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Objective To investigate the effects of total alkaloids in Buxus microphylla leaves (ABML) on isolated rats thoracic aorta rings,and then to explore the possible mechanisms underlying the effects.Methods Thoracic aortas of Wistar rats were isolated,removed,and mounted onto an organ bath.The effects of ABML at different concentration on the contraction of isolated thoracic aorta rings (with and without endothelium) precontracted with KC1 or PE were observed with organ bath technique.Dose-effect curves of CaCl2 were recorded by organ bath technique.The concentration of intracellular Ca2+ ([Ca2+]i) increased by PE,KCI,and caffeine in the presence of ABML was determined using Ca2+ sensitive fluorescence indicator Fura-2/AM loaded thoracic aorta vascular smooth muscle (VSM) cells of rats.Results In aorta rings precontracted with PE and KCI,ABML produced concentrationdependent relaxation in both intact and denuded endothelium ring groups.There was no difference in the inhibition of contraction between the intact and denuded endothelium ring groups at the same concentration.Exposure of isolated thoracic aorta rings to ABML led to a significant reduction in the contracting response induced by CaCI2,and shifted the cumulative concentration-response curves to right.ABML could significantly inhibit the extracellular Ca2+ influx induced by PE and KCI under [Ca2+]0 of 1.5 mmol/L,with inhibitory ratios of 40.2％ and 49.9％,respectively.In the case of Ca2+-free,ABML could significantly inhibit the intracellular Ca2+ release induced by PE,with inhibitory ratio of 72.4％.Conclusion ABML relaxes thoracic aorta VSM cells by suppressing influx of extracellular Ca2+ via voltage-dependent Ca2+ channel and receptor-operated Ca2+ channel.