A simple isolation method for basal-lateral plasma membranes from rat kidney cortex

Calcium transport in basolateral plasma membranes from kidney cortex of Milan hypertensive rats

Ca2+ transport was investigated in basolateral plasma membranes (BLM) isolated from kidney cortex of the Milan strain of genetically hypertensive rats (MHS) and their normotensive controls (MNS) during a pre-hypertensive stage (age 3-4 weeks). It was found that the Vmax of ATP-dependent Ca2+ transport (in the presence of calmodulin) was about 16% lower in MHS than in control rats. In membranes from MNS rats which had been isolated in the presence of EGTA, the ATP-dependent Ca2+ transport showed a hyperbolic Ca2+ concentration dependence, a high Km (Ca2+) and a low Vmax; upon addition of exogenous calmodulin, the kinetics became sigmoidal, the Km (Ca2+) was decreased and the Vmax was increased. In membranes from MHS rats, the Ca2+ concentration dependence of ATP-driven Ca2+ transport was sigmoidal and the Ca2+ affinity was high in the absence of added calmodulin. Addition of exogenous calmodulin to these membranes resulted in an increase in Vmax, but no change in other kinetic parameters. Low-affinity hyperbolic kinetics of Ca2+ transport could only be obtained in MHS rats if the membranes were extracted with hypotonic EDTA and hypertonic KCl. These data suggest that the plasma membrane Ca2+-ATPase, which catalyses the ATP-dependent Ca2+ transport, exists in BLM of pre-hypertensive MHS rats predominantly in an activated, high-affinity form.

Preparation of basolateral membranes that transport p-aminohippurate from primary cultures of rabbit kidney proximal tubule cells

The organic anion p-aminohippurate (PAH) is specifically secreted by the renal proximal tubule. The possibility was examined that the probenecid sensitive PAH transport system (which is involved in this secretory process in renal proximal tubule cells in vivo) is retained in primary cultures of rabbit kidney proximal tubule cells. Significant 3H-PAH uptake into primary cultures of proximal tubule cells was observed. After 10 min, 150 pmole PAH/mg protein had accumulated intracellularly. Given an intracellular fluid volume of 10 microliter/mg protein, the intracellular PAH concentration was estimated to be 15 microM. The initial rate of PAH uptake (when 50 microM PAH was in the uptake buffer) was inhibited 50% by 2 mM probenecid. Intact monolayers also exhibited Na+-dependent alpha methyl-D-glucoside uptake (an apical marker). Basolateral membranes were purified from primary rabbit kidney proximal tubule cell cultures. Probenecid sensitive PAH uptake into the membrane vesicles derived from the primary cultures was observed. The rate of PAH uptake was equivalent to that obtained with vesicles obtained from the rabbit renal cortex. No significant Na+-dependent D-glucose uptake into the vesicles was observed, indicating that primarily basolateral membrane vesicles had indeed been obtained.

Sodium-hydrogen ion exchange in rabbit renal cortical slices incubated in acetate media

1. Thin slices (0.2-0.3 mm) of rabbit renal cortex have been incubated in isosmotic oxygenated acetate media at 25 degrees C with or without ouabain (10(-3) M), amiloride (2 x 10(-3) M) or iodoacetamide (10(-3) M). 2. Slices in normal isosmotic 146 mM-sodium-132 mM-acetate media swelled as reported previously (Cooke & Macknight, 1984). This swelling was not prevented by amiloride or by metabolic inhibition. 3. Slices in isosmotic 132 mM-choline-132 mM-acetate media gained much less water and were little affected by ouabain, amiloride or metabolic inhibition. Choline was able to substitute neither for sodium nor for potassium in activating preparations of renal cortical Na+-K+-ATPase in chloride or in acetate media. 4. Slices in isosmotic 20 mM-sodium-132 mM-acetate medium swelled nearly as much as did slices in normal sodium acetate medium. However, this swelling was impaired markedly by amiloride, by ouabain and by metabolic inhibition. 5. There was a direct correlation between medium sodium concentration and slice water content as sodium was increased from 1.25 to 30 mM in 132 mM-acetate media. However, up to a sodium concentration of 10 mM, amiloride (2 x 10(-3) M) completely prevented this increase in water content. 6. Increasing medium amiloride concentration from 10(-5) to 10(-3) M progressively inhibited cellular swelling in 10 mM-sodium-132 mM-acetate medium. It is concluded that, under these experimental conditions, the dominant pathway for hydrogen ion extrusion from the cells was via amiloride-sensitive sodium-hydrogen exchange. 7. The results are discussed in terms of a model which explains cellular swelling in acetate media in terms of (a) non-ionic diffusion of acetic acid across plasma membranes impermeable to the acetate anion, (b) removal from the cells of the hydrogen ion gained with the acetate by amiloride-sensitive sodium-hydrogen counter-transport and (c) subsequent extrusion of sodium from the cell accompanied by potassium uptake via the ouabain-sensitive Na+-K+-ATPase. 8. The results provide evidence for ion movements across the luminal plasma membrane of proximal tubular cells in rabbit renal cortical slices.

