Pulsatile growth hormone secretion persists in genetic growth hormone-releasing hormone resistance

The Complex World of Regulation of Pituitary Growth Hormone Secretion: The Role of Ghrelin, Klotho, and Nesfatins in It

The classic concept of how pituitary GH is regulated by somatostatin and GHRH has changed in recent years, following the discovery of peripheral hormones involved in the regulation of energy homeostasis and mineral homeostasis. These hormones are ghrelin, nesfatins, and klotho. Ghrelin is an orexigenic hormone, released primarily by the gastric mucosa, although it is widely expressed in many different tissues, including the central nervous system and the pituitary. To be active, ghrelin must bind to an n-octanoyl group (n = 8, generally) on serine 3, forming acyl ghrelin which can then bind and activate a G-protein-coupled receptor leading to phospholipase C activation that induces the formation of inositol 1,4,5-triphosphate and diacylglycerol that produce an increase in cytosolic calcium that allows the release of GH. In addition to its direct action on somatotrophs, ghrelin co-localizes with GHRH in several neurons, facilitating its release by inhibiting somatostatin, and acts synergistically with GHRH stimulating the synthesis and secretion of pituitary GH. Gastric ghrelin production declines with age, as does GH. Klotho is an anti-aging agent, produced mainly in the kidneys, whose soluble circulating form directly induces GH secretion through the activation of ERK1/2 and inhibits the inhibitory effect that IGF-I exerts on GH. Children and adults with untreated GH-deficiency show reduced plasma levels of klotho, but treatment with GH restores them to normal values. Deletions or mutations of the Klotho gene affect GH production. Nesfatins 1 and 2 are satiety hormones, they inhibit food intake. They have been found in GH3 cell cultures where they significantly reduce the expression of gh mRNA and that of pituitary-specific positive transcription factor 1, consequently acting as inhibitors of GH production. This is a consequence of the down-regulation of the cAMP/PKA/CREB signaling pathway. Interestingly, nesfatins eliminate the strong positive effect that ghrelin has on GH synthesis and secretion. Throughout this review, we will attempt to broadly analyze the role of these hormones in the complex world of GH regulation, a world in which these hormones already play a very important role.

Growth Hormone Deficiency: Health and Longevity

The important role of GH in the control of mammalian longevity was first deduced from extended longevity of mice with genetic GH deficiency (GHD) or GH resistance. Mice with isolated GHD (IGHD) due to GHRH or GHRH receptor mutations, combined deficiency of GH, prolactin, and TSH, or global deletion of GH receptors live longer than do their normal siblings. They also exhibit multiple features of delayed and/or slower aging, accompanied by extension of healthspan. The unexpected, remarkable longevity benefit of severe endocrine defects in these animals presumably represents evolutionarily conserved trade-offs among aging, growth, maturation, fecundity, and the underlying anabolic processes. Importantly, the negative association of GH signaling with longevity extends to other mammalian species, apparently including humans. Data obtained in humans with IGHD type 1B, owing to a mutation of the GHRH receptor gene, in the Itabaianinha County, Brazil, provide a unique opportunity to study the impact of severe reduction in GH signaling on age-related characteristics, health, and functionality. Individuals with IGHD are characterized by proportional short stature, doll facies, high-pitched voices, and central obesity. They have delayed puberty but are fertile and generally healthy. Moreover, these IGHD individuals are partially protected from cancer and some of the common effects of aging and can attain extreme longevity, 103 years of age in one case. We think that low, but detectable, residual GH secretion combined with life-long reduction of circulating IGF-1 and with some tissue levels of IGF-1 and/or IGF-2 preserved may account for the normal longevity and apparent extension of healthspan in these individuals.

MECHANISMS IN ENDOCRINOLOGY: The multiple facets of GHRH/GH/IGF-I axis: lessons from lifetime, untreated, isolated GH deficiency due to a GHRH receptor gene mutation

