Interrelationship between secretion, protein phosphorylation and intracellular Ca2+ concentration in platelets...

The interrelationship between ATP-secretion, protein phosphorylation and intracellular Ca2+ concentration ([Ca2+]i) was studied in both 32P and quin 2 loaded human platelets stimulated by thrombin or thromboxane A2 analogue (STA2). In platelets stimulated by thrombin, the degree of 47,000 dalton polypeptides (P47) phosphorylation was observed in completely dose-related manner, regardless of the amount of [Ca2+]i. In the same condition, the degree of myosin light chain (P20) phosphorylation, however, was well correlated with ATP secretion and [Ca2+]i, when platelets were stimulated by lower dose of thrombin. The similar results were obtained in platelets stimulated by STA2. These findings suggested that P20, but not P47, phosphorylation in activated platelets is mediated by a rise of [Ca2+]i and is well correlated with the secretory reaction. It was unlikely that P47 phosphorylation plays any role in promoting platelet activation.