Absorption and lymphatic transport of exogenous and endogenous arachidonic and linoleic acid in the rat

[3H]Arachidonic (20:4) and [14C]linoleic acid (18:2) were fed to thoracic duct-cannulated rats in test meals of either tracers alone, cream, Intralipid, pure arachidonic acid, or pure linoleic acid. Less [3H]20:4 than [14C]18:2 was recovered in chyle during the first 5 h. After cream feeding, the proportion of radioactivity found in phospholipids was high and increased during the first 3 h. After the meal (3-5 h) 61 +/- 6% of the 3H and 57 +/- 10% of the 14C was in phosphatidylcholine, and 11 +/- 3% of the 3H and 3.0 +/- 4% of the 14C was in phosphatidylethanolamine. Changing the fat vehicle to Intralipid or pure 18:2 decreased the proportion of label in the phospholipids and increased the 3H and 14C radioactivity in the triacylglycerol fraction, the distribution of 14C being influenced more than that of 3H. After feeding the tracers in 200 microliters of pure 20:4, greater than 90% of both isotopes was in triacylglycerol. During fasting, triacylglycerol transported 56% (0.7 mumol/h), phosphatidylcholine transported 34% (0.4 mumol/h), and phosphatidylethanolamine transported 10% (0.1 mumol/h) of the 20:4 mass. After cream or Intralipid feeding, the output of 20:4-containing phosphatidylcholine and phosphatidylethanolamine increased 2.1- to 2.8-fold, whereas the transport of 20:4 with triacylglycerol remained constant. Phospholipids thus became the predominant transport form for 20:4. After feeding 200 microliters of 20:4, the intestine produced, however, 20:4-rich triacylglycerols that transported 89% of the chyle 20: