Introduction

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Recent findings from whole animal (Zalups and Barfuss, 1996), luminal membrane-vesicle (Zalups and Lash, 1997), and isolated-perfused-tubule experiments (Cannon et all, 2000) indicate that the primary luminal mechanism by which inorganic mercuric ions gain access to the cytoplasm of proximal tubular epithelial cells is by being transported as a mercuric conjugate of L-cysteine. Based on the bonding characteristics of mercuric ions (Hughes, 1957; Rabenstein, 1989; Ballatori, 1991; Zalups and Lash, 1994) and the thermodynamic stability of mercuric conjugates of sulfhydryl-containing molecules in aqueous solution (Rabenstein, 1989), it has been postulated that the primary mercuric conjugate of L-cysteine that is transported at the luminal plasma membrane is dicysteinylmercury (cysteine-Hg-cysteine). Since the mercuric conjugate dicysteinylmercury is structurally similar to the amino acid L-cystine (cysteine-cysteine), we had hypothesized previously that one or more of the amino acid transport systems that transports L-cystine also transports dicysteinylmercury. Recent findings from isolated perfused tubule experiments of Cannon et al. (2000) strongly support this hypothesis.

Since the majority of amino acid transport systems are not highly specific for any one particular amino acid, it is reasonable to postulate that mercuric conjugates of L-cysteine may also be transported by amino acid transport mechanisms that do not transport L-cystine. These include both Na⁺-dependent and Na⁺-independent transport systems. Some of the better described Na⁺-dependent transporters include system-A, system-ASC, system-B⁰, and system-B^0,+. System A is found in various organs and is involved in the transport of most dipolar amino acids (Hammerman and Sacktor, 1977). Amino acids or amino acid analogs transported by system-A include L-glycine, L-proline, and -methylamino-isobutyric acid (MeAIB) (Mircheff et al., 1982; Tate et al., 1989). Consequently, these compounds can be used as competitive inhibitors of this transporter. System-ASC is also found in various organs and is known to transport L-serine, L-alanine, and L-cysteine, all of which can serve as effective competitive inhibitors. In addition, system-ASC has been implicated in the transport of cationic or protonated anionic amino acids (Hammerman and Sacktor, 1977; Kragh-Hansen and Sheikh, 1984). System-B⁰ is similar to system-ASC in its location and transport specificity, except it has a higher affinity for the larger neutral amino acids than system-ASC. System-B^0,+, which has been localized in oocytes, blastocysts, and the luminal plasma membrane of enterocytes and renal proximal tubular epithelial cells, is involved in the transport of a wide range of amino acids, including the amino acids, L-alanine, L-valine, L-tryptophan, and L-lysine (Boerner et al., 1986).

There are at least two primary Na⁺-independent amino acid transport systems reported to be present in the basolateral (Pineda et al., 1999) and luminal membranes of the proximal tubule, respectively. These are system-L and system-b^0,+. System-L is located in a variety of organ systems and is involved in the transport of amino acids with large bulky hydrophobic side chains. Examples include L-phenylalanine, L-tryptophan, and the bicyclic amino acid analog 2-()-endoamino-bicycloheptane-2-carboxylic acid (BCH) (Hammerman and Sacktor, 1977; Deves and Boyd, 1998). System-b^0,+ also transports a wide variety of amino acids. Although system-b^0,+ is similar to system-B^0,+, it appears to discriminate against amino acids having less extensively branched R-groups attached to the -carbon. This system has a high affinity for neutral and cationic amino acids, and has been shown to transport L-lysine and L-cystine (Boerner et al., 1986).

