EFFECTS OF SEROTONIN ON PLATELET ACTIVATION IN WHOLE-BLOOD

Serotonin (5-hydrotryptamine, 5-HT) is a weak platelet agonist, which has not been evaluated fully with respect to platelet responses in who le blood. We thus re-evaluated platelet responses to 5-HT using three whole blood techniques: flow cytometry, impedance aggregometry and fil tragometry. At concentrations up to 10(-5) mol/l, 5-HT per se failed t o 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 (AD P) 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 pron ounced in thrombin-stimulated samples, as 5-HT at 10(-8), 10(-7), 10(- 6) and 10(-5) mol/l increased fibrinogen binding by 56, 128, 212 and 2 60% (P < 0.05), and P-selectin expression by 31, 56, 89 and 109%, resp ectively (P < 0.01; n = 10). The response to 5-HT was inhibited dose-d ependently by a highly selective 5-HT2A receptor antagonist (SR 46349) , with almost complete inhibition at 10(-6) mol/l. Impedance aggregome try showed a significant enhancement in 5 X 10(-6) mol/l ADP-induced p latelet aggregation caused by 5-HT at 8 X 10(-8) mol/l (from 7.58 +/- 2.50 Ohm to 9.02 +/- 2.43 Ohm, P < 0.02, n = 12). Similarly, filtragom etry readings, i.e. the time taken for platelet aggregates to occlude a microfilter, were shortened by 27% (P < 0.05, n = 9), reflecting inc reased platelet aggregability. Our data suggest that 5-HT per se does not activate platelets, but dose-dependently enhances platelet activat ion 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.