Calcium transport and compartment analysis of free and exchangeable calcium in Plasmodium falciparum-infected red blood cells.

Calcium (Ca2+) is indispensable for normal development of the various stages of the asexual erythrocytic cycle of malaria parasites. However, the mechanisms involved in Ca2+ uptake, compartmentalization and cellular regulation are poorly understood. To clarify some of these issues, we have measured total, exchangeable, and free Ca2+ in normal red cells (RBCs) and Plasmodium falciparum (FCR-3)-infected cells (IRBCs) as a function of parasite development. All three forms of Ca2+ were found to be substantially higher in IRBCs than in RBCs, and to increase with parasite maturation up to the trophozoite stage and decline thereafter. Exchangeable and free [Ca2+] in host cell and parasite compartments were determined by selectively lysing IRBCs with Sendai virus, and estimating these parameters in the lysate (host cytosol) and the pellet (parasite cytosol). Levels of both exchangeable and free [Ca2+] were found to be higher in host cytosol than in parasite cytosol. The Ca2+ gradient across the parasite membrane can be maintained by the pH gradient across this membrane by means of a Ca2+/H+ antiporter. Host cytosol free [Ca2+] reached levels known to produce structural, physiological and biochemical changes in RBCs, and could account for similar features normally seen in malaria-infected red cells. Uptake of Ca2+ into IRBCs was nonsaturable and substantially faster than the saturable Ca2+ uptake into RBCs. The rate of Ca2+ uptake across the parasite membrane was even faster suggesting that the rate-limiting step in uptake into intact IRBCs is the translocation of Ca2+ across the host cell membrane.