Renal Transport of Amino Acids

Pharmacokinetics of S-Allyl-l-cysteine in Rats Is Characterized by High Oral Absorption and Extensive Renal Reabsorption

BACKGROUND

S-Allylcysteine (SAC) is a key component of aged garlic extract, one of many garlic products. However, information on its pharmacokinetics has been scant except for data from a few animal studies.

OBJECTIVE

We designed this study to determine the overall pharmacokinetics of SAC in rats.

METHODS

After oral or intravenous administration of SAC to rats at a dose of 5 mg/kg, the plasma concentration-time profile of SAC and its metabolites, as well as the amounts excreted in bile and urine, were analyzed by using liquid chromatography tandem mass spectrometry.

RESULTS

After oral administration, SAC was well absorbed with a bioavailability of 98%. Two major metabolites of SAC, N-acetyl-S-allylcysteine (NAc-SAC) and N-acetyl-S-allylcysteine sulfoxide (NAc-SACS), were detected in plasma, but their concentrations were markedly lower than those of SAC. SAC was metabolized to a limited extent, but most of the orally absorbed SAC was excreted into urine in the form of its N-acetylated metabolites. The amounts of SAC, NAc-SAC, and NAc-SACS excreted in urine over 24 h were 2.9%, 80%, and 11% of the orally administered SAC, respectively. The very low renal clearance (0.016 L ⋅ h(-1) ⋅ kg(-1)) of SAC indicated that it undergoes extensive renal reabsorption. These results collectively suggested that SAC was ultimately metabolized to NAc-SAC and NAc-SACS through the cycles of urinary excretion, renal reabsorption, and systemic recirculation.

CONCLUSION

The pharmacokinetics of SAC in rats were characterized by high oral absorption, limited metabolism, and extensive renal reabsorption, all of which potentially contribute to its high and relatively long-lasting plasma concentrations.

Iminoglycinuria and hyperglycinuria are discrete human phenotypes resulting from complex mutations in proline and glycine transporters

Iminoglycinuria (IG) is an autosomal recessive abnormality of renal transport of glycine and the imino acids proline and hydroxyproline, but the specific genetic defect(s) have not been determined. Similarly, although the related disorder hyperglycinuria (HG) without iminoaciduria has been attributed to heterozygosity of a putative defective glycine, proline, and hydroxyproline transporter, confirming the underlying genetic defect(s) has been difficult. Here we applied a candidate gene sequencing approach in 7 families first identified through newborn IG screening programs. Both inheritance and functional studies identified the gene encoding the proton amino acid transporter SLC36A2 (PAT2) as the major gene responsible for IG in these families, and its inheritance was consistent with a classical semidominant pattern in which 2 inherited nonfunctional alleles conferred the IG phenotype, while 1 nonfunctional allele was sufficient to confer the HG phenotype. Mutations in SLC36A2 that retained residual transport activity resulted in the IG phenotype when combined with mutations in the gene encoding the imino acid transporter SLC6A20 (IMINO). Additional mutations were identified in the genes encoding the putative glycine transporter SLC6A18 (XT2) and the neutral amino acid transporter SLC6A19 (B0AT1) in families with either IG or HG, suggesting that mutations in the genes encoding these transporters may also contribute to these phenotypes. In summary, although recognized as apparently simple Mendelian disorders, IG and HG exhibit complex molecular explanations depending on a major gene and accompanying modifier genes.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

The molecular basis of neutral aminoacidurias

Recent success in the molecular cloning and identification of apical neutral amino acid transporters has shed a new light on inherited neutral amino acidurias, such as Hartnup disorder and Iminoglycinuria. Hartnup disorder is caused by mutations in the neutral amino acid transporter B(0) AT1 (SLC6A19). The transporter is found in kidney and intestine, where it is involved in the resorption of all neutral amino acids. The molecular defect underlying Iminoglycinuria has not yet been identified. However, two transporters, the proton amino acid transporter PAT1 (SLC36A1) and the IMINO transporter (SLC6A20) appear to play key roles in the resorption of glycine and proline. A model is presented, involving all three transporters that can explain the phenotypic variability of iminoglycinuria.

