The identification and characterization of an uptake system for taurine into rat lung slices

Environmental Tobacco Smoke Alters Metabolic Systems in Adult Rats

Human exposure to environmental tobacco smoke (ETS) is associated with an increased incidence of pulmonary and cardiovascular disease and possibly lung cancer. Metabolomics can reveal changes in metabolic networks in organisms under different physio-pathological conditions. Our objective was to identify spatial and temporal metabolic alterations with acute and repeated subchronic ETS exposure to understand mechanisms by which ETS exposure may cause adverse physiological and structural changes in the pulmonary and cardiovascular systems. Established and validated metabolomics assays of the lungs, hearts. and blood of young adult male rats following 1, 3, 8, and 21 days of exposure to ETS along with day-matched sham control rats (n = 8) were performed using gas chromatography time-of-flight mass spectrometry, BinBase database processing, multivariate statistical modeling, and MetaMapp biochemical mapping. A total of 489 metabolites were measured in the lung, heart, and blood, of which 142 metabolites were identified using a standardized metabolite annotation pipeline. Acute and repeated subchronic exposure to ETS was associated with significant metabolic changes in the lung related to energy metabolism, defense against reactive oxygen species, substrate uptake and transport, nucleotide metabolism, and substrates for structural components of collagen and membrane lipids. Metabolic changes were least prevalent in heart tissues but abundant in blood under repeated subchronic ETS exposure. Our analyses revealed that ETS causes alterations in metabolic networks, especially those associated with lung structure and function and found as systemic signals in the blood. The metabolic changes suggest that ETS exposure may adversely affects the mitochondrial respiratory chain, lung elasticity, membrane integrity, redox states, cell cycle, and normal metabolic and physiological functions of the lungs, even after subchronic ETS exposure.

Chloroquine, a novel inhibitor of amino acid transport by rat renal brush border membrane vesicles

Chloroquine is an antimalarial and antirheumatic lysosomotropic drug which inhibits taurine uptake into and increases efflux from cultured human lymphoblastoid cells. It inhibits taurine uptake by rat lung slices and affects the uptake and release of cystine from cystinotic fibroblasts. Speculations on its mode of action include a proton gradient effect, a non-specific alteration in membrane integrity, and membrane stabilization. In this study, the effect of chloroquine on the uptake of several amino acids by rat renal brush border membrane vesicles (BBMV) was examined. Chloroquine significantly inhibited the secondary active, NaCl-dependent component of 10µM taurine uptake at all concentrations tested, but did not change equilibrium values. Analysis of these data indicated that the inhibition was non-competitive. Taurine uptake was reduced at all osmolarities tested, but inhibition was greatest at the lowest osmolarity. Taurine efflux was not affected by chloroquine, nor was the NaCl-independent diffusional component of taurine transport. Chloroquine (1 mM) inhibited uptake of the imino acids L-proline and glycine, and the dibasic amino acid L-lysine. It inhibited the uptake of D-glucose, but not the neutralα-amino acids L-alanine or L-methionine. Uptake of the dicarboxylic amino acids, L-glutamic acid and L-aspartic acid, was slightly enhanced. With regard to amino acid uptake by BBMV, these findings may support some of the currently proposed mechanisms of the action of chloroquine but further studies are indicated to determine why it affects the initial rate of active amino acid transport.

Comparison of the effects of taurine with those of related sulfur-containing compounds on pyridoxal-induced adrenomedullary catecholamine release and glycogenolysis in the rat

Taurine (2-aminoethanesulfonic acid) is known to attenuate the release of adrenomedullary catecholamines and ensuing hepatic glycogenolysis and hyperglycemia induced by pyridoxal in the rat. Using this animal model, the present study was undertaken to assess the impact that simple structural modifications of the taurine molecule might have on its antagonistic actions against PL. Removal of the amino group (ethanesulfonic acid) or shortening the carbon chain by one methylene (2-aminomethanesulfonic acid) raised the protective actions of taurine. While N-alkylation (N-methyltaurine) or replacement of the amino group by hydroxyl (isethionic acid) had a lowering effect, substituting a sulfhydryl group (2-mercaptoethanesulfonic acid) for the 2-amino group abolished all antagonistic properties associated with taurine. The sulfinic acid analog (hypotaurine) was equipotent with taurine, but the carboxylate isostere (beta-alanine) was inactive. Propranolol, a nonspecific beta-adrenoceptor antagonist, enhanced the antiglycogenolytic effect of taurine and of all structurally related compounds capable of attenuating the outflow of adrenal catecholamines elicited by pyridoxal.

