Ghrelin does not mediate the somatotroph and corticotroph responses to the stimulatory effect of glucagon or insulin-induced hypoglycaemia in humans

The stimulatory effects of glucagon on cortisol and GH secretion occur independently from FGF-21

PURPOSE

Glucagon stimulation test (GST) is used to assess the hypothalamo-pituitary-adrenal (HPA) and growth hormone (GH) axes with an incompletely defined mechanism. We aimed to assess if glucagon acted through fibroblast growth factor-21 (FGF-21) to stimulate cortisol and GH secretion. The secondary outcome was to determine the relationship of FGF-21 with variable GH responses to GST in obesity.

METHODS

A total of 26 healthy participants; 11 obese (body mass index (BMI) > 30 kg/m²) and 15 leans (BMI < 25 kg/m²) were included. Basal pituitary and target hormone levels were measured and GST was performed. During GST, glucose, insulin, cortisol, GH, and FGF-21 responses were measured.

RESULTS

The mean age of the participants was 26.3±3.6 years. Glucagon resulted in significant increases in FGF-21, glucose, insulin, cortisol, and GH levels. The levels of basal cortisol, GH, FGF-21, and IGF-1 were similar in the two groups. The peak GH and area under the curve (AUC)_(GH) responses to GST in the obese group were lower than those of the normal-weight group with a different pattern of response. There were no differences between the groups in terms of peak cortisol, AUC_(cortisol), peak insulin, AUC_(insulin), peak FGF-21, and AUC_(FGF21). Obesity was associated with significantly increased glucose and insulin responses and slightly decreased FGF-21 response to glucagon.

CONCLUSION

Obesity was associated with blunted and delayed GH, but preserved cortisol responses to GST. This is the first study showing that glucagon stimulates the HPA and GH axis independently from FGF-21. The delayed GH response to GST in obesity does not seem to be related to FGF-21.

Investigation of the Hypothalamo-pituitary-adrenal (HPA) axis: a contemporary synthesis

The hypothalamo-pituitary-adrenal (HPA) axis is one of the main components of the stress system. Maintenance of normal physiological events, which include stress responses to internal or external stimuli in the body, depends on appropriate HPA axis function. In the case of severe cortisol deficiency, especially when there is a triggering factor, the patient may develop a life-threatening adrenal crisis which may result in death unless early diagnosis and adequate treatment are carried out. The maintenance of normal physiology and survival depend upon a sufficient level of cortisol in the circulation. Life-long glucocorticoid replacement therapy, in most cases meeting but not exceeding the need of the patient, is essential for normal life expectancy and maintenance of the quality of life. To enable this, the initial step should be the correct diagnosis of adrenal insufficiency (AI) which requires careful evaluation of the HPA axis, a highly dynamic endocrine system. The diagnosis of AI in patients with frank manifestations is not challenging. These patients do not need dynamic tests, and basal cortisol is usually enough to give a correct diagnosis. However, most cases of secondary adrenal insufficiency (SAI) take place in a gray zone when clinical manifestations are mild. In this situation, more complicated methods that can simulate the response of the HPA axis to a major stress are required. Numerous studies in the assessment of HPA axis have been published in the world literature. In this review, the tests used in the diagnosis of secondary AI or in the investigation of suspected HPA axis insufficiency are discussed in detail, and in the light of this, various recommendations are made.

Evaluation of Hypothalamic-Pituitary-Adrenal Axis by the GHRP2 Test: Comparison With the Insulin Tolerance Test

Context

GH-releasing peptide 2 (GHRP2) stimulates the hypothalamic–pituitary–adrenal axis (HPA) through the GH secretagogue receptor (GHSR) in the hypothalamus, in which ghrelin is a natural ligand. Therefore, the GHRP2 test (GHRP2T) could be used instead of the insulin tolerance test (ITT).

Objective

Can the GHRP2T replace the ITT for evaluation of HPA?