Transport of sulfate and phosphate in small intestine and renal proximal tubule: methods and basic properties

1. Isolated brush border membrane vesicles, basolateral membrane vesicles, and cultured renal epithelial cells provide good material for studying transport systems. 2. The vesicle systems have been used to study the transport of labeled phosphate, sodium/phosphate cotransport, sodium/sulfate cotransport, basolateral transport of sulfate and basolateral transport of phosphate via anion exchange. 3. Cultured renal cells show sodium/phosphate cotransport and parathyroid dependent inhibition of phosphate transport.

Sodium-bicarbonate cotransport occurs in rat kidney cortical membranes but not in rat small intestinal basolateral membranes

Basolateral membrane vesicles were isolated from rat kidney cortex and small intestinal enterocytes. Both membrane preparations show ATP-dependent calcium uptake and cytochalasin B-sensitive D-glucose transport. In renal membranes, sodium influx is stimulated by bicarbonate; bicarbonate-dependent sodium flux is membrane-potential-dependent and inhibited by 4,4'-di-isothiocyanato-2, 2'-stilbenedisulphanic acid ('DIDS'). Small intestinal basolateral membranes do not show bicarbonate-dependent sodium fluxes.

Phosphate transport by basolateral plasma membranes of human small intestine

The characteristics of phosphate transport across the human intestinal basolateral membrane were determined using an enriched preparation in which uphill Na1+-dependent D-glucose transport could not be demonstrated but adenosine triphosphate-dependent calcium transport was present. The human basolateral membrane vesicles were oriented as follows: 64% inside-out vesicles and 36% rightside-out vesicles or sheets, or both. Phosphate transport showed a 3.4-fold transient "overshoot" phenomena in the presence of an inwardly directed sodium gradient. Computerized Michaelis-Menten kinetics for the net active transport component indicated a Vmax of 65 +/- 5 pmol/mg protein per 8 s and a Km of 93 +/- 15 microM. The transport process was electroneutral. Gramicidin D and transstimulation studies confirmed the presence of a Na+-phosphate carrier at the basolateral membrane. Adenosine triphosphate enhanced phosphate uptake, indicating an adenosine triphosphate-dependent phosphate transport process. These studies are the first to determine the presence of a phosphate carrier in human intestinal basolateral membrane.

Phosphate transport by rat intestinal basolateral-membrane vesicles

The characteristics of phosphate transport across intestinal basolateral membranes of the rat were determined by using enriched preparations in which uphill Na+-dependent D-glucose transport could not be demonstrated, but ATP-dependent Ca2+ transport was present. Phosphate transport was saturable, Na+-dependent and exhibited Michaelis-Menten kinetics. Vmax. was 51.1 +/- 4.2 pmol/10 s per mg of protein and Km was 14 +/- 3.9 microM. The transport process was electroneutral. Tracer-exchange experiments and counter-transport studies confirmed the presence of a Na+-Pi carrier at the basolateral membrane. The presence of inside-positive membrane potential did not enhance phosphate uptake, indicating that the Na+ effect is secondary to the presence of the Na+-Pi carrier rather than an induction of positive membrane potential. The stoichiometry of this carrier at pH 7.4 was 2 Na+:1 phosphate, as shown by direct studies utilizing the static-head method. These studies are the first to determine the presence of a phosphate carrier at the basolateral membrane.

