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OBJECTIVE:To investigate the effect of osthole on isolated thoracic aortic rings,and to determine the potential mechanism of action.METHODS:Thoracic aortas were isolated from Wistar rats,and were suspended in tissue organ chambers for vascular tension measurement.The effect of cumulative osthole (10-9,10-8,10-7,10-6,and 10-5 mol/L) on endothelium-intact and endothelium-denuded thoracic aortic rings pre-contracted with phenylephrine (PE,10-6 mol/L) or KCl (6 × 10-2 mol/L) was recorded.Histomorphological changes of thoracic aorta were analyzed by hematoxylin-eosin.The effects of different osthole concentrations on endothelium-intact aortic rings,which were pre-inhibited with the non-selective nitric oxide synthase inhibitor L-Arg(NO2)-OMe.HCl (3 × 10-4 mol/L),endothelium-derived nitric oxide synthase inhibitor Nω-nitro-L-arginine (3 × 10-4 mol/L),guanylate cyclase inhibitor 1 H-[1,2,4] oxadiazolo [4,3-α]quinoxaline-1-one (10-5 mol/L),cyclooxygenase inhibitor indometacin (10-5 mol/L),and the Ca2+-activated potassium channel inhibitor tetraethylammonium nitrate (10-5 mol/L),and then contracted with PE,were examined.Aortic rings incubated with osthole (10-5 mol/L),phentolamine (10-5 mol/L),or verapamil (10-5 mol/L) in Ca2+-free Krebs-Henseleit solution (KHS) were stimulated with PE or KCl.RESULTS:There was a dose-dependent increase in vasorelaxation of isolated thoracic aortic rings (both with and without endothelium) with increasing osthole concentration.Hematoxylin-eosin staining showed that osthole significantly improved thoracic aorta ring morphology.Compared with the control group,there were also significant differences after incubation with L-Arg(NO2)-OMe · HCl,Nω-nitro-L-arginine,and 1H-[1,2,4] oxadiazolo [4,3-α] quinoxaline-1-one (P ＜ 0.05 for all).The relaxation rate of the rings in the osthole group incubated with indometacin and tetraethylammonium nitrate were similar to controls.In Ca2+-free KHS,the PE-induced contraction was similar between the osthole (4.37％ + 0.41％) and control (4.21％ + 1.33％)groups.However,after cumulative CaCl2 (0.5,1,1.5,2,2.5,and 3 mmol/L),the Ca2+-induced contraction was significantly inhibited in the osthole and phentolamine groups compared with controls (P ＜ 0.05).After cumulative CaCl2 was added to Ca2+-free KHS (high K+ concentration),the contraction rate was significantly higher than both of the control and the osthole groups (P ＜ 0.05).The contraction rate in the osthole group was higher than the verapamil group (P ＜ 0.05).CONCLUSION:Osthole has a vasorelaxant effect on isolated rat thoracic aortic rings,via inhibition of both receptor-operated and voltage-dependent Ca2+ channels in arterial smooth muscle,leading to decreased Ca2+ influx,and via inhibition of nitric oxide release on arterial endothelial cells.