Exercise training decreases the growth hormone (GH) response to acute constant-load exercise

To assess the influence of exercise training on the growth hormone (GH) response to acute exercise, six untrained males completed a 20-min, high-intensity, constant-load exercise test prior to and after 3 and 6 wk of training (the absolute power output (PO) during each test remained constant, (x) over bar PO = 182.5 +/- 29.5 W). Training increased (pre- vs post-training) oxygen uptake ((V) over dot O-2) at lactate threshold (1.57 +/- 0.33 L.min(-1) vs 1.97 +/- 0.24 L.min(-1), P less than or equal to 0.05), (V) over dot O-2 at 2.5 mM blood lactate concentration ([HLa]) (1.83 +/- 0.38 L.min(-1) vs 2.33 +/- 0.38 L.min(-1), P less than or equal to 0.05), and (V) over dot O-2peak (3.15 +/- 0.54 L.min(-1) vs 3.41 +/- 0.47 L.min(-1), P less than or equal to 0.05). Power output at the lactate threshold (PO-LT) increased with training from 103 +/- 28 to 132 +/- 23W (P less than or equal to 0.05). Integrated GH concentration (20 min exercise + 45 min recovery) (mu g.L-1 x min) after 3 wk (138 +/- 106) and 6 wk (130 +/- 145) were significantly lower (P less than or equal to 0.05) than pre-training (238 +/- 145). Plasma epinephrine and norepinephrine responses to training were similar to the GH response (EPI - pre-training = 2447 +/- 1110; week 3 = 1046 +/- 144; week 6 = 955 +/- 322 pmol.L-1; P less than or equal to 0.05; NE - pre-training = 23.0 +/- 5.2; week 3 = 13.4 +/- 4.8; week 6 = 12.1 +/- 6.8 nmol.L-1 P less than or equal to 0.05). These data indicate that the GH and catecholamine response to a constant-load exercise stimulus are reduced within the first 3 wk of exercise training and support the hypothesis that a critical threshold of exercise intensity must be reached to stimulate GH release.