Calcium mobilization in human platelets by receptor agonists and calcium-ATPase inhibitors

Abstract

Inhibitors of the endoplasmic reticulum Ca 2+-ATPase like thapsigargin (TG) and 2,5-di ( tert-butyl)-1,4-benzohydroquinone ( tBuBHQ) cause increases in cytosolic calcium in intact human platelets resulting from prevention of reuptake. A maximal concentration of TG (0.2 μM) mobilized 100% of sequestered Ca 2+ compared to the action of a receptor agonist like thrombin (0.1 U/ml). A maximal dose of tBuBHQ (50 μM) stimulated release of about 40% of intracellular calcium compared to thrombin and TG. The reduced ability of tBuBHQ to release calcium can be explained with an inhibitory effect on the cyclooxygenase pathway ( K i ≈ 7 μM). Thererore tBuBHQ is not able to cause platelet aggregation compared to TG. In the presence of a cyclooxygenase inhibitor or a thromboxane A 2 receptor antagonist the action of TG is identical to that observed with tBuBHQ. Generally, inhibition of calcium sequestration does not automatically result in platelet activation. In contrast to a receptor mediated activation Ca 2+-ATPase inhibitors require the self-amplification mechanism of endogenously formed thromboxane A 2 to cause a similar response. We conclude that the calcium store sensitive to Ca 2+-ATPase inhibitors is a subset of the receptor sensitive calcium pool.