Alterations in the dynamics of circulating ghrelin, adiponectin, and leptin in human obesity

Rev-erbα gives a time cue to metabolism

Normal physiological processes are under control of circadian rhythms. Moreover, certain pathological events, such as cardiovascular accidents (myocardial infarction, stroke) occur more frequently at specific times of the day. Recent observations demonstrate a causal relationship between alterations in circadian rhythmicity and metabolic disorders. Disruption of clock genes results in dyslipidemia, insulin resistance and obesity, all predisposing to atherosclerosis. The nuclear receptor Rev-erb alpha is part of the clock circuitry and plays an important role in keeping proper timing of the clock. Rev-erb alpha also regulates lipid metabolism, adipogenesis and vascular inflammation. Interestingly, Rev-erb alpha also cross-talks with several other nuclear receptors involved in energy homeostasis. Therefore Rev-erb alpha may serve to couple metabolic and circadian signals.

Growth hormone and ghrelin secretion in severely obese women before and after bariatric surgery

OBJECTIVE

The objective was to evaluate ghrelin and growth hormone (GH) interactions and responses to a growth hormone-releasing hormone (GHRH)/arginine test in severe obesity before and after surgically-induced weight loss.

RESEARCH METHODS AND PROCEDURES

Our study population included 11 severely obese women 39 +/- 12 years of age, with a mean BMI of 48.6 +/- 2.4 kg/m2, re-studied in a phase of stabilized body weight, with a BMI of 33.4 +/- 1.2 kg/m2, 18 months after having successfully undergone biliopancreatic diversion (BPD). A GHRH/arginine test was performed before and 18 months after BPD to evaluate ghrelin and GH interactions. Active ghrelin, measured by radioimmunoassay (RIA), and GH, measured by chemiluminescence assay, were assayed before and after the GHRH/arginine test.

RESULTS

Fasting serum GH levels and GH area under the curve (AUC) significantly increased from 0.2 +/- 0.05 ng/mL to 1 +/- 0.3 ng/mL (p < 0.05) and from 514.76 +/- 98.7 ng/mL for 120 minutes to 1957.3 +/- 665.1 ng/mL for 120 minutes after bariatric surgery (p < 0.05), respectively. Although no significant change in fasting ghrelin levels was observed (573 +/- 77.9 before BPD vs. 574.1 +/- 32.7 after BPD), ghrelin AUC significantly increased from -3253.9 +/- 2180.9 pg/mL for 120 minutes to 1142.3 +/- 916.4 pg/mL for 120 minutes after BPD (p < 0.05). Fasting serum insulin-like growth factor (IGF)-1 concentration did not change significantly (133.6 +/- 9.9 ng/mL before vs. 153.3 +/- 25.2 ng/mL after BPD).

DISCUSSION

Our study demonstrates that the mechanisms involved in ghrelin and GH secretion after the secretagogue stimulus (GHRH/arginine) are consistent with patterns observed in other populations.

Plasma ghrelin levels in various stages of development of iron deficiency anemia

OBJECTIVES

Ghrelin stimulates food intake and induces metabolic changes leading to an increase in body weight and body fat mass. Iron-deficiency anemia (IDA) is the most frequently seen cause of nutritional anemia, that is a type of starvation. There is no available study related to levels of ghrelin in IDA. The aim of this study is to show an association with ghrelin levels and iron deficiency and to demonstrate whether changes seen in iron deficiency (ID) are explained by ghrelin, as opposed to whether ghrelin levels correlate with ID.

MATERIALS AND METHODS

The study group was consisted of children who were admitted in the outpatient clinic of pediatrics. Control group (C) was defined as cases with normal hemoglobin (Hb), serum iron (SI), transferrin saturation (TS), and ferritin (F) (>12 ng/mL) values; group hypoferritinemia (IDec) Hb: N, SI: N, TS: N, F<12 ng/mL; group iron deficiency (IDef), Hb: N, SI: decreased, TS<or=16%, F<12 ng/mL, and group IDA, Hb and SI decreased, TS<or=16%, F<12 ng/mL. The patients were categorized into 4 groups [group 1 (C), n=25, age=82.4+/-16.56 mo, F=40.87+/-6.17 ng/mL; group 2 (IDec), n=30, age=57.5+/-20.71 mo, F=29.95+/-3.77 ng/mL; group 3 (IDef), n=28, age=50.21+/-19.87 mo, F=14.82+/-3.41 ng/mL; group 4 (IDA), n=25, age=31.55+/-13.21 mo, F=11.75+/-4.01 ng/mL].

RESULTS

Mean value of ghrelin was detected to be 396.53+/-85.56 pg/mL, 332.26+/-74.35 pg/mL, 309.66+/-68.62 pg/mL, and 177.66+/-27.81 pg/mL in control, groups IDec, IDef, and IDA, respectively. A statistically significant difference was detected between groups control and IDef (P<0.01), control and IDA, IDec and IDA, IDef and IDA (P<0.001).

CONCLUSIONS

A significant positive correlation was demonstrated between iron status of the body and levels of ghrelin. Decrease in ghrelin levels in IDA can lead to loss of appetite, desire to eat diverse foods with resultant delay in growth and development.

Impact of sleep and sleep loss on glucose homeostasis and appetite regulation

Over the past 30 years there has been an increase in the prevalence of obesity and diabetes, both of which can have serious consequences for longevity and quality of life. Sleep durations may have also decreased over this time period. This chapter reviews laboratory and epidemiologic evidence for an association between sleep loss and impairments in glucose metabolism and appetite regulation, which could increase the risk of diabetes or weight gain.

Constant intravenous ghrelin infusion in healthy young men: clinical pharmacokinetics and metabolic effects

Ghrelin levels fluctuate rapidly and dynamically with surges before meal times and postprandial troughs, and ghrelin increases appetite and food intake. Circulating ghrelin correlates negatively with body mass index (BMI), but obese individuals have a reduced postprandial decrease in ghrelin levels. Whether this reflects changes in secretion or clearance of ghrelin is uncertain. We therefore studied the pharmacokinetics of ghrelin in relation to anthropometric and biochemical measures. We also studied the effects of ghrelin on hormones and metabolites. In fasting humans, we used a constant infusion rate of ghrelin lasting 180 min at 5 pmol.kg body wt(-1).min(-1) in a randomized, double-blind, placebo-controlled crossover study. Serum ghrelin (s-ghrelin; total levels) was distributed and eliminated according to a two-compartment model. s-Ghrelin initial half-life was 24 +/- 2 min and terminal half-life 146 +/- 36 min, respectively. Mean residence time (MRT) of ghrelin was 93 +/- 16 min. MRT correlated positively with both BMI (r = 0.51, P < 0.001) and high-density cholesterol (HDL) levels (r = 0.75, P < 0.001). Serum insulin levels remained constant during ghrelin infusion, whereas plasma glucose increased 0.3 +/- 0.1 mmol/l (P < 0.01) and free fatty acid levels more than doubled (to 1.03 +/- 0.08 mmol/l, P < 0.001), translating into a significant reduction of insulin sensitivity (P < 0.001). In conclusion, 1) we describe novel pharmacokinetics of ghrelin that are useful when tailoring ghrelin infusion rates in clinical experiments, 2) BMI and HDL correlate positively with MRT of infused ghrelin, and 3) supraphysiological ghrelin levels impair insulin sensitivity.

