beta-Amino acid transport across the renal brush-border membrane is coupled to both Na and Cl

Effects of Ischemia-Reperfusion on Tubular Cell Membrane Transporters and Consequences in Kidney Transplantation

Ischemia-reperfusion (IR)-induced acute kidney injury (IRI) is an inevitable event in kidney transplantation. It is a complex pathophysiological process associated with numerous structural and metabolic changes that have a profound influence on the early and the late function of the transplanted kidney. Proximal tubular cells are particularly sensitive to IRI. These cells are involved in renal and whole-body homeostasis, detoxification processes and drugs elimination by a transporter-dependent, transcellular transport system involving Solute Carriers (SLCs) and ATP Binding Cassettes (ABCs) transporters. Numerous studies conducted mainly in animal models suggested that IRI causes decreased expression and activity of some major tubular transporters. This could favor uremic toxins accumulation and renal metabolic alterations or impact the pharmacokinetic/toxicity of drugs used in transplantation. It is of particular importance to understand the underlying mechanisms and effects of IR on tubular transporters in order to improve the mechanistic understanding of IRI pathophysiology, identify biomarkers of graft function or promote the design and development of novel and effective therapies. Modulation of transporters’ activity could thus be a new therapeutic opportunity to attenuate kidney injury during IR.

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.

Is Taurine A Biomarker?

Taurine, a sulfur-containing amino acid present in high concentrations in mammals, plays an important role in several essential biological processes. Taurine is not incorporated into protein and is the most abundant free amino acid in the heart, retina, skeletal muscle, brain, and leukocytes. The ideal biomarker or biological measure should be reliable, reproducible, noninvasive, simple to perform, and inexpensive. Samples for biological measures should be easily obtained from physiological fluids such as blood or urine. Taurine levels in physiologic fluids have been useful for both diagnosing pathology and establishing a disease modifying therapy. In the specific case of taurine, it is important that patient information include nutritional supplementation as well as information on disease status and medications. Taurine has been measured in biological fluids due to the importance of this simple amino acid and its relative ease of determination. Taurine has been measured in animal models of disease as well as a variety of human conditions. However, it remains unclear how taurine should be used as a biomarker and in which situations this measurement would be a good prognostic or diagnostic indicator.

Chloride dependence of glycine betaine transport in Halobacillus halophilus

Growth of Halobacillus halophilus is strictly chloride-dependent but the physiological basis for the chloride dependence remains to be elucidated. To address the function of Cl(-) in H. halophilus, a physiological study was performed. It was found that uptake of the compatible solute glycine betaine under isoosmotic conditions was stimulated by increasing salt concentrations. Uptake of glycine betaine required both, Na(+) and Cl(-). Cl(-) could be substituted by nitrate and bromide, but not by sulfate. Glycine betaine transport was optimal at around 0.7 M Cl(-). Cells responded to an osmotic upshock by accumulating glycine betaine, but only in the presence of chloride. These studies revealed the first chloride-dependent glycine betaine transporter in a prokaryote.

Ser-322 is a critical site for PKC regulation of the MDCK cell taurine transporter (pNCT)

Previous studies have shown that the Madin-Darby canine kidney cell taurine transporter (pNCT) is downregulated by protein kinase C (PKC) activation. In this study, it is hypothesized that the highly conserved serine-322 (Ser-322) located in the fourth intracellular segment (S4) may play an important role in the function of taurine transporter, which is modulated by PKC phosphorylation. It is demonstrated that Ser-322 is the critical site of PKC phosphorylation, as determined by site-directed mutagenesis. When Ser-322 of pNCT was changed to alanine (S322A) and this mutant was evaluated in an oocyte expression system, taurine transport activity increased threefold compared with control (wild-type pNCT). Activation of PKC by the active phorbol ester 12-myristate 13-acetate did not influence taurine transport by mutant S322A. Kinetic analysis showed that the mutation of Ser-322 essentially changed the Vmax, rather than the Km, of the transporter. Mutation of all other PKC consensus sites did not affect transporter activity when expressed in the oocyte system. Western blot analysis showed that expression of taurine transporter protein was similar in oocytes injected with either wild-type or mutant pNCT cRNA, indicating that the enhanced taurine transport activity by mutant S322A was not caused by a greater amount of transporter expressed in the oocyte. Furthermore, this study demonstrated that the taurine transporter was phosphorylated after PKC activation, and this effect was not observed in mutant S322A. In conclusion, Ser-322 is critical in PKC regulation of taurine transporter activity. The steady-state taurine transporter activity is tightly controlled by endogenous PKC phosphorylation of Ser-322, which is located in the fourth intracellular segment of the taurine transporter.

beta-alanine and alpha-fluoro-beta-alanine concentrative transport in rat hepatocytes is mediated by GABA transporter GAT-2

