Characterization of the Intracellular Ca2+ Pools Involved in the Calcium Homeostasis in Herpetomonas sp. Promastigotes

Trypanosomatids of the genus Herpetomonas comprises monoxenic parasites of insects that present pro- and opisthomastigotes forms in their life cycles. In this study, we investigated the Ca2+ transport and the mitochondrial bioenergetic of digitonin-permeabilized Herpetomonas sp. promastigotes. The response of promastigotes mitochondrial membrane potential to ADP, oligomycin, Ca2+, and antimycin A indicates that these mitochondria behave similarly to vertebrate and Trypanosoma cruzi mitochondria regarding the properties of their electrochemical proton gradient. Ca2+ transport by permeabilized cells appears to be performed mainly by the mitochondria. Unlike T. cruzi, it was not possible to observe Ca2+ release from Herpetomonas sp. mitochondria, probably due to the simultaneous Ca2+ uptake by the endoplasmic reticulum. In addition, a vanadate-sensitive Ca2+ transport system, attributed to the endoplasmic reticulum, was also detected. Nigericin (1 μM), FCCP (1 μM), or bafilomycin A1 (5 μM) had no effect on the vanadate-sensitive Ca2+ transport. These data suggest the absence of a Ca2+ transport mediated by a Ca2+/H+ antiport. No evidence of a third Ca2+ compartment with the characteristics of the acidocalcisomes described by A. E. Vercesi et al. (1994, Biochem. J. 304, 227–233) was observed. Thapsigargin and IP3 were not able to affect the vanadate-sensitive Ca2+ transport. Ruthenium red was able to inhibit the Ca2+ uniport of mitochondria, inducing a slow mitochondrial Ca2+ efflux, compatible with the presence of a Ca2+/H+ antiport. Moreover, this efflux was not stimulated by the addition of NaCl, which suggests the absence of a Ca2+/Na+ antiport in mitochondria.