INFLUENCE OF ENDOGENOUS CHOLINERGIC TONE AND GROWTH HORMONE-RELEASINGPEPTIDE-6 ON EXERCISE-INDUCED GROWTH-HORMONE RELEASE

OBJECTIVE The neuroendocrine mechanisms of exercise-induced GH release remain incompletely understood. In this study we have Investigated th e influence of endogenous cholinergic tone and growth hormone-releasin g peptide-6 (GHRP-6) on exercise induced GH release. DESIGN Analysis o f responses of serum GH to administration of pyridostigmine (PD, 120 m g, orally) or GHRP-6 (100 mu g, i.v.), to exercise alone, and to the c ombinations of PD plus exercise or GHRP-6 plus exercise. An indirect e stimation of the secretory pattern of GH was calculated by the deconvo lution 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 s ubjects (age 23.9 +/- 0.3 years, body mass index 23 +/- 0.7 kg/m(2), V over dot (O2) 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 a nalysis of GH (IRMA), lactate (YSI 2300) and haematocrit (micromethod) . RESULTS Irrespective of the tests, peak values of GH were found betw een the 18th and 26th minute. The secretory pattern showed differences between the tests. Exercise alone induced relatively short lasting pe aks of medium amplitude, whereas PO induced long lasting peaks with lo w amplitudes. PD plus exercise showed additive effects on the amplitud e of GH peaks. GHRP-6 induced long lasting peaks with high amplitude, and GHRP-6 plus exercise also had additive effects on the amplitude. C ONCLUSIONS The increase in frequency and amplitude of GH peaks, which occurred during the GHRP-6 plus exercise, indicates that exercise-indu ced GH release is not mediated through an increment in the release of an endogenous GHRP-6-like ligand, favouring the possibility that exerc ise-induced on release is mediated through an increase in endogenous G HRH release.