Unnatural Growth Hormone-Releasing Peptide Begets Natural Ghrelin

Cyril Y. Bowers

Endocrine Section, Tulane University School of Medicine, New Orleans, Louisiana 70112-2699

Address correspondence and requests for reprints to: Cyril Y. Bowers, M.D., Endocrine Section, Tulane University School of Medicine, 1430 Tulane Avenue, SL53, New Orleans, Louisiana 70112-2699.


    Introduction
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 Introduction
 References
 
Unnatural GH-releasing peptide (GHRP) finally has evolved into the highly probable natural GHRP designated ghrelin. The beginning was in 1976, the receptor in 1996, and, at last, the endogenous hormone in 1999 (1, 2, 3). Despite the artificiality of the unnatural GHRPs, results of many talented investigators worldwide are responsible for sustaining the interest in the GHRP/GH secretagogue (GHS) system.

The recent isolation and cloning of both rat and human ghrelin certainly are major milestones in the GHRP story. Ghrelin is a novel molecule with a molecular weight of 3314, which is very possibly more readily biodegradable than GHRH and, thus, may have been the reason GHRH was isolated first. It is a relatively charged, linear, non-C-terminal amidated 28 amino acid peptide with a significant number of chemical functional groups. Unique and a first in mammalian peptides is the posttranslation addition of a straight chain octanoyl group covalently linked to the hydroxyl group of Ser (3) via an ester linkage. The n-octanoyl group adds a hydrophobic property to the N terminus that may facilitate entry and distribution in the brain. Without octanoylation, ghrelin is biologically inactive. As do unnatural GHRPs, natural ghrelin binds with high affinity and specificity to the 7 transmembrane G protein-coupled receptor. To reveal the interrelationships of the chemistry of ghrelin and the many chemically diverse small unnatural peptidyl, partial peptidyl, and nonpeptidyl GHRP/GHSs should impart new understanding and dimension to the structure-activity relationship of peptides (4). Seemingly it is a strong precedent for the feasibility of developing bioactive small molecules that mimic the action of larger peptides. More important, in the future this may even include GHRH 1–44NH2. In addition, a limitation of the purely gene approach for the discovery of biologically active peptides is underscored by the chemical structure of ghrelin.

Involvement in the regulation of GH secretion is the way the GHRP story began and mainly has proceeded, but over time other actions have become of special interest. The recent outstanding accomplishment of Kojima et al. (3) on the isolation and identification of ghrelin and secretion of ghrelin primarily from the fundus of the stomach in addition to the hypothalamus has allowed new and intriguing dimensions to arise on the possible overall physiological role of the ghrelin/GHRP system. The dual action on GH secretion and food intake in conjunction with the dual anatomical localization of ghrelin in the stomach and hypothalamus present an immediate question about the interdependency of these actions and the site of origin. After intracerebroventricular (icv) ghrelin administration to rats, Date et al. (5) demonstrated that GH release was dose-relatedly increased, including even low dosages of ghrelin. The effectiveness of the GHRPs and now ghrelin on increased food intake in rats and mice has been well established (6, 7, 8).

After a single icv injection of ghrelin or neuropeptide Y (NPY) to rats, Wren et al. (7) found a similar effect of the two peptides on food intake that was prolonged in that the effect was sustained over 24 h. In this study, icv GHRP-6 also increased food intake but of shorter duration; however, this may be related to differences in peptide potency and dosage. After a single ip injection of ghrelin or GHRP to satiated rats, food intake was immediately increased during the first hour. As the authors stated, the orexigenic activity of ghrelin after peripheral administration was considerably important in that other hypothalamic peptides that alter food intake are ineffective by this route of administration. Despite the findings that GHRP/ghrelin receptors are coexpressed in NPY neurons, GHRP stimulates c-fos in these neurons, and that NPY is a potent orexigenic peptide, it remains questionable whether the GHRP/ghrelin effect on food intake is mediated via NPY (7, 8, 9). Particularly relevant, Kamegai et al. (10) reported that icv ghrelin increased the expression of the orexigenic agouti-related protein rather than NPY in the hypothalamic arcuate nucleus.

