Kinetics and localization of tubular resorption of "acidic" amino acids. A microperfusion and free flow micropuncture study in rat kidney

Histidine and other amino acids in blood and urine after administration of Bretschneider solution (HTK) for cardioplegic arrest in patients: effects on N-metabolism

Bretschneider (histidine-tryptophan-ketoglutarate, HTK) solution employed for induction of cardioplegic arrest possesses a high histidine concentration (198 mM). Due to the large volume administered, massive amounts of histidine are incorporated. The aim of the study was to evaluate alterations in amino acid and nitrogen metabolism originating from histidine degradation. Between 07/2014 and 10/2014, a total of 29 consecutive patients scheduled for elective isolated coronary artery bypass grafting with cardiopulmonary bypass (CPB) were enrolled in this prospective observational study. The patients received 1.6 L cardioplegic Bretschneider solution on average. Blood gas and urine samples obtained were analyzed for amino acid as well as urea and ammonium concentrations. After CPB initiation, plasma histidine concentration greatly increased to 21,000 µM to reach 8000 µM at the end. Within the operative period, plasma concentrations of aspartate, glutamate, asparagine, alanine, and glutamine increased variable in magnitude. During the same time, urinary analysis revealed histidine excretion of 19,500 µmol in total and marked elevations in glutamate and glutamine excretion. The absolute amounts of urea and ammonium excreted additionally were 3 mmol and 8 mmol, respectively. Already during CPB, distinct amounts of the histidine administered are metabolized, mainly to other amino acids, but only small amounts to urea and ammonia. Thus, the impact of the histidine incorporated on acid–base status in the intraoperative phase is minor. On the other hand, intraoperative provision of several amino acids arising from histidine metabolism might mitigate postaggression syndrome.

Multi-Organ Contribution to the Metabolic Plasma Profile Using Hierarchical Modelling

Hierarchical modelling was applied in order to identify the organs that contribute to the levels of metabolites in plasma. Plasma and organ samples from gut, kidney, liver, muscle and pancreas were obtained from mice. The samples were analysed using gas chromatography time-of-flight mass spectrometry (GC TOF-MS) at the Swedish Metabolomics centre, Umeå University, Sweden. The multivariate analysis was performed by means of principal component analysis (PCA) and orthogonal projections to latent structures (OPLS). The main goal of this study was to investigate how each organ contributes to the metabolic plasma profile. This was performed using hierarchical modelling. Each organ was found to have a unique metabolic profile. The hierarchical modelling showed that the gut, kidney and liver demonstrated the greatest contribution to the metabolic pattern of plasma. For example, we found that metabolites were absorbed in the gut and transported to the plasma. The kidneys excrete branched chain amino acids (BCAAs) and fatty acids are transported in the plasma to the muscles and liver. Lactic acid was also found to be transported from the pancreas to plasma. The results indicated that hierarchical modelling can be utilized to identify the organ contribution of unknown metabolites to the metabolic profile of plasma.

The Genetics of Heteromeric Amino Acid Transporters

Heteromeric amino acid transporters (HATs) are composed of a heavy ( SLC3 family) and a light ( SLC7 family) subunit. Mutations in system b0,+(rBAT-b0,+AT) and in system y+L (4F2hc-y+LAT1) cause the primary inherited aminoacidurias (PIAs) cystinuria and lysinuric protein intolerance, respectively. Recent developments [including the identification of the first Hartnup disorder gene (B0AT1; SLC6A19)] and knockout mouse models have begun to reveal the basis of renal and intestinal reabsorption of amino acids in mammals.

Renal metabolism of amino acids: its role in interorgan amino acid exchange

The kidneys play a role in the synthesis and interorgan exchange of several amino acids. The quantitative importance of renal amino acid metabolism in the body is not, however, clear. We review here the role of the kidney in the interorgan exchange of amino acids, with emphasis on quantitative aspects. We reviewed relevant literature by using a computerized literature search (PubMed) and checking relevant references from the identified articles. Our own data are discussed in the context of the literature. The kidney takes up glutamine and metabolizes it to ammonia. This process is sensitive to pH and serves to maintain acid-base homeostasis and to excrete nitrogen. In this way, the metabolism of renal glutamine and ammonia is complementary to hepatic urea synthesis. Citrulline, derived from intestinal glutamine breakdown, is converted to arginine by the kidney. Renal phenylalanine uptake is followed by stoichiometric tyrosine release, and glycine uptake is accompanied by serine release. Certain administered oligopeptides (eg, glutamine dipeptides) are converted by the kidneys to their constituent components before they can be used in metabolic processes. The kidneys play an important role in the interorgan exchange of amino acids. Quantitatively, for several important amino acids, the kidneys are as important as the gut in intermediary metabolism. The kidneys may be crucial "mediators" of the beneficial effects of specialized, disease-specific feeding solutions such as those enriched in glutamine dipeptides.

