Does Epinephrine Activate Platelets By Binding To A Receptor And Then Reducing Heme In A Membrane Enzyme To Transmit The Activating Signal

It is well established that epinephrine binds to an α- receptor on platelets. How the signal for cell simulation is transmitted is uncertain. Reduction of Fe³⁺-heme to Fe²⁺-heme was evaluated as described previously (Prost. Med. 4:73,1980). Compounds which are known to interact with the plateletoC-receptor reduced heme in the same rank order as their effectiveness as agonists epinephrine > norepinephrine> dopamine > phenylephrine. Phentolamine which binds to the platelet receptor but is an inhibitor not an agonist was ineffective at reducing heme. 1,10 phen- anthroline, 3-chloropyridine, 2,2’-dipyridyl and 4,4’-di- pyridyl which can bind to the Fe in heme all inhibited first wave epinephrine aggregation at lower levels than they inhibited first wave ADP aggregation (IC50s to epinephrine 0.31-0.95; IC50s to ADP 0.93-4.8). The results are consistent with the concept that epinephrine binds to its receptor primarily through interactions involving the methyl-amine and beta-hydroxy groups along with hydrophobic bonds involving the aromatic ring, while intrinsic activity is a function of the catechol moiety which reduces Fe³⁺ heme to Fe²⁺ heme in a membrane enzyme.