Transport of beta-alanine in renal brush border membrane vesicles

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.

Adaptive regulation of taurine and beta-alanine uptake in a human kidney cell line from the proximal tubule

1. The underlying mechanisms involved in the adaptive regulation of beta-amino acid uptake in the human proximal tubule were examined by use of an immortalized human embryonic kidney epithelial cell line (IHKE). 2. The results indicated that the adaptive response to maintain whole-body taurine homeostasis occurs predominantly via changes in the activity of the high-affinity taurine transport system by alterations in the uptake capacity and with an unaffected half-saturation constant. An adaptive response was not observed for the structurally related beta-alanine. 3. Only colchicine, which interferes with microtubule organization, was capable of blocking the response to alterations of taurine in cell medium, whereas inhibition of protein and nucleic acid synthesis by cycloheximide and actinomycin D, respectively, did not change the adaptive pattern. 4. Phorbol 12-myristate 13-acetate (PMA), mimicking the effects of diacylglycerol, induced inhibition of both beta-alanine and taurine uptake. By contrast, the Ca2(+)-ionophore A23187, mimicking the effects of IP3, only stimulated the uptake of taurine but not the influx of beta-alanine. However, the effect of PMA down-regulation and A23187 up-regulation was rapid and short-lived in contrast to the adaptive response, suggesting that the inositol phospholipid pathway involving diacetylglycerol and IP3 is less likely to be linked directly to the adaptive regulation, but rather plays a role in short-term regulation.

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.

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).

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.

Stoichiometric studies of beta-alanine transporters in rabbit proximal tubule

The coupling ratio for the transport of beta-alanine and Na+, H+ and Cl- in luminal membrane vesicles isolated from proximal convoluted tubules (pars convoluta) and proximal straight tubules (pars recta) of rabbit kidney was examined. Indirect evidence indicates that 1 H+ and approx. 2 Na+, 1 Cl- (Na(+)-dependent, high-affinity) or 1 Na+ (Na(+)-dependent, low-affinity) are co-transported with beta-alanine in the pars convoluta. In pars recta, the two Na(+)-dependent transporters exhibited the same stoichiometric properties respectively as in pars convoluta.

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).

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.

Demonstration of H+- and Na+-coupled co-transport of beta-alanine by luminal membrane vesicles of rabbit proximal tubule

1. The characteristics of renal transport of beta-alanine by luminal membrane vesicles isolated from either the proximal convoluted part (pars convoluta) or the proximal straight part (pars recta) of rabbit proximal tubule were investigated. 2. In vesicles from pars convoluta two transport systems have been characterized: (1) a Na+-dependent system with intermediate affinity (half-saturation 2.7 mM), and (2) a Na+-independent system, which in the presence of a H+ gradient (extravesicular greater than intravesicular) can drive the uphill transport of beta-alanine into these vesicles. This is the first demonstration of H+-beta-alanine co-transport across luminal membrane of rabbit kidney proximal convoluted tubule. 3. By contrast, in membrane vesicles from pars recta, transport of beta-alanine was strictly dependent on Na+ and occurred via a dual transport system, namely a high-affinity (half-saturation 0.16 mM) and a low-affinity system (half-saturation 9.3 mM). 4. The demonstration of competition between the Na+-gradient-dependent uptake of beta-alanine and taurine, without appreciable inhibition by alpha-amino acids in vesicles from pars convoluta as well as from pars recta, strongly suggests that the luminal membrane of proximal tubule has transport systems for the reabsorption of beta-amino acids which are distinct from alpha-amino acid transport systems.

