Ca(2+)-dependence of conoid extrusion in Toxoplasma gondii tachyzoites

J Eukaryot Microbiol. Mar-Apr 1996;43(2):120-7. doi: 10.1111/j.1550-7408.1996.tb04491.x.


The role of Ca2+ in conoid extrusion was investigated in isolated Toxoplasma gondii tachyzoites by treatment with Ca(2+)-ionophores, Ca(2+)-chelating agents and an inhibitor of the Ca(2+)-ATPase at the endoplasmic reticulum. The results were evaluated by light phase-contrast microscopy and electron microscopy. Ionomycin (0.5-1 microM) caused an immediate and sustained extrusion of the conoid in up to 80% of the tachyzoites, depending on the concentrations of ionophore and Ca2+ in the medium. However, over 50% of the tachyzoites extruded the conoid when treated with ionomycin in Ca(2+)-free saline complemented with EGTA. The effect of 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 Ca(2+)-free medium was almost completely abolished when the tachyzoites were previously loaded with a permeable compound known to chelate intracellular Ca2+ (BAPTA/AM; 25 microM). On the other hand, exposure of tachyzoites to the Ca(2+)-ATPase inhibitor thapsigargin (0.5-1 microM) produced significant extrusion of the conoid. Tachyzoites loaded with BAPTA/AM as well as those treated with ionomycin, i.e. with conoids paralyzed in opposite positions, 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 depolymerizes actin-filaments. The results suggest that Ca(2+)-release from internal stores may act as a key signal to activate a mechanism of conoid extrusion probably mediated, at least in part, by actin-filaments.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Actins / drug effects
  • Actins / metabolism
  • Animals
  • Calcium / metabolism*
  • Calcium-Transporting ATPases / metabolism
  • Cell Line
  • Chelating Agents
  • Cytochalasin D / pharmacology
  • Dogs
  • Egtazic Acid / analogs & derivatives
  • Fluorescent Dyes
  • Host-Parasite Interactions
  • Ionomycin / pharmacology
  • Ionophores / pharmacology
  • Kinetics
  • Microscopy, Electron
  • Toxoplasma / drug effects
  • Toxoplasma / physiology*
  • Toxoplasma / ultrastructure


  • Actins
  • Chelating Agents
  • Fluorescent Dyes
  • Ionophores
  • 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester
  • Cytochalasin D
  • Egtazic Acid
  • Ionomycin
  • Calcium-Transporting ATPases
  • Calcium