Cloning and expression of a FMRFamide-gated Na+ channel from Helisoma trivolvis and comparison with the native neuronal channel

1. We have cloned a cDNA encoding a Phe:-Met-Arg-Phe-:NH2 (FMRFamide)-gated Na+ channel from nervous tissue of the pond snail Helisoma trivolvis (HtFa NaC) and expressed the channel in Xenopus oocytes. The deduced amino acid s equence of the protein expressed by HtFaNaC is 65 % identical to that of th e FMRFamide-gated channel cloned fr om Helix aspersa (HaFaNaC). 2. HtFaNaC expressed in oocytes was less sensitive to FMRFamide (EC50 = 70 mu M) than HaFaNaC (EC50 = 2 mu M). The two had a similar selectivity for N af. The amplitude of the FMRFamide response of HtFaNsC was increased by red ucing the extracellular concentration of divalent cations. 3. The conductance of the two channels was similar, but the mean open time of unitary events was shorter for expressed HtFaNaC compared to expressed H aFaNaC. Each channel was susceptible to peptide block by high agonist conce ntrations. 4. In marked contrast to HaFaNaC and other amiloride-sensitive Na+ channels , amiloride, and the related drugs benzamil and 5- (N-ethyl-N-isopropyl)-am iloride (EIPA), enhanced the FMRFamide response in oocytes expressing HtFaN aC cRNA. The potentiating effects of EIPA and benzamil were greater than th ose of amiloride. Unitary current analysis showed that with such drugs, the re was channel blockade as well as an increased probability of channel open ing. 5. The similar permeability of the oocyte-expressed HtFaNaC and the Helisom a neuronal channel, and the susceptibility of both to agonist blockade and blockade by divalent cations, suggest that the channels are the same. Howev er, neuronal channels were less susceptible to enhancement by amiloride ana logues and in some patches were more sensitive to FMRFamide than expressed HtFaNaC.