Effect of physical training on the capacity to secrete epinephrine

Epinephrine responses to hypoglycemia and to identical relative work loads have been shown to be higher in endurance-trained athletes than in untrained subjects. To test the hypothesis that training increases the adrenal medullary secretory capacity, we studied the effects of glucagon (1 mg/70 kg iv), acute hypercapnia (inspired O2 fraction = 7%), and acute hypobaric hypoxia (inspired Po2 = 87 Torr), respectively, on the epinephrine concentration in arterialized hand vein blood in eight endurance-trained athletes [T, O2 uptake = 66 (62-70) ml.min-1.kg-1] and seven sedentary males [C, O2 uptake = 46 (41-50)]. In response to identical increments in glucagon concentrations, plasma epinephrine increased more in T than in C subjects [0.87 +/- 0.11 vs. 0.38 +/- 0.14 (SE) nmol/l, P less than 0.05]. In response to hypercapnia [arterial PCO2 = 56 +/- 0.7 Torr (T) and 55 +/- 0.4 (C), P greater than 0.05], the increment in epinephrine was significant in T (0.38 +/- 0.11 nmol/l) but not (P less than 0.1) in C subjects (0.22 +/- 0.11). Hypoxia [arterial PO2 = 42 +/- 2 Torr (T) and 41 +/- 2 (C), P greater than 0.05] increased epinephrine in T (0.22 +/- 0.10 nmol/l, P less than 0.05) but not in C subjects (0.01 +/- 0.07). The plasma norepinephrine concentration never changed, whereas heart rate always increased, the increase being higher (P less than 0.05) in T than in C subjects only during hypercapnia. The results indicate that training increases the capacity to secrete epinephrine.