Effects of sugars and amino acids on sodium and potassium influx in rabbit ileum.

There is compelling evidence that the presence of actively transported sugars or amino acids in the mucosal solution increases sodium absorption and the transmural electrical potential difference across a variety of in vitro and in vivo preparations of small intestine. The mechanism(s) underlying these effects is, however, unsettled. One possibility invokes a direct interaction between sodium and brush border carrier mechanisms for sugars and amino acids which results in an increased influx of sodium from the mucosal solution into the cell, across the mucosal membrane. This is then followed by active extrusion of sodium from the cell into the serosal solution or plasma (1). A second possibility has been suggested by Fordtran, Rector, and Carter (2) as a result of their studies on sodium absorption by human jejunum in vivo. These investigators have proposed that the increase in net sodium absorption observed in the presence of glucose or galactose may be the result of solvent drag, perhaps through tight junctions and the lateral intercellular spaces, secondary to the sugar-induced increase in water absorption. If this solvent drag pathway is cation-selective, the increase in transepithelial electrical potential difference could be attributed to a streaming potential. Needless to say, these proposed mechanisms are not mutually exclusive and may coexist; the contribution of one or both of these mechanisms to the enhancement of sodium absorption by glucose would be expected to vary with species, site of small intestine and whether in vivo or in vitro preparations are employed. Recent studies by Frizzell and Schultz (3) have provided evidence for the presence of an extracellular transepithelial shunt pathway in rabbit ileum which is