The main novel finding of the present study is that treatment with ω-3 PUFA can prevent down-regulation in the activity of mitochondrial enzymes under conditions of severe pressure overload despite no diminution in LV hypertrophy or the classic up-regulation of the mRNA for fetal genes. Importantly, the maintenance in mitochondrial enzyme activity corresponded with attenuation of LV chamber expansion. Lastly, ω-3 PUFA supplementation did not increase adiponectin in TAC animals, suggesting that the protective effects of ω-3 PUFA occur independent of up-regulation of circulating adiponectin.
In the present study we observed a protective effect of ω-3 PUFA in WT mice, with an attenuation of the increase in end diastolic and systolic volumes with TAC and maintenance of cardiac mitochondrial enzyme function. In our previous studies in rats with a similar dose of ω-3 PUFA we observed a large increase (~70%) in total circulating adiponectin [6, 7, 9]. In the current study we did not see the expected increase in total serum adiponectin in WT mice with ω-3 PUFA supplementation. However, we did observe a small non-significant 16% increase in high molecular weight adiponectin in the serum. The lack of effect of ω-3 PUFA on total adiponectin could be due to regulation of adiponectin formation and secretion, thereby changing the amount of the various molecular weight isoforms. In addition, there could be differences between species. Most reports with fish oil supplementation indicate an up-regulation of the expression and secretion of adiponectin in rats [6–8] and humans ; however, there are reports where adiponectin is unaffected by fish oil supplementation in rats and mice [28, 29]. In a study in mice fed a diet substituted with various DHA analogs, serum adiponectin was either reduced or unchanged . Clearly, additional research is needed to address the specific role of ω-3 PUFA in adiponectin regulation.
Long chain fatty acids, including ω-3 PUFA are endogenous ligands for PPARα , which may prevent the decline in PPARα activation and mitochondrial function that is commonly observed in advanced heart failure . In the present investigation we show preservation of mitochondrial enzyme activities with dietary ω-3 PUFA supplementation after TAC. However, the mRNA for genes encoding the mitochondrial proteins citrate synthase, MCAD, PDK4 and UCP3 does not closely mirror the changes observed in activities of citrate synthase and MCAD (Table 3 and Figure 3). The mRNA for all four genes decreased with TAC in WT mice fed STD, but was not increased by ω-3 PUFA feeding. This discrepancy between gene expression and enzyme activity has been previously reported in other models of heart failure and pathological cardiac hypertrophy [10, 32]. Thus, it does not appear that ω-3 PUFA preferentially activate PPARα under physiological conditions to confer cardioprotection in the setting of heart failure. ω-3 PUFA supplementation may be maintaining mitochondrial enzyme activity via changes in mitochondrial membrane composition, as we observed an increase in DHA and decrease in arachidonic acid, which may favorably alter mitochondrial function and structure, as previously suggested .
The effects of ω-3 PUFA on structural and metabolic LV remodeling were markedly different compared to STD-fed controls in each strain. The inherent differences in the response of the LV to pressure overload between the two strains of mice have been detailed in our previous report . We observed the expected chamber dilation and reduction in ejection fraction after TAC in WT mice, but severe concentric hypertrophy with maintenance of ejection fraction in adiponectin-/- mice. In the present investigation we found that WT mice experienced LV dilation that was attenuated by ω-3 PUFA supplementation, but there was no effect of ω-3 PUFA on LV remodeling in adiponectin-/- mice. In addition, the WT mice fed STD displayed a decrease in the activity of mitochondrial oxidative enzymes that classically occurs in advanced LV hypertrophy and heart failure , but the activities were maintained with ω-3 PUFA supplementation. In the adiponectin-/- mice, there was no effect of TAC or dietary ω-3 PUFA on mitochondrial enzyme activities. Taken together, it appears that the beneficial effects of ω-3 PUFA on the heart are not dependent on an increase in adiponectin.
The effects of ω-3 PUFA supplementation on phospholipid composition in cardiac tissue have been well-documented in humans , rats [7, 9], dogs , and mice . Notably, mice have higher DHA levels in cardiac phospholipids without dietary ω-3 PUFA (~30% of total phospholipid fatty acid; Table 1)  compared to humans and dogs (~1%) [35, 36] or rats (~8%) [7, 9]. In the present study ω-3 PUFA supplementation increased the DHA content of membranes in mice to approximately 50% of total fatty acids, while a low level of supplementation increases DHA content to approximately 2-3% in humans . The pronounced difference in membrane DHA content between humans and mice limits the application of these results from mice to human disease.
In summary, treatment with ω-3 PUFA prevented LV chamber expansion and down-regulation of the activity of mitochondrial enzymes under conditions of severe pressure overload despite no diminution in LV hypertrophy or mRNA markers of heart failure. ω-3 PUFA supplementation maintained cardiac mitochondrial enzyme activity, which corresponded with prevention of LV chamber expansion. These effects occurred despite no ω-3 PUFA-induced increase in adiponectin, suggesting that the protective effects of ω-3 PUFA occur independent of up-regulation of circulating adiponectin.