Identification of a novel Na+-independent acidic amino acid transporter with structural similarity to the member of a heterodimeric amino acid transporter family associated with unknown heavy chains

We identified a novel Na(+)-independent acidic amino acid transporter designated AGT1 (aspartate/glutamate transporter 1). AGT1 exhibits the highest sequence similarity (48% identity) to the Na(+)-independent small neutral amino acid transporter Asc (asc-type amino acid transporter)-2 a member of the heterodimeric amino acid transporter family presumed to be associated with unknown heavy chains (Chairoungdua, A., Kanai, Y., Matsuo, H., Inatomi, J., Kim, D. K., and Endou, H. (2001) J. Biol. Chem. 276, 49390-49399). The cysteine residue responsible for the disulfide bond formation between transporters (light chains) and heavy chain subunits of the heterodimeric amino acid transporter family is conserved for AGT1. Because AGT1 solely expressed or coexpressed with already known heavy chain 4F2hc (4F2 heavy chain) or rBAT (related to b(0,+)-amino acid transporter) did not induce functional activity, we generated fusion proteins in which AGT1 was connected with 4F2hc or rBAT. The fusion proteins were sorted to the plasma membrane and expressed the Na(+)-independent transport activity for acidic amino acids. Distinct from the Na(+)-independent cystine/glutamate transporter xCT structurally related to AGT1, AGT1 did not accept cystine, homocysteate, and l-alpha-aminoadipate and exhibited high affinity to aspartate as well as glutamate, suggesting that the negative charge recognition site in the side chain-binding site of AGT1 would be closer to the alpha-carbon binding site compared with that of xCT. The AGT1 message was predominantly expressed in kidney. In mouse kidney, AGT1 protein was present in the basolateral membrane of the proximal straight tubules and distal convoluted tubules. In the Western blot analysis, AGT1 was detected as a high molecular mass band in the nonreducing condition, whereas the band shifted to a 40-kDa band corresponding to the AGT1 monomer in the reducing condition, suggesting the association of AGT1 with other protein via a disulfide bond. The finding of AGT1 and Asc-2 has established a new subgroup of the heterodimeric amino acid transporter family whose members associate not with 4F2hc or rBAT but with other unknown heavy chains.

Regulatory responses to an oral D-glutamate load: formation of D-pyrrolidone carboxylic acid in humans

Previously published studies have shown D-glutamate to be the most potent natural inhibitor of glutathione synthesis known, yet how D-glutamate is handled in humans is unknown. Therefore, we administered an oral D-glutamate load to four healthy volunteers and monitored the plasma D-glutamate concentration and excretion over a 3-h postload period. Compared with time controls, the plasma D-glutamate concentration increased 10-fold in the 1st h and then reached a plateau over the remaining time course. In contrast, plasma D-pyrrolidone carboxylic acid increased progressively throughout the 3-h time course to a level 10-fold higher than the D-glutamate plasma concentration. Excretion of D-glutamate progressively increased despite a constant filtered D-glutamate load rising from only 5 to 95% of the filtered amount. Excretion of D-pyrrolidone carboxylic acid increased with the rise in filtered load without significant reabsorption. The amount of D-pyrrolidone carboxylic acid excreted over the 3-h time course was 10 times the amount excreted as D-glutamate and accounted for almost 20% of the administered D-glutamate. These findings indicate that plasma D-glutamate concentration is tightly regulated through two mechanisms: 1) the transport into cells and metabolic conversion to D-pyrrolidone carboxylic acid and excretion, and 2) the enhancement of D-glutamate clearance by the kidneys.

