Effect of perfusion rate on the fluxes of water, sodium, chloride and urea across the proximal convoluted tubule.

Effect of perfusion rate on the fluxes of water, sodium, chloride and urea across the proximal convoluted tubule. Studies were undertaken to examine the mechanism whereby changes in intraluminal flow rates alter reabsorption in the isolated perfused proximal tubule of the rabbit. All protocols employed the technique of in vitro perfusion of isolated segments of the proximal convoluted tubule. Stepwise elimination of d-glucose and 1-alanine from an artificial perfusate simulating ultrafiltrate decreased the unidirectional flux of sodium, transtubular potential difference, and net water absorption. Using isosmolal ultrafiltrate as the perfusate, net fluid reabsorption and the unidirectional lumen-to-bath flux of sodium and chloride decreased with a decrease in flow rate below 11 n1/min, but neither net fluid reabsorption nor the unidirectional fluxes of sodium and chloride increased further as the perfusion rate was increased above 11 n1/min. The unidirectional flux of 14 C-urea was not affected by changes of perfusion rate from 1.6 to 44 n1/min. The dependence of net fluid reabsorption and unidirectional fluxes of sodium and chloride on flow rate per se , and not on intraluminal hydrostatic pressure or geometry, was established by demonstrating their decrease despite a rise in intraluminal pressure and inside diameter produced by counterpressure at the collecting end of the tubule, while flow was decreased. Ouabain decreased net fluid reabsorption to near zero at all flow rates, but ouabain had no effect on the flow-dependency of unidirectional sodium and chloride fluxes. Furthermore, the flow-dependency of the unidirectional flux of sodium was eliminated with a decrease in bicarbonate concentration and removal of d-glucose and 1-alanine from the perfusate. Thus, the present studies demonstrate that net water and unidirectional sodium and chloride fluxes are flow-dependent. At flow rates somewhere below 11 n1/min, unidirectional fluxes decreased with decreasing perfusion rates; however, at perfusion rates greater than 11 n1/min, there was no further effect of perfusion rate on either net water absorption or the unidirectional fluxes of sodium or chloride. These effects may be partly mediated through the flow-dependent changes in the intraluminal concentration of bicarbonate, d-glucose, and 1-alanine. Effet du debit de perfusion sur les flux d'eau, de sodium, de chlore et d'uree a travers le tube contourne proximal. Ce travail a ete entrepris afin d'etudier le mecanisme par lequel les variations du debit intraluminal modifient la reabsorption dans le tube proximal de lapin isole et perfuse. Tous le protocoles ont utilise la technique de perfusion in vitro de segments isoles de tube contourne proximal. L'elimination progressive du d-glucose et de la l-alanine d'une solution de perfusion artificielle ressemblant a de l'ultrafiltrat diminue le flux unidirectionnel de sodium, la diference de potentiel transtubulaire et la reabsorption nette d'eau. Quand de l'ultrafiltrat iso-osmolaire est utilise comme perfusat, la reabsorption nette de liquide et le flux unidirectionnel, de la lumiere vers le bain, de sodium et de chlore diminue avec le debit de perfusion quand celui-ci est inferieur a 11 n1/min, mais ni la reabsorption de liquide, ni les flux unidirectionnels n'augmentent quand le debit de perfusion est superieur a 11 n1/min. Le flux unidirectional d'uree- 14 C n'est pas affecte par les variations du debit de perfusion entre 1,6 et 44 n1/min. La dependance de la reabsorption nette de liquide et des flux unidirectionnels de sodium et de chlore du debit de perfusion lui-meme, et non de la pression hydrostatique intralumjnale ou de la geometrie, a ete etablie en demontrant leur diminution malgre une augmentation de la pression intraluminale et du diametre interne produite par une contre-pression a l'extremite terminale du tubule alors que le debit de perfusion etait diminue. L'ouabaine diminue la reabsorption nette de liquide a des valeurs voisines de zero a tous les debits de perfusion mais elle n'a pas d'effet sur la dependance vis-a-vis du debit des flux unidirectionnels de sodium et de chlore. De plus, la dependance du flux unidirectionnel de sodium vis-a-vis du debit est annulee par la diminution de la concentration du bicarbonate et la suppression du d-glucose et de la l-alanine du perfusat. Ainsi ce travail demontre que le flux net d'eau et les flux unidirectionnels de sodium et de chlore sont dependants du debit. A des debits inferieurs a 11 n1/min les flux unidirectionnels diminuent en meme temps que le debit. A des debits superieurs a 11 n1/min, cependant, il n'y a pas d'effet supplementaire du debit de perfusion, ni sur la reabsorption nette d'eau, ni sur les flux unidirectionnels de sodium et de chlore. Ces effets pourraient etre en partie exerces par l'intermediaire de modifications de concentrations, dependantes du debit, du bicarbonate, du d-glucose et de la 1-alanine.