Effect of age on pyruvate kinase and lactate dehydrogenase distribution in rat kidney

Sugar or Fat? Renal Tubular Metabolism Reviewed in Health and Disease

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.

Heme protein-induced tubular cytoresistance: expression at the plasma membrane level

Following experimental rhabdomyolysis, animals become resistant to heme protein-induced acute renal failure (ARF). The goals of this study were to: (a) ascertain whether this resistance, previously documented only in vivo, is expressed directly at the proximal tubular cell level; (b) determine whether heme proteinuria (vs. other consequences of rhabdomyolysis) is its trigger; and (c) ascertain some of its subcellular determinants. Rats were injected with a borderline toxic dose of glycerol and 24 hours later proximal tubular segments (PTS) were isolated for study. Their vulnerability to diverse forms of injury (FeSO4-induced oxidant stress, hypoxia, Ca2+ ionophore, cytochalasin D, PLA2) was compared to that found in normal PTS. Post-glycerol PTS manifested significant resistance to each insult (decreased lactate dehydrogenase +/- N-acetyl-beta-D-glucosaminidase release). Protection against FeSO4 was virtually complete and it was associated with a 50% decrease in membrane lipid peroxidation. No decrease in hydroxyl radical generation was noted during the FeSO4 challenge (salicylate trap assessment), suggesting a primary increase in membrane resistance to attack. That PLA2 addition caused less deacylation, plasma membrane enzyme (alanine aminopeptidase) release, and LDH leakage from post-glycerol versus normal tubules supported this hypothesis. To test whether cytoresistance was specifically triggered by heme proteins (vs. being a non-specific filtered protein effect, or a result of endotoxin cascade activation), rats were injected with purified myoglobin, non-heme containing filterable proteins, or endotoxin. Only myoglobin induced cytoresistance. In vivo heme oxygenase inhibition (tin-protoporphyrin) did not block the emergence of cytoresistance and it was expressed despite Na,K-ATPase inhibition (ouabain) or cytoskeletal disruption (cytochalasin D). In vivo heat shock failed to protect. In conclusion, (1) rhabdomyolysis induces broad based proximal tubular cytoresistance; (2) heme proteinuria is its trigger; and (3) it is most easily explained by a primary increase in plasma membrane resistance to attack.

Obstruction of proximal tubules initiates cytoresistance against hypoxic damage

Following acute tubular necrosis (ATN), cytoresistance to further renal injury results. However, the initiating events and the subcellular determinants of this phenomenon have not been defined. Since tubular obstruction is a consequence of ATN, this study evaluated whether it alters tubular susceptibility to hypoxic damage. Extrarenal obstruction (ureteral ligation in rats) was used for this purpose to dissociate obstructive effects from those of ATN. Twenty-four hours following ureteral ligation or sham surgery, cortical proximal tubular segments (PTS) were isolated and subjected to hypoxic (15 or 30 min)/reoxygenation injury. Since oxidant stress, cell Ca2+ overload, and PLA2 attack are purported mediators of hypoxic/reoxygenation injury, degrees of FeS04, Ca2+ ionophore, and phospholipase A2-induced PTS damage also were assessed. The cell injury (% LDH release) which resulted from each of the above was consistently less in PTS obtained from obstructed kidneys. This cytoresistance: (a) did not require prior uremia to develop (seen with unilateral obstruction); (b) it did not appear to correlate with a tubular proliferative response (assessed by proliferating cell nuclear antigen expression); (c) it was uninfluenced by early tubular repair (unchanged by 24 hrs of obstruction release); and (d) it occurred without increased heat shock protein (HSP-70) or antioxidant enzyme (superoxide dismutase, catalase) expression. Total adenylate pools were higher in obstructed versus control PTS during injury; however, this appeared to be a correlate of the protection, rather than a mediator of it. In contrast, obstructed tubules manifested a primary increase in plasma membrane resistance to PLA2 attack (approximately 3-fold less lysophosphatidylcholine and free fatty acid generation in obstructed vs. control PTS during incubation with exogenous PLA2). In sum, these results indicate that: (1) tubular obstruction protects PTS from injury, suggesting that its development during ATN may initiate cytoresistance; and (2) this cytoresistance appears to be mediated, at least in part, by a direct increase in plasma membrane resistance to PLA2 and potentially other forms (such as, oxidant stress, cytosolic Ca2+ loading) of attack.

Relationship between proliferation and glucose metabolism in primary cultures of rabbit proximal tubules

Primary cultures of rabbit proximal tubules, which revert to a glycolytic profile as reflected by increased activity of pyruvate kinase (PK) paralleled by increased glucose consumption and lactate formation, were utilized to explore the relationship between glycolytic metabolism and proliferation. Tubules placed in serum-free, hormonally defined Dulbecco's modified Eagle's medium with 5 mM glucose exhibited logarithmic growth beginning on day 3 in culture. The increase in PK activity lagged approximately 1 day behind, suggesting that the reversion to glycolysis is a consequence of rather than a prerequisite for cellular proliferation. Tubules cultured in 0.5 mM as contrasted with 25 mM glucose exhibited heightened proliferation reflected by an increase in protein content and cell number on day 5 in culture. The heightened proliferation was accompanied by increased PK activity. On day 9, after confluency had been achieved, no differences in protein content or PK activity were detected between tubules cultured in different glucose concentrations. These findings indicate that a low glucose concentration is mitogenic for renal proximal tubules and that the proliferative process in some fashion up-regulates the activity of the glycolytic enzyme PK. Furthermore, because accelerated growth proceeds in the presence of glucose restriction, the energy from glycolysis is not required for the proliferative process.

