Lean, mean, lipolytic machines: lipid mobilization in rainbow trout during graded swimming

The mobilization of mammalian lipid reserves is strongly stimulated during exercise to reach a maximum at moderate intensities, but the effects of swimming speed on fish lipolysis have never been quantified. Continuous infusion of 2-[3H]glycerol was used to measure Ra glycerol (=rate of appearance of glycerol or lipolytic rate) in rainbow trout kept at rest, or during graded exercise in a swim tunnel up to critical swimming speed (Ucrit). Results show that Ra glycerol is 1.67±0.18 µmol kg−1min−1 in control animals, and remains at a steady level of 1.24±0.10 µmol kg−1min−1 in exercising fish at all swimming intensities. Baseline lipolytic rate provides more than enough fatty acids from lipid reserves to accommodate all the oxidative fuel requirements for swimming at up to 2 body lengths per second (BL s−1), and more than 50% of the energy needed at Ucrit (3.4±0.1 BL s−1). Such “excess lipolysis” also means that trout sustain high rates of fatty acid reesterification. Maintaining steady lipolysis at rest and throughout graded swimming is strikingly different from mammals that stimulate Ra glycerol by 2 to 5-fold to support exercise. Instead trout act like “lipolytic machines” that do not modulate Ra glycerol even when their metabolic rate triples: a strategy that eliminates the need to increase lipolytic rate during exercise. This study also supports the notion that maintaining a high rate of reesterification (or triacylglycerol/fatty acid cycling) may be a mechanism widely used by ectotherms to achieve rapid membrane remodelling in variable environments.