The primary objective of the present study was to test the hypothesis that known amino acid transport mechanisms are involved in the absorptive transport of dicysteinylmercury in pars recta (S₂) segments of the rabbit proximal tubule. The strategy used in this study was to coperfuse isolated S₂ segments through the lumen with dicysteinylmercury in the presence or absence of significantly higher concentrations of various amino acids or analogs that are transported by Na⁺-dependent and Na⁺-independent amino acid transport systems. This allowed us to determine whether these compounds inhibit the luminal disappearance flux (J_D) of Hg²⁺ in the form of dicysteinylmercury. It was assumed that significant reductions in the J_D of Hg²⁺ induced by the presence of an amino acid or amino acid analog would provide evidence implicating a role of specific amino acid transport systems in the lumen-to-cell and/or cell-to-bath transport of dicysteinylmercury in proximal tubular epithelial cells.

	Materials and Methods

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Hypothesis and Experimental Design

The primary hypothesis tested in the current study is that the mercuric conjugate dicysteinylmercury is transported across the luminal membrane into the epithelial cells lining S₂ segments of the rabbit proximal tubule via one or more amino acid transport systems. To test this hypothesis, S₂ segments of the rabbit proximal tubule were perfused with a modified Ringer's solution [artificial perfusion medium (APM)] containing a 4:1 ratio of L-cysteine to Hg²⁺ to ensure the formation of the mercuric conjugate dicysteinylmercury (at a concentration of 20 µM). The luminal disappearance flux (J_D) of Hg²⁺, presumably in the form of dicysteinylmercury, was measured with and without purported inhibitory substrates of various amino acid transporters in the perfusing medium. To ensure that the potential inhibitor (amino acid or amino acid analog) was at a great enough concentration to compete with dicysteinylmercury for the transport site, a 50 to 250:1 ratio of inhibitor to dicysteinylmercury was maintained. Perfusion was carried out under two conditions, in the presence or complete absence of Na⁺ in the perfusing and bathing solutions. This allowed us to assess the role of Na⁺-dependent versus Na⁺-independent transporters in the uptake of the putative substrate dicysteinylmercury.

Of necessity, we assumed that a decrease in J_D of Hg²⁺ from the luminal fluid that occurred in the presence an amino acid or amino acid analog in the luminal fluid was the result of competitive inhibition. It is not possible to perfuse segments of the proximal tubule with a great enough range of concentrations of dicysteinylmercury to establish a change of J_max or K_m, which is required to delineate between competitive and noncompetitive inhibition. This limitation is the result of the low specific activity of ²⁰³Hg²⁺ and the fact that higher concentrations (above 100 µM) of dicysteinylmercury induce toxic effects in the perfused tubule. Consequently, we have assumed that any inhibition is competitive unless other investigators have reported noncompetitive inhibition by the particular amino acid or amino acid analog, as is the case for L-lysine.

Note: Notwithstanding the fact that mercuric ions can bind to various nucleophilic groups, the affinity constant for bonding to thiolate anions is on the order of 10¹⁵ to 10²⁰, whereas the affinity constants for bonding to oxygen- or nitrogen-containing ligands, such as carbonyl groups or amino groups, are about 10 orders of magnitude lower (Hughes, 1957). When mercuric ions and at least a 2-fold greater concentration of small thiol-containing molecules are present in aqueous solution, one can predict with high probability [based on the ¹³C NMR findings of Rabenstein (1989)] that each mercuric ion will form a thermodynamically stable linear II coordinate covalent bond with the thiol group of two molecules of the respective thiol-containing compound. Thus, addition of a 4-fold higher concentration of cysteine (relative to the concentration of inorganic mercury) ensures the formation of thermodynamically stable linear II coordinate covalent complexes between each mercuric ion and two molecules of cysteine. Moreover, the bonding characteristics of mercuric ions (Hughes, 1957; Rabenstein, 1989) predict that they would remain bonded to the thiol group of cysteine, even in the presence of low millimolar concentrations of other nonthiol-containing amino acids or amino acid analogs, which provide a substantial pool of nucleophilic functional groups for mercuric ions to bind.

Assessment of Role of Na⁺-Dependent Amino Acid Transporters

System-A. To determine whether system-A is capable of transporting dicysteinylmercury into proximal tubular cells across the luminal membrane, 3 mM L-proline or L-MeAIB were coperfused individually with 20 µM Hg²⁺ and 80 µM L-cysteine.

