Inhibition of human platelet aggregation and thromboxane B2 production by melatonin. Correlation with plasma melatonin levels

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Abstract
Plasma melatonin concentrations and the effect of melatonin on arachidonic acid (AA)‐induced aggregation and thromboxane B2 (TxB2) production by platelet‐rich plasma (PRP) were examined in five normal male volunteers, sampled at 2 hr intervals from 21:30 to 09:30 hr. Peak plasma melatonin concentration was found at 03:30 hr. Inhibition by 10−6 M melatonin of AA‐induced PRP aggregation was observed only in samples taken at 01:30 hr. Assessment of the inhibitory effect of 10−9–10−6 M melatonin on AA‐induced TxB2 production indicated that melatonin activity was greater at 01:30 h as compared to late night. Assessed as a global effect, the inhibitory activity of melatonin on PRP TxB2 showed a maximum at 01:30 hr and minimal effects at 03:30 hr, at the time when plasma concentrations of melatonin were highest. These results indicate the existence of a nocturnal variation in sensitivity of human platelets to melatonin, with a peak that precedes the maximum in circulating melatonin levels.