Quercetin and NPPB-induced diminution of aldosterone action on Na+ absorption and ENaC expression in renal epithelium

In renal epithelial A6 cells, aldosterone applied for 24 h increased the transepithelial Cl− secretion over 30-fold due to activation of the Na+/K+/2Cl− cotransporter and stimulated the transepithelial Na+ absorption, activity of epithelial Na+ channel (ENaC), and α-ENaC mRNA expression. The stimulatory action of aldosterone on the transepithelial Na+ absorption, ENaC activity, and α-ENaC mRNA expression was diminished by 24 h-pretreatment with quercetin (an activator of Na+/K+/2Cl− cotransporter participating in Cl− entry into the cytosolic space) or 5-nitro 2-(3-phenylpropylamino)benzoate (NPPB) (a blocker of Cl− channel participating in Cl− release from the cytosolic space), while 24 h-pretreatment with bumetanide (a blocker of Na+/K+/2Cl− cotransporter) enhanced the stimulatory action of aldosterone on transepithelial Na+ absorption. On the other hand, under the basal (aldosterone-unstimulated) condition, quercetin, NPPB or bumetanide had no effect on transepithelial Na+ absorption, activity of ENaC or α-ENaC mRNA expression. These observations suggest that although aldosterone shows overall its stimulatory action on ENaC (transepithelial Na+ transport), aldosterone has an inhibitory action on ENaC (transepithelial Na+ transport) via activation of the Na+/K+/2Cl− cotransporter, and that modification of activity of Cl− transporter/channel participating in the transepithelial Cl− secretion influences the aldosterone-stimulated ENaC (transepithelial Na+ transport).