Liberation of catecholamines and 5-hydroxytryptamine from human blood-platelets.

Spontaneous and drug-induced liberation of 14C-5-hydroxytryptamine (14C-5HT), 3H-dopamine (3H-DA) and 3H-noradrenaline (3H-NA) from normal and reserpinized human blood-platelets has been determined from measurements of the amine contents before and after incubation in tris-buffer. In normal platelets the spontaneous liberation of 3H-catecholamines was more marked than that of 14C-5HT, but was less in percent for all these labelled amines than in reserpinized platelets. Thrombin lowered amine contents more in normal than in reserpinized platelets. The initial thrombin-induced decrease of 14C-5HT, in contrast to that of 3H-catecholamines, showed a partial recovery after 30 min which was abolished by imipramine. The benzoquinolizine Ro 4-1284 diminished all the amines in normal, but not in reserpinized platelets. In normal platelets tryamine affected 14C-5HT and 3H-DA about equally, whereas 3H-NA much less. Octopamine showed a similar pattern as tryamine, but was less potent. P-chlormethamphetamine (PCMA) and amphetamine decreased 3H-DA less markedly than 14C-5HT and 3H-NA not at all. In reserpinized platelets these arylalkylamines induced a decrease of 14C-5HT but not of 3H-catecholamines. It is concluded that (a) 3H-catecholamines like 14C-5HT are mainly localized in the granular pool of normal human platelets, (b) the pattern of action of a drug on intra- and extragranular amines depends not only on the nature of the drug and the amine to be liberated, but in comparison with previous results also on the species, (c) platelets are not completely satisfactory models for monoaminergic neurons, especially catecholaminergic ones regarding drug-induced amine liberation.