Epidemiological studies have shown that the intake of dietary flavonoids is inversely associated with mortality from coronary heart disease. Many in-vivo studies have demonstrated this protective action: quercetin relaxes the vascular muscles, reduces blood pressure and improves the endothelial function. A Spanish study conducted by the University of Granade and lead by Sanchez M, analyzed the effect of quercetin and its methylated metabolite isorhamnetin on the endothelial function of rats after treatment with angiotensin II, which is a potent direct vasoconstrictor, constricting arteries and veins and increasing blood pressure . Both quercetin and isorhamnetin inhibited the adverse effects of angiotensin II. These phytochemicals prevented angiotensin-induced endothelial dysfunction by reducing the increased production of reactive oxygen species, resulting in increased nitric oxide bioavailability.
HDL protects from arteriosclerosis by inhibiting oxidation, inflammation, activation of the endothelium, coagulation or platelet aggregation. The oxidation of HDL reduces this protective action and therefore increases the risk of arteriosclerosis. A team of researchers, lead by Liu R of the Sichuan University in China, tested the inhibitory effects of two flavonoids, isorhamnetin and hesperidin, on the HDL oxidation induced by copper ions . Their in-vitro test showed that pre-treatment with isorhamnetin or hesperidin reduced the level of thiobarbituric acid reactive substances and carbonyls. Isorhamnetin showed the strongest protective action of the two phytochemicals. Another Chinese study concluded isorhamnetin has protective effects on cultured endothelial cells . Bao M and colleagues found that this protective effect was obtained through the strong antioxidant activity of isorhamnetin and its modulation of different pathways.
A study of University Complutense of Madrid, lead by Ibarra M, concluded that both quercetin and isorhamnetin are candidates to explain the reduction of blood pressure and vascular protective effects of quercetin observed in animal models of hypertension . Iabarra and co-workers tested the effect of both phytochemicals on thoracic aorta, iliac artery and on the isolated perfused mesenteric resistance vascular bed from spontaneously hypertensive rats. The found that isorhamnetin reduced the endothelium-dependent contractile responses induced by acetylcholine.
 Quercetin and isorhamnetin prevent endothelial dysfunction, superoxide production, and overexpression of p47phox induced by angiotensin II in rat aorta. Journal of Nutrition. 2007 Apr;137(4):910-5.
 Inhibitory effect of isorhamnetin and hesperidin on the oxidation of high-density lipoproteins (HDL) induced by Cu2+.
Sichuan Da Xue Xue Bao Yi Xue Ban. 2007 Nov;38(6):961-4.
 Isorhamnetin prevent endothelial cell injuries from oxidized LDL via activation of p38MAPK. European Journal of Pharmacology, 2006 October 10;547(1-3):22-30.
 Effects of the flavonoid quercetin and its methylated metabolite isorhamnetin in isolated arteries from spontaneously hypertensive rats. Planta Medica. 2003 Nov;69(11):995-1000.