Relationship between peritubular oncotic pressure gradients and morphology in isolated proximal tubules.

Relationship between peritubular oncotic pressure gradients and morphology in isolated proximal tubules. Previous studies have demonstrated that net sodium and water flux across the isolated perfused proximal convoluted tubule of the rabbit can be increased by elevating the protein concentration of the bath and at least part of the protein effect is mediated through oncotic forces. This study was designed to determine the pathway of the reabsorbed fluid by examining the morphologic characteristics of the epithelium as the ambient protein concentration was varied. Isolated segments of rabbit proximal convoluted tubule were preserved in vitro for light and electron microscopy by intraluminal perfusion of an isotonic fixative solution as the tubule was bathed in a hyperoncotic, an iso-oncotic or a hypo-oncotic protein-free ultrafiltrate of serum. Widening of lateral and basilar intercellular spaces was observed in proximal convoluted tubules exposed to hyperoncotic and iso-oncotic protein bathing solutions at the time of fixation. The widened intercellular spaces contained protein whose presence was confirmed with electron microscopic autoradiography using 125 I-labeled albumin as a marker. These studies demonstrate an association between the width of the intercellular space and net sodium and water flux in the mammalian proximal convoluted tubule. The results suggest the possibility that the widened lateral and basilar intercellular spaces may represent a component of at least one pathway by way of which fluid is reabsorbed across the epithelium of the proximal convoluted tubule in vitro in response to peritubular oncotic pressure gradients. In addition, since albumin is capable of crossing the basement membrane of the tubule, the oncotic effect of protein in this model is probably not mediated solely across the basement membrane, but may effect an osmotic driving force across either the tight junction or the lateral plasma membrane of the cell or both in the isolated proximal tubule. Relation entre les gradients de pression oncotique peritubulaire et la morphologie du tube proximal isole. Des travaux anterieurs ont montre que le flux net d'eau et de sodium a travers le tube contourne proximal de lapin isole et perfuse peut etre accru par l'augmentation de la concentratione protein des dans le bain et qu'au moins une partie de l'effet des proteines est exerce par l'intermediaire des forces oncotiques. Ce protocole a pour but la determination des voies de reabsorption du liquide par l'etude de la morphologie de l'epithelium alors que la de tube contourne proximal de lapin ont ete prepares in vitro concentration ambiante de proteines varie. Des segments isoles aux fins d'examen en microscopie photonique et electronique par la perfusion intra luminale d'une solution de fixative isotonique alors que le tubule baignait dans un ultrafiltrat de serum hyperoncotique, isooncotique, ou encore hypooncotique exempt de proteines. L'elargissement des espaces intercellulaires lateraux et basilaires est observe dans les tubes contournes proximaux exposes a des solutions hyperoncotiques ou isooncotique au moment de la fixation. Les espaces intercellulaires elargis contiennent des proteines dont la presence est confirmee par des autoradiographies utilisant l'albumine 125 I comme marqueur. Ces resultats demontrent une relation entre la largeur de l'espace intercellulaire et le flux net d'eau et de sodium dans le tube contourne proximal des mammiferes. Ils suggerent que les espaces intercellulaires basilaires et lateraux elargis puissent representer un element d'au moins une voie par laquelle le liquide est reabsorbe in vitro a travers l'epithelium du tube contourne proximal en fonction des gradients peritubulaires de pression oncotique. De plus, du fait que l'albumine peut traverser la membrane basale du tubule, l'effet oncotique des proteines dans ce modele n'est probablement pas exerce seulement a travers la membrane basale mais peut aussi realiser une force osmotique a travers la jonction serree ou la membrane plasmique laterale de la cellule (ou les deux) du tube proximal isole.