Lactate and pyruvate metabolism in isolated renal tubules of normal dogs

Gender-Specific Metabolomics Approach to Kidney Cancer

Renal cell carcinoma (RCC) is the most common form of kidney malignancy. RCC is more common among men with a 2/1 male/female incidence ratio worldwide. Given the underlying epidemiological differences in the RCC incidence between males and females, we explored the gender specific ¹H NMR serum metabolic profiles of RCC patients and their matched controls. A number of differential metabolites were shared by male and female RCC patients. These RCC specific changes included lower lactate, threonine, histidine, and choline levels together with increased levels of pyruvate, N-acetylated glycoproteins, beta-hydroxybutyrate, acetoacetate, and lysine. Additionally, serum lactate/pyruvate ratio was a strong predictor of RCC status regardless of gender. Although only moderate changes in metabolic profiles were observed between control males and females there were substantial gender related differences among RCC patients. Gender specific metabolic features associated with RCC status were identified suggesting that different metabolic panels could be leveraged for a more precise diagnostic.

Protocols and Applications of Cellular Metabolomics in Safety Studies Using Precision-Cut Tissue Slices and Carbon 13 NMR

Numerous xenobiotics are toxic to human and animal cells by interacting with their metabolism, but the precise metabolic step affected and the biochemical mechanism behind such a toxicity remain often unknown. In an attempt to reduce the ignorance in this field, we have developed a new approach called cellular metabolomics. This approach, developed in vitro, provides a panoramic view not only of the pathways involved in the metabolism of physiological substrates of any normal or pathological human or animal cell but also of the beneficial and adverse effects of xenobiotics on these metabolic pathways. Unlike many cell lines, precision-cut tissue slices, for which there is a renewed interest, remain metabolically differentiated for at least 24-48 h and allow to study the effect of xenobiotics during short-term and long-term incubations. Cellular metabolomics (or metabolic flux analysis), which combines enzymatic and carbon 13 NMR measurements with mathematical modeling of metabolic pathways, is illustrated in this brief chapter for studying the effect of insulin on glucose metabolism in rat liver precision-cut slices and of valproate on glutamine metabolism in human renal cortical precision-cut slices. The use of very small amounts of test compounds allows to predict their toxic effect and eventually their beneficial effects very early in the research and development processes. Cellular metabolomics is complementary to other omics approaches, but, unlike them, provides functional, mechanistic, and dynamic pieces of information by measuring enzymatic fluxes.

Use of precision-cut renal cortical slices in nephrotoxicity studies

1.Unlike cell lines and primary cells in culture, precision-cut tissue slices remain metabolically differentiated for at least 24-48 h and allow to study the effect of xenobiotics during short-term and long-term incubations. 2.In this article, we illustrate the use of such an experimental model to study the nephrotoxic effects of (i) chloroacetaldehyde, a metabolite of the anticancer drug ifosfamide, (ii) of cobalt chloride, a potential leakage product of the cobalt-containing nanoparticles, and (iii) of valproate, a widely used antiepileptic drug. 3.Since all the latter test compounds, like many toxic compounds, negatively interact with cellular metabolic pathways, we also illustrate our biochemical toxicology approach in which we used not only enzymatic but also carbon 13 NMR measurements and mathematical modelling of metabolic pathways. 4.This original approach, which can be applied to any tissue, allows to predict the nephrotoxic effects of milligram amounts of test compounds very early during the research and development processes of drugs and chemicals. This approach, combined with the use of cells that retain their in vivo metabolic properties and, therefore, are predictive, reduces the risk, the time and cost of such processes.

Protocols and applications of cellular metabolomics in safety studies using precision-cut tissue slices and carbon 13 NMR

Numerous xenobiotics are toxic to human and animal cells by interacting with their metabolism, but the precise metabolic step affected and the biochemical mechanism behind such a toxicity often remain unknown. In an attempt to reduce the ignorance in this field, we have developed a new approach called cellular metabolomics. This approach, developed in vitro, provides a panoramic view not only of the pathways involved in the metabolism of physiologic substrates of any normal or pathologic human or animal cell but also of the beneficial and adverse effects of xenobiotics on these metabolic pathways. Unlike many cell lines, precision-cut tissue slices, for which there is a renewed interest, remain metabolically differentiated for at least 24-48 h and allow to study the effect of xenobiotics during short-term and long-term incubations. Cellular metabolomics (or cellular metabonomics), which combines enzymatic and carbon 13 NMR measurements with mathematical modeling of metabolic pathways, is illustrated in this brief chapter for studying the effect of insulin on glucose metabolism in rat liver precision-cut slices, and of valproate on glutamine metabolism in human renal cortical precision-cut slices. The use of very small amounts of test compounds allows to predict their toxic effect and eventually their beneficial effects very early in the research and development processes. Cellular metabolomics is complementary to other omics approaches, but, unlike them, provides functional and dynamic pieces of information by measuring enzymatic fluxes.

Targets of chloroacetaldehyde-induced nephrotoxicity

Chloroacetaldehyde, one of the main products of hepatic ifosfamide metabolism, contributes to its nephrotoxicity. However, the pathophysiology of this toxicity is not fully understood. The present work examined the time and dose effects of clinically relevant concentrations of chloroacetaldehyde (25-75microM) on precision-cut rat renal cortical slices metabolizing a physiological concentration of lactate. Chloroacetaldehyde toxicity was demonstrated by the decrease in total glutathione and cellular ATP levels. The drop of cellular ATP was linked to the inhibition of oxidative phosphorylation at the level of complex I of the mitochondrial respiratory chain. The large decrease in glucose synthesis from lactate was explained by the inhibition of some gluconeogenic enzymes, mainly glyceraldehyde 3-phosphate dehydrogenase. The decrease in lactate utilization was demonstrated not only by a defect of gluconeogenesis but also by the decrease in [(14)CO(2)] formation from [U-(14)C]-lactate. All the effects of chloroacetaldehyde were concentration and time-dependent. Finally, the chloroacetaldehyde-induced inhibition of glyceraldehyde 3-phosphate dehydrogenase, which is also a glycolytic enzyme, suggests that, under conditions close to those found during ifosfamide therapy, the inhibition of glycolytic pathway by chloroacetaldehyde might be responsible, at least in part, for the therapeutic efficacy of ifosfamide.

