Regulation of renal ammoniagenesis in metabolic acidosis

Interactive Effects of Empagliflozin and Hyperglycemia on Urinary Amino Acids in Individuals With Type 1 Diabetes

Optimizing energy use in the kidney is critical for normal kidney function. Here, we investigate the effect of hyperglycemia and sodium-glucose cotransporter 2 (SGLT2) inhibition on urinary amino acid excretion in individuals with type 1 diabetes (T1D). The open-label ATIRMA trial assessed the impact of 8 weeks of 25 mg empagliflozin orally once per day in 40 normotensive normoalbuminuric young adults with T1D. A consecutive 2-day assessment of clamped euglycemia and hyperglycemia was evaluated at baseline and posttreatment visits. Principal component analysis was performed on urinary amino acids grouped into representative metabolic pathways using MetaboAnalyst. At baseline, acute hyperglycemia was associated with changes in 25 of the 33 urinary amino acids or their metabolites. The most significant amino acid metabolites affected by acute hyperglycemia were 3-hydroxykynurenine, serotonin, glycyl-histidine, and nicotinic acid. The changes in amino acid metabolites were reflected by the induction of four biosynthetic pathways: aminoacyl-tRNA; valine, leucine, and isoleucine; arginine; and phenylalanine, tyrosine, and tryptophan. In acute hyperglycemia, empagliflozin significantly attenuated the increases in aminoacyl-tRNA biosynthesis and valine, leucine, and isoleucine biosynthesis. Our findings using amino acid metabolomics indicate that hyperglycemia stimulates biosynthetic pathways in T1D. SGLT2 inhibition may attenuate the increase in biosynthetic pathways to optimize kidney energy metabolism.

ARTICLE HIGHLIGHTS

Renal Autocrine and Paracrine Signaling: A Story of Self-protection

Autocrine and paracrine signaling in the kidney adds an extra level of diversity and complexity to renal physiology. The extensive scientific production on the topic precludes easy understanding of the fundamental purpose of the vast number of molecules and systems that influence the renal function. This systematic review provides the broader pen strokes for a collected image of renal paracrine signaling. First, we recapitulate the essence of each paracrine system one by one. Thereafter the single components are merged into an overarching physiological concept. The presented survey shows that despite the diversity in the web of paracrine factors, the collected effect on renal function may not be complicated after all. In essence, paracrine activation provides an intelligent system that perceives minor perturbations and reacts with a coordinated and integrated tissue response that relieves the work load from the renal epithelia and favors diuresis and natriuresis. We suggest that the overall function of paracrine signaling is reno-protection and argue that renal paracrine signaling and self-regulation are two sides of the same coin. Thus local paracrine signaling is an intrinsic function of the kidney, and the overall renal effect of changes in blood pressure, volume load, and systemic hormones will always be tinted by its paracrine status.

Effect of renal tubule-specific knockdown of the Na+/H+ exchanger NHE3 in Akita diabetic mice

Na+/H+ exchanger isoform 3 (NHE3) contributes to Na+/bicarbonate reabsorption and ammonium secretion in early proximal tubules. To determine its role in the diabetic kidney, type 1 diabetic Akita mice with tubular NHE3 knockdown [Pax8-Cre; NHE3-knockout (KO) mice] were generated. NHE3-KO mice had higher urine pH, more bicarbonaturia, and compensating increases in renal mRNA expression for genes associated with generation of ammonium, bicarbonate, and glucose (phosphoenolpyruvate carboxykinase) in proximal tubules and H+ and ammonia secretion and glycolysis in distal tubules. This left blood pH and bicarbonate unaffected in nondiabetic and diabetic NHE3-KO versus wild-type mice but was associated with renal upregulation of proinflammatory markers. Higher renal phosphoenolpyruvate carboxykinase expression in NHE3-KO mice was associated with lower Na+-glucose cotransporter (SGLT)2 and higher SGLT1 expression, indicating a downward tubular shift in Na+ and glucose reabsorption. NHE3-KO was associated with lesser kidney weight and glomerular filtration rate (GFR) independent of diabetes and prevented diabetes-associated albuminuria. NHE3-KO, however, did not attenuate hyperglycemia or prevent diabetes from increasing kidney weight and GFR. Higher renal gluconeogenesis may explain similar hyperglycemia despite lower SGLT2 expression and higher glucosuria in diabetic NHE3-KO versus wild-type mice; stronger SGLT1 engagement could have affected kidney weight and GFR responses. Chronic kidney disease in humans is associated with reduced urinary excretion of metabolites of branched-chain amino acids and the tricarboxylic acid cycle, a pattern mimicked in diabetic wild-type mice. This pattern was reversed in nondiabetic NHE3-KO mice, possibly reflecting branched-chain amino acids use for ammoniagenesis and tricarboxylic acid cycle upregulation to support formation of ammonia, bicarbonate, and glucose in proximal tubule. NHE3-KO, however, did not prevent the diabetes-induced urinary downregulation in these metabolites.

