Laboratory Investigation

Kidney International (1980) 17, 205–222; doi:10.1038/ki.1980.24

Urea and renal concentrating ability in the rabbit

Robert A Gunther1 and Lawrence Rabinowitz1

1Department of Human Physiology, School of Medicine, University of California, Davis, California

Correspondence: Dr L Rabinowitz, Department of Human Physiology, School of Medicine, University of California, Davis, California 95616, USA

Received 22 March 1979; Revised 16 July 1979.

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Abstract

Urea and renal concentrating ability in the rabbit. The hypotheses of passive salt accumulation predict an enhancement of renal concentrating ability by urea. We tested this prediction in rabbits, a species whose nephrons when studied in vitro show transport properties that support these hypotheses. We used calm, unanesthetized, hydropenic, vasopres sin-treated rabbits with intact kidneys fed a 16% protein diet, and we observed the effect of urea administration at two rates of solute excretion (60 and 190 microOsm/min dot kg body wt; N = 10 and 5, respectively). After an i.v. mannitol infusion, when urea was infused, the i.v. solute excretion rate was unchanged, the changes in urine urea concentration were large (a change of 767 and 408 micromoles/ml), but only small and variable changes in urine osmolality occurred (a change of 78plusminus146, and 36plusminus50 microOsm/g H2O). In additional experiments, we removed the kidneys from antidiuretic, or urea- or mannitol-in-fused rabbits and measured the intrarenal distribution of sodium, potassium, urea, and chloride. When the urine urea level was greater than 400 mmoles, the urine-to-papilla ratios for urea were 1.6 to 3.6. This suggested that a low collecting duct permeability to urea could explain the absence of a marked enhancement of concentrating ability during urea administration. Further analysis, based on a model of inner medullary solute compartments, indicated that sodium chloride was the major (86%) osmotically active solute in the medullary central core of these rabbits and that it was not influenced by changes in urinary urea concentration. The results of tissue analysis were consonant with either active or passive sodium chloride reabsorption from the thin ascending limb of Henle's loop in these rabbits.

Urée et pouvoir de concentration rénal chez le lapin. Les hypothèses d'accumulation passive de sel impliquent une augmentation de la capacité de concentration par l'urée. Cette implication a été étudiée chez le lapin, une espèce dont les néphrons ont des propriétés de transport in vitro qui sont en accord avec ces hypothèses. Des lapins nourris avec 16% de protéines, intacts, clames, non anesthésiés, privés d'eau et traités par la vasopressine ont été étudiés. L'effet de l'administration d'urée a été observé à deux débits d'excrétion de substances dissoutes (60 et 190 microOsm/min dot kg de poids corporel; N = 10 et 5, respectifment). Après la perfusion de mannitol i.v., la perfusion d'urée n'a pas modifié le débit d'excrétion de substances dissoutes, a augmenté considérablement la concentration urinaire de l'urée (modification de 767 et 408 micromoles/ml), mais n'a entraîné que des modifications mineures et variables de l'osmolalité urinaire (modification de 78 plusminus 146 et 36 plusminus 50 microOsm/g H2O). Au cours d'autres expériences des reins ont été prélevés chez des lapins en antidiurèse ou en perfusion d'urée ou de mannitol, et la distribution intrarénale de sodium, potassium, urée et dorure a été mesurée. Quand la concentration urinaire d'urée est supérieure à 400 mmoles le rapport de concentration de l'urée urine sur papille (1,6 à 3,6) suggère qu'une faible perméabilité à l'urée du canal collecteur pourrait expliquer l'augmentation importante de la capacité de concentration au cours de l'administration d'urée. Une analyse plus poussée, fondée sur un modèle de compartiments médullaires internes pour les substances dissoutes, a indiqué que chlorure de sodium était la principale substance dissoute osmotiquement active (86%) dans la partie centrale de la médullaire chez ces lapins et n'était pas influencée par les modifications de l'urée urinaire. Les résultats obtenus sur les fragments tissulaires sont compatibles avec une réabsorption, active ou passive, de chlorure de sodium dans la branche grêle ascendante de l'anse de Henle.

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