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

Changes in Ghrelin and Glucagon following a Low Glycemic Load Diet in Women with PCOS

CONTEXT

Altered satiety hormones in women with polycystic ovarian syndrome (PCOS) may contribute to obesity. Diets with a low glycemic load (GL) may influence appetite-regulating hormones including glucagon and ghrelin.

OBJECTIVE

To test the hypothesis that following a 4-week, eucaloric low vs high GL diet habituation, a low vs high GL meal will increase glucagon and decrease ghrelin to reflect greater satiety and improve self-reported fullness.

METHODS

Secondary analysis of a randomized crossover trial.

PARTICIPANTS

Thirty women diagnosed with PCOS.

INTERVENTION

Participants were provided low (41:19:40% energy from carbohydrate:protein:fat) and high (55:18:27) GL diets for 8 weeks each. At each diet midpoint, a solid meal test was administered to examine postprandial ghrelin, glucagon, glucose, insulin, and self-reported appetite scores.

RESULTS

After 4 weeks, fasting glucagon was greater with the low vs high GL diet (P = .035), and higher fasting glucagon was associated with lesser feelings of hunger (P = .009). Significant diet effects indicate 4-hour glucagon was higher (P < .001) and ghrelin was lower (P = .009) after the low vs high GL meal. A trending time × diet interaction (P = .077) indicates feelings of fullness were greater in the early postprandial phase after the high GL meal, but no differences were observed the late postprandial phase.

CONCLUSION

These findings suggest after low GL diet habituation, a low GL meal reduces ghrelin and increases glucagon in women with PCOS. Further research is needed to determine the influence of diet composition on ad libitum intake in women with PCOS.

Ghrelin's Relationship to Blood Glucose

Much effort has been directed at studying the orexigenic actions of administered ghrelin and the potential effects of the endogenous ghrelin system on food intake, food reward, body weight, adiposity, and energy expenditure. Although endogenous ghrelin's actions on some of these processes remain ambiguous, its glucoregulatory actions have emerged as well-recognized features during extreme metabolic conditions. The blood glucose-raising actions of ghrelin are beneficial during starvation-like conditions, defending against life-threatening falls in blood glucose, but they are seemingly detrimental in obese states and in certain monogenic forms of diabetes, contributing to hyperglycemia. Also of interest, blood glucose negatively regulates ghrelin secretion. This article reviews the literature suggesting the existence of a blood glucose-ghrelin axis and highlights the factors that mediate the glucoregulatory actions of ghrelin, especially during metabolic extremes such as starvation and diabetes.

Hypoglycemic Effect of Combined Ghrelin and Glucagon Receptor Blockade

Glucagon receptor (GcgR) blockade has been proposed as an alternative to insulin monotherapy for treating type 1 diabetes since deletion or inhibition of GcgRs corrects hyperglycemia in models of diabetes. The factors regulating glycemia in a setting devoid of insulin and glucagon function remain unclear but may include the hormone ghrelin. Not only is ghrelin release controlled by glucose but also ghrelin has many actions that can raise or reduce falls in blood glucose level. Here, we tested the hypothesis that ghrelin rises to prevent hypoglycemia in the absence of glucagon function. Both GcgR knockout (Gcgr^-/-) mice and db/db mice that were administered GcgR monoclonal antibody displayed lower blood glucose levels accompanied by elevated plasma ghrelin levels. Although treatment with the pancreatic β-cell toxin streptozotocin induced hyperglycemia and raised plasma ghrelin levels in wild-type mice, hyperglycemia was averted in similarly treated Gcgr^-/- mice and the plasma ghrelin level was further increased. Notably, administration of a ghrelin receptor antagonist further reduced blood glucose levels into the markedly hypoglycemic range in overnight-fasted, streptozotocin-treated Gcgr^-/- mice. A lowered blood glucose level also was observed in overnight-fasted, streptozotocin-treated ghrelin receptor-null mice that were administered GcgR monoclonal antibody. These data suggest that when glucagon activity is blocked in the setting of type 1 diabetes, the plasma ghrelin level rises, preventing hypoglycemia.