System-ASC and B⁰. The potential role of system-ASC and/or system-B⁰ in the luminal uptake of dicysteinylmercury was assessed in experiments where 3 mM L-serine was coperfused with 20 µM Hg²⁺ and 80 µM L-cysteine

System-B^0,+. Proximal tubular segments were perfused with 3 mM L-tryptophan, L-lysine, or L-valine with 20 µM Hg²⁺ and 80 µM L-cysteine to determine whether system-B^0,+ is capable of transporting dicysteinylmercury.

Cystine Transporters. Assessment of a specific putative, low-affinity Na⁺-dependent "L-cystine" amino acid transport system in the luminal disappearance of dicysteinylmercury was done by coperfusing 1 mM L-cystine with 20 µM Hg²⁺ and 80 µM L-cysteine.

Effect of D-Enantiomer of Cystine. To determine whether the luminal transport of dicysteinylmercury is stereospecific, S₂ segments were perfused through the lumen with 20 µM Hg²⁺, 80 µM L-cysteine, and 1 mM D-cystine. Additional control experiments were carried out in tubules perfused through the lumen with 20 µM Hg²⁺, 80 µM L-cysteine with or without 1 mM L-cystine. These experiments were carried at the end of all the other transport experiments. Consequently, additional control data were needed to maintain internal consistency.

Assessment of Role of Na⁺-Independent Amino Acid Transporters

In these experiments, Na⁺ in both perfusing and bathing solutions was replaced with N-methyl-D-glucamine.

System-L. The potential role of system-L in the luminal disappearance flux of dicysteinylmercury was assessed by coperfusing 20 µM Hg²⁺ and 80 µM L-cysteine with 3 mM L-histidine, 3 mM L-tryptophan, 3 mM cycloleucine, 5 mM L-phenylalanine, or 5 mM BCH.

System-b^0,+. Coperfusion of 20 µM Hg²⁺ and 80 µM L-cysteine with either 3 mM L-lysine or L-cystine was carried out to assess the potential role of system-b^0,+ in transporting dicysteinylmercury from the luminal fluid into the epithelial cells lining the pars recta of the proximal tubule.

Animals. Female, New Zealand White, specific pathogen-free rabbits (Myrtle's Rabbitry, Inc. Farm, Thompson Station, TN) were used in the present study. Prior to experimentation, the rabbits were maintained on regular rabbit chow and given water ad libitum. All experiments were conducted according to the National Institutes of Health Guide for the Care and Use of Laboratory Animals.

Composition of Perfusing and Bathing Solutions. In all experiments, the perfusing and bathing solutions consisted of simple electrolyte solutions. The perfusing solution (APM) contained the following: 140 mM Na⁺, 140 mM Cl, 5 mM K⁺, 2.5 mM Ca²⁺, 1.2 mM Mg²⁺, 1.2 mM SO₄², 2 mM HPO₄²/H₂PO₄, 1 mM D-glucose, and 0.5 mM glutamine. pH was adjusted to 7.4 with 1 M NaOH. To evaluate cytotoxicity of inorganic mercury, we placed the vital dye FD&C green No.3 (809 Da) in the perfusate at a concentration of 250 nM. Final osmolality was adjusted to 290 mOsmol kg¹ H₂O, with doubly distilled and deionized water. L-[³H]glucose (50 mCi ml¹, 58.8 mCi mg¹) (34 µM) was used as a volume marker in all experiments and was added to the perfusing solution only. The concentration of Hg²⁺ (see note below) in the perfusing solution was 20 µM in all experiments. All perfusing solutions containing Hg²⁺ also contained radioactive mercuric ions (²⁰³Hg²⁺, 33.6 mCi mg¹).