The recent report by Takaya et al. (11) entitled "Ghrelin strongly stimulates GH release in humans" demonstrates the high potency and the extraordinary amount of GH released by ghrelin, which is also characteristic of the GHRP/GHSs. The amount of GH released is much greater than that induced by maximal dosages of GHRH, indicating a distinctive pharmacological action. In these studies in normal young men, the peak GH rise to iv bolus dosages of 0.2, 1, and 5 µg/kg was 43, 81, and 107, respectively, whereas the GH area under the curve was 2451, 6217, and 9581, respectively. Essentially, the same GH responses have been obtained with GHRP-2 at comparable iv bolus dosages in normal young men (12). Dosages calculated on a molar weight basis, however, indicate that ghrelin is more potent in releasing GH than the unnatural GHRP/GHSs. Even at very high iv bolus dosages of ghrelin, there were no unusual clinical effects and the adverse effects were minor.

Despite the very different chemistry of the various GHRP/GHSs, almost all of them and ghrelin as well (3) have the same qualitative biological activity in vivo on GH release as well as ACTH, cortisol, and PRL without any effect on TSH, LH, and FSH. Neither ACTH nor PRL is released in vitro from the pituitary of normal rats by GHRP or ghrelin. In rats, the results of Thomas et al. (13) indicate GHRP releases ACTH by release of CRH whereas in humans Korbonits et al. (14) concluded GHRP releases ACTH via release of vasopressin rather than CRH. The release of PRL by GHRP in rats occurs only after estrogen pretreatment. These results emphasize the necessity of considering species and experimental conditions in assessing the actions of ghrelin/GHRPs. Intravenous bolus administration of ghrelin results in high blood levels and reflects a pharmacological rather than a physiological action of the peptide that suggests, at physiological concentrations, ghrelin possibly will not increase ACTH or PRL. Ghrelin, at the low 0.2 µg/kg iv bolus dosage in humans, released a substantial amount of GH but only minor increases of serum ACTH and PRL. There was a 1.1-fold increase above the baseline for both hormones at the low dosage and, at the higher dosages of 1 and 5 µg/kg, a rise of 1.6 and 3.4 and 1.9 and 2.4, respectively, for ACTH and PRL. When morning vs. evening iv bolus administration of GHRP-2 was compared, ACTH and cortisol responses to 1 µg/kg were greater in the evening. However, during continuous 30-day sc infusion of GHRP-2 to normal older men and women, cortisol and PRL levels measured every 40 min over 24 h were unchanged but the pulsatile GH secretion and the serum insulin-like growth factor I (IGF-I) level on days 0, 14, and 30 were increased. Also, the normal daily circadian rhythm of the cortisol secretion was maintained or slightly increased (15).

What is increasingly more apparent is how closely the GH-releasing activity of GHRPs and ghrelin parallel each other, and, thus, to a significant degree, it probably will be possible to directly interrelate the actions of natural ghrelin and unnatural GHRPs on GH release. Because GHRP-2 and ghrelin both release very large quantities of GH at high dosages in humans, the effect by these two chemically very different peptides has been envisioned to be mediated via the same hypothalamic pituitary receptors and mechanism(s). Results of other investigators and our collaborative studies with Kang Chang in rats further support the commonality of action of the unnatural GHRP-2 and ghrelin (3, 5, 7, 8, 16). In our studies in rats, combined ghrelin and GHRP-2 at maximal dosages induced the same magnitude of GH release as when the peptides were administered alone. GHRH antiserum and a GHRH-antagonist inhibited the GH-releasing action of both GHRP-2 and ghrelin. Combined ghrelin or GHRP-2 with GHRH or an opiate tetrapeptide released GH synergistically. Increasing dosages of ghrelin or GHRP-2 equally attenuate the inhibition of the somatotropin release-inhibiting factor (SRIF) on the GH response of these individual peptides. This attenuation is more effectively induced when ghrelin or GHRP-2 is combined with GHRH. On repeated administration of ghrelin or GHRP-2, desensitization of the GH response is induced homologously and also heterogously in cross-over studies with the two peptides. GHRP-2+GHRH releases GH in vitro additively rather than synergistically, and it seems probable the response of ghrelin+GHRH will be the same. Thus, the synergistic action of these combined peptides on GH release in vivo is not explained by the direct pituitary action of the combined peptides. Nevertheless, because of the substantial chemical difference of these molecules and the primary origin of ghrelin from the stomach rather than the hypothalamus, one is very circumspect of whether the biological activities will be found to be completely the same. Even small biological differences may become significantly important to the physiological action of ghrelin. For example, after repeated administration, we found desensitization of the ghrelin GH response was of shorter duration than that of GHRP-2.

