SOMATOSTATINERGIC TONE IMPAIRES GH RESPONSE TO EXAUSTIVE EXERCISE IN MIDDLE-AGED SUBJECTS

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A decrease of resting pituitary GH secretion and a reduction of GH response to provocative stimuli is well-known in people over 60, possibly for an increase in somatostatinergic tone. Exercise is a powerful stimulus for GH secretion, but the mechanism is not known yet. We have previously observed, that the GH response to maximal exercise is also impaired in middle-aged active men. PURPOSE: To investigate the possible role of somatostatinergic tone in the impaired GH response to maximal exercise in middle-age. METHODS: Six healthy moderately-trained men (mean ± SE, age 43 ± 3y, height 177.2 ± 3.5cm, weight 79.2 ± 3.7kg) participated in two randomised double-blind protocols. In the first protocol (A), GH response to exercise (Ex) was studied comparing Arginine (Arg) infusion (25g, 30 minutes overall, from 15 min before Ex) with saline (S). In the second one (B), GH response to Ex was studied comparing GHRH infusion (1γ/kg body weight, 60 minutes overall, from 30 min before Ex) with S. The Ex protocol was performed on cycle ergometer (50W × 3min) to exhaustion, monitoring gas exchange. The GH response to Arg and GHRH infusion was also studied at rest (R). Blood samples for plasma lactate, GH and glucose have been collected before, at the end of the Ex, and in the recovery time (1-3-5-10-15-20-30 min). RESULTS: Arg, GHRH did not affect VO2 max, lactate and glucose responses. In A, the GH peak was significantly higher in Ex+Arg (25.1 ± 5.8ng/ml) than in Ex+S (9.34 ± 2.06ng/ml, P < .05) and than in Arg+R (15.3 ± 8.7ng/ml, P < .05). In B, the GH peak during Ex+GHRH (52.7 ± 8.7ng/ml) was significantly higher than Ex+S (12,5 ± 4.0ng/ml, P < .01), but not different from GHRH+R (48.9 ± 11.7ng/ml, P=n.s.). CONCLUSIONS: The administration of Arginine, a somatostatinergic tone inhibitor, significantly improves the GH response to exercise. Exercise does not modify the GH response to GHRH administration. Our findings suggest that in middle-aged people the GH response to maximal exercise is impaired by an increase of somatostatinergic tone.