COUNTERCURRENT DIFFUSION IN THE RENAL CORTEX OF THE RABBIT

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 [3H-] water in the 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. This confirmed the existence of arteriovenous diffusion of 14C-butanol and 3H-water in the kidney. In a 2nd set of experiments, 14C-butanol and 3H-water were infused for 10 or 60 s into the renal arteries of anesthetized rabbits, and the kidneys were then removed and frozen in liquid N. Cores were cut from the kidney surface and sectioned in a cryostat, and the ratio of 14C-butanol to 3H-water was calculated at increasing depths in the cortex. This ratio rose from 0.54 .+-. 0.03 (SEM [standard error of the mean]) at the surface to 0.98 .+-. 0.07 at 3 mm beneath the surface in kidneys perfused for 10 s. This gradient was less steep after 60 s of perfusion. The early appearance of 14C-butanol relative to 3H-water in the renal venous outflow and delayed equilibration of 14C-butanol in the outer renal cortical tissue are consistent with countercurrent diffusion. This exchange may occur between adjoining interlobular arteries and veins in the cortex and may contribute to high CO2 tensions found near the renal surface.