Sulphydryl modification inhibits taurine transport in human placental brush border membranes

Taurine, 2-aminoethanesulphonic acid, is a -amino acid required for mammalian development. Although the human fetus accumulates taurine in many tissues, it has limited capacity for synthesis. The majority of fetal taurine is derived from the mother via placental transfer. The objective of this study was to analyse the functional groups involved in the taurine transport system of human placental brush-border membranes. Sulphydryl modifying reagents N-ethylmaleimide (NEM) and pyridyldithioethyl-amine (PDA) caused a dose-dependent inhibition of taurine uptake by brush-border membrane vesicles. Inhibition by PDA was reversible upon reduction by dithiothreitol but not by glutathione indicating that sulphydryl group(s) are located within the bilayer. Preincubation of brush-border membranes with taurine but not with taurocholate, before exposure to NEM, protected taurine transport function. Labelling studies using NEM and chemical cross-linking indicated that a 37.5 kDa protein was protected. These results demonstrate that sulphydryls located within the membrane bilayer are important for taurine transport in human placental brush-border membranes and suggest that a 37.5 kDa protein may be associated with Na(+)-dependent regulation of the taurine transporter.

The uptake and metabolism of cystamine and taurine by isolated perfused rat and rabbit lungs

Cystamine has been reported to be taken up and metabolized to taurine by the rat lung slices. The objectives of the present study were to compare the uptake and metabolism of cystamine and taurine in isolated perfused lungs of rats and rabbits and examine the action of glutathione (GSH) on these processes. The uptake and metabolism of [14C]cystamine and [14C]taurine were studied at 20 microM concentrations each in isolated, ventilated, perfused rat and rabbit lungs. In some experiments, 1 microM GSH was included in the perfusate prior to the addition of cystamine. The perfusate and lung homogenate samples were analyzed for cystamine and its metabolites. [14C]cystamine uptake with and without GSH was 13 and 14% in rat lungs and 37 and 32% in rabbit lungs. [14C]taurine uptake was 10% in rat and 37% in rabbit lungs. The levels of radiolabeled cystamine and its metabolites were (nmol/g lung): 20.0 +/- 10.0 and 11.5 +/- 7.0 cystamine, 4.7 +/- 0.5 and 3.2 +/- 0.5 hypotaurine and 56.0 +/- 16.0 and 49.4 +/- 6.0 taurine, for rat and rabbit lungs, respectively, when perfused without GSH; and 18.0 +/- 1.0 and 2.5 +/- 0.5 cystamine, 6.6 +/- 0.5 and 18 +/- 10 hypotaurine and 60.0 +/- 12.0 and 33.6 +/- 9.0 taurine, when perfused with GSH, for rats and rabbit lungs, respectively. Taurine did not undergo any further metabolism in either of the lungs. These studies show that cystamine is taken up and metabolized to taurine via hypotaurine by both rat and rabbit lungs in a manner similar to that seen in rat lung slices. However, rat lungs have much greater capacity to metabolize cystamine to taurine than rabbit. Inclusion of GSH did not significantly alter the ability of lungs to sequester cystamine from the perfusate but the metabolism of hypotaurine to taurine was markedly decreased in rabbit lungs. Taurine was not metabolized any further. It is concluded that rat and rabbit lungs take up cystamine from the systemic circulation, metabolize it via hypotaurine to taurine, and effuse most of the latter in to the circulation.

