Evidence that free polyunsaturated fatty acids modify Na+ channels by directly binding to the channel proteins

The effects of free polyunsaturated fatty acids (PUFA) on the binding of ligands to receptors on voltage-sensitive Na+ channels of neonatal rat cardiac myocytes were assessed, The radioligand was [benzoyl-2,5-H-3] batrachotoxinin A 20 alpha-benzoate ([H-3]BTXB), a toxin that binds to the Na+ channel, The PUFA that have been shown to be antiarrhythmic, including eicosapentaenoic acid (EPA; C20:5n-3), docosahexaenoic acid (DHA; C22:6n-3), eicosatetraynoic acid (ETYA), linolenic acid (C18:3n-3), and linoleic acid (C18:2n-6), inhibited [H-3]BTXB binding in a dose-dependent fashion with IC50 values of 28-35 mu M, whereas those fatty acids that have no antiarrhythmic effects including saturated fatty acid (stearic acid; C18:0), monounsaturated fatty acid (oleic acid; C18:1n-9), and EPA methyl ester did not have a significant effect an [H-3]BTXB binding. Enrichment of the myocyte membrane with cholesterol neither affected [H-3]BTXB binding when compared with control cells nor altered the inhibitory effects of PUFA on [H-3]BTXB binding. Scatchard analysts of [H-3]BTXB binding showed that EPA reduced the maximal binding without altering the K-d for [H-3]BTXB binding, indicating allosteric inhibition, The inhibition by EPA of [H-3] BTXB binding was reversible (within 30 min) when delipidated bovine serum albumin was added, The binding of the PUFA to this site on the Na+ channel is reversible and structure-specific and occurs at concentrations close to those required for apparent antiarrhythmic effects and a blocking effect on the Na+ current, suggesting that binding of the PUFA at this site relates to their antiarrhythmic action.