Characterization of alpha 1- and alpha 2-adrenoceptors directly associated with basolateral membranes from rat kidney proximal tubules

We have used 2-(beta-(3-125iodo-4-hydroxyphenyl)-ethylaminoethyl)-tetr alo ne ([125I]HEAT or BE2254), an alpha 1-selective antagonist, and [3H]yohimbine, an alpha 2-selective antagonist, to demonstrate and characterize binding sites in basolateral membranes from rat kidney cortex. Parathyroid hormone (PTH) stimulated the adenylate cyclase activity of the basolateral membranes, whereas thyrocalcitonin, arginine vasopressin (AVP) and isoproterenol did not. Therefore, the basolateral membranes were probably derived from the proximal tubules. The specific binding of [125I]HEAT and [3H]yohimbine to basolateral membranes was rapid, reversible, saturable and of high affinity. The maximum densities of alpha 1- and alpha 2-receptors were 364 and 1130 fmoles/mg protein, indicating that the ratio of alpha 1- to alpha 2-adrenoceptors was about 1:3. The specific binding of [125I]HEAT and [3H]yohimbine to the basolateral membranes was displaced by various adrenergic agents in a manner that suggests that the labeled sites probably represent alpha 1- and alpha 2-adrenoceptors respectively. These results suggest that the binding sites of [125I]HEAT and [3H]yohimbine, which appear to be alpha 1- and alpha 2-adrenoceptors, exist in the basolateral membranes of the proximal tubules.

Phosphate transport across the basolateral membrane from rat kidney cortex: sodium-dependence?

Basolateral membrane vesicles were isolated from rat renal proximal tubules by a Percoll-centrifugation method. Transport of phosphate could be stimulated by a sodium gradient but transport of D-glucose was not or only slightly affected by sodium. The Percoll-basolateral membrane fraction showed sodium-independent trans-stimulation of phosphate by itself. To test whether sodium stimulation of phosphate transport is influenced by the crosscontamination of basolateral with brush border membranes, the basolateral membrane fraction obtained by the Percoll-method was applied to free-flow electrophoresis for further purification. Thereby a separation of basolateral from brush border marker enzymes was obtained. It was possible to correlate net sodium-dependent phosphate transport with the brush border marker enzyme activity. ATP-dependent calcium uptake and cytochalasin B sensitive, sodium-independent D-glucose uptake followed basolateral, whereas phlorizin sensitive, sodium-dependent D-glucose followed brush border marker enzymes. Sodium dependent phosphate uptake was inhibited by D-glucose, this inhibition was released by phlorizin. It is concluded that the sodium-dependent phosphate uptake present in the Percoll-basolateral membrane fraction is due to crosscontamination with brush border membranes and that phosphate translocation across the basolateral membrane is carrier-mediated and sodium-independent.

Kinetic studies of sulfate transport in basolateral membrane vesicles from rat renal cortex

The kinetics of sulfate uptake were studied in basolateral membrane vesicles from rat renal cortex. Sulfate uptake exhibits a DIDS-sensitive, saturable component, and a DIDS-insensitive component, which does not saturate in the tested sulfate concentration range (up to 10 mM). Intravesicular (= trans) sulfate strongly stimulates sulfate uptake by increasing Jmax and--to a lesser degree--by decreasing apparent Km. The marked dependence of Jmax on trans-sulfate indicates that the transport system operates as an anion exchanger. Half-maximal sulfate uptake occurs at 0.08-0.14 mmol/l extravesicular sulfate. Half-maximal trans-stimulation is observed at 11 mmol/l intravesicular sulfate indicating that the sulfate transporter is highly asymmetric. Lowering extravesicular pH stimulates sulfate uptake, suggesting that external protons are essential for sulfate uptake. This stimulation is mainly due to a decrease in Km. An inside positive membrane potential stimulates sulfate uptake at pHout = 8.8, but not at pHout = 6.4. These results are compatible with electrogenic sulfate transport at higher and electroneutral 2H+ -SO4(2-) cotransport at lower pH.

Characterization of ATP-driven calcium uptake in renal basal-lateral and renal endoplasmic reticulum membrane vesicles

ATP-driven calcium uptake was studied in basal-lateral membranes and in microsomal fractions, isolated from pig kidney cortex. The uptake is strongly enhanced in conditions where calcium inside the vesicles is precipitated by oxalate (5 mM) or phosphate (40 mM). Both anions were equally effective for the stimulation of calcium uptake in the microsomes but oxalate was less effective than phosphate in the basal-lateral membrane fraction. The active calcium pumps in the renal basal-lateral and microsomal fractions are different transport ATPases characterized by phosphorylated intermediates of 135 kDa and 115 kDa respectively. The subcellular distribution of the 135 kDa and 115 kDa phosphointermediates, reflects the distribution of typical marker enzymes for the basal-lateral membrane and for the endoplasmic reticulum. The calmodulin binding to the 135 kDa polypeptide as estimated by 125I-labelled calmodulin overlay, can be used as a specific marker for the basal-lateral plasma membrane calcium pump.

