Acidocalcisomes in Toxoplasma gondii tachyzoites

Toxoplasma gondii tachyzoites were loaded with the fluorescent indicator fura 2 to investigate the transport mechanisms involved in maintaining their intracellular Ca²⁺ homoeostasis. The mitochondrial ATPase inhibitor oligomycin and the endoplasmic-reticulum Ca²⁺-ATPase inhibitor thapsigargin increased the intracellular Ca²⁺ concentration ([Ca²⁺]_i), thus indicating the requirement for ATP and the involvement of the endoplasmic reticulum in maintaining intracellular Ca²⁺ homoeostasis. The effect of thapsigargin was more accentuated in the presence of extracellular Ca²⁺, clearly showing that, as occurs with other eukaryotic cells, depletion of intracellular Ca²⁺ pools led to an increase in the uptake of Ca²⁺ from the extracellular medium. In addition to these results, we found evidence that, in contrast with what occurs in mammalian cells, T. gondii tachyzoites possess a significant amount of Ca²⁺ stored in an acidic compartment, termed the acidocalcisome, as indicated by: (1) the increase in [Ca²⁺]_i induced by bafilomycin A₁ (a specific inhibitor of H⁺-ATPases), nigericin (a K⁺/H⁺ exchanger) or the weak base NH₄Cl, in the nominal absence of extracellular Ca²⁺ to preclude Ca²⁺ entry; and (2) the effect of ionomycin, a Ca²⁺-releasing ionophore that cannot take Ca²⁺ out of acidic organelles and that was more effective after alkalinization of these compartments by addition of bafilomycin A₁, nigericin or NH₄Cl. Considering the relative importance of the ionomycin-releasable and the ionomycin+NH₄Cl-releasable Ca²⁺ pools, it is apparent that T. gondii tachyzoites contain a significant amount of Ca²⁺ stored in acidocalcisomes.