Mechanism of Excretion of Urea by the Kidney in the Rabbit. ∗

Abstract

Urea, according to the traditional view, is excreted by the kidney as a result of glomerular filtration and partial passive reabsorption in the tubules. Schmidt-Nielsen (1) in a recent review cites observations which necessitate modification of this theory. One such observation is the steep concentration gradient of urea in the medulla of rats and other rodents, increasing from the cortical border up to the papilla. Gottschalk(2), following the theory of Wirz, Hargitay and Kuhn(3), proposes that sodium chloride is actively transported in the ascending limb of the loop and that urea also may be, although he considers it more likely that the urea gradient results from passive diffusion secondary to the sodium chloride gradient. Recently evidence from Gottschalk and his group supports the view of Klümper et al.(4) that the concentration gradient of urea in the medulla is the result of back diffusion of urea from the fluid of the collecting ducts (5).

If urea is diffusible in the kidney medulla it would also move from blood to tubule fluid and because of the close association of the limbs with the vasa recta this might occur rapidly. Labeled urea, injected into the blood stream should appear in the urine before a simultaneously injected glomerular indicator such as ferrocyanide.

We planned to study this question by injecting rapidly a small volume of labeled urea and ferrocyanide intravenously and following their appearance in the urine by free flow technic. In other experiments we followed their excretion by stop-flow technic.

Method. Rabbits were anesthetized with ether and after laparotomy polyethylene tubes were inserted in one or both ureters as close to the renal pelvis as feasible. The ends of the tubes were led out through the abdominal wall and the abdominal incision was closed.