Characteristics of changes in the intracellular potential associated with transport of neutral, dibasic and acidic amino acids in Triturus proximal tubule

Structural properties of the proton translocating complex of the clathrin-coated vesicle

The clathrin-coated vesicle proton pump is a representative member of the new class of endomembrane proton ATPases that share an inhibitor profile which distinguishes them from classic F1F0 and E1E2-type proton pumps. The coated vesicle proton pump is a large (530 kDa) heteroligomer composed of eight polypeptides with molecular masses of 116, 70, 58, 40, 38, 34, 33 and 17 kDa. The 200-fold purified enzyme catalyses ATP-generated proton pumping when reconstituted in liposomes composed of pure lipids. Subunit function has been determined by partial reaction analysis of subunit and subcomplex activities. The isolated 17 kDa subunit, when co-reconstituted with bacteriorhodopsin, forms a dicyclohexylcarbodiimide-inhibitable proton channel. Selective removal of the 116 kDa subunit transforms the proton ATPase from a Mg2+-activatable to a Ca2+-activatable ATPase. Subsequent dissociation and reconstitution of subunits reveals that the 70, 58, 40 and 33 kDa components are required, in composite, to form a functional ATP-hydrolytic core, and that no single subunit or subcomplex deficient in these subunits can catalyse ATP hydrolysis.

Cl- and membrane potential dependence of amino acid transport across the rat renal brush border membrane

The relative roles of the anion present and the membrane potential in the operation of each of the seven amino acid transport systems in the renal tubular brush border membrane were explored by manipulating transmembrane potential and chemical gradients across the membrane. The effect of various external anions with different permeabilities of the membrane and of valinomycin-generated K+ diffusion potential on Na+-coupled amino acid accumulation by rat renal brush border membrane vesicles was examined. Accumulation of all amino acids examined, except for cystine, was membrane potential dependent. The highest voltage dependence was observed for taurine (equivalent to glucose) and l-methionine. Addition of taurine uptake values obtained under each electrical gradient (inside negative) and a chemical gradient (100 mM NaCl out) condition yielded markedly lower values than under conditions where there was a combined electrochemical gradient. Cl- gradient rather than merely imposing a voltage gradient was a specific mediator of Na+-coupled transport of l-proline, taurine, l-glutamic acid, and glycine across the brush border membrane. Cl- gradient alone under Na+-equilibrated conditions could energize an overshoot of taurine accumulation by vesicles providing evidence that taurine is energetically activated by and coupled to Cl- transport. These data suggest that Na+-linked transport of most amino acids across the tubular luminal membrane is an electrogenic positive process and for proline, taurine, glutamic acid, and glycine, a Cl--requiring process. A negative intracellular potential combined with luminal chloride is required for optimal Na+-coupled transport of these amino acids across the luminal membrane of the proximal tubule. The coupling of Cl- to the transport of these osmoprotective amino acids may enhance their volume regulatory effect in kidney cells and other mammalian cells.

Conducted signals within arteriolar networks initiated by bioactive amino acids

Our purpose was to determine the specificity of L-arginine (L-Arg)-induced conducted signals for intra- vs. extracellular actions of L-Arg. Diameter and red blood cell velocities were measured for arterioles [18 +/- 1.6 (SE) micrometer] in the cremaster muscle of pentobarbital sodium-anesthetized (Nembutal, 70 mg/kg) hamsters (n = 53). Remote (conducted) responses were viewed approximately 1,000 micrometer upstream from the local (micropipette) application. Six amino acids were tested: L-arginine, L-cystine, L-leucine, L-lysine, L-histidine, and L-aspartate (100 microM each). Only L-Arg induced a remote dilation; L-lysine and L-aspartate had no effect, and the others each induced a significant remote constriction. There is a second conducted signal initiated by L-arginine that preconditions the arteriolar network and upregulates a direct response of L-arginine to dilate the remote site. This was blocked by inhibition of L-arginine uptake at the local (preconditioning) site (100 microM L-histidine or 1 mM phenformin). Arginine-glycine-aspartate (100 microM)-induced remote dilations (+3. 2 +/- 0.3 micrometer) were not mimicked by a peptide control and were prevented by anti- integrin alphav monoclonal antibody. Remote dilations were greater in animals with a higher wall shear stress for arginine-glycine-aspartate (r2 = 0.92) but not for L-arginine (r2 = 0.12). Thus L-arginine initiates separate conducted signals related to system y+ transport, integrins, and baseline flow.

