Sodium cotransport systems and the membrane potential difference.

Studies with membrane vesicles and with whole cell preparations have shown clearly that the electrochemical gradient of Na+ acting across the cell membrane is closely coupled to the influx and efflux of amino acids or carbohydrates through their cellular pumps. It has been less clear (1) just how tightly solute flow is coupled to that of Na+ in stoichiometrical terms and (2) whether coupling is tight enough to account for the maximum solute gradients that the systems form in vivo. Recent work with ionophores, including nigericin, has revealed circumstances in preparations of mouse ascites-tumor cells where if the sodium gradient hypothesis is correct, electrogenic ion pumping must be supposed to maintain membrane potentials of the order of 80 mV negative. We have used a new fluorescence assay based on an oxonol dye in a search for potentials of that magnitude. Their possible origin is discussed.