Effect of magnolol on coronary vascular resistance in rabbits: Measurementwith pulsed Doppler velocimetry

Background and purpose: Magnolol is an active component purified from Magno lia officinalis that has been reported to protect the myocardium against in farction and reperfusion injury. The purpose of this study was to investiga te the effect of magnolol on the coronary circulation and to determine whet her a change in coronary vascular resistance could be the mechanism underly ing its myocardial protective effect. Methods: Male New Zealand white rabbits were anesthetized. A 3-mm suction-t ype pulsed Doppler velocimetry probe was applied to the proximal part of th e left anterior descending coronary artery after median sternotomy. The 39 rabbits received intravenous injection of either vehicle (n = 5), magnolol (10(-6) g/kg, n = 6; 10(-5) g/kg, n = 5; 10(-4) g/kg, n = 5), or nitroglyce rin (10-5 g/kg, n = 6; 10(-6) g/kg, n = 6; 10-5 g/kg, n = 6). The effects o f magnolol and nitroglycerin on coronary vascular resistance were compared. Results: Magnolol did not change blood pressure or coronary blood flow velo city. However, at a dose of 10(-4) g/kg, it decreased corollary vascular re sistance significantly more than vehicle (88 +/- 1% vs 95 +/- 1% of baselin e coronary vascular resistance, p < 0.001). Nitroglycerin increased coronar y blood flow velocity and decreased coronary vascular resistance in a dose- dependent manner (p < 0.01). Conclusions: Magnolol reduced coronary vascular resistance in anesthetized, open-chest rabbits only at a high concentration. Its effect was modest com pared with that of nitroglycerin. Since magnolol protects the myocardium at relatively low doses, coronary vasodilatation is unlikely to be the underl ying mechanism responsible for its myocardial protective effects.