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Objective: To investigate whether ischemia alters donor kidney metabolism and whether these changes are associated with organ function. Background: An unmet need in kidney transplantation is the ability to predict posttransplant organ function before transplantation. Key to such viability testing is a profound understanding of the organ's complex biochemistry and how ischemia, inevitable during the transplantation process, influences this. Methods: First, metabolic changes in perfusate glucose, lactate, and 20 amino acids, induced by no, 1 hour of warm, or 22 hours of cold ischemia, were investigated during 4-hour perfusion of pig kidneys with autologous whole blood (n = 6/group), simulating the ischemia-reperfusion phase of transplantation. Next, we confirmed similar metabolic changes during normothermic preservation of pigs (n = 3/group; n = 4 for cold ischemia) and discarded human kidneys (n = 6) perfused with a red blood cell-based perfusate. Results: At 2 hours of perfusion with autologous whole blood, abundances of 17/20 amino acids were significantly different between groups, reflecting the type of ischemia. Amino acid changes at 15 minutes and 2 hours of perfusion correlated with future kidney function during perfusion. Similar metabolic patterns were observed during perfusion preservation of pig and discarded human donor kidneys, suggesting an opportunity to assess kidney viability before transplantation. Conclusions: Perfusate metabolite changes during normothermic kidney perfusion represent a unique noninvasive opportunity to assess graft viability. These findings now need validation in transplant studies. [ABSTRACT FROM AUTHOR]

This scoping review summarizes what is known about kidney metabolism during hypothermic perfusion preservation. Papers studying kidney metabolism during hypothermic (<12 °C) perfusion were identified (PubMed, Embase, Web of Science, Cochrane). Out of 14,335 initially identified records, 52 were included [dog (26/52), rabbit (2/52), pig (20/52), human (7/52)]. These were published between 1970–2023, partially explaining study heterogeneity. There is a considerable risk of bias in the reported studies. Studies used different perfusates, oxygenation levels, kidney injury levels, and devices and reported on perfusate and tissue metabolites. In 11 papers, (non)radioactively labeled metabolites (tracers) were used to study metabolic pathways. Together these studies show that kidneys are metabolically active during hypothermic perfusion, regardless of the perfusion setting. Although tracers give us more insight into active metabolic pathways, kidney metabolism during hypothermic perfusion is incompletely understood. Metabolism is influenced by perfusate composition, oxygenation levels, and likely also by pre-existing ischemic injury. In the modern era, with increasing donations after circulatory death and the emergence of hypothermic oxygenated perfusion, the focus should be on understanding metabolic perturbations caused by pre-existing injury levels and the effect of perfusate oxygen levels. The use of tracers is indispensable to understanding the kidney's metabolism during perfusion, given the complexity of interactions between different metabolites. [ABSTRACT FROM AUTHOR]

Background: Understanding kidney metabolism during perfusion is vital to further develop the technology as a preservation, viability assessment, and resuscitation platform. We reviewed the evidence on the use of labeled metabolites (tracers) to understand "on‐pump" kidney behavior. Methods: PubMed, Embase, Web of Science, and Cochrane databases were systematically searched for studies evaluating metabolism of (non)radioactively labeled endogenous compounds during kidney perfusion. Results: Of 5899 articles, 30 were included. All were animal studies [rat (70%), dog (13%), pig (10%), rabbit (7%)] perfusing but not transplanting kidneys. Perfusion took place at hypothermic (4–12°C) (20%), normothermic (35–40°C) (77%), or undefined temperatures (3%). Hypothermic perfusion used albumin or a clinical kidney preservation solution, mostly in the presence of oxygen. Normothermic perfusion was mostly performed with oxygenated crystalloids often containing glucose and amino acids with unclear partial oxygen tensions. Active metabolism of carbohydrate, amino acid, lipids, and large molecules was shown in hypothermic and normothermic perfusion. Production of macromolecules, such as prostaglandin, thromboxane, and vitamin D, takes place during normothermic perfusion. No experiments compared differences in metabolic activity between hypothermic and normothermic perfusion. One conference abstract showed increased anaerobic metabolism in kidneys donated after circulatory death by adding labeled glucose to hypothermically perfused human kidneys. Conclusions: Tracer studies during kidney perfusion contribute to unraveling kidney metabolic behavior in pre‐clinical models. Whether findings are truly translational needs further investigation in large animal models of human kidneys. Furthermore, it is essential to better understand how ischemia changes this metabolic behavior. [ABSTRACT FROM AUTHOR]

