Urea Transport by Rat Ileum Studied in Vitro.

Isolated loops of rat ileum were perfused under a pressure of 25cmH20 at 40'C. The loops were placed in a chamber containing 0.9% NaC!, and the perfusing fluid was Krebs-Ringer-bicarbonate solution. When urea was added in the perfusing fluid, outward transport of urea occured by both of diffusion and solvent drag. On the other hand, when urea was added in the outside medium the inward transport of it would be performed by virtue of diffusion only. Analyzing the directional transfer of urea the following results were concluded. 1. The transport of urea by solvent drag across the wall of the isolated rat ileum did not fit to the Poiseuille's law. That is, the urea movement occuring by solvent drag could not be attributable to the bulk flow through a simple pore model. 2. When the concentration of urea in the perfusing fluid was low, the role of the solvent drag was dominant in the outward transport. On the other hand, when urea concentration was high the transport was performed mostly by diffusion