GTP-binding proteins in luminal and basolateral membranes from pars convoluta and pars recta of rabbit kidney proximal tubule.

The GTP-binding proteins on luminal and basolateral membrane vesicles from outer cortex (pars convoluta) and outer medulla (pars recta) of rabbit proximal tubule have been examined. The membrane vesicles were highly purified, as ascertained by electron microscopy, by measurements of marker enzymes, and by investigating segmental-specific transport systems. The [35S]GTP gamma S binding to vesicles, and to sodium cholate-extracted proteins from vesicles, indicated that the total content of GTP-binding proteins were equally distributed on pars convoluta, pars recta luminal and basolateral membranes. The membranes were ADP-ribosylated with [32P]NAD+ in the presence of pertussis toxin and cholera toxin. Gel electrophoresis revealed, for all preparations, the presence of cholera toxin [32P]ADP-ribosylated 42 and 45 kDa G alpha s proteins, and pertussis toxin [32P]ADP-ribosylated 41 kDa G alpha i1, 40 kDa G alpha i2 and 41 kDa G alpha i3 proteins. The 2D electrophoresis indicated that Go's were not present in luminal nor in basolateral membranes of pars convoluta or pars recta of rabbit proximal tubule.     

Renal transport of neutral amino acids. Cation-dependent uptake of L-alanine by luminal-membrane vesicles.

The characteristics of L-alanine transport in luminal-membrane vesicles isolated either from whole cortex or from pars convoluta or pars recta of rabbit proximal tubules were studied by a rapid filtration technique and by a spectrophotometric method. Uptake of L-alanine by vesicles from whole cortex was mediated by both Na+-dependent and Na+-independent, but electrogenic, processes. The nature, mechanism and tubular localization of the transport systems were studied by the use of vesicles derived from pars convoluta and pars recta. In vesicles from pars recta transport of L-alanine was strictly dependent on Na+ and occurred via a dual transport system, namely a high-affinity (half-saturation 0.14 mM) and a low-affinity system (half-saturation 9.6 mM). The cation-dependent but Na+-unspecific transport system for L-alanine was exclusively localized to the pars convoluta, which also contained an Na+-preferring system of intermediate affinity (half saturation 2.1 mM). A closer examination of the mechanism of transport of L-alanine in vesicles from pars convoluta revealed that an H+ gradient (extravesicular greater than intravesicular) can drive the transport of L-alanine into the vesicles both in the presence and in the absence of Na+. The physiological importance of various L-alanine transporters is briefly discussed.     

Involvement of thiol groups in the function of the dipeptide/proton cotransport system in rabbit renal brush-border membrane vesicles

The role of thiol groups in H+-gradient-dependent dipeptide transport in rabbit renal brush-border membrane vesicles was investigated using glycylsarcosine as the substrate. Treatment of the membrane vesicles with a thiol-group-reducing agent, dimercaptopropanol, stimulated Gly-Sar transport. On the other hand, treatment with thiol group oxidants such as 5,5'-dithiobis(2-nitrobenzoic acid), plumbagin and phenazine methosulfate inhibited Gly-Sar transport. These effects were irreversible, because washing the membranes after treatment failed to reverse the effects. Incubation of the membrane vesicles with phenylarsine oxide, a reagent which interacts specifically with vicinal dithiols, significantly inhibited Gly-Sar transport. In all cases, the stimulation or the inhibition of the dipeptide transport was primarily due to changes in the maximal velocity of the transport system, the apparent affinity constant remaining unaltered. These results demonstrate the involvement of one or more vicinal dithiol groups in the function of the renal dipeptide transport system and that these thiol groups must exist in reduced form to maintain maximal transport activity. In addition, these data indirectly suggest that a dithiol-disulfide interchange may play a role in the function of the renal dipeptide transport system.     