In this review encouraged by original data, we first provided in vivo evidence that the kidney, comparative to the liver or brain, is an organ particularly rich in cysteine. In the kidney, the total availability of cysteine was higher in cortex tissue than in the medulla and distributed in free reduced, free oxidized and protein-bound fractions (in descending order). Next, we provided a comprehensive integrated review on the evidence that supports the reliance on cysteine of the kidney beyond cysteine antioxidant properties, highlighting the relevance of cysteine and its renal metabolism in the control of cysteine excess in the body as a pivotal source of metabolites to kidney biomass and bioenergetics and a promoter of adaptive responses to stressors. This view might translate into novel perspectives on the mechanisms of kidney function and blood pressure regulation and on clinical implications of the cysteine-related thiolome as a tool in precision medicine.

Inhibitors of proximal tubular Na+-glucose cotransporter 2 (SGLT2) are natriuretic, and they lower blood pressure. There are reports that the activities of SGLT2 and Na+-H+ exchanger 3 (NHE3) are coordinated. If so, then part of the natriuretic response to an SGLT2 inhibitor is mediated by suppressing NHE3. To examine this further, we compared the effects of an SGLT2 inhibitor, empagliflozin, on urine composition and systolic blood pressure (SBP) in nondiabetic mice with tubule-specific NHE3 knockdown (NHE3-ko) and wild-type (WT) littermates. A single dose of empagliflozin, titrated to cause minimal glucosuria, increased urinary excretion of Na+ and bicarbonate and raised urine pH in WT mice but not in NHE3-ko mice. Chronic empagliflozin treatment tended to lower SBP despite higher renal renin mRNA expression and lowered the ratio of SBP to renin mRNA, indicating volume loss. This effect of empagliflozin depended on tubular NHE3. In diabetic Akita mice, chronic empagliflozin enhanced phosphorylation of NHE3 (S552/S605), changes previously linked to lesser NHE3-mediated reabsorption. Chronic empagliflozin also increased expression of genes involved with renal gluconeogenesis, bicarbonate regeneration, and ammonium formation. While this could reflect compensatory responses to acidification of proximal tubular cells resulting from reduced NHE3 activity, these effects were at least in part independent of tubular NHE3 and potentially indicated metabolic adaptations to urinary glucose loss. Moreover, empagliflozin increased luminal α-ketoglutarate, which may serve to stimulate compensatory distal NaCl reabsorption, while cogenerated and excreted ammonium balances urine losses of this “potential bicarbonate.” The data implicate NHE3 as a determinant of the natriuretic effect of empagliflozin. [ABSTRACT FROM AUTHOR]

Quercetin is a flavonoid with antioxidant, antiviral, antimicrobial, and anti-inflammatory properties. Therefore, it has been postulated as a molecule with great therapeutic potential. The renoprotective capacity of quercetin against various toxins that produce oxidative stress, in both in vivo and in vitro models, has been shown. However, it is not clear whether quercetin itself or any of its metabolites are responsible for the protective effects on the kidney. Although the pharmacokinetics of quercetin have been widely studied and the complexity of its transit throughout the body is well known, the metabolic processes that occur in the kidney are less known. Because of that, the objective of this review was to delve into the molecular and cellular events triggered by quercetin and/or its metabolites in the tubular cells, which could explain some of the protective properties of this flavonoid against oxidative stress produced by toxin administration. Thus, the following are analyzed: (1) the transit of quercetin to the kidney; (2) the uptake mechanisms of quercetin and its metabolites from plasma to the tubular cells; (3) the metabolic processes triggered in those cells, which affect the accumulation of metabolites in the intracellular space; and (4) the efflux mechanisms of these compounds and their subsequent elimination through urine. Finally, it is discussed whether those processes that are mediated in the tubular cells and that give rise to different metabolites are related to the antioxidant and renoprotective properties observed after the administration of quercetin.