Glutamate transporters in kidney and brain

Glutamate transporters play important roles in the termination of excitatory neurotransmission and in providing cells with glutamate for metabolic purposes. In the kidney, glutamate transporters are involved in reabsorption of filtered acidic amino acids, regulation of ammonia and bicarbonate production, and protection of cells against osmotic stress.

Glutamate transport asymmetry and metabolism in the functioning kidney

Renal glutamate extraction in vivo shows a preference for the uptake of D-glutamate on the antiluminal and L-glutamate on the luminal tubule surface. To characterize this functional asymmetry, we isolated rat kidneys and perfused them with an artificial plasma solution containing either D- or L-glutamate alone or in combination with the system X-AG specific transport inhibitor, D-aspartate. To confirm that removal of glutamate represented transport into tubule cells, we monitored products formed as the result of intracellular metabolism and related these to the uptake process. Perfusion with D-glutamate alone resulted in a removal rate that equaled or exceeded the L-glutamate removal rate, with uptake predominantly across the antiluminal surface; L-glutamate uptake occurred nearly equally across both luminal and antiluminal surfaces. Thus the preferential uptake of D-glutamate at the antiluminal and L-glutamate at the luminal surface confirms the transport asymmetry observed in vivo. Equimolar D-aspartate concentration blocked most of the antiluminal D-glutamate uptake and a significant portion of the luminal L-glutamate uptake, consistent with system X-AG activity at both sites. D-Glutamate uptake was associated with 5-oxo-D-proline production, whereas L-glutamate uptake supported both glutamine and 5-oxo-L-proline formation; D-aspartate reduced production of both 5-oxoproline and glutamine. The presence of system X-AG activity on both the luminal and antiluminal tubule surfaces, exhibiting different reactivity toward L- and D-glutamate suggests that functional asymmetry may reflect two different X-AG transporter subtypes.

Influence of epidermal growth factor (EGF) on renal transport of PAH and amino acids in amino acid loaded rats

In anaesthetized adult female rats, the influence of epidermal growth factor (EGF) on renal transport of p-amino-hippurate (PAH), electrolytes, and amino acids was investigated. After loading with PAH (200 mg/100 g b.wt. iv.), PAH excretion in EGF treated rats (8 microg/100 g b.wt. subcutaneously for 8 days, twice daily 8 a.m. and 4 p.m.) was increased by about 20 %. Continuous infusions of glutamine, arginine (both 50 mg/100 g b.wt. per hour), or alanine (90 mg/ 100 g b.wt. per hour) were followed by an increase in the fractional excretion (FE) of the administered amino acids as well as of the other endogenous amino acids. Under load conditions (alanine, arginine or glutamine), EGF pretreatment was followed by a stimulation of renal amino acid reabsorption. These changes in amino acid transport were connected with a significant reduction of GFR after EGF pre-treatment (0.96+/-0.10 vs. 0.62+/-0.07 ml/min x 100 g b.wt.), with a distinct increase in sodium excretion (2.98+/-0.55 vs. 4.97+/-0.71 microval/100 g b.wt. x 20 min) and with a retarded normal kidney weight gain (874+/-18 vs. 775+/-32 mg/100 g b.wt.). A simultaneous PAH load reduced amino acid reabsorption as a sign of overloading of renal tubular transport capacity, but in EGF pretreated animals the amino acid excretion was only slightly increased under these conditions.

Renal handling of amino acids in 5/6-nephrectomized rats: stimulation of renal amino acid reabsorption after treatment with triiodothyronine or dexamethasone under amino acid load

In anaesthetized adult female rats, the renal amino acid handling was measured six days after 5/6 nephrectomy (5/6NX). The distinct rise in blood urea nitrogen as well as the significant reduction in urine flow and GFR indicate an impairment of kidney function. In principle, in 5/6NX rats amino acid plasma concentrations were comparable to those of control animals with two intact kidneys, whereas the fractional excretions (FEAA) of most endogenous amino acids measured were significantly enhanced. After bolus injection of leucine or taurine (each 20 mg/100 g b.wt.) or glutamine (90 mg/ 100 g b.wt.), dissolved in 2 ml normal saline per 100 g b.wt., the FEAA of both the amino acids administered and the endogenous amino acids increased as a sign of overloaded amino acid reabsorption capacity. This effect was more pronounced in 5/6NX rats than in controls. As early as one hour after amino acid load, plasma concentrations and FEAA returned to baseline values of 5/6NX rats. A pretreatment with triiodothyronine (20 micrograms/100 g b.wt.) or dexamethasone (60 micrograms/100 g b.wt.), both given intraperitoneally once daily for 3 days, stimulated the renal amino acid transport capacity in 5/6NX rats: the increase in FEAA after amino acid load was significantly lower compared to non-pretreated animals. This stimulation could be shown for the bolus amino acids and the endogenous amino acids and was more distinct in 5/6NX rats than in controls with two intact kidneys.

