Interaction between Na+-dependent transport systems for sugars and amino acids. Evidence against a role for the sodium gradient

Summary The concept that interaction between sodium-dependent transport systems represents competition for energy inherent in the transmembrane sodium gradient was examined with the use of isolated intestinal epithelial cells. The isolated cells exhibit transport interactions which are more significant in magnitude than those which have been described for intact tissue preparations. Accumulation of 1mm valine is inhibited 60% by 10mm 3-OMG. Conversely, uptake of 1mm 3-OMG is inhibited only 20% by 10mm valine. These data suggest that 3-OMG must discharge the cellular Na⁺ gradient more effectively than valine, if Na⁺ gradient dissipation can be taken as a basis for the inhibitory interaction. However, entry of 10mm 3-OMG is significantly slower than the entry of 10mm valine. Even if appropriate corrections are made for passive substrate entry and for differences in Na/substrate entry stoichiometry, it appears that valine should be somewhat more effective than 3-OMG in discharging the Na⁺ gradient. In light of these facts, it seems unlikely that the mechanistic basis for interaction between sugar and amino acid transport systems can be related to concomitant co-entry of Na⁺. It is suggested that the interaction results instead from competition for energized intermediates generated at limited rates by basic energy transduction events associated with the cell membrane which serve in support of a variety of active transport systems.