Free Ca2+ requirements of agonist-induced thromboxane A2 synthesis in human platelets.

In quin2-loaded human platelets ionomycin raised cytosolic free calcium to greater than 1 microM and generated less than 1 ng thromboxane. Collagen alone or in the presence of EPO92 generated up to 32 and 16 ng thromboxane respectively; in the latter case at calcium levels around 120 nM. Thrombin maximally raised calcium to greater than 1 microM and generated up to 27 ng thromboxane, although in the presence of 1 mM EGTA these calcium and thromboxane levels were reduced to 200 nM and 5 ng respectively.