Renal ammonia metabolism and transport

Renal ammonia metabolism and transport mediates a central role in acid-base homeostasis. In contrast to most renal solutes, the majority of renal ammonia excretion derives from intrarenal production, not from glomerular filtration. Renal ammoniagenesis predominantly results from glutamine metabolism, which produces 2 NH4(+) and 2 HCO3(-) for each glutamine metabolized. The proximal tubule is the primary site for ammoniagenesis, but there is evidence for ammoniagenesis by most renal epithelial cells. Ammonia produced in the kidney is either excreted into the urine or returned to the systemic circulation through the renal veins. Ammonia excreted in the urine promotes acid excretion; ammonia returned to the systemic circulation is metabolized in the liver in a HCO3(-)-consuming process, resulting in no net benefit to acid-base homeostasis. Highly regulated ammonia transport by renal epithelial cells determines the proportion of ammonia excreted in the urine versus returned to the systemic circulation. The traditional paradigm of ammonia transport involving passive NH3 diffusion, protonation in the lumen and NH4(+) trapping due to an inability to cross plasma membranes is being replaced by the recognition of limited plasma membrane NH3 permeability in combination with the presence of specific NH3-transporting and NH4(+)-transporting proteins in specific renal epithelial cells. Ammonia production and transport are regulated by a variety of factors, including extracellular pH and K(+), and by several hormones, such as mineralocorticoids, glucocorticoids and angiotensin II. This coordinated process of regulated ammonia production and transport is critical for the effective maintenance of acid-base homeostasis.

The role of glutamate dehydrogenase in mammalian ammonia metabolism

Glutamate dehydrogenase (GDH) catalyzes the reversible inter-conversion of glutamate to α-ketoglutarate and ammonia. High levels of GDH activity is found in mammalian liver, kidney, brain, and pancreas. In the liver, GDH reaction appears to be close-to-equilibrium, providing the appropriate ratio of ammonia and amino acids for urea synthesis in periportal hepatocytes. In addition, GDH produces glutamate for glutamine synthesis in a small rim of pericentral hepatocytes. Hence, hepatic GDH can be either a source for ammonia or an ammonia scavenger. In the kidney, GDH function produces ammonia from glutamate to control acidosis. In the human, the presence of two differentially regulated isoforms (hGDH1 and hGDH2) suggests a complex role for GDH in ammonia homeostasis. Whereas hGDH1 is sensitive to GTP inhibition, hGDH2 has dissociated its function from GTP control. Furthermore, hGDH2 shows a lower optimal pH than hGDH1. The hGDH2 enzyme is selectively expressed in human astrocytes and Sertoli cells, probably facilitating metabolic recycling processes essential for their supportive role. Here, we report that hGDH2 is also expressed in the epithelial cells lining the convoluted tubules of the renal cortex. As hGDH2 functions more efficiently under acidotic conditions without the operation of the GTP energy switch, its presence in the kidney may increase the efficacy of the organ to maintain acid base equilibrium.