Carrier-mediated transport systems of tetraethylammonium in rat renal brush-border and basolateral membrane vesicles

Transport of [3H]tetraethylammonium, an organic cation, has been studied in brush-border and basolateral membrane vesicles isolated from rat kidney cortex. Some characteristics of carrier-mediated transport for tetraethylammonium were demonstrated in brush-border and basolateral membrane vesicles; the uptake was saturable, was stimulated by the countertransport effect, and showed discontinuity in an Arrhenius plot. In brush-border membrane vesicles, the presence of an H+ gradient ( [H+]i greater than [H+]o) induced a marked stimulation of tetraethylammonium uptake against its concentration gradient (overshoot phenomenon), and this concentrative uptake was completely inhibited by HgCl2. In contrast, the uptake of tetraethylammonium by basolateral membrane vesicles was unaffected by an H+ gradient. Tetraethylammonium uptake by basolateral membrane vesicles was significantly stimulated by a valinomycin-induced inside-negative membrane potential, while no effect of membrane potential was observed in brush-border membrane vesicles. These results suggest that tetraethylammonium transport across brush-border membranes is driven by an H+ gradient via an electroneutral H+-tetraethylammonium antiport system, and that tetraethylammonium is transported across basolateral membranes via a carrier-mediated system and this process is stimulated by an inside-negative membrane potential.     

Mechanisms of uptake of ketone bodies by luminal-membrane vesicles

The energetics and location of renal transport of acetoacetate, beta-hydroxybutyrate, alpha-hydroxybutyrate and gamma-hydroxybutyrate by luminal-membrane vesicles from either whole cortex or pars convoluta or pars recta of rabbit proximal tubule were studied. Addition of either acetoacetate or beta-hydroxybutyrate or its analogues to dye-membrane-vesicle suspensions in the presence of Na+ gradient (extravesicular greater than intravesicular) resulted in absorbance changes indicative of depolarizing event(s). Valinomycin enhanced the Na+-dependent uptake of monocarboxylic acids, provided a K+ gradient (intravesicular greater than extravesicular) was present. By contrast, Na+-dependent uptake of these compounds was nearly abolished by ionophores that permit Na+ to pass through the luminal-membrane via another channel, either electrogenically (e.g. gramicidin D) or electroneutrally (e.g. nigericin). These results established that the Na+-dependent transport of ketone bodies and analogues by luminal-membrane vesicles is an electrogenic process. Eadie-Hofstee analysis of saturation kinetic data suggested the presence of multiple transport systems in vesicles from whole cortex for these compounds. Tubular localization of the transport systems was studied by the use of vesicles derived from pars convoluta and pars recta. In pars recta uptake of all these compounds was mediated by means of a single high affinity common transport system. Uptake of these compounds by vesicles from pars convoluta was carried out via a relatively low affinity but common transport system. The physiological importance of the transport systems is discussed.     

ATP-dependent renal H+ translocation: regional localization, kinetic characteristics, and chloride dependence.

We characterized Mg(2+)-dependent ATPase activity in membranes from the renal cortex, the outer and inner stripes of the outer medulla, and papillary vesicles. In all regions, there was Mg(2+)-dependent ATPase activity that was resistant to oligomycin and vanadate and sensitive to N,N'-dicyclohexylcarbodiimide (DCCD), N-ethylmaleimide, and filipin. DCCD-Sensitive Mg(2+)-ATPase activity was highest in the inner stripe of the outer medulla and lowest in the cortex, with intermediate values in the outer stripe of the outer medulla and papilla. The Km for ATP, however, was similar among the different regions of the kidney. DCCD-Sensitive Mg(2+)-ATPase activity was critically dependent upon chloride with Km for Cl- in the range of 2-5 mM. In the presence of ATP, this ATPase was capable of H+ translocation, as assessed by acridine orange quenching. Inhibitors of ATPase activity prevented H+ translocation, which suggests that the Mg(2+)-ATPase represents, at least in part, an H(+)-ATPase. H+ transport was likewise critically dependent upon chloride, with similar Km. The effect of chloride on H+ translocation was blocked by the chloride channel inhibitor, diphenylamine-2 carboxylic acid. In the absence of chloride, H+ transport was abolished, but it could be partially restored by the creation of a favorable electric gradient by K+ and valinomycin. These studies demonstrate that the renal H(+)-ATPase exhibits different activities in various regions of the kidney. The ATPase activity and H+ translocation are critically dependent upon the presence of chloride, which suggests that chloride influences H+ translocation by dissipating the H+ gradient and acting at the catalytic site of the ATPase.     

