The degenerin/epithelium sodium channel (DEG/ENaC) gene family represents a class of sodium selective ion channels found in nearly all multicellular animals. However, the function and stimulus of activation of these channels is highly diverse among the different family members. To identify ancient properties of this ion channel superfamily, we have cloned the cDNA of 5 DEG/ENaC subunits from the cnidarian model organism Hydra magnipapillata. We could already show that two of the cloned subunits (HyNaC2/3) form an heteromeric ion channel in Xenopus oocytes, which is activated directly by two neuropeptides (HydraRFamides I or II) from Hydra. Besides FaNaC, a related channel from Helix aspersa, HyNaC is the only known ion peptide gated ion channel.

Now we report the characterisation of a further subunit, HyNaC 5. We found that the current amplitudes that are observed for HyNaC2/3 can be potentiated by coinjection of HyNaC5. Moreover, the apparent peptide affinity for both HydraRFamides was significantly higher for HyNaC5/2/3 (EC50: 0,25–1,5 mM) than for HyNaC2/3 (EC50: 50–150 mM). The peptide activated currents of HyNaC5/2/3 as well as HyNaC2/3 could be blocked by amiloride<phenamil< benzamil at low mM concentrations, but the potency of these antagonists was decreased for HyNaC 5/2/3. Furthermore, the pore of HyNaC2/3, which is rather unselective for monovalent cations, became more selective for Na+ after injection of HyNaC5. This selectivity for sodium is a common characteristic of channels of the DEG/ENaC family.

These results are in accordance with the postulated trimeric structure of ASIC1 und suggest that HyNaC5/2/3 is the ion channel that mediates the effects of HydraRFamides I and II in vivo. Moreover these results suggest that peptides where the first ligands of ion channels belonging to the DEG/ENaC gene family.