EFFECTS OF EPINEPHRINE INFUSION ON MUSCLE GLYCOGENOLYSIS DURING INTENSE AEROBIC EXERCISE

The purpose of this study was to determine whether an epinephrine (Epi) infusion would enhance muscle glycogenolysis during intense aerobic exercise. Epi was infused at rates that produced the same plasma Epi concentrations observed after caffeine (Caf) ingestion. Seven male subjects cycled for 15 min at 80% maximal O-2 uptake during four different trials. Trial 1 was preceded by a 9 mg/kg oral dose of Caf to determine resting and exercise plasma Epi concentrations. Trial 2 was used to determine the Epi infusion rates needed to mimic the plasma Epi levels found with Caf. Trials 3 and 4 were randomized and consisted of either an Epi infusion or a saline infusion (control, Con). During Epi and Con trials muscle samples were obtained from the vastus lateralis at 0, 3, and 15 min of exercise. Plasma Epi levels were similar between Caf and Epi and were elevated twofold compared with Con. At 5 min of exercise the plasma Epi concentrations were 1.51 +/- 0.26, 2.61 +/- 0.34, and 2.97 +/- 0.45 nM for the Con, Caf, and Epi trials, respectively. Plasma Epi increased to 3.08 +/- 0.56, 5.45 +/- 1.11, and 5.86 +/- 1.03 nM at 14 min of exercise in the Con, Caf, and Epi trials, respectively. Muscle glycogenolysis was not different between trials (Con 220.5 +/- 25.3 vs. Epi 240.6 +/- 12.1 mmolikg dry muscle). In addition, the degradation of muscle ATP and phosphocreatine and the accumulation of muscle lactate, ADP, and AMP were similar between trials. The mole fraction of phosphorylase in the a form was similar between trials at 3 min but was significantly elevated at 15 min of exercise in Epi compared with Con (53.3 +/- 6.1 vs. 40.3 +/- 3.5%). It is concluded that muscle glycogenolysis is unaffected by a doubling of the Epi concentration during intense aerobic exercise despite an elevated phosphorylase a fraction. The glycogenolytic rate appears to be related to the ATP demand and posttransformational control of glycogen phosphorylase during exercise of this intensity.