Net interorgan transport of L-glutamate in rats occurs via the plasma, not via erythrocytes

Amino Acid Nutrition and Reproductive Performance in Ruminants

Amino acids (AAs) are essential for the survival, growth and development of ruminant conceptuses. Most of the dietary AAs (including L-arginine, L-lysine, L-methionine and L-glutamine) are extensively catabolized by the ruminal microbes of ruminants to synthesize AAs and microbial proteins (the major source of AAs utilized by cells in ruminant species) in the presence of sufficient carbohydrates (mainly cellulose and hemicellulose), nitrogen, and sulfur. Results of recent studies indicate that the ruminal microbes of adult steers and sheep do not degrade extracellular L-citrulline and have a limited ability to metabolize extracellular L-glutamate due to little or no uptake by the cells. Although traditional research in ruminant protein nutrition has focused on AAs (e.g., lysine and methionine for lactating cows) that are not synthesized by eukaryotic cells, there is growing interest in the nutritional and physiological roles of AAs (e.g., L-arginine, L-citrulline, L-glutamine and L-glutamate) in gestating ruminants (e.g., cattle, sheep and goats) and lactating dairy cows. Results of recent studies show that intravenous administration of L-arginine to underfed, overweight or prolific ewes enhances fetal growth, the development of brown fat in fetuses, and the survival of neonatal lambs. Likewise, dietary supplementation with either rumen-protected L-arginine or unprotected L-citrulline to gestating sheep or beef cattle improved embryonic survival. Because dietary L-citrulline and L-glutamate are not degraded by ruminal microbes, addition of these two amino acids may be a new useful, cost-effective method for improving the reproductive efficiency of ruminants.

Dietary glutamate and the brain: In the footprints of a Jekyll and Hyde molecule

Glutamate is a crucial neurotransmitter of the mammalian central nervous system, a molecular component of our diet, and a popular food-additive. However, for decades, concerns have been raised about the issue of glutamate's safety as a food additive; especially, with regards to its ability (or otherwise) to cross the blood-brain barrier, cause excitotoxicity, or lead to neuron death. Results of animal studies following glutamate administration via different routes suggest that an array of effects can be observed. While some of the changes appear deleterious, some are not fully-understood, and the impact of others might even be beneficial. These observations suggest that with regards to the mammalian brain, exogenous glutamate might exert a double-sided effect, and in essence be a two-faced molecule whose effects may be dependent on several factors. This review draws from the research experiences of the authors and other researchers regarding the effects of exogenous glutamate on the brain of rodents. We also highlight the possible implications of such effects on the brain, in health and disease. Finally, we deduce that beyond the culinary effects of exogenous glutamate, there is the possibility of a beneficial role in the understanding and management of brain disorders.

Ruminal microbes of adult sheep do not degrade extracellular l-citrulline

This study determined whether extracellular citrulline is degraded by ruminal bacteria of sheep. In the first experiment, whole rumen fluid (3 mL) from six adult Suffolk sheep was incubated at 37 °C with 5 mM l-glutamine (Gln), l-glutamate (Glu), l-arginine (Arg), or l-citrulline (Cit) for 0, 0.5, 1, and 2 h or with 0, 0.5, 2, or 5 mM Gln, Glu, Arg, or Cit for 2 h. An aliquot (50 µL) of the incubation solution was collected at the predetermined time points for amino acids (AA) analyses. Results showed extensive hydrolysis of Gln into Glu and ammonia, of Arg into l-ornithine and l-proline, but little or no degradation of extracellular Cit or Glu by ruminal microbes. In the second experiment, six adult Suffolk sheep were individually fed each of three separate supplements (8 g Gln , Cit, or urea) on three separate days along with regular feed (800 g/animal). Blood (2 mL) was sampled from the jugular vein prior to feeding (time 0) and at 0.5, 1, 2, and 4 h after consuming the supplement. Plasma was analyzed for AA, glucose, ammonia, and urea. The concentrations of Cit in the plasma of sheep consuming this AA increased (P < 0.001) by 117% at 4 h and those of Arg increased by 23% at 4 h, compared with the baseline values. Urea or Gln feeding did not affect (P > 0.05) the concentrations of Cit or Arg in plasma. These results indicate that Cit is not metabolized by ruminal microbes of sheep and is, therefore, absorbed as such by the small intestine and used for the synthesis of Arg by extrahepatic tissues.

