The potential dependence of the intestinal Na+-dependent sugar transporter

The unidirectional influx of the lipophilic cation tetraphenylphosphonium (TPP+) into isolated intestinal epithelial cells exhibits a marked dependence on the membrane potential (delta psi) maintained by or experimentally imposed on these cells. By taking advantage of this fact, we have described a "crossover" procedure that allows the relative permeability of a cation and anion pair to be determined. Measurements of such relative permeabilities permits diffusion potentials of defined magnitude to be imposed across the plasma membrane of ATP-depleted cells. This in turn allows description of the relationship between [14C]TPP+ influx and delta psi. We have determined that the flux-potential relationship is that predicted by the Goldman flux equation. Using this relationship as a calibration tool for delta psi, we then determined the quantitative relationship between membrane potential and the Na+-dependent influx of an actively transported sugar, alpha-methylglucoside (alpha-MG). The influx of [14C]alpha-MG also shows an exponential dependence on delta psi although it is more sharply potential dependent than that shown by TPP+. The specific relationship is consistent with that expected for a system with 2:1 Na+ stoichiometry which obeys the potential dependence predicted by Eyring rate theory with a single energy barrier occurring near the midpoint of the membrane. Over the range of potentials from +33 to -61 mV, we find no evidence for a minimum or threshold potential necessary to support transport and no evidence for an optimal potential that can maximize sugar transport. The data raise the possibility for using either [14C]TPP+ or [14C]alpha-MG influx as the basis for a new noninvasive procedure for measurement of delta psi.