The high probability that the actions of ghrelin and the unnatural GHRPs will closely parallel each other are basically of theoretical and practical importance. At the theoretical level, the conceptual models of GH regulation proposed for the unnatural GHRPs should be directly applicable to ghrelin. This includes not only the envisioned physiological role of a putative natural GHRP hormone, probably ghrelin, but also the envisioned pathophysiological role of the natural hormone in the decreased secretion of GH especially in older men and women. At the practical level, the relationship between the actions of GHRP and GHRH already seems applicable to ghrelin and GHRH. This includes the independent and dependent, the additive and synergistic, as well as the permissive relationships of these peptides to each other in the release of GH at the endocrine, anatomical, and molecular levels in animals and humans. Probably applicable to ghrelin and GHRH are the relationships, interpretations, and implications previously described for GHRP-2 and GHRH (1). The full spectrum of biological activity of the unnatural GHRP/GHSs gradually has increased over the years, in part because there is a more widespread distribution of GHRP/GHS receptors in peripheral tissues than first reported as well as the possibility of receptor subtypes. It seems likely that the activity spectrum will be even further expanded from studies of ghrelin, a conclusion that is supported by the existence and synthesis of ghrelin in large amounts in the stomach, secretion directly into the peripheral circulation from the stomach, and the unusual chemistry of ghrelin.

The detailed, elegant studies of Date et al. (17) demonstrate that stomach ghrelin is present in a distinct cell type, X/A-like cells, or now designated Gr cells, mainly located in the fundus of the stomach and infrequently in the rest of the gastrointestinal tract while the number of hypothalamic ghrelin arcuate neurons is small. The Gr cells are not in continuity with the stomach lumen but rather are closely associated with the capillary network of the lamina propria supporting an endocrine role. In rats, food intake decreases and fasting increases ghrelin secretion (Nakazota, M., personal communication).

In the tissue from the gastric fundus, the molecular form of ghrelin was determined by high-performance liquid chromatography (HPLC) and two RIAs, one to C-terminal ghrelin 13–28 and the other to the N-terminal octanoylated ghrelin 1–11 fragment (17). By this N-C-terminal RIA-HPLC approach, bioactive octanoylated ghrelin 1–28 was 2.5 times lower than the nonoctanoylated ghrelin 1–28 (i.e. 1845 vs. 4638 pmol/g wt). In rat plasma, the C-terminal RIA value was 556 vs. 94 fmol/mL for the N-terminal RIA value, indicating the presumed bioactive N-terminal level was much lower than C-terminal ghrelin. Because the plasma level of ghrelin was not determined after HPLC, there is still a possibility that the N-terminal RIA detects a truncated octanoylated molecular fragment such as octanoylated ghrelin 1–12 that probably would be inactive via the original GHS receptor but not necessarily via a GHS receptor subtype. The bioactive ghrelin molecule may be readily inactivated and biodegraded in plasma and by tissue estrases that cleave the octanoyl group as well as by proteolytic enzymes that cleave the peptide chain of the molecule. Until highly sensitive RIAs specific for the entire bioactive form of the ghrelin molecule can be developed, it will be difficult to accurately assess the bioactive plasma levels of this new exciting hormone, which will be an important objective at both the physiological and pathophysiological levels.

