n-3 polyunsaturated fatty acids (PUFAs) produce anti-arrhythmic effects by reducing surface membrane excitability and inhibition of spontaneous release of Ca²⁺ from the sarcoplasmic reticulum (SR) (Kang et al. 1995; Negretti et al. 2000). These effects may be direct actions of PUFAs on ion channels in surface and SR membranes. Alternatively PUFAs act as second messengers and are precursors of biologically active molecules, e.g. prostaglandins and thromboxanes. As second messengers n-3 PUFAs lead to activation and inhibition of various kinases that might be involved in control of contraction in cardiac muscle. Here we report lusitropic effects in cardiac myocytes consistent with activation of protein kinase A (PKA).

Figure 1A shows the effect of the n-3 PUFA eicosapentaenoic acid (EPA, 10 µM) on field-stimulated contractions. In EPA contraction peaks sooner and relaxation is faster. In Fig. 1B the experiment is repeated in the presence of the PKA inhibitor H89 (20 mM). Both effects of EPA are absent. Due to the biphasic nature of the contractions, as a measure of the lusitropic effect we have calculated the area under the contractions after normalising the amplitude. Under control conditions EPA reduces the area of the contraction to 64.7 ± 2.7 % (mean ± S.E.M., n = 6, P < 0.0001, paired t test) of control, in the presence of H89 EPA has no effect on the area under the normalised contraction (103.5 ± 3.1% P > 0.35).

These results suggest that the lusitropic effects of EPA are due to activation of PKA. Although n-3 PUFAs are known to inhibit PKA (Mirnikjoo et al. 2001), they also inhibit phosphodiesterase so cAMP levels rise (Picq et al. 1996). This may overcome the inhibition of PKA, resulting in increased phosphorlylation levels of e.g. phospholamban, leading to more rapid sequestration of Ca²⁺ by the SR following systolic release.

This work was supported by The Wellcome Trust.

Figure 1. EPA (10 µM) causes a positive lusitropic effect that is inhibited by H89 (20 µM). The recontrol solution contained 1 mg ml-1 BSA to help remove EPA.

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