Factors that determine the plasma-membrane potential in bloodstream forms of Trypanosoma brucei

The plasma-membrane potential (Delta psi(p)) in bloodstream forms of Trypanosoma brucei was studied using several different radiolabelled probes: Rb-86(+) and [C-14]SCN- were used to report Delta psi(p) directly because they distribute in easily measured quantities across the plasma membrane only, and [H-3]methyltriphenylphosphonium (MePh3P+) was used to report Delta psi(p) only when Delta psi(m) had been abolished with FCCP because it reports the algebraic sum of the two potentials when used alone. The unperturbed Delta psi(p) had a value of -82 mV and was found to be essentially identical with, and determined almost completely by, the potassium diffusion potential, as evidenced by: (a) the lack of effect of valinomycin on the value obtained under appropriate conditions when any of these probes were used; (b) the close agreement of this measured value with that predicted from the measured distribution of K+ across the plasma membrane (-76 mV); (c) the large effect of changes in the extracellular K+ concentration by substitution with Na+ on Delta psi(p) together with the complete lack of effect of substitution of extracellular Na+ by the choline cation or substitution of extracellular Cl- by the gluconate anion on Delta psi(p). The contribution to Delta psi(p) by electrogenic pumping of Na+/K+-ATPase was found to be small (of the order of 6 mV). H+ was not found to be pumped across the plasma membrane or to contribute to Delta psi(p).