The Membrane Potential of the Intraerythrocytic Malaria Parasite Plasmodium falciparum

The membrane potential (ΔΦ) of the mature asexual form of the human malaria parasite, Plasmodium falciparum, isolated from its host erythrocyte using a saponin permeabilization technique, was investigated using both the radiolabeled ΔΦ indicator tetraphenylphosphonium ([ ³H]TPP⁺) and the fluorescent ΔΦ indicator DiBAC₄(3) (bis-oxonol). For isolated parasites suspended in a high Na⁺, low K⁺ solution, ΔΦ was estimated from the measured distribution of [³H]TPP⁺ to be - 95 ± 2 mV. ΔΦ was reduced by the specific V-type H⁺ pump inhibitor bafilomycin A₁, by the H⁺ ionophore CCCP, and by glucose deprivation. Acidification of the parasite cytosol (induced by the addition of lactate) resulted in a transient hyperpolarization, whereas a cytosolic alkalinization (induced by the addition of NH₄ ⁺) resulted in a transient depolarization. A decrease in the extracellular pH resulted in a membrane depolarization, whereas an increase in the extracellular pH resulted in a membrane hyperpolarization. The parasite plasma membrane depolarized in response to an increase in the extracellular K⁺ concentration and hyperpolarized in response to a decrease in the extracellular K⁺ concentration and to the addition of the K ⁺ channel blockers Ba²⁺ or Cs⁺ to the suspending medium. The data are consistent with ΔΦ of the intraerythrocytic P. falciparum trophozoite being due to the electrogenic extrusion of H⁺ via the V-type H⁺ pump at the parasite surface. The current associated with the efflux of H⁺ is countered, in part, by the influx of K⁺ via Ba²⁺- and Cs ⁺-sensitive K⁺ channels in the parasite plasma membrane.