An in vivo study of the concentrating process in the descending limb of Henle's loop

An in vivo study of the concentrating process in the descending limb of Henle's loop. Two features of the urinary concentrating mechanism were examined in the rat: 1) the relative contribution of water extraction and solute addition to the osmolality of fluid in the descending limb of Henle's loop (defined in this paper as that nephron segment beginning at the junction of the cortex and outer medulla), and 2 ) the difference between the concentration of urea in fluid from the descending limb and from the collecting duct. Rats were studied under three conditions of urinary solute concentration varying from brisk water diuresis to steady-state antidiuresis. Net solute addition contributed substantially (33 to 40%) to the increase in osmolality of descending limb fluid. Urea was the principal solute added: compared to the filtered load, the amount of urea present in fluid at the hairpin turn varied from 334% in water diuresis to 642% in antidiuresis. The differences in urea concentration between fluid from the end of descending limbs and fluid from the tips of collecting ducts (the latter assumed to set an upper limit for interstitial urea concentration) were small, suggesting that the concentration of urea in descending limb fluid approaches equilibrium with that in papillary interstitium. These results are difficult to reconcile with the models recently proposed for generating hypertonicity of the inner renal medulla of the mammalian kidney by passive mechanisms. Etude in vivo du mecanisme de concentration dans la branche descendante de l'anse de Henle. Deux aspects du mecanisme de concentration de l'urine ont ete examines chez le rat: 1) la contribution relative de la soustraction d'eau et de l'addition de substances dissoutes a l'osmolalite du liquide de la branche descendante de l'anse de Henle (definie dans ce travail comme le segment du nephron qui commence a la jonction entre le cortex et la medullaire externe), et 2 ) la difference de concentration de l'uree entre la branche descendante et le canal collecteur. Les rats ont ete etudies dans trois etats de concentration urinaire allant de la diurese aqueuse importante a un etat stationnaire d'antidiurese. L'addition nette de substances dissoutes contribue substantiellement (33 a 40%) a l'augmentation de l'osmolalite du liquide de la branche descendante. L'uree est la principale substance dissoute ajoutee: la quantite d'uree delivree a la pointe de l'anse represente 334%, en diurese aqueuse, et 642%, en antidiurese, de l'uree filtree. La difference de concentration d'uree entre le liquide de la partie terminale des branches descendantes et le liquide de l'extremite des canaux collecteurs (ce dernier est suppose indiquer la limite superieure de la concentration interstitielle d'uree) et petite, ce qui suggere que la concentration de l'uree dans le liquide de la branche descendante est proche de l'equilibre avec la concentration dans l'interstitium de la papille. Ces resultats sont difficiles a accorder avec les modeles recemment proposes pour expliquer l'etablissement de l'hypertonie de la medullaire interne du rein des mammiferes par des mecanismes passifs.