In order to evaluate whether an active transport of urea exists across the collecting duct epithelia of rats the following experiments were performed. Under conditions, where the net transtubular transport of urea, water, and other solutes was zero, tk, e unidirectional flux of Clt-urea out of the collecting duct and the urea concentration di//erence between collecting duct fluid and vasa recta plasma were measured. The following results were obtained 1. In rats on low protein diet the unidirectional outflux of urea was 73Β°/0 higher than in rats on normal diet and 850/0 higher than in rats on high protein diet.
-
Under steady-state conditions with zero net flux the urea concentration ratio of collecting duct fluid to vasa recta plasma water was 0.84 in rats on low protein diet and 0.99 in rats on normal diet.
-
During free flow the urea concentration ratio of collecting duet fluid to vasa recta plasma water was 1.06 in low protein and 1.33 in normal rats.
The data indicate that in normal animals the urea transport across the collecting duct wall is passive while in rats on low protein diet an active outward transport is superimposed.