Complete inhibition of hypothalamic somatostatin activity is only partially responsible for the growth hormone response to strenuous exercise

Abstract

The aim of this study was to investigate whether growth hormone (GH) release during strenuous exercise (EX) is due to complete inhibition of hypothalamic somatostatin (SS) activity. Eight healthy male subjects (age, 22.1 [plusmn] 2.2 years; body mass index [BMI], 22.2 [plusmn] 2.5 kg/m2; maximum oxygen consumption [[Vdot ]o2max], 52.2 [plusmn] 1.5 mL/min/kg [mean [plusmn] SD]) were exposed to strenuous EX on a cycle ergometer, with and without administration of pyridostigmine (PD), and to administration of PD alone. PD is an acetylcholine-esterase inhibitor that stimulates GH secretion by suppressing hypothalamic SS secretion and unmasking endogenous GH-releasing hormone (GHRH) tone. Serial blood samples in the fasted state were taken immediately before the start of each trial, and at appropriate intervals over 2 hours. GH responses were calculated as area under the response curve (AUC) by trapezoidal integration. The mean peak serum GH level to PD alone was 18.3 [mu ]g/L (range, 0.3 to 40.9), which was significantly lower than to EX alone: 64.1 [mu ]g/L (range, 30.5 to 90.5), and to the combined administration of PD and EX (PD+EX): 79.8 [mu ]g/L (range, 37.7 to 98.2) (P [lt ] .05). The arithmetic sum of the individual peak levels of 82.4 [mu ]g/L was not different from the mean peak level to PD+EX: 79.8 [mu ]g/L. AUC (mean [plusmn] SEM) to PD alone (1,721 [plusmn] 358 [mu ]g/L [times ] 180 min) was not significantly different from that to EX alone (2,472 [plusmn] 408 [mu ]g/L [times ] 120 min), but was significantly lower than that to PD+EX: 3,526 [plusmn] 752 (P [lt ] .05). Although the latter AUC was 6% smaller than the AUC obtained by arithmetic addition (3,747 [plusmn] 706), this difference was not statistically significant. In conclusion, the additive effect between PD and EX indicates that PD and EX act independently in evoking GH responses to strenuous EX. Therefore, GH responses to strenuous EX are only partially due to complete inhibition of hypothalamic SS. Additional potentiating factors, such as activation of endogenous GHRH and ghrelin must be operative.