Epidermal growth factor (EGF) increases the renal amino acid transport capacity in amino acid loaded rats

In anaesthetized adult female rats, the influence of epidermal growth factor (EGF) on renal amino acid handling was investigated in glutamine, arginine (both 50 mg/100 g b.wt. per hour), or alanine (90 mg/100 g b.wt. per hour) loaded animals. Continuous infusions of the three amino acids were followed by an increase in the fractional excretion (FE) of the administered amino acids as well as of the other endogenous amino acids. Under load conditions (alanine, arginine or glutamine), EGF pretreatment (8 micrograms/100 g b.wt. subcutaneously for 8 days, twice daily 8 a.m. and 4 p.m.) was followed by a stimulation of renal amino acid reabsorption. The increase in the fractional excretion of the administered amino acids was significantly lower than in non-EGF-treated rats. These changes in amino acid transport were connected with a significant reduction of GFR after EGF pretreatment (0.96 +/- 0.10 vs. 0.62 +/- 0.07 ml/min x 100 g b.wt.) and a distinct increase in sodium excretion (2.98 +/- 0.55 vs. 4.97 +/- 0.71 muval/100 g b.wt. x 20 min). After loading with p-aminohippurate (PAH; 200 mg/100 g b.wt.), PAH excretion in EGF rats was increased by about 20%, whereas urinary protein excretion was lower in EGF pretreated rats (control: 0.45 +/- 0.04 vs. EGF: 0.18 +/- 0.03 mg/100 g b.wt. x 20 min). The PAH load reduced amino acid reabsorption as a sign of overloading of renal tubular transport capacity, but in EGF pretreated animals the amino acid excretion was only slightly increased under these conditions. Furthermore, EGF pretreatment depressed normal kidney weight gain significantly (874 +/- 18 vs. 775 +/- 32 mg/100 g b.wt.). EGF can improve the renal tubular transport capacity, but, compared to well-known stimulators of renal transport like dexamethasone or triiodothyronine, its effect is only of a moderate degree.

Changes of renal taurine transport after treatment with triiodothyronine or dexamethasone in amino acid loaded rats

In adult female anaesthetized rats, the influence of triiodothyronine or dexamethasone on renal amino acid (AA) handling was investigated in taurine (45 mg/100 g b.wt.) loaded animals. Bolus injections of taurine were followed by temporary increase in fractional excretion (FE(AA)) of taurine as well of the endogenous amino acids which were not administered. Under taurine load conditions, triiodothyronine treatment (20 microg/100 g b.wt. for 3 days, i.p. once daily) was followed by a slight stimulation of the renal taurine reabsorption: the increase in FE(taurine) after taurine load was lower than in untreated rats. Dexamethasone (60 microg/100 g b.wt. for 3 days, i.p. once daily) was without significant effect on FE(taurine) in taurine loaded rats. In non taurine loaded rats there was no hormone influence at all. Similarities and differences between the effects of bolus injections of taurine, glutamine, and leucine on the FE(AA) of these three amino acids were compared in detail to further clarify the reason for the increased amino acid reabsorption capacity after pretreatment with triiodothyronine or dexamethasone.

