title: Functional state of the plasma membrane Ca2+ pump in Plasmodium falciparum-infected human red blood cells.
abstract: The active Ca2+ transport properties of malaria-infected, intact red blood cells are unknown. We report here the first direct measurements of Ca2+ pump activity in human red cells infected with Plasmodium falciparum, at the mature, late trophozoite stage. Ca2+ pump activity was measured by the Co2+-exposure method adapted for use in low-K+ media, optimal for parasitised cells. This required a preliminary study in normal, uninfected red cells of the effects of cell volume, membrane potential and external Na+/K+ concentrations on Ca2+ pump performance. Pump-mediated Ca2+ extrusion in normal red cells was only slightly lower in low-K+ media relative to high-K+ media despite the large differences in membrane potential predicted by the Lew-Bookchin red cell model. The effect was prevented by clotrimazole, an inhibitor of the Ca2+-sensitive K+ (KCa) channel, suggesting that it was due to minor cell dehydration. The Ca2+-saturated Ca2+ extrusion rate through the Ca2+ pump (Vmax) of parasitised red cells was marginally inhibited (2-27 %) relative to that of both uninfected red cells from the malaria-infected culture (cohorts), and uninfected red cells from the same donor kept under identical conditions (co-culture). Thus, Ca2+ pump function is largely conserved in parasitised cells up to the mature, late trophozoite stage. A high proportion of the ionophore-induced Ca2+ load in parasitised red cells is taken up by cytoplasmic Ca2+ buffers within the parasite. Following pump-mediated Ca2+ removal from the host, there remained a large residual Ca2+ pool within the parasite which slowly leaked to the host cell, from which it was pumped out.
authors.affiliation: Physiological Laboratory, University of Cambridge, Downing Street, Cambridge CB2 3EG and University Laboratory of Physiology, Oxford University, Parks Road, Oxford OX1 3PT, UK.
authors.forename: T
authors.initials: T
authors.lastname: Tiffert
authors.forename: H M
authors.initials: HM
authors.lastname: Staines
authors.forename: J C
authors.initials: JC
authors.lastname: Ellory
authors.forename: V L
authors.initials: VL
authors.lastname: Lew
mesh_terms: D000818:Animals; D001692:Biological Transport; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D003022:Clotrimazole / Q000494:pharmacology; D003035:Cobalt / Q000378:metabolism; D003470:Culture Media / Q000737:chemistry; D004910:Erythrocyte Membrane / Q000378:metabolism* / Q000469:parasitology; D006801:Humans; D007476:Ionophores / Q000494:pharmacology; D007700:Kinetics; D008564:Membrane Potentials; D010963:Plasmodium falciparum / Q000469:parasitology*
chemical_list: D003470:Culture Media; D007476:Ionophores; D000001:Calcimycin; D003035:Cobalt; D000252:Calcium-Transporting ATPases; D003022:Clotrimazole; D002118:Calcium