Abstract: 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(NO_2)-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 Ca~(2+)-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 Ca~(2+)-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(NO_2)-OMe ω-nitro-L-arginine, and 1 H-[1,2,4] oxadiazo·lo HCl,N [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 Ca~(2+)-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 CaCl_2(0.5, 1, 1.5,2, 2.5, and 3 mmol/L), the Ca~(2+)-induced contraction was significantly inhibited in the osthole and phentolamine groups compared with controls(P < 0.05).After cumulative CaCl_2 was added to Ca~(2+)-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 Ca~(2+)channels in arterial smooth muscle, leading to decreased Ca~(2+) influx, and via inhibition of nitric oxide release on arterial endothelial cells.

Key words: Aorta,thoracic, Osthole, Phentolamine, Verapamil, Vasorelaxation, Calcium, Nitric oxide