Capsazepine is a novel activator of the δ subunit of the human epithelial Na+ channel

Abstract

The amiloride-sensitive epithelial Na+ channel (ENaC) regulates Na+ homeostasis into cells and across epithelia. So far, four homologous subunits of mammalian ENaC have been isolated and are denoted as α, β, γ, and δ. The chemical agents acting on ENaC are, however, largely unknown, except for amiloride and benzamil as ENaC inhibitors. In particular, there are no agonists currently known that are selective for ENaCδ, which is mainly expressed in the brain. Here we demonstrate that capsazepine, a competitive antagonist for transient receptor potential vanilloid subfamily 1, potentiates the activity of human ENaCδβγ (hENaCδβγ) heteromultimer expressed in Xenopus oocytes. The inward currents at a holding potential of -60 mV in hENaCδβγ- expressing oocytes were markedly enhanced by the application of capsazepine (≥1 μM), and the capsazepine-induced current was mostly abolished by the addition of 100 μM amiloride. The stimulatory effects of capsazepine on the inward current were concentration-dependent with an EC50 value of 8 μM. Neither the application of other vanilloid compounds (capsaicin, resiniferatoxin, and olvanil) nor a structurally related compound (dopamine) modulated the inward current. Although hENaCδ homomer was also significantly activated by capsazepine, unexpectedly, capsazepine had no effect on hENaCα and caused a slight decrease on the hENaCαβγ current. In conclusion, capsazepine acts on ENaCδ and acts together with protons. Other vanilloids tested do not have any effect. These findings identify capsazepine as the first known chemical activator of ENaCδ.