Transporters for cationic amino acids in animal cells: discovery, structure, and function

The structure and function of the four cationic amino acid transporters identified in animal cells are discussed. The systems differ in specificity, cation dependence, and physiological role. One of them, system y+, is selective for cationic amino acids, whereas the others (B[0,+], b[0,+], and y+ L) also accept neutral amino acids. In recent years, cDNA clones related to these activities have been isolated. Thus two families of proteins have been identified: 1) CAT or cationic amino acid transporters and 2) BAT or broad-scope transport proteins. In the CAT family, three genes encode for four different isoforms [CAT-1, CAT-2A, CAT-2(B) and CAT-3]; these are approximately 70-kDa proteins with multiple transmembrane segments (12-14), and despite their structural similarity, they differ in tissue distribution, kinetics, and regulatory properties. System y+ is the expression of the activity of CAT transporters. The BAT family includes two isoforms (rBAT and 4F2hc); these are 59- to 78-kDa proteins with one to four membrane-spanning segments, and it has been proposed that these proteins act as transport regulators. The expression of rBAT and 4F2hc induces system b[0,+] and system y+ L activity in Xenopus laevis oocytes, respectively. The roles of these transporters in nutrition, endocrinology, nitric oxide biology, and immunology, as well as in the genetic diseases cystinuria and lysinuric protein intolerance, are reviewed. Experimental strategies, which can be used in the kinetic characterization of coexpressed transporters, are also discussed.

Sodium-coupled amino acid transport in renal tubule

Amino acids are reabsorbed from the tubular lumen by a saturable, carrier-mediated, concentrative transport mechanism driven by a Na+ electrochemical gradient across the luminal membrane. This process is followed by efflux mainly via carrier-mediated, Na+-independent facilitated diffusion across the basolateral membrane. Individual amino acids may have two or more Na+-dependent transport systems with different kinetic characteristics along the luminal membrane of the proximal tubule, thereby enabling very efficient amino acid reabsorption. Dual Na+-coupled transport pathways for some amino acids located in both the luminal and the peritubular membranes may operate in concert to provide the tubular epithelial cell with essential nutrients. One or more Na+ ions, H+, Cl- and in the case of acidic amino acids, K+ ion, may be involved in the translocation of the carrier complex. For most amino acids this process is electrogenic positive, favored by a negative cell interior. At least seven distinct, but largely interacting, Na+-dependent amino acid transport systems have been identified in the brush border membrane. A diet-induced adaptation in Na+-coupled taurine transport and acidosis-induced adaptive response in Na+-dependent glutamine transport are expressed at the luminal and the basolateral membrane surfaces, respectively. The aminoaciduria of early life may be related to a rapid dissipation of the Na+ electrochemical gradient necessary for amino acid reabsorption.

Electrogenic transport of neutral and dibasic amino acids in a cultured opossum kidney cell line (OK)

