The aim of this project is to measure the distributions of ions in Trypanosoma cruzi, the parasitic protozoan that causes Chagas' disease. Of particular interest is the identification of a new type of calcium-buffering organelle (acidocalcisome) that has been proposed from biochemical measurements. This organelle would allow T. cruzi to maintain homeostasis in the extremely low calcium concentrations of the host cell's cytoplasm. Cultured T. cruzi epimastigotes were centrifuged and rapidly frozen in liquid ethane at liquid nitrogen temperature. Blocks were cryosectioned to a thickness of 100 nm, and the sections were cryotransferred into the field-emission STEM. X-ray spectra were recorded from subcellular compartments including nucleus, cytoplasm, kinetoplast, and different classes of granules. The major calcium store in T. cruzi was found to be dense phosphate-rich granules, which contained 300+/-20 mmol/kg of Ca++, in addition to the divalent cations Mg++ and Zn++. All other compartments analyzed contained <1 mmol/kg of Ca++.
