The origin of the glucose dependent increase in the potential difference across the tortoise small intestine.

1. Experiments were carried out to investigate the origin of the glucose dependent increase in the potential difference (p.d.) across the isolated intestinal mucosa of the tortoise.2. In addition to glucose, galactose, alpha-methyl glucoside, 3-0-methyl glucopyranose and sucrose also increased the transepithelial potential difference. There was no increase with either fructose or mannose.3. The use of micro-electrodes demonstrated that the change in the p.d. due to the presence of glucose was wholly accounted for by the increase in the p.d. across the serosal face of the epithelial cells.4. Diffusion potentials were produced across the isolated mucosa by varying the ionic composition of either the mucosal or serosal fluids. However, there was no reduction of the glucose dependent increase in the p.d. when the ionic concentration gradients across the serosal face of the cell were reversed.5. These results suggest that the increase in the p.d. associated with the active transfer of sugars across the small intestine was due to the presence of an electrogenic ion pump at the serosal face of the epithelial cell.