Micropuncture study of net transtubular movement of water and urea in nondiuretic mammalian kidney

¹ Department of Medicine, University of North Carolina School of Medicine, Chapel Hill, North Carolina

Anesthetized, nondiuretic rats and hamsters were infused with C¹⁴-labeled inulin-carboxylic acid or urea, and fluid was subsequently collected by micropuncture from surface tubules in the rats and from loops of Henle and collecting ducts in the hamsters. Osmolality and radioactivity of tubular fluid, ureteral urine, and plasma were determined. There was net loss of both water and solute from all segments of the nephron. In the loop of Henle, water loss occurred primarily from the descending limb and solute loss from the ascending limb. Urea was reabsorbed from the proximal and distal convolutions and collecting ducts but was added to the tubular fluid in the descending limb of the loop of Henle. These results lend support to the countercurrent theory of urine concentration and indicate that the osmotic gradient in the medulla is established by active transport of solute out of the ascending limb of the loop of Henle. The results are compatible with passive movement of water and urea, but the possibility of active transport of urea is not excluded.

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