Vasorelaxation by Samhwangsasim-tang, an herb medicine, is associated with decreased phosphorylation of the myosin phosphatase target subunit

Samhwangsasim-tang (SST) is a widely used herbal medicine with vasodilatory actions in oriental countries. We hypothesized that SST modulates vascular contractility by decreasing phosphorylation of the myosin phosphatase target subunit. Rat aortic ring preparations were mounted in organ baths and subjected to contractions or relaxations. Phosphorylation of 20 kDa myosin light chains (MLC₂₀) and MYPT1, a target subunit of myosin phosphate 1, were examined with immunoblots. SST relaxed aortic ring preparations precontracted with phenylephrine whether endothelium was intact or denuded. Treatment of aortic rings with N^ω-nitro-l-arginine methyl ester (l-NAME), an inhibitor of endothelial nitric oxide synthase or methylene blue, an inhibitor of guanylyl cyclase, did not affect the relaxing action of SST. Furthermore, SST inhibited vascular contractions induced by NaF or phenylephrine, but not by phorbol dibutyrate. SST also decreased vascular tension precontracted by 8.0 mmol/L NaF or 1.0 μmol/L phenylephrine, but not by 1.0 μmol/L phorbol dibutyrate. In vascular strips, SST decreased the phosphorylation level of both MLC₂₀ and MYPT1 induced by 8.0 mmol/L NaF. In conclusion, SST inhibited vascular contraction by decreasing phosphorylation of the myosin phosphatase target subunit.