Transport of sulphate in rat jejunal and rat proximal tubular basolateral membrane vesicles

Basolateral membrane vesicles were isolated by a Percoll density gradient centrifugation method from small intestinal and renal proximal tubular epithelial cells. Transport of sulphate across the basolateral membrane was analyzed by measuring the uptake of tracer sulphate. In both membrane preparations, preloading the vesicles with sulphate- or hydroxyl-anions stimulated tracer sulphate uptake (trans-stimulation); an inwardly directed sodium gradient did not stimulate sulphate influx whether in the absence or in the presence of sulphate- or hydroxyl-ion-trans-stimulation. Under sulphate trans-stimulation conditions, DIDS (10(-4) mol/l) inhibited sulphate influx. In intestinal membranes, trans-stimulation of sulphate influx was obtained by preloading the vesicles with chloride, in renal membranes by preloading with bicarbonate. Under sulphate trans-stimulation conditions, in intestinal membranes, sulphate influx was strongly inhibited by chloride, in renal membranes, chloride inhibition was absent. Under bicarbonate trans-stimulation conditions, in renal membranes, sulphate transport was inhibited by lactate. It is concluded that small intestinal and renal proximal tubular basolateral membrane vesicles contain a transport mechanism for sulphate that cannot be energized by a sodium gradient. The transport system in small intestinal basolateral membranes seems to be different from that in renal membranes. It is suggested that the observed interaction between inorganic and organic anion transport in renal basolateral membranes is indirect.

Calcium transport in canine renal basolateral membrane vesicles. Effects of parathyroid hormone

The effects of parathyroid hormone were studied on Ca2+ fluxes in canine renal proximal tubular basolateral membrane vesicles (BLMV). Efflux of Ca2+ from preloaded BLMV was found to be stimulated by an external Na+ gradient, and this was inhibited by the Na+ ionophore, monensin, and enhanced by intravesicular negative electrical potentials, which indicated electrogenic Na+/Ca2+ exchange activity. There was a Na+ gradient independent Ca2+ flux, but membrane binding of Ca2+ was excluded from contributing to the Na+ gradient-dependent efflux. The Na+ gradient-dependent flux of Ca2+ was very rapid, and even 2- and 5-s points may not fully represent absolute initial rates. It was saturable with respect to the interaction of Ca2+ and Na+ with an apparent (5 s) Km for Na+-dependent Ca2+ uptake of 10 microM, and an apparent (5 s) Vmax of 0.33 nmol/mg protein per 5 s. The Na+ concentration that yielded half maximal Ca2+ efflux (2 s) was 11 mM, and the Hill coefficient was two or greater. Both Na+ gradient dependent and independent Ca2+ efflux were decreased in BLMV prepared from kidneys of thyroparathyroidectomized (TPTX) dogs, and both were stimulated by parathyroid hormone (PTH) infusion to TPTX dogs. BLMV from TPTX dogs exhibited significantly reduced maximal stimulation of Na+ gradient-dependent Ca2+ uptake with an apparent (5 s) Vmax of 0.23 nmol/mg protein per 5 s, but the apparent Km was 8 microM, which was unchanged from normal. The Na+ gradient independent Ca2+ uptake was also reduced in BLMV from TPTX dogs compared with normal. Thus, PTH stimulated both Na+/Ca2+ exchange activity and Na+ independent Ca2+ flux. In vivo, the latter could result in an elevation of cytosolic Ca2+ by PTH, and this might contribute to the observed decrease in solute transport in the proximal tubule.

Identification of rat hepatocyte plasma membrane proteins using monoclonal antibodies