The adipocyte as an active participant in energy balance and metabolism

Obesity is responsible for the mounting incidence of metabolic disease in adult and pediatric populations. Understanding of the pathogenesis and maintenance of the obese state has advanced rapidly over the past 10 years. Bodily energy reserves are managed actively by complex systems that regulate food intake, substrate partitioning, and energy expenditure. An underlying assumption that circulating factors released from storage organs were able to signal bodily energy reserves was confirmed with the discovery of the leptin system. This proof of concept has spurred on the discovery of a multitude of other adipocyte-generated factors. These circulating factors signal to the brain and other organs of metabolic importance, including adipose tissue, liver, muscle, and the immune system. Adipose-derived factors have numerous implications for the basic biology of obesity and provide prospective targets for the amelioration of obesity and its adverse metabolic consequences. In this review we detail the current understanding of leptin as a prototypical adipose tissue-derived hormone related to appetite and obesity. We also describe other important adipose-derived factors in relation to their metabolic effect.

Circulating preprandial ghrelin to obestatin ratio is increased in human obesity

CONTEXT

Obestatin, a sibling of ghrelin derived from preproghrelin, opposes ghrelin's effects on food intake. Plasma obestatin profiles in relation to ghrelin have not been fully investigated in human obesity.

OBJECTIVE

We hypothesize that obesity might present with imbalance of circulating ghrelin and obestatin levels.

PARTICIPANTS AND SETTING

Sixteen obese (eight men, aged 58.8 +/- 4.9 yr; eight women, aged 59.9 +/- 9.6 yr) and 14 normal-weight individuals (seven men, aged 52.7 +/- 5.9 yr; seven women, aged 56.1 +/- 4.9 yr) were evaluated at the in-patient department of Changhai Hospital, Shanghai, China.

MAIN OUTCOME MEASURES

Total plasma ghrelin and obestatin levels, 1 h before and 2 h after breakfast, were measured by RIA.

RESULTS

Both preprandial plasma ghrelin levels (P < 0.01) and obestatin levels (P < 0.01) were lower in the obese compared with normal-weight controls. However, unexpectedly, the ratio of preprandial ghrelin to obestatin was higher in obese compared with normal-weight controls (P < 0.01) even after adjustment for gender and age (P < 0.01). The ratio of postprandial ghrelin to obestatin was decreased both in obese (P < 0.05) and controls (P < 0.01) compared with their preprandial levels. There were no significant differences in the ratio of postprandial ghrelin to obestatin between obese and normal-weight controls. Body mass index was positively correlated with and was a significantly independent determinant of the preprandial ghrelin to obestatin ratio.

CONCLUSION

Circulating preprandial ghrelin to obestatin ratio is elevated in human obesity. We suggest that high preprandial ghrelin to obestatin ratio may be involved in the etiology and pathophysiology of obesity.

Sleep and metabolic control: waking to a problem?

1. The aim of the present review is to outline: (i) the association between sleep and metabolism; (ii) how sleep duration influences the development of disease; and (iii) how sex differences, ageing and obesity may potentially influence the relationship between sleep, metabolic control and subsequent disease. 2. Sleep is associated with a number of endocrine changes, including a change in insulin action in healthy young individuals. Sleep duration shows a prospective U-shaped relationship with all-cause mortality, cardiovascular disease and Type 2 diabetes. 3. Chronic sleep restriction is becoming more common. Experimental sleep restriction impedes daytime glucose control and increases appetite. 4. The sex hormones oestrogen and testosterone influence sleep duration and quality and may account for sex differences in the prevalence of sleep-related disorders. 5. Ageing is associated with a decreased sleep duration, decreased muscle mass and impaired insulin action. 6. Obesity impairs insulin action and is associated with the incidence and severity of obstructive sleep apnoea. 7. Sleep plays an integral role in metabolic control. Consequently, insufficient sleep may represent a modifiable risk factor for the development of Type 2 diabetes. The challenge ahead is to identify how sex differences, ageing and obesity could potentially influence the relationship between sleep and metabolism.

Circadian rhythms in the development of obesity: potential role for the circadian clock within the adipocyte

Obesity is one of the most profound public health problems today, and simplistic explanations based on excessive nutritional consumption or lack of physical activity are inadequate to account for this dramatic and literal growth in our world population. Recent reports have suggested that disruptions in sleep patterns, often linked to our '24-h' lifestyle, are associated with increased body fat and altered metabolism, although the cause-effect relationship for these associations has yet to be elucidated. Abnormal sleep/wake patterns likely alter intracellular circadian clocks, which are molecular mechanisms that enable the cell/tissue/organism to anticipate diurnal variations in its environment. The environment may include circulating levels of nutrients (e.g. glucose, fatty acids and triglycerides) and various hormones (e.g. insulin, glucocorticoids). As such, alterations in this molecular mechanism, in particular within the adipocyte, likely induce metabolic changes that may potentiate disrupted metabolism, adipose accumulation and/or obesity. Although diurnal variations in adipokines and adipose tissue metabolism have been observed, little is known regarding the molecular mechanisms that influence these events.

Acute and chronic regulation of leptin synthesis, storage, and secretion by insulin and dexamethasone in human adipose tissue

Serum leptin levels are upregulated in proportion to body fat and also increase over the short term in response to meals or insulin. To understand the mechanisms involved, we assessed leptin synthesis and secretion in samples of adipose tissue from subjects with a wide range of BMI. Tissue leptin content and relative rates of leptin biosynthesis, as determined by metabolic labeling, were highly correlated with each other and with BMI and fat cell size. To understand mechanisms regulating leptin synthesis in obesity, we used biosynthetic labeling to directly assess the effects of insulin and glucocorticoids (dexamethasone) on leptin synthesis and secretion in human adipose tissue. Chronic treatment (1-2 days in organ culture) with insulin increased relative rates of leptin biosynthesis without affecting leptin mRNA levels. In contrast, dexamethasone increased leptin mRNA and biosynthesis in parallel. Acute treatment with insulin or dexamethasone (added during 1-h preincubation and 45-min pulse labeling) did not affect relative rates of leptin biosynthesis, but pulse-chase studies showed that addition of insulin nearly doubled the release of [35S]leptin after a 1-h chase. We conclude that the higher leptin stores in adipose tissue of obese humans are maintained by chronic effects of insulin and glucocorticoids acting at pre- and posttranslational levels and that the ability of insulin to increase the release of preformed leptin may contribute to short-term variations in circulating leptin levels.

