We investigated the metabolic actions of ghrelin in humans by examining the effects of acute administration of acylated ghrelin, unacylated ghrelin, and the combination in eight adult-onset GH-deficient patients. We followed glucose, insulin, and free fatty acid concentrations before and after lunch and with or without the presence of GH in the circulation. We found that acylated ghrelin, which is rapidly cleared from the circulation, induced a rapid rise in glucose and insulin levels. Unacylated ghrelin, however, prevented the acylated ghrelin-induced rise in insulin and glucose when it was coadministered with acylated ghrelin. Surprisingly, the injection of acylated ghrelin induced an acute increase in unacylated ghrelin and therefore total ghrelin levels. Finally, acylated ghrelin decreased insulin sensitivity up to the end of a period of 6 h after administration. This decrease in insulin sensitivity was prevented by coinjection of unacylated ghrelin. This combined administration of acylated and unacylated ghrelin even significantly improved insulin sensitivity, compared with placebo, for at least 6 h, which warrants studies to investigate the long-term efficacy of this combination in the treatment of disorders with disturbed insulin sensitivity.

Objective: Ghrelin stimulates growth hormone (GH) secretion both in vivo and in vitro. Ghrelin is mainly produced in and released from the stomach but it is probably also produced in the hypothalamic arcuate nucleus. Whether pituitary GH release is under the control of ghrelin from the stomach and/or from the arcuate nucleus is not known. Moreover, no data on the feedback of GH on systemic ghrelin concentrations are available. It has recently been suggested that ghrelin may induce obesity. Design: In this study, we addressed the following two questions: a) are circulating ghrelin levels increased in human GH deficiency (GHD), and b) does GH treatment modify ghrelin levels in human GHD? Methods: The study group consisted of 23 patients with GHD. Eighteen had developed adult-onset GHD and five had developed GHD in their childhood (childhood-onset GHD). Ghrelin was measured with a commercially available radioimmunoassay. All measurements were performed twice, first at baseline, before the start of GH replacement therapy, and then again after one year of therapy. GH doses were adjusted every 3 months, targeting serum total IGF-I levels within the normal gender-and age-related reference values for the healthy population. Maintenance doses were continued once the target serum total IGF-I levels were reached. Results: The sum of skinfolds and body water increased significantly, body fat mass and percentage body fat decreased significantly and body mass index and waist-hip ratio were not significantly changed by one year of GH replacement therapy.Before the start of GH replacement therapy, mean value and range for fasting ghrelin in the studied GHD subjects tended to be lower in comparison with healthy subjects in the control group although the difference did not reach significance (GHD ghrelin mean 67.8 pmol/l, range 37.6-116.3 pmol/l; control mean 83.8 pmol/l, range 35.4 -132 pmol/l; P ¼ 0:11).One year of GH replacement therapy did not modify circulating ghrelin levels (ghrelin before GH therapy: 67.8 pmol/l, range after GH therapy: 65.3 pmol/l,; P ¼ 0:56). Conclusions: We did not observe elevated ghrelin levels in adult GHD subjects and GH replacement therapy did not modify circulating ghrelin levels, despite significant decreases in body fat mass and percentage body fat. It is conceivable that the lack of ghrelin modifications after long-term GH therapy was due to the reduction of adiposity and insulin on one hand, and increased GH secretion on the other. However, it is still possible that systemic ghrelin is involved in the development of obesity, both in normal and GHD subjects.