A study has been made of electrogenic cellular uptake of amino acids resulting in the depolarization of cell membrane potential (PDm) in confluent monolayers of an established opossum kidney (OK) cell line using conventional and pH-selective microelectrodes. Apical superfusion of neutral and dibasic amino acids rapidly depolarized the cell membrane, while application of acidic amino acids had no effect on PDm. The depolarization in response to L-phenylalanine and L-arginine was stereoselective, dose-dependent and saturable. 10 mmol/l of L-phenylalanine reduced PDm by 4.8 +/- 0.4 mV (n = 51) in a completely sodium-dependent way and the concentration necessary for half-maximal depolarization (C1/2) was about 1.5 mmol/l. On the other hand, the C1/2 for L-arginine was about 0.02 mmol/l. The maximal depolarization produced by L-arginine (measured at 10 mmol/l) amounted to 6.8 +/- 1.2 mV (n = 10) and this was not affected when extracellular sodium was replaced by choline (6.3 +/- 1.2 mV; n = 10). The depolarizations induced by L-phenylalanine and L-arginine were significantly additive (p less than 0.001). The intracellular pH of OK cells was 7.09 +/- 0.03 (n = 11) and did not change during L-arginine application. We conclude that (1) carrier-mediated uptake of neutral and dibasic amino acids into OK cells is at least partially electrogenic. (2) L-Phenylalanine is transported by a Na+-symport. (3) In contrast, L-arginine depolarizes PDm independently of extracellular sodium. (4) Electrogenic uptake of acidic amino acids is not detectable in OK cells.

Kinetics of the Na+/alanine cotransporter in pancreatic acinar cells

Electric currents associated with Na+-coupled alanine transport in pancreatic acinar cells were investigated by the technique of tight-seal whole-cell recordings. In a previous study the observed concentration dependence of alanine-dependent currents was found to be consistent with a 'simultaneous' transport mechanism with 1:1 stoichiometry. In the present work the sidedness of the cotransporter was investigated by comparing inward (I") and outward currents (I') measured under mirror-symmetrical conditions. I' and I" were found to be nearly equal (within a factor of approx. 2) in a wide range of Na+ and alanine concentrations. The transport model was further tested by 'infinite-cis' experiments with fixed, saturating concentrations of Na+ and L-alanine on one side of the membrane and variable concentrations on the other. By measuring transmembrane currents as a function of Na+ and alanine concentrations, numerical values of the equilibrium dissociation constants of both substrates could be estimated.

On the nature of delayed repolarization during sustained sodium coupled transport in frog proximal tubules

In proximal tubules of the frog kidney, stimulation of coupled transport of sodium with phenylalanine leads to depolarization of the cell membrane, followed by repolarization within a few minutes. The repolarization is due to a delayed increase of potassium conductance at the peritubular cell membrane. The present study was designed to test for the role of depolarization, of calmodulin and of arachidonic acid metabolites for the delayed increase of potassium conductance. To this end, the potential difference across the peritubular cell membrane of proximal convoluted tubules (PDpt) has been recorded continuously during exposure of the lumen to phenylalanine or during galvanic current injection into a neighbouring cell. During control conditions, PDpt averages -68.6 +/- 1.0 mV (n = 45). Phenylalanine leads to a depolarization of the peritubular cell membrane by +31.5 +/- 1.3 mV (n = 20), followed by a repolarization by -12.9 +/- 1.1 mV (n = 20) within 3 min. Injection of currents from 10 to 80 nAmps leads to a depolarization by +0.83 +/- 0.01 mV/nAmps which is again followed by repolarization. A linear correlation is observed between the magnitude of depolarization (dep) and repolarization (rep) within 3 min: rep (mV) = -(0.24 +/- 0.01) dep (mV) +(2.45 +/- 0.12) mV (r = 0.90). Thus, depolarization is capable to trigger delayed repolarization. The extent of repolarization is a function of the magnitude of depolarization. The possible involvement of calmodulin or arachidonic acid metabolites has been tested for by inducing sodium coupled transport in the presence of 100 mumol/l mepacrine, 10 mumol/l indomethacin or 10 mumol/l trifluoperazine.

