Quercetin-induced induction of the NO/cGMP pathway depends on Ca2+-activated K+ channel-induced hyperpolarization-mediated Ca2+-entry into cultured human endothelial cells

Quercetin is one of the dietary-derived flavonoids that are held responsible for the beneficial effects of red wine drinking in coronary artery disease known as the "French paradox". We examined whether quercetin modulates endothelial function by influencing Ca2+-activated K+ channels with large conductance (BKCa) in cultured human endothelial cells. Membrane potential and intracellular Ca2+ concentrations of cultured human endothelial cells derived from umbilical cord veins (HUVEC) were measured using the fluorescence dyes DiBAC, and FURA-2, respectively. NO production was examined using a cGMP radioimmunoassay. HUVEC proliferation was analyzed by cell counts and thymidine incorporation. A dose-dependent hyperpolarization of HUVEC was recorded when quercetin was added (5 - 100 mu mol/L). The maximum effect (50 mu mol/L) was significantly reduced by the addition of the highly selective BKCa inhibitor iberiotoxin (100 nmol/L), but not by blockers of other Ca2+-activated K+ channels (n = 30; p < 0.05). This BKCa-induced hyperpolarization caused a transmembrane Ca2+ influx, because the quercetin-induced increase of intracellular Call was blocked by iberiotoxin, or by applying 2-aminoethoxydiphenylborate (100 mu mol/L) - an inhibitor of capacitative Ca2+ entry (n = 30; p < 0.05). Quercetin-induced cGMP levels were significantly reduced by the eNOS-inhibitor 1-NMMA (300 mu mol/L), and by iberiotoxin (n = 10; p < 0.05). Endothelial proliferation was significantly reduced by 56% when cells were incubated with quercetin (n = 12; p < 0.05). This effect was due to the increased NO production, because it was reversed when the cells were treated with a combination of quercetin and 1-NMMA. In conclusion quercetin improves endothelial dysfunction by increasing NO synthesis involving BKCa-dependent membrane hyperpolarization-induced capacitative Ca2+ entry. Increased NO production is responsible for the quercetin-dependent inhibition of endothelial proliferation.