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  • 1
    ISSN: 1749-6632
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Natural Sciences in General
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-1440
    Keywords: Epithelial transport ; Kidney ; Lactate transport ; Electrolyte transport ; Epithelialer Transport ; Niere ; Laktattransport ; Elektrolyttransport
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Description / Table of Contents: Zusammenfassung Resorption bzw. Sekretion im proximalen Nierentubulus läuft einmal passiv auf dem parazellulären Weg, d.i. zwischen den Zellen hindurch, ab, zum anderen aktiv, transzellulär, durch die Zellen hindurch. Der transzelluläre aktive Transport ist in der Regel sekundär aktiv. Er verläuft gekoppelt an den Fluß von Na+-Ionen, wobei ein transzellulärer Gradient von Na+-Ionen, der seinerseits durch die kontraluminal gelegene (Na+-K+)-ATPase geschaffen wird, die Triebkraft liefert. Einmal in der Zelle, verlassen die Substanzen die kontraluminale Zellseite vermittels Karrier, die Na+-unabhängig sind. Mit Hilfe von Mikroperfusions- und elektrophysiologischen Techniken sowie mit Hilfe von Bürstensaumvesikeln wurde der Na+-Kotransport von Aminosäuren, Phosphat, Sulfat, Thiosulfat, Gallensäuren, aliphatischen und aromatischen Monokarboxylsäuren (Laktat) sowie der von Dikarboxylsäuren untersucht. Besonderes Augenmerk wurde dem bidirektionalen Transport von Thiosulfat sowie der Spezifität des Mono- und Dikarboxylsäure-Transportsystems gewidmet.
    Notes: Summary 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, where-by 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.
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  • 3
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The transport theory of Kedem and Katchalsky which was derived for passive transport in a two-compartment system is generalized for a multicomponent system with active transport, so that it can be applied to more complicated biological membranes. Equations have been derived to describe the transport of urea through the proximal convolution of the rat kidney and the permeability and the reflection coefficient have been determined. The permeability coefficient $$(\tilde P_u )$$ measured with the microperfusion and stop flow microperfusion methods, was found to be 6.0 and 5.2×10−5 mm2/sec, respectively. The reflection coefficient (σ) was determined in a stationary state situation and found to be 0.68. Earlier free flow micropuncture results together with theP u andσ u of this study indicate that 50% of the filtered urea is reabsorbed proximally and that approximately half of this amount is reabsorbed by solvent drag and the rest by diffusion. In the Appendix, a theoretical treatment of nonelectrolyte transport in a multicomponent system with active transport is given.
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  • 4
    ISSN: 1432-2013
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary In order to evaluate whether an active transport of urea exists across the collecting duct epithelia of rats the following experiments were performed. Under conditions, where the net transtubular transport of urea, water, and other solutes was zero, the unidirectional flux of C14-urea out of the collecting duct and the urea concentration difference between collecting duct fluid and vasa recta plasma were measured. The following results were obtained 1. In rats on low protein diet the unidirectional outflux of urea was 73% higher than in rats on normal diet and 85% higher than in rats on high protein diet. 2. Under steady-state conditions with zero net flux the urea concentration ratio of collecting duct fluid to vasa recta plasma water was 0.84 in rats on low protein diet and 0.99 in rats on normal diet. 3. During free flow the urea concentration ratio of collecting duct fluid to vasa recta plasma water was 1.06 in low protein and 1.33 in normal rats. The data indicate that in normal animals the urea transport across the collecting duct wall is passive while in rats on low protein diet an active outward transport is superimposed.
