The potentiation of adrenaline‐induced in vitro platelet aggregation by ADP, collagen and serotonin and its inhibition by naftopidil and doxazosin in normal human subjects

1. Aggregation in platelet‐rich plasma from normotensive men was induced by adrenaline (0.25‐16 microM), ADP (0.25‐16 microM), collagen (0.25‐8 micrograms ml‐1) or serotonin (10 microM) alone, or by previously sub‐threshold concentrations of adrenaline (0.03‐1 microM) in combination with sub‐threshold concentrations of serotonin (2.5 microM), ADP (0.5 microM) or collagen (0.125 micrograms ml‐1). The effects of the alpha 1‐adrenoceptor blockers naftopidil and doxazosin on platelet aggregation were investigated. 2. The dose‐response curves for collagen and ADP were unaffected by either drug. However, naftopidil (40 microM) inhibited serotonin‐induced platelet aggregation (23.9%, 95% confidence interval (CI) 10.7 to 37.1%; P < 0.01) and caused a slight shift to the right of the adrenaline dose‐response curve with a mean increase in the EC50 value of 0.5 microM (95% CI 0.07 to 0.93 microM; P < 0.05). Doxazosin had no effect on serotonin or adrenaline‐induced aggregation. 3. A marked potentiation of the aggregation induced by subthreshold concentrations of adrenaline resulted from the prior addition of low concentrations of ADP, collagen or serotonin. 4. These potentiated responses were inhibited in a dose‐ dependent manner by naftopidil and to a lesser extent doxazosin. The maximum inhibitions (%) produced by naftopidil (40 microM) on the responses of adrenaline potentiated by ADP were 58.3% (95% CI 36.8 to 79.8%; P < 0.001), serotonin 58.9% (95% CI 40.0 to 77.8%; P < 0.001), and collagen 70.9% (95% CI 52.5 to 89.3%; P < 0.001).