The hepato-renal syndrome: renal amino acid transport in bile duct ligated rats (DL)--influence of treatment with triiodothyronine or dexamethasone on renal amino acid handling in amino acid loaded rats

The influence of triiodothyronine or dexamethasone on renal amino acid handling was investigated in anaesthetized, bile duct-ligated (DL) adult female rats. 3 days after DL, glomerular filtration rate (GFR) was unchanged whereas urine flow was decreased. Plasma concentrations of 5 out of 16 amino acids were significantly enhanced after DL. On the other hand, the fractional excretion (FE) of 11 out of 16 amino acids was significantly reduced as a sign of improved reabsorption capacity. Bolus injections of leucine (20 mg/100 g b.wt.), glutamine (45 mg/100 g b.wt.), or taurine (45 mg/100 g b.wt.) were followed by a temporary increase in the FE of the administered amino acids as well of the endogenous amino acids which were not administered. This phenomenon was more pronounced in DL than in control rats. Under load conditions, dexamethasone (60 microg/100 g b.wt.) or triiodothyronine (20 microg/100 g b.wt.) treatment for 3 days, i.p. once daily, was followed by a stimulation of renal amino acid reabsorption in DL rats. The increase in fractional amino acid excretion after amino acid load was significantly lower than in untreated rats. This effect was also more pronounced in DL rats.

Glutamate transport asymmetry in renal glutamine metabolism

D-Glutamate (Glu) was previously shown to block L-Glu uptake and accelerate glutaminase flux in cultured kidney cells [Welbourne, T. C., and D. Chevalier. Am. J. Physiol. 272 (Endocrinol. Metab. 35): E367-E370, 1997]. To test whether D-Glu would be taken up by the intact functioning kidney and effect the same response in vivo, male Sprague-Dawley rats were infused with D-Glu (2.6 mumol/min), and renal uptake of D- and L-Glu was determined from chemical and radiolabeled arteriovenous Glu concentration differences times renal plasma flow. The amount removed was then compared with that amount filtered to obtain the antiluminal contribution. In the controls, L-Glu uptake measured as net removal was 33% of the arterial L-Glu load and not different from that filtered, 27%; however, the unidirectional uptake was actually 58% of the arterial load, indicating that antiluminal uptake contributes at least half to the overall Glu consumption. Surprisingly, the kidneys showed a more avid removal of D-Glu, removing 73% of the arterial load, indicating uptake predominantly across the antiluminal cell surface. Furthermore, uptake of D-Glu was associated with a 55% reduction in L-Glu uptake, with the residual amount taken up equivalent to that filtered; D-Glu did not increase the excretion of the L-isomer. However, elevating plasma L-Glu concentration reduced uptake of the D-isomer, suggesting a shared antiluminal transporter. Thus there is an apparent asymmetrical distribution of the D-Glu transporter. Under these conditions, kidney cortex L-Glu content decreased 44%, whereas net glutamine (Gln) uptake increased sevenfold (170 +/- 89 to 1,311 +/- 219 nmol/min, P < 0.01) and unidirectional uptake nearly threefold (393 +/- 121 to 1,168 +/- 161 nmol/min, P < 0.05); this large Gln consumption was paralleled by an increase in ammonium production so that the ratio of production to consumption approaches 2, consistent with accelerated Gln deamidation and subsequent Glu deamination. These results point to a functional asymmetry (antiluminal vs. luminal) for Glu transporter activity, which potentially plays an important role in modulating Gln metabolism and renal function.

Localization of the high-affinity glutamate transporter EAAC1 in rat kidney

Most amino acids filtered by the glomerulus are reabsorbed in the kidney via specialized transport systems. Recently, the cDNA encoding a high-affinity glutamate transporter, EAAC1, has been isolated and shown to be expressed at high levels in the kidney. To determine the potential role of EAAC1 in renal acidic amino acid reabsorption, the distribution of EAAC1 mRNA and protein in rat kidney was examined. In situ hybridization revealed that EAAC1 mRNA is expressed predominantly in S2 and S3 segments of the proximal tubules and at low levels in the inner stripe of outer medulla and inner medulla. Polyclonal antibodies raised against the carboxy terminus of EAAC1 recognized a single band of approximately 70 kDa on Western blots of membrane protein from kidney cortex and medulla. Immunofluorescence microscopy revealed intense signals in the luminal membrane of S2 and S3 segments and weaker signals in S1 segments, descending thin limbs of long-loop nephrons, medullary thick ascending limbs, and distal convoluted tubules. These results are consistent with EAAC1 encoding the previously described apical high-affinity glutamate transporter in the kidney that mediates reabsorption of acidic amino acids in tubules beyond early proximal tubule S1 segments. Potential additional roles of EAAC1 in acid/base balance, cell volume regulation, and amino acid metabolism are discussed.