In vivo mesna and amifostine do not prevent chloroacetaldehyde nephrotoxicity in vitro

Chloroacetaldehyde (CAA) is the putative metabolite responsible for ifosfamide-induced nephrotoxicity. Whereas evidence suggests that sodium 2-mercaptoethanesulfonate (mesna) and amifostine protect renal cells against CAA toxicity in vitro, their efficacy in clinical studies is controversial. To better understand the discrepancy between in vivo and in vitro results, we combined the in vivo intraperitoneal administration of either saline or mesna (100 mg/kg) or amifostine (200 mg/kg) in rats and the in vitro study of CAA toxicity to both proximal tubules and precision-cut renal cortical slices. The measured renal cortical concentrations of mesna and amifostine were 0.6+/-0.1 micromol/g and 1.2+/-0.2 micromol/g, respectively; these drugs did not cause renal toxicity. Despite this, none of the adverse effects of 0.5 mM CAA was prevented by the previous in vivo administration of mesna or amifostine. Toxicity of 0.5 mM CAA to rat proximal tubules was shown by the fall of cellular adenosine triphosphate (ATP), total glutathione and coenzyme A + acetyl-coenzyme A levels and by the altered metabolic viability of renal cells. Long-term exposure of cortical slices to CAA concentrations > or =30 microM caused severe cell toxicity (i.e. decrease in cellular ATP, total glutathione, and coenzyme A + acetyl-coenzyme A levels), which was not prevented by the in vivo administration of mesna or amifostine.

Intrinsic gluconeogenesis is enhanced in renal proximal tubules of Zucker diabetic fatty rats

Recent studies indicate that renal gluconeogenesis is substantially stimulated in patients with type 2 diabetes, but the mechanism that is responsible for such stimulation remains unknown. Therefore, this study tested the hypothesis that renal gluconeogenesis is intrinsically elevated in the Zucker diabetic fatty rat, which is considered to be an excellent model of type 2 diabetes. For this, isolated renal proximal tubules from diabetic rats and from their lean nondiabetic littermates were incubated in the presence of physiologic gluconeogenic precursors. Although there was no increase in substrate removal and despite a reduced cellular ATP level, a marked stimulation of gluconeogenesis was observed in diabetic relative to nondiabetic rats, with near-physiologic concentrations of lactate (38%), glutamine (51%) and glycerol (66%). This stimulation was caused by a change in the fate of the substrate carbon skeletons resulting from an increase in the activities and mRNA levels of the key gluconeogenic enzymes that are common to lactate, glutamine, and glycerol metabolism, i.e., mainly of phosphoenolpyruvate carboxykinase and, to a lesser extent, of glucose-6-phosphatase and fructose-1,6-bisphosphatase. Experimental evidence suggests that glucocorticoids and cAMP were two factors that were responsible for the long-term stimulation of renal gluconeogenesis observed in the diabetic rats. These data provide the first demonstration in an animal model that renal gluconeogenesis is upregulated by a long-term mechanism during type 2 diabetes. Together with the increased renal mass (38%) observed, they lend support to the view so far based only on in vivo studies performed in humans that renal gluconeogenesis may be stimulated by and crucially contribute to the hyperglycemia of type 2 diabetes.

Metabolic viability and pharmaco-toxicological reactivity of cryopreserved human precision-cut renal cortical slices

We have tested the suitability of cryopreserved human precision-cut renal cortical slices for metabolic and pharmaco-toxicological studies. The viability of these slices and their pharmaco-toxicological reactivity were assessed using intracellular ATP and protein contents, lactate dehydrogenase (LDH) leakage, lactate and glutamine metabolism and the ammoniagenic effect of valproate. Despite a decrease in ATP and protein contents when compared with those of fresh slices, cryopreserved slices did not show any LDH leakage and retained the capacity to metabolize glutamine and lactate. Glutamine removal and ammonia, lactate and alanine production were similar in fresh and cryopreserved slices; by contrast, cryopreserved slices accumulated more glutamate as a result of decreased flux through glutamate dehydrogenase which catalyses an oxygen-dependent reaction. Valproate markedly and similarly stimulated glutamine metabolism in fresh and cryopreserved slices. Cryopreservation did not alter lactate removal but inhibited lactate gluconeogenesis. In conclusion, these results demonstrate that, although their mitochondrial oxidative metabolism seems to be diminished, cryopreserved human precision-cut renal cortical slices remain metabolically viable and retain the capacity to respond to the ammoniagenic effect of valproate. Thus, this experimental model may be helpful to optimize the use of human renal tissue for metabolic and pharmaco-toxicological studies.

Characteristics of glutamine metabolism in human precision-cut kidney slices: a 13C-NMR study

The metabolism of glutamine, a physiological substrate of the human kidney, plays a major role in systemic acid-base homoeostasis. Not only because of the limited availability of human renal tissue but also in part due to the lack of adequate cellular models, the mechanisms regulating the renal metabolism of this amino acid in humans have been poorly characterized. Therefore given the renewed interest in their use, human precision-cut renal cortical slices were incubated in Krebs-Henseleit medium (118 mM NaCl, 4.7 mM KCl, 1.18 mM KH2PO4, 1.18 mM MgSO4*7H2O, 24.9 mM NaHCO3 and 2.5 mM CaCl2*2H2O) with 2 mM unlabelled or 13C-labelled glutamine residues. After incubation, substrate utilization and product formation were measured by enzymatic and NMR spectroscopic methods. Glutamate accumulation tended to plateau but glutamine removal and ammonia, alanine and lactate production as well as flux through GLDH (glutamate dehydrogenase) increased to various extents with time for up to 4 h of incubation indicating the metabolic viability of the slices. Valproate, a stimulator of renal glutamine metabolism, markedly and in a dose-dependent fashion increased ammonia production. With [3-13C]glutamine as a substrate, and in the absence and presence of valproate, [13C]glutamate, [13C]alanine and [13C]lactate accounted for 81 and 96%, 34 and 63%, 30 and 46% of the glutamate, alanine and lactate accumulations measured enzymatically respectively. The slices also metabolized glutamine and retained their reactivity to valproate during incubations lasting for up to 48 h. These results demonstrate that, although endogenous metabolism substantially operates in the presence of glutamine, human precision-cut renal cortical slices are metabolically viable and strongly respond to the ammoniagenic effect of valproate. Thus, this experimental model is suitable for metabolic and pharmaco-toxicological studies.

