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 c oncentrations observed after caffeine (Caf) ingestion. Seven male subj ects 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 determi ne resting and exercise plasma Epi concentrations. Trial 2 was used to determine the Epi infusion rates needed to mimic the plasma Epi level s found with Caf. Trials 3 and 4 were randomized and consisted of eith er 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 C af and Epi and were elevated twofold compared with Con. At 5 min of ex ercise 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, respectiv ely. 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 fr action of phosphorylase in the a form was similar between trials at 3 min but was significantly elevated at 15 min of exercise in Epi compar ed with Con (53.3 +/- 6.1 vs. 40.3 +/- 3.5%). It is concluded that mus cle glycogenolysis is unaffected by a doubling of the Epi concentratio n during intense aerobic exercise despite an elevated phosphorylase a fraction. The glycogenolytic rate appears to be related to the ATP dem and and posttransformational control of glycogen phosphorylase during exercise of this intensity.