Transient neonatal cystinuria

BACKGROUND

Cystinuria is an inherited disorder of luminal reabsorptive transport for cystine and dibasic amino acids in the renal proximal tubule. Two cystinuria genes have been identified. Mutations of SLC7A9, which encodes the luminal transport channel itself, tend to be dominant and mutations of SLC3A1 (rBAT), which encodes a transporter subunit, are always recessive. Patients who inherit two recessive mutations or two dominant mutations have equally severe forms of cystinuria. Heterozygotes excrete cystine in the normal (type I), moderate (type III), or high stone-forming (type II) range.

METHODS

Infants with cystinuria were identified via the Quebec Newborn Urinary Screening Program. In a subgroup of these infants, cystinuria was severe in the first months of life, but partially resolved by 2 to 4 years postnatally. We assigned each patient a final cystinuria phenotype at 3 to 4 years. In addition, we characterized SLC3A1 gene expression in fetal and postnatal human kidney.

RESULTS

Most infants with transient neonatal cystinuria are eventually classified as type III heterozygotes. All infants with mutant cystinuria genes have exaggerated neonatal cystine excretion except those who inherit two SLC3A1 mutations (type I/I cystinuria); these children have persistent severe cystinuria, implying that wildtype SLC3A1 is required for the maturational effect. Expression of SLC3A1 mRNA was found to be tenfold higher in postnatal vs. fetal kidney; SLC3A1 expression is doubled by the proximal tubule transcription factor, PAX8. rBAT is expressed in the proximal convoluted and straight tubules in both fetal and adult kidney.

CONCLUSION

Maturation of SLC3A1 gene expression between midgestation and 4.5 years postnatal age may account for transient neonatal cystinuria.

Ornithine decarboxylase along the mouse and rat nephron

Renal arginase activity is a potent source of ornithine (Orn) for polyamine synthesis. Ornithine decarboxylase (ODC) was localized along the mouse and rat nephron by incubating viable nephron segments isolated by microdissection from collagenase-treated kidneys with or without D,L-2-(difluoromethyl)ornithine (DFMO), a selective inactivator of ODC. Tubules from either control or DFMO-treated animals were incubated with 100 ¿M L-[1-14C]Orn. In control mice, Orn decarboxylation occurred mainly in the proximal convoluted tubule (PCT). In DFMO-treated mice, Orn decarboxylation was dramatically reduced in PCT and in proximal straight tubules (PST). In rats, Orn decarboxylation also occurred predominantly in the proximal tubule. Addition of 10 mM DFMO to isolated tubules dramatically decreased Orn decarboxylation in PCT and in PST. Thereafter, ODC activity was demonstrated in permeabilized tubules. In Triton X-100-treated tubules from control mice, ODC was exclusively found in proximal tubules (PCT > PST). This ODC activity was strongly inhibited in DFMO-treated mice. In conclusion, the highest ODC activity was found in rat and mouse PCT, a segment devoid of arginase. We hypothesize that the filtered Orn, which is reabsorbed along the PCT,is the main source of Orn for ODC.

Cross-talk and the role of KATP channels in the proximal tubule

Over the last few years it has become evident that an assortment of functionally-related, but diverse, KATP channels provide an important and physiologically-regulated determinant of the K conductive pathways in many, if not all, epithelial cells expressed along the nephron. As such, KATP plays central roles in regulating and maintaining a number of transport processes in concert with physiological demands of the kidney. In the renal proximal tubule, KATP channels and changes in the hydrolytic activity of the Na,K-ATPase permit ATP to act as a coupling modulator of parallel Na,K-ATPase-K recycling. The response insures that cell membrane potential, intracellular K activity and cell volume are protected in the face of physiological variations in transcellular ion transport. In addition to demonstrating the physiological relevance of KATP in renal epithelial, these studies have provided a long awaited answer to the underlying mechanism of pump-leak coupling, a universal and essential homeostatic mechanism observed in nearly all salt translocating epithelia.

