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.     

Characteristics of D-alanine transport by luminal membrane vesicles from pars convoluta and pars recta of rabbit proximal tubule

Uptake of D-alanine against a concentration gradient has been shown to occur with isolated luminal-membrane vesicles from pars convoluta or pars recta of rabbit proximal tubule. Renal D-alanine transport systems, displaying the following characteristics, were shown: (1) In vesicles from pars convoluta, the uptake of D-alanine was mediated by both Na+-dependent and Na+-independent transport processes. It was found that an inwardly directed H+-gradient could drive the transport of D-alanine into the vesicles both in the presence and absence of Na+. Thus, in addition to Na+, the transport of D-alanine is influenced by the H+-gradient. (2) In vesicles from pars recta, the transient accumulation of D-alanine was strictly dependent on Na+, since no 'overshoot' was ever observed in the absence of Na+. Although the Na+-dependent uptake of D-alanine was stimulated at acid pH, H+ did not substitute for Na+, as it apparently does in pars convoluta, but instead potentiated the Na+ effect. (3) Addition of L-alanine to vesicle preparations, both from pars convoluta and from pars recta, specifically inhibited renal uptake of D-alanine. A comparison between the transport characteristics of D- and L-alanine indicated that these two isomers of alanine probably share common transport systems located along the proximal tubule of rabbit kidney.     

Demonstration of Na+-selective channels in the luminal-membrane vesicles isolated from pars recta of rabbit proximal tubule

Characteristics of 22Na+ fluxes through Na+ channels in luminal-membrane vesicles isolated from either pars recta or pars convoluta of rabbit proximal tubule were studied. In NaCl-loaded vesicles from pars recta, transient accumulation of 22Na+ is observed, which is inhibited by amiloride. The isotope accumulation is driven by an electrical diffusion potential as shown in experiments using either these membrane vesicles loaded with different anions, or an outwardly directed K+ gradient with a K+ ionophore valinomycin. The vesicles containing the channel show a cation selectivity with the order Li+ greater than Na+ greater than K+. The amiloride-sensitive 22Na+ flux is dependent on intravesicular Ca2+. In NaCl-loaded vesicles from pars convoluta, no overshoot for 22Na+ uptake is observed. Furthermore, addition of amiloride to the incubation medium did not influence the uptake of 22Na+ in these vesicle preparations. It is concluded that Na+ channels are only present in pars recta of rabbit proximal tubule.     

Junctional complexes of the tubular cells in the human kidney as revealed with freeze-fracture

Intercellular junctions between human kidney tubular cells were studied by the freeze-fracture technique. The number of strands of the zonulae occludentes increased gradually from the proximal segment to the collecting tubule. Only one strand was visible in the proximal segment (in contrast to 2-4 strands of the neighbouring Bowman's capsule). In the thin segment 2-4 strands were revealed. In the distal segment 1-5 strands were present in the pars recta, 4-6 in the pars convoluta. The most extensive and complex zonulae occludentes were found in the collecting tubule. Gap junctions were seen only between proximal tubular cells. The extent of the zonulae occludentes along the tubules in human kidneys is very similar to that observed in the kidney tubules of other mammals. The findings accord well with electrophysiological measurements and with the results of tracer studies on experimental animals.     

Renal transport of monocarboxylic acids. Heterogeneity of lactate-transport systems along the proximal tubule.

The characteristics of D- and L-lactate transport in luminal-membrane vesicles derived from whole cortex, from the pars convoluta and from the pars recta of rabbit kidney proximal tubule were studied. It was found that uptake of both isomers in vesicles from whole cortex occurred by means of dual electrogenic transport systems, namely a low-affinity system and a high-affinity system. Uptake of both isomers in vesicles from the pars recta was strictly Na+-dependent and is mediated via a single high-affinity common transport system. Vesicles from the pars convoluta contained a cation-dependent but Na+-unspecific low-affinity common transport system for these compounds. The physiological importance of this system is briefly discussed.     

Contraluminal uptake of serine in the proximal nephron

Rabbit proximal nephron segments were microperfused in vitro to determine whether active contraluminal uptake of serine occurs in the renal proximal tubule during bath-to-lumen transport (influx) of the L- and D-isomers in the convoluted (pars convoluta) and straight (pars recta) segments. It is known that several amino acids are actively reabsorbed in the proximal nephron by a mechanism involving co-transport with sodium at the luminal membrane. There is some evidence that certain amino acids may also be accumulated across the contraluminal membrane by an energy-dependent mechanism, indicating that net reabsorption is the result of two oppositely directed active transport processes. During in vitro microperfusion of rabbit proximal nephron segments in this study, inward movement of L- and D-serine occurred in a bath-to-cell direction against a concentration gradient in the range 305-2735:1, indicating active uptake at the contraluminal membrane. The concentration gradients were maintained during influx of both isomers of serine in the proximal tubule. L-Serine accumulation by tubular cells was similar in the pars convoluta and recta, and significantly greater than that of D-serine, which was the same in both regions of the proximal tubule. The data support the conclusion that renal handling of serine involves active contraluminal uptake of the L- and D-isomers in both regions of the proximal tubule, and suggest that contraluminal events play an important role in renal handling of amino acids.     