Studies on the compartmentalization of uridine catabolic metabolism in liver have indicated accumulation of beta-alanine as well as alpha-fluoro-beta-alanine (FbetaAL) for 5-fluorouracil in the hepatocytes. Using preparations of rat hepatocytes we were able to identify a Na+-dependent transport with high affinity for beta-alanine and GABA with Michaelis constant (Km) of 35.3 and 22.5 microM, respectively. A second Na+-dependent kinetic component with Km >1 mM was also identified. The sigmoidal profile of beta-alanine uptake with respect to Na+ shows the involvement of multiple ions of sodium in the transport process. A Hill coefficient of 2.6 +/- 0.4 indicates that at least two sodium ions are cotransported with beta-alanine. The flux of beta-alanine was also shown to be chlorine dependent. The substitution of this anion with gluconate, even in the presence of Na+, reduced the intracellular concentrative accumulation of beta-alanine to passive diffusion level, indicating that both Na+ and Cl- are essential for the activity of this transporter. The transport of beta-alanine was inhibited by GABA, hypotaurine, beta-aminoisobutyric acid, and FbetaAL in a competitive manner. However, concentrations up to 1 mM of L- and D-alanine, taurine, and alpha-aminoisobutyric acid did not affect beta-alanine uptake. Considering the similarities in substrate specificity with the rat GAT-2 transporter, extracts of hepatocytes were probed with the anti-GABA transporter antibody R-22. A 80-kDa band corresponding to GAT-2 was present in the hepatocyte and in the GAT-2 transfected Madin-Darby canine kidney cell extract, confirming the extraneural localization of this transporter. In view of these results, the neurotoxic effects related to the administration of uridine and 5-fluorouracil could be explained with the formation of beta-alanine and FbetaAL and their effect on the cellular reuptake of GABA.

Comparative aspects of chloride-dependent amino acid transport across the brush-border membrane of mammalian small intestine

Chloride-dependent amino acid transport has been described in several tissues. This article briefly reviews the evidence of cotransport of chloride and amino acids across the brush-border membrane of rabbit distal ileum. On the basis of amino acid carriers described in the rabbit and the surveys of chloride-dependence reported, a comparison of amino acid carriers in the mammalian small intestine is performed. Additional characteristics of the carriers in the different species are included in the discussion when necessary. From this comparison the rabbit distal ileum and the pig small intestine emerge as the best models of amino acid transport in the human small intestine.

Characterization of taurine transport in human glioma GL15 cell line: regulation by protein kinase C

Data describing characteristics of taurine transport system in human brain cells are not currently available. We have used GL15 cells, a cell line of human brain origin that keeps some properties of normal glial cells, to investigate these characteristics. The human glioma cell line GL15 was found to take up taurine. The uptake was strictly sodium-dependent. Replacement of NaCl with choline chloride almost totally abolished the uptake. There was also an anion requirement for the uptake system, and Cl- was the most potent among several monovalent anions tested. The uptake process was specific for beta-amino acids such as taurine, hypotaurine and beta-alanine. The kinetics of uptake were studied. Apparently, a single transport system with a K(m) of 8.95 +/- 0.26 microM was responsible for the uptake. A maximal velocity of 1.32 +/- 0.03 nmol/mg of protein/10 min was found. Stoichiometric analysis revealed that two Na+ and one Cl- ions were involved in the translocation of one taurine molecule. Phorbol 12-myristate 13-acetate (PMA), a potent stimulator of protein kinase C (PKC), inhibited taurine uptake. Maximal inhibition was obtained at 50 nM after 1 hr of treatment. This effect was prevented by pretreatment of the cells with chelerythrine, a potent and selective inhibitor of PKC. The transport of beta-alanine was inhibited to a comparative extent. The mechanism of this inhibition was not investigated, but it was found that this inhibitory effect was not prevented by cycloheximide, actinomycin D, colchicine or cytochalasin D, indicating that neither protein synthesis, nor microfilament function were involved. The effect of PMA was associated with an impairment of kinetic constants. It is concluded that human GL15 cells have a taurine transporter similar to that expressed in rodent glial cells, and that the activation of PKC can modulate the activity of this transporter.

beta-Amino acid transport in pig small intestine in vitro by a high-affinity, chloride-dependent carrier

This study describes unidirectional influx of amino acids and D-glucose across the small intestinal brush-border membrane of fully weaned eight week old pigs. Influx is minimal in the duodenum and maximal in the distal and/or mid small intestine. Influx of beta-alanine, taurine and N-methyl-aminoisobutyric acid is chloride-dependent. The activation stoichiometry for taurine influx is 1.0 +/- 0.2 chloride/2.4 +/- 0.3 sodium/1 taurine. Influx of D-glucose, lysine, glycine and glutamate is chloride-independent. An ABC test demonstrates a common beta-amino acid carrier: (a) the apparent affinity constant K1/2Taurine is 44 +/- 13 microM (means +/- S.D.) and the inhibitory constant (KiTaurine) against beta-alanine influx is 41 +/- 5 microM (means +/- S.E.). (b) K1/2beta-alanine is 97 +/- 23 microM and Kibeta-alanine against taurine influx is 160 +/- 22 microM. (c) KiHypotaurine against taurine and beta-alanine influx is 43 +/- 4 (n = 7) and 22 +/- 5 microM (n = 7), respectively. In conclusion, a high affinity, low capacity, sodium- and chloride-dependent carrier of beta-amino acids is present in pig small intestine.

