Effects of unilateral sodium replacement on sugar transport across in vitro rabbit ileum

The effects of unilateral sodium replacement with either choline or mannitol on passive and active 3-O-methyl-D-glucose (3-MG) absorption by in vitro rabbit ileum were examined. The results indicate that the predominant factor influencing 3-MG absorption is the presence of sodium in the mucosal bathing solution. Replacement of sodium in the mucosal solution alone markedly inhibits passive and essentially abolishes active 3-MG absorption regardless of the presence or absence of sodium in the serosal bathing solution. The presence or absence of sodium in the serosal solution does not affect transmural 3-MG transport regardless of the sodium concentration in the mucosal solution. Further, when sodium is replaced in the mucosal solution alone, the bidirectional transmural fluxes of 3-MG do not differ markedly from those of D-mannitol, a solute whose transport is probably attributable to simple diffusion. The present findings together with those of previous studies (15, 16) are consistent with the notion that replacement of sodium in the mucosal solution alone essentially abolishes transcellular 3-MG transport and that the remaining transepithelial fluxes are largely due to diffusion through the paracellular or shunt pathway. The relation of these findings to in vivo and in vitro studies in which perfusion of a segment of small intestine with a Na-free solution disclosed little or no inhibition of sugar absorption is discussed.