Lipid accumulation is a pathological feature of every type of kidney injury. Despite this striking histological feature, physiological accumulation of lipids in the kidney is poorly understood. We studied whether the accumulation of lipids in the fasted kidney are derived from lipoproteins or NEFAs. With overnight fasting, kidneys accumulated triglyceride, but had reduced levels of ceramide and glycosphingolipid species. Fasting led to a nearly 5-fold increase in kidney uptake of plasma [14C]oleic acid. Increasing circulating NEFAs using a β adrenergic receptor agonist caused a 15-fold greater accumulation of lipid in the kidney, while mice with reduced NEFAs due to adipose tissue deficiency of adipose triglyceride lipase had reduced triglycerides. Cluster of differentiation (Cd)36 mRNA increased 2-fold, and angiopoietin-like 4 (Angptl4), an LPL inhibitor, increased 10-fold. Fasting-induced kidney lipid accumulation was not affected by inhibition of LPL with poloxamer 407 or by use of mice with induced genetic LPL deletion. Despite the increase in CD36 expression with fasting, genetic loss of CD36 did not alter fatty acid uptake or triglyceride accumulation. Our data demonstrate that fasting-induced triglyceride accumulation in the kidney correlates with the plasma concentrations of NEFAs, but is not due to uptake of lipoprotein lipids and does not involve the fatty acid transporter, CD36.

This study evaluated the effects of high-dose of short-term creatine supplementation (5g·kg-1·day-1 to 1 week) and long-term creatine supplementation (1g·kg-1·day-1 to 4-8 weeks) on kidney and liver structure and function of sedentary and exercised Wistar rats (Exercise sessions consisted of swimming at 80% of maximal work load supported during 5 days per week with daily sessions of 60 minutes throughout the duration of the supplementation). Seventy-two animals (245 ± 5g) were divided into four groups (n = 18): control diet Sedentary (SED), Creatine diet Sedentary (CRE), control diet Exercised (EXE), and Creatine diet Exercised (EXECRE). Histological and blood biochemical studies were performed after one, four, and eight weeks of creatine supplementation and exercise (n = 6). No differences were found when comparing SED, EXE and EXECRE groups for kidney and liver structure and function at one, four and eight weeks. However, the CRE group showed higher levels of creatinine (1.1 ± 0.2 vs. 0.4 ± 0.1 mg·dl-1; p < 0.05), and urea (37 ± 3 vs. 19 ± 1 mg·dl-1; p < 0.05) when compared with all others groups at four and eight weeks. At eight weeks, the CRE group presented increased levels of ALT (41 ± 7 vs. 23 ± 7 U·L-1; p < 0.05), AST (89 ± 6 vs. 62 ± 5 U.L-1; p < 0.05), GGT (8.0 ± 0.9 vs. 3.9 ± 1.0 U.L-1; p < 0.05), and AP (125 ± 10 vs. 69 ± 9 U·L-1; p < 0.05) also when compared with all others groups. Moreover, the CRE group demonstrated some structural alterations indicating renal and hepatic damage at four and eight weeks, respectively. These results suggest that long-term creatine supplementation (up to 4-8 weeks) may adversely affect kidney and liver structure and function of sedentary but not of exercised rats. [ABSTRACT FROM AUTHOR]