Dissection Solution. The tubular dissection solution was a sucrose/phosphate buffer: 125 mM sucrose, 13.3 mM anhydrous monosodium dihydrogen phosphate, and 56 mM anhydrous disodium monohydrogen phosphate. The pH was adjusted to 7.4 with NaOH or HCl. The osmolarity was adjusted to 290 mOsm/kg of water by adding water or NaCl.

Chemicals. All other chemicals were obtained from Sigma (St. Louis, MO), unless otherwise noted. The isotope ²⁰³Hg²⁺, in the form of mercuric chloride, was obtained from Buffalo Materials Corporation (Buffalo, NY). L-[³H]Glucose (14.6 Ci mmol¹, 1 mCi ml¹) was obtained from PerkinElmer Instruments (Shelton, CT).

Obtaining, Identifying, and Perfusing S₂ Segments of Proximal Tubule. The methods used for obtaining, identifying, and perfusing each tubule on the day of experimentation were the same as those we have described previously (Zalups et al., 1991; Zalups and Barfuss, 1996).

Collecting Samples. To measure the J_D [fmol min¹ (mm tubule length)¹] of Hg²⁺ from the luminal compartment, three samples (collectates) of luminal fluid exiting a perfused tubular segment were collected from each perfused tubule. The time required to fill the constant volume pipette (60 nl) was used to calculate the collection rate (nl min¹). Each collectate sample was added to 8 ml of scintillation fluid (Opti-Fluor; Packard, Meriden, CT). To measure the rate of lumen-to-bath leak [fmol min¹ (mm tubule length)¹] of the volume marker L-[³H]glucose, the aspirated bathing solution from the flow-through bath (0.3 ml) was collected at the rate of 0.25 ml min¹ into 20-ml scintillation vials at 5-min intervals. To each vial, 8 ml of scintillation fluid (Opti-Fluor; Packard) was added. The collectate and bathing fluid samples were then counted in a Beckman 5800 scintillation counter to quantify of the amount ³H and ²⁰³Hg present in each sample using standard isotopic methods.

Assessment of Cellular and Tubular Pathology. During each experiment, the perfused tubule was observed microscopically during the entire perfusion process to detect any pathology. Typical pathological changes detected in S₂ segments of the proximal tubule exposed to inorganic mercury include cellular swelling, cytoplasmic vacuolization, shedding of brush-border membrane (blebbing) of the apical plasma membrane, and cellular uptake of the vital dye FD&C green.

Calculations. The calculations used to determine rates of luminal disappearance flux (J_D) of Hg²⁺ and lumen-to-bath leak of the volume maker (L-[³H]glucose) are the same as those described previously (Zalups and Barfuss, 1996).

Statistical Analysis. A minimum of four tubules was perfused under each experimental condition. Moreover, data for each parameter assessed were obtained from tubular segments isolated from at least two animals. In each perfused tubule, three or more measurements of the J_D of Hg²⁺ were averaged. The mean values for J_D from each tubule were used to compute the overall mean and standard error under each experimental condition. Data were first subjected to the Kolmogorov-Smirnov test for normality and then the Levene's test of homogeneity of variance. If both tests were not statistically significant (at P < 0.05) a one-way analysis of variance and Tukey's honestly significant difference post hoc test was performed with a significance level set at P < 0.05. If a set of data failed the normality test or the test for homogeneity of variance, the nonparametric Kruskal-Wallis analysis of variance by ranks, followed by a Mann-Whitney U test analysis was performed with the level of significance set at P < 0.05.

	Results

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Effect of Various Amino Acids on Na⁺-Dependent Transport of Hg²⁺

Control data for the experiments designed to assess inhibition of the J_D of dicysteinylmercury under Na⁺-dependent conditions were obtained from tubular segments perfused with the APM containing 20 µM Hg²⁺ and 80 µM L-cysteine. During 30 min of perfusion, the J_D of Hg²⁺ averaged approximately 102 fmol min¹ (mm tubular length)¹(Fig. 1).

