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.
Zhang
H S
HS
Department of Pathology, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
McDonald
T V
TV
Tanowitz
H B
HB
Wittner
M
M
Weiss
L M
LM
Bilezikian
J P
JP
Morris
S A
SA
D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D006706:Homeostasis; D007476:Ionophores / Q000494:pharmacology; D008985:Monensin / Q000494:pharmacology; D009550:Nigericin / Q000494:pharmacology; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology; D014349:Trypanosoma cruzi / Q000378:metabolism*; D014634:Valinomycin / Q000494:pharmacology
D007476:Ionophores; D013729:Terpenes; D014634:Valinomycin; D019284:Thapsigargin; D008985:Monensin; C045673:manoalide; D009550:Nigericin; D002118:Calcium