In running mammals, endurance training leads to improved FA utilization, but it is biased toward increased reliance on intramyocyte triaclyglycerol ( 44, 45). The rate of utilization of exogenous FA appears to be most limited by transport across the sarcolemma ( 31, 43). Exogenous FA contribute only a small fraction of the energy needed for exercise of even moderate intensity, and near V˙ o 2 maxexogenous FA oxidation contributes ∼10% of energy demand ( 43, 45). Generally, the relative contribution of FA oxidation to total fuel demand declines as exercise intensity increases, with the balance of energy derived mainly from carbohydrate oxidation ( 36). The most complete information on fuel selection during exercise comes from studies of running mammals (including humans). The use of stored fat as a metabolic fuel makes migratory flight possible, yet there currently exists no general mechanistic understanding of how birds achieve the high rates of exogenous FA transport and oxidation required to support such high-intensity endurance exercise. In the special case of migratory flight, during which this intensity of exercise is maintained for as long as 50 or even 100 h, energy metabolism is almost completely dominated (85–95%) by the oxidation of exogenous fatty acids (FA) delivered to flight muscles from extramuscular adipose tissue ( 21, 23, 44). The instantaneous cost of flight is high relative to other forms of locomotion flying birds expend energy at 10 to 15 times basal metabolic rate (BMR), and the minimum cost of flight may be twice the aerobic limit (V˙ o 2 max) of similarly sized running mammals ( 4, 38). Citrate synthase, 3-hydroxyacyl-CoA-dehydrogenase, and carnitine palmitoyl transferase were positively correlated within individuals, suggesting coexpression, but enzyme activities were unrelated to H-FABP levels. The greater relative induction of H-FABP (+70%) with migration than of catabolic enzymes suggests that elevated H-FABP is related to the enhancement of uptake of fatty acids from the circulation. Aerobic capacity, measured by citrate synthase activity, and fatty acid oxidation capacity, measured by 3-hydroxyacyl-CoA-dehydrogenase and carnitine palmitoyl transferase activities, did not change during premigration but increased during migration by 6, 12, and 13%, respectively. Juveniles making their first migration had lower pectoralis H-FABP than adults, further supporting a role for flight training. Premigratory birds increased body fat, but not pectoralis H-FABP, indicating that endurance flight training may be required to stimulate H-FABP expression. Pectoralis H-FABP levels were highest during migration (10%) and declined to 6% in tropically wintering female sandpipers. H-FABP accounted for almost 11% of cytosolic protein in the heart. We developed an ELISA to measure heart-type fatty acid binding protein (H-FABP) in muscles of the western sandpiper ( Calidris mauri), a long-distance migrant shorebird.
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