Sustained activation of renal N-methyl-D-aspartate receptors decreases vitamin D synthesis: a possible role for glutamate on the onset of secondary HPT

N-methyl-D-aspartate (NMDA) receptors (NMDAR) are tetrameric amino acid receptors that act as membrane calcium channels. The presence of the receptor has been detected in the principal organs responsible for calcium homeostasis (kidney, bone, and parathyroid gland), pointing to a possible role in mineral metabolism. The aim of this study was to test the effect of NMDAR activation in the kidney and on 1,25(OH)₂D₃ synthesis. We determined the presence of NMDAR subunits in HK-2 (human kidney cells) cells and proved its functionality. NMDA treatment for 4 days induced a decrease in 1α-hydroxylase levels and 1,25(OH)₂D₃ synthesis through the activation of the MAPK/ERK pathway in HK-2 cells. In vivo administration of NMDA for 4 days also caused a decrease in blood 1,25(OH)₂D₃ levels in healthy animals and an increase in blood PTH levels. This increase in PTH induced a decrease in the urinary excretion of calcium and an increase in urinary excretion of phosphorous and sodium as well as in diuresis. Bone turnover markers also increased. Animals with 5/6 nephrectomy showed low levels of renal 1α-hydroxylase as well as high levels of renal glutamate compared with healthy animals. In conclusion, NMDAR activation in the kidney causes a decrease in 1,25(OH)₂D₃ synthesis, which induces an increase on PTH synthesis and release. In animals with chronic kidney disease, high renal levels of glutamate could be involved in the downregulation of 1α-hydroxylase expression.

Acid loading in vivo and low pH in culture increase angiotensin receptor expression: enhanced ammoniagenic response to angiotensin II

The proximal tubule defends the body against acid challenges by enhancing its production and secretion of ammonia. Our previous studies demonstrated an enhanced ammoniagenic response of the proximal tubule to ANG II added to the lumen in vitro after an in vivo acid challenge. The present study examined the effect of NH4Cl acid loading in vivo on renal cortical type 1 ANG II (AT1) receptor expression, the effect of low pH on AT1receptor expression in a proximal tubule cells in culture, and their response to ANG II. A short-term (18 h) NH4Cl load in vivo resulted in increased renal cortical AT1receptor mRNA expression and increased brush-border membrane AT1receptor protein expression levels. Changing the cell culture pH from 7.4 to 7.0 for at least 2 h increased cell surface expression of AT1receptors and enhanced the stimulatory effect of ANG II on ammonia production rates. This increased ammoniagenic response to ANG II and the early enhancement of cell surface expression induced by exposure of the cultured proximal tubule cells to pH 7.0 were prevented by treatment with colchicine. These results suggest that, after acid challenges, the enhanced ammoniagenic response of the proximal tubule to ANG II is, in part, mediated by increased AT1receptor cell surface expression and that the enhancement of receptor expression plays an important role in the early response of the proximal tubule to acid challenges.

Role of angiotensin II in the enhancement of ammonia production and secretion by the proximal tubule in metabolic acidosis

Acidosis and angiotensin II stimulate ammonia production and transport by the proximal tubule. We examined the modulatory effect of the type 1 angiotensin II receptor blocker losartan on the ability of metabolic acidosis to stimulate ammonia production and secretion by mouse S2 proximal tubule segments. Mice given NH4Cl for 7 days developed metabolic acidosis (low serum bicarbonate concentration) and increased urinary excretion of ammonia. S2 tubule segments from acidotic mice displayed higher rates of ammonia production and secretion compared with those from control mice. However, when losartan was coadministered in vivo with NH4Cl, both the acidosis-induced increase in urinary ammonia excretion and the adaptive increase in ammonia production and secretion of microperfused S2 segments were largely blocked. In renal cortical tissue, losartan blocked the acid-induced increase in brush-border membrane NHE3 expression but had no effect on the acid-induced upregulation of phosphate-dependent glutaminase or phosphoenolpyruvate carboxykinase 1 in cortical homogenates. Addition of angiotensin II to the microperfusion solution enhanced ammonia secretion and production rates in tubules from NH4Cl-treated and control mice in a losartan-inhibitable manner. These results demonstrate that a 7-day acid challenge induces an adaptive increase in ammonia production and secretion by the proximal tubule and suggest that during metabolic acidosis, angiotensin II signaling is necessary for adaptive enhancements of ammonia excretion by the kidney and ammonia production and secretion by S2 proximal tubule segments, as mediated, in part, by angiotensin receptor-dependent enhancement of NHE3 expression.

