Endothelium-independent vasorelaxation by leonurine, a plant alkaloid purified from Chinese motherwort.

Leonurine, a plant alkaloid present in Chinese motherwort, induced concentration- dependent and endothelium-independent relaxation of phenylephrine (PE)- pretreated rat aortic arterial rings. The IC50 values for leonurine were 86.4+/-10.4 and 85.9+/-17.2 microM in the presence and absence of endothelium respectively. It inhibited the responses of aortic smooth muscle to PE in Ca2+ free medium containing 100 microM EGTA, suggesting a possible action on the release of intracellular Ca2+. Leonurine is not a specific alpha-adrenoceptor blocker, since it also caused a concentration-dependent inhibition of vascular contractile responses to KCl with an IC50 value of 96.4+/-13.4 microM, suggesting that leonurine also blocks the L-type Ca2+-channel. In addition, leonurine relaxed the aortic contraction induced by prostaglandin F2alpha (PGF2alpha). These inhibitory effects of leonurine were reversible and did not affect the resting tension. In conclusion, these findings suggest that leonurine is an effective inhibitor of vascular smooth tone, probably acting by inhibiting the Ca2+ influx and the release of intracellular Ca2+.