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

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.