Complexity of glutamine metabolism in kidney tubules from fed and fasted rats

Glutamine is an important renal glucose precursor and energy provider. In order to advance our understanding of the underlying metabolic processes, we studied the metabolism of variously labelled [13C]glutamine and [14C]glutamine molecules and the effects of fasting in isolated rat renal proximal tubules. Absolute fluxes through the enzymes involved, including enzymes of four different cycles operating concomitantly, were assessed by combining mainly the 13C NMR data with an appropriate model of glutamine metabolism. In both nutritional states, unidirectional glutamine removal by glutaminase was partially masked by the concomitant operation of glutamine synthetase; fasting accelerated glutamine removal by increasing flux solely through glutaminase, without changing that through glutamine synthetase. Fasting stimulated net glutamate degradation only by decreasing flux through glutamate dehydrogenase in the reductive amination direction, but surprisingly did not significantly alter complete oxidation of the glutamine carbon skeleton. Finally, gluconeogenesis from glutamine involved not only substantial recycling through the tricarboxylic acid cycle, but also an important anaplerotic flux through pyruvate carboxylase that was accelerated dramatically by fasting. Thus renal glutamine metabolism follows an unexpectedly complex route that is precisely regulated during fasting.

The anaplerotic substrate alanine stimulates acetate incorporation into glutamate and glutamine in rabbit kidney tubules. A (13)C NMR study

Although acetate, the main circulating volatile fatty acid in humans and animals, is metabolized at high rates by the renal tissue, little is known about the precise fate of its carbons and about the regulation of its renal metabolism. Therefore, we studied the metabolism of variously labeled [(13)C]acetate and [(14)C]acetate molecules and its regulation by alanine, which is also readily metabolized by the kidney, in isolated rabbit renal proximal tubules. With acetate as the sole substrate, 72% of the C-1 and 49% of the C-2 of acetate were released as CO(2); with acetate plus alanine, the corresponding values were decreased to 49 and 25%. The only other important products formed from the acetate carbons were glutamine, and to a smaller extent, glutamate. By combining (13)C NMR and radioactive and enzymatic measurements with a novel model of acetate metabolism, fluxes through the enzymes involved were calculated. Thanks to its anaplerotic effect, alanine caused a stimulation of acetate removal and a large increase in fluxes through pyruvate carboxylase, citrate synthase, and the enzymes involved in glutamate and glutamine synthesis but not in flux through alpha-ketoglutarate dehydrogenase. We conclude that the anaplerotic substrate alanine not only accelerates the disposal of acetate but also prevents the wasting of the latter compound as CO(2).

Human kidney tubules detoxify chloroacetaldehyde, a presumed nephrotoxic metabolite of ifosfamide

The nephrotoxic effects of the antineoplastic drug ifosfamide have been attributed to its hepatic metabolite chloroacetaldehyde. The effects of chloroacetaldehyde on isolated human kidney cortex tubules metabolizing lactate (a physiologic substrate in human kidneys) were investigated. At concentrations of > or =0.5 mM, chloroacetaldehyde was toxic to the human kidney tubules, as demonstrated by a dramatic decrease in cellular ATP levels and a large increase in lactate dehydrogenase release; chloroacetaldehyde also stimulated pyruvate accumulation and inhibited lactate removal and glucose synthesis. These effects, which were associated with incomplete disappearance of chloroacetaldehyde and extensive depletion of the cellular CoA, acetyl-CoA, and glutathione contents, were prevented by the addition of thiol-protecting drugs (mesna and amifostine). Human kidney tubules were demonstrated to metabolize chloroacetaldehyde at high rates, presumably via aldehyde dehydrogenase, which is very active in human kidneys. Carbon-13 nuclear magnetic resonance spectroscopy measurements indicated that human kidney tubules converted [2-(13)C]chloroacetaldehyde to [2-(13)C]chloroacetate, the further metabolism of which was very limited. At equimolar concentrations, chloroacetate was much less toxic than chloroacetaldehyde, indicating that chloroacetate synthesis from chloroacetaldehyde by human kidney tubules represents a detoxification mechanism that could play a role in vivo in preventing or limiting the nephrotoxic effects observed during ifosfamide therapy.

Activation of the phosphatidylinositol 3-kinase/Akt pathway protects against interleukin-3 starvation but not DNA damage-induced apoptosis

Baf-3 cells are dependent on interleukin-3 (IL-3) for their survival and proliferation in culture. To identify anti-apoptotic pathways, we performed a retroviral-insertion mutagenesis on Baf-3 cells and selected mutants that have acquired a long term survival capacity. The phenotype of one mutant, which does not overexpress bcl-x and proliferates in the absence of IL-3, is described. We show that, in this mutant, Akt is constitutively activated leading to FKHRL1 phosphorylation and constitutive glycolytic activity. This pathway is necessary for the mutant to survive following IL-3 starvation but is not sufficient or necessary to protect cells from DNA damage-induced cell death. Indeed, inhibition of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway in Baf-3 cells does not prevent the ability of IL-3 to protect cells against gamma-irradiation-induced DNA damage. This protective effect of IL-3 rather correlates with the expression of the anti-apoptotic Bcl-x protein. Taken together, these data demonstrate that the PI3K/Akt pathway is sufficient to protect cells from growth factor starvation-induced apoptosis but is not required for IL-3 inhibition of DNA damage-induced cell death.

