Carbohydrate handling in exercising muscle of obese Zucker rats

OBJECTIVE: To determine the differential substrate utilization of substrates by exercising muscle and in the fatigued state in lean and obese rats. DESIGN: The rats were treadmill-exercised until fatigued, when their oxygen consumption increased by 1.85 x factor. Blood and hind leg tissue (muscle and skin) were sampled at intervals during exercise and recovery. SUBJECTS: Lean and obese adult Zucker female rats. MEASUREMENTS: Three series of rats were used to determine hind-leg glucose, lactate and oxygen arterio-venous balances, blood flow and muscle levels of glucose, hexose-P, glycogen, lactate and skin lactate. The rates of glycosyl unit flow during exercise and recovery were then calculated. RESULTS: In obese rats, exercising muscle showed higher glucose uptake, increased glycogen mobilization and lower lactate production than in the lean. In the obese rats' muscle, there were more glycosyl units available for oxidative metabolism. Excess grycosyl (or 3C) units were used probably for lipid synthesis. Lean rats managed their glycosyl units more efficiently, stretching the available glycogen, thus prolonging the exercise. During recovery, obese rats massively synthesized glycogen and lowered lactate efflux, which left only a few glycosyl units to oxidative metabolism probably the rest of oxidative energy was derived from lipids. CONCLUSION: In lean rats glucose is the main source of muscle energy during exercise and recovery, whereas obese rats use glucose during exercise, and probably synthesize lipid during exercise. Obese rats rely more on lipids for energy during recovery.