Inositol 1,4,5-trisphosphate induced Ca2+ release from chloroquine-sensitive and -insensitive intracellular stores in the intraerythrocytic stage of the malaria parasite P-chabaudi

Isolated P. chabaudi parasites were permeabilized with digitonin and the function of intracellular Ca2+ stores was studied using the Ca2+ indicators arsenate III or Fluo 3-acid in the medium. Addition of the second messenger InsP(3) (5 mu M) to permeabilized parasites leads to Ca2+ release into the medium, with the mean extent of release being 40 nmol Ca2+/10(8) cells. This Ca2+ release was completely abolished in the presence of heparin, an InsP(3) receptor antagonist. The amount of Ca2+ released was approximately 50% reduced when InsP(3) was added subsequent to the discharge of the endoplasmic reticulum (ER) Ca2+ pool with the SERCA (sarcoplasmic ER Ca2+ ATPase) inhibitors thapsigargin and tBHQ (2,5-di(ter-butyl)-1,4 benzohydroquinone). The thapsigargin- and tBHQ-sensitive pool account for 20 nmol of Ca2+/10(8) cells. If InsP(3) was added after the discharge of the residual Ca2+ by addition of either the K+/H+ uncoupler nigericin or the antimalarial drug chloroquine, no further Ca2+ release was observed, This is the first report of InsP(3)-induced Ca2+ release in a parasite protozoa. In addition our finding that chloroquine depletes an InsP(3)-sensitive Ca2+ compartment, raises the possibility that the InsP(3)-dependent Ca2+ release from this store might be important for the regulation of growth and differentiation of the parasite. (C) 1998 Academic Press.