Regulation of intracellular calcium homeostasis in Trypanosoma cruzi. Effects of calmidazolium and trifluoperazine

Trypanosoma cruzi epimastigotes maintained an intracellular free calcium concentration of about 0.15 μM, as measured with the fluorescent indicator Fura-2. The maintenance of low [Ca 2+] i is energy-dependent since it is disrupted by KCN and FCCP. When the cells were permeabilized with digitonin, the steady-state free Ca 2+ concentration in the absence of ATP was about 0.7 μM. The additional presence of ATP resulted in a steady-state level close to 0.1–0.2 μM which compares favorably with the concentration detected in intact cells. Intracellular Ca 2+ uptake at high levels of free Ca 2+ (1 μM) was due to energy-dependent mitochondrial uptake as indicated by its FCCP-sensitivity. However, as the free Ca 2+ concentration was lowered from 1 μM, essentially all uptake was due to the ATP-dependent Ca 2+ sequestration by the endoplasmic reticulum as indicated by its stimulation by ATP, and its inhibition by sodium vanadate. High concentrations of the calrnodulin antagonist trifluoperazine, inhibited both the Ca 2+ uptake by the endoplasmic retlculum and by the mitochondria, while calmidazolium released Ca 2+ from both compartments. In addition, trifluoperazine and calmidazolium inhibited respiration and collapsed the mitochondrial membrane potential of T. cruzi, thus indicating non-specific effects unrelated to calmodulin.