Heart rate regularity analysis obtained from pulse oximetric recordings in the diagnosis of obstructive sleep apnea

Approximate entropy (ApEn) is a technique that can be used to quantify the irregularity or variability of time series. We prospectively evaluated the validity of ApEn of heart rate data obtained from pulse oximetric recordings as a diagnostic test for obstructive sleep apnea (OSA) in patients clinically suspected of suffering this disease. A sample of 187 referred outpatients (147 men and 40 women), with a mean age of 57.9+/-12.8 years and a body mass index of 29.5+/-5.5 kg/m(2), clinically suspected of having OSA were studied using nocturnal pulse oximetric recording performed simultaneously with complete polysomnography. A diagnosis of OSA was confirmed in 111 (59.3%). Patients with OSA presented significantly higher ApEn levels than those without OSA (1.334+/-0.189 vs 1.167+/-0.182). Chronic obstructive pulmonary disease (COPD) was diagnosed for 42 patients. Among these patients, 22 (52.4%) were diagnosed with OSA. COPD patients with OSA showed significantly higher ApEn levels than COPD patients without OSA (1.337+/-0.193 vs 1.184+/-0.173; p=0.01). ApEn correlated significantly with apnea-hypopnea index (r=0.38; p=0.000). There was no significant correlation between ApEn and either age or body mass index. No significant changes were observed in ApEn throughout the night in OSA patients. Using receiver operating characteristic curve analysis, we obtained a diagnostic sensitivity of 71.2%, specificity of 78.9%, positive predictive value of 81.3%, and negative predictive value of 66% at a threshold of 1.272. We conclude that ApEn analysis of heart rate data obtained from pulse oximetric recordings could be a useful tool in the study of OSA.

Plasma levels of acylated ghrelin during an oral glucose tolerance test in obese children

OBJECTIVE

Ghrelin is an acylated peptide with octanoyl modification, which is essential for its GH-releasing ability. Coexpression of GH secretagogue receptor (GHS-R) and ghrelin in the pancreas suggests that this peptide is involved in glucose metabolism. The other form of the molecule, the non-acylated ghrelin, has been reported to be devoid of any pituitaric endocrine activities. Previous reports demonstrated that plasma total ghrelin levels decrease after oral glucose administration in obese children, but no data are available about the plasma levels of acylated ghrelin. Therefore, in the present study the plasma levels of acylated ghrelin were measured in obese and control children during oral glucose tolerance test (OGTT).

MATERIALS AND METHODS

Acylated ghrelin response to OGTT was evaluated in 11 obese and 9 age-matched control children. All subjects received 0.75 g/kg (maximum 75 g) glucose solution orally after an overnight fast. Acylated ghrelin, insulin, glucose, and GH were determined at 0, 30, 60 and 120 min, and leptin at 0 min of the OGTT.

RESULTS

Plasma basal levels of acylated ghrelin were significantly lower in the obese children than in the controls (66.3+/-6.7 vs 97.2+/-14.4 pg/ml, p<0.05). The plasma acylated ghrelin concentration decreased significantly at 30 and 60 min in the control group (53.3+/-9.9 and 57.4+/-7.0 pg/ml, p<0.05), but not in the obese group (64.7+/-9.6 and 49.3+/-4.6 pg/ml) as compared to the basal value. In the obese group the acylated ghrelin level was significantly higher at 120 min, than at 0 min (91.6+/-9.8 vs 66.3+/-6.7 pg/ml, p<0.05).

CONCLUSIONS

There was no rapid fall in plasma levels of acylated ghrelin in obese children after OGTT at 30 min, but there was an increase at 120 min, suggesting that the dynamic of the response to OGTT is slower and there is an upregulation of active ghrelin in the second half of OGTT in obese children.

Association of adiponectin with coronary heart disease and mortality: the Rancho Bernardo study

Hypoadiponectinemia has been implicated in the development of obesity-related conditions, including dyslipidemia and coronary heart disease (CHD). In this study, the authors examined the association of adiponectin with CHD prevalence, incidence, and mortality among 1,513 community-dwelling men and women aged 50-91 years who were followed from 1984-1987 through 2004. In cross-sectional analyses, adiponectin concentrations were positively related to female sex, age, and high density lipoprotein cholesterol level and inversely related to waist girth, triglyceride level, and fasting plasma glucose level (all p's < 0.001). Adiponectin levels in the highest sex-specific quintile, as compared with the lowest, were associated with 44% decreased odds of prevalent CHD (p for trend = 0.03); adjustment for high density lipoprotein cholesterol and/or triglycerides eliminated this association. In 20-year prospective analyses, higher adiponectin concentrations predicted reduced risk of nonfatal myocardial infarction in men only; adiponectin was not associated with fatal incident CHD events or 20-year CHD mortality (n = 215 deaths) in either sex. Adiponectin levels in the highest sex-specific quintile, as compared with lower levels, were associated with almost 40% increased risks of cardiovascular disease death (n = 441) and death from all causes (n = 925), independent of age, sex, waist girth, lipid levels, and glucose level (both p's < 0.001). These results suggest that use of adiponectin for cardiovascular disease risk stratification is premature.

Glucocorticoid inhibition of growth in rats: partial reversal with the full-length ghrelin analog BIM-28125

Glucocorticoids are important immunosuppressive hormones; these steroids also inhibit somatic growth by decreased growth hormone (GH) secretion and induced protein catabolism. The ability of ghrelin, the endogenous ligand for the GHS-1a receptor, to increase body weight is attributed to a combination of enhanced food intake, increased gastric emptying and increased food assimilation, coupled with potent GH releasing activity. The aim of the present study was to evaluate the ability of a full-length, metabolically stabilized ghrelin agonist, BIM-28125, to reverse the dexamethasone-induced decrease of growth rate of prepubertal Sprague-Dawley male rats. Twenty-one days old rats were randomly assigned to two treatment groups. Beginning on day 23 of age, 16 animals were treated ip either with saline or DEX (40 microg/kg/day). On day 33 after birth, these two groups were further subdivided and treated sc with either vehicle or BIM-28125 (80 nmol/kg, t.i.d.). On day 47 after birth, rats were killed and trunk blood was collected for hormone determinations. DEX significantly reduced final body weight and nose-anal length; BIM-28125 increased linear growth in saline-treated rats and reversed growth inhibition in DEX-treated rats. The inhibitory effects of DEX on somatic growth was paralleled by decreased 24 h food intake (FI), decreased food efficiency (FE) and lower plasma IGF-1 levels versus vehicle-treated rats. BIM-28125 induced an increase of FI, FE and plasma IGF-1 in saline-treated rats, and reversed the inhibitory effects of DEX. These preclinical results leads to the conclusion that BIM-28125 may represent a good tool to reverse the catabolic effects induced by glucocorticoids.