The regulation of circulating ghrelin - with recent updates from cell-based assays

Ghrelin is a stomach-derived orexigenic hormone with a wide range of physiological functions. Elucidation of the regulation of the circulating ghrelin level would lead to a better understanding of appetite control in body energy homeostasis. Earlier studies revealed that circulating ghrelin levels are under the control of both acute and chronic energy status: at the acute scale, ghrelin levels are increased by fasting and decreased by feeding, whereas at the chronic scale, they are high in obese subjects and low in lean subjects. Subsequent studies revealed that nutrients, hormones, or neural activities can influence circulating ghrelin levels in vivo. Recently developed in vitro assay systems for ghrelin secretion can assess whether and how individual factors affect ghrelin secretion from cells. In this review, on the basis of numerous human, animal, and cell-based studies, we summarize current knowledge on the regulation of circulating ghrelin levels and enumerate the factors that influence ghrelin levels.

Effect of Arginine Infusion on Ghrelin Secretion in Growth Hormone Sufficient and GH Deficient Children

BACKGROUND

The physiological link between ghrelin and growth hormone (GH) has not yet been fully clarified. Furthermore, the existence of a negative feedback mechanism between growth hormone-insulin-like growth factor (GH-IGF)-I axis and ghrelin and the influence of amino acids on ghrelin secretion in children remain matters of debate.

OBJECTIVES

To understand the regulation of ghrelin secretion and clarify the relationship between ghrelin and GH secretion in GH-deficient (GHD) and GH-sufficient (GHS) children.

PATIENTS AND METHODS

Ten GHD (male/female [M/F], 6/4; age [mean ± SEM], 10.7 ± 0.9 years) and 10 GHS prepubertal children (M/F, 6/4; age [mean ± SEM], 10.3 ± 0.6 years), underwent an arginine (ARG) test (infusion, 0.5 g/kg, iv). Levels of GH, total ghrelin, and acylated ghrelin (AG) were assayed every 30 min from 0 to +120 min.

RESULTS

Peak GH values were lower in GHD subjects than in GHS subjects (P < 0.0001). The baseline levels, peak levels, or area under the curves (AUC) for total ghrelin and AG were similar between GHD and GHS children. ARG infusion was followed by a slight to significant decrease in total ghrelin levels, but not AG levels, both in GHD and GHS subjects with a nadir at +30 min. No correlation was seen between GH, total ghrelin, or AG response and ARG infusion.

CONCLUSIONS

Total ghrelin and AG levels seemed unaffected by GH status in prepubertal children. ARG infusion was unable to blunt ghrelin secretion irrespective of GH status in childhood. Moreover, since ARG influences GH secretion via modulation of somatostatin release, ghrelin secretion seems to be partially refractory to somatostatin action.

Relationship between GH response and glycemic fluctuations in the glucagon stimulation test

OBJECTIVE

Verifying the association between glycemic fluctuation and GH response in the glucagon stimulation test. Basal evaluation of growth hormone (GH) has poor diagnostic accuracy due to its pulsatile secretion. GH-stimulation tests are used for an adequate evaluation of somatotrophic axis. Various stimuli can be employed, among them glucagon, which has an elusive mechanism of action. Since hypoglycemia reportedly occurs during the test, investigation of its role as a stimulus to GH release is granted.

DESIGN

Retrospective analysis of glucagon-stimulated GH tests performed in 128 children (36.7% female; age 12.4+3.3 years), at Fleury Functional Tests Facility from July 2000 to 2006. GH and blood glucose (BG) curves, IGF-1, and IGFBP-3 have been assessed. Positive GH response was defined by a peak GH value >or=3.3 microg/L. Normal IGF-1 levels were defined as those between 2.5th and 97.5th percentiles for age and gender.

