KAM, J.C.*; MILLIGAN, C.L.; The University of Western Ontario, London: Exercise Recovery in Fish: Fuel Substrate Preference duing Muscle Glycogen Resynthesis
In rainbow trout, the relative contributions of carbohydrate, lipid and protein to fuel muscle glycogen resynthesis following exhaustive exercise are unclear. The purpose of this study was to examine fuel preference by trout white muscle undergoing glycogen resynthesis and to quantify the contribution of exogenous substrates in fueling recovery from exhaustive exercise by using an in vitro muscle slice preparation. Muscle slices were isolated post-exercise and incubated for 1 h in glucose-free Cortland�s saline with various combinations of substrates. In the absence of exogenous substrates, all muscle slices cleared the accumulated lactate load within one hour (eg. 20.3±2.9 to 4.3±0.7 µmol/g wet tissue). Only in slices incubated in glycerol or lactate (2.4±0.5 to 4.3±1.1 µmol glucosyl/g wet tissue) or both substrates combined (3.9±0.4 to 9.4±1.1 µmol glucosyl/g) was net glycogen synthesis seen. Radiolabeling of the total glycogen pool was greater with [14C]glycerol (2.8% of the glycogen pool) than [14C]lactate (0.9%). Muscle slices preferentially oxidized lactate (313.8±32.5 nmol lactate/g) over glycerol (3.3±0.8 nmol glycerol/g), and in the presence of glycerol, lactate oxidation increased 2-fold. These results indicate that muscle resynthesizes glycogen in the presence of exogenous glycerol or lactate; yet, glycogen synthesis is enhanced when both substrates are present. Radiolabel experiments show that the primary fate of lactate is oxidative, presumably to fuel glycogen resynthesis, while glycerol is solely used by muscle as a glycogenic substrate. The enhanced oxidation of lactate in the presence of exogenous glycerol suggests a curious interaction between the two substrates that has yet to be uncovered.