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  • 5
    ISSN: 1432-2013
    Keywords: Renal Microperfusion ; Isotonic Reabsorption ; Tracer Permeability ; Glomerulo Tubular Balance ; Renale Mikroperfusion ; Isotone Resorption ; Tracerpermeabilität ; Glomerulotubuläre Balance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary In the first experimental series proximal convolutions of the rat kidney were perfused with a modified Ringer solution and the isotonic fluid absorption was measured. In a second series the tubule was perfused with equilibrium solution which contained36Cl and the chloride permeability was determined. By the recollection method each individual tubule was perfused twice either at constant luminal diameter but different perfusion rates (10:30 or 6:16 nl/min) or at constant perfusion rates but different luminal diameters (20:30 μ). The perfusate was recollected at two different sites which were at least 500 μ distant from the infusion site. The isotonic fluid absorption as well as the36Cl permeability was unchanged when the tubule was distended from 20–30 μ. Both, however, increased about 20% when the perfusion rate was increased 3-fold. The data led to the following conclusions: 1. It is unlikely that there is a flow reactor type dependence of proximal tubular transport on flow rate. 2. The tubular distension cannot be responsible for the glomerulo-tubular balance. 3. It is more advantageous to relate permeability data of the rat nephron to tubular length. 4. In microperfusion experiments non steady sampling does not affect transepithelial fluxes per unit tubular length, provided that the pump delivery is constant.
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  • 6
    ISSN: 1432-2013
    Keywords: l-Histidine Transport ; Proximal Tubule ; Transport Kinetics ; Fanconi Syndrome ; Kidney ; Microperfusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The kinetics ofl-histidine reabsorption by the proximal convolution of the rat nephron have been studied by stationary microperfusion with simultaneous perfusion of peritubular capillaries. Steady-state concentrations (C ∞) and transepithelial concentration differences (Δc ∞) were determined over a wide range of peritubular bistidine concentrations. It was found that Δc ∞ increased hyperbolically with increase in luminal and peritubular histidine concentrations suggesting saturation transport kinetics. Furthermore Δc ∞ declined linearly along the convolution suggesting that nett active transport was not constant throughout the tubule. Using an expression to describe the rate of attainment of steady-state concentration in terms of lummal and peritubular histidine concentrations, histidine permeability coefficient (P), the maximum rate of active histidine transport (J max) and the half saturation constant of the transport reaction (K m ), we were able to determine the cause of the tubule inhomogeneity. We find thatP (14.1×10−5 cm/s) andJ max (45×10−10 mol/cm2· s) are constant along the convolution but thatK m increases markedly from about 5.4 mmol/kg 26% of the way along the convolution to 40 mmol/kg at 86%. These findings suggest that the histidine reabsorptive mechanism would be relatively inefficient with histidinuria occurring at all plasma concentrations but it would have enormous reserve capacity so that saturation would not readily occur. This prediction accords with available data on histidine clearance in the rat.
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  • 7
    ISSN: 1432-2013
    Keywords: Amino Acid Transport ; Microperfusion ; Renal Tubule ; l-Glutamine ; l-Histidine ; Fanconi Syndrome
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Stationary microperfusion of the rat proximal convoluted tubule together with simultaneous perfusion of the peritubular blood capillaries has been used to studyl-histidine andl-glutamine transport in the rat kidney. When histidine and glutamine concentrations in the capillary perfusate were 14.1 and 6.9 mmol/kg respectively, the luminal concentrations stabilized at about 5.6 and 2.0 mmol/kg respectively. The transepithelial concentration differences at steady-state were 8 mmol/kg (histidine) and 5 mmol/kg (glutamine). The results indicated that when peritubular capillary concentrations were high enough, nett passive back-flux of amino acids down a concentration gradient can become of considerable importance in determining nett reabsorptive rates. When the steady-state epithelial concentration differences were analysed in relation to perfusion site within the proximal convolution, it was found that the gradient was greatest near the glomerulus and smallest near thepars recta, the rate of decline along the convolution being approximately linear. This inhomogeneity of the proximal tubule seems to be due to a diminution in nett amino acid transport by about 50%. The results correlate well with the observation in Fanconi syndrome (congenital renal aminoaciduria with rickets) that only the first 1/3 of the proximal tubule usually shows marked pathological changes.
