Enzyme activities in the isolated proximal tubule of rabbit kidney

Höhmann, Bernhard; R, Zwiebel; Yamagata, Akifusa; Kinne, Rolf

doi:10.1007/bf00588536

Cited by 13 publications

(6 citation statements)

References 19 publications

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“…In agreement with this is the observation that the activity of glutamine synthetase is present along the entire proximal tubule of the rabbit [1]. A high glutamate dehydrogenase activity has also been demonstrated along the rabbit proximal tubule [25,26]. Although the activity of the latter enzyme was found to be significantly lower in the pars recta than in the proximal convoluted tubule [25], this does not seem to be a rate-limiting factor in the synthesis of glutamine from physiological concentrations of alanine and aspartate in the S2 and S3 segments, which were found in the present study to synthesize more glutamine than the S1 segment (see Tables 1 and 2).…”

Section: Scheme 1 Schematic Representation Of the Pathway Of Glutaminsupporting

confidence: 76%

“…However, the high rate of glutamine synthesis found in the S1, S2 and S3 segments in the present study clearly indicates that numerous mitochondria are present, and that these mitochondrial enzymes are significantly active along the entire proximal tubule. In agreement with this view is the observation that succinate dehydrogenase has a high activity in the S1, S2 and S3 segments of the rabbit proximal tubule [25].…”

Section: Scheme 1 Schematic Representation Of the Pathway Of Glutaminsupporting

confidence: 72%

“…A high glutamate dehydrogenase activity has also been demonstrated along the rabbit proximal tubule [25,26]. Although the activity of the latter enzyme was found to be significantly lower in the pars recta than in the proximal convoluted tubule [25], this does not seem to be a rate-limiting factor in the synthesis of glutamine from physiological concentrations of alanine and aspartate in the S2 and S3 segments, which were found in the present study to synthesize more glutamine than the S1 segment (see Tables 1 and 2). High activities of phosphoenolpyruvate carboxykinase [27] and pyruvate kinase [28] have also been measured along the entire rabbit proximal tubule.…”

Section: Scheme 1 Schematic Representation Of the Pathway Of Glutaminmentioning

confidence: 87%

“…This would appear to be particularly important in the rabbit, a herbivorous species that forms an alkaline urine and does not need to synthesize and excrete many protons in the form of ammonium ions [2,6]. Given the neurotoxicity of high circulating levels of ammonia, renal glutamine synthesis in the rabbit might also appear to be a major detoxifying mechanism ; indeed, without glutamine synthesis, most of the ammonia synthesized at high rates by the operation of glutamate dehydrogenase, which is very active in the rabbit proximal tubule [25,26] (as also revealed in the present study because one ammonia must be formed for each glutamine synthesized), would be released into the renal vein.…”

Section: Physiological Relevancementioning

confidence: 99%

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Glutamine synthesis is heterogeneous and differentially regulated along the rabbit renal proximal tubule

Ferrier

Conjard

Martin

et al. 1999

Biochemical Journal

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Glutamine synthesis, a major process for ammonia detoxification and the control of acid-base balance, occurs from various precursors in suspensions of rabbit proximal tubules. However, no data are currently available on the distribution of glutamine synthesis along the rabbit proximal tubule, and its modulation by changes of substrate concentration. Therefore we have microdissected and incubated the three parts (S1, S2 and S3) of rabbit proximal tubules and measured glutamine synthesis from alanine and aspartate. With a physiological concentration of alanine (0.25 mM) or aspartate (0.05 mM), glutamine synthesis in the S1 segment was about half of that in the S2 and S3 segments, and was greater from alanine than from aspartate along the entire proximal tubule. Elevation of alanine and aspartate concentrations to 5 mM increased glutamine synthesis in both a substrate- and segment-dependent manner. It is concluded that glutamine synthesis occurs from alanine and aspartate along the entire rabbit proximal tubule; however, contrary to what might have been expected on the basis of measurement of glutamine synthetase activity, the basal rate of glutamine synthesis and its adaptation to increased substrate availability are heterogeneous along this nephron segment.

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Section: Scheme 1 Schematic Representation Of the Pathway Of Glutaminsupporting

confidence: 76%

Section: Scheme 1 Schematic Representation Of the Pathway Of Glutaminsupporting

confidence: 72%

Section: Scheme 1 Schematic Representation Of the Pathway Of Glutaminmentioning

confidence: 87%

Section: Physiological Relevancementioning

confidence: 99%

See 2 more Smart Citations

Glutamine synthesis is heterogeneous and differentially regulated along the rabbit renal proximal tubule

Ferrier

Conjard

Martin

et al. 1999

Biochemical Journal

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“…Because no exogenous nitrogenous substrate was present in the incubation medium in the absence of alanine, the formation of glutamate occurred by transamination of ␣-ketoglutarate with endogenous amino acids derived from proteolysis. Some of the latter glutamate was probably oxidatively deaminated by glutamate dehydrogenase, which is very active in rabbit kidney tubules (29), to provide the ammonia needed for the synthesis of the glutamine accumulated. Note that the minimum flux through the anaplerotic enzyme, pyruvate carboxylase, corresponds to the net conversion of ␣-ketoglutarate into glutamate (Table VI) and that in the presence of acetate as the sole exogenous substrate, the pyruvate metabolized by the pyruvate carboxylase reaction was formed exclusively from endogenous substrates.…”

Section: Discussionmentioning

confidence: 99%

The Anaplerotic Substrate Alanine Stimulates Acetate Incorporation into Glutamate and Glutamine in Rabbit Kidney Tubules