Mechanisms of linoleic acid uptake by rabbit small intestinal brush border membrane vesicles

We examined the initial transport of a long-chain unsaturated fatty acid, linoleic acid, by brush border membrane vesicles isolated from rabbit small intestine. This preparation allowed us to examine the transport of linoleic acid across the brush border membrane without the effect of the unstirred water layer or cytosol binding proteins. Linoleic acid was solubilized in a 2 mM taurocholate solution which did not compromise the functional integrity of the vesicles. Linoleic acid uptake in the range of 1 to 100 microM followed passive diffusion kinetics. Time course study showed that linoleic acid uptake reached maximal levels during the initial 15 seconds. Although the amount of linoleic acid accumulated in the vesicles diminished over the next 30 minutes, the molar quantity was still twentyfold higher than that of D-glucose (6.5 vs 0.33 nmol/mg protein). Uptake of D-glucose by the vesicles demonstrated typical osmotic responsiveness. We found no osmotic effect on linoleic acid uptake. Hypotonic lysis of membrane vesicles loaded with linoleic acid released 40% of the fatty acid. We concluded that a major portion of the accumulated fatty acid was bound to or incorporated into the membrane itself while ca. 40% did traverse the membrane and accumulated in the intravesicular space as nonmicellar aggregates. The known inhibitors of anion transport, diisothiocyanatostilbene and isothiocyanatostilbene did not change the transport of linoleic acid. We conclude that, in the absence of an unstirred layer or cytosol proteins, linoleic acid transport at up to 100 microM concentration is passive with rapid accumulation both by the cell membrane and the lumen of vesicles.

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.

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.

A gamma-aminobutyric acid-specific transport mechanism in mammalian kidney

We describe high-affinity, sodium-dependent transport of gamma-aminobutyric acid in slices exposing basal lateral membranes and brush-border membrane vesicles prepared from rat renal cortex. In the presence of aminooxyacetic acid, to block gamma-aminobutyric acid oxidation, uptake into the intracellular space of slices was saturable (apparent Kt, 26 +/- 4 microM, mean and S.E.) and concentrative (steady-state distribution ratio at 50 microM gamma-aminobutyric acid, 47.7 +/- 2.4, mean and S.E.). Brush-border membrane vesicles accumulated gamma-aminobutyric acid in the presence of an inward-directed sodium chloride gradient, (apparent Kt, 30-36 microM) with the peak of 'overshoot' at 10 min. Uptake by vesicles responded to manipulation of the transmembrane potential gradient with valinomycin or impermeant anion. beta-Alanine inhibited gamma-aminobutyric acid transport by slices and brush-border membrane vesicles; inhibitors of neuronal-type gamma-aminobutyric acid transport (e.g., nipecotic and diaminobutyric acids) did not. An 'ABC test' indicated that gamma-aminobutyric acid and beta-alanine do not share a single carrier in either the brush-border or basal-lateral membrane of renal cortex. Influx of gamma-aminobutyric acid into brush-border membrane vesicles, at transequilibrium NaCl, was stimulated by trans-gamma-aminobutyric acid but not by trans-taurine. Ion gradient-driven gamma-aminobutyric acid co-transport was unaffected in freeze-thawed brush-border membrane vesicles; this treatment abolished beta-alanine and taurine co-transport. We conclude that rat kidney membranes (brush-border and basal-lateral) possess a gamma-aminobutyric acid-preferring, high-affinity transport mechanism.

Factors affecting the transport of beta-amino acids in rat renal brush-border membrane vesicles. The role of external chloride

The effect of a variety of ions and other solutes on the accumulation of the beta-amino acid, taurine, was examined in rat renal brush-border membrane vesicles. Initial taurine uptake (15 and 30 s) is sodium-dependent with a typical overshoot. This Na+ effect was confirmed by exchange diffusion and gramicidin inhibition of taurine uptake. External K+ or Li+ do not increase taurine accumulation more than Na+-free mannitol, except that the combination of external K+ and Na+ in the presence of nigericin enhances uptake. Of all anions tested, including more permeant (SCN- and NO3-) or less permeant (SO4(2-)), chloride supported taurine accumulation to a significantly greater degree. Preloading vesicles with choline chloride reduced taurine uptake, suggesting that external Cl- stimulates uptake. Since this choline effect could be related to volume change, due to the slow diffusion of choline into vesicles, brush-border membrane vesicles were pre-incubated with LiCl, LiNO3 and LiSO4. Internal LiCl, regardless of the final Na+ anion mixture, reduced initial rate (15 and 60 s) and peak (360 s) taurine uptake. Internal LiNO3 or LiSO4 with external NaCl resulted in similar or higher values of uptake at 15, 60 and 360 s, indicating a role for external Cl- in taurine uptake in addition to Na+ effect. Although uptake by vesicles is greatest at pH 8.0 and inhibited at acidic pH values (pH less than 7.0), an externally directed H+ gradient does not influence uptake. Similarly, amiloride, an inhibitor of the Na+/H+ antiporter, had no influence on taurine accumulation over a wide variety of concentrations or at low Na+ concentrations. Taurine uptake is blocked only by other beta-amino acids and in a competitive fashion. D-Glucose and p-aminohippurate at high concentrations (greater than 10(-3) M) reduce taurine uptake, possibly by competing for sodium ions, although gramicidin added in the presence of D-glucose inhibits taurine uptake even further. These studies more clearly define the nature of the renal beta-amino acid transport system in brush-border vesicles and indicate a role for external Cl- in this uptake system.