Chloride-dependent amino acid transport in the small intestine: occurrence and significance

The unidirectional influx of amino acids, D-glucose and ions across the brush-border membrane of the small intestine of different species has been measured in vitro with emphasis on characterization of topographic and species differences and on chloride dependence. The regional differences in transport along the small intestine are outlined and shown to be caused by variation in transport capacity, while the apparent affinity constants are unchanged. Rabbit small intestine is unique by exhibiting maximal rates of transport in the distal ileum and a very steep decline in the oral direction from where tissues are normally harvested for preparation of brush-border membrane vesicles. Transport in the guinea pig and rat is much more constant throughout the small intestine. Since the capacity of nutrient carriers is regulated by their substrates it is possible that bacterial breakdown of peptides and proteins in rabbit distal ileum increases the concentration of amino acids leading to an upregulation of the carriers. Chloride dependence is a characteristics of the carrier rather than the transported amino acid, and is used to improve the classification of amino acid carriers in rabbit small intestine. In this species the imino acid carrier, the beta-amino acid carrier, and the beta-alanine carrier, which should be renamed the B0,+ carrier, are chloride-dependent. The steady-state mucosal uptake of classical substrates for these carriers in biopsies from the human duodenum is also chloride-dependent. The carrier of beta-amino acids emerges as ubiquitous and chloride-dependent, and evidence of cotransport with both sodium and chloride is reviewed. A sodium:chloride:2-methyl-aminoisobutyric acid coupling stoichiometry of approx. 2:1:1 is suggested by ion activation studies. Direct measurements of coupled ion fluxes in rabbit distal ileum confirm that sodium, chloride and 2-methyl-aminoisobutyric acid are cotransported on the imino acid carrier with an identical influx stoichiometry. Control experiments and reference to the literature on the electrophysiology of the small intestine exclude alterations of the membrane potential as a feasible explanation of the chloride dependence. Thus, it is concluded that chloride is cotransported with both sodium and 2-methyl-aminoisobutyric acid across the brush-border membrane of rabbit distal ileum.

Chloride dependent amino acid transport in the human small intestine

Carriers of beta amino acids and imino acids in the small intestine of rabbits and guinea pigs are chloride dependent, and a cotransport of chloride, sodium, and 2-methyl-aminoisobutyric acid has been shown. This study examines the chloride dependence of amino acid transport in the human small intestine. The steady state tissue uptake of amino acids, given as the ratio between substrate concentration in intracellular and extracellular water after 35 minutes incubation at 37 degrees C, was determined in mucosal biopsy specimens from the duodenum of patients undergoing diagnostic upper endoscopy and compared using one way analysis of variance. Uptake of leucine and alpha-methyl-D-glucoside in the duodenum and the distal ileum did not differ. The accumulation of all substrates was sodium dependent. In the absence of mucosal chloride the uptake of taurine, glycine, and 2-methyl-aminoisobutyric acid was significantly reduced while that of leucine and alpha-methyl-D-glucoside was unaffected and the reduction of beta alanine uptake not statistically significant. Uptake of 2-methyl-aminoisobutyric acid and proline showed mutual inhibition. Leucine did not reduce uptake of the beta amino acids. In conclusion, chloride dependent transport processes for 2-methyl-amino-isobutyric acid, taurine, and glycine are present in the human small intestine.

Transport of beta-alanine and biosynthesis of carnosine by skeletal muscle cells in primary culture

Uptake of beta-alanine and synthesis of carnosine (beta-alanyl-histidine) could be demonstrated in primary cell cultures derived from embryonic chick pectoral muscle. Concomitant with the morphological changes, cessation of cell division and the induction of creatine kinase, a rapid increase in the rate of beta-alanine uptake and also in the rate of carnosine synthesis could be observed. The uptake of beta-alanine is sodium and chloride dependent and obeys Michaelis-Menten kinetics with Km values of about 40 microM that are essentially identical for myoblasts and myotubes. In contrast, Vmax increases considerably during differentiation. The beta-alanine transport system is highly specific for beta-amino acids and exhibits a substantial anion dependency (Cl- > J- > CSN- > SO(4)2-). Stoichiometric studies suggest that the transport of one beta-alanine molecule involves two sodium ions and one chloride ion. This ratio is not altered by the process of cell differentiation.

Taurine and beta-alanine transport in an established human kidney cell line derived from the proximal tubule

The transport mechanisms of taurine and beta-alanine by an immortalized human embryonic kidney epithelial cell line (IHKE) were examined. The uptake of these beta-amino acids was characterized by two Na(+)-dependent transport components, whereas an inwardly directed H(+)-gradient only stimulated amino acid influx to a small extent and in the absence of sodium. Competition experiments revealed that taurine and beta-alanine drastically reduced the uptake of one another by the high-affinity Na(+)-dependent transport system. However, some alpha-amino acids could also compete with the beta-amino acids, but with a low affinity. Examinations of the effect of different anions on the Na(+)-dependent uptake of taurine at a low amino acid concentration (240 nM) revealed a specific requirement for Cl-, whereas Cl- had no measurable effect at a higher concentration (1.0 mM) of taurine. In addition, activation of taurine transport as a function of Na+ and Cl- concentration indicated a probable coupling ratio of 3 Na+/1 Cl-/1 taurine for the high-affinity carrier. Finally, cellular regulation of taurine transport was indicated by the finding that pretreatment with taurine containing media decreased the activity of the taurine transporter(s).