The in vivo and in vitro protective properties of taurine

1. Taurine is a ubiquitous, free amino acid found in mammalian systems. 2. The biological functions of taurine are unclear. 3. Various in vivo data suggest that taurine has a variety of protective functions and deficiency leads to pathological changes. 4. Depletion in rats of taurine increases susceptibility to liver damage from carbon tetrachloride. 5. Susceptibility to a variety of hepatotoxicants correlates with the estimated hepatic taurine level. 6. In vitro data suggest that taurine can protect cells against toxic damage. 7. Taurine protects isolated hepatocytes against carbon tetrachloride, hydrazine and 1,4-naphthoquinone but not against allyl alcohol, alpha-naphthylisothiocyanate (ANIT) or diaminodiphenyl methane (DAPM) cytotoxicity. 8. The mechanisms of protection are unclear but may include modulation of calcium levels, osmoregulation and membrane stabilization.

Role of the lung in accumulation and metabolism of xenobiotic compounds--implications for chemically induced toxicity

The mammalian lung is exposed to and affected by many airborne and bloodborne foreign compounds. This review summarizes the role of lung in accumulation and metabolism of xenobiotics, some of which are spontaneously reactive or are metabolically activated to toxic intermediates. The specific architectural arrangement of mammalian lung favors that so-called pneumophilic drugs are filtered out of the blood and are retained within the tissue as shown in particular for amphetamine, chlorphentermine, amiodarone, imipramine, chlorpromazine, propranolol, local anaesthetics, and some miscellaneous therapeutics. There is strong evidence that intrapulmonary distribution activity and regulation of drug-metabolizing enzymes in lung is distinct from liver. This review focuses on the metabolic rate of selected compounds in lung such as 5-fluoro-2'-deoxyuridine, local anesthetics, nicotine, benzo(alpha)pyrene, ipomeanol, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone. It is widely accepted that the formation of radical species is a key event in the pneumotoxic mechanisms induced by bleomycin, paraquat, 3-methylindole, butylhydroxytoluene, or nitrofurantoin. Finally, methodological approaches to assess the capacity of lung to eliminate foreign compounds as well as biochemical features of the pulmonary tissue are evaluated briefly.

Structure-activity relationships of cysteine esters and their effects on thiol levels in rat lung in vitro

Pretreatment with cysteine esters increases cysteine (CySH) levels in rat lung and protects against the lethal effects of inhaled perfluoroisobutene in vivo. There are marked differences in the duration of protection achieved with different cysteine esters. In this study we have compared the uptake and metabolism of CySH, N-acetyl cysteine (NAc), cysteine esters and cystine esters in vitro using rat lung and liver homogenates and lung slices. Liver homogenates metabolized CySH and cysteine esters faster than lung homogenates. The half life (T1/2) of CySH in lung was 58.8 +/- 17.3 min and in liver was 14.0 +/- 1.6 min (mean +/- SEM). T1/2 of the esters in lung ranged between 6.5 and 12.1 min and in liver between 1.9 and 5.3 min. Cysteine tertiary butyl ester, which does not protect in vivo, was not hydrolysed to CySH by lung or liver homogenates. All esters increased and prolonged intracellular CySH concentrations in lung slices to a much greater extent than CySH itself. NAc did not raise intracellular CySH above that of the controls and no NAc appeared within the slice. After CySH incubation intracellular CySH was 0.9 +/- 0.1 nmol/mg wet wt at 10 min whereas after incubation with the esters it ranged between 2.60 and 3.65 nmol/mg wet wt. Cysteine cyclohexyl ester prolonged the increase of CySH the longest and cysteine methyl ester the shortest. CySH levels with cysteine cyclohexyl ester were 2.74 +/- 0.15 and 4.13 +/- 0.37 nmol/mg wet wt at 10 and 60 min, respectively, whereas with cysteine methyl ester, CySH levels were 2.60 +/- 0.5 and 1.25 +/- 0.08 nmol/mg wet wt at similar times. Cystine esters increased intracellular concentrations of both cystine and CySH. CySH concentrations ranged between 2.92 and 3.19 nmol/mg wet wt and cystine between 1.39 and 1.47 nmol/mg wet wt at 60 min. The elevation and duration of CySH in lung slices is well correlated with the duration of protection against perfluoroisobutene achieved in vivo.