Effectors of amino acid transport processes in animal cell membranes

Various effectors, which act upon ion gradients, protein synthesis, membrane components or cellular functional groups, have been employed to provide insights into the nature of amino acid-membrane transport processes in animal cells. Such effectors, for example, include ions, hormones, metabolites and various organic reagents and their judicious use has allowed the following list of conclusions. Sodium ion has been found to stimulate amino acid transport in a wide variety of cell systems, although depending on the tissue and/or substrate, this ion may have no effect on such transport, or even inhibit it. Amino acid transport can be stimulated in some cell systems by other ions such as K+, Li+, H+ or Cl-. Both H+ and K+ have been found to be inhibitory in other systems. Amino acid transport is dependent in many cell systems upon an inwardly directed Na+ gradient and is stimulated by a membrane potential (negative cell interior). In some cell systems an inwardly directed Cl- and H+ gradient or an outwardly directed K+ gradient can energize transport. Structurally dissimilar effectors such as ouabain, Clostridium enterotoxin, aspirin and amiloride inhibit amino acid transport presumably through dissipation of the Na+ gradient. Inhibition by certain sugars or metabolic intermediates of the tricarboxylic acid cycle may compete with the substrate for the energy of the Na+ gradient or interact with the substrate at the carrier level either allosterically or at a common site. Stimulation of transport by other sugars or intermediates may result from their catabolism to furnish energy for transport. Insulin and glucagon stimulate transport of amino acids in a variety of cell systems by a mechanism which involves protein synthesis. Microtubules may be involved in the regulation of transport by insulin or glucagon. Some reports also suggest that insulin has a direct effect on membranes. In addition, a number of growth hormones and factors have stimulatory effects on amino acid transport which are also mediated by protein synthesis. Steroid hormones have been noted to enhance or diminish transport of amino acids depending on the nature of the hormone. These agents appear to function at the level of protein synthesis. While stimulation may involve increased carrier synthesis, inhibition probably involves synthesis of a labile protein which either decreases the rate of synthesis or increases the rate of degradation of a component of the transport system.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of alpha-aminoisobutyric acid transport in rat skeletal muscles

alpha-Aminoisobutyric acid (AIB) transport into the intracellular compartment of extensor digitorum longus and soleus muscles was measured (in vitro) after allowance for the equilibration of the amino acid in the extracellular space. The latter was determined with three markers, [14C]inulin, 60Co-EDTA and [3H]mannitol. Net transport of AIB was subsequently divided into its two components, i.e. influx and efflux. Rates of influx were measured as the intracellular accumulation of [14C]AIB after a short incubation (5 min), and efflux was measured as the release of AIB with time (up to maximum of 50 min) from muscles that had previously been preloaded with AIB. This intracellular efflux was resolved into two phases, which probably represent two separate components of exit. The influence of extracellular Na+ on the transport of this neutral amino acid (representing the A system) was investigated. Na+ depletion resulted in lower accumulations of AIB, the effects becoming more pronounced with progressive depletions of external Na+. These changes arose from an inhibition of AIB influx, concomitant with an enhancement of its efflux. In contrast, all components of tyrosine transport (representing the L system) were unaffected by lowering external Na+ concentrations. The net accumulation of AIB was also suppressed by cortisol. This inhibitory effect was, however, Na+-dependent and resulted solely from the steroid's enhancement of AIB efflux, the hormone being without effect on AIB influx.