View larger version (22K): [in this window] [in a new window]   Fig. 1.   Luminal disappearance flux (JD) of Hg2+in isolated S2 segments of the rabbit proximal tubule perfused with 20 µM Hg2+ and 80 µM L-cysteine, with or without 3.0 mM L-proline, 3.0 mM L-valine, or 5.0 mM MeAIB in the perfusing solution. All tubules were perfused and bathed with solutions containing 140 mM Na+. Each column represents the mean ± S.E. obtained from at least four perfused tubular S2 segments. *, significantly different (P < 0.05) from the mean value obtained from the control tubules perfused with 20 µM Hg2+ and 80 µM cysteine without any additional amino acids or amino acid analogs in the perfusing solution.

To assess whether amino acids transported by systems-A and/or B^0,+ are involved in the luminal transport of dicysteinylmercury, 3 mM L-proline, 3 mM L-valine, or 5 mM MeAIB was added to the control perfusate. The presence of each of these amino acids in the perfusate did not have a significant effect on the J_D of Hg²⁺(Fig. 1). Additionally, L-proline and L-valine had no effect on the concentration of Hg²⁺ in the collectate, while the presence of MeAIB in the perfusate caused a 250% increase in the concentration of Hg²⁺ in the collectate (Table 1).

The mean J_D of Hg²⁺ (in the form of dicysteinylmercury) was reduced markedly when either 3 mM L-lysine or 3 mM L-serine was present in the control perfusion solution (Fig. 2). More specifically, addition of 3 mM L-lysine or 3 mM L-serine to the perfusate caused an approximate 52 or 53% decrease in the J_D of Hg²⁺, respectively. The concentration of Hg²⁺ in the samples of collectate increased by 290 and 345%, respectively, when L-lysine or L-serine were coperfused with dicysteinylmercury (Table 1). These particular experiments were designed to determine whether the amino acids transported by systems-ASC, B⁰, B^0,+, and/or b^0,+ are involved with the luminal transport of dicysteinylmercury.

View larger version (18K): [in this window] [in a new window]   Fig. 2.   Luminal disappearance flux (JD) of Hg2+in isolated S2 segments of the rabbit proximal tubule perfused with 20 µM Hg2+ and 80 µM L-cysteine, with or without 3.0 mM L-lysine or 3.0 mM L-serine in the perfusing solution. All tubules were perfused and bathed with solutions containing 140 mM Na+. Each column represents the mean ± S.E. obtained from at least four perfused tubular S2 segments. *, significantly different (P < 0.05) from the mean value obtained from the control tubules perfused with 20 µM Hg2+ and 80 µM L-cysteine without any additional amino acids or amino acid analogs in the perfusing solution.

Relative to control values, significant decreases in the J_D of Hg²⁺ were detected in S₂ segments perfused through the lumen with 5 mM BCH (38% decrease), 5 mM cycloleucine (57% decrease), 3 mM L-tryptophan (62% decrease), or 5 mM L-phenylalanine (65% decrease) (Fig. 3). Among these conditions evaluated, the J_D of Hg²⁺ was lowest in the tubules perfused with L-phenylalanine and was highest in tubules perfused with BCH. The percentage of change in the concentration of Hg²⁺ in the samples of collectate increased by 240, 330, 360, and 470%, respectively, when BCH, cycloleucine, L-tryptophan, or L-phenylalanine was present in the perfusate (Table 1). The effect of these amino acids on the J_D of Hg²⁺ was examined to determine whether the amino acids transported System-L participates in the absorptive transport of dicysteinylmercury.

View larger version (23K): [in this window] [in a new window]   Fig. 3.   Luminal disappearance flux (JD) of Hg2+in isolated S2 segments of the rabbit proximal tubule perfused with 20 µM Hg2+ and 80 µM L-cysteine, with or without 5.0 mM BCH, 5.0 mM cycloleucine, 3.0 mM L-tryptophan, or 3.0 mM L-phenylalanine in the perfusing solution. All tubules were perfused and bathed with solutions containing 140 mM Na+. Each column represents the mean ± S.E. obtained from at least four perfused tubular S2 segments. *, significantly different (P < 0.05) from the mean value obtained from the control tubules perfused with 20 µM Hg2+ and 80 µM cysteine without any additional amino acids or amino acid analogs in the perfusing solution.