Taurine prevents galactose-induced cataracts

Intact lenses from New Zealand white rabbits were incubated in tissue culture media containing either 5 mM glucose or 5 mM glucose plus 30 mM galactose. The standard media did not contain taurine. Lenses were also cultured in a third medium containing 30 mM galactose plus 0.2 mM taurine. The frequency of cataract formation was evaluated as a function of the culture media. One lens (1/10), in media containing 5 mM glucose, developed a lenticular opacification during a 72-h incubation. Lenses (12/15) incubated in 30 mM galactose, without taurine, developed cataracts; fewer lenses (2/13) exposed to 30 mM galactose plus 0.2 mM taurine developed cataracts (p < 0.005). Galactose cataracts have been associated with lens edema attributed to the osmotic stress of tissue polyol (galactitol) accumulation. The water content of the noncataractous and cataractous lenses in this experiment did not differ. Lens edema, therefore, was not thought to be important in cataract pathogenesis. Taurine, an organic osmolyte was lower (5.1 +/- 1.5 mumol/g protein) in cataractous lenses than in control lenses (10.0 +/- 1.0 mumol/g protein). Malondialdehyde, an indicator of lipid peroxidation, was higher (36.6 +/- 5.0 mumol/g protein) in lens-containing opacifications than in noncataractous lenses (10.1 +/- 1.9 mumol/gm protein) (p < 0.01). The levels of malondialdehyde suggest that lipid peroxidation was increased in the process of sugar cataractogenesis. The malondialdehyde content of all the lenses correlated inversely (r = -0.53, p < 0.01) with the coincident lens taurine levels. Taurine appears to protect the lens against the development of sugar cataracts; its inverse relationship with lens malondialdehyde suggests this is an antioxidant effect.

"Activation" of alveolar leukocytes isolated from cats fed taurine-free diets

Taurine is a ubiquitous amino sulfonic acid in mammals, present in high concentrations in tissues, including those exposed to elevated levels of oxidants. Experiments were designed to examine the consequences of taurine deficiency on production of ROI in leukocytes isolated from the lungs and blood of cats fed taurine-deficient diets. Cats were maintained on taurine-free or taurine-supplemented diets for at least 12 months at which time taurine deficiency was evident. To analyze alveolar cells, lungs were lavaged to recover lung macrophages and PMNs. Lung lavage fluid from cats contained macrophages and PMNs, although taurine deficiency was associated with a decrease in the percentage of PMNs in the lungs. This is similar to our findings in blood that taurine deficiency reduced the proportion of PMNs. Taurine measurements revealed 2.1 +/- 1.6 mumol/g wet wt of taurine in the lungs from cats fed a taurine-deficient diet versus 8.3 +/- 2.6 in lungs from cats fed a diet supplemented with taurine (n = 16). The effects of taurine deficiency on the functional activity of lung macrophages and PMNs were analyzed including the production of ROI. Alveolar leukocytes from cats fed taurine-deficient diets produced more superoxide anion in response to phorbol myristate acetate than cats fed taurine supplemented diets. Similar results were obtained using a chemiluminescence assay. Using the highly specific H2O2 indicator dye, dichlorofluorescin, and flow cytometry we found that alveolar leukocytes made more H2O2 than cells from cats fed taurine-supplemented diets. Forty-two percent of the cells from cats fed a taurine-supplemented diet expressed class II antigens. In contrast, 72% of cells from the taurine-deficient cats expressed this antigen. We hypothesize that taurine functions to prevent terminal activation and release of cytotoxic mediators by lung macrophages. Thus, a deficiency of taurine will indeed cause an activation of leukocytes, as evidenced by our data which show an increase in ROI, as well as an increase in class II antigen.