Electrophysiology of L-lysine entry across the brush-border membrane of Necturus intestine

Microelectrode measurements of apical membrane potentials (Va) in absorptive cells of isolated Necturus intestine showed that, in the presence or absence of external Na+, 10 mM lysine added to the mucosal medium caused rapid depolarization followed by slower repolarization of Va. In Na+-free media the effects of 10 mM lysine on Va were abolished by 10 mM leucine which alone had no effect on Va under these conditions. This indicates that uncoupled electrodiffusion of lysine plays little or no role in lysine entry across the brush-border membrane. When external Na+ was greater than 10 mM the maximum depolarization of Va (delta Va') induced by [Lys] ranging from 5 to 30 mM was a simple saturable function of [Lys]. In Na+-free media, the relationship between delta Va' and [Lys] was biphasic. At first, delta Va' increased with increasing [Lys] reaching a maximum at 10 mM lysine. When [Lys] was further increased, delta Va' declined progressively to reach zero or near zero values. A single transport pathway model is proposed to account for rheogenic lysine entry across the brush-border membrane in the presence and absence of Na+. This postulates an amino acid transporter in the membrane with two binding sites. One is an amino acid site specific for the alpha-amino-alpha-carboxyl group. The other is a Na+ site. Neutral amino acids (e.g. leucine) compete with lysine for the amino acid site. The Na+ site has some affinity for the epsilon-amino group of lysine. When external Na+ is high the Na+ site is essentially 'saturated' with Na+ and formation of a mobile complex between an amino acid and the transporter depends in a saturable fashion on amino acid concentration. In Na+-free media or in media containing low [Na+]; at low external [Lys] the epsilon-amino group of a lysine molecule (simultaneously attached to the amino acid site) interacts with the Na+ site to form a mobile complex, as external [Lys] is increased, attachment of different lysine molecules to each site of an increasing number of transporters to form nontransported or poorly transported complexes results in substrate inhibition of the rheogenic lysine transport process.

Acidic amino acid transport in animal cells and tissues

1. The occurrence and characterization of acidic amino acid transport in the plasma membrane of a variety of cells and tissues of a number of organisms is reviewed. 2. Several cell types, especially in brain, possess both high- and low-affinity transport systems for acidic amino acids. 3. High-affinity systems in brain may function to remove neurotransmitter amino acid from the extracellular environment. 4. Many cell systems for acidic amino acid transport are energized by an inwardly directed Na+ gradient. Moreover, certain cell types, such as rat brain neurons, human placental trophoblast and rabbit and rat kidney cortex epithelium, respond to an outwardly directed K+ gradient as an additional source of energization. This simultaneous action may account for the high accumulation ratios seen with acidic amino acids. 5. Rabbit kidney has been found to have a glutamate-H+ co-transport system which is subject to stimulation by protons in the medium. 6. Acidic amino acid transport in rat brain neurons occurs with a stoichiometric coupling of 1 mol of amino acid to 2 mol of Na+. For rabbit intestine, one Na+ is predicted to migrate for each mol of amino acid. 7. Uptake in rat kidney cortex and in high-K+ dog erythrocytes is electrogenic. However, uptake in rabbit and newt kidney and in rat and rabbit intestine is electroneutral. 8. Na+-independent acidic amino acid transport systems have been described in the mouse lymphocyte, the human fibroblast, the mouse Ehrlich cell and in rat hepatoma cells. 9. In a number of cell systems, D-acidic amino acids have substantial affinity for transport; D-glutamate, in a number of systems, however, appears to have little reactivity. 10. Acidic amino acid transport in some cell systems appears to occur via the "classical" routes (Christensen, Adv. Enzymol. Relat. Areas Mol. Biol. 49, 41-101, 1979). For example, uptake in the Ehrlich cell is partitioned between the Na+-dependent A system (which transports a wide spectrum of neutral amino acids), the Na+-dependent ASC system (which transports alanine, serine, threonine, homoserine, etc.), and the Na+-independent L system (which shows reactivity centering around neutral amino acids such as leucine and phenylalanine). Also, a minor component of uptake in mouse lymphocytes occurs by a route resembling the A system. 11. Human fibroblasts possess a Na+-independent adaptive transport system for cystine and glutamate that is enhanced in activity by cystine starvation.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultracytochemistry of ouabain-sensitive K+-dependent p-nitrophenyl phosphatase in rat incisor enamel organ