Twenty years ago, we described kindred of 105 individuals with isolated GH deficiency (IGHD) in Itabaianinha County, in northeast Brazil, carrying a homozygous mutation in the GH-releasing hormone receptor gene. These subjects exhibit markedly reduced GH responsiveness to stimulatory tests, and anterior pituitary hypoplasia. Serum concentrations of IGF-I, IGF binding protein type 3 and the acid-labile subunit are markedly reduced, with a lesser reduction of IGF-II. The most striking physical findings of these IGHD individuals are the proportionate short stature, doll facies, high-pitched voice and visceral obesity with reduced fat-free mass. There is neither microphallus, nor neonatal hypoglycemia. Puberty is delayed, menopause anticipated, but fertility is preserved in both genders. The reduction in bone sizes is not even, with mean standard deviation scores for height of -7.2, total maxillary length of -6.5, total facial height of -4.3 and cephalic perimeter of -2.7. In addition, the non-osseous growth is not uniform, preserving some organs, like pancreas, liver, kidney, brain and eyes, and compromising others such as thyroid, heart, uterus and spleen. These subjects present higher prevalence of dizziness, mild high-tones sensorineural hearing loss, reduction of vascular retinal branching points, increase of optic disk, genu valgum and increased systolic blood pressure. Biochemically, they have high low density lipoprotein cholesterol and C-reactive protein levels, but maintain increased insulin sensitivity, and do not show premature atherosclerosis. Finally, they have normal immune function, and normal longevity. This review details the findings and summarizes 20 years of clinical research carried out in this unique population.

Growth hormone axis and aging

Growth hormone (GH) and/or ghrelin mimetics represent potential treatment and/or prevention options for musculoskeletal impairment associated with aging. Use of improvement in muscle function as an outcome in studies of GH and ghrelin mimetics is complicated by the lack of a standardized definition for clinically meaningful efficacy of this end point. Based on preliminary study results, the use of ghrelin mimetics may be more suitable for use in this age group than GH itself. There are still several unanswered questions related to the use of ghrelin mimetics in the elderly, which prevents recommendation for its use at the current time.

Growth hormone doping in sports: a critical review of use and detection strategies

GH is believed to be widely employed in sports as a performance-enhancing substance. Its use in athletic competition is banned by the World Anti-Doping Agency, and athletes are required to submit to testing for GH exposure. Detection of GH doping is challenging for several reasons including identity/similarity of exogenous to endogenous GH, short half-life, complex and fluctuating secretory dynamics of GH, and a very low urinary excretion rate. The detection test currently in use (GH isoform test) exploits the difference between recombinant GH (pure 22K-GH) and the heterogeneous nature of endogenous GH (several isoforms). Its main limitation is the short window of opportunity for detection (~12-24 h after the last GH dose). A second test to be implemented soon (the biomarker test) is based on stimulation of IGF-I and collagen III synthesis by GH. It has a longer window of opportunity (1-2 wk) but is less specific and presents a variety of technical challenges. GH doping in a larger sense also includes doping with GH secretagogues and IGF-I and its analogs. The scientific evidence for the ergogenicity of GH is weak, a fact that is not widely appreciated in athletic circles or by the general public. Also insufficiently appreciated is the risk of serious health consequences associated with high-dose, prolonged GH use. This review discusses the GH biology relevant to GH doping; the virtues and limitations of detection tests in blood, urine, and saliva; secretagogue efficacy; IGF-I doping; and information about the effectiveness of GH as a performance-enhancing agent.

Growth hormone response to growth hormone-releasing peptide-2 in growth hormone-deficient little mice

OBJECTIVE

To investigate a possible direct, growth hormone-releasing, hormone-independent action of a growth hormone secretagogue, GHRP-2, in pituitary somatotroph cells in the presence of inactive growth hormone-releasing hormone receptors.

MATERIALS AND METHODS

The responses of serum growth hormone to acutely injected growth hormone-releasing P-2 in lit/lit mice, which represent a model of GH deficiency arising from mutated growth hormone-releasing hormone-receptors, were compared to those observed in the heterozygous (lit/+) littermates and wild-type (+/+) C57BL/6J mice.

RESULTS

After the administration of 10 mcg of growth hormone-releasing P-2 to lit/lit mice, a growth hormone release of 9.3±1.5 ng/ml was observed compared with 1.04±1.15 ng/ml in controls (p<0.001). In comparison, an intermediate growth hormone release of 34.5±9.7 ng/ml and a higher growth hormone release of 163±46 ng/ml were induced in the lit/+ mice and wild-type mice, respectively. Thus, GHRP-2 stimulated growth hormone in the lit/lit mice, and the release of growth hormone in vivo may be only partially dependent on growth hormone-releasing hormone. Additionally, the plasma leptin and ghrelin levels were evaluated in the lit/lit mice under basal and stimulated conditions.