Renal handling of drugs and amino acids after impairment of kidney or liver function--influences of maturity and protective treatment

Renal tubular cells are involved both in secretion and in reabsorption processes within the kidney. Normally, most xenobiotics are secreted into the urine at the basolateral membrane of the tubular cell, whereas amino acids are reabsorbed quantitatively at the luminal side. Under different pathological or experimental circumstances, these transport steps may be changed, e.g., they may be reduced by renal impairment (reduction of kidney mass, renal ischemia, administration of nephrotoxins) or they may be enhanced after stimulation of transport carriers. Furthermore, a distinct interrelationship exists between excretory functions of the kidney and the liver. That means liver injury can influence renal transport systems also (hepato-renal syndrome). In this review, the following aspects were included: based upon general information concerning different transport pathways for xenobiotics and amino acids within kidney cells and upon a brief characterization of methods for testing impairment of kidney function, the maturation of renal transport and its stimulation are described. Similarities and differences between the postnatal development of kidney function and the increase of renal transport capacity after suitable stimulatory treatment by, for example, various hormones or xenobiotics are reviewed. Especially, renal transport in acute renal failure is described for individuals of different ages. Depending upon the maturity of kidney function, age differences in susceptibility to kidney injury occur: if energy-requiring processes are involved in the transport of the respective substance, then adults, in general, are more susceptible to renal failure than young individuals, because in immature organisms, anaerobic energy production predominates within the kidney. On the other hand, adult animals can better compensate for the loss of renal tissue (partial nephrectomy). With respect to stimulation of renal transport capacity after repeated pretreatment with suitable substances, age differences also exist: most stimulatory schedules are more effective in young, developing individuals than in mature animals. Therefore, the consequences of the stimulation of renal transport can be different in animals of different ages and are discussed in detail. Furthermore, the extent of stimulation is different for the transporters located at the basolateral and at the luminal membranes: obviously the tubular secretion at the contraluminal membrane can be stimulated more effectively than reabsorption processes at the luminal side.

The influence of acetazolamide and amlodipine on the intracellular sodium content of rat proximal tubular cells

1. This investigation set out to use 23Na n.m.r. spectroscopy to measure changes in intracellular levels of sodium in isolated suspensions of rat proximal tubules. The effects of temperature, an inhibitor of the sodium pump and known natriuretic drugs on intracellular sodium content of such tubular preparations were measured and compared with calcium channel antagonists where action at this level is unclear. 2. Rat kidneys were perfused with collagenase, roughly chopped, subjected to mechanical dispersion and washed to remove all traces of the enzyme. The proximal tubules were then purified and concentrated by Percoll density gradient centrifugation and then resuspended in buffer containing dysprosium tripolyphosphate shift reagent. 3. Distinct peaks corresponding to intracellular and extracellular sodium signals were observed when the tubules were placed into the n.m.r. spectrometer. As the temperature of the suspension rose to 37 degrees C, there was an exponential decrease in sodium content, with a decay constant of 0.15 +/- 0.02 min-1, which reached a stable level within 20 to 25 min. Addition of ouabain, 10(-3) M, resulted in a significant (P < 0.01) 30% increase in intracellular sodium content within 5 min which peaked at 70% 20 min later. Although acetazolamide (10(-3) M) significantly (P < 0.01) increased intracellular sodium content by 45%, amlodipine (10(-4) M) had no effect. 4. These data show that changes in the activity of the Na+/K+/ATPase have a considerable influence on the intracellular levels of sodium in proximal tubule cells. Inhibition of carbonic anhydrase activity resulted in a rise in intracellular sodium content which is compatible with its action to reduce the turnover rate of the Na+/(HCO3-)3 symporter. The lack of effect of amlodipine was consistent with the suggestion that it does not have a direct action on the sodium handling processes at the level of the proximal tubule.