RESULTS

Hypoglycemia under 2.2 mmol/L did not occur during the test. BG decrease occurred with lower magnitude and was not associated to GH response. Comparison between patients with negative and positive GH response showed, respectively, BG nadir 3.74 vs. 3.62 mmol/L, glucose AUC 23.3 vs. 22.4, and glycemic decrease (below 3.3 mmol/L) 19% vs. 35.5% (with P non-significant for all comparisons).

CONCLUSION

Hypoglycemia was not seen after glucagon stimulation and decrease in BG occurred above levels physiologically expected to stimulate GH release, being apparently not associated to GH response.

Secretion of ghrelin from rat stomach ghrelin cells in response to local microinfusion of candidate messenger compounds: a microdialysis study

Ghrelin is produced by A-like cells (ghrelin cells) in the mucosa of the acid-producing part of the stomach. The mobilization of ghrelin is stimulated by nutritional deficiency and suppressed by nutritional abundance. In an attempt to identify neurotransmitters and regulatory peptides that may contribute to the physiological, nutrient-related regulation of ghrelin secretion, we challenged the ghrelin cells in situ with a wide variety of candidate messengers, including known neurotransmitters (e.g. acetylcholine, catecholamines), candidate neurotransmitters (e.g. neuropeptides), local tissue hormones (e.g. serotonin, histamine, bradykinin, endothelin), circulating gut hormones (e.g. gastrin, CCK, GIP, neurotensin, PYY, secretin) and other circulating hormones/regulatory peptides (e.g. calcitonin, glucagon, insulin, PTH). Microdialysis probes were placed in the submucosa of the acid-producing part of the rat stomach. Three days later, the putative messenger compounds were administered via the microdialysis probe (reverse microdialysis) at a screening dose of 0.1 mmol l(-1) for regulatory peptides and 0.1 and 1 mmol l(-1) for amines and amino acids. The rats were awake during the experiments. The resulting microdialysate ghrelin concentration was monitored continuously for 3 h (radioimmunoassay), thereby revealing stimulators or inhibitors of ghrelin secretion. Dose-response curves were constructed for each candidate messenger that significantly (p<0.05) affected ghrelin mobilization at the screening dose. Peptides that showed a (non-significant) tendency to affect ghrelin release at the screening dose were also given at a dose of 0.3 or 1 mmol l(-1). Adrenaline, noradrenaline, endothelin and secretin stimulated ghrelin release, while somatostatin and GRP inhibited. Whether these agents act directly or indirectly on the ghrelin cells remains to be investigated. All other candidate messengers were without measurable effects, including acetylcholine, serotonin, histamine, GABA, aspartic acid, glutamic acid, glycine, VIP, PACAP, CGRP, substance P, NPY, PYY, PP, gastrin, CCK, GIP, insulin, glucagon, GLP and glucose.

A Comparison of Ghrelin, Glucose, Alpha-amylase and Protein Levels in Saliva from Diabetics

During the past decade, many salivary parameters have been used to characterize disease states. Ghrelin (GAH) is recently-discovered peptide hormone secreted mainly from the stomach but also produced in a number of other tissues including salivary glands. The aim of this work was to examine the relationship between active (aGAH) and inactive (dGAH) ghrelin in the saliva and other salivary parameters in type II diabetic patients and healthy controls. Salivary parameters were assessed in a single measurement of unstimulated whole saliva from 20 obese and 20 non-obese type II diabetes patients, and in 22 healthy controls. Total protein and alpha-amylase were determined by colorimetric methods, and glucose by the glucose-oxidase method. Saliva aGAH and dGAH levels were measured using a commercial radioimmunoassay (RIA) kit. Salivary concentrations of aGAH and dGAH ghrelin were more markedly decreased in obese diabetic subjects than in the two other groups. Glucose and alpha-amylase levels were higher in diabetic subjects than in controls. Furthermore, there were correlations between GAH levels and BMI, and between GAH and blood pressure. However, there was no marked variability in saliva flow rates among the groups. These results indicate that measurement of salivary GAH and its relationship to other salivary parameters might help to provide insight into the role of ghrelin in diabetes.

