Ketone body transport in renal brush border membrane vesicles

Urinary metabolite quantitative trait loci in children and their interaction with dietary factors

Human metabolism is influenced by genetic and environmental factors. Previous studies have identified over 23 loci associated with more than 26 urine metabolites levels in adults, which are known as urinary metabolite quantitative trait loci (metabQTLs). The aim of the present study is the identification for the first time of urinary metabQTLs in children and their interaction with dietary patterns. Association between genome-wide genotyping data and 44 urine metabolite levels measured by proton nuclear magnetic resonance spectroscopy was tested in 996 children from the Human Early Life Exposome project. Twelve statistically significant urine metabQTLs were identified, involving 11 unique loci and 10 different metabolites. Comparison with previous findings in adults revealed that six metabQTLs were already known, and one had been described in serum and three were involved the same locus as other reported metabQTLs but had different urinary metabolites. The remaining two metabQTLs represent novel urine metabolite-locus associations, which are reported for the first time in this study [single nucleotide polymorphism (SNP) rs12575496 for taurine, and the missense SNP rs2274870 for 3-hydroxyisobutyrate]. Moreover, it was found that urinary taurine levels were affected by the combined action of genetic variation and dietary patterns of meat intake as well as by the interaction of this SNP with beverage intake dietary patterns. Overall, we identified 12 urinary metabQTLs in children, including two novel associations. While a substantial part of the identified loci affected urinary metabolite levels both in children and in adults, the metabQTL for taurine seemed to be specific to children and interacted with dietary patterns.

Renal transport of organic anions and cations

Organic anions and cations (OAs and OCs, respectively) comprise an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. The kidney, primarily the renal proximal tubule, plays a critical role in regulating the plasma concentrations of these organic electrolytes and in clearing the body of potentially toxic xenobiotics agents, a process that involves active, transepithelial secretion. This transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. Basolateral and luminal OA and OC transport reflects the concerted activity of a suite of separate proteins arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney, now allows the development of models describing the molecular basis of the renal secretion of OAs and OCs. New information on naturally occurring genetic variation of many of these processes provides insight into the basis of observed variability of drug efficacy and unwanted drug-drug interactions in human populations. The present review examines recent work on these issues.

Renal Urate Transport

Serum uric acid is determined by a balance between production and renal excretion. Luminal reabsorption of urate by the proximal tubule from the glomerular ultrafiltrate involves coupling between sodium-anion cotransport and urate-anion exchange. Apical sodium-coupled cotransport of lactate, ketoacids, nicotinate, and pyrazinoate increases intracellular levels of these anions in proximal tubular cells, stimulating the apical absorption of luminal urate via anion exchange. Hyperuricemia occurs when plasma levels of these anions increase; for example, hyperuricemia is a well-recognized concomitant of lactic acidosis and ketoacidosis. Relevant developments in the molecular and renal physiology of urate homeostasis are reviewed.

D(-)3-hydroxybutyrate cotransport with Na in rat renal brush border membrane vesicles

Which are the driving forces for D(-)3-hydroxybutyrate (HB) transport in rat renal brush border membranes (RBB)? Sodium, even in the absence of gradients, accelerates the unidirectional (1-5 s) flux of HB into rat RBB vesicles. Valinomycin (and Ki = Ko) does not significantly alter the NaCl gradient driven HB influx. Thus, the Na-dependent HB influx is driven by the chemical Na+ gradient but it is not driven by changes in the transmembrane electrical potential. Indeed, in valinomycin-treated membranes, vesicle-inside more negative potentials (K-gluconate in-Na-gluconate out) sufficient to accelerate Na-glucose cotransport, did not stimulate HB influx, in the presence of inwardly directed Na+ gradients, and did not significantly inhibit when in the absence of Na+. Thus, cotransport of HB with Na in rat RBB membranes does not involve the net transfer of positive charge and the passive conductance of this membrane for HB- is not large. However, vesicle inside more negative potentials (induced by inwardly directed NaNO3 gradients or by outwardly directed K+ gradients and valinomycin in the presence of inwardly directed Na+ gradients) inhibited HB influx, suggesting that another potential sensitive mechanism, perhaps redistribution of intramembrane charges, may influence HB influx. Acidification (pHi = pHo = 6.4 vs. 7.4) or inwardly directed H+ gradients (pHo/pHi = 6.4/7.4) did not alter HB influx, in the absence of Na+. Thus there is no evidence for a H+ driven HB influx. HB influx is significantly inhibited by high (100 mEq/l) trans concentration of Na+. Also, influx of 2.25 mM 14C-HB was significantly increased by 5-10 mM intravesicular HB under Na-equilibrated conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport of D-beta-hydroxybutyrate across rat liver mitochondrial membranes

