Calmidazolium leads to an increase in the cytosolic Ca2+ concentration in Dictyostelium discoideum by induction of Ca2+ release from intracellular stores and influx of extracellular Ca2+.
The Ca2+ stores of Dictyostelium discoideum amoebae take part in control of homoeostasis of the cytosolic free Ca2+ concentration ([Ca2+]i) and the cyclic-AMP-induced [Ca2+]i-signalling cascade. In order to characterize regulatory mechanisms of these stores, we incubated cells with the calmodulin antagonist calmidazolium. Measurement of permeabilized and intact cells in suspension with a Ca(2+)-sensitive electrode revealed that calmidazolium induced Ca2+ release from intracellular stores, influx of Ca2+ across the plasma membrane and subsequent efflux. In single fura-2-loaded cells calmidazolium evoked rapid and global transient elevations of [Ca2+]i. Other calmodulin antagonists (trifluoperazine, chlorpromazine, fendiline and W7) also induced transient elevations of [Ca2+]i, which were, however, slower and observed in fewer cells. The calmidazolium-induced influx of extracellular Ca2+ was inhibited by preincubation with 2,5-di-(t-butyl)-1, 4-hydroquinone (BHQ) and 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-Cl), both known to interact with pumps of the inositol 1,4,5-trisphosphate (IP3)-sensitive store, and by the V-type H(+)-ATPase inhibitor bafilomycin A1, which affects the acidosomal Ca2+ store. Incubation with pump inhibitors did not itself induce changes in [Ca2+]i. We conclude that the effects of calmidazolium are, at least in part, mediated by its calmodulin-antagonizing properties, that it acts by inducing Ca2+ release from filled storage compartments, and that its target of action is both the IP3-sensitive store and the acidosome; emptying of these stores leads to influx of extracellular Ca2+.
Schlatterer
C
C
Fakultät für Biologie, Universität Konstanz, Federal Republic of Germany.
Schaloske
R
R
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002147:Calmodulin / Q000037:antagonists & inhibitors; D002462:Cell Membrane / Q000378:metabolism; D003600:Cytosol / Q000187:drug effects* / Q000378:metabolism; D004023:Dictyostelium / Q000187:drug effects* / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D016257:Fura-2; D007093:Imidazoles / Q000494:pharmacology*
D002147:Calmodulin; D004791:Enzyme Inhibitors; D007093:Imidazoles; C031938:calmidazolium; D000252:Calcium-Transporting ATPases; D002118:Calcium; D016257:Fura-2