The metabolism of 20‐ and 22‐carbon unsaturated acids in rat heart and myocytes as mediated by feeding fish oil

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Abstract
When rats were fed 5% corn oil, the heart phospholipids contained large amounts of 22‐carbon (n−6) acids. When half of the corn oil was replaced with fish oil, the reduced level of arachidonate and 22‐carbon (n−6) acids in phospholipids was accompanied by increases in the levels of 22‐carbon (n−3) acids while only small amounts of 20∶5(n−3) were acylated. Heart myocytes readily took up and acylated [1‐14C]‐labeled 20∶4(n−6), 20∶5(n−3) and 22∶6(n−3) into phospholipids. The uptake and acylation of 20∶4(n−6) was greater than for 20∶5(n−3) but the intracellular labeling profiles were similar. Uptake and acylation of 22∶6(n−3) was somewhat lower. In addition the intracellular labeling profile differed in that more 22∶6(n−3) was incorporated into the ethanolamine‐containing phospholipids than when 20∶4(n−6) or 20∶5(n−3) were the substrates. Neither 20∶4(n−6) nor 20∶5(n−3) was chain elongated. When [3‐14C]‐labeled 22∶4(n−6) and 22∶5(n−3) were the substrates, it was not possible to detect radioactive 22∶5(n−6) or 22∶6(n−3). Both [3‐14]‐labeled substrates were acylated into phospholipids and retroconverted with the subsequent esterification of radioactive 20∶4(n−6) and 20∶5(n−3) into triglycerides and phospholipids. These studies show that cardiomyocytes lack the ability to make 22‐carbon acids from 20‐carbon precursors but they retroconvert 22‐carbon acids to 20‐carbon acids. The high levels of 22‐carbon polyunsaturated acids in total heart lipids thus cannot be attributed to the synthetic capacities of cardiomyocytes.

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