Characteristics of uptake of short chain fatty acids by luminal membrane vesicles from rabbit kidney

The mechanisms of renal transport of short chain fatty acids by luminal membrane vesicles prepared from pars convoluta or pars recta of rabbit proximal tubule were studied by a Millipore filtration technique and by a spectrophotometric method using a potential-sensitive carbocyanine dye. Both luminal membrane vesicle preparations take up propionate and butyrate by strictly Na+-dependent transport systems, although with different characteristics. The uptake of short chain fatty acids by membrane vesicles from the pars convoluta was insensitive to changes in membrane potential, which is indicative of electroneutral transport of these compounds. Furthermore, kinetic studies showed that the Na+-dependent, but electrically silent transport of propionate is saturable (Km = 10.9 +/- 1.1 mM and Vmax = 3.6 +/- 0.2 nmol/mg protein per 20 s) and is unaffected by the presence of L- and D-lactate, indicating that these monocarboxylic acids did not share the same common transport system. In the luminal membrane vesicles from the pars recta, the uptake of propionate and butyrate was mediated by an Na+-dependent electrogenic transport process, since addition of the organic compounds to these vesicle/dye suspensions depolarized the membrane vesicles and the renal uptake of propionate and butyrate was enhanced by K+ diffusion potential induced by valinomycin. Competition experiments revealed that in contrast to the transport of propionate by vesicles from the pars convoluta, the Na+-dependent electrogenic transport of short chain fatty acids in vesicles from the pars recta occurred via the same transport system that is responsible for the reabsorption of L- and D-lactate in this region of rabbit kidney proximal tubule.     

Na+-H+ exchange in luminal-membrane vesicles from rabbit proximal convoluted and straight tubules in response to metabolic acidosis.

Na+-H+-exchanger activity of pars convoluta and pars recta luminal-membrane vesicles prepared from the proximal tubule of acidotic and control rabbits were assayed by a rapid-filtration technique and an Acridine Orange method. Both experimental approaches revealed the existence of an antiporter, sensitive to metabolic acidosis, in pars convoluta membrane vesicles. Kinetic data, obtained with the pH-sensitive dye, showed that the Km for Na+ transport was unchanged by acidosis, whereas Vmax. for exchanger activity was increased, on an average, by 44%. The fluorescence method, in contrast with the rapid-filtration technique, was able to detect exchanger activity in pars recta membrane vesicles. The Km value for the antiporter located in pars recta is comparable with that calculated for pars convoluta membrane vesicles. By contrast, the Vmax. of this exchanger is only about 25% of that found for pars convoluta. Furthermore, metabolic acidosis apparently does not increase Na+-H+-exchanger activity of pars recta luminal-membrane vesicles.     

The postnatal development of the rat kidney,with special reference to the chemodifferentiation of the proximal tubule

Summary New nephron anlages appear in the renal cortex up to the 4th postnatal day (PD). The last anlages to be formed develop into functional nephrons by PD 10, and the cortex appears mature at PD 12 after formation of the cortex corticis. The renal medulla develops by the longitudinal growth of loops of Henle and collecting ducts. The immature medulla cannot be divided into different zones and corresponds structurally to the later inner stripe of the outer zone. The inner zone is formed by PD 8, and the outer stripe of the outer zone by PD 12. The renal medulla is mature at PD 21.From the start of its development, the renal proximal tubule consists of the pars convoluta and pars recta. In both parts the formation of the brush border is accompanied by the simultaneous appearance of brush border enzymes (alkaline phosphatase, -glutamyltranspeptidase, dipeptidylamino-peptidase IV) and lysosomal enzymes (acid phosphatase, acid -galactosidase, N-acetylglucosaminidase, dipeptidylaminopeptidase II) over the full length of the proximal tubule. During the course of proximal tubule maturation, however, the lysosomal enzyme activities decline in the pars convoluta (with constant brush border enzyme activities), while the brush border enzyme activities increase in the pars recta (with constant lysosomal enzyme activities). The two parts further differ in that they exhibit different lysosomal patterns from the outset, the pars convoluta containing numerous large, highly enzyme-active lysosomes arranged in groups, and the pars recta containing only a few very small lysosomes with low enzyme activity. Thus, even in the newborn rat, the lysosomal pattern of the pars recta already corresponds to that of the mature S₃ segment. The S₁ and S₂ segments of the pars convoluta first differentiate between PD 10 and 21, as the groups of large lysosomes are progressively broken up and the extent of the lysosomal apparatus is diminished, this proceeding in a retrograde direction from the end of the immature pars convoluta.Supported by the Deutsche Forschungsgemeinschaft (SFB 105)     

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.     

