Activation of phospholipases A and C in human platelets exposed to epinephrine: role of glycoproteins IIb/IIIa and dual role of epinephrine.
Human platelets stimulated by epinephrine undergo enhanced turnover of phosphatidylinositol 4,5-bisphosphate, accumulate inositol trisphosphate, 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 (EIPA)], 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 by 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)-hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic acid (U46619) and epinephrine, aspirin-treated platelets show a potentiation of phospholipase 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 further amplifies the initial stimulatory response.
D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000097:blood; D001241:Aspirin / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology*; D002352:Carrier Proteins / Q000097:blood; D004789:Enzyme Activation; D004837:Epinephrine / Q000494:pharmacology*; D005340:Fibrinogen / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D010740:Phospholipases / Q000097:blood*; D010741:Phospholipases A / Q000097:blood*; D010980:Platelet Membrane Glycoproteins / Q000502:physiology*; D011450:Prostaglandin Endoperoxides, Synthetic / Q000097:blood; D044262:Prostaglandin H2; D011463:Prostaglandins H / Q000097:blood; D017923:Sodium-Hydrogen Exchangers; D013928:Thromboxane A2 / Q000378:metabolism; D010738:Type C Phospholipases / Q000097:blood*
D001095:Arachidonic Acids; D002352:Carrier Proteins; D010980:Platelet Membrane Glycoproteins; D011450:Prostaglandin Endoperoxides, Synthetic; D011463:Prostaglandins H; D017923:Sodium-Hydrogen Exchangers; D016718:Arachidonic Acid; D044262:Prostaglandin H2; D013928:Thromboxane A2; D005340:Fibrinogen; D010740:Phospholipases; D010741:Phospholipases A; D010738:Type C Phospholipases; D001241:Aspirin; D004837:Epinephrine