Pyloric ceca are significant sites of newly synthesized 22 : 6n-3 in rainbow trout (Oncorhynchus mykiss)

In this pulse-chase study, rainbow trout fed a diet containing deuterated (D-5) (17,17,18,1 8,18)-18:3n-3 ethyl ester accumulated D-5-22:6n-3 in pyloric ceca to a greater extent than in liver 2 d post-dose. The ratio of newly synthesized D-5-22:6n-3 in ceca to that in liver 2 d after feeding D-5-18:3n-3 was 4.7 +/-1.2 when expressed as per mg tissue and 5.2 +/- 2.4 when expressed as per mg protein. The amount of D-5-22:6n-3 in ceca. then declined whereas that in liver and blood increased, with the ratio of ceca to liver falling to 1.7 and 1.4, respectively, by day 5 and approaching unity by day 9. A crude cecal mucosa fraction contained 123 +/- 50, ng D-5-22:6n-3/mg protein/mg D-5-18:3n-3 eaten 2 d after feeding the tracer, compared with 35 21 ng D-5-22:6n-3/mg protein/mg D-5-18:3 n-3 eaten in liver. Three days later the amount in cecal mucosa had fallen by one-third and that in liver had increased threefold. Most of the D-5-18:3n-3 was catabolized very rapidly. The ratio of D-5-18:3n-3 to 21:4n-6 (a relatively inert FA marker) in the diet was 4.0, but this fell to 0.30 in ceca and ca. 0.8 in liver, blood, and whole carcass one day after feeding. These results indicate that ceca are active in the synthesis of 22:6n-3 and the oxidation of 18:3n-3.