Application of branched-chain amino acids in human pathological states: renal failure

During renal failure, abnormalities of BCAA and branched-chain keto acid (BCKA) metabolism are due to both the lack of renal contribution to amino acid metabolism and the impact of renal failure and acidosis on whole-body nitrogen metabolism. Abnormal BCAA and BCKA metabolism result in BCAA depletion as reflected by low plasma BCAAs and cellular valine. BCAA metabolic disturbances can alter tissue activities, particularly brain function, and nutritional status. In dialysis patients, BCAA oral supplementation can induce an improvement of appetite and nutritional status. During chronic renal failure, the aims of nutritional interventions are to minimize uremic toxicity, avoid malnutrition and delay progression of kidney disease. BCAA and BCKA supplements have been proposed to decrease further protein intake while maintaining satisfactory nutritional status. In this setting, BCAAs or BCKAs have not been administrated solely but in association with other essential AA or keto analogs. Therefore, the proper effects of BCAAs and/or BCKAs have not been studied separately. Protein restriction together with keto acids and/or essential AAs has been reported to improve insulin sensitivity and hyperparathyroidism and to be compatible with a preservation of nutritional status. Nonetheless, a careful monitoring of protein-calorie intake and nutritional status is needed. A recent meta-analysis concluded that reducing protein intake in patients with chronic renal failure reduces the occurrence of renal death by approximately 40% as compared with larger or unrestricted protein intake. The additional effect of essential amino acids and keto acids on retardation of progression of renal failure has not been demonstrated.

Regulatory and functional interaction of vasoactive factors in the kidney and extracellular pH

A growing body of evidence suggests that vasoactive factors produced in the kidney such as nitric oxide, endothelins, angiotensin, and prostaglandins participate actively in the regulation of acid-base homeostasis under physiologic conditions. In addition, recent reports indicate that alterations in the systemic acid-base status may also influence the generation of vasoactive cytokines in the kidney, which in turn may mediate the renal effector processes that tend to restore normality under such conditions. Metabolic acidosis, which so frequently accompanies many forms of chronic renal failure (CRF), may contribute to down-regulation of intrarenal nitric oxide production that characterizes CRF. Reduced extracellular pH inhibits inducible nitric oxide production in mesangial cells by altering the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) oxidation, an important posttranslational mechanism in the inducible nitric oxide synthase (iNOS) activation. The underlying defects resulting in the uncoupling of NADPH oxidation in acidemic microenvironment are discussed. Acidosis stimulates renal production of endothelins, which mediate proximal tubular acidification by enhancing sodium-hydrogen exchanger-3 (NHE-3) activity. Renal endothelins mediate enhanced urinary acid excretion following dietary acid ingestion, an effect that is effectively blocked by endothelin receptor blockers. Reduced extracellular pH stimulates endothelin secretion from renal microvascular endothelial cells, which may promote enhanced acid excretion from the distal tubule under conditions of acidosis. These phenomena as well as the role of angiotensin and renal prostaglandins in mediating renal acidification in normal and acidotic conditions are discussed in this review, which describe the regulatory interaction between extracellular pH and renal vasoactive factors.