CONCLUSIONS

Here, we have demonstrated that lit/lit mice, which harbor a germline mutation in the Growth hormone-releasing hormone gene, maintain a limited but statistically significant growth hormone elevation after exogenous stimulation with GHRP-2. The present data probably reflect a direct, growth hormone-independent effect on Growth hormone S (ghrelin) stimulation in the remaining pituitary somatotrophs of little mice that is mediated by growth hormone S-R 1a.

Comparative study of approximate entropy and sample entropy robustness to spikes

OBJECTIVE

There is an ongoing research effort devoted to characterize the signal regularity metrics approximate entropy (ApEn) and sample entropy (SampEn) in order to better interpret their results in the context of biomedical signal analysis. Along with this line, this paper addresses the influence of abnormal spikes (impulses) on ApEn and SampEn measurements.

METHODS

A set of test signals consisting of generic synthetic signals, simulated biomedical signals, and real RR records was created. These test signals were corrupted by randomly generated spikes. ApEn and SampEn were computed for all the signals under different spike probabilities and for 100 realizations.

RESULTS

The effect of the presence of spikes on ApEn and SampEn is different for test signals with narrowband line spectra and test signals that are better modeled as broadband random processes. In the first case, the presence of extrinsic spikes in the signal results in an ApEn and SampEn increase. In the second case, it results in an entropy decrease. For real RR records, the presence of spikes, often due to QRS detection errors, also results in an entropy decrease.

CONCLUSIONS

Our findings demonstrate that both ApEn and SampEn are very sensitive to the presence of spikes. Abnormal spikes should be removed, if possible, from signals before computing ApEn or SampEn. Otherwise, the results can lead to misunderstandings or misclassification of the signal regularity.

The role of ghrelin in GH secretion and GH disorders

In humans, growth hormone (GH) is secreted from the anterior pituitary in a pulsatile pattern. The traditional view is that this secretory pattern is driven by two counter regulatory neurohormones, GHRH and somatostatin. Ghrelin, the natural ligand for the growth hormone (GH)-secretagogue receptor (GHS-R), is produced in the stomach. Ghrelin is the strongest GH secretagogue known to date, but the role of endogenous ghrelin in the regulation of circulating GH levels remains controversial. The following review examines the evidence suggesting that endogenous ghrelin may be a key regulator of GH peak amplitude and discusses studies of diseases with altered GH levels, where it is found that in these states GH and ghrelin levels change in a similar way.

AutoDecon: a robust numerical method for the quantification of pulsatile events

This work presents a new approach to the analysis of aperiodic pulsatile heteroscedastic time-series data, specifically hormone pulsatility. We have utilized growth hormone (GH) concentration time-series data as an example for the utilization of this new algorithm. While many previously published approaches used for the analysis of GH pulsatility are both subjective and cumbersome to use, AutoDecon is a nonsubjective, standardized, and completely automated algorithm. We have employed computer simulations to evaluate the true-positive, the false-positive, the false-negative, and the sensitivity percentages of several of the routinely employed algorithms when applied to GH concentration time-series data. Based on these simulations, it was concluded that this new algorithm provides a substantial improvement over the previous methods. This novel method has many direct applications in addition to hormone pulsatility, for example, to time-domain fluorescence lifetime measurements, as the mathematical forms that describe these experimental systems are both convolution integrals.

Evidence for acyl-ghrelin modulation of growth hormone release in the fed state

CONTEXT

The timing and frequency of GH secretory episodes is regulated by GHRH and somatostatin. This study provides evidence for amplification of these GH pulses by endogenous acyl-ghrelin.

DESIGN

Blood was sampled every 10 min for 26.5 h during a fed admission with standardized meals and also during the final 24 h of a 61.5-h fast. GH secretion profiles were derived from deconvolution of 10-min sampling data, and full-length acyl-ghrelin levels were measured using a newly developed two-site sandwich assay.

SETTING

The study was conducted at a university hospital general clinical research center.

PARTICIPANTS

Participants included eight men with mean (+/- sd) age 24.5 +/- 3.7 yr (body mass index 24 +/- 2.1 kg/m(2)).