Conjard

Dugelay

Chauvin

et al. 2002

Journal of Biological Chemistry

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Although acetate, the main circulating volatile fatty acid in humans and animals, is metabolized at high rates by the renal tissue, little is known about the precise fate of its carbons and about the regulation of its renal metabolism. Therefore, we studied the metabolism of variously labeled [13 C]acetate and [ 14 C]acetate molecules and its regulation by alanine, which is also readily metabolized by the kidney, in isolated rabbit renal proximal tubules. With acetate as the sole substrate, 72% of the C-1 and 49% of the C-2 of acetate were released as CO 2 ; with acetate plus alanine, the corresponding values were decreased to 49 and 25%. The only other important products formed from the acetate carbons were glutamine, and to a smaller extent, glutamate. By combining 13 C NMR and radioactive and enzymatic measurements with a novel model of acetate metabolism, fluxes through the enzymes involved were calculated. Thanks to its anaplerotic effect, alanine caused a stimulation of acetate removal and a large increase in fluxes through pyruvate carboxylase, citrate synthase, and the enzymes involved in glutamate and glutamine synthesis but not in flux through ␣-ketoglutarate dehydrogenase. We conclude that the anaplerotic substrate alanine not only accelerates the disposal of acetate but also prevents the wasting of the latter compound as CO 2 .Acetate is the main circulating volatile fatty acid in humans and other mammalian species. Its blood concentration is low (less than 0.2 mM) in fed and starved humans and starved herbivores but may reach the millimolar range in humans after alcohol consumption and in fed herbivorous species (1-7). The sources of blood acetate are on the one hand absorption of the acetate formed as a result of gastrointestinal bacterial fermentation, and on the other hand, the acetate formed and released by various tissues containing acetyl-CoA hydrolase activity (2,3,6,8).On the basis of experiments performed in vivo with labeled acetate, it has been shown that the turnover of circulating acetate is rapid and that, depending on the species and nutritional state, the oxidation of this compound provides from 6 to 70% of the whole body energy expenditure (3,4,7,9). This means that acetate is removed and metabolized by peripheral tissues. Indeed, acetyl-CoA synthetase, the enzyme that initiates acetate degradation, has been demonstrated to be active in many tissues including the liver, kidney, heart, brain, adipose tissue, and skeletal muscle (3). It has been found that, besides the heart, the kidney contains a high activity of this enzyme (3).In agreement with this observation, we have shown in a recent study that acetate is readily metabolized by suspensions of rabbit renal proximal tubules (10). In the same study (10), we have demonstrated that acetate significantly altered the metabolism of alanine, a major precursor of glutamine in these tubules. For this, we used 13 C-labeled alanine and unlabeled acetate in combination with enzymatic and 13 C NMR spectroscopy measurements to calculate metabo...

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SEM of the proximal tubule of the adult rabbit kidney

1978

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The present study utilizes the scanning electron microscope (SEMI to reveal the surface morphology of proximal tubular cells and the parietal cells of Bowman's capsule of the adult rabbit nephron. To facilitate the examination of the basal surface of these cells, proximal tubules were dissected free and treated with collagenase t o remove the basememt membrane. Other blocks of tissue were cryofractured to expose the lateral cell surfaces of the proximal tubules. Our investigation has shown that the lateral and basal surfaces of both the convoluted and straight segments of the proximal tubule have numerous processes. However, the arrangement and degree of branching is distinctly different in the two segments. The convoluted segment has large lateral ridges which form a t the base of the microvilli and fan out to divide into lateral-basal processes. Many of the lateral-basal processes reach the basement membrane intact, interdigitating with complementary processes from adjacent cells. However, some of the lateral-basal processes branch into short, knobby projections (basal villi) which may also reach the basement membrane. Patches of basal villi are interspersed between broad regions of interdigitating lateralbasal processes. Therefore, in the convoluted segment, the lateral-basal processes form the major part of the basal cell surface. In tubular cells of the pars recta, unlike convoluted tubular cells, the majority of the ridges remain unbranched and pass directly to the basal surface where they divide into elaborate basal villi. Thus the basal surface of the pars recta cells is highly complex, appearing leaf-like, being a composite of numerous basal villi with a few lateral ridges.The basal surface of some parietal cells of Bowman's capsule have parallel ridges, which results in patches of striations.The complex morphology of cells of the proximal tubule (PT) of the kidney has interested investigators for many years (Wedl, 1850; Heidenhain, 1874; Schachawa, 1876; Landauer, 1895; Zimmermann, 1898; Zimmermann, '11). Heidenhain (1874) was one of the early light microscopists who noted extensive lateral projections on the proximal tubule cells. Subsequent workers have studied kidneys from a variety of animals and corroborated the data of the earlier studies (von Mollendorff, '30; Grafflin and Foote, '39). A renewed interest in the shape of the PT cells has developed with the advent of the scanning and transmission electron microscopes and functional data that suggests a direct relationship of cell shape to function (Rhodin, '58; Bulger, '65; Waugh et al., '67; Andrews and Porter, '74; Bulger et al., '74; Curran and MacIntosh, '62; Diamond and Bossert, '67). Several models of proximal tubule cell shape have appeared in the literature. Welling and Welling ('76) have recently published an elegant quantitative three-dimensional model of the shape of the epithelial cells and intercellular channels in the rabbit proximal tubule. However, the lack of data on the precise ultrastructure of the basal surface has limit...

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Enzyme activities in the isolated proximal tubule of rabbit kidney

Cited by 13 publications

References 19 publications

Glutamine synthesis is heterogeneous and differentially regulated along the rabbit renal proximal tubule

Glutamine synthesis is heterogeneous and differentially regulated along the rabbit renal proximal tubule

The Anaplerotic Substrate Alanine Stimulates Acetate Incorporation into Glutamate and Glutamine in Rabbit Kidney Tubules

SEM of the proximal tubule of the adult rabbit kidney

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