Renal amino acid transport in immature and adult rats during thallium-induced nephrotoxicity

The effect of Tl2SO4 (Tl, 2 mg/100 g b.wt.) on renal amino acid excretion and plasma amino acid composition was investigated in 10- and 55-day-old rats. Tl decreased glomerular filtration rate only in adult rats. On the other hand, the renal fractional excretion (FE) of amino acids was distinctly higher in adult rats as a sign of lower amino acid reabsorption capacity after Tl. In immature animals FE was increased only for a few amino acids. However, in both age groups Tl administration significantly decreased plasma amino acid concentrations, and was more pronounced in immature rats. The investigation of renal amino acid handling (1) confirms that Tl was more nephrotoxic in 55-day-old animals as demonstrated before using other parameters for nephrotoxicity testing and (2) showed that determination of renal amino acid handling is a suitable marker for nephrotoxicity in adult rats.

Kinetics of receptor-mediated endocytosis of albumin in cells derived from the proximal tubule of the kidney (opossum kidney cells): influence of Ca2+ and cAMP

In this study we investigated the effects of Ca2+ and cyclic adenosine monophosphate (cAMP) on the kinetic of receptor-mediated (RME) and fluid-phase (FPE) endocytosis in opossum kidney (OK) cells, derived from the proximal tubule of the kidney. We used fluorescein isothiocyanate (FITC)-labelled albumin and FITC-labelled dextran as endocytotic substrates for RME and FPE, respectively. Removal of extracellular Ca2+ led to a dramatic decrease of the apparent affinity of RME, but did not influence the maximum endocytotic uptake rate (Jmax). Reduction of extracellular Ca2+ to 1 mumol/1 had no effect. Apparent affinity of specific binding of albumin to the plasma membrane was increased to 200% of control in the absence of extracellular Ca2+, whereas maximum binding capacity was slightly decreased. FPE was not affected by removal of extracellular Ca2+. Additional removal of cytoplasmic Ca2+, using ionomycin, had no further effect on RME and did not affect FPE. Increases of cytoplasmic (using ionomycin at extracellular Ca2+ concentrations of 1 mumol/l or 1.2 mmol/l) or extracellular Ca2+ did not alter the kinetics of RME or FPE. Dibutyryl-cAMP reduced Jmax but left the apparent affinity of RME unchanged. FPE and albumin binding to the plasma membrane were not changed in the presence of cAMP. Removal of extracellular Ca2+ and addition of cAMP led to an alkalinization of endocytotic vesicles. Yet the alkalinization induced by removal of Ca2+ was significantly greater as compared to the alkalinization in the presence of cAMP. Endosomal alkalinization with bafilomycin A1 had no further effect in the absence of Ca2+, but reduced RME in the presence of cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Cationic amino acid fluxes beyond the proximal convoluted tubule of rat kidney

To investigate the fluxes of cationic amino acids beyond the proximal convolution, we micropunctured and microperfused superficial tubules of male Wistar rats in vivo et situ. In free-flow micropuncture experiments, the concentrations of endogenous L-arginine+, [Arg], and of intravenously infused L-homoarginine+, [HoArg], were determined by HPLC. Fluorescein isothiocyanate-labeled inulin was detected on-line in the same tubular fluid samples. To determine undirectional fluxes, radiolabeled Arg and inulin were (1) microperfused through short loops of Henle and (2) microinfused into different tubule segments to measure urinary recovery of the radiolabel. At a mean [Arg]plasma of 116 mumol/l, [Arg] was 9.3 mumol/l in the late proximal tubule (LPT), and 35.6 mumol/l in the early distal tubule (EDT) corresponding to fractional deliveries (FD) of 0.055 in LPT and 0.078 in EDT. Fractional urinary excretion (FE) of Arg was 0.00033 (P < 0.05 vs FDEDT). Infusion of HoArg (2.5 or 7.5 mumol/min) led to respective mean [HoArg]plasma values of 1.44 and 3.73 mmol/l, and resulted in respective FDLPT values for HoArg of 0.23 and 0.53, respective FDEDT values of 0.29 and 0.41, and finally, respective FE values for HoArg of 0.25 and 0.58. When short loops of Henle were microperfused with 1 or 50 mmol/l [14C]Arg (+[3H]inulin), fractional recovery (FR) of 14C (relative to inulin) in the EDT was 0.13 and 0.36, respectively. During microinfusion of radiolabeled Arg (1 or 50 mmol/l) and inulin into LPT, the urinary FR of the radiolabel was 0.14, or 0.59, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Age dependent different consequences of bilateral nephrectomy (NX) in rats

