Mucosal and serosal fluxes of alanine in rabbit ileum.

The four unidirectional fluxes of alanine across the mucosal and serosal borders of rabbit ileum were evaluated as functions of the alanine concentration on a single piece of tissue using a method previously described (Naftalin, R.J. and Curran, P.F. (1974) J. Membrane Biol. 16, 257-278). The effects of Na+ removal and of ouabain on these fluxes were investigated. Alanine was actively transported across the mucosal membrane under control conditions; Na+ removal or ouabain inhibited this process as a result of a decrease in flux from the mucosal solution to the cell and an increase in the flux in the opposite direction. The results concerning mucosal efflux of alanine are apparently inconsistent with the carrier model for alanine transport at this border. Alanine transfer across the serosal membrane appeared to involve a facilitated transfer mechanism. Alanine movement at the serosal side of the cell was not influenced by Na+.