The influence of epinephrine on washed human platelets: shape change and protein phosphorylation

Protein phosphorylation is an important regulator of the properties or functions of many proteins and is associated with the platelet activation response to a number of chemically and functionally different agents such as thrombin, platelet-activating factor, serotonin and collagen. The physiological responses of platelets to these agents are similar, and the common intracellular messenger for activation is an elevated concentration of calcium. Platelets possess alpha-2-receptors, and treatment with epinephrine produces an elevation in platelet cytosolic free calcium concentrations. Methods are described for studying hormone sensitive shape change and protein phosphorylation in washed human platelets. Epinephrine induces platelet shape change, and this process is independent of extracellular calcium. Treatment of [32P]-orthophosphate-labelled platelets with epinephrine produces an increase in 32P-incorporation into two platelet proteins with molecular weights of 47000 and 20000. This phosphorylation response is both dose and time dependent. Extracellular calcium is not absolutely essential for epinephrine-induced phosphorylation, but does enhance the maximum levels of 32P-incorporation. Epinephrine sensitive phosphorylation is completely inhibited following pretreatment with verapamil or nitrendipine. Shape change in response to epinephrine occurs in the absence of enhanced protein phosphorylation. The data suggest that epinephrine mobilizes intracellular calcium, and induces platelet shape change and phosphorylation responses characteristic of platelet activation.