Basolateral uptake and tubular metabolism of L-citrulline in the isolated-perfused non-filtering kidney of the African clawed toad (Xenopus laevis)

The kidney forms arginine (Arg) by using citrulline (Cit) as precursor, and is the main source of Arg for systemic protein synthesis. Even if the filtered and reabsorbed load (in rats) is sufficient for normal Arg synthesis, the following questions remain. (a) Can Cit be taken up across the contraluminal membrane of the tubule cells also? If so, (b) by what kind of mechanism? And (c) is this Cit, entering the cell from the peritubular side, metabolized to Arg and ornithine (Orn)? Although these questions are raised mainly in connection with mammals, we used the amphibian kidney, which is especially suitable because of its double blood supply, for an initial approach to the problem. After the toad was decapitated, the portal vein, the caval vein and the ureters were catheterized, and the kidneys were perfused through the portal vein (Ringer solution + L- or D-Cit + inulin + p-aminohippurate + L-aspartate). Exclusive peritubular perfusion was assured by showing that inulin perfused into the portal vein did not appear in the urine. During perfusion of the portal vein with L-Cit in a physiological concentration (65 mumol/l), an initial peritubular net uptake of L-Cit of 170 +/- 27 (n = 10) nmol.h-1.g kidney-1 (wet weight) was observed, whereas the value for D-Cit (65 mumol/l) was only 18 +/- 7 (n = 6) nmol.h-1.g-1. After perfusion for 50 min, the uptake of L-Cit reached a steady state with an uptake rate of 108 +/- 5 nmol.h-1.g-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport of nutrients into the renal brush border membrane vesicles as marker in evaluating the role of antipili antibodies in modulation of ascending pyelonephritis in rats

The uptake of D-glucose, L-aspartate, L-lysine and L-proline was investigated in renal brush border membrane (BBM) vesicles prepared from control, infected or passively-immunized-infected rats. Except L-aspartate, a progressive decrease in the uptake of these nutrients in both infected and immunized-infected groups during the course of infection was observed, but the changes were less apparent in immunized-infected rats than in non-immunized ones. The uptake of L-aspartate was increased in vesicles from early stages of infection but decreased in those from later stages. Also in L-aspartate uptake, the changes were smaller in immunized animals. The uptake of nutrients was detectable earlier than were histopathological alterations of both kidneys. The observations demonstrated that uptake of D-glucose and amino acids in the kidneys is disturbed prior to appearance of histopathological lesions and thus can be used for early detection of the disease. The data also demonstrate that antipili antibodies afford partial protection against ascending pyelonephritis.

Na+-sugar cotransport system as a polarization marker during organization of epithelial membrane

The present study analyzed the changes in Na+-dependent sugar transport and transepithelial electrical resistance as LLC-PK1 cells reorganize into epithelial membranes. Sugar influx increased to reach a maximum 9 h after plating. The increase in the transepithelial electrical resistance, however, showed a significant delay, reaching steady state 15 h after plating. No changes in the electrochemical Na+ gradient were observed during the reorganization of the epithelial membranes. Kinetic analysis and [3H]phlorizin-binding studies showed that the increase in sugar influx resulted from an increase in the number of carriers. Unidirectional sugar influx measurements indicated that the sugar transporters were primarily located at the apical surface of the epithelial cells. These observations are consistent with the hypothesis that the sorting of native proteins occurs intracellularly before their insertion in the apical membrane, or as an alternative that they are randomly inserted, but then immediately sorted such as any carrier could be detected in the basolateral side during the reorganization process. In addition, the results suggest that the functional development of the apical membrane may occur before the complete sealing of the intercellular space during the development of the occluding junctions. Furthermore, development of the sugar transport system and occluding junctions was inhibited by cycloheximide and puromycin but not by actinomycin D, suggesting that the expression of epithelial cell polarization is probably a posttranslational event in the protein synthesis.

