Effects of eicosapentaenoic acid on cardiac SR Ca(2+)-release and ryanodine receptor function

n-3 polyunsaturated fatty acids (PUFAs) can prevent life-threatening arrhythmias but the mechanisms responsible have not been established. There is strong evidence that part of the antiarrhythmic action of PUFAs is mediated through inhibition of the Ca(2+)-release mechanism of the sarcoplasmic reticulum (SR). It has also been shown that PUFAs activate protein kinase A (PKA) and produce effects in the cardiac cell similar to beta-adrenergic stimulation. We have investigated whether the inhibitory effect of PUFAs on the Ca(2+)-release mechanism is caused by direct inhibition of the SR Ca(2+)-release channel/ryanodine receptor (RyR) or requires activation of PKA. Experiments in intact cells under voltage-clamp show that the n-3 PUFA eicosapentaenoic acid (EPA) is able to reduce the frequency of spontaneous waves of Ca(2+)-release while increasing SR Ca(2+) content even when PKA activity is inhibited with H-89. This suggests that the EPA-induced inhibition of SR Ca(2+)-release is not dependent on activation of PKA. Consistent with this, single-channel studies demonstrate that EPA (10-100 microM), but not saturated fatty acids, reduce the open probability (Po) of the cardiac RyR incorporated into phospholipid bilayers. EPA also inhibited the binding of [3H]ryanodine to isolated heavy SR. Our results indicate that direct inhibition of RyR channel gating by PUFAs play an important role in the overall antiarrhythmic properties of these compounds.