Osmotic regulation of taurine transport via system beta and novel processes in mouse preimplantation conceptuses

Taurine was shown recently to increase the frequency at which 2-cell mouse conceptuses develop into blastocysts in vitro. For this reason and because taurine helps cells adapt to external stresses, we studied transport of this and related amino acids by preimplantation mouse conceptuses. The most conspicuous component of taurine transport in conceptuses at the 1-cell through blastocyst stages of development was both Na(+)- and Cl(-)-dependent. This Na(+)- and Cl(-)-dependent transport system interacted relatively strongly with beta- but not alpha-amino acids. By these criteria, transport system beta is responsible for Na(+)-dependent taurine transport in preimplantation mouse conceptuses. Moreover, detection of mRNA encoding the taurine transport protein (TAUT) in early conceptuses supports the theory that TAUT is a major component of system beta. Transport of taurine by system beta in 1-cell conceptuses was slower in hypotonic than in hypertonic media, whereas the reverse was true for system beta in blastocysts. In contrast, hypotonically stimulated Na(+)-independent taurine transport was, of course, more rapid in hypotonic than in hypertonic media in both 1-cell conceptuses and blastocysts. Transport via this hypotonically stimulated process also showed no sign of saturation by up to 10 mM taurine. Hypotonically stimulated taurine transport appeared transiently in 1-cell conceptuses under hypotonic conditions until they had recovered their initial volumes. Hence, we suggest that a decrease in taurine uptake via system beta and an increase in taurine exodus via the Na(+)-independent, nonsaturable transport process could contribute to the regulatory volume decrease in 1-cell conceptuses in hypotonic medium. Since taurine uptake by system beta in blastocysts is, however, higher in hypotonic than in hypertonic media, taurine uptake by system beta in blastocysts might intensify a tendency to increase cell volume in hypotonic medium. Such an increase in taurine uptake could further favor anabolic changes associated with cell swelling. In addition to contributing to regulation of cellular volume and perhaps metabolism, the hypotonically stimulated Na(+)-independent transport processes in early embryos have novel characteristics. Hypotonically stimulated Na(+)-independent taurine transport was inhibited by niflumate, N-ethylmaleimide and NaN3 but not by furosemide, iodoacetate, KCN, ouabain or alpha- or beta-amino acids. Furthermore, 4,4'-diisothiocyanostilbene-2,2'-disulfonate inhibited this transport in 1-cell conceptuses but not in blastocysts. Hence, different hypotonically stimulated Na(+)-independent taurine transport processes appear to be present in 1-cell conceptuses vs. blastocysts. The functions of these and other instances of developmental regulation of expression of transport processes in preimplantation conceptuses remain largely to be elucidated.(ABSTRACT TRUNCATED AT 400 WORDS)

Solubilization and functional reconstitution of the human placental taurine transporter

The taurine transporter from purified human placental brush-border membranes was solubilized and reconstituted into proteoliposomes in a functional form. Solubilization was done with 2.5% cholate in the presence of 4 M urea. The proteins in the solubilizate were precipitated with 6% poly(ethylene glycol) and the precipitated proteins were reconstituted into proteoliposomes with an asolectin/protein ratio of 10:1. Under these experimental conditions, the taurine transport activity in the proteoliposomes was maximal. SDS-PAGE analysis of proteins, however, revealed that the proteoliposomes still contained a majority of the proteins originally present in the brush-border membranes. Uptake of taurine in the reconstituted proteoliposomes was obligatorily dependent on the presence of Na+ as well as Cl-. Substitution of Na+ with other monovalent cations such as K+ and Li+ reduced the taurine transport activity drastically. Similarly, substitution of Cl- with other monovalent anions such as SCN-, F-, I- and NO3- could support the transport activity only to a maximum of 30% of the control activity. In the presence of Cl-, the uptake rate was sigmoidally related to Na+ concentration, resulting in a Na+/taurine coupling ratio of 2:1. The apparent dissociation constant for Na+ was about 195 mM. In the presence of Na+, the uptake rate was hyperbolically related to Cl- concentration, indicating a Cl-/taurine coupling ratio of 1:1. The apparent dissociation constant for Cl- was about 205 mM. The NaCl-dependent taurine uptake was stimulated by an inside-negative membrane potential, showing that the uptake process was electrogenic. The uptake system was specific for beta-amino acids. The affinity of the system for taurine was high with an apparent dissociation constant of 2.7 +/- 0.1 microM. It is concluded that the taurine transporter can be dislodged from the placental brush-border membranes and reconstituted in a catalytically active form in proteoliposomes with no significant change in its characteristics.

The role of chloride in taurine transport across the human placental brush-border membrane

Taurine, a sulfated beta-amino acid, is conditionally essential during development. A maternal supply of taurine is necessary for normal fetal growth and neurologic development, suggesting the importance of efficient placental transfer. Uptake by the brush-border membrane (BBM) in several other tissues has been shown to be via a selective Na(+)-dependent carrier mechanism which also has a specific anion requirement. Using BBM vesicles purified from the human placenta, we have confirmed the presence of Na(+)-dependent, carrier-mediated taurine transport with an apparent Km of 4.00 +/- 0.22 microM and a Vmax of 11.72-0.36 pmol mg-1 protein 20 s-1. Anion dependence was examined under voltage-clamped conditions, in order to minimize the contribution of membrane potential to transport. Uptake was significantly reduced when anions such as thiocyanate, gluconate, or nitrate were substituted for Cl-. In addition, a Cl(-)-gradient alone (under Na(+)-equilibrated conditions) could energize uphill transport as evidenced by accelerated uptake (3.13 +/- 0.8 pmol mg-1 protein 20 s-1) and an overshoot compared to Na+, Cl- equilibrated conditions (0.60 +/- 0.06 pmol mg-1 protein 20 s-1). A Cl(-)-gradient (Na(+)-equilibrated) also stimulated uptake of [3H]taurine against its concentration gradient. Analysis of uptake in the presence of varying concentrations of external Cl- suggested that 1 Cl- ion is involved in Na+/taurine cotransport. We conclude that Na(+)-dependent taurine uptake in the placental BBM has a selective anion requirement for optimum transport. This process is electrogenic and involves a stoichiometry of 2:1:1 for Na+/Cl-/taurine symport.