Ammonia production and secretion by S3 proximal tubule segments from acidotic mice: role of ANG II

ANG II has potent effects on ammonia production and secretion rates by the proximal tubule and is found in substantial concentrations in the lumen of the proximal tubule in vivo. Because our previous studies demonstrated that acid loading enhanced the stimulatory effects of ANG II on ammonia production and secretion by S2 proximal tubule segments, we examined the effect of ANG II on ammonia production and secretion by isolated, perfused S3 segments from nonacidotic control mice and acidotic mice given NH4Cl for 7 days. In the absence of ANG II, ammonia production and secretion rates were no different in S3 segments from acidotic and control mice. By contrast, when ANG II was present in the luminal perfusion solution, ammonia production and secretion rates were stimulated, in a losartan-inhibitable manner, to a greater extent in S3 segments from acidotic mice. Ammonia secretion rates in S3 segments were largely inhibited by perfusion with a low-sodium solution containing amiloride in the presence or absence of ANG II. These results demonstrated that isolated, perfused mouse S3 proximal tubule segments produce and secrete ammonia, that NH4Cl-induced acidosis does not affect the basal rates of ammonia production and secretion, and that ANG II, added to the luminal fluid, stimulates ammonia production and secretion to a greater extent in S3 segments from acidotic mice. These findings suggest that S3 segments, in the presence of ANG II, can contribute to the enhanced renal excretion that occurs with acid loading.

Renal metabolism of amino acids: its role in interorgan amino acid exchange

The kidneys play a role in the synthesis and interorgan exchange of several amino acids. The quantitative importance of renal amino acid metabolism in the body is not, however, clear. We review here the role of the kidney in the interorgan exchange of amino acids, with emphasis on quantitative aspects. We reviewed relevant literature by using a computerized literature search (PubMed) and checking relevant references from the identified articles. Our own data are discussed in the context of the literature. The kidney takes up glutamine and metabolizes it to ammonia. This process is sensitive to pH and serves to maintain acid-base homeostasis and to excrete nitrogen. In this way, the metabolism of renal glutamine and ammonia is complementary to hepatic urea synthesis. Citrulline, derived from intestinal glutamine breakdown, is converted to arginine by the kidney. Renal phenylalanine uptake is followed by stoichiometric tyrosine release, and glycine uptake is accompanied by serine release. Certain administered oligopeptides (eg, glutamine dipeptides) are converted by the kidneys to their constituent components before they can be used in metabolic processes. The kidneys play an important role in the interorgan exchange of amino acids. Quantitatively, for several important amino acids, the kidneys are as important as the gut in intermediary metabolism. The kidneys may be crucial "mediators" of the beneficial effects of specialized, disease-specific feeding solutions such as those enriched in glutamine dipeptides.

pH-responsive stabilization of glutamate dehydrogenase mRNA in LLC-PK1-F+ cells

During chronic metabolic acidosis, the adaptive increase in rat renal ammoniagenesis is sustained, in part, by increased expression of mitochondrial glutaminase (GA) and glutamate dehydrogenase (GDH) enzymes. The increase in GA activity results from the pH-responsive stabilization of GA mRNA. The 3'-untranslated region (3'-UTR) of GA mRNA contains a direct repeat of an eight-base AU-rich element (ARE) that binds zeta-crystallin/NADPH:quinone reductase (zeta-crystallin) with high affinity and functions as a pH-response element. RNA EMSAs established that zeta-crystallin also binds to the full-length 3'-UTR of GDH mRNA. This region contains four eight-base sequences that are 88% identical to one of the two GA AREs. Direct binding assays and competition studies indicate that the two individual eight-base AREs from GA mRNA and the four individual GDH sequences bind zeta-crystallin with different affinities. Insertion of the 3'-UTR of GDH cDNA into a beta-globin expression vector (pbetaG) produced a chimeric mRNA that was stabilized when LLC-PK1-F+ cells were transferred to acidic medium. A pH-responsive stabilization was also observed using a betaG construct that contained only the single GDH4 ARE and a destabilizing element from phosphoenolpyruvate carboxykinase mRNA. Therefore, during acidosis, the pH-responsive stabilization of GDH mRNA may be accomplished by the same mechanism that affects an increase in GA mRNA.

