Adrenal hormones enhance glycogenolysis in nonexercising muscle during exercise

The purpose of this study was to investigate whether epinephrine exerts an effect on glycogen metabolism in nonexercising (Non-Ex) as well as in exercising (Ex) skeletal muscle. Rats ran (15 m/min; 8% grade) on their forelimbs while their hindlimbs (Non-Ex) were suspended above the treadmill. Electromyographic records confirmed the lack of significant contractile activity in muscles during suspension. Plasma epinephrine levels were manipulated in three experimental groups (n = 20 for each group): adrenalectomized (ADX), intact adrenals (IA), and IA + epinephrine injection (+Ep). Another group of rats performed normal exercise on all four limbs (15 m/min; 8% grade). Muscle glycogen levels were measured in selected hindlimb muscles at t = 0 and after 90 min exercise (15 m/min; 8% grade) or suspended rest. In the absence of epinephrine (ADX), no glycogen loss was found (P greater than 0.05) in Non-Ex muscles during the exercise period. In the IA group (epinephrine levels elevated sixfold above basal at t = 90 min), glycogen levels in the nonexercising soleus, plantaris, and red and white gastrocnemius were significantly (P less than 0.05) depleted to 62 +/- 6, 67 +/- 6, 58 +/- 5, and 67 +/- 9% of control values, respectively. Similar decrements occurred in these muscles when exercise was performed on all four limbs (P greater than 0.05). We conclude that glycogenolysis occurs in nonexercising skeletal muscle independent of contractile activity, probably due to the effect of epinephrine. Furthermore, the present data strongly suggest that glycogen depletion patterns in muscles during exercise cannot be used as an index of motor unit recruitment.