Effects of serotonin on platelet activation in whole blood.
Li, N.; Wallen, N. H.; Ladjevardi, M.; Hjemdahl, P.
Blood Coagulation & Fibrinolysis.
8(8):517-524, November 1997.
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Serotonin (5-hydrotryptamine, 5-HT) is a weak platelet agonist, which has not been evaluated fully with respect to platelet responses in whole blood. We thus re-evaluated platelet responses to 5-HT using three whole blood techniques: flow cytometry, impedance aggregometry and filtragometry. At concentrations up to 10-5 mol/l, 5-HT per se failed to induce platelet aggregation in whole blood, or to increase platelet fibrinogen binding or P-selectin expression. However, 5-HT potentiated platelet responses to low concentrations of adenosine diphosphate (ADP) or thrombin dose-dependently. 5-HT (10-7, 10-6 and 10-5 mol/l) increased ADP-induced platelet fibrinogen binding by 40, 59 and 79%, while P-selectin expression increased by 45, 64 and 95%, respectively (P < 0.05; n = 10). The enhancing effects of 5-HT were even more pronounced in thrombin-stimulated samples, as 5-HT at 10-8, 10-7, 10-6 and 10-5 mol/l increased fibrino gen binding by 56, 128, 212 and 260% (P < 0.05), and P-selectin expression by 31, 56, 89 and 109%, respectively (P < 0.01; n = 10). The response to 5-HT was inhibited dose-dependently by a highly selective 5-HT2A receptor antagonist (SR 46349), with almost complete inhibition at 10-6 mol/l. Impedance aggregometry showed a significant enhancement in 5 x 10-6 mol/l ADP-induced platelet aggregation caused by 5-HT at 8 x 10-8 mol/l (from 7.58 x 2.50 [OMEGA] to 9.02 x 2.43 [OMEGA], P < 0.02, n = 12). Similarly, filtragometry readings, i.e. the time taken for platelet aggregates to occlude a microfilter, were shortened by 27% (P < 0.05, n = 9), reflecting increased platelet aggregability. Our data suggest that 5-HT per se does not activate platelets, but dose-dependently enhances platelet activation induced by ADP and, in particular, thrombin in whole blood. Thus, the idea that 5-HT is a 'helper agonist' is supported; this effect is mediated by 5-HT2A receptors.
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