We have localized and identified five rat hepatocyte plasma membrane proteins using hybridoma technology in combination with morphological and biochemical methods. Three different membrane preparations were used as immunogens: isolated hepatocytes, a preparation of plasma membrane sheets that contained all three recognizable surface domains of the intact hepatocyte (sinusoidal, lateral, and bile canalicular), and a glycoprotein subfraction of that plasma membrane preparation. We selected monoclonal IgGs that were hepatocyte specific and localized them using both immunofluorescence on 0.5-micron sections of frozen liver and immunoperoxidase at the ultrastructural level. One antigen (HA 4) was localized predominantly to the bile canalicular surface, whereas three (CE 9, HA 21, and HA 116) were localized predominantly to the lateral and sinusoidal surfaces. One antigen (HA 16) was present in all three domains. Only one antigen (HA 116) could be detected in intracellular structures both in the periphery of the cell and in the Golgi region. The antigens were all integral membrane proteins as judged by their stability to alkaline extraction and solubility in detergents. The apparent molecular weights of the antigens were established by immunoprecipitation and/or immunoblotting. In a related study (Bartles, J.R., L.T. Braiterman, and A.L. Hubbard, 1985, J. Cell. Biol., 100:1126-1138), we present biochemical confirmation of the domain-specific localizations for two of the antigens, HA 4 and CE 9, and demonstrate their suitability as endogenous domain markers for monitoring the separation of bile canalicular and sinusoidal lateral membrane on sucrose density gradients.

Effects of trypsin and protein modification on the renal transporter ofp-aminohippurate

Basal-lateral membranous vesicles prepared from rabbit renal cortex exhibited Mg2+-stimulated, probenecid-inhibitable transport of p-aminohippurate (PAH). This uptake could be completely eliminated by incubating the membranes with trypsin at a weight ratio of 1:700 (trypsin/membrane protein). The loss of PAH uptake activity occurred in two stages. Over the first ten minutes of the vesicles' exposure to trypsin, there was a nearly linear loss, with respect to time, of about 80% of the PAH uptake activity. The remaining 20% of activity was resistant to further trypsin digestion for the next ten minutes, but by twenty-five minutes a total inactivation of the uptake activity occurred. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of normal and trypsin-treated vesicles showed very little degradation of proteins. However, two minor polypeptides (Mr - 410,000 and 388,000) were degraded during the first ten minutes of the membranes' exposure to trypsin. After twenty minutes of exposure, two other polypeptides (Mr = 94,500 and 87,500) were degraded. Chymotrypsin and clostripain also caused a loss of PAH transport activity. However, compared to the effects of trypsin, the effects of these two proteases were less complete, slower in onset, and for clostripain, a much higher concentration of enzyme was required. Other functions or properties of the vesicles including morphological appearance, degree of vesiculation, glucose space or Na+-dependent L-glutamate transport and Na+,K+-ATPase activity were not altered by the concentration of trypsin which abolished 80% of the transport of PAH. Thus, it is possible that one or more of the degraded polypeptides detected by polyacrylamide gel electrophoresis comprises the PAH transporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoaffinity labeling of organic anion transport system in proximal tubule

The present investigation compares brush-border (BBM) and basolateral membrane (BLM) vesicles in terms of purity, function, appearance on sodium dodecyl sulfate (SDS) gels, and labeling pattern by use of N-(4-azido-2-nitrophenyl)-2-aminoethanesulfonic acid (NAP-taurine), a photoaffinity analogue of p-aminohippurate (PAH). Both BLM and BBM were vesicular by demonstration of PAH uptake into an osmotically active space and had probenecid-inhibitable uptake of PAH. Time courses for uptake were similar. 250 microM NAP-taurine resulted in a 35% inhibition of PAH uptake in BLM but it did not significantly effect PAH uptake into BBM. The latter was affected by 1 mM NAP-taurine. A comparison of Coomassie blue SDS gels of BLM and BBM showed markedly different staining patterns. A major band at approximately 52,000 daltons was more intensely stained in BLM than BBM. Major bands at approximately 40,000 and approximately 80,000 were stained more heavily in BBM than BLM. A minor protein at 26,000 in the BLM did not appear in BBM. An irreversible inhibition of PAH uptake in BLM was observed after photolysis in the presence of NAP-taurine. This was associated with the labeling of four protein bands on SDS polyacrylamide gels. In contrast no labeling was observed in BBM.

Sodium-dependent dicarboxylate transport in rat renal basolateral membrane vesicles

