Intracellular Ca2+ homeostasis in trypomastigotes of Trypanosoma cruzi

Trypomastigotes of Trypanosoma cruzi maintain an intracellular Ca2+ concentration ([Ca2+]i) of 64 +/- 30 nM. Equilibration of trypomastigotes in an extracellular buffer containing 0.5 mM [Ca2+]o (preloaded cells) increased [Ca2+]i < 20 nM whereas total cell Ca2+ increased by 1.5 to 2.0 pmole/cell. This amount of Ca2+ would be expected to increase [Ca2+]i to > 10 microM suggesting active sequestration of Ca2+. We tested the hypothesis that maintenance of [Ca2+]i involved both the sequestration into intracellular storage sites and extrusion into the extracellular space. Pharmacological probes known to influence [Ca2+]i through well characterized pathways in higher eukaryotic cells were employed. [Ca2+]i responses in the presence or absence of [Ca2+]o were measured to asses the relative contribution of sequestration or extrusion processes in [Ca2+]i homeostasis. In the presence of 0.5 mM [Ca2+]o, the ability of several agents to increase [Ca2+]i was magnified in the order ionomycin >>> nigericin > thapsigargin > monensin > valinomycin. In contrast, preloading markedly enhanced the increase in [Ca2+]i observed only in response to monensin. Manoalide, an inhibitor of phospholipase A2, enhanced the accumulation of [Ca2+]i due to all agents tested, particularly ionomycin and thapsigargin. Our results suggest that sequestration of [Ca2+]i involved storage sites sensitive to monensin and ionomycin whereas extrusion of Ca2+ may involve phospholipase A2 activity. A Na+/Ca2+ exchange mechanism did not appear to contribute to Ca2+ homeostasis.