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 the rate of appearance of glycerol or lipolytic rate (R_a glycerol) in rainbow trout kept at rest, or during graded exercise in a swim tunnel up to critical swimming speed (U_crit). Results show that R_a glycerol is 1.67±0.18 µmol kg^-1 min^-1 in control animals, and remains at a steady level of 1.24±0.10 µmol kg^-1 min^-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 U_crit (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 R_a glycerol by twofold to fivefold to support exercise. Instead, trout act like 'lipolytic machines' that do not modulate R_a 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.