In this review encouraged by original data, we first provided in vivo evidence that the kidney, comparative to the liver or brain, is an organ particularly rich in cysteine. In the kidney, the total availability of cysteine was higher in cortex tissue than in the medulla and distributed in free reduced, free oxidized and protein-bound fractions (in descending order). Next, we provided a comprehensive integrated review on the evidence that supports the reliance on cysteine of the kidney beyond cysteine antioxidant properties, highlighting the relevance of cysteine and its renal metabolism in the control of cysteine excess in the body as a pivotal source of metabolites to kidney biomass and bioenergetics and a promoter of adaptive responses to stressors. This view might translate into novel perspectives on the mechanisms of kidney function and blood pressure regulation and on clinical implications of the cysteine-related thiolome as a tool in precision medicine. [ABSTRACT FROM AUTHOR]

The collecting duct of the kidney is composed of two morphologically and physiologically distinct cell types, principal and intercalated cells. To better understand intercalated cell function we generated a transgenic mouse expressing Cre recombinase under the control of a cell type- specific promoter. We used 7 kb of the ATP6V1B1 5′ untranslated region (B1 promoter), a gene found in the intercalated cells of the kidney and the male reproductive tract. We first crossed these B1-Cre transgenic mice with the ROSA26-loxP-stop-loxP-yellow fluorescent protein reporter mice to assess the specificity of Cre expression. Immunohistochemistry and confocal fluorescence microscopy showed that Cre is selectively active in all intercalated cells (type A, type B, and non-A/B cells) within the collecting duct and most cells of the connecting segment. About half of the principal cells of the connecting segment also expressed Cre, a pattern also seen in B1-driven enhanced green fluorescent protein transgenic mice. Cre was found to be active in the male reproductive tract and at a low level in limited non-ATP6V1B1 expressing tissues. The B1-Cre transgenic mice are healthy, breed normally, produce regular sized litters, and transmit the transgene in Mendelian fashion. This new cell-specific Cre expressing mouse should prove useful for the study of intercalated cell physiology and development.Kidney International (2009) 75, 435–439; doi:10.1038/ki.2008.569; published online 3 December 2008 [ABSTRACT FROM AUTHOR]

Alterations of protein metabolism by metabolic acidosis in children with chronic renal failure. Background. Several reports suggest that metabolic acidosis may induce significant alterations in protein metabolism and that its outbreak may even result in growth retardation in children with chronic renal failure (CRF). However, the effects of metabolic acidosis on protein metabolism kinetics have never been investigated in these settings. Methods. Postabsorptive leucine metabolism, a marker of whole-body protein metabolism, was measured by using a primed, continuous intravenous infusion of L-[1-13 C]leucine in 10 CRF children who were one to four years old. Results. Interindividual values of whole-body protein turnover exhibited a very large range, which was mainly accounted for by acidotic status (plasma HCO3 - ) and body composition [fat-free mass (FFM)]. After correction for differences in FFM, plasma HCO3 - was highly correlated with protein breakdown (R 2 = 0.65, P < 0.001), so that CRF children were divided in two groups according to their acid-base status: Group A had a mean plasma HCO3 - level of 15.8 ± 1.5 mmol · L-1 (mean ± SD, N = 5), whereas group B had near-normal values (HCO3 - , 22.6 ± 3.0 mmol · L-1 , N = 5). The leucine rate of appearance from protein breakdown was markedly higher in group A than in group B (4.15 ± 1.43 vs. 2.46 ± 0.47 μmol · kg-1 · min-1 , respectively, P < 0.05), and the net leucine balance tended to be more negative in group A (-0.73 ± 0.34 vs. -0.44 ± 0.26 μmol · kg-1 · min-1 , respectively). Conclusions. Metabolic acidosis in children with CRF results in an excessive catabolic state, suggesting that acidosis-related protein wasting could contribute to growth retardation. [ABSTRACT FROM AUTHOR]

