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 Ca(2+)-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 Ca(2+)-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.
Moreno
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Department of Veterinary Pathobiology, University of Illinois at Urbana-Champaign 61801, USA.
Zhong
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D000643:Ammonium Chloride / Q000494:pharmacology; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006706:Homeostasis; D006863:Hydrogen-Ion Concentration; D015759:Ionomycin / Q000494:pharmacology; D018942:Macrolides; D009550:Nigericin / Q000494:pharmacology; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D014122:Toxoplasma / Q000254:growth & development / Q000378:metabolism*
D000900:Anti-Bacterial Agents; D004791:Enzyme Inhibitors; D018942:Macrolides; D013729:Terpenes; D000643:Ammonium Chloride; D015759:Ionomycin; D019284:Thapsigargin; C040929:bafilomycin A1; D009550:Nigericin; D002118:Calcium