Ghrelin and feedback systems

Ghrelin is produced primarily in the stomach in response to hunger, and circulates in the blood. Plasma ghrelin levels increase during fasting and decrease after ingesting glucose and lipid, but not protein. The efferent vagus nerve contributes to the fasting-induced increase in ghrelin secretion. Ghrelin secreted by the stomach stimulates the afferent vagus nerve and promotes food intake. Ghrelin also stimulates pituitary gland secretion of growth hormone (GH) via the afferent vagus nerve. GH inhibits stomach ghrelin secretion. These findings indicate that the vagal circuit between the central nervous system and stomach has a crucial role in regulating plasma ghrelin levels. Moreover, body mass index modulates plasma ghrelin levels. In a lean state and anorexia nervosa, plasma ghrelin levels are increased, whereas in obesity, except in Prader-Willi syndrome, plasma ghrelin levels are decreased and the feeding- and sleeping-induced decline in plasma ghrelin levels is disrupted. There are two forms of ghrelin: active n-octanoyl-modified ghrelin and des-acyl ghrelin. Fasting increases both ghrelin types compared with the fed state. Hyperphagia and obesity are likely to decrease plasma des-acyl ghrelin, but not n-octanoyl-modified ghrelin levels. Hypothalamic serum and glucocorticoid-inducible kinase-1 and serotonin 5-HT2C/1B receptor gene expression levels are likely to be proportional to plasma des-acyl ghrelin levels during fasting, whereas they are likely to be inversely proportional to plasma des-acyl ghrelin levels in an increased energy storage state such as obesity. Thus, a dysfunction of the ghrelin feedback systems might contribute to the pathophysiology of obesity and eating disorders.

From Neuroendocrinology to Neuroimmunomodulation - a tribute to Prof. Dr. Samuel McCann

One of the leading experts in the field of Neuroendocrinology and Neuroimmunmodulation, Samuel Mac Donald McCann, known by all his friends as 'Don', passed away in 2007. This article pays tribute to his outstanding scientific contribution and a glimpse on his fascinating personality. A member of the National Academy of Sciences of the United States and pioneer in the field of neuroendocrine regulation, he identified numerous hormones and peptides and set the stage for basic concepts in physiology and clinical medicine.

The role of leptin and ghrelin in the regulation of food intake and body weight in humans: a review

Leptin and ghrelin are two hormones that have been recognized to have a major influence on energy balance. Leptin is a mediator of long-term regulation of energy balance, suppressing food intake and thereby inducing weight loss. Ghrelin on the other hand is a fast-acting hormone, seemingly playing a role in meal initiation. As a growing number of people suffer from obesity, understanding the mechanisms by which various hormones and neurotransmitters have influence on energy balance has been a subject of intensive research. In obese subjects the circulating level of the anorexigenic hormone leptin is increased, whereas surprisingly, the level of the orexigenic hormone ghrelin is decreased. It is now established that obese patients are leptin-resistant. However, the manner in which both the leptin and ghrelin systems contribute to the development or maintenance of obesity is as yet not clear. The purpose of this review is to provide background information on the leptin and ghrelin hormones, their role in food intake and body weight in humans, and their mechanism of action. Possible abnormalities in the leptin and ghrelin systems that may contribute to the development of obesity will be mentioned. In addition, the potentials of leptin and ghrelin as drug targets will be discussed. Finally, the influence of the diet on leptin and ghrelin secretion and functioning will be described.

Endogenous ApoE expression modulates adipocyte triglyceride content and turnover

Apolipoprotein E (apoE) is highly expressed in adipose tissue and adipocytes in which its expression is regulated by peroxisome proliferator-activated receptor (PPAR)-gamma agonists and tumor necrosis factor-alpha. There is, however, no information regarding a role for endogenous apoE in differentiated adipocyte function. In this report, we define a novel role for apoE in modulating adipocyte lipid metabolism. ApoE(-/-) mice have less body fat and smaller adipocytes compared with wild-type controls. Freshly isolated adipose tissue from apoE(-/-) mice contains lower levels of triglyceride and free fatty acid, and these differences are maintained in cultured adipocytes derived from preadipocytes. Adenoviral expression of apoE in apoE(-/-)-cultured adipocytes increases triglyceride and fatty acid content. During incubation with apoE-containing triglyceride-rich lipoproteins, apoE(-/-) adipose tissue accumulates less triglyceride than wild type. The absence of apoE expression in primary cultured adipocytes also leads to changes in the expression of genes involved in the metabolism/turnover of fatty acids and the triglyceride droplet. Markers of adipocyte differentiation were lower in freshly isolated and cultured apoE(-/-) adipocytes. Importantly, PPAR-gamma-mediated changes in lipid content and gene expression are markedly altered in cultured apoE(-/-) adipocytes. These results establish a novel role for endogenous apoE in adipocyte lipid metabolism and have implications for constructing an integrated model of adipocyte physiology in health and disease.

Role of endogenous ghrelin in growth hormone secretion, appetite regulation and metabolism

Ghrelin, a 28-amino acid hormone that is acylated post-translation, is the endogenous ligand for the growth hormone (GH) secretagogue (GHS) receptor (GHS-R). The highest concentrations of ghrelin are found in the stomach; however ghrelin peptide is also present in hypothalamic nuclei known to be important in the control of GH and feeding behavior. Exogenous ghrelin potently stimulates pituitary GH release through a mechanism that is dependent, in part, on endogenous GH-releasing hormone. Whether endogenous ghrelin plays a role in the control of GH secretion and growth is not clear and ghrelin deficient animals appear to grow normally. In contrast, experimental animal and clinical data suggest that abnormalities in GHS-R signaling could impact growth. Ghrelin or other GHS are clinically useful for GH-testing and limited data suggest that they might be useful in the treatment of some patients with GH deficiency. Substantial data have implicated ghrelin as an important regulator of feeding behavior and energy equilibrium. Ghrelin has a potent orexigenic effect in both animals and humans and this effect is mediated through hypothalamic neuropeptide Y (NPY) and Agouti-related peptide (AgRP). Appetite simulation coupled with other metabolic effects promotes weight gain during chronic treatment with ghrelin. These metabolic effects are in part mediated through an increase in respiratory quotient (VQ). Presence of ghrelin appears to be necessary for the development of obesity in some animal models. Whether abnormalities in ghrelin signaling are involved in human obesity is not yet known.

