Regulation of hormone-induced Ca2+ mobilization in the human platelets.

alpha-Thrombin, gamma-thrombin, and platelet-activating factor each stimulated the mobilization of intracellular Ca2+ stores in aspirin-treated human platelets. This was followed by desensitization of the receptors, as shown by the return of the Ca2+ level to basal values and by the fact that a subsequent addition of a second different agonist, but not the same agonist, could again elicit a response. Epinephrine, acting on alpha 2-adrenergic receptors, was by itself ineffective at mobilizing Ca2+ stores. However, when added after the thrombin-induced response, epinephrine could evoke a considerable release of Ca2+ from cellular stores. This appeared to be due to epinephrine recoupling thrombin receptors to phospholipase C. In support of this, epinephrine was able to induce the formation of inositol triphosphate when added after the response to thrombin had also become desensitized. Alone, epinephrine was without effect. Pre-activation of protein kinase C with the phorbol ester abolished these effects of epinephrine, suggesting that epinephrine was working by activating a protein which could be inactivated by phosphorylation. Our current work is to characterize this protein that may be a member of the Gi, GTP-binding protein family.