Icilin activates the δ-subunit of the human epithelial Na+ channel

The amiloride-sensitive epithelial Na+ channel ( ENaC) regulates Na+ homeostasis in cells and across epithelia. Four homologous ENaC subunits (alpha, beta, gamma, and delta) have been isolated in mammals. The chemical activators acting on ENaC, however, are largely unknown. More recently, we have found that capsazepine activates human ENaC delta (hENaC delta), which is mainly expressed in the brain. In addition, here we show that icilin, which is a tetrahydropyrimidine-2-one derivative unrelated structurally to capsazepine, markedly enhanced the activity of hENaC delta beta gamma heteromultimer expressed in Xenopus laevis oocytes. The inward currents at a holding potential of - 60 mV in hENaC delta beta gamma-expressing oocytes were increased by the application of icilin in a concentration-dependent manner with an EC50 value of 33 mu M. The icilin-elicited current was mostly abolished by the addition of 100 mu M amiloride or by the removal of external Na+. Homomeric hENaC delta was also significantly activated by icilin, whereas hENaC alpha activity was not affected by icilin, and icilin caused a slight inhibition of the hENaC alpha beta gamma current. Furthermore, icilin acted together with protons or capsazepine on hENaC delta beta gamma. These findings identify icilin as a novel chemical activator of ENaC delta, providing us with a lead compound for drug development in the degenerin/ENaC superfamily.