Moderate Intensity Endurance Exercise Training Increases Muscle Glycogen Content but Does Not Alter Substrate Oxidation in C57BL6 Mice

Abstract

INTRODUCTIONAerobic exercise training induces a range of desirable cardiovascular and metabolic adaptations but questions remain regarding the minimum thresholds of intensity, duration, and frequency of training required to elicit measurable changes in health outcomes. We examined the effects of a moderate intensity treadmill endurance training protocol in mice in order to determine whether this intensity level would induce changes in body composition and serum parameters. Furthermore, we investigated the extent to which any such changes would be accompanied by similar changes in muscle and liver substrate oxidation in an effort to better characterize the suite of metabolic adaptations that occur in response to moderate intensity exercise.METHODS20 male C57BL6 mice, approximately 12 weeks of age, were randomly assigned to either an exercise (EX) or sedentary (CON) group. All animals were maintained on low‐fat (13%) rodent chow (LabDiet 5001) and group housed. The EX group underwent a 6‐week program daily treadmill exercise at speeds no greater than 20 meters/min at an incline of 5°. Moderate‐intensity training sessions elicited a 50% increase in circulating lactate, but remained below a lactate threshold level. Body composition was measured weekly by NMR. Mice were euthanized following the cessation of the exercise training protocol. Muscle and liver tissues were collected and substrate oxidation was assessed using 14C labeled substrates. Glycogen content was assayed in liver and quadriceps muscle. Serum parameters were also assessed.RESULTSThe EX group exhibited reduction in body weight and fat mass. Glycogen storage in muscle was significantly increased (p<0.05) by exercise training, whereas intramuscular triglycerides were decreased (p<0.05). There were no statistically significant changes observed in pyruvate, leucine, palmitate, or lignocerate oxidation between the groups, which was supported by a relative lack of changes in gene/protein expression of relevant pathways.CONCLUSIONA moderate intensity 6‐week treadmill endurance exercise program was sufficient to induce a set of “traditional” training adaptations (reduced body weight and fat mass, increased glycogen); however, the stimulus was not sufficient to alter substrate oxidative capacity in skeletal muscle. Collectively these data suggest that in order for exercise training to induce remodeling of substrate metabolism pathways at a molecular level, protocols using higher intensities may be required.Support or Funding InformationThis work was supported by NIH R01 DK103860‐01 (R.C.N.).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.