PubMed1997
OBJECTIVES: Hexarelin (HEX) is a synthetic hexapeptide belonging to the growth hormone-releasing peptide (GHRP) family. The exact mechanism underlying the strong GH-releasing activity of GHRPs is still unclear, though it has been shown that they act both at the pituitary and the hypothalamic level, where they have specific receptors. To clarify the influence of the cholinergic system on the GH-releasing activity of GHRPs in man, we investigated the effects of pyridostigmine, a cholinergic agonist which stimulates GH secretion by inhibiting somatostatin release, on the GH response to various HEX doses. DESIGN: We studied the GH release induced by various HEX doses (0.25, 0.5 and 2.0 micrograms/kg i.v.) and pyridostigmine (PD, 120 mg po), both alone and coadministered. The interactions between the lowest HEX dose or PD and the maximally effective GHRH dose (1.0 microgram/kg i.v.) were also studied. SUBJECTS: Six normal male volunteers, aged 24-30 years, were studied. MEASUREMENTS: Serum GH was measured in duplicate by immunoradiometric assay. RESULTS: The GH response to HEX administration was dose-dependent. In fact, the GH response to 0.25 microgram/kg HEX (AUC, mean +/- SEM: 816.4 (235.6 mU/l/120 min) was lower, although not significantly, than that to 0.5 microgram/kg HEX (2154.6 +/- 491.6 mU/l/120 min), which, in turn, was lower (p < 0.05) than that after 2.0 micrograms/kg HEX (4819.2 +/- 668.0 mU/l/120 min). The GH rise after GHRH (1299.2 +/- 222.8 mU/l/120 min) was lower (P < 0.05) than that after 2.0 micrograms/kg HEX, but not different from the responses to either 0.25 or 0.5 microgram/kg HEX. PD induced a significant GH rise (559.0 +/- 129.8 mU/l/120 min, P < 0.05 vs saline), similar to that after 0.25 microgram/kg HEX, and lower than those after both 0.5 and 2.0 micrograms/kg HEX (P < 0.05 and p < 0.01, respectively) and GHRH (p < 0.05). PD pretreatment enhanced the GH response to the lowest HEX dose (1961.4 +/- 253.8 mU/l/120 min, p < 0.05) in an additive way, but failed to modify the GH response to either 0.5 or 2.0 micrograms/kg HEX (2753.6 +/- 444.6 and 5179.0 +/- 770.8 mU/l/120 min, respectively). Notably, the GH response to 0.25 microgram/kg HEX + PD was still lower (P < 0.05) than that to 2.0 micrograms/kg HEX. PD pretreatment as well as 0.25 microgram/kg HEX truly potentiated the GH response to GHRH to the same extent (4926.6 +/- 912.8 mU/l/120 min, p < 0.05 and 5958.8 +/- 750.0 mU/l/120 min, p < 0.05 respectively). The GH responses to PD + GHRH and 0.25 microgram/kg HEX + GHRH were similar to that after 2.0 micrograms/kg HEX alone. CONCLUSIONS: Our results demonstrate that pyridostigmine is able to enhance the GH response only to a very low dose Hexarelin which, in turn, potentiates the GHRH-induced GH rise to the same extent as pyridostigmine. As there is evidence that GHRPs do not inhibit hypothalamic somatostatin release, these findings are consistent with the hypothesis that they act by antagonizing somatostatin activity and/or through unknown factors. On the other hand, though there is evidence showing that GHRH activity is needed for GHRP action, our findings indicate that GHRPs act, at least partially, independently of GHRH.