Figure 7: Model of host Ca²⁺ influx during intracellular growth.

Top left: An infected host cell with tachyzoites both with cytosolic Ca²⁺ at resting levels. Intracellular Stores (IS) are filled. Top right: A host cell Ca²⁺ signaling event triggers an increase in cytosolic Ca²⁺. Given that the PV is in equilibrium with the host cell cytosol, PV Ca²⁺ rises simultaneously. Bottom: Rise in PV Ca²⁺ is followed by Ca²⁺ influx into the parasite via a plasma membrane Ca²⁺ channel causing a rise in the cytosolic Ca²⁺ of the parasite. Host cytosolic Ca²⁺ returns to resting level and the tachyzoite cytosolic Ca²⁺ is pumped into intracellular stores (IS). Parasites will continue replicating within the host cell while utilizing the Ca²⁺ influx from the host cell to maintain IS Ca²⁺ filled to be eventually utilized and released during egress. The red scale estimates the concentration of Ca²⁺ in the host cell and PV and the green scale in the parasite cytosol.