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 KCl or PE were observed with organ bath technique.Dose-effect curves of CaCl2 were recorded by organ bath technique.The concentration of intracellular Ca 2+ ([Ca 2+ ]i)increased by PE,KCI,and caffeine in the presence of ABML was determined using Ca 2+ sensitive fluorescence indicator Fura-2/AM loaded thoracic aorta vascular smooth muscle (VSM)cells of rats.Results In aorta rings precontracted with PE and KCl,ABML produced concentration- dependent 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 CaCl2,and shifted the cumulative concentration-response curves to right.ABML could significantly inhibit the extracellular Ca 2+ influx induced by PE and KCl under[Ca 2+ ]0 of 1.5 mmol/L,with inhibitory ratios of 40.2%and 49.9%,respectively.In the case of Ca 2+ -free,ABML could significantly inhibit the intracellular Ca 2+ release induced by PE,with inhibitory ratio of 72.4%.Conclusion ABML relaxes thoracic aorta VSM cells by suppressing influx of extracellular Ca 2+ via voltage-dependent Ca 2+ channel and receptor-operated Ca 2+ channel. 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 contraction on the contraction of isolated thoracic aorta rings (with and without endothelium) precontracted with KCl or PE were observed with organ bath technique. Need-effect curves of CaCl2 were recorded by organ bath technique. The concentration of intracellular Ca 2+ ([Ca 2+] i) increased by PE, KCI, and caffeine in the presence of ABML was determined using Ca 2+ sensitive fluorescence indicator Fura-2 / AM loaded thoracic aorta vascular smooth muscle (VSM) cells of rats. Results In aorta rings precontracted with PE and KCl, ABML produced concentration- dependent relaxation in both intact and denuded endothelium ring groups. There was no difference in the inhibition of contracti on between the intact and denuded endothelium ring groups at the same concentration. Expose of isolated thoracic aorta rings to ABML led to a significant reduction in the contracting response induced by CaCl2, and shifted the cumulative concentration-response curves to right. the extracellular Ca 2+ influx induced by PE and KCl under [Ca 2+] 0 of 1.5 mmol / L with inhibitory ratios of 40.2% and 49.9%, respectively.In the case of Ca 2+ -free, ABML could significantly inhibit the intracellular Ca 2+ release induced by PE, with inhibitory ratio of 72.4% .Conclusion ABML relaxes thoracic aorta VSM cells by suppressed influx of extracellular Ca 2+ via voltage-dependent Ca 2+ channel and receptor-operated Ca 2+ channel.