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

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