There is the potential for the side effects associated with use of growth hormone when growth hormone secretagogues are used, particularly if the use is not under medical supervision. There are limited data on the safety of intravenous and subcutaneous use of AOD-9604 and on the long-term oral use of AOD-9604 in doses in excess of those used in clinical trials.
Years ago, our group examined the cytoprotective effects exerted by the GHRP-6 preventive administration in the hepatic tissue subjected to I/R, as in other distal organs from the ischemic site (ie, lungs, kidneys, and small intestine). Histological and biochemical results allowed us to conclude that the pharmacological preconditioning induced by the GHRP-6 treatment attenuated I/R liver damage. Besides respiratory distress syndrome like pulmonary changes, intestinal transmural infarct and acute tubular necrosis in kidneys were significantly reduced. These results indicated for the first time a systemic cytoprotective effect for the GHRP-6, suggesting its potential efficacy to control the inflammatory response associated with acute I/R and shock, which eventually originated multiple organs damage (MOD). Cytoprotection induced by GHRP-6 treatment was also related to the attenuation in the generation of ROS and preservation of the antioxidant defense reserves. Histological analysis as the assessment of myeloperoxidase activity evidenced a clear anti-inflammatory GHRP-6-induced effect in the liver and remote organs. Moreover, the molecular mechanism mediating the action of GHRP-6 peptide was shown to involve the phosphatidylinositol 3-kinase/RAC-alpha serine/threonine-protein kinase (PI-3K/AKT1) pathway, as the induction of the hypoxia-inducible factor-1 alpha (HIF-1α) all committed in cellular survival.51 Subsequently, Granado et al52 examined the potential anti-inflammatory impact of GHRP-2 in lipopolysaccharide (LPS)-challenged rats. GHRP-2 administration attenuated the effects of LPS on the elevation of circulating levels of transaminases, nitrites/nitrates, and tumor necrosis factor-alpha (TNF-α), via direct interaction with liver nonparenchymal cells. Globally, the exogenous administration of these two synthetic GHRPs appeared to exert a potent hepatoprotective role by attenuating the inflammatory response orchestrated by liver-resident macrophages. Another line of evidences document the benefits of 15-daily injections of GHRP-2 (100 μg/kg) in arthritic rats, so that the treatment ameliorated the external symptoms of arthritis and decreased the circulating levels of interleukin 6 (IL-6) as the nitrite/nitrate release from peritoneal macrophages in vitro. This experiment extrapolated the counter-inflammatory properties of GHRP-2 to a nonepithelial organ and suggested again a direct interaction with ghrelin receptor of immune cells.53 Similarly, effects have been attributed to ghrelin by inhibiting the inflammatory response via AKT1-activated pathway with a concomitant reduction of myeloperoxidase activity, the rate of apoptosis, and oxidative stress.54 All these data suggest that GHRPs exert a mutually inclusive beneficial effect by directly protecting parenchymal organs epithelial cells, and simultaneously by modulating the magnitude of the inflammatory response by direct interaction with the effector immune cells. Supporting the protective effect of GHRP-6 on epithelial organs, a recent study has excellently described and dissected the mechanistic bases on how GHRP-6 prevented gastric mucosal damage induced by water immersion restraint (WRS) and other forms of stress. The data indicated that the protective effect of GHRP-6 on WRS-induced gastric mucosal injury is somehow mediated by peripherally suppressing the vagal efferent effect on the stomach, including gastric acid secretion. Although more studies are clearly demanded, the present findings open the possibility to use GHRP-6 in preventing Curling ulcers.55
Results in Fig. 1.8 of normal young men (left panel) and women (right panel) demonstrate that iv bolus combined GHRP-2 and GHRH at the respective doses of 1 μg/kg GHRH and a subthreshold GH-releasing dose of 0.03 μg/kg GHRP-2 released GH synergistically (Bowers, 1998). From these studies, GHRP is envisioned to act on the hypothalamus to release an unknown factor (U factor) rather than endogenous GHRH which subsequently acts concomitantly with GHRH on the pituitary somatotroph to release GH synergistically. In this study, the important specific finding is that GHRP-2 augments GHRH release even when GHRH is present in excess amounts, and the concomitant GHRP-2 dose of 0.03 μg/kg is a subthreshold GH-releasing amount. Thus, GHRP + GHRH is not releasing GH in this study by augmenting endogenous GHRH release and, furthermore, GHRP+GHRH release in vitro is additive and not synergistic. In addition, from other high-dosage GHRP-2 data, that is, 10 μg/kg sc (not shown), we have postulated that at high doses GHRPs do act on the hypothalamus to release endogenous GHRH because high-dose GHRP-2 (10 μg/kg sc) releases the same large amount of GH released by combined GHRH + GHRP-2 at 1 + 1 μg/kg iv (Bowers, 1998a,b).
Growth hormone-releasing peptide 6 (GHRP-6) (developmental code name SKF-110679), also known as growth hormone-releasing hexapeptide, is one of several synthetic met-enkephalin analogues that include unnatural D-amino acids, were developed for their growth hormone-releasing activity and are called growth hormone secretagogues. They lack opioid activity but are potent stimulators of growth hormone (GH) release. These secretagogues are distinct from growth hormone releasing hormone (GHRH) in that they share no sequence relation and derive their function through activation of a completely different receptor. This receptor was originally called the growth hormone secretagogue receptor (GHSR), but due to subsequent discoveries, the hormone ghrelin is now considered the receptor's natural endogenous ligand, and it has been renamed as the ghrelin receptor. Therefore, these GHSR agonists act as synthetic ghrelin mimetics.