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

Summary

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 tyramine affected ¹⁴C-5HT and ³H-DA about equally, whereas ³H-NA much less. Octopamine showed a similar pattern as tyramine, 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.