Interpretation of disaccharide-dependent electrical potential differences in the small intestine.
The nature of maltose- and trehalose-induced electrical potential increments across everted small intestines of toads were investigated. A Michaelis-Menten-like relation was seen between the amplitude of PD increments (deltaPD) and the mucosal concentration of disaccharides over a wide range of concentration, but, at a higher concentration range, Lineweaver-Burk type plot of data always deviated from the linear line for the low and moderate concentration range. The extrapolation of the linear segment of the plots intercepted the ordinate at the same point as that of the line for the glucose-induced potential increments. Both the disaccharide- and the glucose-evoked potentials were not additive and were equally sensitive to phlorizin. Tris depressed the disaccharide-evoked potentials to about the same extent as that of inhibition of enzyme activities. The amplitude and time course of the disaccharide-induced potentials, however, could not be accounted for by the mucosal concentration of liberated glucose. These qualitative and quantitative characteristics were explained properly on the basis of a simple well-type local pool for liberated glucose assumed to exist at the brush border. In conclusion, a close functional linkage between brush border membrane disaccharidase activities and the electrogenic hexose transport is well reflected in the disaccharide-evoked potentials in the small intestine.