Effects of external divalent cations on the FMRFamide-gated sodium channel

Abstract

FMRFamide-gated Na⁺ channel (FaNaC) is the peptide-gated sodium channel belonging to the epithelial Na⁺ channel/degenerin family. In the Aplysia kurodai FaNaC (AkFaNaC), external Ca²⁺ and Mg²⁺ affect the channel differently (Furukawa et al, 2006). Ca²⁺ but not Mg²⁺ blockes the channel, high Mg²⁺ increases the currents, and Mg²⁺ but not Ca²⁺ enhances the desensitization. In the present study, Mg²⁺ was found to inhibit Ca²⁺ block of AkFaNaC. The concentration-response relationship by FMRFamide was shifted to the left by Mg²⁺, and shifted to the opposite direction by Ca²⁺. The maximum acitivatable current was significantly enhanced by Mg²⁺, but noticeably reduced by Ca²⁺. Because two aspartate residues (D552 and D556) are conserved in the presumed pore region of FaNaCs, we examined the mutant channels in these sites. Most of the actions of Mg²⁺ and Ca²⁺ on AkFaNaC were either abolished or depressed in a mutant D552N. In D552N, Mg²⁺ also blocked the channel. At D556, we made three mutants (D556N, D556C, D556E) but none of them were functional. We propose that D552 is involved in a binding site of divalent cations, and that the binding of divalent cations to the site affects the affinity and/or efficacy of FaNaC as well as the channel block. [J Physiol Sci. 2008;58 Suppl:S86]