Renal transport of neutral amino acids. Demonstration of Na+-independent and Na+-dependent electrogenic uptake of L-proline, hydroxy-L-proline and 5-oxo-L-proline by luminal-membrane vesicles

Uptake of L-proline, hydroxy-L-proline and 5-oxo-L-proline by luminal-membrane vesicles isolated either from whole cortex or from pars convoluta or pars recta of proximal tubules was studied by a spectrophotometric method. Uptake of L-proline and hydroxy-L-proline by vesicles from whole cortex was mediated by both Na+-dependent and Na+-independent, but electrogenic, processes, whereas transport of 5-oxo-L-proline in these vesicles was strictly Na+-dependent. Eadie-Hofstee analysis of saturation-kinetic data suggested the presence of multiple transport systems in luminal-membrane vesicles from whole renal cortex for the uptake of all these amino acids. Tubular localization of the transport systems was studied by the use of vesicles derived from pars convoluta and from pars recta. In pars recta transport of all three amino acids was strictly dependent on Na+ and occurred via a high-affinity system (half-saturation: 0.1-0.3 mM). Cation-dependent but Na+-unspecific transport of low affinity for L-proline and hydroxy-L-proline was exclusively localized to the pars convoluta, which also contained a Na+-preferring system of intermediate affinity (half-saturation: L-proline, 0.75 mM; hydroxy-L-proline, 1.3 mM). 5-Oxo-L-proline was transported by low-affinity and Na+-dependent systems in both pars convoluta and pars recta. Competition experiments revealed that transport systems for L-proline and hydroxy-L-proline are common, but indicated separate high-affinity transport systems for 5-oxo-L-proline and L-proline in luminal-membrane vesicles from pars recta. The physiological importance of the presence of various neutral amino acid-transport systems in different segments of the proximal tubule is discussed.

Renal adaptation to altered dietary sulfur amino acid intake occurs at luminal brushborder membrane

The beta-amino acid transport capabilities of rat renal epithelium were assessed using brushborder membrane vesicles (BBMV). Taurine, a metabolically inert sulfur-containing amino acid, was studied with emphasis on the renal adaptation to dietary sulfur amino acid alteration. Three isoproteinic diets were given to Sprague-Dawley rats: low-sulfur-amino-acid diet (LTD), normal-sulfur-amino-acid diet (NTD), and high-taurine diet (HTD). Our studies demonstrated that taurine is actively transported into membrane vesicles by a sodium-dependent transport system. This transport is enhanced by hyperpolarization with valinomycin and decreased by dissipation of the sodium gradient by gramicidin. On LTD (compared to NTD), plasma taurine, urinary taurine, and fractional excretion of taurine were reduced. On HTD (compared to NTD), plasma taurine, urinary taurine, and fractional excretion of taurine were elevated. In vitro studies in BBMV from NTD animals revealed a Km of 40 microM and Vmax of 102 pmoles/mg protein/30 sec. Other beta-amino acids significantly inhibited BBMV taurine accumulation. BBMV taurine uptake was enhanced after LTD (compared to NTD) and diminished after HTD (compared to NTD). These studies indicate that a renal adaptation to dietary alterations in sulfur-containing amino acids occurs and that the luminal brushborder membrane participates in the adaptation. Renal adaptative mechanisms to dietary change may serve to help conserve amino acids during deprivation and to excrete amino acids during periods of excess.

Electrogenic transport of 5-oxoproline in rabbit renal brush-border membrane vesicles. Effect of intravesicular potassium

