MELATONIN EFFECT ON SEROTONIN UPTAKE AND RELEASE IN RAT PLATELETS - DIURNAL-VARIATION IN RESPONSIVENESS

The present study was conducted to examine whether melatonin impairs s erotonin (5HT) release and uptake in rat platelets. Exposure of platel et-rich plasma samples (PRP) to melatonin induced a concentration-depe ndent inhibition of 5HT uptake and the value of IC50 was 1.3x10(-3)M. We have also investigated the melatonin effect on the kinetic paramete rs of platelet 5HT uptake. Transport capacity was inhibited (Vmax; Con trol: 2.28+/-0.52, Melatonin: 0.74+/-0.13 pmol/10(7) platelet.min; p<0 .05) while the affinity of 5HT for its uptake carriers remained unalte red, thus indicating a non-competitive effect. Studies carried out to determine the existence of a differential morning (8:00h)-evening (21: 00h) melatonin effect showed a higher platelet uptake sensitivity at 8 :00h (two-way ANOVA, p<0.001). Spontaneous 5HT release was not impared by the hormone and no daily variation in sensitivity was detected. Th e possible mechanism of action of melatonin on platelet transport is d iscussed, and the results support the suitability of the platelet mode l for studying sensitivity changes in target cells to the hormone.