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Oxidant stress stimulates active transport of serotonin by platelets.

The effect of oxidant stress on the active transport of serotonin (5-HT) into mouse platelets was examined. Oxidant stress was produced using either H2O2 or the xanthine-xanthine oxidase generating system that yields both superoxide anion and H2O2. H2O2 (6.25-100 microM) caused a rapid (2-4 min) stimulation of platelet 5-HT transport that returned to control levels after 15 min of incubation. Catalase (1500 U/ml) completely prevented the stimulation, and the hydroxyl radical trapping agents mannitol (1 nM) and thiourea (1 mM) failed to alter the stimulation. Fluoxetine (1 microM) totally blocked all 5-HT uptake into stimulated platelets. The xanthine-xanthine oxidase (3.12-25 mU/ml) generating system produced a response similar to that of H2O2. In this system, superoxide dismutase (250 U/ml) did not alter the stimulatory response, whereas catalase (1500 U/ml) totally prevented the stimulation. The kinetics of 5-HT transport showed that oxidant stress did not alter the Km of 5-HT transport (Km control = 8.0 +/- 1.0 x 10(-7) M versus Km H2O2 = 9.5 +/- 1.1 x 10(-7) M) but markedly increased the maximal rate of transport (Vmax control = 36.1 +/- 4.8 pmol/10(8) platelets/4 min versus Vmax H2O2 = 79.9 +/- 9.1 pmol/10(8) platelets/4 min). Washed platelets failed to be stimulated by H2O2; however, the addition of small amounts of plasma to the buffer medium fully restored the stimulating response to H2O2. These data suggest that a plasma factor regulates the active transport of 5-HT by platelets that are oxidatively stressed.