Characteristics of D-galactose transport systems by luminal membrane vesicles from rabbit kidney

The characteristics of renal transport of D-galactose by luminal membrane vesicles from either whole cortex, pars recta or pars convoluta of rabbit proximal tubule were investigated by a spectrophotometric method using a potential-sensitive carbocyanine dye. Uptake of D-galactose by luminal membrane vesicles prepared from whole cortex was carried out by an Na+-dependent and electrogenic process. Eadie-Hofstee analysis of saturation-kinetic data suggested the presence of multiple transport systems in vesicles from whole cortex for the uptake of D-galactose. Tubular localization of the transport systems was studied by the use of vesicles derived from pars recta and pars convoluta. In pars recta, Na+-dependent transport of D-galactose and D-glucose occurred by means of a high-affinity system (half-saturation: D-galactose, 0.15 +/- 0.02 mM; D-glucose, 0.13 +/- 0.02 mM). These results indicated that the "carrier' responsible for the uptake of these hexoses does not discriminate between the steric position of the C-4 hydroxyl group of these two isomers. This is further confirmed by competition experiments, which showed that D-galactose and D-glucose are taken up by the same and equal affinity transport system by these vesicle preparations. Uptake of D-galactose and D-glucose by luminal membrane vesicles isolated from pars convoluta was mediated by a low-affinity common transport system (half-saturation: D-galactose, 15 +/- 2 mM; D-glucose, 2.5 +/- 0.5 mM). These findings strongly suggested that the "carrier' involved in the transport of monosaccharides in vesicles from pars convoluta is specific for the steric position of the C-4 hydroxyl group of these sugars and presumably interacts only with D-glucose at normal physiological concentration.     

Phenylalanine uptake in isolated renal brush border vesicles.

The uptake of L-phenylalanine into brush border microvilli vesicles and basolateral plasma membrane vesicles isolated from rat kidney cortex by differential centrifugation and free flow electrophoresis was investigated using filtration techniques. Brush border microvilli but not basolateral plasma membrane vesicles take up L-phenylalanine by an Na+-dependent, saturable transport system. The apparent affinity of the transport system for L-phenylalanine is 6.1 mM at 100 mM Na+ and for Na+ 13mM at 1 mM L-phenylalanine. Reduction of the Na+ concentration reduces the apparent affinity of the transport system for L-phenylalanine but does not alter the maximum velocity. In the presence of an electrochemical potential difference of Na+ across the membrane (etaNao greater than etaNai) the brush border microvilli accumulate transiently L-phenylalanine over the concentration in the incubation medium (overshoot pheomenon). This overshoot and the initial rate of uptake are markedly increased when the intravesicular space is rendered electrically more negative by membrane diffusion potentials induced by the use of highly permeant anions, of valinomycin in the presence of an outwardly directed K+ gradient and of carbonyl cyanide p-trifluoromethoxyphenylhydrazone in the presence of an outward-directed proton gradient. These results indicate that the entry of L-phenylalanine across the brush border membrane into the proximal tubular epithelial cells involves cotransport with Na+ and is dependent on the concentration difference of the amino acid, on the concentration difference of Na+ and on the electrical potential difference. The exit of L-phenylalanine across the basolateral plasma membranes is Na+-independent and probably involves facilitated diffusion.     

Evidence for an organic cation-proton antiport system in brush-border membranes isolated from the human term placenta

