Tachyzoite calcium changes during cell invasion by Toxoplasma gondii

The invasion of host cells by the obligate intracellular protozoan parasite Toxoplasma gondii is calcium dependent. We have identified two calcium sto rage areas in tachyzoites, the endoplasmic reticulum and vesicles that cont ain high concentrations of calcium as amorphous calcium phosphate precipita tes. Our data indicate that these vesicles slowly lose their calcium during the intracellular development of the tachyzoite as their nucleus phosphoru s content increases. We found fluctuations in the sulfur content of the tac hyzoite during invasion following the exocytosis of protein from the secret ory organelles, with a loss of sodium and chlorine, and the uptake of potas sium from the host cell cytoplasm. We demonstrated that penetration of the tachyzoite into the host cell was accompanied by increases in the concentra tions of phosphorus and sulfur in the host cell nucleus, probably due to in creased transcription. The cytosol sodium concentrations decreased, while t he potassium content increased. Thus, the subcellular element distribution of tachyzoites and host cells changes during invasion and intracellular gro wth of the parasites. In addition, our results indicate that tachyzoite cal cium might be involved in the egress of the parasite from the host cell.