Design

The present retrospective study analyzed the clinical features and laboratory data from 254 patients admitted for evaluation of hypopituitarism who underwent both GHRP2T and ITT. We analyzed the association between the maximum cortisol level (Fmax) during both tests. Adrenocortical insufficiency was diagnosed by ITT. The suitability of GHRP2T was examined using the receiver operating characteristic curve.

Results

A strong correlation was found between Fmax measured using both tests (r = 0.777, P < 0.0001). However, the sensitivity (64%) and specificity (79%) showed that the GHRP2T was not suitable for clinical use. Various factors influenced the correlation, probably through their effects on ghrelin and/or GHSR, including functional adenoma (P < 0.05) and sex (P < 0.05). No substantial correlation was found between Fmax measured using both tests in patients with prolactinoma (n = 30). The exclusion of patients with functional adenoma revealed no factors that affected the association in male patients; however, age and menstruation significantly influenced it in female patients (P < 0.05). Analysis of the data from male subjects without functional adenoma (n = 104) showed high sensitivity (95%) and specificity (85%) for the GHRP2T.

Conclusion

ITT can be substituted with GHRP2T for assessment of HPA in male patients free of functional adenoma.

A significant proportion of thalassemia major patients have adrenal insufficiency detectable on provocative testing

Advances in chelation therapy and noninvasive monitoring of iron overload have resulted in substantial improvements in the survival of transfusion-dependent patients with thalassemia major. Myocardial decompensation and sepsis remain the major causes of death. Although endocrine abnormalities are a well-recognized problem in these iron-overloaded patients, adrenal insufficiency and its consequences are underappreciated by the hematology community. The aims of this study were to determine the prevalence of adrenal insufficiency in thalassemia major subjects, to identify risk factors for adrenal insufficiency, and to localize the origin of the adrenal insufficiency within the hypothalamic-pituitary-adrenal axis. Eighteen subjects with thalassemia major (18.9±9.3 y old, 7 female) were tested for adrenal insufficiency using a glucagon stimulation test. Those found to have adrenal insufficiency (stimulated cortisol <18 µg/dL) subsequently underwent an ovine corticotropin-releasing hormone (oCRH) stimulation test to define the physiological basis for the adrenal insufficiency. The prevalence of adrenal insufficiency was 61%, with an increased prevalence in males over females (92% vs. 29%, P=0.049). Ten of 11 subjects who failed the glucagon stimulation test subsequently demonstrated normal ACTH and cortisol responses to oCRH, indicating a possible hypothalamic origin to their adrenal insufficiency.

Ghrelin levels after a cold pressor stress test in obese women with binge eating disorder

OBJECTIVE

Ghrelin, a peptide hormone secreted mainly by the stomach, increases appetite and food intake. Surprisingly, ghrelin levels are lower in obese individuals with binge eating disorder (BED) than in obese non-BED individuals. Acute psychological stress has been shown to raise ghrelin levels in animals and humans. Our aim was to assess ghrelin levels after a cold pressor test (CPT) in women with BED. We also examined the relationship between the cortisol stress response and changes in ghrelin levels.

METHODS

Twenty-one obese (mean [standard deviation] body mass index = 34.9 [5.8] kg/m(2)) women (10 non-BED, 11 BED) underwent the CPT, hand submerged in ice water for 2 minutes. Blood samples were drawn for 70 minutes and assayed for ghrelin and cortisol.

RESULTS

There were no differences between the groups in ghrelin levels at baseline (-10 minutes). Ghrelin rose significantly after the CPT (F = 2.4, p = .024) peaking at 19 minutes before declining (F = 17.9, p < .001), but there were no differences between the BED and non-BED groups. Area under the curve for ghrelin was not related to ratings of pain, stress, hunger, or desire to eat after CPT. In addition, there were no observed relationships between the area under the curves for ghrelin or cortisol after stress.

CONCLUSIONS

Although there were no differences between BED groups, there was a significant rise in ghrelin in obese humans after a stressor, consistent with other recent reports suggesting a stress-related role for ghrelin.

