Effects of magnolol on vascular contraction in rat aortic rings

1. Magnolol (5,5′‐diallyl‐2,2′‐dihydroxybiphenyl) is a major phenolic compound purified from Magnolia officinalis. The aim of the present study was to elucidate the effects of magnolol on vascular contractions. 2. Rat aortic rings were mounted in organ baths. Magnolol was added cumulatively (0.3–30 μmol/L) to elicit relaxation in endothelium‐intact and ‐denuded rat aortic rings precontracted with U46619 (30 nmol/L, 20 min), NaF (8.0 mmol/L, 40 min), phenylephrine (1.0 or 0.1 μmol/L, 15 min) or phorbol‐12,13‐dibutyrate (PDBu, 0.3 or 0.1 μmol/L, 40 min). In separate experiments, cumulative concentration?response curves were obtained for NaF (2.0–12 mmol/L), U46619 (1.0 nmol/L?1.0 μmol/L) or PDBu (1.0 nmol/L?1.0 μmol/L) after pretreatment with either magnolol or vehicle for 30 min in endothelium‐denuded aortic rings. After completion of the functional study, we measured the amount of guanosine triphosphate (GTP) RhoA by using a G‐LISA RhoA Activation Assay, as well as the phosphorylation of 20 kDa myosin light chain (MLC₂₀), myosin phosphatase‐targeting subunit 1 (MYPT1) and protein kinase C‐potentiated inhibitory protein for heterotrimeric myosin light‐chain phosphatase of 17 kDa (CPI‐17) by immunobloting. 3. Magnolol (0.3–30 μmol/L) reduced vascular tension induced by the thromboxane A₂ agonist U46619 (30 nmol/L), sodium fluoride (NaF) (8.0 mmol/L) and the α₁‐adrenoceptor agonist phenylephrine (1.0 or 0.1 μmol/L) in both endothelium‐intact and ‐denuded rings. The magnitude of the relaxation effects of magnolol on the contraction induced by each of the drugs were similar. The magnitude of the effect of magnolol in endothelium‐intact and ‐denuded rings were similar. 4. Pretreatment of rat aortic rings with 1.0, 3.0 or 10 μmol/L magnolol for 30 min dose‐dependently inhibited the maximum response on the concentration–response curves to NaF and U46619, but not to phorbol‐12,13‐dibutyrate (PDBu). 5. Magnolol (3.0 or 10 μmol/L) decreased RhoA activation, as well as the phosphorylation of MLC₂₀, MYPT1_Thr855 and CPI‐17_Thr38 induced by either 8.0 mmol/L NaF or 30 nmol/L U46619. In contrast, magnolol did not affect PDBu (0.1 μmol/L)‐induced phosphorylation of CPI‐17_Thr38. 6. In conclusion, magnolol reduces vascular contraction by inhibiting the RhoA/Rho kinase pathway in endothelium‐denuded rat aorta.