OBJECTIVETo determine whether matrine, a kind of traditional Chinese medicinal alkaloid, can relax the aortic smooth muscles isolated from guinea pigs and to investigate the mechanism of its relaxant effects.

METHODSPhenylephrine or potassium chloride concentration-dependent relaxation response of aortic smooth muscles to matrine was studied in the precontracted guinea pigs.

RESULTSMatrine (1 x 10(-4) mol/L -3.3 x 10(3) mol/L) relaxed the endothelium-denuded aortic rings pre-contracted sub-maximally with phenylephrine, in a concentration-dependent manner, and its pre-incubation (3.3 x 10(-3) mol/L) produced a significant rightward shift in the phenylephrine dose-response curve, but had no effects on the potassium chloride-induced contraction. The anti-contractile effect of matrine was not reduced by the highly selective ATP-dependent K+ channel blocker glibenclamide (10(-5) mol/L), either by the non-selective K+ channel blocker tetraethylammonium (10(-3) mol/L), or by the beta-antagonist propranolol (10(-5) mol/L). In either "normal" or "Ca(2+)-free" bathing medium, the phenylephrine-induced contraction was attenuated by matrine (3.3 x 10(-3) mol/L), indicating that the vasorelaxation was due to inhibition of intracellular and extracellular Ca2+ mobilization.

CONCLUSIONMatrine inhibits phenylephrine-induced contractions by inhibiting activation of alpha-adrenoceptor and interfering with the release of intracellular Ca2+ and the influx of extracellular Ca2+.