Enhanced ammonia secretion by proximal tubules from mice receiving NH(4)Cl: role of angiotensin II

Acidosis and angiotensin II (ANG II) stimulate ammonia production and transport by the proximal tubule. We examined the effect of short-term (18 h) in vivo acid loading with NH(4)Cl on ammonia production and secretion rates by mouse S2 proximal tubule segments microperfused in vitro with or without ANG II in the luminal microperfusion solution. S2 tubules from NH(4)Cl-treated mice displayed higher rates of luminal ammonia secretion compared with those from control mice. The adaptive increase in ammonia secretion in NH(4)Cl-treated mice was eliminated when losartan was coadministered in vivo with NH(4)Cl. Ammonia secretion rates from both NH(4)Cl-treated and control mice were largely inhibited by amiloride. Addition of ANG II to the microperfusion solution enhanced ammonia secretion and production rates to a greater extent in tubules from NH(4)Cl-treated mice compared with those from controls, and the stimulatory effects of ANG II were blocked by losartan. These results demonstrate that a short-term acid challenge induces an adaptive increase in ammonia secretion by the proximal tubule and suggest that ANG II plays an important role in the adaptive enhancement of ammonia secretion that is observed with short-term acid challenges.

Differential expression and acid-base regulation of glutaminase mRNAs in gluconeogenic LLC-PK(1)-FBPase(+) cells

LLC-PK(1)-FBPase(+) cells, which are a gluconeogenic substrain of porcine renal LLC-PK(1) cells, exhibit enhanced oxidative metabolism and increased levels of phosphate-dependent glutaminase (PDG) activity. On adaptation to acidic medium (pH 6.9, 9 mM HCO(-)(3)), LLC-PK(1)-FBPase(+) cells also exhibit a greater increase in ammonia production and respond with an increase in assayable PDG activity. The changes in PDG mRNA levels were examined by using confluent cells grown on plastic dishes or on permeable membrane inserts. The latter condition increased the state of differentiation of the LLC-PK(1)-FBPase(+) cells. The levels of the primary porcine PDG mRNAs were analyzed by using probes that are specific for the 5.0-kb PDG mRNA (p2400) or that react equally with both the 4.5- and 5.0-kb PDG mRNAs (p930 and r1500). In confluent dish- and filter-grown LLC-PK(1)-FBPase(+) cells, the predominant 4.5-kb PDG mRNA is increased threefold after 18 h in acidic media. However, in filter-grown epithelia, which sustain an imposed pH and HCO(-)(3) gradient, this adaptive increase is observed only when acidic medium is applied to both the apical and the basolateral sides of the epithelia. Half-life experiments established that induction of the 4. 5-kb PDG mRNA was due to its stabilization. An identical pattern of adaptive increases was observed for the cytosolic PEPCK mRNA. In contrast, no adaptive changes were observed in the levels of the 5. 0-kb PDG mRNA in either cell culture system. Furthermore, cultures were incubated in low-potassium (0.7 mM) media for 24-72 h to decrease intracellular pH while maintaining normal extracellular pH. LLC-PK(1)-FBPase(+) cells again responded with increased rates of ammonia production and increased levels of the 4.5-kb PDG and PEPCK mRNAs, suggesting that an intracellular acidosis is the initiator of this adaptive response. Because all of the observed responses closely mimic those characterized in vivo, the LLC-PK(1)-FBPase(+) cells represent a valuable tissue culture model to study the molecular mechanisms that regulate renal gene expression in response to changes in acid-base balance.

Renal responses of trout to chronic respiratory and metabolic acidoses and metabolic alkalosis