Our studies indicate that octanoylated ghrelin 1–5 amide or octanoylated ghrelin 1–13 free acid along with nonoctanoylated ghrelin 1–28 have no GH-releasing bioactivity (16). Furthermore, neither of these two truncated molecular forms of ghrelin nor nonoctanoylated ghrelin 1–28 inhibit the in vivo GH-releasing activity of ghrelin or GHRP-2. The absence of agonist or antagonist activity of these peptides strongly suggests that both the octanoylation and the C terminus part of the ghrelin molecule probably play a definite role in establishing the bioactive conformation of the intact octanoylated ghrelin molecule. Obviously, the molecular mechanism and regulation of the essential octanoylation step eventually will be critical to elucidate. Except for the possible substitution or deletion of one or two amino acids, the entire intact ghrelin 1–28 molecule in addition to the n-octanoyl group of Ser3 seems to be required for GH-releasing activity. To be noted is that the amino acid sequence of human and rat ghrelin is different by two conservative substitutions at positions 11 and 12, Arg11 and Val12 in the human vs. Lys11 and Ala12 in the rat, and they are essentially equally potent (3). Furthermore, octanoylated Ser3 des Gln14 ghrelin has been isolated from rat stomach, and this octanoylated ghrelin 1–27 molecule was found to be equally potent to octanoylated ghrelin 1–28 in releasing GH in rats (18). Interestingly, rather than being produced as usual by a distinct separate gene, the ghrelin 1–27 molecule is encoded by the ghrelin 1–28 gene. Octanoylated des Gln14 ghrelin is produced after the unusual alternative splicing of the intron within the ghrelin 1–28 gene coding region at the encoded Gln13 site. Rather than evolving by the usual process, one gene and one messenger RNA (mRNA), the two equally potent bioactive molecules evolve from the same single gene and two mRNAs.

The gradual development of GHRPs as new GH-releasing diagnostic agents has resulted recently in three publications (19, 20, 21) that propose and demonstrate the probable value of administering iv bolus GHRP+GHRH. The synergistic GH-releasing effects of ghrelin+GHRH in rats justifiably predicts that these combined peptides will have complementary effects on GH release in humans; however, the advantage of ghrelin compared with GHRPs as a diagnostic agent so far is not apparent. In the above three diagnostic studies GHRH at a maximal GH-releasing dosage of 1 µg/kg was administered together with 0.1, 1.0, or 0.25 µg/kg GHRP-2, GHRP-6, or hexarelin, respectively (19, 20, 21). The greater safety, ease, and simplicity of performance, dose-related response, reproducibility, and more global action on the hypothalamic-pituitary unit together with the isolation of ghrelin are cogent reasons for proposing and developing the GHRP+GHRH or ghrelin+GHRH diagnostic approach rather than using the current "gold standard" insulin tolerance test. A problem that arises in the utilization of the GHRP+GHRH or the future ghrelin+GHRH diagnostic approach for GH deficiency of hypothalamic origin concerns the unknowns that still exist at both the approach and clinical disorder levels. Now that ghrelin has been discovered it seems possible that both may be better understood. Previously, the pathophysiology of idiopathic decreased GH secretion of hypothalamic origin was considered to be secondary to decreased GHRH secretion and/or increased SRIF release. With the availability of the unnatural GHRPs and the considerable evidence that they are ghrelin surrogates, indirect evidence from acute iv bolus studies with GHRP-2 and GHRH alone and together indicate that endogenous ghrelin deficiency alone or possibly even more likely ghrelin plus GHRH may be responsible for the decreased GH secretion in select normal older men and women with low serum IGF-I levels (12, 15). From the results obtained, GHRH deficiency alone or excess SRIF secretion seems unlikely. These studies emphasize that new approaches and viewpoints about the pathophysiology of decreased GH secretion have arisen from the availability of the GHRPs and ghrelin.