1. D-beta-hydroxybutyrate, a major ketone body, is produced or converted in mitochondria from various animal tissues. 2. It is an easy permeate anion of the inner mitochondrial membrane. However, its translocation is not a passive diffusion process since it is inhibited by pyruvate transport inhibitors like alpha-cyanocinnamate and derivatives. 3. This carrier mediated process is associated with proton movements. Besides, dicarboxylate anions strongly inhibit the penetration into mitochondria. 4. This is in agreement with the existence of a second transport process related to the dicarboxylate carrier.

Contraluminal transport of small aliphatic carboxylates in the proximal tubule of the rat kidney in situ

In order to study the characteristic of contraluminal transport of hydrophylic small fatty acids the in situ stopped flow microperfusion technique [12] has been applied. By measuring with 4 s contact time the decrease in the contraluminal concentration of the respective radiolabelled substances the concentration dependence of the influx into the cortical cells was tested. The 4 s decrease in contraluminal concentration of chloroacetate, L-lactate, D-lactate, 3-hydroxybutyrate and acetoacetate was between 26% and 31%. For each substance the percent decrease was the same, no matter whether it was offered in a concentration of 0.1 or 10 mmol/l. Contraluminal disappearance of 0.1 mmol/l L-lactate was not influenced by 5 mmol/l H2DIDS, probenecid, phloretin, mersalyl or cyanocinnamate, but it was significantly (37%) inhibited by 5-nitro-2-(phenyl-propyl-amino) benzoate, a blocker of the nonspecific anion channel. The percent decrease in propionate uptake was somewhat larger - between 36% and 39% - but again not different at 0.01, 0.1, 1.0 and 10 mmol/l. With pyruvate the contraluminal decrease was 20% at 0.1 mmol/l and 31% at 10 mmol/l. The percent disappearance of the aromatic pyrazinoate was 38% and 34% at 0.1 and 10 mmol/l and for nicotinate 42% and 22%, respectively. The disappearance of nicotinate (0.1 mmol/l) was significantly inhibited by 10 mmol/l pyrazinoate and paraaminohippurate (PAH). The data are in agreement with the hypothesis that the hydrophilic small fatty acids traverse the contraluminal cell side by simple diffusion, possibly via the unspecific anion channel [14], pyruvate via the dicarboxylic acid pathway in a cooperative manner and pyrazinoate, as well as nicotinate, via the PAH pathway.