Quercetin is a flavonoid with antioxidant, antiviral, antimicrobial, and anti-inflammatory properties. Therefore, it has been postulated as a molecule with great therapeutic potential. The renoprotective capacity of quercetin against various toxins that produce oxidative stress, in both in vivo and in vitro models, has been shown. However, it is not clear whether quercetin itself or any of its metabolites are responsible for the protective effects on the kidney. Although the pharmacokinetics of quercetin have been widely studied and the complexity of its transit throughout the body is well known, the metabolic processes that occur in the kidney are less known. Because of that, the objective of this review was to delve into the molecular and cellular events triggered by quercetin and/or its metabolites in the tubular cells, which could explain some of the protective properties of this flavonoid against oxidative stress produced by toxin administration. Thus, the following are analyzed: (1) the transit of quercetin to the kidney; (2) the uptake mechanisms of quercetin and its metabolites from plasma to the tubular cells; (3) the metabolic processes triggered in those cells, which affect the accumulation of metabolites in the intracellular space; and (4) the efflux mechanisms of these compounds and their subsequent elimination through urine. Finally, it is discussed whether those processes that are mediated in the tubular cells and that give rise to different metabolites are related to the antioxidant and renoprotective properties observed after the administration of quercetin. [ABSTRACT FROM AUTHOR]

Tissue engineering of a bioartificial renal tubule assist device: In vitro transport and metabolic characteristics. Background. Current renal substitution therapy for acute or chronic renal failure with hemodialysis or hemofiltration is life sustaining, but continues to have unacceptably high morbidity and mortality rates. This therapy is not complete renal replacement therapy because it does not provide active transport nor metabolic and endocrinologic functions of the kidney, which are located predominantly in the tubular elements of the kidney. Methods. To optimize renal substitution therapy, a bioartificial renal tubule assist device (RAD) was developed and tested in vitro for a variety of differentiated tubular functions. High-flux hollow-fiber hemofiltration cartridges with membrane surface areas of 97 cm2 or 0.4 m2 were used as tubular scaffolds. Porcine renal proximal tubule cells were seeded into the intraluminal spaces of the hollow fibers, which were pretreated with a synthetic extracellular matrix protein. Attached cells were expanded in the cartridge as a bioreactor system to produce confluent monolayers containing up to 1.5 × 109 cells (3.5 × 105 cells/cm2 ). Near confluency was achieved along the entire membrane surface, with recovery rates for perfused inulin exceeding 97 and 95% in the smaller and larger units, respectively, compared with less than 60% recovery in noncell units. Results. A single-pass perfusion system was used to assess transport characteristics of the RADs. Vectorial fluid transport from intraluminal space to antiluminal space was demonstrated and was significantly increased with the addition of albumin to the antiluminal side and inhibited by the addition of ouabain, a specific inhibitor of Na+ ,K+ -ATPase. Other transport activities were also observed in these devices and included active bicarbonate transport, which was decreased with... [ABSTRACT FROM AUTHOR]

The effects of wide concentration ranges of furosemide, ethacrynic acid, and chlorothiazide on glucose formation by rat kidney cortex slices were investigated from fructose, pyruvate, lactate, and α-ketoglutarate and partly at two different pH. Under control conditions without the drugs, more glucose was synthetized in μmoles/g dry weight · hour at the lower pH of 7.21 than at pH 7.49 without substrate as well as with the 5 substrates studied. Furosemide stimulated the rate of gluconeogenesis concentration dependent if fructose, pyruvate, or lactate were present as substrates. The percent increase was higher at pH 7.49 but the rates achieved with the maximal effective concentrations were about the same at both pH. The rate of glucose formation was only slightly enhanced from the substrates malate and α-ketoglutarate. Ethacrynic acid enhanced the rate of glucose synthesis from fructose, pyruvate, and lactate, too, but had no stimulating effects, if malate or α-ketoglutarate were the substrates. It exerted its effects at even lower concentrations than furosemide, but the maximal rates which could be observed were in the same range. Gluconeogenesis was not stimulated by chlorothiazide. Various and different sites of action on enzyme activities in the Embden-Meyerhof pathway are discussed for the two drugs influenced either directly by them or indirectly by changing concentrations of ATP or cyclic AMP. [ABSTRACT FROM AUTHOR]