Dicarboxylate transport in basolateral membrane vesicles prepared from rat kidney cortex was studied using 3H-methylsuccinate as a substrate. A sodium gradient (out greater than in) simulated methylsuccinate uptake and led to a transient overshoot. Lithium inhibited methylsuccinate uptake in the presence of sodium. The dependence of methylsuccinate uptake on sodium concentration indicated the interaction of more than one sodium ion with the transporter. Half-maximal stimulation was observed at 24 mmol/l sodium. Sodium-driven methylsuccinate uptake was electrogenic carrying a net positive charge. The basolateral dicarboxylate transport system exhibited an optimum at pH 7.0-7.5. In contrast, the sodium-dependent dicarboxylate transport system of brush border membranes depended much less on pH and had no optimum in the tested range. Cis-inhibition studies showed a preference of the system for dicarboxylates in the trans-configuration (fumarate) over cis-dicarboxylates (maleate). Citrate was accepted but oxalate and L-glutamate were not. DIDS exhibited a small inhibition. Among the monocarboxylates, gluconate and pyruvate inhibited methylsuccinate uptake whereas probenecid and p-aminohippurate (1 mmol/l) were without effect. The data indicate the presence of a sodium-dependent transport system in the basolateral membrane which accepts tricarboxylic acid cycle intermediates. This system is most likely not identical to the transport system responsible for organic anion secretion.

A domain-specific marker for the hepatocyte plasma membrane. II. Ultrastructural localization of leucine aminopeptidase to the bile canalicular domain of isolated rat liver plasma membranes

Leucine aminopeptidase (LAP) is an integral membrane glycoprotein localized to the apical membrane domain of intestinal and kidney epithelial cells. By indirect immunofluorescence, we have shown that antibodies raised against rat intestinal LAP recognized a similar protein concentrated in the bile canalicular (BC) domain of the hepatocyte in situ (Roman, L.M., and A.L. Hubbard, 1983, J. Cell Biol., 96:1548-1558). We have extended this localization to the ultrastructural level. When a saponin-permeabilized, agarose-embedded plasma membrane (PM) fraction was incubated with affinity-purified anti-LAP, 85% of the protein A-gold particles associated with the three recognizable PM domains were present in the BC. The levels of labeling on the other two domains (sinusoidal and lateral) did not exceed that observed with nonimmune controls. The concentration of LAP in the BC domain in isolated PM sheets prompted us to use this antigen for the affinity isolation of BC membrane (Roman, L.M., and A.L. Hubbard, 1984, J. Cell Biol., 98:1497-1504, companion paper).

Isopycnic centrifugation of rat-liver microsomes in isoosmotic gradients of Percoll and release of microsomal material by low concentrations of sodium deoxycholate

Rat-liver microsomes were subjected to isopycnic centrifugation under isoosmotic conditions in gradients of Percoll containing 0%, 0.05%, 0.1% and 0.2% sodium deoxycholate, respectively. The buoyant density of the microsomes was in the range 1.043 to 1.046 g/ml, independent of the detergent concentration. Absorbance measurements and polyacrylamide gel electrophoresis revealed that the detergent causes an increase in the release of material from the microsomes. Electron microscopy studies showed that membrane disassembly was avoided if the microsomes were isolated at concentrations below 0.1% sodium deoxycholate.

Isolation of basolateral and brush-border membranes from the rabbit kidney cortex. Vesicle integrity and membrane sidedness of the basolateral fraction

A rapid and reproducible method has been developed for the simultaneous isolation of basolateral and brush-border membranes from the rabbit renal cortex. The basolateral membrane preparation was enriched 25-fold in (Na+ + K+)-ATPase and the brush-border membrane fraction was enriched 12-fold in alkaline phosphatase, whereas the amount of cross-contamination was low. Contamination of these preparations by mitochondria and lysosomes was minimal as indicated by the low specific activities of enzyme markers, i.e., succinate dehydrogenase and acid phosphatase. The basolateral fraction consisted of 35-50% sealed vesicles, as demonstrated by detergent (sodium dodecyl sulfate) activation of (Na+ + K+)-ATPase activity and [3H]ouabain binding. The sidedness of the basolateral membranes was estimated from the latency of ouabain-sensitive (Na+ + K+)-ATPase activity assayed in the presence of gramicidin, which renders the vesicles permeable to Na+ and K+. These studies suggest that nearly 90% of the vesicles are in a right-side-out orientation.

A high-affinity, calmodulin-dependent Ca2+ pump in the basal-lateral plasma membranes of kidney cortex

A purified preparation of kidney basolateral membrane vesicles is capable of ATP-dependent Ca2+ uptake. The reaction has high affinity for Ca2+ (Km about 0.1 microM) and a V of 5.8 nmol Ca2+ X mg-1 protein X min-1 in the predominantly right-side-out vesicular preparation used. It is inhibited by vanadate (K0.5 about 5 microM) and by anti-calmodulin drugs. A stimulatory effect of calmodulin is visible in membranes depleted of the activator. Exposure of basolateral membranes to 125I-azido-modified calmodulin results in the specific labeling of a membrane protein of Mr 141 000, which is tentatively suggested to be the Ca2+-pumping ATPase.

