title: Sodium-proton exchange stimulates Ca2+ release from acidocalcisomes of Trypanosoma brucei.
abstract: Acidocalcisomes are acidic vacuoles present in trypanosomatids that contain a considerable fraction of intracellular Ca2+ [Vercesi, Moreno and Docampo (1994) Biochem. J. 304, 227-233; Scott, Moreno and Docampo (1995) Biochem. J. 310, 789-794; Docampo, Scott, Vercesi and Moreno (1995) Biochem. J. 310, 1005-1012]. The data presented here indicate that Na+ stimulates Ca2+ release from the acidocalcisomes of digitonin-permeabilized Trypanosoma brucei procyclic trypomastigotes in a dose-dependent fashion, this effect being enhanced by increasing pH of the medium from 7.0 to 7.8. The hypothesis that this Na+ effect was mediated by alkalinization of the acidocalcisomes via a Na+/H+ antiporter was supported by experiments showing that Na+ promotes release of Acridine Orange previously accumulated in these vacuoles. This putative antiporter did not transport Li+ and was not sensitive to the amiloride analogue 5-(N-ethyl-N-isopropyl)amiloride. Addition of the Na+/H+ ionophore monensin to intact cells loaded with fura 2, in the nominal absence of extracellular Ca2+ to preclude Ca2+ entry, was followed by an increase in cytosolic Ca2+ concentration ([Ca2+]i), which was more accentuated in the presence of extracellular Na+. An increase in intracellular pH (pHi) of BCECF-loaded cells was detected after addition of monensin in the presence of extracellular Na+, whereas a dramatic decrease in pHi was detected in its absence, thus indicating the presence of a significant amount of releasable protons in the acidic compartments. These results are consistent with the presence of a Na+/H+ antiporter in the acidocalcisomes that could be involved in the regulation of pH1 and [Ca2+]1 in these parasites.
authors.affiliation: Department of Veterinary Pathobiology, University of Illinois at Urbana-Champaign, Urbana, IL 71801, U.S.A.
authors.forename: A E
authors.initials: AE
authors.lastname: Vercesi
authors.forename: R
authors.initials: R
authors.lastname: Docampo
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002463:Cell Membrane Permeability; D006863:Hydrogen-Ion Concentration; D007476:Ionophores / Q000494:pharmacology; D008985:Monensin / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*; D013268:Stimulation, Chemical; D014346:Trypanosoma brucei brucei / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D014617:Vacuoles / Q000187:drug effects / Q000378:metabolism
chemical_list: D007476:Ionophores; D017923:Sodium-Hydrogen Exchangers; D008985:Monensin; D012964:Sodium; D002118:Calcium