Plasma membrane potential of Lettré cells does not depend on cation gradients but on pumps.

The plasma membrane potential of Lettré cells has been determined with the optical indicator oxonol-V and found to be -57 mV at 37 degrees C (range -20 to -80 mV depending on the physiological condition of the cells). Increasing extracellular K+ does not depolarize cells: even in the presence of 155 mM K+ the potential is -41 mV; membrane potential is also insensitive to the chemical gradient of Na+, Mg2+, Ca2+ or Cl-. Ouabain depolarizes the cells; H+ efflux from cells is stimulated by extracellular Na+. We propose that in Lettré cells the plasma membrane potential is generated by electrogenic cation pumps. The balancing fluxes of Na+ and K+ are mainly through electroneutral cation exchanges (Na+/K+ and Na+/H+) and the magnitude of the potential is limited by organic anion leaks. Such a mechanism may operate in other biological membranes also.