The rabbit kidney tubule simultaneously degrades and synthesizes glutamate. A 13C NMR study

The rabbit kidney does not readily metabolize but synthesizes glutamine at high rates by pathways that remain poorly defined. Therefore, the metabolism of variously labeled [13C]- and [14C]glutamates has been studied in isolated rabbit kidney tubules with and without acetate. CO2, glutamine, and alanine were the main carbon and nitrogenous end products of glutamate metabolism but no ammonia accumulated. Absolute fluxes through enzymes involved in glutamate metabolism, including enzymes of four different cycles operating simultaneously, were assessed by combining mainly the 13C NMR data with a new model of glutamate metabolism. In contrast to a previous conclusion of Klahr et al. (Klahr, S., Schoolwerth, A. C., and Bourgoignie, J. J. (1972) Am. J. Physiol. 222, 813-820), glutamate metabolism was found to be initiated by glutamate dehydrogenase at high rates. Glutamate dehydrogenase also operated at high rates in the reverse direction; this, together with the operation of the glutamine synthetase reaction, masked the release of ammonia. Addition of acetate stimulated the operation of the "glutamate --> alpha-ketoglutarate --> glutamate" cycle and the accumulation of glucose but reduced both the net oxidative deamination of glutamate and glutamine synthesis. Acetate considerably increased flux through alpha-ketoglutarate dehydrogenase and citrate synthase at the expense of flux through phosphoenolpyruvate carboxykinase; acetate also caused a large decrease in flux through alanine aminotransferase, pyruvate dehydrogenase, and the "substrate cycle" involving oxaloacetate, phosphoenolpyruvate, and pyruvate.

Advantages and limitations of the use of isolated kidney tubules in pharmacotoxicology

Among the cellular models used in in vitro renal pharmacotoxicology, isolated kidney tubules, used as suspensions mainly of proximal tubules, offer important advantages. They can be prepared in large amounts under nonsterile conditions within 1-2 h; thus, it is possible to employ a great number of experimental conditions simultaneously and to obtain rapidly many experimental results. Kidney tubules can be prepared from the kidney of many animal species and also from the human kidney; given the very limited availability of healthy human renal tissue, it is therefore possible to choose the most appropriate species for the study of a particular problem encountered in man. Kidney tubules can be used for screening and prevention of nephrotoxic effects and to identify their mechanisms as well as to study the renal metabolism of xenobiotics. When compared with cultured renal cell, a major advantage of kidney tubules is that they remain differentiated. The main limitations of the use of kidney tubules in pharmacotoxicology are (1) the necessity to prepare them as soon as the renal tissue sample is obtained; (2) their limited viability, which is restricted to 2-3 h; (3) the inability to expose them chronically to a potential nephrotoxic drug; (4) the inability to study transepithelial transport; and (5) the uncertainty in the extrapolation to man of the results obtained using animal kidney tubules. These advantages and limitations of the use of human and animal kidney tubules in pharmacotoxicology are illustrated mainly by the results of experiments performed with valproate, an antiepileptic and moderately hyperammonemic agent. The fact that kidney tubules, unlike cultured renal cells, retain key metabolic properties is also shown to be of the utmost importance in detecting certain nephrotoxic effects.

Alanine metabolism in isolated human kidney tubules--Use of a mathematical model

To gain insight into the fate of alanine nitrogen and carbon taken up by the human kidney under certain conditions, isolated human kidney cortex tubules were incubated in Krebs-Henseleit medium with L-alanine as substrate. The tubules metabolized alanine at high rates and in a dose-dependent manner. Most of the alanine nitrogen removed was recovered as ammonia and to a lesser extent as glutamate. Glucose, lactate and glutamate were also found to be significant products of alanine carbon metabolism. A simple mathematical model allowing one to calculate flux of alanine carbon through the various metabolic steps involved is proposed and applied to data obtained in experiments in which 5 mM [U-14C]-,[1-14C]-, [2-14C]- and [3-14C]alanine were used as substrates in parallel. About 40% of the alanine carbon removed was recovered as CO2 and oxidation of C1 of alanine accounted for most of the CO2 released from alanine. Calculations reveal that the ATP produced exceeded 3.2-fold the ATP consumed in relation to alanine metabolism. It is concluded that, in human kidney, alanine may serve as an energy supplier and as a precursor of glucose and ammonia.

Improved culture conditions stimulate gluconeogenesis in primary cultures of renal proximal tubule cells

Unlike renal proximal tubule cells (RPTC) in vivo, RPTC cultured in standard conditions are hypoxic, glycolytic, and not gluconeogenic. This study investigated the effects of glucose and lactate on glycolysis and gluconeogenesis in rabbit RPTC cultured in conditions of increased oxygen supply (Shake). Confluent Shake cultures grown in the presence of glucose exhibited increased oxygen consumption and decreased glycolysis compared with stationary (Still) cultures. Addition of 5 mM lactate to a 5 mM glucose medium decreased net glucose consumption and glucose oxidation in Shake cultures by 34 and 50%, respectively, and resulted in net lactate consumption. Addition of 5 mM lactate to a glucose-free medium resulted in a threefold increase in net glucose production (0.024 +/- 0.003 vs. 0.074 +/- 0.013 mumol.mg protein-1.day-1) in Shake cultures. Net glucose production further increased to 0.430 +/- 0.020 and 1.640 +/- 0.040 mumol.mg protein-1.day-1 when glucose reuptake was inhibited by 1 mM phloridzin or 1 mM phloridzin + 1 mM phloretin, respectively. These results show that, under conditions of improved oxygenation and in the presence of lactate and physiological levels of glucose and insulin, RPTC aerobic metabolism increases and glucose metabolism changes from glycolysis and net lactate production to gluconeogenesis and net lactate consumption.