D, McKeever K. Effects of exogenous ghrelin infusion on feed intake and metabolic parameters of energy homeostasis in Standardbred mares

Six Standardbred mares (age 12±2 years, body weight 502±63 kg; mean ± standard deviation) were given 1.6 µg/kg acylated human ghrelin or vehicle treatment as an intravenous bolus in a randomised, cross-over design to test the hypothesis that exogenous ghrelin infusion would increase feed intake and alter metabolic parameters of energy homeostasis, leptin, glucose, insulin and cortisol. After the horses had initial access to hay cubes for 1.5 h, doses were given and hay cubes were available once again. Leftover feed was weighed 6 times over each of the 24 h testing periods. Blood samples for measurement of active ghrelin, growth hormone, leptin, glucose, insulin and cortisol were taken at time 0 (immediately before infusion) and 20, 40, 60, 80, 100, 120, 240, 480, and 720 min post-infusion. Every 10 min, the horses’ behaviour was recorded for eating, drinking, resting, and other behaviours. Ghrelin infusion did not increase (P≯0.05) feed intake in the mares as a group, but did increase feed intake (P<0.04) in horses that had the highest growth hormone response to ghrelin infusion. Plasma concentrations of active ghrelin growth hormone, glucose, insulin and cortisol were all increased (P<0.05) by ghrelin infusion. There was no significant change in plasma leptin concentration due to treatment. Ghrelin infusion did not cause a significant change in the number of eating episodes either 2 h post-treatment or for the 24 h testing period. Regression analysis suggests that the increase in feed intake in horses with the highest growth hormone response to ghrelin infusion may be due to their lower (P<0.05) body condition score and % body fat compared with horses that did not increase feed intake and had lower growth hormone response to ghrelin infusion.

Serum ghrelin levels are suppressed in hypopituitary patients following insulin-induced hypoglycaemia irrespective of GH status

OBJECTIVE

Circulating ghrelin is suppressed during insulin-induced hypoglycaemia in healthy subjects, but it is unknown whether this is determined by feedback inhibition from counter-regulatory hormones. We therefore investigated the impact of GH and cortisol on ghrelin secretion during insulin-induced hypoglycaemia.

DESIGN

Serum levels of ghrelin, GH, and cortisol were measured in 41 adult patients with suspected hypopituitarism during insulin-induced hypoglycaemia. Based on their peak GH response (cut-off level: 3 microg/l), the patients were divided into a GH-sufficient group (GHS) and a GH-deficient group (GHD).

RESULTS

The GHS patients (n = 16) were younger (P < 0.01), had lower baseline cortisol levels [255 +/- 37 vs. 372 +/- 38 nmol/l (P = 0.04)], and tended to have a lower body mass index (P = 0.09) as compared to GHD patients (n = 25). By definition, peak GH (microg/l) was higher in GHS patients [15.0 +/- 1.8 vs. 1.0 +/- 0.2 (P < 0.0001)]. The increase in serum cortisol during the ITT (insulin-tolerance test) was higher and occurred later in GHS patients [Cmax (nmol/l): 561 +/- 41 vs. 412 +/- 50 (P = 0.04); Tmax (minutes): 65 +/- 5 vs. 49 +/- 5 (P = 0.03)]. Serum ghrelin levels changed significantly with time during ITT in both groups (P < 0.0001), characterized by a moderate decline during the initial 50-60 min and a return to baseline after 2 h. No significant differences were recorded in AUCghrelin during the ITT between the two groups. No gender differences in ghrelin levels were recorded.

CONCLUSIONS

(i) Like in healthy subjects serum ghrelin levels are suppressed during an ITT in patients with suspected hypopituitarism. (ii) The suppression of ghrelin was similar in GHD and GHS subjects and was not determined by cortisol. (iii) We hypothesize that insulin rather than hypoglycaemia accounts for ghrelin suppression during an ITT.

Short-Term Secretory Regulation of Ghrelin during Growth Hormone Provocative Tests in Prepubertal Children with Various Growth Hormone Secretory Capacities

BACKGROUND AND OBJECTIVE

Ghrelin is a novel gastric peptide which stimulates GH secretion and has been demonstrated to have orexigenic and adipogenic properties. Insulin is a physiological and dynamic modulator of plasma ghrelin, and insulinemia possibly mediates the effect of the nutritional state on the plasma concentrations of ghrelin in adults. No data on the regulation of GH secretion by ghrelin have so far been reported, nor has the possible influence of hypoglycemia on the plasma ghrelin levels in children been reported.