Sprague-Dawley strain rats of 4-5 weeks old were perfusion-fixed with either a mixture containing 0.1 or 0.25% glutaraldehyde and 2% formaldehyde, or a 2% formaldehyde in 0.1 M sodium cacodylate buffer for 10 minutes. Non-decalcified 30-50-micron sections of the enamel organ taken from lower incisors were then processed for ultracytochemical demonstration of ouabain-sensitive, K+-dependent, p-nitrophenyl phosphatase, by use of the one-step lead method, representing the second dephosphorylative step of Na+-K+-ATPase. Throughout the secretory, transition, and maturation stages of amelogenesis, the enzymatic activity was demonstrated along the cytoplasmic side of the plasma membranes of the stratum intermedium and the papillary layer cells, especially along their numerous microvilli. The plasma membranes forming gap junctions and desmosomes were free of reaction or showed slight focal precipitates of reaction products. The stellate reticulum and the outer enamel epithelium exhibited either a weak reaction or were reaction negative. Secretory ameloblasts showed a weak trace-like reaction along the basal and lateral cell surfaces; however, the latter surfaces were sometimes completely free of reaction. Tomes' processes were usually reaction negative. Ameloblasts in the transition and maturation stages were devoid of enzymatic activity, except for a slight reaction along the plasma membranes of the basal cell surfaces of transition ameloblasts facing the papillary layer. The enzymatic activity described above was completely dependent on the presence of potassium and substrate in the incubation media and was almost completely inhibited by an addition of 10 mM ouabain to the incubation media.

Influence of potassium depletion on potassium conductance in proximal tubules of frog kidney

In order to test for the contribution of intracellular potassium activity to the link of sodium/potassium-ATPase activity and potassium conductance, studies with conventional and potassium selective microelectrodes were performed on proximal tubules of the isolated perfused frog kidney. The peritubular transference number for potassium (tk), i.e., the contribution of peritubular slope potassium conductance to the slope conductance of the cell membranes (luminal and peritubular), was estimated from the influence of peritubular potassium concentration on the potential difference across the peritubular cell membrane (PDpt). During control conditions, PDpt is -65 +/- 1 mV, intracellular potassium activity (Ki) 57 +/- 2 mmol/l and tk 0.41 +/- 0.05. The resistance in parallel of the luminal and peritubular cell membranes (Rm) is 44 +/- 4 k omega cm, the resistance of the cellular cable (Rc) 137 +/- 13 M omega/cm. When the cells are exposed 10 min to potassium free perfusates (series I), PDpt increases by -28 +/- 3 mV within 2 min and then decreases gradually to approach the control value within 10 min. Ki decreases by 22 +/- 3 mmol/l and Rc increases by 35 +/- 10%. After a transient decrease, Rm increases by 36 +/- 9%. Readdition of peritubular potassium leads to a transient increase of PDpt, a gradual decrease of Rm and Rc as well as a gradual increase of Ki. tk recovers only slowly to approach 65 +/- 8% of control value within 3 and 79 +/- 10% within 6 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: I. Tight-seal whole-cell recordings

Electrical currents associated with sodium-coupled alanine transport in mouse pancreatic acinar cells were studied using the method of whole-cell recording with patch pipettes. Single cells or small clusters of (electrically coupled) cells were isolated by collagenase treatment. The composition of the intracellular solution could be controlled by internal perfusion of the patch pipette. In this way both inward and outward currents could be measured under "zero-trans" conditions, i.e., with finite concentrations of sodium and L-alanine on one side and zero concentrations on the other. Inward and outward currents for equal but opposite concentration gradients were found to be of similar magnitude, meaning that the cotransporter is functionally nearly symmetric. The dependence of current on the concentrations of sodium and L-alanine exhibited a Michaelis-Menten behavior. From the sodium-concentration dependence of current as well as from the reversal potential of the current in the presence of an alanine-concentration gradient, a sodium/alanine stoichiometric ratio of 1:1 can be inferred. The finding that N-methylated amino acids may substitute for L-alanine, as well as the observed pH dependence of currents indicate that the pancreatic alanine transport system is similar to (or identical with) the "A-system" which is widespread in animal cells. The transport system is tightly coupled with respect to Na+; alanine-coupled inward flow of Na+ is at least 30 times higher than uncoupled Na+ flow mediated by the cotransporter. The current-voltage characteristic of the cotransporter could be (approximately) determined from the difference of transmembrane current in the presence and in the absence of L-alanine. The sodium-concentration dependence of the current-voltage characteristic indicates that a Na+ ion approaching the binding site from the extracellular medium has to cross part of the transmembrane electric field.