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  • 8
    ISSN: 1432-2013
    Keywords: Renal Tubule ; Phosphate Transport ; Sodium Dependence ; Micropuncture ; Microperfusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The standing droplet method has been used in combination with the peritubular perfusion of blood capillaries to determine the build up of transtubular concentration differences of phosphate (P i ) in the renal proximal convoluted tubule of parathyroidectomized rats. Electron probe analysis was used to estimate P i . At zero time both the intraluminal and the contraluminal P i concentration was 2 mM. The time dependent decrease of the intraluminal P i concentration was approximately 4 times faster in the early than in the late proximal convoluted tubule. After 45 sec an intraluminal steady state concentration of 0.20 mM P i was achieved in the early part. In the late part the intraluminal P i concentration approached a steady state value of 0.54 mM at 120 sec. When sodium free solutions were used the intraluminal P i concentration increased to 2.22 mM in the earlier and to 2.76 mM in the late part. The data indicate that in the proximal convoluted tubule 1. The rate of phosphate reabsorption is greater in the early part than in the later part, and 2. phosphate reabsorption might occur as co-transport with Na+ ions.
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Pflügers Archiv 364 (1976), S. 223-228 
    ISSN: 1432-2013
    Keywords: Renal calcium transport ; Renal calcium permeability ; Sodium dependence ; H+ transport ; Ouabain
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Using the stop flow microperfusion technique with simultaneous capillary perfusion the rate of active Ca2+ reabsorption was evaluated by measuring the static head electrochemical potential difference as well as the permeability of the tubular wall for Ca2+ ions. Under control conditions the active Ca2+ transport was calculated to be 3.35×10−13 mol/cm·s. It declined toward zero if the ambient Na+ was replaced by choline or lithium. Parallel experiments in the golden hamster showed that active Ca2+ transport, vanished completely if active Na+ transport was blocked by ouabain (1 mM). These data indicate that the active Ca2+ reabsorption from the proximal tubule depends on the active reabsorption of Na2+ presumably via a Na+−Ca2+ countertransport at the contraluminal cell membrane. The static head electrochemical potential difference of Ca2+ is the same in late and early proximal tubules. It is also not affected by the presence of acetazolamide (10−4 M) by the absence of bicarbonate or glycodiazine buffer or by the absence or presence of phosphate (2 mM).
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  • 10
    ISSN: 1432-2013
    Keywords: Kidney cortex structure ; Tissue compartments ; Transport models ; Inhibitory kinetics ; Facilitated diffusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract In order to study the transport of dicarboxylic acids through the contraluminal cell membrane of proximal tubular cells,3H-methylsuccinate has been synthetized by catalytic hydration of methylfumarate. As the chromatography of radioactive material excreted in the urine after i.v. injection of3H-methylsuccinate shows, no metabolite is detectable during the first 3 min. After 10 min, less than 10% of the excreted radiolabel is metabolized. To measure the contraluminal influx of3H-methylsuccinate from the interstitium into cortical tubular cells, the renal vessels were clamped so that the proximal tubular lumina collapsed. Then Ringer solution was injected into the blood capillaries. It contained different concentrations of3H-methylsuccinate and14C-inulin as extracellular space marker. After contact times between 1 and 10 s, this fluid was withdrawn from the capillaries and the disappearance of3H-methylsuccinate relative to14C-inulin was measured. The morphological compartments in the outer cortex of the clamped glutaraldehyde-fixed kidney were evaluated by a stereological method. For proximal tubular cells a ratio of extracellular water space to intracellular space of 1:3.1 and a ratio extracellular water space to free cell water space of 1:2 was found. It was tested whether the experimental disappearance curves with 4 different starting concentrations of3H-methylsuccinate fit with the data from four model calculations. It was found that the data and the conditions of transport are consistent with the predictions of a facilitated diffusion model. In this model, a transport coefficient occurs which depends on the concentration of3H-methylsuccinate following saturation kinetics. The calculated parameters wer:K m for3H-methylsuccinate=0.12 mmol/l,J max=0.50 pmol/s ·cm (related to tubular length in cm). Furthermore, equations are given to calculate inhibitory constantsK i of competing dicarboxylic acids.
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