RESULTS

Correlations were computed between amplitudes of individual GH secretory events and the average acyl-ghrelin concentration in the 60-min interval preceding each GH burst. In the fed state, the peak correlations were positive for all subjects and significantly higher than in the fasting state when acyl-ghrelin levels declined [mean (+/- sem): 0.7 (0.04) vs. 0.29 (0.08), P = 0.017]. In addition, long-term fasting was associated with an increase in the GH secretory pulse mass and amplitude but not frequency [fed vs. fasting pulse mass: 0.22 (0.05) vs. 0.44 (0.06) microg/liter, P = 0.002; amplitude: 5.2 (1.3) vs. 11.8 (1.9) microg/liter/min, P = 0.034; pulses per 24 h: 19.4 (0.5) vs. 22.0 (1.4), P = 0.1].

CONCLUSION

Our data support the hypothesis that under normal conditions in subjects given regular meals endogenous acyl-ghrelin acts to increase the amplitude of GH pulses.

Evidence that ghrelin is as potent as growth hormone (GH)-releasing hormone (GHRH) in releasing GH from primary pituitary cell cultures of a nonhuman primate (Papio anubis), acting through intracellular signaling pathways distinct from GHRH

Ghrelin is more effective than GHRH in stimulating GH release in normal adult humans and monkeys in vivo. This robust effect of ghrelin has been largely attributed to regulation of hypothalamic input, whereas the direct effect of ghrelin on pituitary GH release has been minimized by the observation that ghrelin has only a modest impact on GH release, compared with GHRH, in cultures prepared from human fetal pituitaries and GH-producing adenomas, as well as pituitaries from nonprimate species. However, comparable in vitro studies have not been performed to test the direct effect of ghrelin on normal adult primates. Therefore, in the present study, primary pituitary cell cultures from female baboons (Papio anubis) were used as a model system to test the direct effects of ghrelin on primate somatotrope function. In this model, both ghrelin and GHRH increased GH release in a dose-dependent fashion. Surprisingly, at maximal concentrations (10 nM), both ghrelin and GHRH elicited a robust increase in GH release (4 and 24 h, respectively), and both up-regulated GH secretagogue-receptor and GHRH-receptor mRNA levels (24 h). Combined treatment with ghrelin and GHRH resulted in an additive effect on GH release, suggesting that distinct intracellular signaling pathways are activated by each ligand, as confirmed by the use of specific inhibitors of intracellular signaling. Together, these results present the first evidence that a direct effect of ghrelin on somatotrope function may play a major role in stimulating GH release in primates.

Metabolic effects of growth hormone (GH) replacement in children and adolescents with severe isolated GH deficiency due to a GHRH receptor mutation

BACKGROUND

The interpretation of the true effect of GH replacement therapy (GHRT) on metabolic status in GH deficiency (GHD) is often complicated by differing aetiologies of GHD and by the presence of additional hormone deficits.

OBJECTIVE

To study the growth and response of the lipid profile and body composition to GHRT in a cohort of children with the same mutation in the GHRH receptor gene. Design Nine GH-deficient subjects (mean age 12.8 years, range 5-17.5 years; three male) in a rural community in Northeast Brazil were treated with GHRT for 2 years and compared with indigenous normal controls.

MAIN OUTCOME MEASURES

Total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), high density lipoprotein cholesterol (HDL-C), triglycerides (TG) and body composition were measured at baseline and after 3, 12 and 24 months of GHRT.

RESULTS

At baseline, the subjects with GHD had an adverse lipid profile, including elevated TC, elevated LDL-C and elevated TG. GHRT normalized TG in 3 months, LDL-C in 12 months and TC in 24 months. At baseline, older pubertal subjects with GHD had adverse body composition, including higher percentage fat mass (%FM), and GHRT induced a reduction in %FM that was maintained after 24 months. By contrast, younger prepubertal subjects did not have an adverse body composition.

CONCLUSIONS

Lipid profile was abnormal at baseline, while abnormal body composition was only seen in older subjects in late puberty, indicating that body composition is less sensitive to the effect of GHD than lipid profile. GHRT improves lipid profile at all ages, while it affects body composition only towards the end of growth, emphasizing its importance in achieving normal somatic development in the transition period.

Factors regulating growth hormone secretion in humans

Growth hormone (GH) secretion is pulsatile in nature in all species. The periodic pattern of GH release plays an important role in transmitting the GH message in a tissue-specific manner. The question of what regulates the pulsatile GH secretion pattern is an issue of not only theoretical interest but of considerable practical importance for designing different GH therapies for a variety of human diseases. This article provides a brief introductory overview of the different regulators of GH secretion and concentrates primarily on human studies.