Carbohydrate metabolism by primary cultures of rabbit proximal tubules

Renal proximal tubular epithelia were used to assess the factors responsible for the induction of glycolysis in cultured cells. Primary cultures of rabbit proximal tubules, which achieved confluency at 6 days, exhibited hormonal responsiveness and brush-border characteristics typical of proximal tubular cells. Beginning at day 4, these cultured cells exhibited increased glycolytic metabolism reflected by enhanced glucose uptake and lactate production, along with parallel increases in activity of the glycolytic enzymes, pyruvate kinase and lactate dehydrogenase. The gluconeogenic enzymes, phosphoenolpyruvate carboxykinase (PEPCK) and fructose-1,6-bisphosphatase (FDP), were downregulated, and the cultured cells exhibited lower oxygen consumption rates than fresh tubules. Cells grown on a rocker, to mitigate hypoxia, exhibited a metabolic and enzymatic profile similar to cells grown under still conditions. ATP levels in cultured cells were higher than in fresh tubules. Furthermore, pyruvate kinase activity was higher in cells grown in media containing 0.5 as contrasted with 25 mM glucose. The enhanced glycolytic metabolism exhibited by cultured proximal tubular cells appears to be a characteristic of proliferation and is not a response to hypoxia, the Pasteur effect, or environmental glucose.

The relationship between renal metabolism and proximal tubule transport during ontogeny

The proximal tubules of newborn and adult animals reabsorb a similar fraction of the filtered load of Na+ and H2O (65%-70%). In tubules from adult animals, transcellular, active Na+ reabsorption accounts for one-third of the total, while two-thirds occur passively through the paracellular pathway, driven by hydrostatic and oncotic forces (one-third) and by cell-generated effective osmotic and ionic gradients (one-third). Since two-thirds of the Na+ is reabsorbed passively and does not require energy, the mature proximal tubule has a high Na+/O2 molar ratio (48 Eq of Na+/mol of O2). Measurements of ouabain-sensitive oxygen consumption in suspensions of proximal tubules indicate that in newborn, aerobic metabolism can support about 50% of the net Na+ transport rate compared with the 33% in tubules from adult animals. Independent confirmation of the direct and proportional relationship between active Na+ transport and ouabain-sensitive O2 consumption exists for the adult but not for the newborn. However, measurements of epithelial conductances and of transepithelial hydrostatic and oncotic pressure differences indicate that passive paracellular fluxes can account for the remaining 50% of the proximal Na+ reabsorption in newborn. The high permeability of the proximal tubules of newborn animals to small molecular weight solutes suggests that cell-generated osmotic and ionic transepithelial gradients are minimal in the tubules of newborn animals. Yet in the newborn, the osmolality of the end proximal tubule fluid was found to exceed that in plasma. This indicates that osmotic gradients due to differences in reflection coefficients for preferentially reabsorbed solutes and Cl- do exist across the proximal tubules of the newborn and suggests that these gradients may contribute to Na+ and H2O reabsorption. If this is indeed the case, then the contribution of active and of hydrostatic and oncotic pressure-driven flows to the overall reabsorption of Na+ and fluid has been overestimated. Resolution of this discrepancy requires measurements of the reflection coefficients for HCO3- and Cl- in the proximal tubule of the newborn. The metabolic processes by which energy is supplied to renal proximal cells during development are also incompletely characterized. There is evidence that maturation of aerobic metabolism, Krebs cycle enzymes activity, and of the mitochondrial membrane surface area precede the development of net reabsorptive transport (Na+, H2O, HCO3, glucose). By contrast, maturation of Na(+)-K(+)-ATPase activity at the basolateral cell membrane follows that in reabsorptive transport and does not limit its development.(ABSTRACT TRUNCATED AT 400 WORDS)

Change in energy reserves in different segments of the nephron during brief ischemia

Rat kidneys were made ischemic for 5 to 120 seconds. Segments of individual nephrons were dissected from freeze dried sections and analyzed for ATP, phosphocreatine, glycogen, glucose, glucose-6-phosphate, lactate and creatine kinase. ATP fell most rapidly in proximal convoluted and straight tubules (PCT, PST) and distal convoluted tubules (DCT), and most slowly in glomerulus and papilla. Phosphocreatine levels ranged fivefold and was highest in DCT, where it approached that of brain. Creatine kinase ranged 100-fold with lowest level in PCT, where the ischemic fall in phosphocreatine was so slow as to suggest a function other than that of an energy reserve. Glycogen varied tenfold from modest levels in distal segments to very low levels in PST, and was not used rapidly in any segment. Glucose consumption and lactate production were most rapid in distal portions. High-energy phosphate consumption for the first 7.5 seconds of ischemia, calculated from these data, indicates roughly-equal energy metabolism in proximal and distal segments, with lower levels in papilla, and especially in glomerulus. The absolute values suggest that the in vivo metabolic rate of the nephron continued almost unabated for 5 or 10 seconds of ischemia.