Glutamine synthesis from glucose and ammonium chloride by guinea-pig kidney tubules

1. At a physiological concentration (5 mM), glucose was found to be metabolized by isolated kidney cortex tubules prepared from fed guinea pigs. 2. The release of 14CO2 from [U-14C]glucose indicated that oxidation of the glucose carbon skeleton represented about 50% of the glucose removed; significant amounts of lactate and glutamine also accumulated. 3. Addition of 0.1-10 mM NH4Cl led to a dose-dependent stimulation of glucose metabolism which was accompanied by a large increase in lactate and glutamine accumulation and, to a lesser extent, in glucose oxidation. 4. Comparison of the release of 14CO2 from [1-14C]- and [6-14C]glucose indicates that, in both the absence and the presence of NH4Cl, the pentose phosphate shunt was only a minor pathway of glucose metabolism. 5. The central role of pyruvate carboxylase in the conversion of glucose carbon into glutamine carbon was demonstrated by using a bicarbonate-free medium and measuring the fixation of 14CO2 from [14C]bicarbonate, which was recovered mostly at C-1 of glutamine plus glutamate. 6. The NH4Cl-induced stimulation of glucose removal was secondary not only to increased glutamine synthesis, as shown by the effect of methionine sulphoximine, an inhibitor of glutamine synthetase, but also to the stimulation of phosphofructokinase activity by NH4Cl. 7. Renal arterio-venous difference measurements revealed that, in vivo, the guinea-pig kidney removed glucose from the circulating blood, which suggests that glucose carbon may contribute to the carbon skeleton of the glutamine released by this organ.

Release and fixation of CO2 by guinea-pig kidney tubules metabolizing aspartate

1. The metabolism of L-[U-14C]aspartate, L-[1-14C]aspartate and L-[4-14C]aspartate was studied in isolated guinea-pig kidney tubules. 2. Oxidation of C-1 plus that of C-4 of aspartate accounted for 90-92% of the CO2 released from aspartate, whereas oxidation of the inner carbon atoms of aspartate (which occurs beyond the 2-oxoglutarate dehydrogenase step) represented only 8-10% of aspartate carbon oxidation. 3. The formation of [1-14C]glutamine and [1-14C]glutamate from [1-14C]aspartate and [4-14C]aspartate indicated that about one-third of the oxaloacetate synthesized from aspartate underwent randomization at the level of fumarate. 4. With [U-14C]aspartate as substrate, the percentage of the C-1 of glutamate and glutamine found radiolabelled after 60 min of incubation was 92.7% and 47.5% in the absence and the presence of bicarbonate respectively. 5. That CO2 fixation occurred at high rates in the presence of bicarbonate was demonstrated by incubating tubules with aspartate plus [14C]bicarbonate; under this condition, the label fixed was found in C-1 of glutamate, glutamine and aspartate, as well as in C-4 of aspartate, demonstrating not only randomization of aspartate carbon but also aspartate resynthesis secondary to oxaloacetate cycling via phosphoenolpyruvate carboxykinase, pyruvate kinase and pyruvate carboxylase. 6. The importance of CO2 fixation in glutamine synthesis from aspartate is discussed in relation to the possible role of the guinea-pig kidney in systemic acid-base regulation in vivo.

Glutamine synthesis from aspartate in guinea-pig renal cortex

1. Glutamine was found to be the main carbon and nitrogen product of the metabolism of aspartate in isolated guinea-pig kidney-cortex tubules. Glutamate, ammonia and alanine were only minor products. 2. Carbon-balance calculations and the release of 14CO2 from [U-14C]aspartate indicate that oxidation of the aspartate carbon skeleton occurred. 3. A pathway involving aspartate aminotransferase, glutamate dehydrogenase, glutamine synthetase, phosphoenolpyruvate carboxykinase, pyruvate kinase, pyruvate dehydrogenase and enzymes of the tricarboxylic acid cycle is proposed for the conversion of aspartate into glutamine. 4. Evidence for this pathway was obtained by: (i) inhibiting aspartate removal by amino-oxyacetate, an inhibitor of transaminases, (ii) the use of methionine sulphoximine, an inhibitor of glutamine synthetase, which induced a large increase in ammonia release from aspartate, (iii) the use of quinolinate, an inhibitor of phosphoenolpyruvate carboxykinase, which inhibited glutamine synthesis from aspartate, (iv) the use of alpha-cyano-4-hydroxycinnamate, an inhibitor of the mitochondrial transport of pyruvate, which caused an accumulation of pyruvate from aspartate, and (v) the use of fluoroacetate, an inhibitor of aconitase, which inhibited glutamine synthesis with concomitant accumulation of citrate from aspartate.

Effect of the antiepileptic drug sodium valproate on glutamine and glutamate metabolism in isolated human kidney tubules

We studied the effects of sodium valproate, a widely used antiepileptic drug and a hyperammonemic agent, on L-[1-14C]glutamine and L-[1-14C]glutamate metabolism in isolated human kidney-cortex tubules. Valproate markedly stimulated glutamine removal as well as the formation of ammonia, 14CO2, pyruvate, lactate and alanine, but it inhibited glucose synthesis; the increase in ammonia formation was explained by a stimulation by valproate mainly of flux through glutaminase (EC 3.5.1.2) and to a much lesser extent of flux through glutamate dehydrogenase (EC 1.4.1.3). By contrast, valproate did not stimulate glutamate removal or ammonia formation, suggesting that the increase in flux through glutamate dehydrogenase observed with glutamine as substrate was secondary to the increase in flux through glutaminase. Accumulation of pyruvate, alanine and lactate in the presence of valproate was less from glutamate than from glutamine. Inhibition by aminooxyacetate of accumulation of alanine from glutamine caused by valproate did not prevent the acceleration of glutamine utilization and the subsequent stimulation of ammonia formation. It is concluded from these data, which are the first concerning the in vitro metabolism of glutamine and glutamate in human kidney-cortex tubules, that the stimulatory effect of valproate is primarily exerted at the level of glutaminase in human renal cortex.

