Calcium efflux from platelet vesicles of the dense tubular system. Analysis of the possible contribution of the Ca2+ pump

The ATP dependent Ca²⁺ uptake of platelet vesicles was inhibited by the two hydrophobic drugs trifluoperazine (TFP) and propranolol (PROP). Inhibition was significantly lowered when Pi was used instead of oxalate as a precipitant agent. When the ATPase ligands substrate (Mg²⁺ and Pi) were absent of the efflux medium, a slow release of Ca²⁺ which did not couple with ATP synthesis (passive Ca²⁺ efflux) was observed. Both, TFP and PROP enhanced the passive Ca²⁺ efflux. This enhanced efflux was partially inhibited only when Mg²⁺ and Pi were added together to the efflux reaction media, but it was not affected by spermidine, ruthenium red or thapsigargin (TG). The Ca²⁺ ionophores A23187 and ionomycin, also enhanced passive Ca²⁺ efflux. However, in this case, Ca²⁺ efflux was inhibited just by inclusion of Mg²⁺ to the medium. Ca²⁺ efflux promoted by Triton X-100 was not affected by either Mg²⁺ or Pi, included together or separately into the efflux medium. The ATP Pi measured in the presence of Triton X-100 and millimolar Ca²⁺ concentrations was inhibited by both TFP and PROP, but not by Ca²⁺ ionophores up to 4 M. The data suggest that the observed enhancement of passive Ca²⁺ efflux promoted by TFP and PROP could be attributed to a direct effect of these drugs over the platelet Ca²⁺ pump isoforms (Sarco Endoplasmic Reticulum Calcium ATPase, SERCA_2b and SERCA₃) themselves, as it was reported for the sarcoplasmic reticulum Ca²⁺ ATPase (SERCA₁).