Hypoxic pulmonary vasoconstriction in isolated rat pulmonary arteries is not inhibited by antagonists of H2 S-synthesizing pathways

An increase in the H₂S (hydrogen sulphide, hereafter sulphide) concentration in pulmonary artery smooth muscle cells (PASMCs) has been proposed to mediate hypoxic pulmonary vasoconstriction (HPV). We evaluated this hypothesis in isolated rat intrapulmonary arteries (IPAs) by examining the effects of the sulphide precursor cysteine and sulphide-synthesis blockers on HPV and also on normoxic pulmonary vasoconstriction (NPV) stimulated by prostaglandin F2α (PGF_2α) and by the drug LY83583, which causes contraction in IPAs by increasing cellular reactive oxygen species levels. Experiments with several blockers of cystathionine γ-lyase (CSE), the enzyme responsible for sulphide synthesis in the vasculature, demonstrated that propargylglycine (PAG, 1 mm) had little or no effect on the NPV caused by PGF_2α or LY83583. Conversely, other CSE antagonists tested, aminooxyacetic acid (AOAA, 100 μm), β-cyanoalanine (BCA, 500 μm) and hydroxylamine (HA, 100 μm), altered the NPV to PGF_2α (BCA increased, HA inhibited) and/or LY83583 (BCA increased, AOAA and HA inhibited). Preincubating IPAs in physiological saline solution (PSS) containing 1 mm cysteine increased the amplitude of the NPV to PGF2^α by ∼50%, and had a similar effect on HPV elicited by hypoxic challenge with 0% O₂. The enhancement of both responses by cysteine was abolished by pretreatment with 1 mm PAG. Measurements carried out with an amperometric electrode demonstrated that incubation with 1 mm cysteine under anoxic conditions (to minimize sulphide oxidation) greatly potentiated the release of sulphide from pieces of rat liver and that this release was strongly antagonized by PAG, indicating that at this concentration PAG could enter cells intact and antagonize CSE. PAG at 1 mm had no effect on HPV recorded in control PSS, or in PSS supplemented with physiological concentrations of cysteine (10 μm), cystine (50 μm) and glutamate (100 μm) in order to prevent the possible depletion of intracellular cysteine during experiments. Application of a combination of 1 mm cysteine and 1 mm α-ketoglutarate to promote sulphide synthesis via the cysteine aminotransferase/mercaptopyruvate sulphurtransferase (CAT/MST) pathway caused an increase in HPV similar to that observed for cysteine. This was partially blocked by the CAT antagonist aspartate (1 mm) and also by PAG. However, HPV was not increased by 1 mm α-ketoglutarate alone, and HPV in the absence of α-ketoglutarate and cysteine was not attenuated by aspartate. Pretreatment of IPAs with dithiothreitol (DTT, 1 mm), proposed to promote the conversion of mitochondrial thiosulphate to sulphide, did not increase the release of sulphide from pieces of rat liver in either the presence or the absence of 1 mm cysteine, and virtually abolished HPV. The results provide evidence that the sulphide precursor cysteine can promote both NPV and HPV in rat IPA by generating sulphide via a PAG-sensitive pathway, presumably CSE. However, HPV evoked under control conditions was unaffected by the blockade of CSE. Moreover, HPV was not affected by the CAT antagonist aspartate and was blocked rather than enhanced by DTT. The data therefore indicate that sulphide generated by CSE or CAT/MST or from thiosulphate is unlikely to contribute to O₂ sensing during HPV in these arteries.

Skeletal muscle protein synthesis and the abundance of the mRNA translation initiation repressor PDCD4 are inversely regulated by fasting and refeeding in rats

