Biochemical mechanisms in 5-hydroxytryptamine-induced human platelet aggregation.

The activation of human platelets by 5-hydroxytryptamine (5-HT) is not accompanied by detectable release of ATP or TXB2. The process is unaffected by cyclooxygenase, thromboxane synthetase or combined cyclooxygenase/lipoxygenase inhibition (suprofen, indomethacin, R 19091, dazoxiben, N.D.G.A, BW755C, esculetin), indicating the absence of involvement of arachidonic acid metabolites. Transmembrane Ca2+-entry blockers (flunarizine, nifedipine, nimodipine) have no effect either, indicating that the activator calcium released by 5-HT comes from intracellular stores. The 5-HT-induced platelet activation is inhibited by stimulators of adenylate cyclase (PGE1, PGE2, isoprenaline, adenosine) and inhibitors of cAMP phosphodiesterase (papaverine, anagrelide, RA233), indicating that also for this type of platelet activation cAMP behaves as a unidirectional, inhibitory regulator.