Objective: The neuroendocrine mechanisms of exercise-induced GH release remain incompletely understood. In this study we have investigated the influence of endogenous cholinergic tone and growth hormone-releasing peptide-6 (GHRP-6) on exercise induced GH release.
Design: Analysis of responses of serum GH to administration of pyridostigmine (PD, 120 mg, orally) or GHRP-6 (100 micrograms, i.v.), to exercise alone, and to the combinations of PD plus exercise or GHRP-6 plus exercise. An indirect estimation of the secretory pattern of GH was calculated by the deconvolution technique. Exercise was performed on a bicycle ergometer with a 20-minute workload near the individual lactate threshold of 4 mmol/l. The five tests were performed in random order after an overnight fast, and at least 3 days apart.
Subjects: Eleven healthy, non-obese male subjects (age 23.9 +/- 0.3 years, body mass index 23 +/- 0.7 kg/m2, VO2 max: 52.4 +/- 2.0 ml/min/kg body weight; mean +/- SEM) participated in this study.
Measurements: Serial blood samples from an indwelling catheter were taken before, during and after exercise for analysis of GH (IRMA), lactate (YSI 2300) and haematocrit (micromethod).
Results: Irrespective of the tests, peak values of GH were found between the 18th and 26th minute. The secretory pattern showed differences between the tests. Exercise alone induced relatively short lasting peaks of medium amplitude, whereas PD induced long lasting peaks with low amplitudes. PD plus exercise showed additive effects on the amplitude of GH peaks. GHRP-6 induced long lasting peaks with high amplitude, and GHRP-6 plus exercise also had additive effects on the amplitude.
Conclusions: The increase in frequency and amplitude of GH peaks, which occurred during the GHRP-6 plus exercise, indicates that exercise-induced GH release is not mediated through an increment in the release of an endogenous GHRP-6-like ligand, favouring the possibility that exercise-induced GH release is mediated through an increase in endogenous GHRH release.