Effects of 5-hydroxykynurenamine-a serotonin metabolite- and its analogues on platelet functions

Abstract

Effects of 5-hydroxykynurenamine (5-HK), produced from serotonin by the action of indoleamine 2, 3-dioxygenase, and its anologues on platelet functions were investigated. Platelet aggregation was studied by using a dual sample aggregometer (Sienco, Model DP-247) and serotonin uptake was examined by incubating platelet-rich plasma with 1μM 14C-serotonin at 37°C for 2min. 5-HK showed regulatory effects on human platelet functions both by antagonizing against the promoting effect of serotonin on arachidonic acid-induced platelet aggregation as well as that on ADP-induced aggregation and by inhibiting 14Cserotonin uptake by platelets. LASS production by incubation of platelet microsomes with arachidonic acid was estimated by the maximal changes in light transmission of citrated human platelet-rich plasma which was added to the incubation mixture, containing microsomes and arachidonic acid, and was found to be enhanced by 5-HK. Kynurenamine, N, N′-dimethyl-5-HK, N, N′-dimethyl-3-HK, 2-(3′-aminopropyl) aniline and 5-methoxykynurenamine inhibited both 14C-serotonin uptake by platelets and serotonin-induced aggregation, while N-acetyl-5-methoxykynurenamine, kynurenine and 5-hydroxykynurenine affected neither of these platelet functions. However, the degree of inhibitory effects of these analogues on serotoninuptake was not always in parallel with that on serotonin-induced aggregation. These findings support the concept that the inhibition of the serotoninuptake by these substances is intimately, if not directory, related to their ability to inhibit serotonin-induced platelet aggregation. LASS production was enhanced by N, N′-dimethyl-5-HK, N, N′-dimethyl-3-HK, 5-methoxykynurenamine and 5-hydroxykynurenine, while kynurenamine, 2-(3′-aminopropyl) aniline, N-acetyl-5-methoxykynurenamine and kynurenine showed no effect upon it. These studies with analogues indicated that the substitutions of o-aminobenzyl moiety with hydroxy or methoxy groups were somewhat tolerated for their inhibitory effects on platelet aggregation and serotoninuptake, whereas such groups were essential for their promoting effect on the LASS production by platelet microsomes. When the amine was masked by N-acetylation, both the inhibitory activities on serotonin-induced aggregation as well as on serotonin uptake and the promoting activity on LASS production were completely lost, suggesting that the alkylamine in the side chain is essential for these activities.