METHODS

Provocative studies were performed using a variety of stimuli, including insulin-induced hypoglycemia, and glucagon, arginine and L-dopa loading. We studied a group of 27 children with short stature being investigated for GH deficiency (10 F, 17 M; age 4-14 years; height SDS -0.92 to -3.27); the subjects were instructed to fast overnight, and the following morning, the relationships among the plasma ghrelin, GH and glucose levels were investigated by determining the plasma ghrelin profiles during those provocative tests. Using a new method for determining the two types of ghrelin, samples were obtained for determination of the plasma ghrelin, serum glucose and serum GH levels after the administration of the aforementioned stimulating agents.

RESULTS

All the four stimuli caused a significant decrease in the circulating C- and N-ghrelin levels with a nadir at +30 min, with the exception of the N-ghrelin level following the L-dopa loading. During the same period, the plasma GH level increased following insulin, arginine and L-dopa loading, and the plasma glucose level increased significantly following glucagon loading. In the arginine and L-dopa load connected, a significant correlation was observed between the 30-min change in the serum GH level and the 30-min change in the plasma C-ghrelin level. In the multiple regression analysis to explain the 30-min change in the plasma level of C-ghrelin, the baseline plasma level of C-ghrelin (basal), height and % overweight were the only three significant parameters, accounting for 85.2% of the variance.

CONCLUSION

This study demonstrated that the inverse relation between the circulating GH and ghrelin levels may indicate the existence of a feedback loop, and also lends support to the assumption of a GH-independent relationship between plasma ghrelin and glucose levels. These observations constitute further evidence to suggest that peripheral ghrelin is a direct growth-promoting hormone.

Prandial regulation of ghrelin secretion in humans: does glucagon contribute to the preprandial increase in circulating ghrelin?

OBJECTIVE

Glucagon secretion is stimulated by fasting and inhibited postprandially, a pattern that mimics the secretory profiles of both ghrelin and GH. We thus hypothesized that glucagon may be a determinant of the changes in circulating ghrelin and GH that occur in relation to meals. The objective of the study was to explore this hypothesis by determining the ghrelin and GH response to a bolus of glucagon or saline in healthy subjects.

SUBJECTS AND MEASUREMENTS

Nine healthy volunteers, mean age 47 years (range 33-58) and body mass index (BMI) 24 kg/m2 (range 20.9-27.6) were recruited and received either 1 mg glucagon (n = 9) or 1 ml saline (n = 6) subcutaneously on separate days between 0800 and 0830 h after an overnight fast. Venous blood was then sampled at 15-min intervals during the first hour, followed by 30-min intervals up to 4 h for glucose, insulin, GH, cortisol, somatostatin and ghrelin.

RESULTS

Mean +/- SE basal ghrelin was 213.1 +/- 34.3 pmol/l and decreased significantly by 15 min after glucagon administration to 179.3 +/- 28 pmol/l (P = 0.01), then remaining suppressed relative to the basal value until 240 min after glucagon. Plasma insulin increased from a basal value of 46.7 +/- 7.7 pmol/l to a peak of 327.1 +/- 54.9 pmol/l (P < 0.0001). There was an inverse statistical relationship between the increase in insulin over the first 120 min and the decrease in ghrelin (P = 0.005), while somatostatin, GH and glucose were not significant contributors to the decrease in ghrelin (P > 0.05). Mean +/- SE basal GH was 7.3 +/- 2.9 microg/l and increased by 150 min after glucagon to a peak of 20.5 +/- 6.8 microg/l (P = 0.006). Changes in neither ghrelin nor glucose were related to the increase in GH (P = 0.7). Saline administration did not produce any significant change in ghrelin, insulin or somatostatin although the expected diurnal reduction in cortisol (P < 0.05) was observed.