In the future, GH results that have been inadequately or only provisionally explained may be understood in terms of actions and/or abnormal secretion of ghrelin. A functional basis for this approach is that ghrelin augments the GH response of GHRH and, like GHRP-2, presumably GHRH will be found to augment the GH response of ghrelin. Additionally, ghrelin+GHRH more effectively attenuates the inhibition of GH secretion by SRIF than either peptide alone. Two possible examples where ghrelin may play a role is in the decreased GH response in obesity and during the chronic administration of dexamethasone. Because food intake decreases ghrelin secretion from the stomach, the decreased GHRH GH response in obesity may be due to suppressed synthesis and release of stomach ghrelin. Also, because dexamethasone is known to decrease GHRP/GHS receptors in the hypothalamic arcuate nucleus, the GH- releasing action of ghrelin on the hypothalamus may be attenuated and subsequently the GH releasing actions of GHRH may be impaired. A special yet undetermined pertinent issue that will arise in these types of studies will involve to what degree stomach ghrelin vs. hypothalamic ghrelin or both play a role in the regulation of GH secretion. Undoubtedly, in the future a number of unexplained GH-releasing results will be reevaluated from the ghrelin viewpoint.

A series of studies in humans on the continuous infusion of unnatural GHRPs demonstrating an increase in normal pulsatile GH secretion and an elevation of serum IGF-I levels also presumably will reflect the action of endogenous ghrelin. Because of the early in vitro and in vivo findings in rats and those in humans that the GH response of the GHRPs rapidly and markedly becomes desensitized after repeated administration, the sustained increase of the normal pulsatile GH secretion during continuous GHRP infusion to humans, indeed, was surprising. Like GHRP, even though the GH response of ghrelin after repeated administration to rats becomes rapidly and markedly desensitized, it is predicted that continuous ghrelin infusion as well as continuous endogenous ghrelin secretion also will induce sustained increases in pulsatile GH secretion and IGF-I levels. The degree to which endogenous ghrelin is secreted continuously as well as intermittently from the stomach and/or hypothalamus as well as whether hypothalamic or stomach ghrelin or both regulate GH release are variables in need of detailed elucidation to reveal the physiological and pathophysiological role of ghrelin.

The advantages and disadvantages of therapeutic administration of ghrelin/GHRP vs. GHRH or GHRH combined with one of these peptides eventually will need specific assessment. Nevertheless, especially noteworthy is the physiological action that these peptides (GHRP-2/GHRH) have on GH secretion when administered continuously alone or together (15). As more data are obtained on distinguishing when decreased GH secretion is due to hormonal deficiency rather than normal aging in older adults or due to hormonal deficiency in short statured children, the therapeutic approach of administering ghrelin/GHRP or the combination of one of these peptides with GHRH continually would be considered ideal for restoring the physiological secretion of GH and IGF-I to normal. When one considers therapeutic hormonal replacement for thyroid, adrenal, and gonadal hypofunction, none of these therapeutic hormonal approaches seemingly would be as physiological in effect as the ghrelin/GHRP/GHRH neuroendocrine therapeutic approach, which involves a normal intact feedbackward system, restoration of a deficient feedforward system, and the induction of a physiological pattern of secretion of GH with an associated elevation of IGF-I.

