CA2-DEPENDENCE OF CONOID EXTRUSION IN TOXOPLASMA-GONDII TACHYZOITES()

The role of Ca2+ in conoid extrusion was investigated in isolated Toxo plasma gondii tachyzoites by treatment with Ca2+-ionophores, Ca2+-chel ating agents and an inhibitor of the Ca2+-ATPase at the endoplasmic re ticulum. The results were evaluated by light phase-contrast microscopy and electron microscopy. Ionomycin (0.5-1 mu M) caused an immediate a nd sustained extrusion of the conoid in up to 80% of the tachyzoites, depending on the concentrations of ionophore and Ca2+ in the medium. H owever, over 50% of the tachyzoites extruded the conoid when treated w ith ionomycin in Ca2+-free saline complemented with EGTA. The effect o f ionomycin was reversible and could be induced a second time in about half of the responsive population. Similar results were obtained with A23187. Conoid extrusion induced by ionomycin in Ca2+-free medium was almost completely abolished when the tachyzoites were previously load ed with a permeable compound known to chelate intracellular Ca2+ (BAPT A/AM; 25 mu M). On the other hand, exposure of tachyzoites to the Ca2-ATPase inhibitor thapsigargin (0.5-1 mu M) produced significant extru sion of the conoid. Tachyzoites loaded with BAPTA/AM as well as those treated with ionomycin, i.e. with conoids paralyzed in opposite positi ons, had a diminished capacity to invade cultured epithelial cells. A substantial reduction in the response to stimulation by ionomycin was found also in parasites treated with cytochalasin-D, a drug that depol ymerizes actin-filaments. The results suggest that Ca2+-release from i nternal stores may act as a key signal to activate a mechanism of cono id extrusion probably mediated, at least in part, by actin-filaments.