Optimal skeletal muscle mass is vital to human health, because defects in muscle protein metabolism underlie or exacerbate human diseases. The mammalian target of rapamycin complex 1 is critical in the regulation of mRNA translation and protein synthesis. These functions are mediated in part by the ribosomal protein S6 kinase 1 (S6K1) through mechanisms that are poorly understood. The tumor suppressor programmed cell death 4 (PDCD4) has been identified as a novel substrate of S6K1. Here, we examined 1) the expression of PDCD4 in skeletal muscle and 2) its regulation by feed deprivation (FD) and refeeding. Male rats (~100 g; n = 6) were subjected to FD for 48 h; some rats were refed for 2 h. FD suppressed muscle fractional rates of protein synthesis and Ser(67) phosphorylation of PDCD4 (-50%) but increased PDCD4 abundance (P < 0.05); refeeding reversed these changes (P < 0.05). Consistent with these effects being regulated by S6K1, activation of this kinase was suppressed by FD (-91%, P < 0.05) but was increased by refeeding. Gavaging rats subjected to FD with a mixture of amino acids partially restored muscle fractional rates of protein synthesis and reduced PDCD4 abundance relative to FD. Finally, when myoblasts were grown in amino acid- and serum-free medium, phenylalanine incorporation into proteins in cells depleted of PDCD4 more than doubled the values in cells with a normal level of PDCD4 (P < 0.0001). Thus feeding stimulates fractional protein synthesis in skeletal muscle in parallel with the reduction of the abundance of this mRNA translation inhibitor.

A flow-cytometric method to investigate glutamate-receptor-sensitivity in whole blood platelets - results from healthy controls and patients with schizophrenia

Hypofunction of glutamate receptors may contribute to the symptoms of schizophrenia. Human platelets express glutamate receptors and can serve as peripheral surrogate model for neuronal cells. Aim of this study was to establish a fast and sensitive flow-cytometric method to determine the glutamate-dependent kinetics of intracellular calcium ([Ca++]i) mobilization in platelets of schizophrenic patients. Glutamate stimulated [Ca++]i response was measured with a flow-cytometer in anti-CD-41a-labelled whole blood platelets of treated schizophrenic patients (n=18) and controls (n=18). In two control experiments the NMDA-receptor antagonist MK-801 and the dopamine antagonist amisulpride, respectively, were added to probes from healthy subjects. Stimulation with glutamate led dose-dependently to a mobilization of [Ca++]i in both healthy controls and patients. This effect was significantly reduced in patients. In vitro NMDA-antagonism inhibited the glutamate response, whereas dopamine-antagonism had no effect. Our flow-cytometric method allows to measure glutamate-receptor mediated [Ca++]i response in whole blood platelets, without requiring platelet rich preparations. The reduced glutamate-response in the patients was not explained by a direct inhibitory treatment effect. However, further studies with drug naive patients will be necessary to find out whether or not the observed hypoglutamergic function of platelets is endogenous to the disorder.

Acute, transient hemorrhagic hypotension does not aggravate structural damage or neurologic motor deficits but delays the long-term cognitive recovery following mild to moderate traumatic brain injury

OBJECTIVES

Posttraumatic hypotension is believed to increase morbidity and mortality in traumatically brain-injured patients. Using a clinically relevant model of combined traumatic brain injury with superimposed hemorrhagic hypotension in rats, the present study evaluated whether a reduction in mean arterial blood pressure aggravates regional brain edema formation, regional cell death, and neurologic motor/cognitive deficits associated with traumatic brain injury.

DESIGN

Experimental prospective, randomized study in rodents.

SETTING

Experimental laboratory at a university hospital.

SUBJECTS

One hundred nineteen male Sprague-Dawley rats weighing 350-385 g.

INTERVENTIONS

Experimental traumatic brain injury of mild to moderate severity was induced using the lateral fluid percussion brain injury model in anesthetized rats (n = 89). Following traumatic brain injury, in surviving animals one group of animals was subjected to pressure-controlled hemorrhagic hypotension, maintaining the mean arterial blood pressure at 50-60 mm Hg for 30 mins (n = 47). The animals were subsequently either resuscitated with lactated Ringer's solution (three times shed blood volume, n = 18) or left uncompensated (n = 29). Other groups of animals included those with isolated traumatic brain injury (n = 34), those with isolated hemorrhagic hypotension (n = 8), and sham-injured control animals receiving anesthesia and surgery alone (n = 22).