The Na+-dependent transport of 5-oxoproline into rabbit renal brush-border vesicles was stimulated by a K+ diffusion potential (interior-negative) induced by valinomycin. Na+ salts of two anions of different epithelial permeabilities also affected 5-oxoproline transport. These results show that the Na+-dependent 5-oxoproline transport in renal brush-border vesicles is an electrogenic process which results in a net transfer of positive charge. Maximum transport of 5-oxoproline occurred at an extravesicular pH of 6.0 to 8.0 and over that pH range, 5-oxoproline exists completely as an anion with a negative charge. The simplest stoichiometry consistent with this process is, therefore, the cotransport of one 5-oxoproline anion with two sodium ions. The presence of K+ inside the vesicles stimulated the Na+-dependent transport of 5-oxoproline. This stimulatory effect was specific for K+ and required the presence of Na+. The presence of Na+ gradient was not mandatory for the K+ action. The stimulation by the intravesicular K+ was seen in the presence as well as in the absence of a K+ gradient. Therefore, the increased influx of 5-oxoproline was not coupled to the simultaneous efflux of K+. The presence of K+ in the extravesicular medium alone did not affect the Na+-dependent transport of 5-oxoproline, showing that the site of K+ action was intravesicular. Glutamate did not interact with the Na+-dependent 5-oxoproline transport even in the presence of an outward K+ gradient.

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.

Peptide transport in rabbit kidney. Studies with L-carnosine

L-Carnosine was shown to be transported into rabbit renal brush-border membrane vesicles by an Na+ -independent mechanism. The transport was competitively inhibited by glycyl-L-proline. Various di- and tripeptides inhibited L-carnosine transport, whereas free amino acids did not. Inhibition studies showed that blocking the free amino and carboxyl groups of the peptide reduced its affinity for the transport carrier. Under the conditions in which there was no detectable hydrolysis of L-carnosine in the medium, intravesicular contents showed a 30% hydrolysis of the peptide within the vesicles. Disruption of membrane vesicles with deoxycholate resulted in a 3-fold increase in L-carnosine hydrolyzing activity over untreated intact vesicles. Based on these observations, a model for peptide transport is proposed in which transport of the intact peptide across the membrane is followed by its partial or complete hydrolysis by a membrane peptidase whose active site is on the cytoplasmic side of the membrane.

Na+-dependent transport of glycine in renal brush border membrane vesicles. Evidence for a single specific transport system

The uptake of glycine in rabbit renal brush border membrane vesicles was shown to consist of glycine transport into an intravesicular space. An Na+ electrochemical gradient (extravesicular greater than intravesicular) stimulated the initial rate of glycine uptake and effected a transient accumulation of intravesicular glycine above the steady-state value. This stimulation could not be induced by the imposition of a K+, Li+ or choline+ gradient and was enhanced as extravesicular Na+ was increased from 10 mM to 100 mM. Dissipation of the Na+ gradient by the ionophore gramicidin D resulted in diminished Na+-stimulated glycine uptake. Na+-stimulated uptake of glycine was electrogenic. Substrate-velocity analysis of Na+-dependent glycine uptake over the range of amino acid concentrations from 25 microM to 10 mM demonstrated a single saturable transport system with apparent Km = 996 microM and Vmax = 348 pmol glycine/mg protein per min. Inhibition observed when the Na+-dependent uptake of 25 microM glycine was inhibited by 5 mM extravesicular test amino acid segregated dibasic amino acids, which did not inhibit glycine uptake, from all other amino acid groups. The amino acids D-alanine, D-glutamic acid, and D-proline inhibited similarly to their L counterparts. Accelerative exchange of extravesicular [3H]glycine was demonstrated when brush border vesicles were preloaded with glycine, but not when they were preloaded with L-alanine, L-glutamic acid, or with L-proline. It is concluded that a single transport system exists at the level of the rabbit renal brush border membrane that functions to reabsorb glycine independently from other groups of amino acids.

Na+-independent L-arginine transport in rabbit renal brush border membrane vesicles

