EFFECTS OF EPINEPHRINE ON MUSCLE GLYCOGENOLYSIS, PDH ACTIVATION AND CARBOHYDRATE OXIDATION DURING EXERCISE

Abstract

To evaluate the role of epinephrine in regulating carbohydrate metabolism during moderate exercise, 10 recreationally active men completed two 20 min exercise bouts at 58 ± 2% VO2peak. On one occasion, saline was infused (CON), and on the other epinephrine was infused intravenously for five min prior to and throughout exercise (EPI). Glucose kinetics were measured by a primed, continuous infusion of [6,6-2H] glucose and muscle samples were obtained prior to and after 1 and 20 min of exercise. The infusion of epinephrine elevated (p < 0.01) plasma epinephrine during exercise. Total carbohydrate oxidation during exercise increased by 18% and this effect was associated with a greater (p < 0.05) skeletal muscle glycogenolysis (EPI: 187 ± 31, CON: 96 ± 31 mmol/kg dw) and PDH activation (EPI: 3.2 ± 0.3, CON: 2.8 ± 0.4 mmol acetyl CoA/kg wet wt/min at 20 min of exercise). Glucose Ra was not different between trials; however, the infusion of epinephrine decreased (p < 0.05, treatment effect) glucose Rd. During exercise muscle glucose 6-phosphate (P = 0.055) and lactate (p < 0.05) were elevated in EPI compared with CON and no changes were observed for pyruvate, creatine, phosphocreatine, ATP and the calculated free concentrations of ADP and AMP. The data demonstrate that elevated plasma epinephrine levels during moderate exercise in untrained men increase skeletal muscle glycogen breakdown, PDH activation and carbohydrate oxidation. The decreased glucose uptake observed during exercise in EPI is likely to be due to increased intracellular glucose 6-phosphate and a subsequent decrease in glucose phosphorylation.