Adult rats survived the removal of both kidneys (NX) for about 48 hours; survival time was distinctly lower in 10- and 20-day-old rats (about 24 hrs). Some of the measured parameters indicate age dependent differences in uremic impairment. In the prefinal period after NX blood urea nitrogen concentrations were distinctly higher in adult than in young rats. After nephrectomy diminution of the concentrations of GSH and GSSG in liver tissue is more distinct in adult than in young rats. Similarly, 24 hrs after NX the content of lipid peroxides in liver tissue was higher in adult than in 10- and 20-day-old rats. Furthermore, the prolongation of bleeding time in uremic rats was more distinct in adult than in young rats. The following parameters indicate uremic impairment clearly, but age dependent differences do not exist: Increase of potassium concentration in plasma, enhanced activities of liver specific enzymes in plasma, reduced concentration of various amino acids in plasma and distinct increase in plasma concentration of taurine.

Contribution of amino acids in protective solutions to postischemic functional recovery of canine kidneys

Amino acids are known to increase glomerular filtration rate (GFR). There is also an early resumption of filtration following 2-h renal ischemic stress under protection by histidine-buffered histidine-tryptophan-ketoglutarate solution (HTK), possibly due in part to an amino acid effect. Hence, we have examined the possibility of further enhancing the postischemic GFR by adding 32 (ASP I; 4 mM Mg2+) or 36 (ASP II; 6 mM Mg2+) mM L-aspartate (asp) or 32 mM DL-aspartate (ASP III) to the HTK solution in place of chloride. After infusion of 500 ml 5% glucose, canine kidneys were protected by an 8-min perfusion with HTK (n = 5), ASP I (n = 4), ASP II (n = 5) or ASP III-solution (n = 3). The subsequent ischemia lasted for 2 h at 27-31 degrees C. During reperfusion, both GFR and filtration fraction (FF) were higher in kidneys protected by L-aspartate-containing solutions. ASP III showed no improvement against HTK. An additional preischemic intra-aortal application of HTK or ASP I solution just above the exit of the renal arteries prior to the intrinsic protective perfusion further raised the postischemic GFR. The present results suggest that L-aspartate but also histidine may have favorable amino acid effects in renal protective solutions in addition to known positive effects of histidine.

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.

Fate of intraluminal glutamine in the perfused renal tubule

To determine the fate of intraluminal glutamine and specifically the role of brush border gamma glutamyltransferase in its hydrolysis and reabsorption, proximal convoluted tubules of rabbits were isolated and perfused with an artificial ultrafiltrate containing 1 mM 14C-glutamine and 3H-PEG as a volume absorption marker. The tubules, average length 0.80 +/- 0.09 mm, were bathed in perfusate containing albumin, 6.5 percent but no glutamine. Aliquots of collectate and bathing media were monitored for total 14C counts while the distribution of radioactive 14C between glutamine and glutamate in the collectate was determined by separation on a Dowex X8 formate form ion-exchange column. After 3 ten minute control periods the perfusate was switched to one containing 1 mM AT-125 in addition to glutamine and after equilibration an additional 3 collections were obtained. Control period glutamine load averaged 16.1 +/- 2.4 pmole/min of which 35 percent was absorbed and 38 and 27 percent excreted as glutamine and glutamate respectively; of the absorbed glutamine 25 percent was metabolized. During AT-125 administration, glutamine delivery averaged 15.0 +/- 2.1 pmole/min of which 57 percent was absorbed; increased absorption occurred at the expence of intraluminal glutamate formation which fell to less than 10 percent. Thus luminal transport and gamma glutamyltransferase mediated hydrolysis appear to compete for available glutamine. Significantly, reducing intraluminal glutamine hydrolysis doubles the cellular metabolism of absorbed glutamine suggesting that extracellular conversion of glutamine to glutamate alters the metabolic fate of filtered glutamine.