Very recently, another series of provocative studies on food intake and metabolic effects of ghrelin in mice and rats were published in an article entitled "Ghrelin induces adiposity in rodents" by Tschop et al. (8). These studies led the authors to conclude that "ghrelin may be a new link between the GH/IGF-I axis and the neuroendocrine regulation of energy balance." During once daily sc ghrelin administration to wild-type mice for 2 weeks, body weight and fat mass increased without a change in food intake, locomotor activity, lean body mass, or bone mass. Because ghrelin releases GH, increases food intake, and GH is lipolytic rather than lipogenic, these are unexpected results. As concluded, ghrelin induced a positive energy balance and body weight gain by decreasing fat utilization. Within a 24-h period, a single injection of ghrelin and GH to the mice resulted in differential effects on the respiratory quotient (RQ) and the energy expenditure (EE). The RQ was increased and the EE was unchanged by ghrelin whereas the RQ was unchanged and the EE increased during the light but not the dark photoperiod by GH. A general conclusion proposed was that the metabolic and GH effects of ghrelin can be dissociated. Daily sc administration of ghrelin to GH-deficient dwarf rats produced the same effects observed in the wild-type mice. Even very low dosages of ghrelin administered icv for 7 days to normal adult male rats produced a dose-dependent weight gain, food intake, and an increased RQ without an effect on EE or locomotor activity. This indicates these effects were mediated by a direct central nervous system action of ghrelin. Other results of note were that ghrelin increased fat mass in NPY-deficient mice and that plasma ghrelin levels increased in fasted rats and decreased in fed rats. Furthermore, it was stated that GHRP-2 produced similar effects that again indicate the commonality of action of GHRP and ghrelin. After chronic once daily administration of ghrelin/GHRP, desensitization of these metabolic effects were not induced. The authors were intrigued with the possibility that ghrelin may play the expanded role of regulating energy balance via a direct hypothalamic action and proposed hypothalamic ghrelin regulates positive energy balance and stomach ghrelin regulates GH secretion. The positive energy balance, in turn, would maximize the anabolic action of GH, whereas during starvation higher plasma levels of ghrelin would be viewed as an integrator of a more energy efficient state and the secretion of GH. Obviously, the authors’ results, interpretations, and conceptualizations are most thought provoking.

Besides this exciting aspect of ghrelin and the evidence supporting a wider distribution of the GHS/ghrelin receptor or receptor subtypes (22, 23) than the GHS receptors reported earlier (24), there are a number of studies in rats and humans and now the rabbit indicating a peripheral cardioprotective action of GHRPs (25, 26, 27). The results continue to support that the GHRPs have a direct favorable cardioprotectant action on the heart independent of a GH effect. Another important aspect of these studies has been evidence that supports the possibility that these effects seem to be mediated via a GHRP receptor subtype (22, 23). From the substantial variety of biologically active GHRP chemical types that have been developed, subtypes of GHRP/GHS receptors, indeed, are a like possibility. The possible direct cardiac action of ghrelin or modified endogenous ghrelin molecules will be of particular interest because of the unusual ghrelin chemistry and its secretion from the stomach directly to the heart.

Now, with the increasing stature of the natural ghrelin hormone, what is known about the unnatural GHRP/GHSs, and the high probability that GHRP is a ghrelin surrogate, the provocative question arises that if ghrelin had been isolated before GHRH, would ghrelin rather than GHRH have been considered the primary regulator of GH secretion? From this viewpoint and from what is already known about the GH-releasing action of ghrelin/GHRP and the interactions of these two peptides with endogenous GHRH, consideration of this hypothetical possibility seemingly helps one to become even more enthusiastic about the new hormone ghrelin. In this context, a deficiency of endogenous GHRH may result in a normal or increased peripheral plasma level of ghrelin and, depending on the degree of GHRH deficiency, a greater but nevertheless probably still attenuated GH response may be induced by GHRH rather than by ghrelin or GHRP. Without endogenous GHRH the ghrelin GH response, like that of the GHRPs, would be markedly impaired whereas the magnitude of the GHRH GH response probably would depend on the pituitary content of GH as well as how dependent the GHRH GH response would be on endogenous ghrelin. Pandya et al. (28) showed that a GHRH antagonist markedly inhibited the GH response of GHRP-6 in normal men and in our studies a GHRH antagonist and a GH antiserum markedly inhibited the GH response of ghrelin in rats. In addition, the GH-releasing activity of combined ghrelin+GHRH is established to be most complementary. A significant still unknown link is whether ghrelin itself stimulates pituitary GH synthesis either directly or indirectly via the release and/or augmentation of the pituitary action of GHRH. The increase of GH synthesis by GHRPs in vivo has been reported to be both positive and negative whereas a positive effect by GHRH is well established in vitro and in vivo. From the known actions of GHRP on enhancement of the pituitary action of GHRH on pituitary cAMP and also its increase of endogenous GHRH release, theoretically both ghrelin and GHRP should increase pituitary GH synthesis at least in vivo even though in vitro they may be ineffective. The colocalization of the ghrelin/GHRP/GHS receptor has been demonstrated in a subpopulation of arcuate GHRH neurons further supporting these neurons as a site of action of these GHSs (29). Important basic implications that seem to evolve from these theoretical considerations is that the smaller the in vivo effect of ghrelin or GHRP on pituitary GH synthesis, the less likely the GH-releasing activity of these peptides results primarily from their augmentation of endogenous GHRH release. The permissive, active, and interactive roles of these peptides are envisioned to reflect physiological as well as pharmacological actions of ghrelin.

