Icilin activates the delta-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 ENaCdelta (hENaCdelta), 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 hENaCdeltabetagamma heteromultimer expressed in Xenopus laevis oocytes. The inward currents at a holding potential of -60 mV in hENaCdeltabetagamma-expressing oocytes were increased by the application of icilin in a concentration-dependent manner with an EC(50) value of 33 microM. The icilin-elicited current was mostly abolished by the addition of 100 microM amiloride or by the removal of external Na(+). Homomeric hENaCdelta was also significantly activated by icilin, whereas hENaCalpha activity was not affected by icilin, and icilin caused a slight inhibition of the hENaCalphabetagamma current. Furthermore, icilin acted together with protons or capsazepine on hENaCdeltabetagamma. These findings identify icilin as a novel chemical activator of ENaCdelta, providing us with a lead compound for drug development in the degenerin/ENaC superfamily.