Fish cardiac sodium channels are tetrodotoxin sensitive

ABSTRACT

Aim: Sodium current (I_Na) of the mammalian heart is resistant to tetrodotoxin (TTX) due to low TTX affinity of the cardiac sodium channel (Na_v) isoform Na_v1.5. To test applicability of this finding to other vertebrates, TTX sensitivity of the fish cardiac I_Na and its molecular identity were examined.

Methods: Molecular cloning and whole-cell patch-clamp were used to examine α-subunit composition and TTX inhibition of the rainbow trout (Oncorhynchus mykiss) cardiac Na_v respectively.

Results:  I _Na of the trout heart is about 1000 times more sensitive to TTX (IC₅₀ = 1.8–2 nm) than the mammalian cardiac I_Na and it is produced by three Na_vα-subunits which are orthologs to mammalian skeletal muscle Na_v1.4, cardiac Na_v1.5 and peripheral nervous system Na_v1.6 isoforms respectively. Oncorhynchus mykiss (om) omNa_v1.4a is the predominant isoform of the trout heart accounting for over 80% of the Na_v transcripts, while omNa_v1.5a forms about 18% and omNa_v1.6a only 0.1% of the transcripts. OmNa_v1.4a and omNa_v1.6a have aromatic amino acids, phenylalanine and tyrosine, respectively, in the critical position 401 of the TTX binding site of the domain I, which confers their high TTX sensitivity. More surprisingly, omNa_v1.5a also has an aromatic tyrosine in this position, instead of the cysteine of the mammalian TTX-resistant Na_v1.5.

Conclusions: The ortholog of the mammalian skeletal muscle isoform, omNa_v1.4a, is the predominant Na_vα-subunit in the trout heart, and all trout cardiac isoforms have an aromatic residue in position 401 rendering the fish cardiac I_Na highly sensitive to TTX.