Potential impact of carbohydrate and fat intake on pathological left ventricular hypertrophy

Currently, a high carbohydrate/low fat diet is recommended for patients with hypertension; however, the potentially important role that the composition of dietary fat and carbohydrate plays in hypertension and the development of pathological left ventricular hypertrophy (LVH) has not been well characterized. Recent studies demonstrate that LVH can also be triggered by activation of insulin signaling pathways, altered adipokine levels, or the activity of peroxisome proliferator-activated receptors (PPARs), suggesting that metabolic alterations play a role in the pathophysiology of LVH. Hypertensive patients with high plasma insulin or metabolic syndrome have a greater occurrence of LVH, which could be due to insulin activation of the serine-threonine kinase Akt and its downstream targets in the heart, resulting in cellular hypertrophy. PPARs also activate cardiac gene expression and growth and are stimulated by fatty acids and consumption of a high fat diet. Dietary intake of fats and carbohydrate and the resultant effects of plasma insulin, adipokine, and lipid concentrations may affect cardiomyocyte size and function, particularly in the setting of chronic hypertension. This review discusses potential mechanisms by which dietary carbohydrates and fats ca affect cardiac growth, metabolism, and function, mainly in the context of pressure overload-induced LVH.

Ghrelin increases neuropeptide Y and agouti-related peptide gene expression in the arcuate nucleus in rat hypothalamic organotypic cultures

Ghrelin, which was identified from the rat stomach, is a potent stimulant for food intake. Several lines of evidence suggest that the orexigenic action of ghrelin is mediated via the neuropeptide Y (NPY) neurons in the arcuate nucleus, although the detailed mechanisms by which ghrelin stimulates NPY neurons are not clear. In this study, we examined the gene regulation of NPY and agouti-related peptide (AGRP), another orexigenic peptide synthesized in the NPY neurons, in the arcuate nucleus by ghrelin in hypothalamic organotypic cultures. Incubation of the hypothalamic explants with ghrelin significantly increased NPY and AGRP mRNA expression in the presence, but not absence, of dexamethasone. Glucocorticoids were also necessary for ghrelin action in vivo because an intracerebroventricular injection of ghrelin significantly increased NPY and AGRP mRNA expression in the arcuate nucleus only in sham-operated, but not in adrenalectomized rats. The stimulatory effects of ghrelin on gene expression were not blocked by a sodium channel blocker tetrodotoxin in the organotypic cultures. Ghrelin also increased NPY heteronuclear (hn) RNA expression, the first transcript that has been used as an indicator for gene transcription. The stimulatory effects of ghrelin on NPY gene expression were abolished in the presence of cycloheximide, which blocks translation, suggesting that de novo protein synthesis is required for ghrelin action. These data suggest that ghrelin stimulates NPY and AGRP gene expression independently of action potentials only in the presence of glucocorticoids. Furthermore, our data demonstrate stimulatory action of ghrelin on NPY gene transcription, which requires de novo protein synthesis.

Ghrelin: a hormone regulating food intake and energy homeostasis

Regulation of energy homeostasis requires precise coordination between peripheral nutrient-sensing molecules and central regulatory networks. Ghrelin is a twenty-eight-amino acid orexigenic peptide acylated at the serine 3 position mainly with an n-octanoic acid, which is produced mainly in the stomach. It is the endogenous ligand of the growth hormone secretagogue (GHS) receptors. Since plasma ghrelin levels are strictly dependent on recent food intake, this hormone plays an essential role in appetite and meal initiation. In addition, ghrelin is involved in the regulation of energy homeostasis. The ghrelin gene is composed of four exons and three introns and renders a diversity of orexigenic peptides as well as des-acyl ghrelin and obestatin, which exhibit anorexigenic properties. Ghrelin stimulates the synthesis of neuropeptide Y (NPY) and agouti-related protein (AgRP) in the arcuate nucleus neurons of the hypothalamus and hindbrain, which in turn enhance food intake. Ghrelin-expressing neurons modulate the action of both orexigenic NPY/AgRP and anorexigenic pro-opiomelanocortin neurons. AMP-activated protein kinase is activated by ghrelin in the hypothalamus, which contributes to lower intracellular long-chain fatty acids, and this appears to be the molecular signal for the expression of NPY and AgRP. Recent data suggest that ghrelin has an important role in the regulation of leptin and insulin secretion and vice versa. The present paper updates the effects of ghrelin on the control of energy homeostasis and reviews the molecular mechanisms of ghrelin synthesis, as well as interaction with GHS receptors and signalling. Relationships with leptin and insulin in the regulation of energy homeostasis are addressed.

Predictors of postabsorptive ghrelin secretion after intake of different macronutrients

CONTEXT

Release of ghrelin, a gastrointestinal hormone regulating feeding and energy balance, is blunted in obesity, a condition associated with insulin resistance.

OBJECTIVE

The objective was to identify anthropometric and metabolic predictors of postabsorptive ghrelin secretion.

DESIGN

We evaluated ghrelin, insulin, glucose, and leptin secretion overnight and after intake of different macronutrients.

SUBJECTS

Ten obese subjects (age, 31.8 +/- 2.5 yr; body mass index, 43.4 +/- 0.8 kg/m(2)) and six lean subjects (age, 33.5 +/- 2.4 yr; body mass index, 21.8 +/- 1.4 kg/m(2)) participated in the study.

MAIN OUTCOME MEASURES

The main outcome measures were resting energy expenditure (REE); fat mass; nighttime approximate entropy (ApEn) and synchronicity (cross-ApEn) of ghrelin, insulin, and leptin; insulin sensitivity by homeostatic model approach insulin-sensitivity (HOMA-S%); postabsorptive area under the curve (AUC); and Delta of ghrelin, insulin, glucose, and leptin after carbohydrate-, lipid-, and protein-rich test meals.