Kidney brush-border membrane transporters: differential sensitivity to diethyl pyrocarbonate

The effects of the histidine modifier, diethyl pyrocarbonate (DEPC), on brush-border membrane transport systems were studied in rat kidney. DEPC caused a strong inhibition of sodium-dependent phosphate and D-glucose uptake. Phosphate uptake remained linear up to 10 s in control and DEPC-treated membrane vesicles. The D-glucose carrier was more sensitive than the phosphate carrier with half-times of inhibition being 4 and 7 min, respectively. Sodium-independent phosphate and D-glucose uptake remained unaffected by DEPC. Intravesicular volume and two enzyme activities endogenous to the luminal membrane (alkaline phosphatase and aminopeptidase M) remained unaffected by DEPC. Increasing the preincubation pH from 5 to 9 increased phosphate transport inhibition caused by DEPC from 73 to 88% in the presence of DEPC. Hydroxylamine was able to completely reverse phosphate uptake inhibition by DEPC (100%), but only partially reversed the D-glucose uptake inhibition (16%). Sodium or substrate (D-glucose or phosphate) in the preincubation media were unable to protect their respective carriers from DEPC. Sodium-dependent transport of L-glutamine, L-phenylalanine, L-leucine, L-alanine, L-glycine, beta-alanine and L-proline were inhibited at different levels ranging from 70 to 90%. Three transport processes were found insensitive to DEPC modification: L-glutamate, L-lysine and D-fructose. None of the amino acid transporters was protected against DEPC by sodium and/or their respective substrates. Sodium influx was inhibited by DEPC (47%) in the absence of any substrate. Our results show a differential sensitivity of sodium-dependent transporters to DEPC and suggest an important role for histidine residues in the molecular mechanisms of these transporters. More experiments are in progress to further characterize the residue(s) involved in these transport inhibitions by DEPC.

Both Na+ and Cl- gradients energize NaCl/L-glutamate cotransport in lobster hepatopancreatic brush border membrane vesicles

Previous work with L-[3H]glutamate transport by lobster (Homarus americanus) hepatopancreatic brush border membrane vesicles (BBMV) indicated that the transport of this amino acid was stimulated by the presence of both Na+ and Cl- ions in the external medium, however, the specific catalytic or energetic role of each monovalent ion in amino acid transfer was not established (Ahearn and Clay (1987) J. Exp. Biol. 130, 175-191). The present study employs a variety of experimental treatments with this membrane preparation to clarify the nature of the ion dependency in the cotransport process. A zero-trans time course experiment using inwardly-directed transmembrane Na+ or Cl- gradients led to similar transient accumulations of the amino acid above equilibrium values in the presence of equilibrated concentrations of the respective counterions. The uptake overshoots observed in the presence of single ion gradients were significantly increased when gradients of both Na+ and Cl- were used simultaneously. When vesicles were pre-equilibrated with L-[3H]glutamate and either of the monovalent ions, an inwardly-directed gradient of each counterion led to the transient accumulation of additional labelled amino acid above its equilibrium concentration, indicating that either ion gradient was capable of energizing the net flow of L-glutamate. A cotransport stoichiometry of 1 Na+/1 Cl-/1 L-glutamate was established using the Static Head analysis where a balance of ion and amino acid driving forces were attained with a 7:1 Na+ or Cl- gradient (o greater than i) against a 7:1 L-glutamate gradient (i greater than o).

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).

Chloride-dependence of amino acid transport in rabbit ileum

Chloride-dependence of influx across the brush-border membrane of distal rabbit ileum was examined for beta-alanine, 2-methylaminoisobutyric acid (MeAIB), leucine, lysine, proline and D-glucose. Influx of leucine at 2 mM and of D-glucose at 0.5 mM was chloride-independent indicating that substitution of isethionate for chloride has no unspecific effect on sodium gradient driven transport processes. In contrast influx of beta-alanine and MeAIB was totally dependent on the presence of chloride ions. In the absence of chloride, proline transport was reduced to 20% of its control level. This remaining transport can be accounted for by the function of the carrier of alpha-amino-monocarboxylic acids. Transport of leucine at 0.1 mM was reduced by absence of chloride. This is in accordance with the observation of leucine transport by the beta-alanine carrier. The kinetics of chloride and sodium activation of transport of MeAIB were examined at 1 mM MeAIB. Chloride activation was characterized by a Hill coefficient of 1 and a K1/2 of 23.5 mM, and sodium activation by a Hill coefficient of 2 and a K1/2 of 51 mM. Thus cotransport of chloride with an imino acid would be compatible with the known rheogenic nature of this transport. This study adds the imino acid carrier and the beta-alanine carrier to the group of chloride-dependent, epithelial amino acid transport systems.

