Ca2+ release from platelet intracellular stores by thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone : relationship to Ca2+ pools and relevance in platelet activation

The effects of the Ca(2+)-ATPase inhibitors thapsigargin (Tg) and 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBuBHQ) were examined by using Ca(2+)-regulatory systems of platelet mixed membranes, saponin-permeabilized and intact platelets. Both agents inhibit Ca(2+)-ATPase activities of platelet mixed membranes, without any effect on the basal Mg(2+)-ATPase activity. Tg is more effective (EC50 = 35 nM) than tBuBHQ (EC50 = 580 nM). The effect of the two inhibitors on 45Ca2+ release from saponin-permeabilized platelets has also been characterized. 45Ca2+ uptake into non-mitochondrial intracellular stores occurs via an ATP-dependent mechanism, and if added at equilibrium the second messenger Ins(1,4,5)P3 releases 50% of the accumulated 45Ca2+. Maximally effective concentrations of Tg (1 microM) and tBuBHQ (50 microM) release 77% and 68% of the accumulated 45Ca2+. Addition of Ins(1,4,5)P3 together with either Tg or tBuBHQ resulted in a non-additive release which was the same as with either Tg or tBuBHQ alone, indicating that the Ins(1,4,5)P3-sensitive Ca2+ pool was a subset of the pool that is sensitive to the Ca(2+)-ATPase inhibitors. Release of 45Ca2+ by either Tg or tBuBHQ was not affected by heparin, which totally blocked Ins(1,4,5)P3-induced Ca2+ release, and Tg was found not to affect [32P]Ins(1,4,5)P3 binding to its receptor on mixed membranes. Thus both Tg and tBuBHQ release Ca2+ from a pool that totally overlaps the Ins(1,4,5)P3-sensitive pool without affecting Ins(1,4,5)P3 function. In intact indomethacin-treated Fura 2-loaded platelets, Tg and tBuBHQ cause Ca2+ elevation, arising from release from intracellular stores and influx from the outside. Both Tg and tBuBHQ elevated Ca2+ to similar levels, which were less and slower than those observed with thrombin. Addition of thrombin to cells already treated with Tg or tBuBHQ produced further elevation of Ca2+, indicating agonist utilization of a Ca(2+)-ATPase inhibitor-insensitive pool. In aggregation experiments Tg and tBuBHQ showed different functional effects. In indomethacin-treated cells Tg induces slow aggregation and secretion responses, whereas tBuBHQ only induces shape change. Both agents show synergistic secretory responses with the protein kinase C activator dioctanoylglycerol (DiC8). Tg also showed greater ability than tBuBHQ to release [3H]arachidonic acid (AA) from [3H]AA-labelled platelets. Additionally, in [32P]Pi-labelled platelets both Tg and tBuBHQ induced phosphorylation of myosin light