GH response to exercise: Assessment of the pituitary refractory period, and relationship with circulating components of the GH-IGF-I axis in adolescent females

The concept of pituitary refractory period for GH secretion has been previo usly described. To measure the length of this refractory period we performe d an exercise provocation test for GH secretion immediately following multi ple overnight GH blood sampling. In addition, we correlated the magnitude o f the GH response to a single exercise input with mean overnight GH, IGF-I and circulating IGFBP levels. 23 healthy adolescent females (15-17 yr) perf ormed 10-min constant cycle ergometry at a power normalized to each subject 's aerobic and anaerobic capacity. GH was measured every 10 min starting 10 min before exercise and then for 60 min after the exercise bout. Mean noct urnal GH was calculated from overnight values obtained every 20 min over a 12-h period. Pre-exercise GHBP, IGF-I and IGFBPs 1-5 were assessed using st andard techniques. In five subjects, a spontaneous GH peak had preceded the exercise test by 1 hour or less, and no response to exercise was found. In the remaining 18 subjects, a GH peak (6.8+/-1.3 ng/ml, p<0.0001) was obser ved at 32+/-4 min after the onset of exercise. The GH response to exercise was not correlated with fitness, mean GH or IGF-I but was correlated with I GFBP-3 (r-0.65, p<0.05). Spontaneous GH pulses may acutely render the pitui tary refractory to exercise stimuli. The length of this refractory period i s approximately 1 hour. The data corroborate the idea that while relationsh ips exist among the various components of the GH-IGF-I axis, no single fact or identified to date fully reflects GH-IGF-I "tone".