CARBOHYDRATE HANDLING IN EXERCISING MUSCLE OF OBESE ZUCKER RATS

OBJECTIVE: To determine the differential substrate utilization of subs trates by exercising muscle and in the fatigued state in lean and obes e rats. DESIGN: The rats were treadmill-exercised until fatigued, when their oxygen consumption increased by 1.85 x factor. Blood and hind l eg tissue (muscle and skin) were sampled at intervals during exercise and recovery. SUBJECTS: Lean and obese adult Zucker female rats. MEASU REMENTS: Three series of rats were used to determine hind-leg glucose, lactate and oxygen arterio-venous balances, blood flow and muscle lev els of glucose, hexose-P, glycogen, lactate and skin lactate. The rate s of glycosyl unit flow during exercise and recovery were then calcula ted. RESULTS: In obese rats, exercising muscle showed higher glucose u ptake, increased glycogen mobilization and lower lactate production th an in the lean. In the obese rats' muscle, there were more glycosyl un its available for oxidative metabolism. Excess grycosyl (or 3C) units were used probably for lipid synthesis. Lean rats managed their glycos yl units more efficiently, stretching the available glycogen, thus pro longing the exercise. During recovery, obese rats massively synthesize d glycogen and lowered lactate efflux, which left only a few glycosyl units to oxidative metabolism probably the rest of oxidative energy wa s derived from lipids. CONCLUSION: In lean rats glucose is the main so urce of muscle energy during exercise and recovery, whereas obese rats use glucose during exercise, and probably synthesize lipid during exe rcise. Obese rats rely more on lipids for energy during recovery.