Mechanisms of uptake of ketone bodies by luminal-membrane vesicles

The energetics and location of renal transport of acetoacetate, beta-hydroxybutyrate, alpha-hydroxybutyrate and gamma-hydroxybutyrate by luminal-membrane vesicles from either whole cortex or pars convoluta or pars recta of rabbit proximal tubule were studied. Addition of either acetoacetate or beta-hydroxybutyrate or its analogues to dye-membrane-vesicle suspensions in the presence of Na+ gradient (extravesicular greater than intravesicular) resulted in absorbance changes indicative of depolarizing event(s). Valinomycin enhanced the Na+-dependent uptake of monocarboxylic acids, provided a K+ gradient (intravesicular greater than extravesicular) was present. By contrast, Na+-dependent uptake of these compounds was nearly abolished by ionophores that permit Na+ to pass through the luminal-membrane via another channel, either electrogenically (e.g. gramicidin D) or electroneutrally (e.g. nigericin). These results established that the Na+-dependent transport of ketone bodies and analogues by luminal-membrane vesicles is an electrogenic process. Eadie-Hofstee analysis of saturation kinetic data suggested the presence of multiple transport systems in vesicles from whole cortex for these compounds. Tubular localization of the transport systems was studied by the use of vesicles derived from pars convoluta and pars recta. In pars recta uptake of all these compounds was mediated by means of a single high affinity common transport system. Uptake of these compounds by vesicles from pars convoluta was carried out via a relatively low affinity but common transport system. The physiological importance of the transport systems is discussed.

Specificity of the Na+-dependent monocarboxylic acid transport pathway in rabbit renal brush border membranes

The substrate specificity of a Na+-dependent transport pathway for L-lactate was studied in rabbit renal brush border membrane vesicles. Jmax for L-lactate transport was unaffected by the presence of a fixed concentration of two different short-chain monocarboxylic acids, while the apparent Kt(Ka) for L-lactate increased, and this is compatible with competitive inhibition. The inhibitor constants ("Ki"'s) for the transport pathway for the two solutes examined closely corresponded to the respective "Ki"'s derived from a Dixon plot. A broad range of compounds were then tested as potential inhibitors of L-lactate transport, and the "Ki"'s thereby derived yielded specific information regarding optimal substrate recognition by the carrier. A single carboxyl group is an absolute requirement for recognition, and preference is given to 3 to 6 C chain molecules. Addition of ketone, hydroxyl and, particularly, amine groups at any carbon position, diminishes substrate-carrier interaction. Intramolecular forces, notably the inductive effects of halogens, may play a role in enhancing substrate-carrier interaction; however, no correlation was found between pKa and "Ki" for the substrates examined. We conclude that a separate monocarboxylic acid transport pathway, discrete from either the D-glucose, alpha or beta neutral amino-acid, or dicarboxylic acid carriers, exists in the renal brush border, and this handles a broad range of monocarboxylates.

Tryptophan transport into plasma membrane vesicles derived from rat brain synaptosomes

Tryptophan uptake by membrane vesicles derived from rat brain was investigated. The uptake is dependent on the Na+ gradient [Na+] outside greater than [Na+] inside, and is maximal when both Na+ and Cl- are present. The uptake represents transport into an osmotically active space and not a binding artifact, as indicated by the effect of increasing the medium osmolarity. The uptake of tryptophan is stimulated by a membrane potential (interior negative) as demonstrated by the effects of the ionophores valinomycin and carbonyl cyanide m-chlorophenylhydrazone and anions with different permeabilities. Kinetic data show that tryptophan is accumulated by two systems with different affinities. Ouabain, an inhibitor of Na+,K+-activated ATPase, does not affect tryptophan transport. The uptake of tryptophan is inhibited by high concentrations of phenylalanine, tyrosine, leucine, and 3,4-dihydroxyphenylalanine.

Changes in glucose and lipid metabolism in starved or starved-refed Japanese quail (coturnix coturnix japonica) in relation to fine structure of liver cells

1. Metabolic response of adult quail to fasting or refeeding was studied by measuring the main blood and hepatic metabolites. Moreover, the fine structure of hepatocytes in these physiological conditions was described. 2. Starvation or refeeding did not affect glycemia in male as in female quails. 3. Fasting had no effect on plasma free fatty acids in female quails, whereas plasma triglycerides were markedly decreased. 4. In fasted quails, there was an active ketogenesis with a high 3-hydroxybutyrate/acetoacetate ratio. 5. Ultrastructural aspect of liver parenchymal cells from fasted quails revealed alterations in the quantity of glycogen, smooth endoplasmic reticulum, lysosomes and in the form of the rough endoplasmic reticulum. 6. The significance of these morphological changes was discussed in relation to an hormonal stimulation.

