page 103, Gazarini et al. now show that Plasmodia get around this problem by maintaining a high Ca2+ level within the parasitophorous vacuole (PV), their home in the red blood cell.
Previous studies suggested that the PV membrane (PVM) was something of a sieve, and that the ionic environment inside it should be similar to the host cytoplasm. The researchers tested this by using indicator dyes to measure Ca2+ levels in the cytosol and inside the PVM. They found that Plasmodia maintain local Ca2+ levels that are 100–1,000 times higher than those in the red blood cell cytoplasm—easily enough to supply their internal stores. Those stores are essential for regulation of parasite activity, including a Ca2+-based signaling system that responds to the host hormone melatonin. Ca2+ is also required for parasite maturation, as even a temporary two hour decrease in PV Ca2+ concentration strongly decreased the number of Plasmodia that developed to the infectious stage.
It is not yet clear how Ca2+ levels are kept so high in the PVM. The orientation of the host plasma membrane is inverted to form the PVM during parasite invagination, so inverted Ca2+-ATPase pumps from the host membrane may be able to pump Ca2+ into the PVM. A drug that interferes with this PVM Ca2+ accumulation without affecting the red blood cells themselves might be a valuable tool in our arsenal against this devastating parasite. ▪