Influence of barium on the effects of phenylalanine in proximal tubules

The present study was designed to further test for the role of peritubular potassium conductance in the repolarization of peritubular cell membrane during sustained stimulation of sodium coupled transport by phenylalanine. To this end the potential difference across the peritubular cell membrane (PDpt) has been recorded continuously, while 10 mmol/l phenylalanine (Phe) were added to the luminal perfusate, both in the presence or absence of peritubular or luminal barium (1 mmol/l). In the absence of phenylalanine and barium, PDpt amounts to -65.5 +/- 2.2 mV. Phe leads to a rapid depolarization of the peritubular cell membrane by +36.2 +/- 2.2 mV within 30 s, followed by an almost complete repolarization by -28.9 +/- 2.6 mV within 7 min. In the presence of barium in peritubular perfusate, the depolarization following Phe is +24.3 +/- 2.6 mV and the repolarization almost abolished (-4.3 +/- 0.9 mV). In the presence of barium in luminal perfusate, Phe leads to a depolarization by +35.7 +/- 2.4 mV followed by a repolarization of -17.0 +/- 3.2 mV within 7 min. It is concluded that the repolarization during sustained stimulation of sodium coupled transport is in large part due to alterations of peritubular potassium conductance.

Electrophysiological analysis of rat renal sugar and amino acid transport. IV. Basic amino acids

Electrophysiological techniques were used to study the transport of the basic amino acids L-arginine, L-lysine and L-ornithine in rat kidney proximal tubule in vivo. Tubular cells were punctured with microelectrodes and the response of the cell membrane potential to sudden applications of the amino acids was measured. In the presence of physiological Na+ concentrations luminal perfusion with millimolar concentrations of basic amino acids depolarized the tubular cells in a concentration dependent fashion by up to 15 mV, while in the absence of Na+ no significant potential changes were observed. These observations indicate that the basic amino acids are taken up into the cell across the brushborder in coupling with Na+ ions in a similar way as neutral and acidic amino acids, and that simple conductive pathways for uncoupled flow of the basic amino acids do either not exist or are quantitatively negligible in the brushborder. From kinetic measurements and competition experiments it was concluded that all basic amino acids are transported by the same transport system, which however does not accept acidic or neutral amino acids (with the possible exception of L-cystine). Perfusion of the peritubular capillaries with millimolar concentrations of basic amino acids depolarized the cells only by approximately 1 mV, both in the presence and absence of Na+. This observation may indicate that a passive uncoupled transport pathway for basic amino acids is present in the peritubular cell membrane to allow exit from cell to interstitial space, if the intracellular concentration rises high enough to overcome the cell membrane potential.

Electrophysiological analysis of rat renal sugar and amino acid transport. V. Acidic amino acids

