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  • 1
    ISSN: 1432-2013
    Keywords: d-Glucose ; Microperfusion ; Proximal Tubule ; Active Reabsorption ; Kinetic Study ; d-Glucose ; Mikroperfusion ; proximaler Tubulus ; aktive Reabsorption ; kinetische Studien
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Description / Table of Contents: Zusammenfassung Proximale Konvolute von Rattennieren wurden bei fehlendem Nettofluß von Natriumionen und Wasser kontinuierlich mit Lösungen perfundiert, die eined-Glucosekonzentration zwischen 0,5 und 2,0 mmol/l enthielten. Der Abfall der intraluminalend-Glucosekonzentration entlang eines Konvolutes wurde durch Absaugen der perfundierten Lösung in abnehmender Entfernung von der Perfusionsstelle verfolgt. Die pro innere Tubulusoberfläche und Zeit transportierted-Glucosemenge wird mit Abnahme der intraluminalen Glucosekonzentration kleiner. Dieses Verhalten läßt sich durch eine 2-parametrige Gleichung analog der Michealis-Menten-Kinetik beschreiben. Es errechnet sich eine maximale Transportrate,V max, von 6 · 10−10 mol · cm−2 · sec−1 und eine Halbsättigungskonzentration,K m, von 0,6 mmol/l. Die so beschriebene aktive Resorption und die von uns gefundene passive Permeabilität des proximalen Konvolutes fürd-Glucose reichen, nach angestellten Computerberechnungen zu schließen, allein nicht aus, um den Nettoglucosetransport der Gesamtniere unter Freiflußbedingungen quantitativ zu beschreiben.
    Notes: Summary The proximal convoluted tubule of rat kidney was continuously perfused with a steady state solution containing 0.5 to 2.0 mM ofd-glucose. The gradual decrease of intraluminald-glucose concentration was investigated with repeated collections of perfusate from the same tubule whereby the sequence of punctures proceeded towards the site of perfusion. The rate ofd-glucose transport per unit area decreased with decreasing intraluminald-glucose concentration. This relationship could be expressed by a two parameter system corresponding to the Michaelis-Menten equation. It was found that the local maximal transport rateV max equals 6×10−10 mol×cm−2×sec−1 andK m equals 0.6 mM. Our data on active resorption and passive permeability ofd-glucose in the proximal convolution have been subjected to computer analysis. The sum of both components ofd-glucose transport alone as measured under the condition of zero netflux of sodium chloride and water did not match the amount of net glucose transport found for the whole kidney under free-flow-conditions.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Pflügers Archiv 305 (1969), S. 155-166 
    ISSN: 1432-2013
    Keywords: l-Glucose ; Micropuncture and Microperfusion ; Proximal Tubule ; Active Secretion ; Kinetic Study ; l-Glucose ; Mikropunktion und Mikroperfusion ; proximaler Tubulus ; aktive Sekretion ; kinetische Studien
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Studies with the free flow micropuncture technique have shown that the ratio of TF/Pl-glucose to TF/PInulin in proximal tubular fluid, in distal tubular fluid, and in more than half of the final urine samples measured was greater than one, which suggests thatl-glucose was actively secreted. Studies with the microperfusion technique confirmed this finding and showed thatl-glucose was secreted by the proximal tubules. A maximum rate of secretion was reached at a plasma concentration of 4 mM. The tubular secretion ofl-glucose was augmented by the presence of 16.6 mMd-glucose in tubular lumen and inhibited by 10−4 M phlorizin. Kinetic analysis showed that theV max values forl-glucose secretion in the absence and in the presence ofd-glucose are 5.0×10−10 and 6.3×10−10 mol×cm−2×sec−1 respectively which were very close to the value reported for theV max ford-glucose reabsorption. However, theK m forl-glucose secretion was 3.1 mM and was reduced to 1.6 mM whend-glucose was present in the perfusion fluid. TheK m ford-glucose reabsorption has been reported to be 0.6 mM (8). The results of this investigation were interpreted as being consistant with the hypothesis thatl-glucose secretion andd-glucose reabsorption share the same carrier system.
