EFFECTS OF EXERCISE DURING NORMOXIA AND HYPOXIA ON THE GROWTH-HORMONEINSULIN-LIKE GROWTH-FACTOR-I AXIS

The response of plasma insulin-like growth factor I (IGF I) to exercis e-induced increase of total human growth hormone concentration [hGH(to t)] and of its molecular species [hGH(20kD)] was investigated up to 48 h after an 1-h ergometer exercise at 60% of maximal capacity during n ormoxia (N) and hypoxia (H) (inspiratory partial pressure of oxygen = 92 mmHg (12.7 kPa); n = 8). Lactate and glucose concentrations were di fferently affected during both conditions showing higher levels under H. Despite similar maximal concentrations, the increase of human growt h hormone (hGH) was faster during exercise during H than during N[hGH( tot) 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 [hGH(20kD)] were close ly correlated to those of [hGH(tot)], but its absolute concentration d id not exceed 3% of the [hGH(tot)]. Plasma IGF I concentration was sig nificantly decreased 24 h after both experimental conditions [N from 3 19 (SD 71) ng . ml(-1) to 228 (SD 72) ng . ml(-1), P < 0.05; H from 25 3 (SD 47) to 200 (SD 47) ng . ml(-1): P < 0.01], and was still lower t han basal levels 48 h after exercise during H [204 (SD 44) ng . ml(-1) , P < 0.01]. Linear regression analysis yielded no significant correla tion between increase in plasma [hGH(tot)] or [hGH(20kD)] during exerc ise and the plasma IGF I concentration after exercise. It was conclude d that the exercise-associated elevated plasma [hGH] did not increase the hepatic IGF I production. From our study it would seem that the hi gh energy demand during and after the long-lasting intensive exercise may have overridden an existing hGH stimulus on plasma IGH I, which wa s most obvious during hypoxia.