Among the experiments in which both perfusing and bathing solutions contained 140 mM Na⁺, the greatest effect on the J_D of Hg²⁺ was detected between the group of tubules perfused with 1 mM L-cystine and the group of control tubules (Fig. 4). The J_D of Hg²⁺ in the tubules perfused with 1 mM L-cystine averaged approximately 25 fmol min¹ (mm tubular length)¹, which is 76% lower than the average J_D of Hg²⁺ in the control tubules. In addition, the concentration of Hg²⁺ in the samples of collectate from the tubules perfused with 1 mM L-cystine was 470% greater than that in the samples of collectate from the control tubules (Table 1). These experiments were designed to determine whether similarity in molecular homology between dicysteinylmercury and the amino acid cystine plays an important role in the transport of dicysteinylmercury by transport-proteins involved in proximal tubular absorption of cystine.

View larger version (21K): [in this window] [in a new window]   Fig. 4.   Luminal disappearance flux (JD) of Hg2+in isolated S2 segments of the rabbit proximal tubule perfused with 20 µM Hg2+ and 80 µM L-cysteine, with or without 1.0 mM L-cystine in the perfusing solution. All tubules were perfused and bathed with solutions containing 140 mM Na+. Each column represents the mean ± S.E. obtained from at least four perfused tubular S2 segments. *, significantly different (P < 0.05) from the mean value obtained from the control tubules perfused with 20 µM Hg2+ and 80 µM L-cysteine without any additional amino acids or amino acid analogs in the perfusing solution.

To determine whether the effect on L-cystine on the luminal uptake of dicysteinylmercury is stereospecific, the effect of D-cystine on the luminal uptake of dicysteinylmercury was examined. With 140 mM Na⁺ present in the luminal and basolateral fluid compartments, coperfusing S₂ segments with 20 µM Hg²⁺ and 80 µM L-cysteine with 1 mM D-cystine did not result in a significant reduction in the J_D of Hg²⁺ (in the form of dicysteinylmercury). The transport data obtained from the separate set of control tubules (matched for these experiments) confirmed that 1 mM L-cystine inhibited the J_D of dicysteinylmercury to a greater extent than any other amino acid (by 85% in these additional tubules). Thus, two independent series of experiments indicate that L-cystine is a very effective inhibitor of the luminal uptake of inorganic mercury (in the form of dicysteinylmercury) in proximal tubular segments.

Cellular Toxicity with Na⁺ Being Present in Both Perfusing and Bathing Solutions. In all of the aforementioned experiments, no visual evidence of acute cellular toxicity was detected. However, lumen-to-bath leak of L-[³H]glucose was slightly but significantly greater (2-3 times) in four groups (control, MeAIB, BCH, and L-phenylalanine) than that in normal tubules not perfused with Hg²⁺ [2.5 fmol min¹ (mm tubule length)¹] (Barfuss and Schafer, 1981). By contrast, the presence of L-valine, L-proline, L-lysine, L-serine, cycloleucine, L-tryptophan, or L-cystine in the luminal compartment had no effect on the rate of leak of L-[³H]glucose (Table 1).

Effect of Various Amino Acids on Na⁺-Independent Transport of Hg²⁺

Replacement of Na⁺ in both the perfusing and bathing solutions with N-methyl-D-glucamine caused a significant reduction (40%) in J_D of Hg²⁺(Fig. 5). The concentration of Hg²⁺ in the samples of collectate increased by 260%, compared with the concentration of Hg²⁺ in the samples of collectate from the tubules perfused in the presence of Na⁺ (control group in Table 1).

