['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(v)1.5. To test applicability of this finding to other vertebrates, TTX sensitivity of the fish cardiac I(Na) and its molecular identity were examined.', 'Molecular cloning and whole-cell patch-clamp were used to examine alpha-subunit composition and TTX inhibition of the rainbow trout (Oncorhynchus mykiss) cardiac Na(v) respectively.', '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(v)alpha-subunits which are orthologs to mammalian skeletal muscle Na(v)1.4, cardiac Na(v)1.5 and peripheral nervous system Na(v)1.6 isoforms respectively. Oncorhynchus mykiss (om) omNa(v)1.4a is the predominant isoform of the trout heart accounting for over 80% of the Na(v) transcripts, while omNa(v)1.5a forms about 18% and omNa(v)1.6a only 0.1% of the transcripts. OmNa(v)1.4a and omNa(v)1.6a 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(v)1.5a also has an aromatic tyrosine in this position, instead of the cysteine of the mammalian TTX-resistant Na(v)1.5.'] Are fish cardiac sodium channels tetrodotoxin sensitive?