Endothelium-dependent vasorelaxant and antiproliferative effects of apigenin

This study was designed to determine whether the relaxant effect of apigenin was endothelium dependent and to examine the possible antiproliferative effect of apigenin. Apigenin relaxed the phenylephrine-precontracted endothelium-intact aortic rings with IC₅₀ value of 3.7±0.5 μM and removal of a functional endothelium significantly attenuated this relaxation (IC₅₀=8.2±0.9 μM). However, apigenin did not affect the 0.1 μM phorbol 12,13-dibutyrate-induced contraction (IC₅₀=34.6±1.2 μM) within the concentration range that relaxed the phenylephrine-contracted arteries, suggesting that apigenin did not influence protein kinase C-mediated contractile mechanisms in rat aorta. Pretreatment of apigenin significantly potentiated the relaxant effect of acetylcholine on phenylephrine-induced contraction. Pretreatment with N^G-nitro-l-arginine methyl ester (l-NAME) or methylene blue reduced the relaxant effect of apigenin. Apigenin (10 μM) increased the guanosine 3′,5′-cyclic monophosphate (cGMP) content of endothelium-intact tissues. Pretreatment with l-NAME (100 μM) or removal of endothelium significantly suppressed the effect of apigenin on cGMP production. In addition, apigenin significantly inhibited [³H]thymidine incorporation into DNA of primary cultured rat aortic smooth muscle cell in a dose-dependent manner. These findings suggest that besides influx and release of Ca²⁺, nitric oxide (NO) and cGMP may account for the apigenin-induced endothelium-dependent relaxation and hypotensive activity. Both vasorelaxant and antiproliferative activities may contribute to a benefit of apigenin in the vascular system.