INHIBITORY EFFECT OF MELATONIN ON PLATELET ACTIVATION-INDUCED BY COLLAGEN AND ARACHIDONIC-ACID

Melatonin, an indolamine synthesized in the pineal gland, is known to have antiprostanoid activity. The inhibition of platelet aggregation i nduced by melatonin has been proposed to take place through the cycloo xygenase pathway. In the present study, we found that melatonin has a marked inhibitory effect on collagen, arachidonic acid (AA), adenosine diphosphate (ADP), epinephrine, and A23187-induced aggregation in pla telet-rich plasma. On the other hand, using metrizamide-filtered plate lets resuspended in Tyrode's buffer, melatonin fails to suppress AA-in duced platelet aggregation and C-14-5-HT release. Under the same condi tions, melatonin inhibits collagen-induced platelet activation; howeve r, the addition of threshold doses of AA (0.3 mM) abrogates this effec t. These studies suggest that melatonin also inhibits platelet functio n at a stage preceding the cyclooxygenase-dependent pathway.