Ghrelin, the peripheral hunger hormone

In the current review we summarize the available data concerning the gastric hormone ghrelin and its receptor. Ghrelin stimulates short-term food intake and long-term body weight regulation via its adipogenic and diabetogenic effects. Ghrelin stimulates gastric emptying, and these effects could be explored from a therapeutic point of view. Ghrelin levels change profoundly in anorexia, in states of insulin resistance, in obesity, and after bariatric surgery, suggesting that this is an important hormone in body weight regulation.

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.

Glucagon inhibits ghrelin secretion in humans

OBJECTIVE

It is well known that i.m. glucagon administration stimulates GH and cortisol release in humans, although the mechanisms are unclear. These effects are similar to those described for ghrelin on somatotroph and corticotroph function. The aim of the present study was to investigate the role of ghrelin in mediating the stimulatory effects of glucagon and to evaluate the effect of glucagon on ghrelin secretion.

DESIGN AND METHODS

We studied the endocrine and metabolic response to i.m. glucagon administration in 24 subjects (14 men, 10 women; age 19-65 years; body mass index, 25.3 +/- 1 kg/m(2)), who were shown to have an intact anterior pituitary function as evaluated before enclosure.

RESULTS

Serum ghrelin concentrations fell significantly at 30, 60, 120 and 180 min after glucagon administration (means +/- s.e.m.; baseline, 377.9 +/- 34.5 pg/ml; nadir, 294.6 +/- 28.3 pg/ml (60 min); P < 0.01). Conversely, i.m. glucagon elicited an increase in GH (baseline, 1.5 +/- 0.4 microg/l; peak, 14.2 +/- 2.7 microg/l (180 min); P < 0.01) and cortisol concentrations (baseline, 452.6 +/- 35.2 nmol/l; peak, 622.1 +/- 44 nmol/l (180 min); P < 0.01). The changes in ghrelin concentration at both 120 and 180 min were still significant after correction for glucose and insulin (P < 0.05).

CONCLUSIONS

We show that i.m. glucagon decreases ghrelin significantly. Therefore, the already known stimulatory effects of i.m. glucagon on cortisol and GH are not mediated by a change in ghrelin concentrations. The mechanisms underlying the ghrelin suppression after i.m. glucagon are unlikely to include glucose or insulin variations and need to be further elucidated.

Ghrelin in Growth and Development

Exogenous administration of ghrelin increases caloric intake and stimulates growth hormone (GH) secretion, two effects that are mediated through binding of ghrelin to the GH secretagogue receptor (GHS-R). In addition, ghrelin is thought to inhibit adipogenesis by GHS-R-independent mechanisms. In adults, ghrelin is mainly produced by the stomach. In contrast, in the fetal and early postnatal period, ghrelin gene expression is abundant in the pancreas but not in the stomach. While knockout animal studies demonstrate that ghrelin is not required for perinatal development under normal nutritional conditions, the characteristics of ghrelin metabolism during fetal development suggest that ghrelin could contribute to the programming of mechanisms involved in energy balance, such as beta-cell maturation, orexigenic pathways and adipogenesis. In humans, ghrelin concentrations progressively decrease during childhood and adolescence, as well as with advancing puberty. In adolescents, similar to adults, ghrelin concentrations are inversely related to body mass index and to circulating insulin. One notable exception is the presence of elevated ghrelin concentrations in subjects with Prader-Willi syndrome, raising the possibility that ghrelin could be part of the etiology of excess food intake in this condition. These data raise a number of fascinating questions on the potential physiologic role of this hormone during growth and development.