MEASUREMENTS AND MAIN RESULTS

The withdrawal of 6-7 mL of arterial blood significantly reduced mean arterial blood pressure by 50% without decreasing arterial oxygen saturation or Pao2. Brain injury induced significant cerebral edema (p < .001) in vulnerable brain regions and cortical tissue loss (p < .01) compared with sham-injured animals. Neither regional brain edema formation at 24 hrs postinjury nor the extent of cortical tissue loss assessed at 7 days postinjury was significantly aggravated by superimposed hemorrhagic hypotension. Brain injury-induced neurologic deficits persisted up to 20 wks after injury and were also not aggravated by the hemorrhagic hypotension. Cognitive dysfunction persisted for up to 16 wks postinjury. The superimposition of hemorrhagic hypotension significantly delayed the time course of cognitive recovery.

CONCLUSIONS

A single, acute hypotensive event lasting 30 mins did not aggravate the short- and long-term structural and motor deficits but delayed the speed of recovery of cognitive function associated with experimental traumatic brain injury.

Monosodium glutamate in standard and high-fiber diets: metabolic syndrome and oxidative stress in rats

OBJECTIVE

This study determined the effects of adding monosodium glutamate (MSG) to a standard diet and a fiber-enriched diet on glucose metabolism, lipid profile, and oxidative stress in rats.

METHODS

Male Wistar rats (65 +/- 5 g, n = 8) were fed a standard diet (control), a standard diet supplemented with 100 g of MSG per kilogram of rat body weight, a diet rich in fiber, or a diet rich in fiber supplemented with 100 g of MSG per kilogram of body weight. After 45 d of treatment, sera were analyzed for concentrations of insulin, leptin, glucose, triacylglycerol, lipid hydroperoxide, and total antioxidant substances. A homeostasis model assessment index was estimated to characterize insulin resistance.

RESULTS

Voluntary food intake was higher and feed efficiency was lower in animals fed the standard diet supplemented with MSG than in those fed the control, fiber-enriched, or fiber- and MSG-enriched diet. The MSG group had metabolic dysfunction characterized by increased levels of glucose, triacylglycerol, insulin, leptin, and homeostasis model assessment index. The adverse effects of MSG were related to an imbalance between the oxidant and antioxidant systems. The MSG group had increased levels of lipid hydroperoxide and decreased levels of total antioxidant substances. Levels of triacylglycerol and lipid hydroperoxide were decreased in rats fed the fiber-enriched and fiber- and MSG-enriched diets, whereas levels of total antioxidant substances were increased in these animals.

CONCLUSIONS

MSG added to a standard diet increased food intake. Overfeeding induced metabolic disorders associated with oxidative stress in the absence of obesity. The fiber-enriched diet prevented changes in glucose, insulin, leptin, and triacylglycerol levels that were seen in the MSG group. Because the deleterious effects of MSG, i.e., induced overfeeding, were not seen in the animals fed the fiber-enriched diets, it can be concluded that fiber supplementation is beneficial by discouraging overfeeding and improving oxidative stress that is induced by an MSG diet.

Glutathione metabolism and its implications for health

Glutathione (gamma-glutamyl-cysteinyl-glycine; GSH) is the most abundant low-molecular-weight thiol, and GSH/glutathione disulfide is the major redox couple in animal cells. The synthesis of GSH from glutamate, cysteine, and glycine is catalyzed sequentially by two cytosolic enzymes, gamma-glutamylcysteine synthetase and GSH synthetase. Compelling evidence shows that GSH synthesis is regulated primarily by gamma-glutamylcysteine synthetase activity, cysteine availability, and GSH feedback inhibition. Animal and human studies demonstrate that adequate protein nutrition is crucial for the maintenance of GSH homeostasis. In addition, enteral or parenteral cystine, methionine, N-acetyl-cysteine, and L-2-oxothiazolidine-4-carboxylate are effective precursors of cysteine for tissue GSH synthesis. Glutathione plays important roles in antioxidant defense, nutrient metabolism, and regulation of cellular events (including gene expression, DNA and protein synthesis, cell proliferation and apoptosis, signal transduction, cytokine production and immune response, and protein glutathionylation). Glutathione deficiency contributes to oxidative stress, which plays a key role in aging and the pathogenesis of many diseases (including kwashiorkor, seizure, Alzheimer's disease, Parkinson's disease, liver disease, cystic fibrosis, sickle cell anemia, HIV, AIDS, cancer, heart attack, stroke, and diabetes). New knowledge of the nutritional regulation of GSH metabolism is critical for the development of effective strategies to improve health and to treat these diseases.