Uptake of guanidine, an endogenous organic cation, into brush-border membrane vesicles isolated from human term placentas was investigated. Initial uptake rates were manyfold greater in the presence of an outward-directed H+ gradient ([pH]o greater than [pH]i) than in the absence of a H+ gradient ([pH]o = [pH]i). Guanidine was transiently accumulated inside the vesicles against a concentration gradient in the presence of the H+ gradient. The H+ gradient-dependent stimulation of guanidine uptake was not due to a H+-diffusion potential because an ionophore (valinomycin or carbonylcyanide p-trifluoromethoxyphenylhydrazone)-induced inside-negative membrane potential failed to stimulate the uptake. In addition, uphill transport of guanidine could be demonstrated even in voltage-clamped membrane vesicles. The H+ gradient-dependent uptake of guanidine was inhibited by many exogenous as well as endogenous organic cations (cis-inhibition) but not by cationic amino acids. The presence of unlabeled guanidine inside the vesicles stimulated the uptake of labeled guanidine (trans-stimulation). These data provide evidence for the presence of an organic cation-proton antiporter in human placental brush-border membranes. Kinetic analysis of guanidine uptake demonstrated that the uptake occurred via two saturable, carrier-mediated transport systems, one being a high affinity, low capacity type and the other a low affinity, high capacity type. Studies on the effects of various cations on the organic cation-proton antiporter and the Na+-H+ exchanger revealed that these two transport systems are distinct.     

Neutral amino acid transport by the blood-brain barrier. Membrane vesicle studies.

Endothelial cell membranes, the site of the blood-brain barrier, were obtained from the capillaries of cow brain. The luminal and abluminal membranes were separated by centrifugation on a discontinuous Ficoll gradient. Electron microscopy revealed that the membrane preparations consisted almost entirely of sealed vesicles. The release of latent enzyme activity showed that both membrane preparations were primarily right side out. Radiolabeled L-phenylalanine uptake by luminal vesicles was proportional to membrane protein concentration, with less than 10% binding. Transport was by a high affinity carrier (Km 11.8 +/- 0.1 microM, asymptotic standard error) that showed little or no stereospecificity, and was independent of Na+ or H+ gradients. Transport was inhibited by L-tryptophan, L-leucine, 2-aminobicyclo[2,2,1]heptane-2-carboxylate and D-phenylalanine, but not by N-(methylamino)-isobutyrate. Abluminal membranes showed an additional component in which a Na+ gradient accelerated the transport of both phenylalanine and N-(methylamino)-isobutyrate. These studies demonstrate the utility of membrane vesicles as a model to characterize the transport properties of the distinct membranes of the polar endothelial cells that form the blood-brain barrier.     

Cimetidine transport in rat renal brush border and basolateral membrane vesicles

The transport of cimetidine by rat renal brush border and basolateral membrane vesicles has been studied in relation to the transport system of organic cation. Cimetidine inhibited [3H]tetraethylammonium uptake by basolateral membrane vesicles in a dose dependent manner, and the degree of the inhibition was almost the same as that by unlabeled tetraethylammonium. In contrast, cimetidine inhibited the active transport of [3H]tetraethylammonium by brush border membrane vesicles more strongly than unlabeled tetraethylammonium did. In agreement with the transport mechanism of tetraethylammonium in brush border membranes, the presence of an H+ gradient ([H+]i greater than [H+]o) induced a marked stimulation of cimetidine uptake against its concentration gradient (overshoot phenomenon), and this concentrative uptake was inhibited by unlabeled tetraethylammonium. These results suggest that cimetidine can share common carrier transport systems with tetraethylammonium in renal brush border and basolateral membranes, and that cimetidine transport across brush border membranes is driven by an H+ gradient via an H+-organic cation antiport system.     

Renal transport of taurine in luminal membrane vesicles from rabbit proximal tubule

