Countercurrent diffusion in the renal cortex of the rabbit.

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Abstract
Evidence was obtained for countercurrent diffusion of 14C-n-butanol in the rabbit renal cortex. After injection into the renal artery of perfused, isolated rabbit kidneys, concentrations of both 14C-butanol and tritiated water in the initial samples collected from the renal venous outflow exceeded those of 125I-albumin when the rate of perfusion was low. When flow was increased, the 125I-albumin curve was shifted to earlier times relative to the other labels. These observations confirm the existence of arteriovenous diffusion of 14C-butanol and tritiated water in the kidney. In a second set of experiments, 14C-butanol and tritiated water were infused for 10 or 60 seconds into the renal arteries of anesthetized rabbits, and the kidneys were then removed and frozen in liquid nitrogen. Cores were cut from the kidney surface and sectioned in a cryostat, and the ratio of 14C-butanol to tritiated water was calculated at increasing depths in the cortex. This ratio rose from 0.54 +/- 0.03 (SEM) at the surface to 0.98 +/- 0.07 at 3 mm beneath the surface in kidneys perfused for 10 seconds. This gradient was less steep after 60 seconds of perfusion. The early appearance of 14C-butanol relative to tritiated water in the renal venous outflow and delayed equilibration of 14C-butanol in the outer renal cortical tissue are consistent with counter-current diffusion. It is suggested that this exchange may occur between adjoining interlobular arteries and veins in the cortex and may contribute to high carbon dioxide tensions found near the renal surface.