View larger version (22K): [in this window] [in a new window]   Fig. 5.   Effect of replacing Na+ with N-methyl-D-glucamine, in both bathing and perfusing solutions, on the luminal disappearance flux (JD) of Hg2+in isolated S2 segments of the rabbit proximal tubule perfused with 20 µM Hg2+ and 80 µM L-cysteine. Each column represents the mean ± S.E. obtained from at least four perfused tubular S2 segments. *, significantly different (P < 0.05) from the mean value obtained from the control tubules perfused with 20 µM Hg2+ and 80 µM L-cysteine with Na+-containing perfusion and bathing solutions.

Effects of L-Lysine. After Na⁺ had been replaced with N-methyl-D-glucamine, in both the perfusing and bathing solutions, no significant difference could be detected in the J_D of Hg²⁺ between S₂ segments perfused through the lumen with 3.0 mM L-lysine and the corresponding group of control S₂ segments (Fig. 6). The concentration of Hg²⁺ in the samples of collectate from these two groups was also not significantly different. These experiments were preformed to assess the extent the amino acid transport systems L, y⁺, and b^0,+ play in the luminal transport of dicysteinylmercury.

View larger version (22K): [in this window] [in a new window]   Fig. 6.   Luminal disappearance flux (JD) of Hg2+ in isolated S2 segments of the rabbit proximal tubule perfused with 20 µM Hg2+ and 80 µM L-cysteine, with or without 3.0 mM lysine, 5.0 mM BCH, 3.0 mM L-tryptophan, or 3.0 mM L-histidine in the perfusing solution under Na+-free conditions. All tubules were perfused and bathed with solutions containing 140 mM N-methyl-D-glucamine. Each column represents the mean ± S.E. obtained from at least four perfused tubular S2 segments. *, significantly different (P < 0.05) from the mean value obtained from the control tubules perfused with 20 µM Hg2+ and 80 µM L-cysteine without any additional amino acids or amino acid analogs in the perfusing solution.

Cellular Toxicity in Na⁺-Free Conditions. Much like when 140 mM Na⁺ was present in the perfusing and bathing solutions, no visible signs of toxicity were observed during the experiments in which L-lysine, BCH, L-tryptophan, or L-histidine was used as a competitive inhibitor in the presence of 140 mM N-methyl-D-glucamine. There were, however, significant increases in the lumen-to-bath flux of the leak indicator (L-[³H]glucose) with the use of some of the amino acids. The rate of leak was not elevated significantly when L-lysine, BCH, or L-tryptophan was present in the perfusate containing N-methyl-D-glucamine, when compared with that detected in the Na⁺-free control experiments. Only when L-histidine was present in the perfusate, was the rate of leak of L-[³H]glucose increased (3 times).

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

Several lines of in vivo and in vitro evidence indicate that the luminal absorptive transport of Hg²⁺ in the renal proximal tubule is linked to the activity of -glutamyltransferase and cysteinylglycinase (Berndt et al., 1985; Tanaka et al., 1990; Tanaka-Kagawa et al., 1993; deCeaurriz et al., 1994; Zalups, 1995; Cannon et al., 2000). In the kidney, these enzymes are found almost exclusively in the luminal plasma membrane of proximal tubular epithelial cells. Consequently, the actual luminal uptake of Hg²⁺ in the proximal tubule appears to involve the transport of some cleavage product(s) formed by the actions of these enzymes. Because the activities of both -glutamyltransferase and cysteinylglycinase are very high in the luminal compartment of proximal tubular segments, the most likely primary species of Hg²⁺ transported at the luminal membrane would appear to be a mercuric conjugate of L-cysteine (dicysteinylmercury).