Notably, the biological actions of the unnatural GHRPs and the natural ghrelin closely mimic each other. Nevertheless, the transition from the unnatural GHRPs to natural ghrelin is viewed as proceeding from relative simplicity to greater complexity. This emanates from the novel and unique chemistry of ghrelin, its primary synthesis in the stomach and, in contrast to the GHRH receptor, the proposed wide spread of the ghrelin receptor sites including the possibility of receptor subtypes in peripheral tissues. A question even arises about the major physiological role(s) of ghrelin. As envisioned by others, its primary physiological role may be as an integrator of food intake, GH secretion, and energy balance. Also, secretion of ghrelin from the stomach to the heart uniquely would support a direct action on the heart.

Conceptually, our working physiological model for the role of ghrelin nevertheless consists of the regulation of GH secretion by the endocrine activity of stomach ghrelin and the regulation of food intake by the paracrine activity of hypothalamic ghrelin. In this model daytime secretion of stomach ghrelin is suppressed by food intake and presumably is augmented by fasting during the nighttime to regulate GH secretion. In contrast, daytime secretion of hypothalamic ghrelin is increased and, during the nighttime, decreased to regulate food intake. The endocrine secretion of stomach ghrelin during the nighttime inhibits the paracrine action of hypothalmic ghrelin via a homologous desensitization action. During the daytime, the endocrine secretion of stomach ghrelin is suppressed and the paracrine secretion and action of ghrelin on food intake is augmented. Thus despite the classical hypothalamic hypophysiotrophic model of neuroregulation, it is the peripheral stomach ghrelin rather than hypothalamic ghrelin that appears to prevail in the regulation of GH secretion. In further support of this conclusion, peripheral ghrelin receptors are in low concentration and the functional activities of these receptors have been negligible after acute and chronic administration of even large dosages of GHRP, which seems to closely mimic the action of ghrelin. Nevertheless, it will be challenging to determine the relative physiological roles and actions of ghrelin at peripherial vs. central anatomical sites.

Because the GH-releasing activity of ghrelin/GHRP in vivo is very dependent on endogenous GHRH and GH secretion is not maintained in the absence of GHRH, the primary feedforward regulator of GH secretion as currently viewed would be GHRH. Ghrelin would be best considered as a helper and modulator of GHRH in the regulation of GH secretion. A GH role requires an explanation of how stomach or hypothalamic ghrelin or both might interact with GHRH as well as SRIF, GH, and IGF-I to regulate pulsatile secretion of GH. The finding that food intake decreases and fasting increases the secretion of ghrelin from the stomach suggests that ghrelin would be uniquely positioned to support the enhanced physiological secretion of GH that occurs nocturnally. Collectively, the dramatic, unique, and selective actions of ghrelin/GHRP on GH secretion is a strong impetus for envisioning that the major role of ghrelin still could be the regulation of GH secretion.

Received October 25, 2000.

Revised December 8, 2000.

Accepted December 20, 2000.