Adaptive regulation of taurine transport in two continuous renal epithelial cell lines

Taurine is actively transported by a beta-amino acid transporter located on the proximal tubule apical surface. We have characterized taurine transport into confluent monolayers of two continuous renal epithelial cell lines: LLC-PK1, a cell of porcine proximal tubular origin, and the Madin-Darby canine kidney cell line (MDCK) of distal origin. Taurine uptake is linear up to 90 minutes in LLC-PK1 cells and 180 minutes in MDCK cells. This process is highly dependent upon Na+ as the cation and either Cl- or Br- as the anion. Taurine uptake is inhibited by another beta-amino acid, beta-alanine, to a greater extent than the alpha-analog, L-alanine or other alpha-amino acids. Incubation of cell monolayers with taurine-free medium (0 microM taurine) induces an increase in Na(+)-dependent taurine uptake when compared to cells exposed to standard medium (50 microM taurine). When cells were incubated in medium containing high taurine (500 microM), uptake was decreased as compared to control cells. This adaptive response is evident by 12 hours in both cell lines and is the result of changes in the apparent transport maximum (Jmax) rather than the apparent Km for taurine. The changes in transport observed after manipulation of medium taurine concentration were not associated with differences in taurine efflux. In summary, taurine is transported by a beta-specific, Na-Cl dependent process in both renal epithelial cell lines. Although the factors which regulate taurine transport are not known, an increased transport maximum is observed in cells which have been taurine-starved, and a decreased Jmax is seen in cells supplied with excess taurine.(ABSTRACT TRUNCATED AT 250 WORDS)

The renal transport of taurine and the regulation of renal sodium-chloride-dependent transporter activity

A model for the beta-amino acid taurine transport is presented to help define the ionic, pH, and voltage requirements for the movement of taurine into the rat proximal tubule brush border membrane vesicle (BBMV). Sodium-(Na+)-taurine symport across the apical surface of the proximal tubule has a highly specific requirement for Cl- and Br-. Active taurine transport operates with a 2 Na+:1 Cl-:1 taurine-carrier complex. Complexes like the one required for maximal taurine transport may be pertinent for many other amino acids whose uptake is Na(+)-dependent. Renal epithelial cell lines LLC-PK and MDCK were used to define the nature of taurine uptake; they express Na(+)-Cl(-)-taurine cotransport that is inhibited by beta-alanine. The cell lines up- or down-regulate taurine transport in response to changes in the taurine concentration of the medium in a manner similar to that seen in BBMV. The adaptation is present by 12 h and depends on new protein synthesis and protein import to the cell membrane. The role of trafficking in the adaptive response was also explored in brush border vesicles. During dietary surfeit, transporter could be down-regulated and transporters could be shifted back into the microtubule system, resulting in taurinuria. Use of continuous renal cell lines allowed a more mechanistic exploration of intracellular trafficking in the up- and down-expression of the Na(+)-Cl(-)-taurine cotransporter. Colchicine appeared to be a more potent inhibitor of the rapid (over hours) adaptive response to a reduction in media and, therefore, intracellular taurine content.(ABSTRACT TRUNCATED AT 250 WORDS)

Taurine transport by microvillous membrane vesicles and the perfused cotyledon of the human placenta

Human placental uptake and maternal-to-fetal (M-to-F) net transfer of taurine were evaluated in purified microvillous membrane vesicles (MMV) and the isolated perfused cotyledon. Taurine uptake by MMV was specifically stimulated by an inward Na+ gradient [maximum velocity (Vmax), 24.5 +/- 0.6 pmol.mg-1.30 s-1; Michaelis constant (Km), 6.2 +/- 0.7 microM], with uptake stoichiometry showing approximately 2 Na+/taurine molecule. In the presence or absence of Na+, Cl- did not stimulate uptake. Na(+)-stimulated uptake was enhanced by an outward K+ gradient. beta-Alanine and hypotaurine competitively inhibited uptake of taurine in MMV. Two-way uptake inhibition studies showed no interaction between taurine and non-beta-amino acids. When MMV were preloaded with taurine there was enhancement of Na(+)-stimulated uptake. In the perfused placentas, saturable M-to-F net transfer of taurine was not observed, despite saturation of tissue uptake from the maternal circulation. During 3 h of perfusion, no fetal-to-maternal (F-to-M) gradient formed for taurine; yet, a 2:1 gradient simultaneously occurred for histidine. This study demonstrates that taurine uptake by the microvillous membrane of the human placenta is highly specific, of high affinity, and Na+ coupled.

Tyrosine residues are essential for the activity of the human placental taurine transporter

Treatment of human placental brush-border membrane vesicles with four tyrosine group-specific reagents, N-acetylimidazole, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl), tetranitromethane and p-nitrobenzesulfonyl fluoride, inhibited NaCl gradient-driven taurine uptake in these vesicles without affecting the vesicle integrity. The relative potency of these reagents to inhibit the transporter was in the following order: tetranitromethane greater than NBD-Cl greater than p-nitrobenzenesulfonyl fluoride greater than N-acetylimidazole. The inhibition by N-acetylimidazole was reversible with hydroxylamine and the inhibition by NBD-Cl was reversible with 2-mercaptoethanol. Kinetic analysis of taurine uptake in control and in N-acetylimidazole-treated membrane vesicles revealed that the inhibition was primarily due to a reduction in the maximal velocity. There was no change in the affinity of the transporter for taurine in control and treated vesicles. The transporter could be protected from the N-acetylimidazole-induced inhibition by Na+. The dependence of taurine uptake rate on extravesicular Na+ concentration was sigmoidal and analysis of the data revealed that two Na+ ions were involved per transport of one taurine molecule. It is concluded that tyrosine residues are essential for optimal transport function of the human placental taurine transporter and that these critical tyrosine residues are located at or near the Na+-binding site of the transporter.