The uptake of taurine by luminal membrane vesicles from pars convoluta and pars recta of rabbit proximal tubule was examined. In pars convoluta, the transport of taurine was characterized by two Na(+)-dependent (Km1 = 0.086 mM, Km2 = 5.41 mM) systems, and one Na(+)-independent (Km = 2.87 mM) system, which in the presence of an inwardly directed H(+)-gradient was able to drive the transport of taurine into these vesicles. By contrast, in luminal membrane vesicles from pars recta, the transport of taurine occurred via a dual transport system (Km1 = 0.012 mM, Km2 = 5.62 mM), which was strictly dependent on Na+. At acidic pH with or without a H(+)-gradient, the Na(+)-dependent flux of taurine was drastically reduced. In both kind of vesicles, competition experiments only showed inhibition of the Na(+)-dependent high-affinity taurine transporter in the presence of beta-alanine, whereas there was no significant inhibition with alpha-amino acids, indicating a beta-amino acid specific transport system. Addition of beta-alanine, L-alanine, L-proline and glycine, but not L-serine reduced the H(+)-dependent uptake of taurine to approx. 50%. Moreover, only the Na(+)-dependent high-affinity transport systems in both segments specifically required Cl-. Investigation of the stoichiometry indicated 1.8 Na+: 1 Cl-: 1 taurine (high affinity), 1 Na+: 1 taurine (low affinity) and 1 H+: 1 taurine in pars convoluta. In pars recta, the data showed 1.8 Na+: 1 Cl-: 1 taurine (high affinity) and 1 Na+: 1 taurine (low affinity).     

A proton gradient is the driving force for uphill transport of lactate in human placental brush-border membrane vesicles

The characteristics of lactate transport in brush-border membrane vesicles isolated from normal human full-term placentas were investigated. Lactate transport in these vesicles was Na+-independent, but was greatly stimulated when the extravesicular pH was made acidic. In the presence of an inwardly directed H+ gradient ([H+]o greater than [H+]i), transient uphill transport of lactate could be demonstrated. This H+ gradient-dependent stimulation was not a result of a H+ diffusion potential. Transport of lactate in the presence of the H+ gradient was not inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid or by furosemide, ruling out the participation of an anion exchanger in placental lactate transport. Many monocarboxylates strongly interacted with the lactate transport system, whereas, with the single exception of succinate, dicarboxylates did not. The monocarboxylates pyruvate and lactate, but not the dicarboxylate succinate, when present inside the vesicles, were able to exert a trans-stimulatory effect on the uptake of radiolabeled lactate. Kinetic analyses provided evidence for a single transport system with a Kt of 4.1 +/- 0.4 mM for lactate and a Vmax of 54.2 +/- 9.9 nmol/mg of protein/30 s. Pyruvate inhibited lactate transport competitively, by reducing the affinity of the system for lactate without altering the maximal velocity. It is concluded that human placental brush-border membranes possess a transport system specific for lactate and other monocarboxylates and that this transport system is Na+-independent and is energized by an inwardly directed H+ gradient. Lactate-H+ symport rather than lactate-OH- antiport appears to be the mechanism of the H+ gradient-dependent lactate transport in these membranes.     

Two substrate sites in the renal Na+-d-glucose cotransporter studied by model analysis of phlorizin binding and-d-glucose transport measurements

Summary Time courses of phlorizin binding to the outside of membrane vesicles from porcine renal outer cortex and outer medulla were measured and the obtained families of binding curves were fitted to different binding models. To fit the experimental data a model with two binding sites was required. Optimal fits were obtained if a ratio of low and high affinity phlorizin binding sites of 1:1 was assumed. Na⁺ increased the affinity of both binding sites. By an inside-negative membrane potential the affinity of the high affinity binding site (measured in the presence of 3 mM Na⁺) and of the low affinity binding site (measured in the presence of 3 or 90 mM Na⁺) was increased. Optimal fits were obtained when the rate constants of dissociation were not changed by the membrane potential. In the presence of 90 mM Na⁺ on both membrane sides and with a clamped membrane potential,K _D values of 0.4 and 7.9 M were calculated for the low and high affinity phlorizin binding sites which were observed in outer cortex and in outer medulla. Apparent low and high affinity transport sites were detected by measuring the substrate dependence ofd-glucose uptake in membrane vesicles from outer cortex and outer medulla which is stimulated by an initial gradient of 90 mM Na⁺(out>in). Low and high affinity transport could be fitted with identicalK _m values in outer cortex and outer medulla. An inside-negative membrane potential decreased the apparentK _m ofhigh affinity transport whereas the apparentK _m of low affinity transport was not changed. The data show that in outer cortex and outer medulla of pighigh and low affinity Na⁺-d-glucose cotransporters are present which containlow and high affinity phlorizin binding sites, respectively. It has to be elucidated from future experiments whether equal amounts of low and high affinity transporters are expressed in both kidney regions or whether the low and high affinity transporter are parts of the same glucose transport moleculc.     

