Urea transport in nephron segments from medullary rays of rabbits.

To evaluate possible routes of urea delivery to the renal medulla, urea transport was studied in cortical thick ascending limbs and proximal straight tubules dissected from inner cortical medullary rays of rabbit kidneys. Urea was measured colorimetrically in the perfused, collected, and bath fluids. No evidence for active transport of urea was found in either segment. With imposed urea concentration differences between perfusion and bath fluids, there were significant passive fluxes of urea in both segments. The magnitude of the flux was independent of the direction of the concentration difference. Apparent permeability coefficients (X10(-5) cm/s) for urea were 2.0 for the cortical thick ascending limbs and 1.5 for the proximal straight tubules. Based on the measured permeability in the cortical thick ascending limb, substantial passive absorption of urea is predicted in vivo. This will contribute to the dilution of tubular fluid in this segment. The results in proximal straight tubules are compatible with passive urea secretion but not with a significant degree of active secretion.