Taurocholate transport by basolateral plasma membrane vesicles isolated from human liver

Transport of taurocholate into the hepatocyte against unfavorable chemical and electrical gradients occurs via a sodium-dependent, carrier-mediated transport system. Although this cotransporter has been characterized in the rodent, it has not been demonstrated in man. Therefore, we utilized human liver, obtained via multiorgan donation but not used for transplantation, to prepare basolateral (sinusoidal) liver plasma membrane vesicles by a Percoll gradient method. Na+,K+-ATPase, a marker enzyme for the basolateral domain, was enriched 28.9-fold in the final membrane fraction compared with homogenate, whereas the bile canalicular membrane enzymes Mg++-ATPase and alkaline phosphatase were enriched only 3.4- and 6.4-fold, respectively. Marker enzyme activities for endoplasmic reticulum, lysosomes and mitochondria were not enriched compared with homogenate. Integrity of the membrane vesicles was confirmed by the demonstration of Na+-dependent concentrative uptake of the amino acid L-alanine (estimated intravesicular volume of 0.59 microliter per mg protein). An inwardly directed 100 mM Na+ gradient stimulated the initial rate of 2.5 microM taurocholate uptake and energized a transient 2-fold accumulation of the bile acid above equilibrium ("overshoot"). In contrast, uptake was slower and no overshoot occurred with a K+ gradient. A negative intravesicular potential, created by altering accompanying anions or by valinomycin-induced K+ diffusion potentials, did not enhance taurocholate uptake, suggesting an electroneutral cotransport mechanism. Chloride as the accompanying anion stimulated the initial rate of uptake compared with anions of lesser or greater lipid permeability. Na+-dependent taurocholate (4 microM) uptake was significantly inhibited by 250 microM cholate, taurocholate, glycocholate, taurochenodeoxycholate and bromsulfophthalein.(ABSTRACT TRUNCATED AT 250 WORDS)

Ionic requirements for amino acid transport

The reabsorption of amino acids by the proximal tubule is remarkably efficient. Current evidence indicates that this process occurs by Na+-amino acid cotransport or symport. The energy for amino acid entry is derived from the chemical and voltage gradient for Na+ entry across the apical surface of the renal cell maintained by pumping Na+ out of the cell by Na+-K+-adenosine triphosphatase (ATPase) activity at the basolateral membrane. We chose the beta-amino acid taurine to study the anionic requirements as well as voltage- and pH-dependence of Na+-taurine symport into rat proximal tubule brush border membrane vesicles. Maximal uptake was found when Cl- or Br- were the anions. The addition of various ionophores (amiloride, carbonyl cyanide-n chlorophenyl-hydrazone, and valinomycin) under pH-equilibrated conditions did not change taurine entry into the vesicle. Hill equation analysis of the initial rate of taurine uptake into vesicles indicates that transport operates by means of a 2 Na+:1 Cl-:1 taurine-carrier complex. Because taurine is a zwitterion, this complex has a net positive charge. Its entry into the vesicle is favored by the imposition of an outwardly directed K+ gradient in the presence of valinomycin. The movement of a quaternary complex of this type across the apical surface of the proximal tubular cell would assure that the movement of both Cl- and the amino acid is energized by the Na+ gradient. Because most amino acids are zwitterions at physiologic pH this complex would be positively charged, favoring entry into the voltage negative renal cell interior.

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.

Taurine and cell volume maintenance in the shark rectal gland: cellular fluxes and kinetics

Tissue slices of shark rectal gland are studied to examine the kinetics of the cellular fluxes of taurine, a major intracellular osmolyte in this organ. Maintenance of high steady-state cell taurine (50 mM) is achieved by a ouabain-sensitive active Na+-dependent uptake process and a relatively slow efflux. Uptake kinetics are described by two saturable taurine transport components (high-affinity, Km 60 microM; and low-affinity, Km 9 mM). [14C]Taurine uptake is enhanced by external Cl-, inhibited by beta-alanine and unaffected by inhibitors of the Na+/K+/2Cl- co-transport system. Two cellular efflux components of taurine are documented. Incubation of slices in p-chloromercuribenzene sulfonate (1 mM) reduces taurine uptake, increases efflux of taurine and induces cell swelling. Studies of efflux in isotonic media with various cation and anion substitutions demonstrate that high-K+ markedly enhances taurine efflux irrespective of cell volume changes (i.e. membrane stretching is not involved). Moreover, iso-osmotic cell swelling induced in media containing propionate is not associated with enhanced efflux of taurine from the cells. It is suggested that external K+ exerts a specific effect on the cytoplasmic membrane to increase its permeability to taurine.

