Cyclovirobuxine D Induced Vasodilatation Effect And Its Mechanism In Rat Thoracic Aortas

Background: Cyclovirobuxine D , an active principle extracted from plant Buxus microphylla, is used in china for the treatment angina pectoris, arrhythmias and cardiac insufficiency. It has been reported that Cyclovirobuxine D can bi-directional regulate blood pressure. In isolate porcine coronary segments, Cyclovirobuxine D significantly reduced the degree of potassium chlorid-induced contraction. However, the mechanism(s) of action has not been fully elucidated.Aims: The objective of the present study were to examine the effect of Cyclovirobuxine D on contraction or relaxation of isolated rat aortic rings and to elucidate the underlying mechanism.Methods: Male Wistar rats were stunned by a sharp blow to the neck and quickly decapitated. The thoracic aorta was removed and carefully cleaned of adherent fat and connective tissue, then cut into strips (approx. 2 mm ³mm length). In some experiments, the endothelium was removed by gently rubbing with a pare of forceps. Aorta rings with or without endothelium were prepared in organ bath tomeasure the vascular tone. Relaxation responses to cyclovirobuxine D are reported here as a percentage relaxation from the precontracted state. Isometric contractions induced by PE, KCl, caffeine were measured, and calculated as the percentage of the maximal tension induced by KCl. Data were expressed as mean ± sd. The statistical analysis of difference was performed with ANOVA-test for comparisons between two groups. Differences were accepted as statistically significant at P values < 0.05.Results: In aorta rings precontracted with phenylephrine or potassium chloride, cyclorvirobuxine D produced concentration-dependent relaxation in both endothelium-intact and denuded rings (p<0.05).Cyclorvirobuxine D induced significantly greater relaxation in intact rings than in those denuded of endothelium, regardless of whether they were precontracted with phenylephrine or KCl (P<0.05). There were no significant difference between acetic acid groups and controls. In intact rings, pretreatment with L-NAME or indomethacin reduced the degree of cyclorvirobuxine D-induced relaxation.Exposure of endothelium-denuded aortic arteries to cyclovirobuxine D 3.2 X 10^-4 mol L^-1 caused a significant reduction in the contractile response to PE (.P<0.05). Exposure of endothelium-denuded aortic rings to cyclovirobuxine D 6.4 X10^-4 mol L^-1 caused a significant reduction in the contractile response to KCl (P<0.05).Cumulative application of Ca²+ induced a concentration-dependent contraction in rat aortic rings without endothelium in Ca²+ free K-H solution pre-depolarized with KCl. The presence of cyclovirobuxine D decreased the contractile response to CaCl₂. Contraction elicited by PE either via intracellular Ca²+ release or Ca²+ influx through receptor-operated Ca ⁺ channels were depressed. In Ca²+-free media, PE induced contraction were inhibited in a dose-dependent manner by cyclorvirobuxine D. But cyclorvirobuxine D can not inhibit caffeine induced contraction.Conclusion: Cyclovirobuxine D could produce a vasodilatation in KCl or PE pre-contracted aorta rings. It vasodilates via both endothelium-dependent and -independent mechanisms. The endothelium-dependent effect is probably via prostacyclin and NO release from the endothelium, whereas endothelium-independent vasodilation probably accurs via the suppression of voltage-sensitive Ca2+ channels and inhibition of IP3-sensitive Ca2+ release in vascular smooth muscles.