Experimental evidence indicates that polyunsaturated fatty acids (PUFAs) have a protective effect against ischaemia-induced arrhythmias when present in the diet (Leaf et al. 1999). More recently, it has been shown that two PUFAs, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), reduce the frequency of the potentially arrhythmogenic spontaneous waves of Ca2+ release in isolated cardiac cells (Negretti et al. 2000) and part of this action appears to result from an inhibitory effect on sarcoplasmic reticulum (SR) Ca2+ release.
To identify a possible mechanism for the inhibition of SR calcium release, we examined the effects of the n-3 PUFA eicosapentaenoic acid (EPA) on the gating properties of native ryanodine receptors (RyR). Pig skeletal and sheep cardiac muscle was obtained from an abattoir. Native sheep cardiac or pig skeletal RyR were incorporated into planar phospholipid bilayers under voltage-clamp conditions and single-channel current fluctuations were monitored at 0 mV with Ca2+ as the permeant ion as described previously (Chan et al. 2000).
In the presence of 1 mM ATP and 1 mM free cytosolic Ca2+, cardiac RyR open probability (Po) was 0.65 ± 0.08 (n = 3). With the addition of 10, 20 and 100 mM EPA to the cytosolic channel side, the Po was 0.52 ± 0.25, 0.34 ± 0.17 and 0.20 ± 0.08, respectively (mean ± S.D.; n = 3), demonstrating a dose-dependent inhibitory effect. Under identical experimental conditions, EPA exerted similar effects on the skeletal RyR: control Po was 0.47 ± 0.25 and 0.36 ± 0.29, 0.3 ± 0.32 and 0.11 ± 0.08 with the addition of 10, 20 and 100 mM EPA, respectively (mean ± S.D.; n = 7). The effect of EPA on RyR Po is therefore not isoform specific.
The inhibitory action of EPA results from both a decrease in mean open times and a reduction in the frequency of channel opening. Control mean open and closed times were 7.02 ± 6.17 and 2.26 ± 1.05 ms-1, respectively, and were 1.94 ± 1.8 and 27.54 ± 44.12 ms-1, respectively, in the presence of 10 mM EPA (mean ± S.D.; n = 3).
These results demonstrate that EPA reduces RyR Po in a dose-dependent manner and at concentrations that could be achieved with supplementation of the diet with fish oils (Negretti et al. 2000). The inhibitory effect of EPA on RyR channel gating is a possible mechanism to explain the inhibition of SR Ca2+ release in cardiac cells by PUFAs and may contribute to their anti-arrhythmic properties.
This work was supported by the British Heart Foundation.