Carbon flux through tricarboxylic acid cycle in rat renal tubules

Our aim was to delineate the effect(s) of chronic metabolic acidosis on renal TCA-cycle metabolism. Renal tubules isolated from control and chronically acidotic rats were incubated at pH 7.4 with either 2 mM [2,3-13C]pyruvate or [2-13C]acetate. GC-MS and/or 13C-NMR were utilized to monitor the flux of 13C through pyruvate dehydrogenase, pyruvate carboxylase and the TCA-cycle. With either, precursor acidosis was associated with significantly decreased formation of 13C-labelled citrate, malate, aspartate and alanine and increased formation of glucose, lactate and acetyl-CoA as compared with the control. The results indicate that adaptation of renal metabolism to chronic metabolic acidosis is associated with diminished flux through citrate synthetase and concomitantly increased flux through pyruvate carboxylase. The data suggest that depletion of TCA-cycle intermediates and enhanced ammoniagenesis in the kidney of chronically acidotic rats may be regulated at the site of mitochondrial citrate-condensing enzyme.

Stimulation of glutamine metabolism by the antiepileptic drug, sodium valproate, in isolated dog kidney tubules

The effects of sodium valproate, a widely used antiepileptic drug and an hyperammonemic agent, on glutamine and glutamate metabolism were studied in isolated dog kidney tubules. Valproate markedly stimulated glutamine removal as well as the formation of ammonia, aspartate, pyruvate, lactate, alanine and glucose; the increase in ammonia formation was explained by a stimulation by valproate of flux not only through glutaminase (EC 3.5.1.2) but also through glutamate dehydrogenase (EC 1.4.1.3). By contrast, valproate did not stimulate glutamate removal or ammonia, aspartate and glucose formation from glutamate; this suggests that the increase in flux through glutamate dehydrogenase with glutamine as substrate was secondary to the increase in flux through glutaminase. Accumulation of pyruvate, alanine and lactate in the presence of valproate was much less from glutamate than from glutamine. Inhibition by amino-oxyacetate of accumulation of aspartate and alanine from glutamine caused by valproate did not prevent the acceleration of glutamine utilization and the subsequent stimulation of ammonia formation. These data are consistent with a stimulatory effect of valproate primarily exerted at the level of glutaminase in dog kidney tubules. However, the fact that assayed activity of glutaminase remained unchanged in the presence of valproate suggests that this compound accelerates flux through the latter enzyme by an indirect mechanism probably related to the renal metabolism of this compound.

Pyruvate carboxylation in glutamine synthesis from alanine by isolated guinea-pig renal cortical tubules

Isolated guinea-pig kidney cortex tubules were incubated in Krebs-Henseleit buffer containing NaH14CO3 (25 mM) and L-alanine (5 mM). A high rate of alanine metabolism was found to be accompanied by a high rate of both 14CO2 fixation and glutamine synthesis. The fixation of 14CO2 was virtually abolished in the presence of oxalate, a known inhibitor of pyruvate carboxylase, indicating that, in guinea-pig renal cortex, this enzyme is responsible for the synthesis of oxaloacetate in the conversion of alanine into glutamine. More than 90% of the label fixed was found in carbon 1 mainly of glutamine and to a lesser extent of glutamate. In the presence of alanine + NaH14CO3 + MSO, an inhibitor of glutamine synthetase, most of the 14CO2 fixed by pyruvate carboxylase was subsequently released and carbon 1 of glutamate was the only site of labelling. In the presence of alanine + NaH14CO3, the fact that not all the glutamine found was labelled in carbon 1 could be explained by glutamine synthesis from endogenous substrates as well as by glutamine synthesis from alanine after prior equilibration of [4-14C]-oxaloacetate with fumarate; that such equilibration occurred was demonstrated by the observation that [1-14C]-glutamine and [1-14C]-glutamate were synthesized from [1-14C]-alanine.

Nephrotoxicity: a rational approach to target cell injury in vitro in the kidney

1. The kidney is a complex organ in which there is cellular heterogeneity. Many nephrotoxic chemicals target preferentially for discrete cell types, but adjacent, morphologically different cells are unaffected. This selectivity has made the assessment of nephrotoxicity in vivo (and the study of underlying mechanisms) difficult. Discrete renal injury can, however, be exploited in vitro, to study the interactions between the toxic compound and the target cell. 2. Several in vitro models have been used to study the potential interaction between the target cells and chemicals, including perfusion of the isolated kidney, renal slices, freshly isolated fragments, primary cultures and continuous cell lines. Where appropriate, isolated organelles and purified enzymes can also be used. 3. The target cell toxicity in vivo of adriamycin, 2-bromoethanamine and hexachlorobutadiene N-acetyl cysteine conjugate is selectively maintained towards glomerular epithelial, medullary interstitial and proximal tubular cells, respectively, in vitro, showing that the "in vivo-in vitro gap" can be bridged. Characteristics unique to each of these renal cell types, such as the selective uptake of a toxin, enzyme systems for generating biologically reactive intermediates, and the presence of lipid droplets (rich in polyunsaturated fatty acid) and peroxidase activity have been identified, and one or more of these may explain the mechanisms of selective injury in discrete regions of the kidney.