Exposure to hyperoxia (500-600 torr) or low pH (4.5) for 72 h or NaHCO(3) infusion for 48 h were used to create chronic respiratory (RA) or metabolic acidosis (MA) or metabolic alkalosis in freshwater rainbow trout. During alkalosis, urine pH increased, and [titratable acidity (TA) - HCO(-)(3)] and net H(+) excretion became negative (net base excretion) with unchanged NH(+)(4) efflux. During RA, urine pH did not change, but net H(+) excretion increased as a result of a modest rise in NH(+)(4) and substantial elevation in [TA - HCO(-)(3)] efflux accompanied by a large increase in inorganic phosphate excretion. However, during MA, urine pH fell, and net H(+) excretion was 3.3-fold greater than during RA, reflecting a similar increase in [TA - HCO(-)(3)] and a smaller elevation in phosphate but a sevenfold greater increase in NH(+)(4) efflux. In urine samples of the same pH, [TA - HCO(-)(3)] was greater during RA (reflecting phosphate secretion), and [NH(+)(4)] was greater during MA (reflecting renal ammoniagenesis). Renal activities of potential ammoniagenic enzymes (phosphate-dependent glutaminase, glutamate dehydrogenase, alpha-ketoglutarate dehydrogenase, alanine aminotransferase, phosphoenolpyruvate carboxykinase) and plasma levels of cortisol, phosphate, ammonia, and most amino acids (including glutamine and alanine) increased during MA but not during RA, when only alanine aminotransferase increased. The differential responses to RA vs. MA parallel those in mammals; in fish they may be keyed to activation of phosphate secretion by RA and cortisol mobilization by MA.

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 x 109 cells (3. 5 x 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 acetazolamide, a carbonic anhydrase inhibitor, active glucose transport, which was suppressed with phlorizin, a specific inhibitor of the sodium-dependent glucose transporters, and para-aminohippurate (PAH) secretion, which was diminished with the anion transport inhibitor probenecid. A variety of differentiated metabolic functions was also demonstrated in the RAD. Intraluminal glutathione breakdown and its constituent amino acid uptake were suppressed with the irreversible inhibitor of gamma-glutamyl transpeptidase acivicin; ammonia production was present and incremented with declines in perfusion pH. Finally, endocrinological activity with conversion of 25-hydroxy(OH)-vitamin D3 to 1,25-(OH)2 vitD3 was demonstrated in the RAD. This conversion activity was up-regulated with parathyroid hormone and down-regulated with increasing inorganic phosphate levels, which are well-defined physiological regulators of this process in vivo.

CONCLUSIONS

These results clearly demonstrate the successful tissue engineering of a bioartificial RAD that possesses critical differentiated transport, and improves metabolic and endocrinological functions of the kidney. This device, when placed in series with conventional hemofiltration therapy, may provide incremental renal replacement support and potentially may decrease the high morbidity and mortality rates observed in patients with renal failure.

Replacement of renal function in uremic animals with a tissue-engineered kidney

Current renal substitution therapy with hemodialysis or hemofiltration has been the only successful long-term ex vivo organ substitution therapy to date. Although this approach is life sustaining, it is still unacceptably suboptimal with poor clinical outcomes of patients with either chronic end-stage renal disease or acute renal failure. This current therapy utilizes synthetic membranes to substitute for the small solute clearance function of the renal glomerulus but does not replace the transport, metabolic, and endocrinologic functions of the tubular cells. The addition of tubule cell replacement therapy in a tissue-engineered bioartificial kidney comprising both biologic and synthetic components will likely optimize renal replacement to improve clinical outcomes. This report demonstrates that the combination of a synthetic hemofiltration device and a renal tubule cell therapy device containing porcine renal tubule cells in an extracorporeal perfusion circuit successfully replaces filtration, transport, metabolic, and endocrinologic functions of the kidney in acutely uremic dogs.

Regulation of phosphoenolpyruvate carboxykinase (GTP) gene expression

Phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) (PEPCK) is a key enzyme in the synthesis of glucose in the liver and kidney and of glyceride-glycerol in white adipose tissue and the small intestine. The gene for the cytosolic form of PEPCK (PEPCK-C) is acutely regulated by a variety of dietary and hormonal signals, which result in alteration of synthesis of the enzyme. Major factors that increase PEPCK-C gene expression include cyclic AMP, glucocorticoids, and thyroid hormone, whereas insulin inhibits this process. PEPCK-C is absent in fetal liver but appears at birth, concomitant with the capacity for gluconeogenesis. Regulatory elements that control transcription of the PEPCK-C gene in liver, kidney, and adipose tissue have been delineated, and many of the transcription factors that bind to these elements have been identified. Transgenic mice have been especially useful in elucidating the physiological roles of specific sequence elements in the PEPCK-C gene promoter and in demonstrating the key role played at these sites by the isoforms of CAAT/enhancer binding protein in patterning of PEPCK-C gene expression during the perinatal period. The PEPCK-C gene provides a model for the metabolic control of gene transcription.

