Relationship between intracellular free calcium concentrations and the intracellular development of Toxoplasma gondii

We measured intracellular free calcium concentrations ([Ca++](i)) in the subcellular compartments of Toxoplasma gondii infected living cells using microspectrofluorometry and Indo-1 staining. [Ca++](i) mapping was defined in infected and uninfected cells and in the neoformed parasitophorous vacuole (PV) 24 and 48 hr after parasite inoculation. At 24 hr after infection, a [Ca++](i) gradient (PV/cytoplasm) was observed in favor of the PV in 72% of infected cells (p<0.001). Inside of the PV (lumen and parasites) , [Ca++](i) values appeared to be homogeneously distributed. At 48 hr after infection, the parasites had replicated and formed typical rosettes of more than 16 parasites. At this step, a positive [Ca++](i) gradient (PV/cytoplasm) was detected in all analyzed cells (p<0.001). This result suggests that the PV (lumen and parasites) represents an individual subcellular compartment within the host cell that includes an independent [Ca++](i). Moreover, after 48 hr the cytoplasmic [Ca++](i) decreased significantly (39 nM) compared with that measured from uninfected cells (53 nM) (p<0.05). Furthermore, the exit of Toxoplasma mediated by the calcium ionophore 4BrA23187 was preceded by a rise of [Ca++](i) to 1 mM in the PV. The [Ca++](i) rise and the liberation of parasites from their host appear to be correlated. On the basis of these observations, we suggest that the increase of [Ca++](i) in the vacuole may act as a signal that triggers the egress of T. gondii.