Ghrelin possesses endocrine and non-endocrine actions mediated by the GH Secretagogue (GHS)-Receptors (GHS-R). The regulation of ghrelin secretion is still largely unknown. Somatostatin (SRIF) modulates central and gastroenteropancreatic hormonal secretions and functions. SRIF actions are partially shared by cortistatin (CST), a natural SRIF analogue, that binds all SRIF receptors and also GHS-R. Herein, we studied the effects of SRIF-14 or CST-14 (2.0 micro g/kg/h i.v. over 120 min) and of placebo on ghrelin, GH, insulin, glucagon and glucose levels in 6 normal young men. Placebo unaffected GH, insulin, glucagon, glucose and ghrelin levels. SRIF and CST similarly inhibited (p < 0.05) spontaneous GH secretion of about 90%. After SRIF or CST withdrawal, GH levels recovered to baseline levels. Both SRIF and CST similarly inhibited (p<0.01) insulin secretion of about 45%. In both sessions, after SRIF or CST withdrawal, insulin overrode baseline levels. Both SRIF and CST similarly inhibited (p < 0.01) glucagon levels of about 40%. After SRIF or CST withdrawal, glucagon persisted lower (p < 0.05) than at baseline. Neither SRIF nor CST modified glucose levels. Both SRIF and CST similarly inhibited (p < 0.01) circulating ghrelin levels of about 55%. Ghrelin levels progressively decreased from time +15 min, reaching the nadir at 120 and 105 min for SRIF and CST, respectively. Even 30 min after SRIF or CST withdrawal, ghrelin levels persisted lower (p < 0.05) than those at baseline. In conclusion, this study first shows that SRIF and CST strongly inhibits ghrelin secretion that, differently from GH and insulin secretion, persists inhibited even after stopping the infusion of SRIF or CST.

Objectives: In humans, fasting leads to elevated serum GH concentrations. Traditionally, changes in hypothalamic GH-releasing hormone and somatostatin release are considered as the main mechanisms that induce this elevated GH secretion during fasting. Ghrelin is an endogenous ligand of the GH secretagogue receptor and is synthesized in the stomach. As ghrelin administration in man stimulates GH release, while serum ghrelin concentrations are elevated during fasting in man, this increase in ghrelin levels might be another mechanism whereby fasting results in stimulation of GH release. Design and subjects: In ten healthy non-obese males we performed a double-blind placebo-controlled crossover study comparing fasting with and fasting without GH receptor blockade. GH, ghrelin, insulin, glucose and free fatty acids were assessed. Results: While ghrelin levels do not vary considerably in the fed state, fasting rapidly induced a diurnal rhythm in ghrelin concentrations. These changes in serum ghrelin concentrations during fasting were followed by similar, profound changes in serum GH levels. The rapid development of a diurnal ghrelin rhythm could not be explained by changes in insulin, glucose, or free fatty acid levels. Compared with fasting without pegvisomant, fasting with pegvisomant did not change the ghrelin rhythm. Conclusions: These data indicate that ghrelin is the main driving force behind the enhanced GH secretion during fasting.

Ghrelin secretion has been reportedly increased by fasting and energy restriction but decreased by food intake, glucose, insulin, and somatostatin. However, its regulation is still far from clarified. The cholinergic system mediates some ghrelin actions, e.g. stimulation of gastric contractility and acid secretion and its orexigenic activity. To clarify whether ghrelin secretion undergoes cholinergic control in humans, we studied the effects of pirenzepine [PZ, 100 mg per os (by mouth)], a muscarinic antagonist, or pyridostigmine (PD, 120 mg per os), an indirect cholinergic agonist, on ghrelin, GH, insulin, and glucose levels in six normal subjects. PD increased (P < 0.05) GH (change in area under curves, mean +/- SEM, 790.9 +/- 229.3 microg(*)min/liter) but did not modify insulin and glucose levels. PZ did not significantly modify GH, insulin, and glucose levels. Circulating ghrelin levels were increased by PD (11290.5 +/- 6688.7 pg(*)min/ml; P < 0.05) and reduced by PZ (-23205.0 +/- 8959.5 pg(*)min/ml; P < 0.01). The PD-induced ghrelin peak did not precede that of GH. In conclusion, circulating ghrelin levels in humans are increased and reduced by cholinergic agonists and antagonists, respectively. Thus, ghrelin secretion is under cholinergic, namely muscarinic, control in humans. The variations in circulating ghrelin levels induced by PD and PZ are unlikely to mediate the cholinergic influence on GH secretion.