The kidney in acid-base balance

The practitioner's approach to the pediatric patient with metabolic acidosis begins with calculation of the serum anion gap, which allows the clinician to place the patient in one of two categories of acid-base disturbance: a normal anion gap acidosis or high anion gap acidosis. Likewise, the patient with metabolic alkalosis can be categorized by urinary chloride concentration and the response to chloride replenishment as either chloride-responsive or chloride-resistant. The disease states associated with each category are reviewed in this article.

Effects of methionine sulphoximine treatment on renal amino acid and ammonia metabolism in rats

Renal glutamine metabolism in relation to ammoniagenesis has been extensively studied during chronic metabolic acidosis, when arterial glutamine levels are reduced. However, little is known about the effects of reduced glutamine delivery on renal glutamine and ammonia metabolism at physiological systemic pH values. Therefore, a model of decreased arterial glutamine concentrations at normal pH values was developed using methionine sulphoximine (MSO). Renal glutamine and ammonia metabolism was measured by determining fluxes and intracellular concentrations after an overnight fast in ether anaesthetized normal rats, MSO-treated rats and their pair-fed controls. Moreover, fluxes and intracellular concentrations of several other amino acids were determined concomitantly. After 2 and 4 days of MSO treatment, arterial glutamine concentrations were reduced to 55%, while arterial ammonia concentrations increased by 70%. Kidney glutamine uptake reduced, but systemic pH was unchanged. Fractional extraction of glutamine remained unchanged, suggesting that also in vivo net uptake of glutamine by the kidney at subnormal levels is related to arterial glutamine concentrations. As a result, at day 2 but not at day 4, the kidney reduced the net release of ammonia into the renal vein and thus reduced net renal ammonia addition to body ammonia pools. Therefore at day 2, the kidney seems to play an important role in adaptation to both hyperammonaemia and hypoglutaminaemia.

Metabolic adaptation of the kidney to hyperammonemia during chronic liver insufficiency in the rat

The aim of this study was to evaluate the role of renal ammonia and glutamine metabolism in the metabolic adaptation to chronic liver insufficiency-induced hyperammonemia in the rat. To this purpose, urinary excretion, renal net exchange and tissue concentrations of ammonia and amino acids were measured in anesthetized, normal control rats that did not undergo surgery, in control rats that underwent sham surgery, in rats that underwent portacaval shunting and in rats that underwent both portacaval shunting and bile duct ligation. Rats that underwent sham surgery and portacaval shunting were pair-fed with rats that underwent portacaval shunting and biliary obstruction, to correct for anorexia in that group, and all rats that were operated on were studied 7 and 14 days after surgery. Arterial ammonia and glutamine levels were elevated in groups that underwent portacaval shunting and portacaval shunting plus biliary obstruction at all time points. At days 7 and 14, total renal ammonia production decreased in rats that underwent portacaval shunting and in rats that underwent portacaval shunting plus biliary obstruction, associated with a 50% decrease in net renal glutamine uptake and strongly diminished net ammonia release into the renal vein, which was most prominent in the group that underwent portacaval shunting plus biliary obstruction. Urinary ammonia excretion was similar in rats that underwent portacaval shunting and in those that underwent sham surgery but was increased more than 200% at days 7 and 14 in rats that underwent portacaval shunting plus biliary obstruction. In this group, in contrast to portacaval-shunted rats, the kidney appeared to be an organ of net ammonia disposal from the body. In separate experiments in unanesthetized, unrestrained rats, similar changes in urinary ammonia excretion were observed without changes in arterial pH, excluding an effect of anesthesia or pH on the obtained results. These results indicate that the kidney plays an important role in the metabolic adaptation to hyperammonemia during chronic liver insufficiency in the rat.