Activation of phospholipases A and C in human platelets exposed to epinephrine: role of glycoproteins IIb/IIIa and dual role of epinephrine.

Publisher Website
Google Scholar
Abstract
Human platelets stimulated by epinephrine undergo enhanced turnover of phosphatidylinositol 4,5-bisphosphate, accumulate inositol triphosphate, diacylglycerol, and phosphatidic acid, and phosphorylate a 47-kDa protein. All of these phenomena indicate stimulation of phospholipase C. These responses are blocked completely by inhibitors of .alpha.2-adrenergic receptors (yohimbine), cyclooxygenase (aspirin or indomethacin), phospholipase A [2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid (ONO-RS-082)], Na+/H+ exchange [ethylisopropylamiloride (EOPA)], fibrinogen binding to glycoprotein IIb/IIIa (antibody A2A9), Ca2+/Mg+ binding (EDTA), or removal of fibrinogen. Epinephrine evokes (i) an increased turnover of ester-linked arachidonic acid in aspirin-treated platelets that is inhibited by ONO-RS-082, EDTA, yohimbine, or the absence of fibrinogen and (ii) a rapid cytoplasmic alkalinization that is inhibited partially to blockage of cyclooxygenase activity and completely by A2A9 or EIPA. In contrast, when incubated with subaggregatory concentrations of the prostaglandin H2/thromboxane A2 analogue [(15S)-hydroxyl-11.alpha.,9.alpha.-(epoxymethano)prosta-5,13-dienoic acid (U46619)] and epinephrine, aspirin-treated platelets show a potentiation of phosholipase C activation that is unaffected by the above inhibitors. We propose that epinephrine, in promoting exposure of glycoprotein IIb/IIIa sites for fibrinogen binding, leads to a cytoplasmic alkalinization, which, in conjunction with local shifts in Ca2+, promotes low-level activation of phospholipase A. The resulting free arachidonic acid is converted to cyclooxygenase products, which, potentiated by epinephrine, activate phospholipase C. This furhter ampiflies the initial stimulatory response.