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

Abstract

The plasma-membrane potential (Δψ(p)) in bloodstream forms of Trypanosoma brucei was studied using several different radiolabelled probes: 86Rb+ and [14C]SCN- were used to report Δψ(p) directly because they distribute in easily measured quantities across the plasma membrane only, and [3H]methyltriphenylphosphonium (MePh3P+) was used to report Δψ(p) only when Δψ(m) had been abolished with FCCP because it reports the algebraic sum of the two potentials when used alone. The unperturbed Δψ(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 Δψ(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 Δψ(p). The contribution to Δψ(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 Δψ(p).