GLUCAGON AND PLASMA CATECHOLAMINES DURING BETA-RECEPTOR BLOCKADE IN EXERCISING MAN

Seven men ran at 60% of individual maximal O2 uptake .ovrhdot.VO2 max to exhaustion during .beta.-adrenergic blockade with propranolol (P), during lipolytic blockade with nicotinic acid (N) or without drugs (C). The total work times (83 .+-. 9 (P), 122 .+-. 8 (N), 166 .+-. 10 (C) min, mean and SE) differed significantly. Epinephrine rose progressively above preexercise levels (0.06 .+-. 0.01 ng/ml); at exhaustion concentrations in P experiments (2.15 .+-. 0.41) were larger than in N (1.08 .+-. 0.31) and C (0.72 .+-. 0.28) experiments. Norepinephrine increased consistently while insulin decreased. After an initial decrease glucagon concentrations increased progressively in parallel with declining plasma glucose and were at exhaustion always 3 times preexercise values. .beta.-adrenergic blockade did not diminish the glucagon response. Nor was this response increased when .alpha.-recptor stimulation in P experiments was intensified. Carbohydrate combustion was smaller and NEFA [nonesterified fatty acids] and glycerol concentrations in serum larger during C experiments. Alanine concentrations were never raised at exhaustion. Neither stimulation of adrenergic receptors nor NEFA and alanine concentrations are major determinants for the exercise-induced glucagon secretion in man. Decreased glucose availability apparently enhances the secretion of glucagon and epinephrine during prolonged exercise.