CONCLUSIONS

Our study found no evidence that glucagon stimulates ghrelin secretion in humans and supports the hypothesis that insulin is a negative regulator of ghrelin secretion in the postprandial state. We did not find a negative relationship between endogenous somatostatin and ghrelin despite earlier reports that exogenously administered somatostatin analogues suppress plasma ghrelin. Finally, glucagon-induced GH secretion is not mediated by an increase in plasma ghrelin.

The negative association between total ghrelin levels, body mass and insulin secretion is lost in hypercortisolemic patients with Cushing's disease

OBJECTIVE

Ghrelin exerts a wide spectrum of endocrine and non-endocrine actions. The stomach is the major source of circulating ghrelin levels that are negatively associated with body mass, insulin and glucose levels. The role of glucocorticoids in ghrelin secretion and action is still unclear.

DESIGN

In 8 patients with Cushing's disease (CD, BMI 29.8 +/- 1.6 kg/m(2)), 7 normal (NS) and 6 obese subjects (OB, BMI 32.9 +/- 1.1 kg/m(2)) we studied: a) total ghrelin levels (every 15 min over 3 h) and their correlation with BMI, insulin, glucose, homeostatic model assessment (HOMA) index, ACTH and cortisol levels; b) GH, ACTH, cortisol, insulin and glucose responses to acylated ghrelin administration (1.0 mug/kg i.v. at 0 min).

RESULTS

CD patients had BMI, insulin and glucose levels as well as HOMA index higher than those in NS (P < 0.05) but similar to those in OB. Despite this, total ghrelin levels in CD were similar to those in NS and both were higher (P < 0.05) than those in OB. No correlation was found among total ghrelin and BMI, insulin, glucose, ACTH and cortisol levels in CD patients. The GH responses to ghrelin in CD and OB were similar and both were lower (P < 0.002) than those in NS. In CD ghrelin induced exaggerated ACTH and cortisol responses clearly higher (P < 0.005) than in OB and NS. Ghrelin administration increased glucose in all groups; insulin levels showed slight decrease that was significant (P < 0.05) in OB only.

CONCLUSIONS

Hypercortisolism in humans is associated with impaired ghrelin secretion and action. In fact, total ghrelin secretion in CD is not reduced despite increased BMI, insulin and glucose levels, while the GH and ACTH responses to acylated ghrelin are clearly reduced and enhanced, respectively.

Ghrelin Is Suppressed by Glucagon and Does Not Mediate Glucagon-Related Growth Hormone Release

BACKGROUND

Glucagon stimulation is routinely used as a provocative test to assess growth hormone (GH) sufficiency in pediatrics. Ghrelin also markedly stimulates GH secretion. Because glucagon stimulates the promoter of the ghrelin gene in vitro as well as ghrelin secretion by the perfused rat stomach, we sought to determine whether ghrelin mediates glucagon-induced GH secretion.

METHODS

We compared ghrelin, GH, insulin and glucose responses following administration of 0.03 mg/kg intravenously (iv; max. 1 mg) and 0.1 mg/kg intramuscularly (im; max. 2 mg) of glucagon in two groups (n = 10-11/group) of GH-sufficient children. We also measured ghrelin before and 6 min after iv administration of 1 mg glucagon in 21 adult subjects.

RESULTS

In children, glucagon caused a 26% decrease in ghrelin and a 72% increase in glucose concentrations that were independent of the dose or administration route of glucagon. In contrast, the insulin response was 2-3 times higher following administration of 0.1 mg/kg im compared to 0.03 mg/kg of glucagon iv. There was a significant correlation between the maximum decrease in ghrelin and increases in glucose (p = 0.03) but not in insulin. There was a significant correlation between ghrelin and GH area under the curve after controlling for the dose of glucagon (p = 0.03) but not for the maximum increase in glucose.In normal adults, glucagon administration caused a 7% decrease in ghrelin concentrations after 6 min (p = 0.0002).

CONCLUSION

Ghrelin does not play a causal role in the GH response to pharmacological glucagon administration, which suppresses ghrelin levels starting a few minutes after injection.