Taurine transport by rabbit kidney brush-border membranes: coupling to sodium, chloride, and the membrane potential

Ion dependence and electrogenicity of taurine uptake were studied in rabbit renal outer cortical brush-border membrane vesicles isolated by differential precipitation. Na+-D-glucose cotransport was followed in parallel to monitor changes in the membrane potential. Concentrative taurine flux was dependent on a chemical and/or an electrical Na+ gradient (K+ diffusion potential) and could be completely inhibited by other beta-amino acids. It displayed a specific anion requirement (Cl- greater than or equal to Br- much greater than SCN- greater than I- greater than NO-3). At chemical Na+ equilibrium, Cl- gradients, depending on their orientation, stimulated or inhibited taurine uptake more than could be attributed solely to electrical anion effects, although a Cl- gradient alone could not energize an overshoot. Furthermore, taurine tracer exchange was significantly stimulated by Cl- as well as Br-. The Cl- stoichiometry was found to be one, whereas taurine transport, in the presence of Cl-, was sigmoidally related to the Na+ concentration, resulting in a coupling ratio of 2 to 3 Na+: 1 taurine. Upon Cl- replacement with gluconate, taurine uptake showed a reduced potential sensitivity and was no longer detectably affected by the Na+ concentration (up to 150 mM). These results suggest a 2 to 3 Na+ :1 Cl- :1 taurine cotransport mechanism driven mainly by the Na+ gradient, which is sensitive to the membrane potential due to a negatively charged empty carrier. Cl- appears to stimulate taurine flux primarily by facilitating the formation of the translocated solute-carrier complex.

Na+ + Cl- -gradient-driven, high-affinity, uphill transport of taurine in human placental brush-border membrane vesicles

Uptake of taurine in human placental brush-border membrane vesicles was greatly stimulated in the presence of an inwardly-directed Na+ + Cl- -gradient and uphill transport of taurine could be demonstrated under these conditions. Na+ as well as Cl- were obligatory for this uptake and both ion gradients could energize the uphill transport. This Na+ + Cl- -gradient-dependent taurine uptake was stimulated by an inside-negative membrane potential, demonstrating the electrogenicity of the process. The uptake system was highly specific for beta-amino acids and the Km of the system for taurine was 6.5 +/- 0.4 microM.

Amino acid transport in the gill epithelium of a marine bivalve

1. Amino acid transport across the intestine of bivalve molluscs is reviewed. 2. Transport of alanine or taurine is dependent on the sodium gradient across the intestine. 3. The time course of the uptake of alanine into the brush border membrane vesicles is also sodium dependent.

Chloride dependence of the sodium-dependent glycine transport in pig kidney cortex brush-border membrane vesicles

The Na+-dependent glycine uptake in pig kidney cortex brush-border membrane vesicles is specifically enhanced by the presence of Cl-. The Na+-independent glycine uptake is not affected by Cl-. Various anions tested could not substitute Cl- in the activation of the Na+-dependent glycine transport. Cl- is specifically required on the outer membrane side. The Na+-dependent glycine uptake is higher in the presence of an inwardly directed Cl- gradient than the one measured in the presence of equilibrated Cl-. The Na+-dependent glycine uptake depends on, and is saturable at increasing Cl- concentrations. By studying the activation of glycine uptake by Na+ in the presence and in the absence of Cl-, evidence was found that two different Na+-dependent glycine transport pathways are present in pig kidney cortex brush-border membrane vesicles. The kinetics of the glycine uptake measured in the presence of an inwardly directed NaCl gradient show the presence of two glycine transport systems, a low-affinity, high-capacity one and a high-affinity, low capacity one. In the absence of Cl- the high-affinity, low-capacity transport is almost suppressed, thus indicating the presence of a high-affinity glycine transport system simultaneously dependent on both Na+ and Cl- ions.

Renal handling of taurine, L-alanine, L-glutamate and D-glucose in Opsanus tau: studies on isolated brush border membrane vesicles

Renal brush border membrane vesicles (bbmv) from the aglomerular toadfish (Opsanus tau), isolated by differential precipitation, were tested for their ability to actively translocate (i) taurine, known to be secreted by the kidney of several marine teleosts, and (ii) L-alanine, L-glutamic acid, and D-glucose, solutes that are normally reabsorbed in the filtering nephron. Vesicular taurine uptake displayed a Na+ dependence. Transport was greatest under conditions of an inward-directed Na+ gradient, but a significant stimulation by Na+ over K+ could also be observed in the absence of a salt gradient. At high extravesicular K+, the addition of valinomycin reduced taurine uptake. Na+-dependent 3H-taurine flux was almost completely inhibited by non-labeled taurine (tracer replacement) or beta-alanine, but was unaffected by L-alanine. Replacement of medium chloride by SCN- or NO3- in the presence of Na+ resulted in significantly lower uptake rates under both anion gradient and anion equilibrium conditions, whereas Br- could almost fully substitute for the stimulatory Cl- action. These results indicate the presence of an electrogenic Na+-cotransport mechanism with specificity for beta-amino acids in the toadfish renal brush border. Whether the system under physiological conditions mediates reabsorption or secretion of taurine remains to be determined. Toadfish bbmv also translocated L-alanine and L-glutamic acid in a Na+-dependent manner. Possible roles for these most likely reabsorptive transport systems in a non-filtering kidney are discussed. D-glucose uptake, however, appeared to occur via Na+-independent pathways, since it was not affected by phlorizin in the presence of Na+, or by Na+ replacement.