Peroxidation of docosahexaenoic acid is responsible for its effects on ITO and ISS in rat ventricular myocytes

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Abstract
Exposure to docosahexaenoïc acid (DHA), a long‐chain polyunsaturated fatty acid, is known to block several ionic currents such as the transient outward current ITO. It has also been reported to activate certain potassium channels. It has been suggested that these effects, observed in single‐cell experiments, participate in the antiarrhythmic properties of these compounds in vivo. DHA is highly prone to peroxidation. To investigate the influence peroxidation may have on the effects of DHA on ion channels, we studied ITO and the steady‐state outward current ISS in isolated rat ventricular myocytes under ruptured whole‐cell patch‐clamp conditions. A measure of 10 μM DHA alone reduced ITO, evoked by a pulse to +70 mV, by 74.8±10.8% (n=7) and activated a delayed outward current with kinetic properties different from ISS. When an antioxidant, alpha‐tocopherol (1 μM), was added together with DHA, the blockade of ITO was reduced to 38.5±7.7% (n=8) and the delayed outward current was not activated. α‐Tocopherol alone had no effect on these currents. When an oxidant, hydrogen peroxide (1 μM), was applied together with DHA, the blockade of ITO was almost complete (98.4±1.0%, n=7) and a large delayed outward current was activated. A measure of 1 μM hydrogen peroxide alone had no effect on these currents. Measurements of nonperoxidized DHA in experimental solutions confirmed the negative relation between DHA concentration and the effects on the currents. We conclude that rather than DHA itself, it is the peroxidation products of DHA that block ITO and activate a delayed outward current in in vitro single‐cell experiments. These findings have important implications for the extrapolation of in vitro experimental findings to the antiarrhythmic effects of DHA in vivo because, in vivo, peroxidation of DHA is unlikely to occur. British Journal of Pharmacology (2003) 139, 816–822. doi:10.1038/sj.bjp.0705308