Studies on the pathophysiology of acute renal failure. II. A histochemical study of the proximal tubule of the rat following administration of mercuric chloride.

Acute renal failure was induced in male rats by the subcutaneous injectioon of 4 mg HgC12 per kg body weight. Enzyme activities of the proximal tubule were studied histochemically at six time intervals from 15 min to 24 h. The enzyme studied were alkaline phosphatase, 5'-nucleotidase, acid phosphatase, alpha-glycerophosphate dehydrogenase (NAD-independent), malic dehydrogenase, succinic dehydrogenase, latic dehydrogenase, glucose-6-phosphate dehydrogenase and glucose-6-phosphatase. Decreases in activity were observed for alkaline phosphatase and 5'-nucleotidase after 15 min. Acid phosphatase was decreased after 30 min. These three enzymes returned to control levels after 3 h, but malic dehydrogenase and alpha-glycerophosphate dehydrogenase were decreased at this time interval. Succinic dehydrogenase was first decreased after 6 h. The earliest morphological changes detectable by light microscopy were observed in pars recta tubules in the medullary rays after 6 h, a time when all enzymes studied showed widespread decreased activity throughout the proximal tubule. After 24 h, the pars convoluta appeared morphologically normal but the pars recta was necrotic and exhibited calcification, whereas enzyme activity was decreased (absent in some cases) in both pars convoluta and pars recta. These results support the hypothesis that Hg++, when given in a sublethal dose, is associated with early histochemical changes in the brush border of the proximal tubule, which may be related to early changes in sodium reabsorption and to the subsequent development of acute renal failure. The observation that changes in plasma membrane-associated enzymes occur early and prior to alterations in enzymes of mitochondria and the endoplasmic reticulum suggests that Hg++ interacts initially with the plasma membrane.     

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.     

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.     

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).     

Localization ofd-amino acid oxidase mRNA in the mouse kidney and the effect of testosterone treatment

d-Amino acid oxidase (DAO), which catalyzes oxidative deamination ofd-amino acids, is known to be highly expressed in the kidney. This study was designed to examine the localization of DAO mRNA in the mouse kidney using in situ hybridization histochemistry (ISH). For comparison, ISH for mRNA of ornithine decarboxylase (ODC), which is also highly expressed in the mouse kidney, was simultaneously performed. Adult, male mice which received 1 mg of testosterone propionate or vehicle injection, were sacrificed 14 h after injection and their kidneys were removed and processed for ISH. Hybridization signals for both mRNAs were exclusively located over the epithelial cells of the proximal tubule in the vehicle-treated animals. Signals for the DAO mRNA were observed at nearly the same hybridization intensity throughout the proximal tubule, whereas hybridization signals for the ODC mRNA were observed exclusively in the pars convoluta. Following testosterone treatment, ODC mRNA in the pars convoluta was expressed with a stronger intensity than that in the vehicle-injected animals. ODC mRNA was also expressed in the pars recta with a weaker intensity than in the pars convoluta. On the other hand, DAO mRNA expression was little affected by testosterone treatment. These results indicate that, although both genes are possibly expressed in the same cells, the expression of these genes is regulated by different mechanisms.     

Effect of mannitol, dextran (macrodex), allopurinol, and methylprednisolone on the morphology of the proximal tubule of the rat kidney made ischemic in vivo.

Rats were anesthetized and their lift kidneys were made ischemic for 1 h by clamping of the aorta just above the left renal artery. Mannitol (2.5 g/kg), Dextran 70 (0.6 g/kg), methylprednisolone (50 and 100 mg/kg), and allopurinol (100 mg/kg body weight) were administered before, during, or after the ischemia period in order to test the effect of each of these drugs upon this model of renal injury. At 24 h after the release of the aortic clamp the left kidneys of the drug treated animals wwere perfusion fixed and processed for light and electron microscopy. Dextran administration to animals with ischemic kidneys gave rise to a pronounced vacuolization ("osmotic nephrosis"), in the entire proximal tubule and especially in the pars recta. This was in contrast to dextran administration to rats with nonischemic kidenys, which showed no or very mild "osmotic nephrosis." This demonstrates that ischemia makes rat kidneys more susceptible to the development of "osmotic nephrosis." In controls (no drug treatment) one hour of renal ischemia gave partial necrosis of pars recta of the proximal tubule, while the pars convoluta tubule survived. Mannitol treatment significantly reduced the amount of necrosis of the pars recta, whereas dextran, methylprednisolone, and allopurinol had no or a negative effect on the survival of the cells of the pars recta segment. It is suggested that mannitol protects against the development of necrosis by increasing medullary blood flow in combination with a counteractive influence on the cellular swelling, which is known to occur in ischemia.     