GH response to hypoglycemia and clonidine in the GH-releasing hormone resistance syndrome

GH secretion by the pituitary is the result of the balance between the stimulatory effect of GHRH and the inhibitory effect of SS. Patients with mutations in GHRH receptor (GHRH-R) gene (GHRH-R) offer a unique model to study the mechanism of action of different GH secretion stimuli. In the past, we have demonstrated a small but significant GH response to a GH secretagogue (GHRP-2) in a homogenous cohort of patients with severe GH deficiency (GHD) due to a homozygous null mutation in GHRH-R (IVS1+1G-->A). Now, we sought to determine if we could detect a GH response to hypoglycemia (ITT: insulin tolerance test) or clonidine (CL) in these patients. Nine young GHD subjects underwent both ITT and CL tests, and 2 additional subjects underwent only CL test. There was a small but significant GH increase during ITT, but not during CL test. These results indicate that a minimal albeit significant GH response to ITT can occur despite complete lack of GHRH-R function.

Biological, physiological, pathophysiological, and pharmacological aspects of ghrelin

Ghrelin is a peptide predominantly produced by the stomach. Ghrelin displays strong GH-releasing activity. This activity is mediated by the activation of the so-called GH secretagogue receptor type 1a. This receptor had been shown to be specific for a family of synthetic, peptidyl and nonpeptidyl GH secretagogues. Apart from a potent GH-releasing action, ghrelin has other activities including stimulation of lactotroph and corticotroph function, influence on the pituitary gonadal axis, stimulation of appetite, control of energy balance, influence on sleep and behavior, control of gastric motility and acid secretion, and influence on pancreatic exocrine and endocrine function as well as on glucose metabolism. Cardiovascular actions and modulation of proliferation of neoplastic cells, as well as of the immune system, are other actions of ghrelin. Therefore, we consider ghrelin a gastrointestinal peptide contributing to the regulation of diverse functions of the gut-brain axis. So, there is indeed a possibility that ghrelin analogs, acting as either agonists or antagonists, might have clinical impact.

Familial Growth Hormone Deficiency and Mutations in the GHRH Receptor Gene

Growth hormone (GH)-releasing hormone (GHRH) is necessary for the proliferation of the somatotropic cells of the anterior pituitary and the synthesis and secretion of GH. GHRH is released by the hypothalamus into the portal hypophysial circulation to bind to a membrane surface receptor [GHRH receptor (GHRHR)] expressed by the somatotropic cells. Because of the need of GHRH for GH secretion, it is to be expected that alterations in synthesis or action of GHRH would result in isolated GH deficiency (IGHD). Indeed, although GHRH gene mutations have never been reported, mutations in the GHRHR gene (GHRHR) are emerging as a relatively common cause of inherited autosomal recessive IGHD. The first human GHRHR mutations were discovered in families with a history of parental consanguinity. More recently, kindreds in which IGHD subjects are compound heterozygotes for two distinct mutations indicate that faulty GHRHR alleles may be prevalent and that these mutations may need to be suspected even in sporadic IGHD cases. Patients with two faulty GHRHR alleles have normal weight at birth. Growth failure becomes apparent during the first year of life. Biochemical studies show low serum insulin-like growth factor-1 level, and absent or markedly reduced GH response to a variety of stimuli. Magnetic resonance imaging shows hypoplasia of the anterior pituitary. In this chapter, we describe the GHRHR mutations reported to date and the phenotype of affected individuals.

Hexarelin modulates the expression of growth hormone secretagogue receptor type 1a mRNA at hypothalamic and pituitary sites

Ghrelin and the synthetic growth hormone secretagogues (GHSs) activate a G-protein-coupled receptor (GHS-R) originally cloned from the pituitary, but which is also expressed in the hypothalamus, in other areas of the brain and in numerous peripheral tissues. Several studies have shown that growth hormone (GH)-releasing hormone (GHRH) is necessary for GHSs to exert maximal GH release in vivo. The exact mechanism of this synergism is not clear. Previous data suggest that GHSs can affect pituitary GHS-R mRNA expression; however, it is unknown whether this effect is age dependent and whether hypothalamic GHS-Rs are also affected. In this study, we tested whether (a) the synthetic GHS hexarelin regulates mRNA expression of its own receptor at the pituitary and/or hypothalamus and whether this effect is age dependent, and (b) whether short-term treatment with GHRH or, conversely, passive immunization against GHRH affects pituitary GHS-R1a mRNA expression in infant (10 days old) and young adult rats. GHS-R1a mRNA expression was measured with competitive reverse transcriptase-polymerase chain reaction. Hexarelin treatment significantly increased pituitary and hypothalamic GHS-R1a mRNA levels in normal infant rats, but not in normal young adult rats. In addition, hexarelin administration also stimulated pituitary GHS-R1a mRNA in infant as well as in young adult rats passively immunized against GHRH. GHRH treatment significantly enhanced pituitary GHS-R1a mRNA expression in GHRH-deprived young adult rats, though it did not affect the basal levels of GHS-R1a mRNA in normal infant and adult rats. These data further support the hypothesis that GHRH can affect GHS-R1a expression and that hexarelin upregulates the expression of its own receptor at the pituitary as well as the hypothalamus in an age-dependent fashion.