Low plasma ghrelin level in gastrectomized patients is accompanied by enhanced sensitivity to the ghrelin-induced growth hormone release

Ghrelin is a brain-gut peptide with potent GH-releasing activities. It has been suggested that the majority of circulating ghrelin originates from the stomach, with a smaller portion from the small intestine. Gastrectomy (GASTRX) significantly reduces circulating ghrelin concentrations. The implication of decreased circulating ghrelin on the somatotropic axis post GASTRX has not been studied. Therefore, we aimed to investigate the somatotropic axis in 10 gastrectomized patients who underwent total GASTRX for various reasons at least 2 yr ago. At baseline circulating total ghrelin, GH, IGF-I, and IGF binding protein (IGFBP)-3 levels were measured. The GH stimulation test consisted of an insulin-induced hypoglycemia, ghrelin in two iv bolus doses (0.1 and 1 microg/kg), and a GHRH test. GH sensitivity was assessed by an IGF-I generation test. All the tests were performed 2 wk apart. At baseline serum ghrelin levels were reduced by 55% in GASTRX patients, compared with the control group (P < 0.05). IGF-I (P < 0.05) and IGFBP-3 (P < 0.01) levels were also significantly lower than in controls. GH response to the insulin-induced hypoglycemia test in both GASTRX and control subjects was of similar magnitude, whereas circulating plasma ghrelin levels in GASTRX patients were not modified during hypoglycemia. Both doses (0.1 and 1.0 microg/kg) of ghrelin stimulated GH release significantly more in GASTRX than control subjects, respectively (peak mean GH +/- se: 18.2 +/- 5.6 vs. 5.4 +/- 1.3 microg/liter, P < 0.03; and 58.7 +/- 7.5 vs. 35.3 +/- 1.9 microg/liter, P < 0.01). There was no difference in GHRH-induced GH response between GASTRX patients and control subjects (P > 0.05). Concomitantly, increased increments in IGF-I and IGFBP-3 to a single bolus of GH were found (P < 0.03). In conclusion, our data suggest that low circulating ghrelin levels, found in GASTRX patients, are accompanied by enhanced ghrelin sensitivity with respect to GH response. This is associated with increased GH responsiveness. GASTRX is a state of acquired chronic hypoghrelinemia that may require replacement with ghrelin, and it is tempting to speculate that this may affect the GH-IGF-IGFBP axis.

Ghrelin: more than a natural GH secretagogue and/or an orexigenic factor

Ghrelin, an acylated peptide produced predominantly by the stomach, has been discovered to be a natural ligand of the growth hormone secretagogue receptor type 1a (GHS-R1a). Ghrelin has recently attracted considerable interest as a new orexigenic factor. However, ghrelin exerts several other neuroendocrine, metabolic and also nonendocrine actions that are explained by the widespread distribution of ghrelin and GHS-R expression. The likely existence of GHS-R subtypes and evidence that the neuroendocrine actions, but not all the other actions, of ghrelin depend on its acylation in serine-3 revealed a system whose complexity had not been completely explored by studying synthetic GHS. Ghrelin secretion is mainly regulated by metabolic signals and, in turn, the modulatory action of ghrelin on the control of food intake and energy metabolism seems to be among its most important biological actions. However, according to a recent study, ghrelin-null mice are neither anorectics nor dwarfs and this evidence clearly depicts a remarkable difference from leptin null mice. Nevertheless, the original and fascinating story of ghrelin, as well as its potential pathophysiological implications in endocrinology and internal medicine, is not definitively cancelled by these data as GHS-R1a null aged mice show significant alterations in body composition and growth, in glucose metabolism, cardiac function and contextual memory. Besides potential clinical implications for natural or synthetic ghrelin analogues acting as agonists or antagonists, there are several open questions awaiting an answer. How many ghrelin receptor subtypes exist? Is ghrelin 'the' or just 'a' GHS-R ligand? That is, are there other natural GHS-R ligands? Is there a functional balance between acylated and unacylated ghrelin forms, potentially with different actions? Within the next few years suitable answers to these questions will probably be found, making it possible to gain a better knowledge of ghrelin's potential clinical perspectives.

