Fig. 1. Effect of extracellular calcium on attachment and invasion. Host cells were challenged with parasites in the presence of 1 mM BAPTA, a diluent control (ctl; refers to water or DMSO), 5 mM CaCl₂, or 5 mM EGTA in pH 6.0 or pH 7.2 low calcium Ringer's. To examine the role of intracellular calcium, parasites were pretreated with 50 µM BAPTA-AM (T. gondii pretreatment). In parallel, host cells were loaded with 50 µM BAPTA-AM (host cell pretreatment) or a diluent control (ctl) prior to contact with parasites. Invasion (solid bars) and attachment (open bars) were monitored as described in the methods and average percentages calculated from the results of three independent experiments (mean±s.e.).