Characteristics of acetaldehyde metabolism in isolated dog, rat and guinea-pig kidney tubules

The metabolism of acetaldehyde was studied in isolated dog, rat and guinea-pig kidney-cortex tubules. In contrast with previous observations of Cederbaum and Rubin in rat kidney mitochondria (Archs Biochem. Biophys. 179, 46-66 1977) acetaldehyde was found to be metabolized by the tubules at high rates and in a dose-dependent manner at concentrations up to 5-10 mM. At high acetaldehyde concentrations (1-10 mM) acetaldehyde removal was accompanied by a high rate of acetate accumulation which explained most of the acetaldehyde metabolized in dog and guinea-pig but not in rat kidney tubules. These species differences in acetaldehyde metabolism can be explained by the differences in activities of aldehyde dehydrogenase (EC 1.2.1.3) and acetyl-CoA synthetase (EC6.2.1.1), the enzymes involved in renal acetaldehyde metabolism which were measured in the renal cortex of the three species. The acetaldehyde carbon removed and not accounted for by acetate accumulation was completely oxidized to CO2 as demonstrated by the measurement of [U-14C]-acetaldehyde conversion into 14CO2. At "physiological" acetaldehyde concentrations (0.1 and 0.2 mM) acetaldehyde utilization was also concentration-dependent but no acetate accumulation was observed.

Metabolism of acetaldehyde in human and baboon renal cortex. Ethanol synthesis by isolated baboon kidney-cortex tubules

Acetaldehyde (1-20 mM) was metabolized at high rates and in a dose-dependent manner in isolated human and baboon kidney-cortex tubules. Acetaldehyde removal was accompanied by a large accumulation of acetate in both human and baboon tubules. By contrast, a large synthesis of ethanol was observed only in baboon tubules. Consistent with the latter finding, ethanol was found to be metabolized at significant rates in baboon but not human tubules. In the tubules from both species, a significant fraction of the acetaldehyde removed was also completely oxidized to CO2 and H2O. These results suggest that, in both man and baboon, the kidneys participate in the in vivo metabolism of acetaldehyde; they also suggest that, in contrast with the human kidneys, the baboon kidneys contribute to the detoxication of circulating ethanol.

Substrate-dependent effect of 1-34 human parathyroid hormone fragment, dibutyryl cAMP and cAMP on gluconeogenesis in rabbit renal tubules

In the presence of 0.5 mM extracellular Ca2+ concentration both 1-34 human parathyroid hormone fragment (0.5 micrograms/ml) as well as 0.1 mM dibutyryl cAMP stimulated gluconeogenesis from lactate in renal tubules isolated from fed rabbits. However, these two compounds did not affect glucose synthesis from pyruvate as substrate. When 2.5 mM Ca2+ was present the stimulatory effect of the hormone fragment on gluconeogenesis from lactate was not detected but dibutyryl cAMP increased markedly the rate of glucose formation from lactate, dihydroxyacetone and glutamate, and inhibited this process from pyruvate and malate. Moreover, dibutyryl cAMP was ineffective in the presence of either 2-oxoglutarate or fructose as substrate. Similar changes in glucose formation were caused by 0.1 mM cAMP. As concluded from the 'crossover' plot the stimulatory effect of dibutyryl cAMP on glucose formation from lactate may result from an acceleration of pyruvate carboxylation due to an increase of intramitochondrial acetyl-CoA, while an inhibition by this compound of gluconeogenesis from pyruvate is likely due to an elevation of mitochondrial NADH/NAD+ ratio, resulting in a decrease of generation of oxaloacetate, the substrate of phosphoenolpyruvate carboxykinase. Dibutyryl cAMP decreased the conversion of fracture 1,6-bisphosphate to fructose 6-phosphate in the presence of both substrates which may be secondary to an inhibition of fructose 1,6-bisphosphatase.

Vasopressin promotes the metabolism of near-physiological concentration of glutamine in isolated rat liver cells

In isolated rat liver cells, vasopressin, like glucagon, promotes the metabolism of glutamine used at near-physiological concentration (1 mM). These findings indicate that, in vivo, both hormones might participate in the control of hepatic gluconeogenesis and ureogenesis from glutamine.

Stimulation of glutamine metabolism by 3-aminopicolinate in isolated dog kidney-cortex tubules

1. The effects of 3-aminopicolinate, a known hyperglycaemic agent in the rat, on glutamine metabolism were studied in isolated dog kidney tubules. 2. 3-Aminopicolinate greatly stimulated glutamine (but not glutamate) removal and glutamate accumulation from glutamine as well as formation of ammonia, aspartate, lactate, alanine and glucose. 3. The increased accumulation of aspartate from glutamine and glutamate, and the inhibition of glucose synthesis from various non-nitrogenous gluconeogenic substrates, as well as the increased accumulation of malate from succinate, support the proposal that 3-aminopicolinate is an inhibitor rather than a stimulator of phosphoenolpyruvate carboxykinase (EC 4.1.1.32) in dog kidney tubules. 4. With glutamine as substrate, the increase in flux through glutamate dehydrogenase (EC 1.4.1.3) could not explain the large increase in glutamine removal caused by 3-aminopicolinate. 5. Inhibition by amino-oxyacetate of accumulation of aspartate and alanine from glutamine caused by 3-aminopicolinate did not prevent the acceleration of glutamine utilization. 6. These data are consistent with a direct stimulation of glutaminase (EC 3.5.1.2) by 3-aminopicolinate in dog kidney tubules.

Evidence that 3-aminopicolinate stimulates glutamine metabolism by rat renal cortical tissue in vitro

The hyperglycemic agent 3-aminopicolinate used at concentrations of 0.05 and 0.1 mM stimulates the removal of 5 mM glutamine. It also stimulates the accumulation of glutamate and the formation of ammonia, glucose and aspartate by isolated rat kidney-cortex tubules. These effects are consistent with a stimulation of glutaminase and an inhibition of phosphoenolpyruvate carboxy-kinase by this compound. Higher concentrations (0.5 and 1 mM) of 3-aminopicolinate fail to affect the removal of 5 mM glutamine, but greatly alter the fate of both glutamine carbon and amino nitrogen. Similar effects of 3-aminopicolinate are observed when glutamine is used at a near-physiological concentration.