Ghrelin secretion is inhibited by glucose load and insulin-induced hypoglycaemia but unaffected by glucagon and arginine in humans

OBJECTIVE

Circulating ghrelin levels are increased by fasting and decreased by feeding, glucose load, insulin and somatostatin. Whether hyperglycaemia and insulin directly inhibit ghrelin secretion still remains matter of debate. The aim of the present study was therefore to investigate further the regulatory effects of glucose and insulin on ghrelin secretion.

DESIGN AND SUBJECTS

We studied the effects of glucose [oral glucose tolerance test (OGTT) 100 g orally], insulin-induced hypoglycaemia [ITT, 0.1 IU/kg insulin intravenously (i.v.)], glucagon (1 mg i.v.), arginine (0.5 mg/kg i.v.) and saline on ghrelin, GH, insulin, glucose and glucagon levels in six normal subjects.

MEASUREMENTS

In all the sessions, blood samples were collected every 15 min from 0 up to + 120 min. Ghrelin, GH, insulin, glucagon and glucose levels were assayed at each time point.

RESULTS

OGTT increased (P < 0.01) glucose and insulin while decreasing (P < 0.01) GH and ghrelin levels. ITT increased (P < 0.01) GH but decreased (P < 0.01) ghrelin levels. Glucagon increased (P < 0.01) glucose and insulin without modifying GH and ghrelin. Arginine increased (P < 0.01) GH, insulin, glucagon and glucose (P < 0.05) but did not affect ghrelin secretion.

CONCLUSIONS

Ghrelin secretion in humans is inhibited by OGTT-induced hyperglycaemia and ITT but not by glucagon and arginine, two substances able to increase insulin and glucose levels. These findings question the assumption that glucose and insulin directly regulate ghrelin secretion. On the other hand, ghrelin secretion is not associated with the GH response to ITT or arginine, indicating that the somatotroph response to these stimuli is unlikely to be mediated by ghrelin.

Ghrelin, hypothalamus-pituitary-adrenal (HPA) axis and Cushing's syndrome

Ghrelin, a peptide predominantly produced by the stomach, has been discovered as a natural ligand of the GH Secretagogue receptor type 1a (GHS-R1a), known as specific for synthetic GHS. Ghrelin has recently attracted considerable interest as a new orexigenic factor. However, ghrelin exerts pleiotropic actions that are explained by the widespread distribution of ghrelin and GHS-R expression. Besides strong stimulation of GH secretion, the neuroendocrine ghrelin actions also include significant stimulation of both lactotroph and corticotroph secretion; all these actions depend on acylation of ghrelin in serine-3 that allows binding and activation of the GHS-R1a. However, GHS-R subtypes are likely to exist; they also bind unacylated ghrelin that is, in fact, the most abundant circulating form and exerts some biological actions. Ghrelin secretion is mainly regulated by metabolic signals, namely inhibited by feeding, glucose and insulin while stimulated by energy restriction. The role of glucocorticoids on ghrelin synthesis and secretion is still unclear although morning ghrelin levels have been found reduced in some patients with Cushing's syndrome; this, however, would simply reflect its negative association to body mass. Ghrelin, like synthetic GHS, stimulates ACTH and cortisol secretion in normal subjects and this effect is generally sensitive to the negative glucocorticoid feedback. It is remarkable that, despite hypercortisolism, ghrelin as well as synthetic GHS display marked increase in their stimulatory effect on ACTH and cortisol secretion in patients with Cushing's disease. This is even more intriguing considering that the GH response to ghrelin and GHS is markedly reduced by glucocorticoid excess. It has been demonstrated that the ACTH-releasing effect of ghrelin and GHS is purely mediated at the central level in physiological conditions; its enhancement in the presence of ACTH-secreting tumours is, instead, likely to reflect direct action on GHS receptors present on the neoplastic tissues. In fact, peculiar ACTH hyperresponsiveness to ghrelin and GHS has been observed also in ectopic ACTH-secreting tumours.