The kidney is a highly metabolically active organ that relies on specialized epithelial cells comprising the renal tubules to reabsorb most of the filtered water and solutes. Most of this reabsorption is mediated by the proximal tubules, and high amounts of energy are needed to facilitate solute movement. Thus, proximal tubules use fatty acid oxidation, which generates more adenosine triphosphate (ATP) than glucose metabolism, as its preferred metabolic pathway. After kidney injury, metabolism is altered, leading to decreased fatty acid oxidation and increased lactic acid generation. This review discusses how metabolism differs between the proximal and more distal tubular segments of the healthy nephron. In addition, metabolic changes in acute kidney injury and chronic kidney disease are discussed, as well as how these changes in metabolism may impact tubule repair and chronic kidney disease progression. [ABSTRACT FROM AUTHOR]

The kidneys are metabolically active organs with importance in several physiological tasks such as the secretion of soluble wastes into the urine and synthesizing glucose and oxidizing fatty acids for energy in fasting (non-fed) conditions. Once damaged, the metabolic capability of the kidneys becomes altered. Here, we define metabolic tasks in a computational modeling framework to capture kidney function in an update to the iRno network reconstruction of rat metabolism using literature-based evidence. To demonstrate the utility of iRno for predicting kidney function, we exposed primary rat renal proximal tubule epithelial cells to four compounds with varying levels of nephrotoxicity (acetaminophen, gentamicin, 2,3,7,8-tetrachlorodibenzodioxin, and trichloroethylene) for six and twenty-four hours, and collected transcriptomics and metabolomics data to measure the metabolic effects of compound exposure. For the transcriptomics data, we observed changes in fatty acid metabolism and amino acid metabolism, as well as changes in existing markers of kidney function such as Clu (clusterin). The iRno metabolic network reconstruction was used to predict alterations in these same pathways after integrating transcriptomics data and was able to distinguish between select compound-specific effects on the proximal tubule epithelial cells. Genome-scale metabolic network reconstructions with coupled omics data can be used to predict changes in metabolism as a step towards identifying novel metabolic biomarkers of kidney function and dysfunction. • Updated metabolic network reconstruction for rat kidney metabolism. • Transcriptomics and metabolomics reveal distinct markers of in vitro kidney injury. • Metabolic networks connect changes in transcriptomics and metabolomics. • Potential biomarkers of in vitro kidney injury. [ABSTRACT FROM AUTHOR]

The leptin receptor-deficient db/db mouse model is an accepted in vivo model to study obesity, type 2 diabetes, and diabetic kidney disease. Healthy gastrointestinal (GI) microbiota has been linked to weight loss, improved glycemic control, and physiological benefits. We investigated the effect of various drugs on the GI microbiota of db/db mice as compared to control db/m mice. Treatment with long-acting pirfenidone (PFD) increased gut microbial diversity in diabetic db/db mice. Firmicutes, the most abundant phylum in db/m mice, decreased significantly in abundance in db/db mice but showed increased abundance with long-acting PFD treatment. Several bacterial taxa, including Lactobacillus and some Bacteroides, were less abundant in db/db mice and more abundant in long-acting-PFD-treated db/db mice. Long-acting PFD treatment reduced the abundance of Akkermansia muciniphila (5%) as compared to db/db mice (~15%). We conclude that gut microbial dysbiosis observed in db/db mice was partially reversed by long-acting PFD treatment and hypothesize that PFD has beneficial effects, in part, via its influence on the gut microbial metabolite profile. In quantitatively assessing urine metabolites, we observed a high abundance of diabetic ketoacidosis biomarkers, including 3-hydroxybutyric acid and acetoacetic acid in db/db mice, which were less abundant in the long-acting-PFD-treated db/db mice. [ABSTRACT FROM AUTHOR]