The relation between the reabsorption of urea and of water in the distal tubule of the rat kidney

Summary

Micropuncture studies on rats with hereditary hypophysial diabetes insipidus were undertaken to examine the effects of water and osmotic diuresis and dietary protein on renal tubular urea handling.

The results show that in a massive water diuresis the urea present at end of the proximal tubules is practically quantitatively exreted in the final urine. In osmotic diuresis an ever increasing amount of urea reaches the end of the proximal tubule, sinceTF/P _urea of 1.31±0.16 in water diuresis remains constant in osmotic diuresis, whilstTF/P _inulin falls from 2.41±0.21 in water diuresis to values lower than 2.00 in osmotic diuresis. In this case fractional amounts of urea of the same order as in the proximal tubule were measured in the distal convolution and in the final urine. A value of 0.99±0.17×10⁻⁴ mm/sec for the distal tubular urea permeability was obtained by direct measurement at high plasma and high intratubular urea concentrations and did not change significantly after pretreatment on low or high protein diet (1.00±0.20 and 1.05±0.23×10⁻⁴ mm/sec, respectively). From these results we conclude: 1. The magnitude of the urea excretion in a massive diuresis is primarily determined by the water reabsorption in the distal nephron. 2. The urea permeability of the distal convolution is independent of the concentration thus excluding the suggested possibility that there exists a reabsorption mechanism for urea autonomous of water reabsorption. 3. The fall in fractional urea excretion in animals on low protein diet is due entirely to mechanisms located in the collecting duct.