Transport of inorganic and organic substances in the renal proximal tubule

The transport through the epithelial cell layer of the renal proximal tubule proceeds in principle by passive paracellular and active transcellular transport. The active transcellular transport is mostly secondary active. This means it proceeds coupled with the flux of Na+ ions, whereby the transcellular gradient of sodium, created by the (Na+ + K+)-ATPase, located at the contraluminal cell side, provides the main driving force. Once in the cell the substances leave the other cell side by a Na+ -independent, but carrier-mediated transport system. Using microperfusion and electrophysiological techniques as well as brush border membrane vesicle preparation the Na+ -H+ countertransport and the Na+-cotransport of amino acids, phosphate, sulfate, thiosulfate, bile acids, aliphatic-aromatic monocarboxylic acids (lactate) and dicarboxylic acids was studied. Special emphasis will be given to the bidirectional transport of thiosulfate as well as to the specificity of the monocarboxylic acid and dicarboxylic acid transport system.

L-Aspartate transport into plasma membrane vesicles derived from rat brain synaptosomes

Aspartate uptake by membrane vesicles derived from rat brain was investigated. The uptake is dependent on a Na+ gradient ([Na+] outside greater than [Na+] inside). Active transport of aspartate is strictly dependent upon the presence of sodium and maximal extent of transport is reached when both Na+ and Cl- ions are present. The uptake is transport into an osmotically active space and not a binding artifact as indicated by the effect of increasing the medium osmolarity. The uptake of aspartate is stimulated by a membrane potential (negative inside), as demonstrated by the effect of the ionophore carbonyl cyanide m-chlorophenylhydrazone and anions with different permeabilities. The presence of ouabain, an inhibitor of (Na+ + K+)-ATPase, does not affect aspartate transport. The kinetic analysis shows that aspartate is accumulated by two systems with different affinities, showing Km and Vmax values of similar order to those found in slightly "cruder" preparations. Inhibition of the L-aspartate uptake by D-aspartate and D- and L-glutamate indicates that a common carrier is involved in the process, this being stereospecific for the D- and L-glutamate stereoisomers.

Glycine transport into plasma-membrane vesicles derived from rat brain synaptosomes

1. Transport of glycine has been demonstrated in membrane vesicles isolated from rat brain, using artificially imposed ion gradients as the sole energy source. 2. The uptake of glycine is strictly dependent on the presence of Na+ and Cl- in the medium, and the process can be driven either by an Na+ gradient (out greater than in) or by a C1- gradient (out greater than in) when the other essential ion is present. 3. The uptake of glycine is stimulated by a membrane potential (interior negative), as demonstrated by the effects of the ionophores valinomycin and carbonyl cyanide m-chlorophenylhydrazone and anions of different permeabilities. 4. The kinetic analysis shows that glycine is accumulated by two systems with different affinities. 5. The presence of ouabain, an inhibitor (Na+ + K+)-activated ATPase, does not affect glycine transport. 6. The existence of a high-affinity, Na+-dependent glycine-uptake system in membrane vesicles derived from rat brain suggests that this amino acid may have a transmitter role in some areas of the rat brain.

Tyrosine transport by membrane vesicles isolated from rat brain

Tyrosine uptake by membrane vesicles derived from rat brain has been investigated. The uptake is dependent on an Na+ gradient ([Na+]outside greater than [Na+]inside). The uptake is transport into an osmotically active space and not a binding artifact as indicated by the effect of increasing the medium osmolarity. The process is stimulated by a membrane potential (negative inside) as demonstrated by the effect of the ionophores valinomycin and carbonyl cyanide m-chlorophenylhydrazone and anions with different permeabilities. Kinetic data show that tyrosine is accumulated by two systems with different affinities. Tyrosine uptake is inhibited by the presence of phenylalanine and tryptophan.