Expression of amino acid transport systems in Xenopus oocytes injected with mRNA of rat small intestine and kidney

Xenopus and Cynops oocytes were injected with exogenous mRNA prepared from rat small intestine and kidney and their electrical responses to amino acids were measured by both the current clamped and the voltage clamped methods. Oocytes injected with mRNA of rat small intestine showed a depolarization response to several neutral and basic amino acids, and almost no response to acidic amino acids. The responses to amino acids increased with incubation time after injection of mRNA, and followed Michaelis-Menten type kinetics. The responses were dependent on both Na+ concentration and membrane potential, and were inactivated by a sulfhydryl reagent, 5,5-dithiobis(2-nitrobenzoate). These results are interpreted as due to the expression of Na+/amino acid cotransporter(s) in oocytes injected with rat small intestine mRNA. On the other hand, the oocyte injected with rat kidney mRNA showed a hyperpolarization response to neutral amino acids, a depolarization response to basic ones, and almost no response to acidic ones in frog Ringer solution. These responses were independent of Na+ concentration and followed Michaelis-Menten type kinetics. These amino acid response characteristics in oocytes injected with rat kidney mRNA are interpreted as due to the expression of facilitated diffusion carrier protein(s) (uniporter) of amino acids in the oocyte.

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.

Transport of L-cystine in isolated perfused proximal straight tubules

Unidirectional fluxes of L-35S-cystine and intracellular 35S activity were measured in isolated perfused segments of rabbit proximal straight tubule. The absorptive (lumen-to-both) flux of L-35S-cysteine showed a tendency toward saturation within the concentration limits imposed by the low solubility of cystine (0.3 mmol . l-1). In contrast, for the bath-to-lumen fluxes, there was a linear relation between the bathing solution concentration of L-35S-cystine and the rate of 35S appearance in the lumen. Nonlinear fitting of both sets of unidirectional flux data gave a maximal cystine transport rate (Jmax) of 1.45 +/- 0.27 (SEM) pmol min-1 mm-1, a Michaelis constant (Km) of 0.20 +/- 0.07 mmol . l-1, and an apparent permeability coefficient of 0.27 +/- 0.11 pmol min-1 mm-1 (mmol . l-1)-1 (approximately 0.06 micrometer/s). The 35S concentration in the cell exceeded that in the lumen by almost 60-fold during the lumen-to-bath flux, and exceeded the bathing solution concentration by 4.7-fold during the bath-to-lumen flux. Thus cystine was accumulated by the cells across either membrane, but over 77% of the intracellular activity was in the form of cysteine. Although the presence of luminal L-lysine or cycloleucine inhibited the absorptive flux of cystine, neither amino acid affected the bath-to-lumen flux.

Proton-coupled L-lysine uptake by renal brush border membrane vesicles from mullet (Mugil cephalus)

The uptake of the basic amino acid, L-lysine, was studied in brush border membrane vesicles isolated from the kidney of the striped mullet (Mugil cephalus). The uptake of L-lysine was not significantly stimulated by a Na+ gradient and no overshoot was observed. However, when a proton gradient (pHo = 5.5; pHi = 8.3) was imposed across the membrane in the absence of Na+, uptake was transiently stimulated. When the proton gradient was short circuited by the proton ionophore, carbonylcyanide p-triflouromethoxyphenyl hydrazone, proton gradient-dependent uptake of lysine was inhibited. Kinetics of lysine uptake determined under equilibrium exchange conditions indicated that the Vmax increased as available protons increased (2.1 nmol/min/mg protein at pH 7.5 to 3.7 nmol/min/mg at pH 5.5), whereas the apparent Km (4.9 +/- 0.6 mM) was not altered appreciably. When membrane potential (inside negative) was imposed by K+ diffusion via valinomycin, a similar (but smaller) stimulation of lysine uptake was observed. When the membrane potential and the proton gradient were imposed simultaneously, a much higher stimulation in lysine uptake was shown, and the uptake of lysine was approximately the sum of the components measured separately. These results indicate that the uptake mechanism for basic amino acids is different from that of neutral or acidic amino acids and that the proton-motive force can provide the driving force for the uptake of L-lysine into the isolated brush border membrane vesicles.