We have used electrophysiological techniques to study various aspects of the transport of glutamate and aspartate in proximal tubules of the rat kidney in vivo. Single tubular cells were punctured with microelectrodes and the response of the cell membrane potential to sudden luminal or peritubular applications of these amino acids was measured. The experiments indicated that a specific transport system exists for L-glutamate and L-aspartate in the brushborder membrane, which does not transport neutral or basic amino acids. The uptake of both L-amino acids from the lumen into the cell was found to be rheogenic, probably reflecting the cotransport of two Na+ ions together with one amino acid molecule. The transport system has a slightly greater affinity for L-glutamate, but transports the smaller L-aspartate somewhat faster. Besides the L-isomers also D-glutamate and D-aspartate were found to depolarize the tubular cells which suggests that also the D-isomers are absorbed in the tubule, however they do not seem to use the same transport system as the L-isomers. In addition to the transport system in the brushborder, a similar Na+-dependent, rheogenic transport system for L-glutamate and L-aspartate was also found in the peritubular cell membrane, as deduced from cell cell depolarizations in response to these substrates applied peritubularly. The simultaneous presence of Na-driven transport systems in the apical and basal cell membrane which is not found with other amino acids, may explain the high intracellular accumulation of L-glutamate and L-aspartate in the kidney and provides a rational basis for explaining clinically observed cases of dicarboxylic aminoacidurias.

L-glutamate transport in renal plasma membrane vesicles

This review describes the uptake of L-glutamate by well-characterized preparations of renal brush border (liminal) and baso-lateral membrane vesicles derived from the plasma membrane of the polar proximal tubular cell. L-glutamate is taken up against its concentration gradient, from both sides, by co-transport systems in which the movement of the amino acid into the cell is coupled to the influx of Na+ and efflux of K+ down their respective electrochemical gradients. The presence of these ion gradient-energized systems, specific for L-glutamate, may account for the exceedingly high intracellular concentration of their metabolically important amino acid in the renal tubule.

Effects of changes in sodium electrochemical potential gradient on p-aminohippurate transport in newt kidney

1. The relation between p-aminohippurate uptake and the electrochemical potential gradient of Na+ (delta muNa+) across the peritubular membrane was examined in newt (Triturus pyrrhogaster) kidney. The delta muNa+ was modified by changing cellular Na+ concentration and/or lowering the electrical potential difference across the peritubular membrane (peritubular membrane potential) 2. Elevation of external K+ concentration or addition of alanine at 40 mM to the medium decreased the delta muNa+ mainly through the depolarization of the cells. Addition of 1 mM ouabain resulted in a decrease in the peritubular membrane potential and increase in cellular Na+ concentration, thus decrease in the delta muNa+. 3. p-Aminohippurate uptake decreased in proportion to the decrease in the delta muNa+ under all experimental conditions, indicating that the maintenance of the delta muNa+ is required for p-aminohippurate transport. 4. All three different experimental conditions, high medium K+ concentration, 40 mM alanine or 1 mM ouabain, increased the apparent Michaelis constant, Kt, without affecting the maximal uptake rate, V, for p-aminohippurate. These results suggests that the delta muNa+, largely the peritubular membrane potential, may affect the association and/or dissociation of p-aminohippurate and Na+ at both interfaces of the peritubular membrane of the proximal tubular cells.

Role of sodium ions in p-aminohippurate transport by newt kidney

1. The effect of external Na+ concentration on p-aminohippurate uptake by isolated kidneys of newt (Triturus pyrrhogaster) was studied kinetically and electrophysiologically. 2. p-Aminohippurate uptake conformed to Michaelis-Menten type kinetics in regard to both p-aminohippurate and Na+ concentrations in the incubation medium. Kinetic studies revealed that reduction of Na+ concentration increased the values of Kt without altering the maximal rate (V) of p-aminohippurate uptake. The values of Kt were a linear function of the reciprocal of Na+ concentration. These results suggest the presence of interaction between p-aminohippurate and Na+ at the carrier level, i.e. Na+-coupled cotransport. 3. p-Aminohippurate had no effect on the electrical potential difference across the peritubular membrane in both 10 and 100 mM Na+ solutions, suggesting that p-aminohippurate is transported across the peritubular membrane in a form of electrically neutral carrier complex. This is consistent with the results of the kinetic studies. 4. p-Aminohippurate uptake was proportional to the electrochemical potential gradient of Na+ (delta mu Na) across the peritubular membrane. This result indicates that the maintenance of sufficient delta mu Na appears to be necessary for the accumulation of p-aminohippurate against its electrochemical potential gradient, supporting Na+ gradient hypothesis.