Glycerol and fatty acid kinetics in rainbow trout: Effects of endurance swimming

Continuous infusions of 2-[H-3]glycerol and 1[C-14]palmitate were performed ill vivo in rainbow trout to measure the effects of prolonged swimming on (1) the rate of appearance of glycerol (R-a glycerol or lipolytic rate), (2 ) the rate of appearance of non-esterified fatty acids (R-a NEFA) and (3) t he rate of triacylglycerol:fatty acid cycling (TAG:FA cycling or re-esterif ication). Our goals were to test the hypothesis that sustained exercise for up to 4 days causes the progressive mobilization of triacylglycerol reserv es to supply fuel to contracting muscles, and to assess whether TAG:FA cycl ing plays a role in the regulation of NEFA availability in teleosts. Contra ry to expectation, the rates of lipolysis and fatty acid release in resting trout are not affected by endurance exercise. Unlike mammals, which increa se the rate of lipolysis by two- to fourfold during submaximal exercise, th ese active teleosts do not mobilize triacylglycerol reserves beyond resting levels to supply more NEFAs to working muscles. Furthermore, they maintain R-a glycerol and R-a NEFA well in excess of oxidative fuel requirements ev en at rest. More than two-thirds of the NEFAs produced are re-esterified, b ut the results show that TAG:FA cycling is not involved in the regulation o f NEFA availability during or after swimming. We propose that the observed high rates of re-esterification represent an important feature of ectotherm ic metabolism that allows the restructuring of membrane phospholipids to be synchronized with frequent changes in body temperature.