Proton pathways in rat renal brush-border and basolateral membranes

The quenching of acridine orange fluorescence was used to monitor the formation and dissipation of pH gradients in brush-border and basolateral membrane vesicles isolated from rat kidney cortex. The fluorescence changes of acridine orange were shown to be sensitive exclusively to transmembrane delta pH and not to membrane potential difference. In brush-border membrane vesicles, an Na+ (Li+)-H+ exchange was confirmed. At physiological Na+ concentrations, 40-70% of Na+-H+ exchange was mediated by the electroneutral Na+-H+ antiporter; the remainder consisted of Na+ and H+ movements through parallel conductive pathways. Both modes of Na+-H+ exchange were saturable, with half-maximal rates at about 13 and 24 mM Na+, respectively. Besides a Na+ gradient, a K+ gradient was also able to produce an intravesicular acidification, demonstrating conductance pathways for H+ and K+ in brush-border membranes. Experiments with Cl- or SO2-4 gradients failed to demonstrate measurable Cl--OH- or SO2-4-OH- exchange by an electroneutral antiporter in brush-border membrane vesicles; only Cl- conductance was found. In basolateral membrane vesicles, neither Na+(Li+)-H+ exchange nor Na+ or K+ conductances were found. However, in the presence of valinomycin-induced K+ diffusion potential, H+ conductance of basolateral membranes was demonstrated, which was unaffected by ethoxzolamide and 4,4'-diisothiocyanostilbene-2,2-disulfonic acid. A Cl- conductance of the membranes was also found, but antiporter-mediated electroneutral Cl--OH- or SO2-4-OH- exchange could not be detected by the dye method. The restriction of the electroneutral Na+-H+ exchanger to the luminal membrane can explain net secretion of protons in the mammalian proximal tubule which leads to the reabsorption of bicarbonate.

A domain-specific marker for the hepatocyte plasma membrane: localization of leucine aminopeptidase to the bile canalicular domain

Indirect immunofluorescence was used to establish a domain-specific marker for hepatocyte plasma membranes. In frozen sections of fixed rat liver (0.5-4 microns), antibodies directed against rat intestinal leucine aminopeptidase (LAP) recognized an antigen that was restricted to the bile canalicular plasma membrane. Fluorescence was not observed on the sinusoidal or lateral membranes, and intracellular staining was not detected. The liver antigen was identified as LAP, based on its chemical similarity to intestinal LAP. First, immunoprecipitation experiments using trypsin-solubilized intestinal LAP (G-200 fraction, 91% pure) established a correlation between the loss of LAP enzyme activity from the soluble fraction and the appearance in the specific immunoprecipitates of polypeptides migrating on SDS PAGE between 110,000 and 130,000 daltons. The antigen precipitated from a detergent extract of liver plasma membranes had the same electrophoretic mobility. Second, the chymotryptic map of the major band in the liver immunoprecipitate was similar to that of purified intestinal LAP.

Renal sulfate transport at the basolateral membrane is mediated by anion exchange

The transport of sulfate was studied in basolateral membrane (BLM) vesicles isolated from rat kidney cortex by centrifugation on a Percoll self-generating gradient. In contrast to sulfate transport at the luminal membrane, sulfate uptake by BLM vesicles was not sodium dependent. However, imposition of an inside greater than outside bicarbonate gradient stimulated BLM sulfate uptake nearly 10-fold and produced a transient overshoot of about 4-fold. This process appeared to become saturated at high concentrations of either bicarbonate or sulfate. Sulfate itself, thiosulfate, and hydroxyl, but not chloride or thiocyanate, were able to substitute for bicarbonate. None of these anions was as effective as bicarbonate. The sulfate/bicarbonate exchange was unaltered by manipulation of membrane potential, suggesting that it was electroneutral. Both bicarbonate stimulation and overshoot could be prevented by known inhibitors of anion transport, including mercuric chloride, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, and phloretin. Bicarbonate-stimulated sulfate uptake also was inhibited by thiosulfate, probenecid, and acetazolamide. Thus, rat kidney BLM vesicles showed carrier-mediated anion exchange. Its properties indicate that this carrier may participate in both reabsorptive and secretory sulfate transport.

