Flavonoids stimulate Cl conductance of human airway epithelium in vitro and in vivo

Abstract

The ability of the flavonoids genistein, apigenin, kaempferol, and quercetin to activate cystic fibrosis transmembrane conductance regulator-mediated Cl currents in human airway epithelium was investigated. We used the patch-clamp technique on single Calu-3 cells, transepithelial measurements in Calu-3 monolayers, and in vivo measurements of nasal potential difference. All flavonoids stimulated Cl currents in transepithelial experiments dose dependently. Half-maximal stimulatory concentrations were kaempferol (5.5 ± 1.7 μM) ≤ apigenin (11.2 ± 2.1 μM) ≤ genistein (13.6 ± 3.5 μM) ≤ quercetin (22.1 ± 4.5 μM). Stimulation of monolayers with forskolin significantly increased their sensitivity to flavonoids: kaempferol (2.5 ± 0.7 μM) ≤ apigenin (3.4 ± 0.9 μM) ≤ quercetin (4.1 ± 0.7 μM) ≤ genistein (6.9 ± 2.2 μM). Forskolin pretreatment significantly reduced the Hill coefficient ( n H) for all flavonoids. Control monolayers showed n H = 2.00 ± 0.21 (all flavonoids combined), and forskolin-stimulated monolayers showed n H = 1.07 ± 0.07, which was not different among the flavonoids. These data imply that the activation kinetics and the binding site(s) for flavonoids were significantly altered by forskolin stimulation. In whole cell patch-clamp experiments, maximal flavonoid-stimulated currents (percentage of forskolin-stimulated currents) were apigenin (429 ± 86%) ≥ kaempferol (318 ± 45%) ≥ genistein (258 ± 20%) = quercetin (256 ± 26%). Stimulation of the currents was caused by an increase in channel open probability. No other Cl conductances contributed significantly to the flavonoid-activated Cl currents in Calu-3 cells. In vivo, flavonoids significantly stimulated nasal potential difference by, on average, 27.8% of isoproterenol responses.