THE DISTRIBUTION OF PERMEANT IONS DEMONSTRATES THE PRESENCE OF AT LEAST 2 DISTINCT ELECTRICAL GRADIENTS IN BLOOD-STREAM FORMS OF TRYPANOSOMA-BRUCEI

The distribution of Rb-86+ and the radiolabelled lipophilic cation [H-3]methyltriphenylphosphonium (MePh3P+) was used to investigate the membrane potentials that exist in bloodstream forms of Trypanosoma brucei. Even after correction for binding to cellular constituents, the accumulation of MePh3P+ was approximately tenfold greater than the accumulation of Rb+ under resting conditions. The addition of low concentrations of carbonylcyanide p-trifluoromethoxyphenylhydrazone or valinomycin reduced the accumulation of MePh3P+ tenfold without perturbing the accumulation of Rb+. Although selective permeabilization of the plasma membrane abolished the accumulation of Rb+ and caused a substantial decrease in the accumulation of MePh3P+, a significant carbonylcyanide-p-trifluoromethoxyphenylhydrazone-sensitive accumulation of MePh3P+ persisted under these conditions. These data were consistent with the presence of at least two distinct membrane potentials (DELTA-PSI) in bloodstream forms of T. brucei; a potential across the plasma membrane (DELTA-PSI(p)) and an additional DELTA-PSI, generated by the electrogenic movement of H+, across the membrane of an intracellular organelle that possesses no electrical permeability to Rb+ or K+.