Substantive in vivo and in vitro data support the hypothesis that dicysteinylmercury (cysteine-Hg-cysteine) is one of the primary forms of Hg²⁺ taken up by proximal tubular epithelial cells (Zalups and Barfuss, 1996; Zalups and Lash, 1997). By far, the most convincing evidence supporting the luminal transport of a mercuric conjugate of L-cysteine comes from the isolated perfused tubule studies of Cannon et al. (2000). These investigators demonstrated in isolated perfused S₂ proximal tubular segments that the rates of luminal uptake (luminal disappearance flux, J_D) of Hg²⁺ in the form of dicysteinylmercury were approximately 2-fold greater than the rates of luminal uptake of Hg²⁺ in the form of mercuric conjugates of either glutathione or cysteinylglycine. They also demonstrated that addition of millimolar concentrations of L-lysine or cycloleucine to a perfusate containing 20 µM Hg²⁺ and 80 µM L-cysteine (the same as in the present study) caused an approximate 50% reduction in the net rate of absorptive transport of Hg²⁺. Collectively, these findings led the investigators to postulate that luminal uptake of Hg²⁺, presumably in the form of dicysteinylmercury, occurs through one or more transporters shared by L-cystine and the dibasic amino acid L-lysine.

Consistent with the isolated perfused tubule-findings of Cannon et al. (2000) are the results of the Richardson et al. (1975). These researchers demonstrated that the uptake of Hg²⁺ in renal cortical slices was reduced when the slices were exposed to mercuric conjugates of L-cysteine and an excess of the amino acid L-histidine or L-lysine. In addition, Wei et al. (1999) have demonstrated in isolated fragments of the rabbit proximal tubule that proximal tubular cells may sequester several mercuric conjugates, including mercuric conjugates of cysteine, by a neutral amino acid transport mechanism. It should be kept in mind, however, that uptake in tissue slices and nonperfused isolated tubular fragments probably results primarily via transport mechanisms located on the basolateral membrane. Other than these findings, there has been a paucity of data dealing with potential mechanisms involved in the renal tubular uptake and transport of Hg²⁺ in the kidney. This deficit served as the primary impetus for the present study. The experiments in the present study were designed specifically to investigate the potential role of some of the better characterized amino acid transport systems in the disappearance of the mercuric conjugate dicysteinylmercury from the luminal fluid of the S₂ segment of the rabbit proximal tubule. The Na⁺-dependent amino acid transporters targeted were systems A, ASC, B⁰, and B^0,+, while the Na⁺-independent amino acid transporters targeted were systems L and b^0,+.

Na⁺-Dependent Transport of Dicysteinylmercury. According to our present findings, system-A does not appear to be involved significantly in the luminal uptake of dicysteinylmercury in the proximal tubule. This conclusion is based on the findings that the luminal uptake of 20 µM dicysteinylmercury was not inhibited significantly by millimolar concentrations of L-proline or MeAIB, which are established substrates for the system-A amino acid transporter (Fig. 1) (Hammerman and Sacktor, 1977).

1

1

1

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Na⁺-Independent Transport of Dicysteinylmercury. Findings from the current study show clearly that the luminal transport of dicysteinylmercury uses both Na⁺-dependent and Na⁺-independent transport mechanisms to absorb this mercuric conjugate from the luminal fluid. Primary support for this conclusion comes from the data showing that an approximate 40% reduction in the J_D of Hg²⁺ occurs in S₂ segments when Na⁺ in both perfusing and bathing solutions is replaced with N-methyl-D-glucamine (Fig. 5).

Potential Nonspecific Effects. One might be led to speculate that some of the decreases in J_D detected with the use of millimolar concentrations of amino acids could be due, in part, to competitive binding of mercuric ions to the nucleophilic functional groups (such as carboxyl, amide, and amine groups) present on the amino acids. However, this is very unlikely based on the tremendously strong and thermodynamically stable bond formed between mercuric ions and the sulfur atom of the thiol group of cysteine. To put some perspective on this issue, the reported affinity (constant) for mercuric ions bonding to a thiolate anion is on the order of 10¹⁰ times greater than that for mercury bonding to any other nucleophilic group (Hughes, 1957).