The conversion of alanine into glutamine in guinea-pig renal cortex. Essential role of pyruvate carboxylase

1. The metabolism of L-alanine was studied in isolated guinea-pig kidney-cortex tubules. 2. In contrast with previous conclusions of Krebs [(1935) Biochem. J. 29, 1951-1969], glutamine was found to be the main carbon and nitrogenous product of the metabolism of alanine (at 1 and 5 mM). Glutamate and ammonia were only minor products. 3. At neither concentration of alanine was there accumulation of glucose, glycogen, pyruvate, lactate, aspartate or tricarboxylic acid-cycle intermediates. 4. Carbon-balance calculations and the release of 14CO2 from [U-14C]alanine indicate that oxidation of the alanine carbon skeleton occurred at both substrate concentrations. 5. A pathway involving alanine aminotransferase, glutamate dehydrogenase, glutamine synthetase, pyruvate dehydrogenase, pyruvate carboxylase and enzymes of the tricarboxylic acid cycle is proposed for the conversion of alanine into glutamine. 6. Strong evidence for this pathway was obtained by: (i) suppressing alanine removal by amino-oxyacetate, and inhibitor of transaminases, (ii) measuring the release of 14CO2 from [1-14C]alanine, (iii) the use of L-methionine DL-sulphoximine, an inhibitor of glutamine synthetase, which induced a large increase in ammonia release from alanine, and (iv) the use of fluoroacetate, an inhibitor of aconitase, which inhibited glutamine synthesis with concomitant accumulation of citrate from alanine. 7. In this pathway, the central role of pyruvate carboxylase, which explains the discrepancy between our results and those of Krebs (1935), was also demonstrated.

Fate of glutamate carbon and nitrogen in isolated guinea-pig kidney-cortex tubules. Evidence for involvement of glutamate dehydrogenase in glutamine sythesis from glutamate

1. The pathways and the fate of glutamate carbon and nitrogen were investigated in isolated guinea-pig kidney-cortex tubules. 2. At low glutamate concentration (1 mM), the glutamate carbon skeleton was either completely oxidized or converted into glutamine. At high glutamate concentration (5 mM), glucose, lactate and alanine were additional products of glutamate metabolism. 3. At neither concentration of glutamate was there accumulation of ammonia. 4. Nitrogen-balance calculations and the release of 14CO2 from L-[1-14C]glutamate (which gives an estimation of the flux of glutamate carbon skeleton through alpha-oxoglutarate dehydrogenase) clearly indicated that, despite the absence of ammonia accumulation, glutamate metabolism was initiated by the action of glutamate dehydrogenase and not by transamination reactions as suggested by Klahr, Schoolwerth & Bourgoignie [(1972) Am. J. Physiol. 222, 813-820] and Preuss [(1972) Am. J. Physiol. 222, 1395-1397]. Additional evidence for this was obtained by the use of (i) amino-oxyacetate, an inhibitor of transaminases, which did not decrease glutamate removal, or (ii) L-methionine DL-sulphoximine, an inhibitor of glutamine synthetase, which caused an accumulation of ammonia from glutamate. 5. Addition of NH4Cl plus glutamate caused an increase in both glutamate removal and glutamine synthesis, demonstrating that the supply of ammonia via glutamate dehydrogenase is the rate-limiting step in glutamine formation from glutamate. NH4Cl also inhibited the flux of glutamate through glutamate dehydrogenase and the formation of glucose, alanine and lactate. 6. The activities of enzymes possibly involved in the glutamate conversion into pyruvate were measured in guinea-pig renal cortex. 7. Renal arteriovenous-difference measurements revealed that in vivo the guinea-pig kidney adds glutamine and alanine to the circulating blood.

Is pyruvate carboxylase involved in the renal tubular reabsorption of bicarbonate?

The co-existence of a hereditary defect of pyruvate carboxylase activity along with proximal renal tubular acidosis in several patients prompted the following theories: (1) Some of the bicarbonate which is normally reabsorbed from the glomerular filtrate is trapped in the mitochondria by pyruvate carboxylase in the conversion of pyruvate to oxaloacetate. The subsequent conversion of oxaloacetate to phosphoenol pyruvate releases CO2 in the cytosol. (2) The trapping of HCO-3 by pyruvate (or other carboxylases) provides an important route for the recovery of filtered HCO-3. (3) The process of trapping HCO-3 from the glomerular filtrate followed by release of CO2 in the cytosol contributes to the apparently high RQ of kidney, since the CO2 does not originate from a metabolic fuel. (4) Lactate and possibly other fuels are actively taken up by the kidney and are used as energy sources. Diversion of lactate for gluconeogenesis may contribute to the "excess substrate uptake" phenomenon. (5) It is possible that some of the glucose which is synthesized in the cortex is used for glycolysis in the medulla. Conversely, lactate produced in the medulla may be available to the cortex for bicarbonate trapping and thus for gluconeogenesis.

A role for bicarbonate in the regulation of mammalian glutamine metabolism

1. The concentration of HCO3- (independent of any change of pH) exerts different effects on glutamine metabolism in rat kidney-cortex tubules, hepatocytes and enterocytes.2. In kidney tubules HCO3- (10.5-50 MM) has no effect on glutaminase (EC 3.5.1.2), whereas glutamate dehydrogenase (EC 1.4.1.3) is inhibited as HCO3- concentration is increased. The result is that flux through the entire glutamate-to-glucose pathway is inhibited by increasing HCO3- concentrations. A large proportion (more than 30%) of the glutamine removed undergoes complete oxidation. 3. In hepatocytes, and to a smaller extent in enterocytes, HCO3- is an accelerator of glutaminase. Synthesis of glucose and urea from glutamine in hepatocytes increases as HCO3- concentration is increased. Calculations show that fumarate, formed via aspartate aminotransferase and arginino-succinate lyase, is the precursor of the glucose. There is no complete oxidation of the carbon skeleton of glutamine in hepatocytes. 4. Leucine at near-physiological concentrations (0.1-1 mM) is an accelerator of glutaminase in hepatocytes, but not in kidney tubules or in enterocytes. 5. The results are discussed in relation to regulation of acid/base balance in vivo.