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).(ABSTRACT TRUNCATED AT 250 WORDS)

We examined the effects of the alpha(1)-adrenoreceptor blockers naftopidil and doxazosin and the Ca(2+) antagonist nifedipine on platelet function with reference to stimulus-induced thromboxane (TxB(2)) generation and platelet-derived growth factor (PDGF) efflux. Collagen (5 micro g/ml) caused a 12.5-fold increase in TxB(2) generation, from a basal level of 7.69 +/- 1.28 ng/10(8) platelets to 96.34 +/- 13.37 ng/10(8) platelets (P<0.001). Adrenaline (16 micro M) increased TxB(2) production 3-fold from 2.44 +/- 0.61 to 8.02 +/- 1.08 ng/10(8) platelets (P<0.01). Adrenaline-induced TxB(2) generation was inhibited 42.5 +/- 10.3% (P<0.05) and 81.8 +/- 7.5% (P<0.05) by 10 and 40micro M naftopidil, respectively. Collagen-stimulated TxB(2) generation was inhibited 59.5 +/- 9.2% (P<0.01) by 40 micro M naftopidil and 53.7 +/- 11.3% (P<0.01) by 28 micro M nifedipine. Doxazosin (7.5 and 30 micro M) did not influence adrenaline- or collagen-induced TxB(2) synthesis. Collagen increased PDGF efflux from 1.17 +/- 0.39 to 4.25 +/- 0.51 ng/10(8) platelets (P<0.01), whilst adrenaline raised concentrations from 1.08 +/- 0.19 to 5.37 + 1.02 ng/10(8) platelets (P<0.01). Naftopidil had no effect on collagen-induced PDGF release. Adrenaline-stimulated PDGF efflux was, however, inhibited 82.9 +/- 13.7% (P<0.001) and 125.7 +/- 16.3% (P<0.001) by 10 and 40 micro M naftopidil, respectively. Doxazosin (30 micro M) inhibited adrenaline-induced PDGF release by 70.3 +/- 31.5% (P<0.05), whilst nifedipine (28 micro M) had no effect on collagen-stimulated release. We conclude that naftopidil, like nifedipine, may block stimulated TxB(2) generation via inhibition of phospholipase A(2), the Ca(2+)-dependent, rate-limiting enzyme in thromboxane synthesis. Although adrenaline-induced PDGF release was inhibited by naftopidil and doxazosin, collagen-induced release was unaffected by either alpha(1)-adrenoreceptor blocker or nifedipine, indicating that platelet alpha-granular release is not dependent on Ca(2+) mobilisation or thromboxane generation. Thus, the effects of these drugs on PDGF release may be mediated through alternative cellular signalling mechanisms.

Aims The aim of the study was to obtain further information regarding the modes of action of doxazosin, naftopidil and nifedipine on platelet function. Methods We conducted an in vitro study of drug influences on adrenaline and collagen-induced mobilization of platelet calcium. Results In the presence of fibrinogen (300 mg ml −1 ) both collagen (5 mg ml −1 ) and Conclusions These data indicate that the a 1 -adrenoreceptor blockers, naftopidil and doxazosin, inhibit Ca 2+ mobilization, this mechanism being possibly the means whereby these drugs inhibit platelet aggregation.

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.