RESULTS

Nighttime ApEn scores were higher in obese than lean subjects (P < 0.01). Cross-ApEn revealed a synchronicity between ghrelin-insulin, ghrelin-leptin, and insulin-leptin in both groups. Compared with baseline, ghrelin decreased significantly (P < 0.01) in lean and obese subjects after carbohydrates (42.2 vs. 28.5%; P < 0.05), lipids (40.2 vs. 26.2%; P < 0.01), and proteins (42.2 vs. 26.3%; P < 0.01) devoid of between-meal ghrelin differences. Significant associations occurred between nocturnal ghrelin ApEn and insulin (r = 0.53; P < 0.05), postmeal ghrelin AUCs and REE (r = -0.57; P < 0.05), and HOMA-S% (r = 0.52; P < 0.05), postmeal ghrelin Delta and HOMA-S% (r = 0.60; P < 0.05). REE (beta = -0.57; P = 0.02) and ghrelin ApEn (beta = -0.62; P = 0.01) were predictors of postmeal ghrelin AUC and Delta, respectively.

CONCLUSIONS

Obesity determined a decreased orderliness of ghrelin secretion and a relative loss of ghrelin-insulin synchrony. Postabsorptive ghrelin secretion decreased significantly both in obese and lean subjects, was related to insulin sensitivity, and was predicted by energy expenditure and hormone pulsatility.

Sex-specific determinants of serum adiponectin in older adults: the role of endogenous sex hormones

OBJECTIVE

To assess the sex-specific association of adiponectin with multiple factors thought to influence its levels, with a special emphasis on endogenous sex hormones.

DESIGN AND METHODS

A cross-sectional study of determinants of serum adiponectin in 873 men and 673 postmenopausal women, ages 50-92. Factors evaluated include age, body size, fat distribution, lifestyle (exercise, smoking, alcohol intake), insulin resistance, renal function and endogenous sex hormone levels (total and bioavailable testosterone and estradiol).

RESULTS

Median serum adiponectin was 50% higher in women than men (P<0.001). In unadjusted analyses, adiponectin was positively related to age, alcohol intake, high-density lipoprotein (HDL) and testosterone, and negatively related to waist girth, body mass index, Homeostasis Model Assessment for Insulin Resistance (HOMA-IR), triglycerides and bioavailable estradiol in both sexes (all P<0.01). Adiponectin was positively related to blood urea nitrogen, a measure of renal function, in men only (P<0.001). Sex-specific multivariate linear regressions adjusting for HDL and triglycerides showed that only age, HOMA-IR and sex hormones independently predicted circulating adiponectin for both men and women. Higher levels of endogenous testosterone and lower bioavailable estradiol concentrations each predicted higher adiponectin; this was true for both sexes, and was not explained by differences in age, adiposity, alcohol intake, insulin resistance or lipoprotein levels.

CONCLUSIONS

The association of adiponectin with the factors studied here is strikingly similar for men and women. Sex differences in circulating adiponectin levels in older adults cannot be explained by sex hormone regulation.

Obesity and androgens: facts and perspectives

OBJECTIVE

This review discusses androgen status in male and female obesity, according to their specific phenotype, and the main mechanisms responsible.

DESIGN

Published data in the literature of the last 20 years represented the basis of most of the data and concepts incorporated in the review.

RESULT(S)

Obesity is associated with profound alterations in androgen secretion, transport, metabolism, and action, according to a dichotomous behavior depending on sex. Obese men are characterized by a progressive decrease of testosterone levels with increasing body weight, whereas obese women, particularly those with the abdominal phenotype, tend to develop a condition of functional hyperandrogenism. Reduced sex hormone-binding globulin synthesis and circulating blood levels represent the sole common mechanism which is responsible in both sexes. Among other still partially undefined factors, mechanisms potentially responsible for the sex dichotomy in androgen levels involve specific alterations of gonadotropin secretion, estrogens, the hypothalamic-pituitary-adrenal axis, leptin, androgen receptors, specific steroidogenic enzymes in the peripheral tissues, and, possibly, ghrelin. In both sexes, androgens play an important role in determining the sex-dependent pattern of body fat distribution. Moreover there are theoretical possibilities that low testosterone in men and high free testosterone fraction in women may play a role in the development of the metabolic syndrome. This is exemplified by the well defined association between obesity and other features of the metabolic syndrome in women with polycystic ovary syndrome and in hypogonadal men. The effects of androgen and antiandrogens in obese men and women also represent arguments in favor of this association.

CONCLUSION(S)

Given the fundamental role of sex hormones in the regulation of body composition, fuel homeostasis, and reproduction in humans, more emphasis should be placed on the potential role of androgen dysregulation in the pathophysiology of different obesity phenotypes and the metabolic syndrome.

Biological, physiological, and pharmacological aspects of ghrelin

Ghrelin, identified as an endogenous ligand for the growth hormone secretagogue receptor, functions as a somatotrophic and orexigenic signal from the stomach. Ghrelin has a unique post-translational modification: the hydroxyl group of the third amino acid, usually a serine but in some species a threonine, is esterified by octanoic acid and is essential for ghrelin's biological activities. The secretion of ghrelin increases under conditions of negative energy-balance, such as starvation, cachexia, and anorexia nervosa, whereas its expression decreases under conditions of positive energy-balance such as feeding, hyperglycemia, and obesity. In addition to having a powerful effect on the secretion of growth hormone, ghrelin stimulates food intake and transduces signals to hypothalamic regulatory nuclei that control energy homeostasis. Thus, it is interesting to note that the stomach may play an important role in not only digestion but also pituitary growth hormone release and central feeding regulation. We summarized recent findings on the integration of ghrelin into neuroendocrine networks that regulate food intake, energy balance, gastrointestinal function and growth.

Adiponectin stimulates proliferation and cytokine secretion in colonic epithelial cells

Adiponectin is a recently described mediator secreted by adipose tissue. Here we report the growth promoting and pro-inflammatory actions of adiponectin on colonic epithelial cancer cells. Full-length and globular adiponectin produced an identical stimulation of HT-29 cell growth that was blocked by inhibition of adenylate cyclase and protein kinase A and partially inhibited by a pan-specific protein kinase C inhibitor, but was unaffected by specific inhibition of extracellular signal-related kinase (ERK) or p38 MAP kinase. Globular adiponectin but not full-length adiponectin significantly increased the secretion and mRNA levels of IL-8, GM-CSF and MCP-1. Globular adiponectin doubled IL-1beta-stimulated IL-8 and GM-CSF secretion. Adiponectin-stimulated cytokine secretion was blocked by pharmacological inhibitors of NF-kappaB, ERK and p38 MAP kinase. Globular adiponectin increased phosphorylation of both ERK and p38 MAP kinase and increased the nuclear translocation of active NF-kappaB. Adiponectin has pro-proliferative and pro-inflammatory actions on colonic epithelial cells; these appear to be differentially activated by the adiponectin isoforms. Adiponectin may have a role in the regulation of gastrointestinal mucosal function, inflammation and colon carcinogenesis.

