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 (36)Cl(-) 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. (36)Cl(-) 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 approximately 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.