An Acid-loading Chloride Transport Pathway in the Intraerythrocytic Malaria Parasite, Plasmodium falciparum

The intraerythrocytic malaria parasite exerts tight control over its ionic composition. In this study, a combination of fluorescent ion indicators and Cl-36(-) flux measurements was used to investigate the transport of Cl- and the Cl--dependent transport of "H+-equivalents" in mature (trophozoite stage) parasites, isolated from their host erythrocytes. Removal of extracellular Cl-, resulting in an outward [Cl-] gradient, gave rise to a cytosolic alkalinization (i.e. a net efflux of H+-equivalents). This was reversed on restoration of extracellular Cl-. The flux of H+-equivalents was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and, when measured in ATP-depleted parasites, showed a pronounced dependence on the pH of the parasite cytosol; the flux was low at cytosolic pH values < 7.2 but increased steeply with cytosolic pH at values > 7.2. Cl-36(-) influx measurements revealed the presence of a Cl- uptake mechanism with characteristics similar to those of the Cl--dependent H+-equivalent flux. The intracellular concentration of Cl- in the parasite was estimated to be similar to 48 mM in situ. The data are consistent with the intraerythrocytic parasite having in its plasma membrane a 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive transporter that, under physiological conditions, imports Cl- together with H+-equivalents, resulting in an intracellular Cl- concentration well above that which would occur if Cl- ions were distributed passively in accordance with the parasite's large, inwardly negative membrane potential.