Ghrelin for the gastroenterologist: history and potential

Ghrelin, a novel 28-amino acid orexigenic peptide discovered in 1999, has given us further insights into the control of energy homeostasis and growth hormone secretion. As a natural endogenous ligand of the growth hormone secretagogue receptor, it potently stimulates growth hormone release but is also implicated in many other homeostatic mechanisms. Released from the stomach, it stimulates lactotroph and corticotroph secretion, increases appetite and adiposity, has beneficial hemodynamic effects, has prokinetic and gastric acid secretory functions in the stomach, and may even be implicated in sleep. As advances in the understanding of appetite and obesity are made, it is timely to review the possibly central role of ghrelin in these physiological and pathophysiological states. This review will discuss the recent literature concerning this exciting novel neuropeptide and discuss the possible therapeutic possibilities it may open up to us.

Pulsatile and nocturnal growth hormone secretions in men do not require periodic declines of somatostatin

Using a continuous subcutaneous octreotide infusion to create constant supraphysiological somatostatinergic tone, we have previously shown that growth hormone (GH) pulse generation in women is independent of endogenous somatostatin (SRIH) declines. Generalization of these results to men is problematic, because GH regulation is sexually dimorphic. We have therefore studied nine healthy young men (age 26 +/- 6 yr, body mass index 23.3 +/- 1.2 kg/m2) during normal saline and octreotide infusion (8.4 microg/h) that provided stable plasma octreotide levels (764.5 +/- 11.6 pg/ml). GH was measured in blood samples obtained every 10 min for 24 h. Octreotide suppressed 24-h mean GH by 52 +/- 13% (P = 0.016), GH pulse amplitude by 47 +/- 12% (P = 0.012), and trough GH by 39 +/- 12% (P = 0.030), whereas GH pulse frequency and the diurnal rhythm of GH secretion remained essentially unchanged. The response of GH to GH-releasing hormone (GHRH) was suppressed by 38 +/- 15% (P = 0.012), but the GH response to GH-releasing peptide-2 was unaffected. We conclude that, in men as in women, declines in hypothalamic SRIH secretion are not required for pulse generation and are not the cause of the nocturnal augmentation of GH secretion. We propose that GH pulses are driven primarily by GHRH, whereas ghrelin might be responsible for the diurnal rhythm of GH.

The role of circulating ghrelin in growth hormone (GH) secretion in freely moving male rats

To examine the physiological significance of plasma ghrelin in generating pulsatile growth hormone (GH) secretion in rats, plasma GH and ghrelin levels were determined in freely moving male rats. Plasma GH was pulsatilely secreted as reported previously. Plasma ghrelin levels were measured by both N-RIA recognizing the active form of ghrelin and C-RIA determining total amount of ghrelin. Mean +/- SE plasma ghrelin levels determined by N-RIA and C-RIA were 21.6 +/- 8.5 and 315.5 +/- 67.5 pM, respectively, during peak periods when plasma GH levels were greater than 100 ng / ml. During trough periods when plasma GH levels were less than 10 ng / ml, they were 16.5 +/- 4.5 and 342.1 +/- 29.8 pM, respectively. There were no significant differences in plasma ghrelin levels between two periods. Next, effect of a GH secretagogue antagonist, [D-Lys-3]-GHRP-6, on plasma GH profiles was examined. There were no significant differences in both peak GH levels and area under the curves of GH (AUCs) between [D-Lys-3]-GHRP-6-treated and control rats. These findings suggest circulating ghrelin in peripheral blood does not play a role in generating pulsatile GH secretion in freely moving male rats.