Multiple transport pathways for neutral amino acids in rabbit jejunal brush border vesicles

Amino acids enter rabbit jejunal brush border membrane vesicles via three major transport systems: (1) simple passive diffusion; (2) Na-independent carriers; and (3) Na-dependent carriers. The passive permeability sequence of amino acids is very similar to that observed in other studies involving natural and artificial membranes. Based on uptake kinetics and cross-inhibition profiles, at least two Na-independent and three Na-dependent carrier-mediated pathways exist. One Na-independent pathway, similar to the classical L system, favors neutral amino acids, while the other pathway favors dibasic amino acids such as lysine. One Na-dependent pathway primarily serves neutral L-amino acids including 2-amino-2-norbornanecarboxylic acid hemihydrate (BCH), but not beta-alanine or alpha-methylaminoisobutyric acid (MeAIB). Another Na-dependent route favors phenylalanine and methionine, while the third pathway is selective for imino acids and MeAIB. Li is unable to substitute for Na in these systems. Cross-inhibition profiles indicated that none of the Na-dependent systems conform to classical A or ACS paradigms. Other notable features of jejunal brush border vesicles include (1) no beta-alanine carrier, and (2) no major proline/glycine interactions.

Taurine transport by isolated flounder renal tubules

Previous in vivo clearance studies (Schrock et al., '82) have revealed that taurine is secreted by marine fish kidneys. In the present study taurine secretion by the flounder (Pseudopleuronectes americanus) renal tubule was investigated by assaying the transport of 14C-taurine in vitro. Collections from isolated fluid-secreting flounder tubules confirmed the presence of a tubular mechanism for taurine secretion. The flounder renal tubule concentrated taurine in the lumen at a lumen/bath ratio of 25, with the movement across the peritubular membrane identified as the concentrating step of taurine transport. Studies with teased flounder renal tubules identified transport as Na+ and C1- dependent. Taurine transport was inhibited by beta alanine, gamma-aminobutyric acid, and hypotaurine. In a study of the hormonal control of taurine transport, only the adrenal steroid dexamethasone stimulated taurine uptake by the flounder renal tubules. Transport was not affected by the second messengers adenosine 3'5'-cyclic monophosphate, guanosine 3'5'-cyclic monophosphate, adenosine, or Ca++ ionophore (A12384).

Characterization of neutral amino acid uptake by cultured epithelial cells from pig kidney

Two transport systems for neutral amino acids have been characterised in LLC-PK, cells. The first, which transports alanine in a sodium-dependent manner, also mediates alanine exchange and is perferentially inhibited by serine, cysteine, and alpha-amino-n-butyric acid. This system resembles the ASC system in Ehrlich ascites and some other cell types. There is only a small contribution of other systems to alanine uptake. The second, which transports leucine with no requirement for sodium and mediates leucine exchange, is blocked by 2-aminonorbornane-2-carboxylic acid and hydrophobic amino acids. This system is similar to the L system described in other cell types. LLC-PK1 cells retain several other features implying renal proximal tubule origin; our results thus suggest that these transport systems may be involved in the reabsorption of neutral amino acids by the nephron in vivo.

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.

Disorders of proximal nephron function

The proximal nephron is responsible for reabsorbing 80 to 99 percent of several filtered solutes, including amino acids, glucose and bicarbonate. Separate, high-affinity sodium co-transport mechanisms are used. Increasing luminal concentration of each of these solutes stimulates its active transcellular reabsorption until there is saturation. Slightly less than half of the filtered chloride is reabsorbed, partly by passive mechanisms that are linked to the reabsorption of organic solutes and bicarbonate, as well as by less well defined independent cellular and/or paracellular mechanisms that appear to be sensitive to transepithelial osmotic pressure gradients. Proximal tubule reabsorption is isosmotic and isonatric, and about 50 to 60 percent of the filtered sodium and water in reabsorbed. Disorders or proximal nephron function include conditions in which luminal, cellular and/or peritubular factors affecting reabsorption are altered. Clinical disorders caused by modification of the luminal reabsorptive determinants include conditions in which tubular flow rate is increased or luminal composition is altered, as when non-reabsorbable solutes (mannitol) are filtered or when reabsorbable solutes (glucose) are filtered in concentrations exceeding their tubular transport capacity. Other disorders occur due to loss of affinity or capacity of the cellular active transport systems for specific solutes, such as amino acids (renal aminoacidurias), glucose (renal glycosurias) and bicarbonate (proximal renal tubular acidosis), or for all solutes (Fanconi syndrome). Finally, disorders due to changes in the peritubular factors affecting reabsorption include states of altered peritubular Starling forces or pH, which modify sodium chloride or sodium bicarbonate reabsorption, respectively.

