Exercise endurance capacity is markedly reduced due to impaired energy homeostasis during prolonged fasting in FABP4/5 deficient mice.

BACKGROUND: Skeletal muscle prefers carbohydrate use to fatty acid (FA) use as exercise intensity increases. In contrast, skeletal muscle minimizes glucose use and relies more on FA during fasting. In mice deficient for FABP4 and FABP5 (double knockout (DKO) mice), FA utilization by red skeletal muscle and the heart is markedly reduced by the impairment of trans-endothelial FA transport, with an increase in glucose use to compensate for reduced FA uptake even during fasting. We attempted to determine whether prolonged fasting affects exercise performance in DKO mice, where constant glucose utilization occurs. RESULTS: A single bout of treadmill exercise was performed in the fed and fasted states. The initial speed was 10 m/min, and gradually increased by 5 m/min every 5 min up to 30 m/min until the mice stopped running. Running distance was significantly reduced by DKO genotype and prior fasting, leading to the shortest distance in fasted DKO mice. Levels of glycogen in skeletal muscle and the liver were nearly depleted in both WT and DKO mice during prolonged fasting prior to exercise. Levels of TG in skeletal muscle were not reduced by exercise in fasted DKO mice, suggesting that intramuscular TG was not utilized during exercise. Hypoglycaemia was accelerated in fasted DKO mice, and this acceleration could be due to constant glucose utilization by red skeletal muscle and the heart where FA uptake is diminished due to defective trans-endothelial FA transport. Taken together, energy supply from serum and storage in skeletal muscle were very low in fasted DKO mice, which could lead to a significant reduction in exercise performance. CONCLUSIONS: FABP4/5 have crucial roles in nutrient homeostasis during prolonged fasting for maintaining exercise endurance capacity.

CD36 is indispensable for nutrient homeostasis and endurance exercise capacity during prolonged fasting.

During fasting, most tissues including skeletal muscle heavily rely on utilization of fatty acids (FA) and minimize glucose use. In contrast, skeletal muscle prefers carbohydrate use as exercise intensity increases. In mice deficient for CD36 (CD36-/- mice), FA uptake is markedly reduced with a compensatory increase in glucose uptake in skeletal muscle even during fasting. In this study, we questioned how exercise endurance is affected during prolonged fasting in CD36-/- mice where glucose utilization is constantly increased. With or without a 24-h fast, a single bout of treadmill exercise was started at the speed of 10 m/min, and the speed was progressively increased up to 30 m/min until mice were exhausted. Running distance of wild type (WT) and CD36-/- mice was comparable in the fed state whereas that of CD36-/- mice was significantly reduced after a 24-h fast. Glycogen levels in liver and skeletal muscle were depleted both in WT and CD36-/- mice after a 24-h fast. In CD36-/- mice, FA uptake by skeletal muscle continued to be reduced during fasting. Glucose utilization also continued to be enhanced in the heart and oxidative skeletal muscle and glucose supply relative to its demand was diminished, resulting in accelerated hypoglycemia. Consequently, available energy substrates from serum and in muscle for exercise performance were very limited in CD36-/- mice during prolonged fasting, which could cause a remarkable reduction in exercise endurance. In conclusion, our study underscores the importance of CD36 for nutrient homeostasis to maintain exercise performance of skeletal muscle when nutrient supply is limited.

Serum levels of fatty acid binding protein 4 and fat metabolic markers in relation to catecholamines following exercise.

BACKGROUND: Lipolysis is stimulated by activation of adrenergic inputs to adipose tissues. Our recent study showed that serum concentrations of fatty acid binding protein 4 (FABP4) are robustly elevated in patients with acute myocardial infarction and ventricular tachyarrhythmia, that display a marked activation of the sympathetic nervous system (SNS). However, it remains unknown whether circulating FABP4 concentrations are associated with exercise-induced SNS activation. METHODS: Thirty one healthy volunteers underwent cardiopulmonary exercise testing on a cycle ergometer up to the workload levels below and above anaerobic threshold, low- and high-intensity exercise, respectively. Serial blood samplings were performed before and after exercise. RESULTS: High-intensity exercise significantly increased serum concentrations of FABP4 and catecholamines, and their concentrations declined fast thereafter in a similar fashion. These changes were accompanied by little, if any, changes in other metabolic markers. Regardless of adiposity, percent change from baseline to peak FABP4 levels (%FABP4) was comparable in all subjects. Stepwise regression analysis revealed that %FABP4 was highly correlated with that in norepinephrine. CONCLUSIONS: Our study reveals the significant correlation between circulating FABP4 and norepinephrine levels during exercise testing. Together with the fact that FABP4 is secreted from adipocytes via ß-adrenergic-mediated lipolytic mechanisms, this study suggests FABP4 as a potential biomarker for adrenergic overdrive.