transport system may be due to steric hindrance posed by the -OCH3 group rather than by a specific requirement for a free hydroxyl group at the position in the ring. It is concluded that sugars are transported across the basolateral membrane of the intestinal epithelium by a facilitated diffusion system reminiscent of that in human red blood cells.

Testing carrier models of cotransport using the binding kinetics of non-transported competitive inhibitors

The kinetic equations representing the binding of a non-transported competitive inhibitor are derived from three variations of the carrier model of cotransport. These are (a) the model in which the binding sequence of activator and substrate is random (random bi-bi); (b) the model in which activator must bind before substrate (ordered bi-bi, activator essential), and (c) the model in which substrate must bind before activator (ordered bi-bi, activator non-essential). In general it is found that the kinetic equations for inhibitor binding are considerably simpler and easier to test than the corresponding transport equations. The effect of trans-inhibitor, transported substrate, activator concentration and membrane potential on inhibitor binding are examined in some detail. The use of these results to test and characterize the three transport models is emphasized. Applications to transport mechanisms which are not of the mobile carrier type are also discussed. A summary of relevant experimental data interpreted in terms of the theoretical models concludes the paper.

A rapid method for the isolation of kidney brush border membranes

A simple rapid method for the preparation of purified brush border membranes from rabbit kidney proximal tubules is described. The method is based on hypotonic lysis, Ca2+ aggregation of contaminants and differential centrifugation. In contrast to most other published methods, the brush border membranes are free of contamination by basolateral membranes.

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

The kinetics of D-glucose transport over the concentration range 0.07--20 mM have been investigated in a vesiculated membrane preparation from dog kidney cortex. 1. A sodium-dependent and a sodium-independent component of D-glucose uptake are observed. The sodium-dependent component is phlorizin sensitive (KI approximately 0.6 micron) and electrogenic. 2. The sodium-dependent component of D-glucose uptake yields non-linear Eadie-Hofstee plots consistent with the presence of high (GH) and low (GL) affinity sites (KH approximately 0.2 mM, KL approximately 4.5 mM, VL/VH approximately 7 at pH 7.4, 25 degrees C, 100 mM NaC1 gradient). Alternative explanations are cooperative effects of non-Michaelis-Menten kinetics. 3. The initial uptake of D-glucose increases as the intravesicular membrane potential become more negative but the numerical values of KH and KL show little, if any, change. 4. alpha-Methyl-D-glucoside transport is also sodium dependent and phlorizin sensitive (KI approximately 1.9 micron). 5. In contrast to the results for D-glucose, the sodium-dependent component of alpha-methyl-D-glucoside uptake exhibits a nearly linear Eadie-Hofstee plot consistent with a single carrier site with Km approximately 1.9 mM and Vmax approximately 27 nmol/min per mg protein at pH 7.4, 25 degrees C, 100 mM NaCl gradient. 6. The kinetics of D-glucose transport in newborn dog kidney are similar to those in the adult except that the low affinity (GL) system appears to be less well developed.