Two classes of agonist-sensitive Ca2+ stores in platelets, as identified by their differential sensitivity to 2,5-di-(tert-butyl)-1,4-benzohydroquinone and thapsigargin

Abstract

In the absence of extracellular Ca2+, extensive Ca2+ release from the platelet intracellular stores [monitored as an increase of intracellular Ca2+ concentration ([Ca2+]i)] is produced by the combined action of the endomembrane Ca(2+)-ATPase inhibitor thapsigargin and 2 nM ionomycin. The titration of Ca2+ unloading with thapsigargin (plus ionomycin) shows that a substantial fraction of the store-associated Ca2+ is released by 8-10 nM thapsigargin, but that 100-200 nM thapsigargin is required for the complete release. The store depletion obtained in similar conditions with a different endomembrane Ca(2+)-ATPase inhibitor, 2,5-di-(tert-butyl)-1,4-benzohydroquinone (TBHQ), is always incomplete. It is completed by thrombin or by 10 nM thapsigargin. We conclude that two different types of Ca2+ pumps exist in platelets, one sensitive to TBHQ and to high thapsigargin, the other insensitive to TBHQ and sensitive to low thapsigargin. They are distributed separately in discrete subpopulations of the agonist-sensitive stores. The influx of external Ca2+ is maximal when both types of stores are Ca(2+)-depleted, either by high thapsigargin or by the combined action of low thapsigargin and TBHQ.

Biogenesis of endoplasmic reticulum proteins involved in Ca2+ signalling during megakaryocytic differentiation: an in vitro study

“…First, a large body of evidence suggests that human platelet Ca# + signalling is controlled by the content of ER intracellular Ca# + pools. Secondly, in platelets InsP $ -sensitive and -insensitive Ca# + pools co-exist [10][11][12]. Thirdly, the identification of platelet ER proteins showed co-expression of multiple and diverse InsP $ -Rs [13][14][15][16] and SERCAs [17].…”

Modulation of Endoplasmic Reticulum Calcium Pump Expression during T Lymphocyte Activation

“…This enzyme, at least in platelets, is specifically associated with the IP 3 mobilizable calcium pool (20,21,43). Lymphocyte activation as well as apoptosis are strictly dependent on the mobilization of the IP 3 -sensitive calcium pool (1, 8, 44 -46).…”

Regulation of Plasma Membrane Ca2+-ATPase by Small GTPases and Phosphoinositides in Human Platelets

“…applied to pancreatic acinar cells by Toescu and Petersen (20) we have ascertained the importance of Ca 2ϩ extrusion during Ca 2ϩ signaling in human platelets. In a Ca 2ϩ -free medium (100 M EGTA added), inhibition of the endomembrane Ca 2ϩ -ATPase (sarco/endoplasmic reticulum Ca 2ϩ ATPase) using 1 M TG in the presence of a low concentration of IONO (50 nM; required for extensive depletion of the intracellular Ca 2ϩ stores in platelets where two Ca 2ϩ stores with high and low Ca 2ϩ leakage rates have been described (21,22)), resulted in a transient increase in [Ca 2ϩ ] i due to release of Ca 2ϩ from intracellular stores (Fig. 1B, trace a).…”