Low-dose leptin reverses skeletal muscle, autonomic, and neuroendocrine adaptations to maintenance of reduced weight

Maintenance of a reduced body weight is accompanied by decreased energy expenditure that is due largely to increased skeletal muscle work efficiency. In addition, decreased sympathetic nervous system tone and circulating concentrations of leptin, thyroxine, and triiodothyronine act coordinately to favor weight regain. These "weight-reduced" phenotypes are similar to those of leptin-deficient humans and rodents. We examined metabolic, autonomic, and neuroendocrine phenotypes in 10 inpatient subjects (5 males, 5 females [3 never-obese, 7 obese]) under 3 sets of experimental conditions: (a) maintaining usual weight by ingesting a liquid formula diet; (b) maintaining a 10% reduced weight by ingesting a liquid formula diet; and (c) receiving twice-daily subcutaneous doses of leptin sufficient to restore 8 am circulating leptin concentrations to pre-weight-loss levels and remaining on the same liquid formula diet required to maintain a 10% reduced weight. During leptin administration, energy expenditure, skeletal muscle work efficiency, sympathetic nervous system tone, and circulating concentrations of thyroxine and triiodothyronine returned to pre-weight-loss levels. These responses suggest that the weight-reduced state may be regarded as a condition of relative leptin insufficiency. Prevention of weight regain might be achievable by strategies relevant to reversing this leptin-insufficient state.

Resistin as a putative modulator of insulin action in the daily feeding/fasting rhythm

Resistin and adiponectin are adipokines with postulated opposite functions. Resistin has been related with insulin resistance in obesity, while adiponectin could be associated to higher insulin sensitivity. We have determined whether the production of these two adipokines during the day is related to the feeding rhythm in rats. Resistin mRNA levels in adipose tissue correlated positively with the gastric contents and serum insulin concentration, showing higher levels during the dark phase (period of the highest food intake), especially in the mesenteric depot, while levels decreased during the light phase. The diurnal pattern of resistin expression was not directly reflected in the circulating levels, but it showed a 6-h delay and correlated negatively with the gastric contents and serum insulin. Adiponectin expression followed an opposite pattern, not apparently related to feeding or insulin release, and not translated into changes in circulating levels. Moreover, considering that insulin stimulates resistin expression and that circulating resistin follows a contrary circadian pattern in comparison to insulin, resistin, apart from its role in the increased insulin resistance associated to obesity, could also act as a putative modulator of insulin in the daily feeding/fasting rhythm through a negative feedback regulation of its action.

Serum ghrelin levels are enhanced in patients with epilepsy

PURPOSE

In patients with epilepsy, although many changes in the physiology of hormones in the neuroendocrine system can occur (especially in the sex hormones, for example), the causes of these changes have not been fully elucidated. There are also relations between seizure activity and stages of sleep. Ghrelin is the peptide hormone, which has been shown to affect both endocrine function and sleep. The purpose of this study was to evaluate serum levels of ghrelin in epilepsy patients.

METHODS

A total of 35 patients currently receiving antiepileptic drug therapy (of these patients, 20 had primary generalized seizure and 15 had partial seizure) were studied. The control group consisted of 30 healthy volunteers matched for age and gender. In all participants, serum levels of ghrelin, cholesterol and triglycerides were measured and body mass index (BMI) was determined. Patients with endocrine, immune or any other chronic diseases were excluded.

RESULTS

In the epilepsy patients, the mean serum ghrelin level was 158.81+/-55.97 pg/ml, and this was significantly higher than the control group's level of 93.43+/-21.33 pg/ml (p<0.001). In terms of serum cholesterol, triglycerides and BMI, no significant differences were found between the epilepsy patients and the control group (p>0.05).

CONCLUSIONS

The origin of higher serum ghrelin levels in epilepsy and their relation to seizures are not completely known. However, this elevation of serum ghrelin could contribute to the lengthening of NREM sleep in epilepsy patients, thereby playing a role in seizure occurrence. From another direction, high serum ghrelin levels could cause changes and/or dysfunction in hormone secretion and physiology via its effects on growth hormone, and thereby play a facilitating role in seizure occurrence.

Rhythmic messenger ribonucleic acid expression of clock genes and adipocytokines in mouse visceral adipose tissue

Various peripheral tissues show circadian rhythmicity, which is generated at the cellular level by their own core oscillators that are composed of transcriptional/translational feedback loops involving a set of clock genes. Although the circulating levels of some adipocytokines, i.e. bioactive substances secreted by adipocytes, are on a 24-h rhythmic cycle, it remains to be elucidated whether the clock gene system works in adipose tissue. To address this issue, we investigated the daily mRNA expression profiles of the clock genes and adipocytokines in mouse perigonadal adipose tissues. In C57BL/6J mice, all transcript levels of the clock genes (Bmal1, Per1, Per2, Cry1, Cry2, and Dbp) and adipocytokines (adiponectin, resistin, and visfatin) clearly showed 24-h rhythms. On the other hand, the rhythmic expression of these genes was mildly attenuated in obese KK mice and greatly attenuated in more obese, diabetic KK-A(y) mice. Obese diabetes also diminished the rhythmic expression of the clock genes in the liver. Interestingly, a 2-wk treatment of KK and KK-A(y) mice with pioglitazone impaired the 24-h rhythmicity of the mRNA expression of the clock genes and adipocytokines despite the antidiabetic effect of the drug. In contrast, pioglitazone improved the attenuated rhythmicity in the liver. These findings suggest that the intracellular clock gene system acts in visceral adipose tissues as well as liver and is influenced by the conditions of obesity/type 2 diabetes and pioglitazone treatment.

Diurnal variation of ghrelin, leptin, and adiponectin in Standardbred mares

Twelve Standardbred mares underwent blood sampling for 24 h to test the hypothesis that there is diurnal variation of humoral mediators of peripheral energy balance including active ghrelin, adiponectin, leptin, glucose, insulin, and cortisol. The experiment was conducted under acclimated conditions. Grass hay and pelleted grain were provided at 0730 and 1530. Plasma concentrations of active ghrelin and leptin concentrations both peaked (47.3 +/- 6.5 pg/ mL and 5.9 +/- 1.1 ng/mL, respectively; P < 0.05) at 1550, 20 min after feeding. Active ghrelin decreased (P < 0.05) to 28.9 +/- 4.5 pg/mL overnight. The nadir of leptin (4.6 +/- 0.9 ng/mL) occurred at 0650. Neither hormone showed variation (P > 0.05) after the morning feeding. Plasma glucose and insulin concentrations increased (P < 0.05) in response to feeding; however, the morning responses (glucose = 96.9 +/- 2.6 mg/dL; insulin = 40.6 +/- 7.3 uIU/mL) were greater (P < 0.05) than the afternoon responses (glucose = 89.9 +/- 1.8 mg/dL; insulin = 23.2 +/- 4.3 uIU/mL at 180 and 60 min after feeding, respectively). Cortisol concentrations increased (P < 0.05) during the morning hours, but did not respond to feeding, whereas adiponectin concentrations remained stable throughout the study. Hence, active ghrelin and leptin may be entrained to meal feeding in horses, whereas adiponectin seems unaffected. We concluded that there seems to be a diurnal variation in glucose and insulin response to a meal in horses. Furthermore, elevated glucose and insulin concentrations resulting from the morning feeding may be responsible for the increase in leptin concentration in the afternoon.