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  • 3
    ISSN: 1432-2013
    Keywords: d-Glucose ; Microperfusion ; Proximal Tubule ; Active Reabsorption ; Passive Permeability ; d-Glucose ; Mikroperfusion ; proximaler Tubulus ; aktive Resorption ; passive Permeabilität
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Description / Table of Contents: Zusammenfassung Der Nettotransport und der unidirektionale Transport vond-Glucose wurden in Mikroperfusionsversuchen am proximalen Konvolut der Rattenniere unter der Bedingung des fehlenden Nettoflusses von Natriumchlorid und Wasser gemessen. Nach vollständiger Hemmung des aktiven Transportanteils durch Phlorrhizin (10−4 mol/l in der Perfusionslösung) läßt sich eine passive Komponente desd-Glucosetransports nachweisen. Diese ist als additives Glied zum aktiven Transportanteil auch im nicht phlorrhizin-vergifteten Zustand zu beobachten. Der aktive Transportanteil beträgt imV max Bereich 6·10−10 mol·cm−2·sec−1. Die passive Transportkomponente ist derd-Glucosekonzentrationsdifferenz zwischen Perfusat und Serum im angewandten Konzentrationsbereich direkt proportional. Zwischen den Ergebnissen der Ein- und Ausstrommessung besteht kein signifikanter Unterschied. Es wird daraus geschlossen, daß der passived-Glucosefluß ein einfacher Diffusionsprozeß ist. Der sich aus drei verschiedenen Versuchsanordnungen ergebende mittlere Permeabilitätskoeffizient der proximalen Tubuluswand für Glucose beträgt 1,7·10−5 cm ·sec−1. Es ist experimentell möglich, den aktiven Austransport durch passiven Einwärtstransport vollständig zu kompensieren. Dazu ist allerdings eine Konzentrationsdifferenz von 33 mmol/l nötig. Wird der unidirektionale Austransport mit Hilfe von14C markierter Glucose gemessen, der Nettotransport jedoch chemisch, so kann man die experimentell gemessene Änderung der spezifischen Aktivität mit den aus der Zweikomponentenhypothese (aktiver Transport + passive Diffusion) voraussagbaren Veränderungen vergleichen. Eine Übereinstimmung von Voraussage und Befund ist bei Benutzung der oben angeführten Werte gegeben. Unter normalen Freiflußbedingungen spielt die passive Transportkomponente keine wesentliche Rolle. Nur bei größeren transtubulären Konzentrationsdifferenzen — wie sie bei niedrigem Glomerulumfiltrat und hoher Serumglucosekonzentration auftreten — wird der passive Glucoseeinstrom für died-Glucoseresorption der Gesamtniere bedeutend.
    Notes: Summary Both, the net and unidirectional transport ofd-glucose across the proximal tubule of rat kidney were studied by the technique of continuous microperfusion under the condition of zero netflux of water and sodium chlorid. When the active transport component was completely abolished by 10−4 M of phlorizin a small passive component could be demonstrated. The passive component was also observed in the non-phlorizin-poisoned stat to be additive to the active transport.V max of the actived-glucose transport was 6×10−10 mol×cm−2×sec−1. The passive glucose flux is proportional to the concentration difference between perfusate and serum. There is no difference between data of influx and efflux measurements. From this it is concluded that the passived-glucose flux is a simple diffusion process. The permeability coefficient ford-glucose (P G) calculated from 3 different sets of experiments is 1.7×10−5 cm×sec−1. Active and passive glucose transport component can cancel out each other at a transtubular concentration difference of 33 mmol/l. Changes of the specific activity in the perfusate were measured under three different conditions (d-glucose concentration lumen: 〉 plasma, lumen = plasma, lumen 〈 plasma). These changes were in agreement with our theoretical prediction using the above mentioned values ofV max andP G for the calculation (see appendix). The permeability ofd-glucose across the proximal tubules is small. Therefore, under normal free flow condition the passive transport component plays no role in thed-glucose absorption. However, with greatly increasing transtubular concentration difference ofd-glucose, e.q. due to a low glomerular filtration rate and a high serumd-glucose concentration, the passive transport component may be the rate limiting factor for the net transport ofd-glucose.
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