Na+-independent L-arginine uptake was studied in rabbit renal brush border membrane vesicles. The finding that steady-state uptake of L-arginine decreased with increasing extravesicular osmolality and the demonstration of accelerative exchange diffusion after preincubation of vesicles with L-arginine, but not D-arginine, indicated that the uptake of L-arginine in brush border vesicles was reflective of carrier-mediated transport into an intravesicular space. Accelerative exchange diffusion of L-arginine was demonstrated in vesicles preincubated with L-lysine and L-ornithine, but not L-alanine or L-proline, suggesting the presence of a dibasic amino acid transporter in the renal brush border membrane. Partial saturation of initial rates of L-arginine transport was found with extravesicular [arginine] varied from 0.005 to 1.0 mM. L-Arginine uptake was inhibited by extravesicular dibasic amino acids unlike the Na+-independent uptake of L-alanine, L-glutamine, glycine or L-proline in the presence of extravesicular amino acids of similar structure. L-Arginine uptake was increased by the imposition of an H+ gradient (intravesicular pH less than extravesicular pH) and H+ gradient stimulated uptake was further increased by FCCP. These findings demonstrate membrane-potential-sensitive, Na+-independent transport of L-arginine in brush border membrane vesicles which differs from Na+-independent uptake of neutral and acidic amino acids. Na+-independent dibasic amino acid transport in membrane vesicles is likely reflective of Na+-independent transport of dibasic amino acids across the renal brush border membrane.

Delineation of sodium-stimulated amino acid transport pathways in rabbit kidney brush border vesicles

We have confirmed previous demonstrations of sodium gradient-stimulated transport of L-alanine, phenylalanine, proline, and beta-alanine, and in addition demonstrated transport of N-methylamino-isobutyric acid (MeAIB) and lysine in isolated rabbit kidney brush border vesicles. In order to probe the multiplicity of transport pathways available to each of these 14C-amino acids, we measured the ability of test amino acids to inhibit tracer uptake. To obtain a rough estimate of nonspecific effects, e.g., dissipation of the transmembrane sodium electrochemical potential gradient, we measured the ability of D-glucose to inhibit tracer uptake. L-alanine and phenylalanine were completely mutually inhibitory. Roughly 75% of the 14C-L-alanine uptake could be inhibited by proline and beta-alanine, while lysine and MeAIB were no more effective than D-glucose. Roughly 50% of the 14C-phenylalanine uptake could be inhibited by proline and beta-alanine; lysine was as effective as proline and beta-alanine, and the effects of pairs of these amino acids at 50 mM each were not cumulative. MeAIB was no more effective than D-glucose. We conclude that three pathways mediate the uptake of neutral L, alpha-amino acids. One system is inaccessible to lysine, proline, and beta-alanine. The second system carries a major fraction of the L-alanine flux; it is sensitive to proline and beta-alanine, but not to lysine. The third system carries half the 14C-phenylalanine flux, and it is sensitive to proline, lysine, and beta-alanine. Since the neutral, L, alpha-amino acid fluxes are insensitive to MeAIB, we conclude that they are not mediated by the classical A system, and since all of the L-alanine flux is inhibited by phenylalanine, we conclude that it is not mediated by the classical ASC system. L-alanine and phenylalanine completely inhibit uptake of lysine. MeAIB is no more effective than D-glucose in inhibiting lysine uptake, while proline and beta-alanine appear to inhibit a component of the lysine flux. We conclude that the 14C-lysine fluxes are mediated by two systems, one, shared with phenylalanine, which is inhibited by proline, beta-alanine, and L-alanine, and one which is inhibited by L-alanine and phenylalanine but inaccessible to proline, beta-alanine, and MeAIB. Fluxes of 14C-proline and 14C-MeAIB are completely inhibited by L-alanine, phenylalanine, proline, and MeAIB, but they are insensitive to lysine. Proline and MeAIB, as well as alanine and phenylalanine, but not lysine, inhibit 14C-beta-alanine uptake. However, beta-alanine inhibits only 38% of the 14C-proline uptake and 57% of the MeAIB uptake. We conclude that two systems mediate uptake of proline and MeAIB, and that one of these systems also transports beta-alanine.

L-glutamate transport in renal plasma membrane vesicles

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

GABA transport in the rat thyroid

1. The uptake of gamma-aminobutyric acid (GABA) into rat thyroid slices was studied. 2. Uptake of 14C-GABA was concentration-dependent: one unsaturable (diffusion) and two saturable components obeying Michaelis-Menten kinetics contributed to transport. 3. The kinetic constants of saturable GABA transport systems were: Km1 = 1.5 microM, V1 = 4.0 nmol x (g wet weight)-1 x (20 min)-1 (high-affinity uptake): Km2 = 800 microM, V2 = 260 nmol x (g wet weight)-1 x (20 min)-1 (low-affinity uptake). 4. Uptake mediated by each of the carrier systems was concentrative, entirely Na+-dependent, and required activation energies characteristic for active transport. 5. High-affinity transport was structurally specific for GABA. The substrate specificity of low-affinity uptake resembled that of beta-amino acid transport systems.