Leptin pulsatility in formerly obese women

Plasma leptin and growth hormone (GH) profile and pulsatility have been studied in morbidly obese subjects before and 14 months after bilio-pancreatic diversion (BPD), a bariatric technique producing massive lipid malabsorption. The maximum leptin diurnal variation (acrophase) decreased (10.27+/-1.70 vs. 22.60+/-2.79 ng x ml(-1); P=0.001), while its pulsatility index (PI) increased (1.084+/-0.005 vs. 1.050+/-0.004 ng x ml(-1) x min(-1); P=0.02) after BPD. Plasma GH acrophase increased (P=0.0001) from 0.91+/-0.20 to 4.58+/-0.80 microg x l(-1) x min(-1) after BPD as well as GH PI (1.70+/-0.13 vs. 1.20+/-0.04 microg x l(-1) x min(-1); P=0.024). Whole-body glucose uptake (M), assessed by euglycemic-hyperinsulinemic clamp, almost doubled after BPD (from 0.274+/-0.022 to 0.573+/-0.027 mmol x kgFFM(-1) x min(-1); P<0.0001), while 24 h lipid oxidation was significantly (P<0.0001) reduced (131.94+/-35.58 vs. 44.56+/-15.10 g). However, the average lipid oxidation was 97.2+/-3.1% (P<0.01) of the metabolizable lipid intake after the bariatric operation, while it was 69.2+/-8.5% before. After the operation, skeletal muscle ACC2 mRNA decreased (P<0.0001) from 452.82+/-76.35 to 182.45+/-40.69% of cyclophilin mRNA as did the malonyl-CoA (from 0.28+/-0.02 to 0.16+/-0.01 nmol x g(-1); P<0.0001). Leptin changes negatively correlated with M changes (R2=0.69, P<0.001). In a stepwise regression (R2=0.87, P=0.0055), only changes in 24 h free fatty acids (B=0.105+/-0.018, P=0.002) and glucose/insulin ratio (B=0.247+/-0.081, P=0.029) were the best predictors of leptin variations. In conclusion, the reversion of insulin resistance after BPD might allow reversal of leptin resistance, restoration of leptin pulsatility, and consequent inhibition of ACC2 mRNA expression, translating to a reduced synthesis of malonyl-CoA, which, in turn, results in increased fatty acid oxidation. Finally, since leptin inhibits GH secretion, a reduction of circulating leptin levels might have produced an increase in GH secretion, as observed in our series.

Ghrelin: a gastric peptide that regulates food intake and energy homeostasis

Ghrelin, an endogenous ligand for the growth-hormone-secretagogue receptor, was isolated from human and rat stomach. It is a 28-amino acid peptide with a posttranslational acyl modification that is indispensable for its activity. In addition to stimulating growth-hormone secretion, food intake, and body weight gain, ghrelin also plays a role in a variety of other systems, including circulation, digestion, and cell proliferation. This review will focus on the discovery, structural characteristics, tissue distribution, and physiological functions of ghrelin, as well as the regulation of its expression and secretion.

Ghrelin: structure and function

Small synthetic molecules called growth hormone secretagogues (GHSs) stimulate the release of growth hormone (GH) from the pituitary. They act through the GHS-R, a G protein-coupled receptor whose ligand has only been discovered recently. Using a reverse pharmacology paradigm with a stable cell line expressing GHS-R, we purified an endogenous ligand for GHS-R from rat stomach and named it "ghrelin," after a word root ("ghre") in Proto-Indo-European languages meaning "grow." Ghrelin is a peptide hormone in which the third amino acid, usually a serine but in some species a threonine, is modified by a fatty acid; this modification is essential for ghrelin's activity. The discovery of ghrelin indicates that the release of GH from the pituitary might be regulated not only by hypothalamic GH-releasing hormone, but also by ghrelin derived from the stomach. In addition, ghrelin stimulates appetite by acting on the hypothalamic arcuate nucleus, a region known to control food intake. Ghrelin is orexigenic; it is secreted from the stomach and circulates in the bloodstream under fasting conditions, indicating that it transmits a hunger signal from the periphery to the central nervous system. Taking into account all these activities, ghrelin plays important roles for maintaining GH release and energy homeostasis in vertebrates.

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.

Influencing the between-feeding and endocrine responses of plasma ghrelin in healthy dogs

OBJECTIVES

Ghrelin has recently been isolated from rat and human stomach as an endogenous ligand for the growth hormone (GH) secretagog receptor. Using beagle dogs, we investigated the distribution of ghrelin in the stomach and its possible role.

METHODS

We examined: (i) GH release in response to ghrelin injection (0.5 or 5 microg/kg, i.v.), (ii) gastric localization of ghrelin-immunostained cells, (iii) changes in daily food consumption after ghrelin injection (3, 10, and 20 microg/kg, i.v.), (iv) plasma ghrelin levels under regular, but restricted feeding conditions, and (v) variations in plasma ghrelin levels in relatively lean, normal and obese dogs.

RESULTS

Administration of ghrelin to dogs promptly increased circulating GH concentrations, although this effect was transitory and was maintained for only 20 min. Ghrelin was localized in the stomach fundus and body, but none was detected in either the pylorus or cardia. Administration of ghrelin at a dose of 20 microg/kg increased the daily food intake of beagle dogs. Plasma ghrelin levels peaked just before meal times, and then returned to basal levels. Obese dogs had higher plasma ghrelin levels than did normal and lean dogs.

CONCLUSIONS

These results indicate that ghrelin is a potent GH secretagog in dogs. The distribution of ghrelin-immunoreactive cells in the canine stomach resembles that of both the murine and human stomach. Ghrelin participates in the control of feeding behavior and energy homeostasis in dogs and may, therefore, be involved in the development of obesity.