L-threonine transport in pig jejunal brush border membrane vesicles. Functional characterization of the unique system B in the intestinal epithelium.

Uptake and inhibitory kinetics of [3H]L-threonine were evaluated in preparations of pig jejunal brush border membrane vesicles. Uptake of [3H]L-threonine under O-trans, Na+ gradient, and O-trans, Na(+)-free conditions was best described by high affinity transport (Km < 0.01 mM) plus a nonsaturable component. The maximal velocity of transport was 3-fold greater under Na+ gradient conditions. 100 mM concentrations of all of the dipolar amino acids and 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid caused complete inhibition of [3H]L-threonine transport under Na+ gradient and Na(+)-free conditions. Imino acids, anionic amino acids, cationic amino acids, and methylamino-isobutyric acid caused significant partial inhibition of L-threonine uptake. Inhibitor concentration profiles for proline and lysine were consistent with low affinity competitive inhibition. The Ki values of alanine and phenylalanine approximated 0.2 and 0.5 mM, respectively, under both Na+ gradient and Na(+)-free conditions. These data indicate that the transport system available for L-threonine in the intestinal brush border membrane (system B) is functionally distinct from other amino acid transport systems. Comparison of kinetics parameters in the presence and absence of a Na+ gradient suggests that both partially and fully loaded forms of the carrier can function to translocate substrate and that Na+ serves to accelerate L-threonine transport by a mechanism that does not involve enhanced substrate binding.     

Molecular evidence for two renal Na+/glucose cotransporters.

Previous studies have shown that two kinetically and genetically distinct Na+/glucose cotransporters exist in mammalian kidney. We have recently cloned and sequenced one of the rabbit renal Na+/glucose cotransporters (SGLT1) and have found that it is identical in sequence to the intestinal Na+/glucose cotransporter. Northern blots showed that SGLT1 mRNA was found predominantly in the outer medulla of rabbit kidney. Injection of mRNA from outer medulla and outer cortex into Xenopus oocytes resulted in equal expression of Na(+)-dependent sugar uptake, indicating that the outer cortex sample contained mRNA encoding both SGLT1 and a second Na+/glucose cotransporter. Western blots using antipeptide antibodies against SGLT1 showed that the SGLT1 protein is more abundant in outer medulla than outer cortex. However, brush border membrane vesicles prepared from outer cortex had a greater capacity for Na(+)-dependent glucose transport, indicating the presence of a second transporter in the vesicles from outer cortex. It appears that the cloned renal Na+/glucose cotransporter, SGLT1, is the 'high affinity, low capacity' transporter found predominantly in outer medulla. There is evidence that a second transporter, the 'low affinity, high capacity' transporter, is in outer cortex. Finally, the cDNA and protein sequences of the two renal Na+/glucose cotransporters are predicted to differ by more than 20%.     

The high and low affinity transport systems for dipeptides in kidney brush border membrane respond differently to alterations in pH gradient and membrane potential

The principal aim of the present study was to investigate the effects of variation in proton gradient and membrane potential on the transport of glycyl-L-glutamine (Gly-Gln) by renal brush border membrane vesicles. Under our conditions of transport assay, Gly-Gln was taken up by brush border membrane vesicles almost entirely as intact dipeptide. This uptake was mediated by two transporters shared by other dipeptides and characterized as the high affinity (Kt = 44.1 +/- 11.2 microM)/low capacity (Vmax = 0.41 +/- 0.03 nmol/mg protein/5 s) and low affinity (Kt = 2.62 +/- 0.50 mM)/high capacity (Vmax 4.04 +/- 0.80 nmol/mg protein/5 s) transporters. In the absence of a pH gradient, only the low affinity system was operational, but with a reduced transport capacity. Imposing a pH gradient of 1.6 pH units increased the Vmax of both transporters. Kinetic analysis of the rates of Gly-Gln uptake as a function of external pH revealed Hill coefficients of close or equal to 1, indicating that transporters contain only one binding site for the interaction with external H+. The effects of membrane potential on Gly-Gln uptake were investigated with valinomycin-induced K+ diffusion potentials. The velocity of the high affinity system but not of the low affinity system increased linearly with increasing inside-negative K+ diffusion potentials (p less than 0.01). The Kt of neither system was affected by alterations in either pH gradient or membrane potential. We conclude that (a) the high affinity transporter is far more sensitive to changes in proton gradient and membrane potential than the low affinity transporter and (b) in the presence of a pH gradient, transport of each dipeptide molecule requires cotransport of one hydrogen ion to serve as the driving force.     

