Calcium channel blocking activity of calycosin, a major active component of Astragali Radix, on rat aorta

Aim: To investigate the vasoactivity of calycosin, a major active component of Astragali Radix. Methods: Experiments were performed on isolated rat thoracic aortic rings pre-contracted with phenylephrine (PHE) or KCl. Results: Calycosin produced a concentration-dependent relaxation on the tissue pre-contracted using PHE with 4.46 +/- 0.13 of pD(2) and 95.85%+/- 2.67% of E-max; or using KCl with 4.27 +/- 0.05 of pD(2) and 99.06%+/- 2.15% of E-max, and displaced downwards the concentration-response curves of aortic rings to PHE or KCl. The relaxant effect of calycosin on denuded endothelium aortic rings was the same as on intact endothelium aortic rings, and its vasorelaxant effect was not influenced by L-NAME or indomethacin. In Ca2+-free solution, calycosin (30 mu mol/L) did not have an effect on PHE (1 x 10(-6) mol/L)-induced aortic ring contraction. The effects of calycosin and nifedipine where somewhat different; calycosin decreased aortic ring contractions induced by the two agonists, but nifedipine displayed a more potent inhibitory effect on KCl-induced contractions than on PHE-induced contractions, and the vascular relaxing effects of calycosin and nifidipine were additive on PHE-induced contraction but not KCl-induced. Conclusion: Calycosin is a vasorelaxant. Its action is endothelium-independent and is unrelated to intracellular Ca2+ release. It is a noncompetitive Ca2+ channel blocker. The effect of calycosin on Ca2+ channel blockade may be different from that of dihydropyridines. This study demonstrated a novel pharmacological activity of calycosin, and supplied a theoretic foundation for Astragali Radix application.