Sodium-dependent sugar and amino acid transport in isolated goldfish intestinal epithelium: electrophysiological evidence against direct interactions at the carrier level.

The effects of mucosal application of monosaccharides and amino acids on transepithelial and membrane potentials in isolated goldfish intestinal epithelium were investigated. Isosmotic replacement of mucosal mannitol by sugars or L-amino acids resulted in a rapid depolarization of the mucosal membrane potential psi mc followed by a slow repolarization. Phlorizin inhibited the responses to sugar but not those to amino acids. D-Amino acids did not induce any electrical response in the epithelium. Dose-response curves for L-amino acids showed simple saturation. Simultaneous application of L-amino acid and glucose induced transepithelial responses of about 80% of the sum of the separate responses to the application of amino acid or glucose alone. Simultaneous application of different amino acids in saturating concentrations did not increase the magnitude of the electrical responses. From the measured changes in potentials we calculated the change in electromotive force across the mucosal (delta Em) and serosal (delta Es) membrane. The change in Em induced by combined application of alanine and glucose was 90% of the sum of the calculated values induced by glucose and alanine alone. The response of Es to both substrates was accelerated with respect to that of separate substrates alone. We conclude that by application of glucose in addition to alanine the influx of sodium is increased, thereby stimulating the basolaterally located electrogenic Na+/K+-pump. There are no indications for direct interaction of sugars and amino acids at the mucosal membrane of the intestinal epithelial cell.