Role of pH gradient and membrane potential in dipeptide transport in intestinal and renal brush-border membrane vesicles from the rabbit. Studies with L-carnosine and glycyl-L-proline

We examined the role of pH gradient and membrane potential in dipeptide transport in purified intestinal and renal brush-border membrane vesicles which were predominantly oriented right-side out. With an intravesicular pH of 7.5, changes in extravesicular pH significantly affected the transport of glycyl-L-proline and L-carnosine, and optimal dipeptide transport occurred at an extravesicular pH of 5.5-6.0 in both intestine and kidney. When the extravesicular pH was 5.5, glycyl-L-proline transport was accelerated 2-fold by the presence of an inward proton gradient. A valinomycin-induced K+ diffusion potential (interior-negative) stimulated glycyl-L-proline transport, and the stimulation was observed in the presence and absence of Na+. A carbonyl cyanide p-trifluoromethoxyphenylhydrazone-induced H+ diffusion potential (interior-positive) reduced dipeptide transport. It is suggested that glycyl-L-proline and proton(s) are cotransported in intestinal and renal brush-border membrane vesicles, and that the process results in a net transfer of positive charge.     

H+ coupled uphill transport of aminocephalosporins via the dipeptide transport system in rabbit intestinal brush-border membranes

The transport characteristics of aminocephalosporin antibiotics, possessing an alpha-amino group and a carboxyl group, in brush-border membranes isolated from rabbit small intestine have been studied by a rapid filtration technique. The uptake of cephradine by brush-border membrane vesicles was stimulated by the countertransport effect of dipeptides, which indicates the existence of a common carrier transport system. An inward H+ gradient ([pH]i = 7.5 to 8.4, [pH]o = 6.0) stimulated cephradine uptake against a concentration gradient (overshoot phenomenon), and this stimulation was reduced when the H+ gradient was subjected to rapid dissipation by the presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone, a protonophore. A valinomycin-induced K+ diffusion potential (interior-negative) stimulated H+ gradient-dependent cephradine uptake without altering the equilibrium value. The uptake of other aminocephalosporins (cefadroxil, cefaclor, cephalexin) was also stimulated in the presence of an inward H+ gradient, while the uptake of cephalosporins without the alpha-amino group (cefazolin, cefotiam) was not changed in the presence or absence of the H+ gradient. These results suggest that the transport of aminocephalosporins can be driven actively by an inward H+ gradient via the dipeptide transport system in the intestinal brush-border membranes, and that the process results in the transfer of a positive charge.     

Glycodeoxycholate transport in brush border membrane vesicles isolated from rat jejunum and ileum.

The transport of the bile salt, glycodeoxycholate, was studied in vesicles derived from rat jejunal and ileal brush border membranes using a rapid filtration technique. The uptake was osmotically sensitive, linearly related to membrane protein and resembled D-glucose transport. In ileal, but not jejunal, vesicles glycodeoxycholate uptake showed a transient vesicle/medium ratio greater than 1 in the presence of an initial sodium gradient. The differences between glycodeoxycholate uptake in the presence and absence of a Na+ gradient yielded a saturable transport component. Kinetic analysis revealed a Km value similar to that described previously in everted whole intestinal segments and epithelial cells isolated from the ileum. These findings support the existence of a transport system in the brush border membrane that: (1) reflects kinetics and characteristics of bile salt transport in intact intestinal preparations, and (2) catalyzes the co-transport of Na+ and bile salt across the ileal membrane in a manner analogous to D-glucose transport.