Localization and role of pyruvate kinase isoenzymes in the regulation of carbohydrate metabolism and pyruvate recycling in rat kidney cortex

This work was performed to gain more information on the role of pyruvate kinase isoenzymes in the regulation of renal carbohydrate metabolism. Immunohistochemically, pyruvate kinase type L is shown to be localized in the proximal tubule of the nephron and pyruvate kinase type M2 in the distal tubule and the collecting duct. a tight relationship between gluconeogenesis and pyruvate recycling was found. The rate of gluconeogenesis (8 mumol/g wet wt. per 30 min) was of the same order of magnitude as the rate of pyruvate recycling (10.92 mumol/g wet wt. per 30 min). Stimulation of gluconeogenesis from 20 mM lactate in kidney cortex slices of 24-h-starved rats by dibutyryl-cAMP, alanine and parathyroid hormone was connected with a decrease in pyruvate recycling; inhibition of gluconeogenesis due to a lack of Ca2+ in the incubation medium was linked with an increase in pyruvate recycling. The degradation of [6-14C]glucose to lactate, pyruvate, ketone bodies and CO2 and of [2-14C]lactate was unaffected by dibutyryl-cAMP, alanine, epinephrine, vasopressin or the omission of Ca2+ from the incubation medium. 1 mM dibutyryl-cAMP or 5 mM alanine did not alter the activities of oxaloacetate decarboxylase, 'malic' enzyme and malate dehydrogenase from rat kidney cortex. Since aerobic glycolysis in the distal tubules and the collecting ducts is not influenced by hormones, dibutyryl-cAMP and Ca2+, pyruvate kinase type M2 residing in this tissue is unlikely to be a control point of glycolysis. Since this tissue degrades only one-seventh of the glucose formed via gluconeogenesis, it does not contribute significantly to pyruvate recycling. Therefore, the decrease of pyruvate recycling in the presence of dibutyryl-cAMP and alanine in rat kidney cortex slices, leading to increased renal gluconeogenesis, has to be ascribed to the regulation of pyruvate kinase type L.

Immunohistochemical localization of pyruvate kinase isoenzymes in chicken tissues

A method for the localization of pyruvate kinase isoenzymes type L, M2 and M1 in tissue sections is described. Mono-specific antibodies directed against isoenzymes of pyruvate kinase from chicken and the peroxidase antiperoxidase method were used. The following preferential localizations of the isoenzymes in chicken tissues were observed: Pyruvate kinase M1 was found in skeletal muscle. The white muscle fibers were more intensely stained than the red. Some dark muscles (e.g., anterior latissimus dorsi) and the heart muscle showed no reaction with antiserum against pyruvate kinase M1. Pyruvate kinase type L was found in the hepatocytes and in kidney cortex. Pyruvate kinase type M2 was seen in the distal tubules of kidney, in hepatocytes and sinusoidal cells in liver, in lung, adipose tissue, and in the spleen mainly in the bursa dependent areas. Pyruvate kinase type M2 was detected in high concentrations in the granulation tissue of regenerating liver after partial hepatectomy. Liver sections of a hen bearing a pancreatic tumor showed an unusually high content of pyruvate kinase type M2 in some hepatocytes, which were each clustered to spots in the liver parenchyma. Thus, contrary to previous reports, the tissue distribution of isoenzymes in chicken is similar to that of other vertebrates.

Localization of pyruvate kinase isozymes in bovine kidney and comparison of these patterns with those of lactate dehydrogenases and aldolases

Electrophoretic and immunofluorescence analysis were used to study the distribution of pyruvate kinase isozymes in the bovine kidney. Electrophoretic analysis demonstrated the presence of large amounts of K4 plus small amounts of K-M hybrids in cortical, medullary, and papillary sections cut from the kidney. Nearly all of the K-L hybrids seen in whole kidney extracts were found in cortical sections. Immunofluorescence of frozen sections revealed the presence of type L subunits in the tubules but the complete absence of this subunit type in flomeruli. Glomeruli do contain large quantities of pyruvate kinase isozymes, probably K4 and K-M hybrids, that cross-react with antibodies produced against type M pyruvate kinase. Type L-containing forms of pyruvate kinase and aldolase type B both appear to be found in cell types thought to be capable of catalyzing of gluconeogenesis, while type K pyruvate kinase and type A aldolase are found in predominantly glycolytic cell types of the kidney. Lactate dehydrogenase isozymic patterns appear to be less closely correlated with glycolytic versus gluconeogenic functions of the kidney but may be determined more directly by other metabolic functions.