In vitro adrenaline and collagen-induced mobilization of platelet calcium and its inhibition by naftopidil, doxazosin and nifedipine

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.‘fn2Results In the presence of fibrinogen (300 μg ml⁻¹ ) both collagen (5 μg ml⁻¹ ) and adrenaline (16 μm ) stimulated the aggregation of washed platelets. Collagen induced a transient rise (+4.97±0.63 μm ) in platelet Ca²⁺ concentration, [Ca²⁺]_i, as measured using the photoprotein aequorin, which coincided with the onset of aggregation. Adrenaline induced a smaller rise (+3.6±0.96 μm ) which, however, occurred after the onset of aggregation. Naftopidil, an α₁-adrenoreceptor antagonist produced a concentration-dependent inhibition of collagen-induced Ca²⁺ mobilization, maximum inhibition (22.9±4%, P<0.05) occurring with 40 μm naftopidil. The inhibition of Ca²⁺ mobilization was not reflected by a concentration-dependent inhibition of platelet aggregation, although 40 μm naftopidil produced statistically significant inhibition (23.3±11.7%, P<0.05). The adrenaline-induced rise in [Ca²⁺]_i was inhibited dose dependently by naftopidil (e.g. 40 μm naftopidil, 100±0%, P<0.05), as was aggregation (40 μm naftopidil, 100±0%, P<0.05). Doxazosin, another α₁-adrenoreceptor blocker, inhibited Ca²⁺ mobilization induced by collagen to similar extents as for naftopidil (30 μm doxazosin, 17.4±2.5%, P<0.05), but did not inhibit platelet aggregation. It also inhibited the adrenaline-induced rise in [Ca²⁺]_i in a concentration-dependent manner (30 μm doxazosin, 37.6±13.7%, P<0.05), significant inhibitions of platelet aggregation also being produced (30 μm, 49.6±17.2%, P<0.05). As expected, the calcium channel blocker nifedipine produced concentration-dependent inhibitions of both collagen-induced Ca²⁺ mobilization (e.g. 28 μm nifedipine, 47.8±2.7%, P<0.05) and aggregation (28 μm, 55.1±9.2%, P<0.05).Conclusions These data indicate that the α₁-adrenoreceptor blockers, naftopidil and doxazosin, inhibit Ca²⁺ mobilization, this mechanism being possibly the means whereby these drugs inhibit platelet aggregation.