The beta-amino acid transport system in Friedreich's ataxia

Taurine and beta-alanine uptake in cultured skin fibroblasts proceeds through at least two distinct amino acid transport systems. The predominant beta amino acid uptake system which we refer to as the "Beta" system, incorporates taurine in a proportion of 95%. Beta-alanine in a proportion of 80% and does not incorporate beta-amino-isobutyric acid. A second transport system for beta-alanine seems to be operative cultured skin fibroblasts and this system shares the characteristics of system "L" for branched-chain and ring-side neutral amino acids. Results of ion depletion experiments, metabolic inhibition by drugs and blocking agents and previous kinetic studies of taurine and beta-alanine uptake in cultured skin fibroblasts failed to disclose any major difference in beta-amino acid transport between control individuals and patients with Friedreich's ataxia.

Amino acid transport in plasma-membrane vesicles from rat liver. Characterization of L-alanine transport

Plasma-membrane vesicles, isolated from rat liver, catalyze active transport of L-alanine. The transient accumulation of L-alanine requires the presence of Na+ concentration gradient (outside greater than inside). The alanine-Na+ symport is an electrogenic process, since it is stimulated under conditions that allow compensatory ion movements: both co-transported anions as well as counter-transported cations influence the rate of alanine-Na+ symport. However, no uptake is observed in the presence of a membrane potential, when no Na+ concentration gradient is present. Sodium-gradient-stimulated alanine uptake is dependent on temperature and pH, stereospecific, and is affected by the presence of other amino acids. The activity of the L-alanine transport system is influenced both by the Na+ and by the L-alanine concentration. In the presence of 100 mM Na+, an apparent Km for alanine of 2 mM is observed; lowering the Na+ concentration results in an increase in the apparent Km, and a decrease in the apparent V. The apparent Km for Na+ is 34 mM in the presence of 0.2 mM L-alanine. Increasing the L-alanine concentration also results in a lower apparent Km for Na+, and a higher V.

Renal transport of amino acids

According to recent experimental data the renal transport of amino acids (AA) is characterized as follows. 1. Kinetics: Several reabsorption systems remove AA from the tubular fluid by active transport with Michaelis-Menten type kinetics. Passive diffusion does play only a relatively small role in reabsorption, but determines the pump leak steady state concentration at the end of the tubule. 2. Stereospecificity: Except for aspartate the naturally occurring L-analogs show a much larger affinity to the transport "carriers" than the D-isomers do. 3. Specificity: Separate transport mechanisms exist for a) the "acidic" AA (Glu and Asp); b) the "dibasic" AA (Arg, Lys, Orn); c) cystine/cystine; d) the "imino" acids (Pro, OH-Pro and other N-substituted AA); e) the beta- and gamma-AA (beta-Ala, GABA, Taurine); f) all other "neutral" AA. For the group (d) and maybe also for (b) and glycine additional low capacity/high affinity systems exist. 4. Localization: Except for glycine and taurine under normal conditions more than 80% of the filtered load are reabsorbed within the first third of the proximal tubule. At an elevated load the rest of the proximal tubule (including pars recta) but not the distal nephron is included into the reabsorptive process. AA are also taken up from the peritubular blood. 5. Energy sources: At least the main part of AA uptake at the brushborder membrane is dependent from a transmembranal Na+-gradient which in turn is established by the ATP driven Na+-pumps at the basolateral side of the cell (Secondary active transport or co-transport of AA). 6. Biochemistry: The biochemical nature of the AA-"carriers" is unknown. The recent hypothesis than a "gamma-glutamyl cycle" plays a major role in this context has been disproved to great extent. 7. Peptides: Oligopeptides (Angiotensin, Gluthathion) filtered at the glomerulum are hydrolyzed by brushborder peptidases within the tubule lumen. The splitting products, the free constituent amino acids, are reabsorbed subsequently by their respective transport systems.