Cells and intercellular contacts in glomeruli and tubules of the frog kidney

Summary By the use of thin sections and freeze-fracture replicas the glomerular and tubular structures of the kidney of the frog (Rana esculenta) were studied with special reference to intercellular junctions.In the glomerulus the filtration barrier is of very variable thickness, and frequent tight and gap junctional contacts occur between podocyte processes.Although structurally less elaborate, the proximal tubule resembles its mammalian counterpart. In the initial part the tight junctions are relatively shallow but become very broad in the mid and distal portions of the proximal tubule. The proximal tubular cells are extensively linked by gap junctions. In some animals the shapes of the cells in the proximal and distal portions of the proximal tubule were markedly different.The distal tubule consists of two segments which differ mainly in the pattern of interdigitations and the structure of the zonulae occludentes. Similarities with the tight junctional morphology of the mammalian distal tubule are striking. In the first part of the distal tubule (diluting segment) a narrow band of parallel tight junctions is found closely resembling that found in the mammalian straight distal tubule; in the more distal part of the distal tubule, however, a broad band of anastomosing tight junctional strands exists, like the zonula occludens of the mammalian convoluted distal tubule.The connecting tubule displays cellular dimorphism: its wall contains a mixture of light and dark (flask) cells. The luminal and basolateral membranes of the flask cells are covered with numerous rod-shaped particles. The tight junctions of the connecting tubule are broad and increase in depth and number of strands along its length; they are typical of a very tight epithelium.In spite of several dissimilarities with phylogenetically younger kidneys our findings suggest that many structural principles of the mammalian kidney are also represented in the kidneys of amphibians. The structural-functional relationships are discussed.     

Distribution and properties of the glycylsarcosine-transport system in rabbit renal proximal tubule. Studies with isolated brush-border-membrane vesicles.

The distribution and properties of the peptide-transport system in rabbit renal proximal tubule was examined with glycylsarcosine as the substrate and using brush-border-membrane vesicles derived from pars convoluta (outer cortex) and pars recta (outer medulla). The dipeptide was transported into these vesicles against a concentration gradient in the presence of an inward-directed H+ gradient, demonstrating the presence of a H+-coupled peptide-transport system in outer-cortical as well as outer-medullary brush-border membranes. Even though the transport was electrogenic and was energized by a H+ gradient in both membranes, the system was more active in outer medullary membranes than in outer cortical membranes. Kinetic analysis showed that, although the affinity of the transport system for glycylsarcosine was similar in both membrane preparations, the capacity of the system was significantly greater in outer medulla than in outer cortex. In addition, the pH profiles of the peptide-transport systems in these membrane preparations also showed dissimilarities. The greater dipeptide uptake in one membrane vis-à-vis the other may probably be due to the difference in the affinity of the transport system for H+ and/or the difference in peptide/H+ stoichiometry.     

Potassium channels in basolateral membrane vesicles from pars convoluta of rabbit proximal tubule

The characteristics of 86Rb+ fluxes through conductive channels in basolateral-membrane vesicles isolated from pars convoluta of rabbit proximal tubule were investigated. In KCl loaded vesicles a transient accumulation of 86Rb+ was observed which was inhibited by BaCl2. The accumulation was driven by an electrical diffusion potential, as shown in experiments using membrane vesicles loaded with Li2SO4 and an outwardly directed Li+ gradient established with a Li(+)-ionophore. The vesicles containing the channel showed a cation selectivity with the order K+ = Rb+ much greater than Li+ greater than or equal to Na+ greater than choline+. The 86Rb+ flux was dependent on intravesicular Ca2+. Increasing concentrations of Ca2+ gradually decreased the 86Rb+ uptake.     

Ba2+-sensitive K+ channels in luminal-membrane vesicles from pars convoluta of rabbit proximal tubule

This paper describes properties of 86Rb+ fluxes through a novel K+ channel in luminal-membrane vesicles isolated from pars convoluta of rabbit proximal tubule. The uptake of 86Rb+ into potassium salt loaded vesicles was specifically inhibited by Ba2+. The isotope accumulation is driven by an electrical diffusion potential as shown in experiments using these membrane vesicles loaded with anions of different membrane permeability and was as follows: gluconate greater than SO4(2-) greater than Cl-. Furthermore, the vesicles containing the channels show a cation selectivity with the order K+ greater than Rb+ greater than Li+ greater than Na+ = choline+.