The effect of urea infusion on the urinary concentrating mechanism in protein-depleted rats.

To explore the role of urea in the urinary concentrating mechanism, the contents of vasa recta, Henle's descending limbs and collecting ducts were sampled by micropuncture of the renal papilla before and after infusion of urea in 10 protein-depleted rats. Eight protein-depleted rats not given urea were similarly studied as a control group. After urea administration, osmolality and the concentrations of urea and nonurea solute of urine from both exposed and contralateral kideny increased significantly. The osmolality and urea concentration of fluid from the end of Henle's descending limb and vasa recta plasma and the tubule fluid-to-plasma inulin ratio in the end-descending limb all increased significantly after urea infusion. We interpret these observations to indicate that urea enhances urinary concentration by increasing the abstraction of water from the juxtamedullary nephron (presumably the descending limb), in agreement with the prediction of recent passive models of the urinary concentrating mechanism. However, the concentration of urea in fluid from the descending limb after urea infusion was high (261 plus or minus 31 mM) and the difference in solium concentration between descending limb fluid and vasa recta was small and statistically insignificant.