Renal handling of L-histidine studied by continuous microperfusion and free flow micropuncture in the rat

Renal tubular reabsorption of L-histidine (His) was measured in vivo et situ by continuous microperfusion and free flow micropuncture of single proximal convoluted tubules of the rat kidney. The reabsorption is shown to be saturable. A permeability coefficient (P) of less than 29 microns 2 . s-1, a maximum reabsorption rate (J max) of 2.75 +/- 1.05 greater than J max greater than 1.97 +/- 0.86 (SEM) nmol . m-1 . s-1 and an affinity constant (Km) of 13.8 +/- 4.2 greater than Km greater than 10.9 +/- 4.0 (SEM) mol . 1-1 (lower values for P = 29 microns 2 . s-1, higher values for P = 0) were calculated from the microperfusion data. Using these constants and taking backflux of His and water reabsorption into account a good fit with the concentration profile of His along the proximal tubule--measured by free flow micropuncture--was obtained. Varying the buffered pH-values of the perfusion fluids (5.0 or 7.4) influenced neither the active reabsorption nor passive permeability of His. This indicates that the charge of the imidazol group of His does not play a significant role in His reabsorption. Further experiments showed that the addition of 20 mmol . 1-1 L-arginine--a strong inhibitor of the reabsorption system for dibasic amino acids--did not have a significant effect on the reabsorption of L-histidine. It is concluded, therefore, that His is reabsorbed by a system for neutral amino acids. Non ionic diffusion does not play an important role for His reabsorption.

Molecular specificity of tubular reabsorption of L-proline. A microperfusion study in rat kidney

In microperfusion experiments the reabsorption of 3H and 14C labelled L-proline by two recently defined transport systems (one with high capacity and low affinity, the other one having the opposite characteristics) was measured in vivo et situ on addition of several amino acids and some N-methylated derivatives. The high capacity system is apparently an unspecific system for neutral amino acids. The methylation of the amino group does not change the affinity to the system. The affinity decreases in the order phenylalanine > glutamine > alanine > proline, hydroxyproline > glycine. The low capacity system seems to be a specific reabsorption mechanism for imino acids like proline, hydroxyproline, sarcosine an N-methylalanine. Common neutral amino acids are not accepted. The different characteristics of both transport systems are also demonstrated by the finding that the affinity of phenylalanine for the high capacity system is about 5 times higher but its affinity for the low capacity system is about 50 times lower than the affinity for proline.

The effects of potassium and membrane potential on sodium-dependent glutamic acid uptake

The uptake of L-glutamic acid into brush-border membrane vesicles isolated from rat renal proximal tubules is NA+-dependent. In contrast to Na+-dependent uptake of D-glucose, pre-equilibration of the vesicles with K+ stimulates L-glutamic acid uptake. Imposition of a K+ gradient ([Ki+] > [Ko+]) further enhances Na+-dependent L-glutamic acid uptake, but leaves K+-dependent glucose transport unchanged. If K+ is present only at the outside of the vesicles, transport is inhibited. Intravesicular Rb+ and, to a lesser extent, Cs+ can replace intravesicular K+ to stimulate L-glutamic acid uptake. Changes in membrane potential incurred by the imposition of an H+-diffusion potential or anion replacement markedly affect Na+-dependent glutamic acid uptake only in the presence of K+. Experiments with a potential-sensitive cyanine dye also indicate that, in the presence of intravesicular K+ a charge movement is involved in Na+-dependent transport of L-glutamic acid. The data indicate that Na+-dependent L-glutamic acid transport can be additionally energized by a K+ gradient. Furthermore, intravesicular K+ render Na+-dependent L-glutamic acid transport sensitive to changes in the transmembrane electrical potential difference.