Transport of glutathione by renal basal-lateral membrane vesicles

Transport of glutathione was studied in membrane vesicles derived from the basal-lateral region of the plasma membrane of rat kidney proximal tubules. The integrity of the vesicle preparation was demonstrated by showing that vesicles were osmotically sensitive, with GSH uptake at equilibrium varying inversely with medium osmolality. Analysis of vesicle content by high-pressure liquid chromatography and competition experiments with glycine and cysteinylglycine confirmed that measured uptake of GSH represented transport of intact tripeptide rather than transport of degradation products. The initial rate of GSH uptake in the presence of either the sodium or potassium salt of the permeant thiocyanate anion showed that the uptake was sodium-dependent. This suggests that a GSH-Na+ cotransport system exists in these membranes.

An efficient method for the isolation and separation of basolateral-membrane and luminal-membrane vesicles from rabbit kidney cortex

A procedure for isolation and separation of purified luminal-membrane and basolateral-membrane vesicles from adult and newborn rabbit renal cortex by using Ca2+/Mg2+ precipitation, differential centrifugation and a self-orienting Percoll-gradient centrifugation is described. The purity of the membrane-vesicle suspensions was examined by electron microscopy and by measuring the activity of several marker enzymes. The activity of Na+ + K+-stimulated ATPase in the fraction mainly containing adult rabbit basolateral-membrane vesicles was enriched 16-fold, and the activity of alkaline phosphatase in the fraction mainly containing luminal-membrane vesicles was increased 13-fold, compared with the homogenate. Similar results were obtained with kidneys from newborn rabbits. Uptake studies, with a rapid filtration technique and the spectrophotometric method described in an accompanying paper [Kragh-Hansen, Jørgensen & Sheikh (1982) Biochem. J. 208, 359-368], showed that both adult and newborn rabbit luminal-membrane vesicles, in contrast with the basolateral-membrane preparations, possess an Na+-dependent electrogenic transport system for L-proline. Adult rabbit luminal-membrane vesicles take up citrate and L-malate by Na+-dependent electrogenic processes, whereas adult rabbit basolateral membrane vesicles do not exhibit electrogenic uptake of citrate. By contrast, these vesicles show Na+-dependent electrogenic uptake of L-malate.

Basal-lateral membranes from rabbit renal cortex prepared on a large scale in a zonal rotor

Basal-lateral membranes from the renal cortex of the rabbit were isolated by sucrose gradient centrifugation in a zonal rotor which allows for a large-scale preparation of these membranes. A heterogeneous population of membranes (P4) which contained 29% of the (Na+ + K+)-ATPase found in the homogenate of renal cortex was prepared by differential centrifugation. When pellet P4 was subjected to centrifugation in a sucrose gradient the activity of (Na+ + K+)-ATPase, a marker for basal-lateral membranes, could be separated from enzymatic markers of other organelles. The specific activity of (Na+ + K+)-ATPase was enriched 12-fold at a density of 1.141 g/cm3. Membranes (P alpha) contained in the (Na+ + K+)-ATPase-rich fractions consisted primarily of closed vesicles which exhibited probenecid inhibitable transport of rho-aminohippurate. These membranes did not exhibit Na+-dependent, phlorizin-inhibitable D-glucose transport. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of proteins from P alpha revealed at least six major protein bands with molecular weights of 91000, 81000, 73000, 65000, 47000 and 38000. A small fraction of total alkaline phosphatase found in the homogenate was found in pellet P4. Membranes containing this alkaline phosphatase activity were distributed widely over the gradient, with peak activity found at a density of 1.141 g/cm3. In contrast, when brush borders were subjected to gradient centrifugation under the same conditions as P4, alkaline phosphatase was found in a narrow distribution, with peak activity at a density of 1.158 g/cm3. The principle subcellular localization of the alkaline phosphatase found in P4 could not be determined unambiguously from the data, but the activity did not seem to be primarily associated with classical brush borders.

A simple method for the isolation of basolateral plasma membrane vesicles from rat kidney cortex. Enzyme activities and some properties of glucose transport

A procedure for preparing basolateral membrane vesicles from rat renal cortex was developed by differential centrifugation and Percoll density gradient centrifugation, and the uptake of D-[3H] glucose into these vesicles was studied by a rapid filtration technique. (Na+ + K+)-ATPase, the marker enzyme for basolateral membranes, was enriched 22-fold compared with that found in the homogenate. The rate of D-glucose uptake was almost unaffected by Na+ gradient (no overshoot).