The relation between membrane potential and the transport activity of system A and L in plasma membrane vesicles of the Ehrlich cell.

Plasma membrane vesicles isolated from Ehrlich ascites tumor cells have been used to investigate the role of the transmembrane potential in the energetics of Systems A and L. As expected, Na+ -dependent System A was responsive to changes in membrane potential. System L activity, as measured by transport of 2-amino-norbornane-2-carboxylic acid (BCH), was shown to be Na+ -independent and was not altered by changes in the membrane potential. The combination of valinomycin and nigericin decreased accumulation of MeAIB but not that of BCH. The presence of nigericin alone caused a significant decrease in uptake by System A and a decrease in uptake by System L to a lesser degree. The inhibitory action of nigericin might reflect its ability to dissipate the Na+ gradient rather than an effect on K+ or H+ flows. The results indicate that modes of energization not produced through the transmembrane potential must account for any uphill operation of System L.