RESUMO
Human platelets contain a2-adrenergic receptors, which are coupled with guanine nucleotide proteins [G Proteins]. Stimulation of a2 adrenergic receptors leads to the activation of Gi adenylyl cyclase cascade in platelets. Recent evidence suggests the role of cross regulation between adenylyl cyclase and phospholipase signaling pathways. Calcium ionophore, A23187 is thought to activate cellular phospholipases. In the present study, we have investigated the mechanism involved in the action of epinephrine on platelet aggregation induced by A23187. The results show that epinephrine at low concentrations [0.01 - 0.2 pM] and/or A23187 [0.1-0.5 micro M] itself did not produce platelet aggregation. However, when added together, a marked potentiation of platelet aggregation was observed. This synergistic effect was inhibited by a2-receptor blocker [yohimbine; IC50 = 0.05 micro M] showing that the response is receptor mediated. To find out the molecular basis of this potentiation, we used Wortmannin, selective inhibitor of phosphatidylinositol 3. kinase [P13K]. Wortmannin inhibited the platelet aggregation induced by A23187 and epinephrine with IC50 value of 0.35 pM. The data suggest that epinephrine and calcium influx act synergistically and P13K plays an important role in platelet aggregation