Influence of the alpha-adrenoreceptor naftopidil and doxazosin, on adrenaline-induced serotonin platelets: comparison with the antagonists, collagen and efflux by human effects of nifedipine.

Collagen (5 microg/ml) stimulation of washed platelets increased endogenous serotonin (5-HT) release to the medium from 13.88 1.39 to 188.67 26.37 pmol/108 platelets ( P < 0.001). Adrenaline (16 microM) also increased 5-HT release, from 11.0 1.46 to 110.6 29.9 pmol/108 platelets ( P < 0.02). Naftopidil enhanced collagen-induced 5-HT efflux; significant increases occurred with 2 microM (+71.6%, P < 0.01), 10 microM (+89.1%, P < 0.01) and 40 microM (+69.7%, P < 0.01). With 0.4 muM and 2 microM naftopidil, adrenaline-induced 5-HT release was enhanced, albeit non-significantly, whilst with 10 microM and 40 muM naftopidil release was reduced (40 microM,-58.5%, P < 0.05). Doxazosin increased collagen-induced 5-HT release, significant increases being recorded with 7.5 microM (+81.7%, P < 0.05) and 30 microM (+78.4%, P < 0.05). Adrenaline-induced 5-HT release was also increased by doxazosin, but not significantly. Collagen-stimulated 5-HT release was inhibited by nifedipine (7 microM,-38.8%, P < 0.05; 28 microM, -61.2%, P < 0.001). These data suggest that the-antagonists, naftopidil and doxazosin, and the Ca2+ channel blocker, nifedipine, influence agonist-induced platelet 5-HT release through different mechanisms. Thus naftopidil and doxazosin may possess 5-HT transporter-blocking activity. The observation that naftopidil inhibited, adrenaline-induced 5-HT release may indicate that naftopidil also inhibits adrenaline uptake and exchange with dense granular 5-HT, with consequent inhibition of 5-HT release and platelet aggregation. The data obtained with nifedipine are consistent with 5-HT release being reduced as a result of its inhibitory action on platelet Ca2+ mobilisation.