L-Tryptophan transport in human red blood cells

1. The initial rate of L-tryptophan uptake into human red cells as a function of the concentration in the medium was studied at 25 and 37 degrees C. 2. Uptake was resolved into saturable and linear components. Kinetic constants at 37 degrees C were, apparent Km 1.55 mM, V 0.145 mmol/l cell water per min and apparent KD 0.0103 min-1. 3. Inhibitor studies showed that L-tryptophan transport via the saturable component represents uptake by a previously unidentified transport system, designated the T-system. The linear component represents L-tryptophan transport via the L-system. 4. The substrate specificity of the T-system is apparently limited to the aromatic amino acids, L- and D-tryptophan, L-tyrosine and L-phenylalanine. The main route of L-phenylalanine transport is, however, via the L system. L-Tyrosine is partly transported via the T-system, partly via the L-system.

Amino acid reabsorption in the proximal tubule of rat kidney: stereospecificity and passive diffusion studied by continuous microperfusion

Renal tubular reabsorption of glycine and of the L- and D-isomers of histidine, serine, phenyl-alanine, methionine, proline and cystine was investigated in vivo et situ by continuous microperfusion of single proximal convolutions of the rat kidney. In the case of glycine and the L-isomers, tubular reabsorption is saturable to a great extent. The D-amino acids are reabsorbed much more slowly than the respective L-forms. Furthermore in the case of methionine and perhaps also of proline, serine and phenylalanine, the fractional reabsorption decreases in the presence of high concentrations of the L-form. This indicates that the D-isomers also have a measurable affinity for the reabsorption mechanisms of the renal tubule. The very poor reabsorption of D-amino acids in the presence of their L-isomers indicates that simple passive diffusion plays only a relatively small role in tubular amino acid reabsorption. Permeability coefficients estimated from these findings are in the range from 1--5 X 10(-7) cm2 - s-1. These values are very similar to those found for other organic molecules of comparable molecular weights.

Renal tubular reabsorption of taurine, gamma-aminobutyric acid (GABA) and beta-alanine studied by continuous microperfusion

Renal tubular reabsorption of taurine, gamma-aminobutyric acid (GABA), and beta-alanine was studied in vivo et situ by continuous microperfusion of single proximal tubules of the rat. In each case, reabsorption was much slower than that for other amino acids that have been studied. With a concentration of 0.1 mmol/l in initial load was reabsorbed over perfusion distance of 3 mm. Taurine reabsorption saturated with only 2.17 mmol/l in initial perfusate. Assuming simple two-parameter kinetics, upper limits for Km of 0.54 mmol/l and forVmax of 0.59 pmol-cm-1--s-1 for tubular reabsorption of taurine were estimated. High (20 mmol/l) concentrations of taurine or beta-alanine in perfusate completely inhibited GABA reabsorption, but L-phenylalanine (20 mmol/l) had no significant effect. The results indicate that the three amino acids are reabsorbed slowly from the proximal tubule by what may be a common transport system. This system appears to have a high affinity but low capacity and to be different from other known renal tubular transport systems for amino acids.

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

(1)Introduction of L-alanine and L-lysine into the lumen of the proximal tubule of Triturus kidney evoked an immediate and sustained depolarization of the peritubular membrane potential (Epm) and a small increase in the transtubular potential (Ett). L-Aspartate had no effect. (2) The alanine-induced depolarization was absolutely dependent on the presence of Na+, whereas the lysine-induced one was partially dependent on Na+. In the absence of Na+, alanine usually evoked a transient hyperpolarization of the Epm, while lysine evoked a diffusion potential-like PD change. (3) Addition of alanine or lysine to the peritubular fluid did not cause any immediate change in the Epm, but the cells depolarized with a marked time delay. The delayed depolarization could be ascribed to the entrance of amino acids into the lumen through the nephrostromes and the paracellular pathways. (4) Cellular uptake of alanine and lysine was partially dependent on Na+, while that of aspartate was completely dependent on Na+. (5) Characteristics of the observed electrical events were explained in terms of the differences in the charge transfer associated with transport of these amino acids across the luminal membrane.