Liberation of catecholamines from blood platelets

1 Platelet-rich plasma (PRP) of guinea-pigs with or without reserpine was preincubated either with [(14)C]-5-hydroxytryptamine ([(14)C]-5-HT) plus [(3)H]-dopamine or with [(14)C]-5-HT plus [(3)H]-noradrenaline ([(3)H]-NA). After isolation on two successive dextran gradients the double-labelled platelets were incubated in Tris-buffer in the presence or absence of various drugs. The decrease in radioactivity in the platelets was measured in order to determine the amount of the amine that had been liberated.2 Spontaneous liberation of the labelled amines was more marked in reserpine-treated platelets than in normal ones and somewhat more pronounced for the (3)H-catecholamines than for [(14)C]-5-HT.3 The reserpine-like benzoquinolizine, Ro 4-1284, caused liberation of all three labelled amines in normal but not in reserpine-treated platelets. More [(3)H]-dopamine was liberated than [(14)C]-5-HT and less [(3)H]-NA.4 The arylalkylamines, tyramine and p-chloromethamphetamine (PCMA), liberated all three labelled amines from normal platelets, and [(14)C]-5-HT and [(3)H]-dopamine, but not [(3)H]-NA from reserpine-treated ones. In normal platelets dopamine was reduced to a greater extent than [(14)C]-5-HT and [(3)H]-NA to a smaller extent, whereas in reserpine-treated platelets [(14)C]-5-HT was more markedly diminished than [(3)H]-dopamine.5 The 5-HT uptake inhibitor, imipramine, had little influence on the spontaneous and drug-induced liberation of [(14)C]-5-HT and [(3)H]-dopamine.6 It is concluded that (3)H-catecholamines like [(14)C]-5-HT are mostly localized in the granular pool of platelets; the three drugs tested liberate [(3)H]-dopamine [(3)H]-NA and [(14)C]-5-HT from the granular pool. Ro 4-1284 does not liberate (3)H-catecholamines and [(14)C]-5-HT from extragranular sites whereas tyramine and PCMA also act on the extragranular pool of [(3)H]-dopamine and [(14)C]-5-HT but not [(3)H]-NA.7 The liberation of catecholamines from platelets differs from that of 5-HT in several respects and platelets are only partly comparable to neurones as far as drug-induced liberation of biogenic amines is concerned.