Membrane characteristics regulating potassium transport out of the isolated perfused descending limb of Henle

Membrane characteristics regulating potassium transport out of the isolated perfused descending limb of Henle. Previous micropuncture studies have noted that the potassium concentration in the descending limb of Henle (DLH) is significantly higher than in the plasma. The mechanisms by which this occurs is not clear. The present studies were designed to examine this issue by elucidating the characteristics of potassium transport across the isolated perfused rabbit descending limb of Henle. Three different types of measurements were made: 1) simultaneous isotopic permeability coefficients for 42 K (P K ) and 22 Na (K Na ) were determined under conditions of zero net fluid transport; 2) the collected-to-perfused potassium concentration ratio was measured when tubules were perfused with isosmolal ultrafiltrate of same rabbit serum as bath; and 3) the fractional rise in potassium concentration was compared to fractional movement of water secondary to induced osmotic forces. The results indicate that P K and P Na , expressed per unit length of tubule, were 2.43 ± 0.43 × 10 -7 cm 2 sec -1 and 1.86 ± 0.34 × 10 -7 cm 2 sec -1 , respectively. These values indicate that potassium is almost as impermeant as sodium across the DLH and is consistent with the previous observations which suggested that the isolated rabbit DLH is relatively impermeant to solutes. The chemical concentration of potassium was measured in two sets of studies. In the first, the collected-to-perfused (C/P) potassium concentration ratio did not rise above the (C/P) volume marker ratio when tubules were perfused with isosmolal ultrafiltrate. However, in the second set of studies a significant rise was noted in the (C/P) potassium concentration ratio when DLH was perfused with 302 mOsm/liter ultrafiltrate in hyperosmolar bath made up by addition of 298 mOsm NaCl to rabbit serum (final osmolality of bath was 600 mOsm/liter). Under these circumstances the (C/P) potassium ratio was 1.53 ± 0.07 while (C/P) volume marker ratio was 1.62 ± 0.09. It is concluded that the combination of DLH permeation characteristics are such that potassium concentrations within the DLH are raised to high values by sieving processes secondary to osmotic abstraction of water as the fluid within the DLH courses through progressively more hyperosmolar surroundings. Caracteristiques de membrane regulant le transport de potassium hors de la branche descendante de l'anse de Henle isolee et perfusee. Des travaux anterieurs utilisant les techniques de microponctions ont montre que la concentration du potassium dans la branche descendante de l'anse de Henle (DHL) est significativement superieure a celle du plasma. Le mecanisme par lequel cela survient n'est pas clair. Ce travail a ete concu pour envisager ce probleme en elucidant les caracteristiques du transport de potassium a travers la branche descendante de l'anse de Henle de lapin isolee et perfusee. Trois categories de mesures ont ete faites: 1) les coefficients de permeabilite pour les isotopes 42 K (P K ) et 22 Na (P Na ) ont ete determines dans des conditions de flux net nul d'eau; 2) le rapport de concentration du potassium collection/perfusat a ete determine alors que les tubules etaient perfuses avec un ultrafiltrat iso-osmotique du meme serum de lapin que celui utilise dans le bain; 3) l'augmentation fractionelle de la concentration du potassium a ete comparee aux mouvements d'eau consecutifs aux forces osmotiques imposees. Les resultats de ces experiences sont indiques ci-dessous. P K et P Na , exprimes par unite de longueur de tubule, sont respectivement de 2,43 ± 0,43 · 10 -7 · cm 2 · sec -1 et 1,86 ± 0,34 · 10 -7 · cm 2 · sec -1 . Ces valeurs indiquent que le potassium est presque aussi impermeant que le sodium a travers la DHL, ce qui est en accord avec les observations anterieures qui suggeraient que la DHL isolee de lapin est relativement impermeable aux substances dissoutes. La concentration chimique du potassium a ete mesuree dans deux series d'etudes. Dans la premiere, le rapport de concentration du potassium collection/perfusion (C/P) n'a pas depasse le meme rapport pour l'indicateur de volume alors que les tubules etaient perfuses avec un ultrafiltrat iso osmotique. Dans la deuxieme, cependant, une augmentation significative de (C/P) du potassium a ete obtenue alors que la DHL etait perfusee avec un ultrafiltrat a 302 mOsm/kg dans un bain rendu hyperosmolaire pas l'addition de NaCl, 298 mOsm/kg, a du serum de lapin (l'osmolalite finale du bain etait de 600 mOsm/litre). Dans ces conditions le (C/P) du potassium etait de 1,53 ± 0,07 alors que le (C/P) du marqueur de volume etait de 1,62 ± 0,09. Il est conclu que la combinaison des caracteristiques de permeation de la DHL est telle que les concentrations de potassium a l'interieur de la DHL sont rendues elevees par un processus de filtration, secondaire a la soustraction osmotique d'eau, au fur et a mesure que le liquide, dans la DHL, parcourt un milieu environnant d'hyperosmolarite croissante.