Effects of exercise during normoxia and hypoxia on the growth hormone-insulin-like growth factor I axis

The response of plasma insulin-like growth factor I (IGF I) to exercise-induced increase of total human growth hormone concentration [hGHtot] and of its molecular species [hGH20kD] was investigated up to 48 h after an 1-h ergometer exercise at 60% of maximal capacity during normoxia (N) and hypoxia (H) (inspiratory partial pressure of oxygen = 92 mmHg (12.7 kPa); n = 8). Lactate and glucose concentrations were differently affected during both conditions showing higher levels under H. Despite similar maximal concentrations, the increase of human growth hormone (hGH) was faster during exercise during H than during N[hGHtot after 30 min: 8.6 (SD 11.4) ng.ml-1 (N); 16.2 (SD 11.6) ng.ml-1 (H); P < 0.05]. The variations in plasma [hGH20kD] were closely correlated to those of [hGHtot], but its absolute concentration did not exceed 3% of the [hGHtot]. Plasma IGF I concentration was significantly decreased 24 h after both experimental conditions [N from 319 (SD 71) ng.ml-1 to 228 (SD 72) ng.ml-1, P < 0.05; H from 253 (SD 47) to 200 (SD 47) ng.ml-1, P < 0.01], and was still lower than basal levels 48 h after exercise during H [204 (SD 44) ng.ml-1, P < 0.01]. Linear regression analysis yielded no significant correlation between increase in plasma [hGHtot] or [hGH20kD] during exercise and the plasma IGF I concentration after exercise. It was concluded that the exercise-associated elevated plasma [hGH] did not increase the hepatic IGF I production. From our study it would seem that the high energy demand during and after the long-lasting intensive exercise may have overridden an existing hGH stimulus on plasma IGH I, which was most obvious during hypoxia.