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

The plasma-membrane potential (Delta psi(p)) in bloodstream forms of Trypan osoma brucei was studied using several different radiolabelled probes: Rb-8 6(+) and [C-14]SCN- were used to report Delta psi(p) directly because they distribute in easily measured quantities across the plasma membrane only, a nd [H-3]methyltriphenylphosphonium (MePh3P+) was used to report Delta psi(p ) only when Delta psi(m) had been abolished with FCCP because it reports th e algebraic sum of the two potentials when used alone. The unperturbed Delt a psi(p) had a value of -82 mV and was found to be essentially identical wi th, and determined almost completely by, the potassium diffusion potential, as evidenced by: (a) the lack of effect of valinomycin on the value obtain ed under appropriate conditions when any of these probes were used; (b) the close agreement of this measured value with that predicted from the measur ed distribution of K+ across the plasma membrane (-76 mV); (c) the large ef fect of changes in the extracellular K+ concentration by substitution with Na+ on Delta psi(p) together with the complete lack of effect of substituti on of extracellular Na+ by the choline cation or substitution of extracellu lar Cl- by the gluconate anion on Delta psi(p). The contribution to Delta p si(p) by electrogenic pumping of Na+/K+-ATPase was found to be small (of th e order of 6 mV). H+ was not found to be pumped across the plasma membrane or to contribute to Delta psi(p).