Ammoniagenesis catalyzed by hippurate-activated gamma-glutamyltransferase in the lumen of the proximal tubule. A microperfusion study in rat kidney in vivo

gamma-Glutamyltransferase (gamma-GT) is located in the brushborder membrane of the proximal tubule where the catalytic site of the enzyme faces the lumen. The (phosphate-independent) glutaminase activity of gamma-GT in vitro is activated by hippurate. In order to investigate glutamine deamidation in the tubule lumen in vivo, 14C-L-glutamine-containing solutions were continuously microperfused through sections of the proximal convoluted tubule in vivo and in situ. D-aspartate and L-phenylalanine (10 mmol/l, each) were added to the perfusate in order keep the reabsorption of L-glutamine as such low and to block reabsorption of any glutamate possibly formed, respectively. Intraluminal formation of glutamate from glutamine in the absence of hippurate is small. In presence of 10 mmol/l hippurate, 5%-70% of the recovered 14C-activity was 14C-glutamate at an initial 14C-L-glutamine concentration of 1 mmol/l. The respective absolute rate (+/- SEM) of glutamate formation, i.e., 36 +/- 5 pmol X s-1 X m-1, was increased 1.4-fold at an initial L-glutamine concentration of 3 mmol/l, but dropped to one third at initially 0.3 mmol/l. A rough estimate of the apparent kinetic constants resulted in a Km of 0.58 (0.19-0.97) mmol/l and a Vmax of 56 (40-93) pmol X s-1 X m-1. Deamidation of glutamine occurred also in the absence of L-phenylalanine. Acivicin (AT 125), a gamma-GT inhibitor, completely blocked glutamate formation. Endogenous hippurate concentrations determined by free flow micropuncture and HPLC were 0.16 mmol/l in the late proximal convolution, 0.6 mmol/l in the early distal convolution, and 4.9 mmol/l in the final urine.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular specificity of the tubular resorption of "acidic" amino acids. A continuous microperfusion study in rat kidney in vivo

Single sections of superficial proximal convolutions of rat kidney were microperfused in vivo and in situ. The perfusion fluids contained radioactively labelled L- or D-aspartate, L-glutamate, L-pyroglutamate, or N-methyl-D-aspartate. L-gamma-Carboxyglutamate as well as the other amino acids were added in the unlabelled form. Results. L- and D-Aspartate (0.073 mmol X 1(-1)) are quickly resorbed at about the same rate. D-Aspartate resorption was blocked by L-aspartate (5 mmol X 1(-1)) but not by beta-alanine (5 mmol X 1(-1)). L-Aspartate resorption was inhibited by L-glutamate (2 mmol X 1(-1)) but not by D-glutamate, L-asparagine, L-phenylalanine or by succinate (2 mmol X 1(-1), each). The fast resorption of L-glutamate (0.073 mmol X 1(-1)) was blocked by D-aspartate, L-cysteate (2 mmol X 1(-1)), but not by 3-mercaptopicolinic acid (0.15 mmol X 1(-1)), L-glutamine, 2-oxoglutarate, taurine, N-methyl-L-glutamate or kainic acid (2 mmol X 1(-1), each). L-gamma-Carboxyglutamate (0.66 mmol X 1(-1)) and N-methyl-D-aspartate (2 mumol X 1(-1)) were found to be resorbed only at an extremely small rate. L-Pyroglutamate (0.076 mmol X 1(-1)) resorption was not influenced by L-glutamate (1 mmol X 1(-1)). Fractional excretion of gamma-carboxyglutamate was 7-25% (L-form) or 45-70% (D-form) at an artificially elevated plasma level of 12 mumol X 1(-1). It is concluded that L- and D-aspartate, L-glutamate, L-cysteate and, to a much smaller extent, L-gamma-carboxyglutamate, are accepted by the tubular resorption mechanism highly specific for "acidic" amino acids. N-Substitution, the amidation of the beta- or gamma-carboxyl group, or the removal of the alpha-amino moiety almost completely abolish the ability of such compounds to be resorbed via this carrier; N-methylated or gamma-carboxylated derivatives of "acidic" amino acids are not resorbed at all from the proximal tubule. The resorption of glutamate, but not of aspartate, is highly stereospecific.