Flow dependence of fluid transport in the isolated superficial pars recta: Evidence that osmotic disequilibrium between external solutions drives isotonic fluid absorption

Flow dependence of fluid transport in the isolated superficial pars recta: Evidence that osmotic disequilibrium between external solutions drives isotonic fluid absorption. The present studies tested the hypothesis that osmotic disequilibrium between luminal and peritubular fluids is the driving force for net volume absorption in the isolated proximal straight tubule. Isolated tubule segments from superficial rabbit renal cortex were perfused at varying rates with a high chloride and bicarbonate-free solution as they were bathed with a normal bicarbonate-Krebs-Ringer buffer solution at 38° C. Increasing the perfusion rate from ≃4 to ≃30 nl/min produced a monotonie increase in net volume absorption (J v ) from 0.18 ± ( SEM ) 0.03 to 0.62 ± 0.08 nl · min -1 · mm -1 . The chloride concentration in collected fluid samples rose from ≃137 to ≃147 mEq/ liter over the same perfusion rate range. Ouabain (10 -4 M ) added to the bathing solution inhibited J v by a rate which varied from 0.20 to 0.28 nl · min -1 · mm -1 depending on the perfusion rate. A mathematical model of the axial flows and transepithelial transport processes was developed. This model, and the experimental data, is consistent with the view that the driving force for isotonic fluid absorption in these tubules depends on the axial maintenance of osmotic disequilibrium between the perfusate and the bathing solution. Increasing the perfusion rate opposes osmotic equilibration by minimizing the extent to which dissipative fluxes of chloride and bicarbonate ions change the transepithelial chloride and bicarbonate concentration gradients, and by minimizing the tendency of the luminal cryoscopic osmolality to increase as volume absorption occurs. Dependance du transport de liquide vis a vis du debit dans la pars recta superficielle isolee: Preuve de ce que le desequilibre osmotique entre les solutions externes gouverne l'absorption isotonique. Ce travail est destine a evaluer l'hypothese selon laquelle le desequilibre osmotique entre les liquides luminaux et peritubulaires est la force motrice de l'absorption nette dans le tube proximal droit isole. Des segments tubulaires isoles a partir du cortex superficiel de lapin ont ete perfuses a des debits variables avec une solution riche en chlorure et depourvue de bicarbonate et immerges dans une solution tampon bicarbonate-Krebs-Ringer a 38° C. L'augmentation du debit de perfusion de ≃4 a ≃30 nl/min a produit une augmentation du debit d'absorption net (J v ) de 0,18 ± ( SEM ) 0,03 a 0,62 ± 0,08 nl · min -1 · mm -1 . La concentration de chlorure dans les echantillons de liquide collecte a augmente de ≃137 a ≃147 mEq/ litre pour les memes valeurs de debit de perfusion. Un modele mathematique des debits axiaux et des processus de transport transepitheliaux a ete developpe. Ce modele est, ainsi que les resultats experimentaux, en accord avec l'idee selon laquelle la force motrice de l'absorption isotonique dans ces tubules est fonction du maintien axial d'un desequilibre osmotique entre le perfusat et le bain. L'augmentation du debit de perfusion s'oppose a l'equilibration osmotique en diminuant la capacite des flux dissipatifs de ions des chlorure et bicarbonate de modifier les gradients de concentration transepitheliaux de chlorure et bicarbonate et en diminuant la tendance a l'augmentation de l'osmolalite luminale en fonction de l'absorption.