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Binding of serotonin by rat lipids was examined in an organic solvent-aqueous partition system. Only phospholipids and sulfatide were found to have appreciable activity; this technique was unsuitable for gangliosides due to their poor extractibility. Binding by phospholipid was abolished and that by sulfatide was greatly inhibited by increasing ionic strength in the aqueous phase. At an ionic strength of 0.3 M the apparent affinity of sulfatide for serotonin was about 3 X 10(3) M. Both tryptamine and 5-methoxytryptamine were much more effective than serotonin in inhibiting the binding of radioactive serotonin, suggesting that the observed binding is simply a charge neutralization with little specificity. Binding of serotonin by mixed brain gangliosides was examined in an equilibrium dialysis system. Without adequate precautions, the chemical lability of serotonin was found to produce spurious data when binding was assessed by the distribution of radiolabel. Binding of serotonin by ganglioside was also greatly inhibited by increasing ionic strength: at 0.3 M an apparent affinity of about 10(3) M was found. While dopamine did not inhibit the binding of radioactive serotonin, tryptamine, 5-methoxytryptamine, and serotonin were equally effective inhibitors.
Binding of serotonin by rat lipids was examined in an organic solvent-aqueous partition system. Only phospholipids and sulfatide were found to have appreciable activity; this technique was unsuitable for gangliosides due to their poor extractibility. Binding by phospholipid was abolished and that by sulfatide was greatly inhibited by increasing ionic strength in the aqueous phase. At an ionic strength of 0.3 M the apparent affinity of sulfatide for serotonin was about 3 X 10(3) M. Both tryptamine and 5-methoxytryptamine were much more effective than serotonin in inhibiting the binding of radioactive serotonin, suggesting that the observed binding is simply a charge neutralization with little specificity. Binding of serotonin by mixed brain gangliosides was examined in an equilibrium dialysis system. Without adequate precautions, the chemical lability of serotonin was found to produce spurious data when binding was assessed by the distribution of radiolabel. Binding of serotonin by ganglioside was also greatly inhibited by increasing ionic strength: at 0.3 M an apparent affinity of about 10(3) M was found. While dopamine did not inhibit the binding of radioactive serotonin, tryptamine, 5-methoxytryptamine, and serotonin were equally effective inhibitors.
The influence of aspirin on 5-hydroxytryptamine (5-HT) uptake and storage by human blood platelets has been investigated. Uptake of 5-HT was strongly inhibited. In 30 min aspirin released 50% of the 5-HT that had been incorporated into the platelets prior to the addition of the aspirin. These results are discussed in terms of possible interference with a 5-HT membrane receptor and the impairment of 5-HT storage in the dense granules.
A B S T R A C T Human platelets contain the cuprozinc (cytoplasmic) and manganese (mitochondrial) forms of superoxide dismutase. Nevertheless, superoxide radicals were detectable in the surrounding medium of metabolically viable platelet suspensions by using two assay systems: cytochrome c and nitroblue tetrazolium. The quantity of superoxide generated by platelets (5 x 105 superoxide radicals/platelet per 10 min) was constant and did not increase after aggregation by agents such as collagen and thrombin. The superoxide-generating system was present in the supernate of both aggregated and resting platelets and therefore was not platelet-bound. Platelet superoxide production was unaffected by prior ingestion of aspirin, indicating that the prostaglandin and thromboxane pathways were not involved. Both resting and aggregated platelets exhibited a reductive capacity toward cytochrome c and nitroblue tetrazolium which was unrelated to superoxide production. Furthermore, the aggregation process always resulted in a marked increase in this reduction. The nonsuperoxide reduction associated with aggregation was found to be membrane bound and to decrease with an apparent first order reaction rate (k1 = 0.067 min-'). In addition, accumulative, time-dependent nonsuperoxide-related cytochrome c reduction was also detected. Since there is no superoxide dismutase in plasma, the presence of superoxide radicals in the surrounding medium of platelets may have in vitro significance for platelet and leukocyte concentration and storage and in vivo significance for hemostasis, coagulation, and thrombosis. The nonsuperoxiderelated reducing activities may represent a biochemical basis for platelet-blood vessel interactions, with particular reference to blood vessel integrity.
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