    References
 Top
 Introduction
 References
 

  1. Bowers CY. 1999 GH releasing peptides (GHRPs). In: Kostyo J, Goodman H, eds. Handbook of physiology. New York: Oxford University Press; 267–297.
  2. Howard AD, Feighner SD, Cully DF, et al. 1996 A receptor in pituitary and hypothalamus and functions in growth hormone release. Science. 273:974–977.[Abstract]
  3. Kojima M, Hosada H, Date Y, Nakazato M, Matsui H, Kangawa K. 1999 Ghrelin is a growth-hormone releasing acylated peptide from stomach. Nature. 402:656–660.[CrossRef][Medline]
  4. Feighner SD, Howard AD, Prendergast K, et al. 1998 Structural requirements for the activation of the human growth hormone secretagogue receptor by peptide and nonpeptide secretagogues. Mol Endocrinol. 12:137–145.[Abstract/Free Full Text]
  5. Date Y, Murakami N, Kojima M, et al. 2000 Central effects of a nobel acylated peptide, ghrelin on growth hormone release in rats. Biochem Biophys Res Commun. 275:477–480.[CrossRef][Medline]
  6. Kuriyama H, Hotta M, Wakabayashi I, Shibasaki T. 2000 A 6-day intracerebroventricular infusion of the growth hormone-releasing peptide KP-102 stimulates food intake in both non-stressed and intermittently-stressed rats. Neuroscience. 282:109–112.
  7. Wren AM, Small CJ, Ward HL, et al. 2000 The novel hypothalamic peptide ghrelin stimulates food intake and growth hormone secretion. Endocrinology. 141:4325–4328.[Abstract/Free Full Text]
  8. Tschop M, Smiley DL, Behman ML. 2000 Ghrelin induces adiposity in rodents. Nature. 47:908–913.
  9. Willesen MG, Kristensen P, Romer J. 1999 Co-localization of growth hormone secretagogue receptor and NPY mRNA in the arcuate nucleus of the rat. Neuroendocrinology. 70:306–316.[CrossRef][Medline]
  10. Kamegai J, Tamura H, Shimizu T, Ishii S, Sugihara H, Wakabayashi I. 2000 Central effect of ghrelin, an endogenous growth hormone secretagogue, on hypothalamic peptide gene expression. Endocrinology. 141:4797–4800.[Abstract/Free Full Text]
  11. Takaya K, Ariyasu H, Kanamoto N, et al. 2000 Ghrelin strongly stimulates growth hormone (GH) release in humans. J Clin Endocrinol Metab. 85:4908–4911.[Abstract/Free Full Text]
  12. Bowers CY. 1999 GHRP: unnatural toward the natural. In: Diegues C, Ghigo E, Boghen M, Casanueva FF, eds. Growth hormone secretagogues. Amsterdam: Elsevier Science; 5–18.
  13. Thomas GB, Fairhall KM, Robinson ICAF. 1997 Activation of hypothlamo-pituitary-adrenal axis by the growth hormone (GH) secretagogue, GH-releasing peptide-6, in rats. Endocrinology. 138:1585–1591.[Abstract/Free Full Text]
  14. Korbonits M, Kaltsas G, Perry LA, et al. 1999 The growth hormone secretagogue hexarelin stimulates the hypothlamo-pituitary-adrenal axis via arginine vasopressin. J Clin Endocrinol Metab. 84:2489–2495.[Abstract/Free Full Text]
  15. Bowers CY, Granda-Ayala R. GH/IGF-I response to acute and chronic GHRP-2, GHRH 1–44NH2 and the combination in older men and women with decreased GH secretion. Third International Symposium on Growth Hormone Secretagogues. Endocr J. In press.
  16. Bowers CY, Reynolds GA, Chang K. 2000 Unnatural GHRP and natural ghrelin link. Proceedings of the 82nd Annual Meeting of The Endocrine Society, Toronto, Canada, 2000; pp 170.
  17. Date Y, Kojima M, Hososda H, et al. 2000 Ghrelin, a novel growth-hormone-releasing acylated peptide, is synthesized in a distinct endocrine cell type in the gastrointestinal tracts of rats and humans. Endocrinology. 141:4255–4261.[Abstract/Free Full Text]
  18. Hosoda H, Kojima M, Matson H, Kangawa K. 2000 Purification and characterization of rat des Gln14-ghrelin, a second endogenous ligand for the growth hormone secretagogue receptor. J Biol Chem. 275:21995–22000.[Abstract/Free Full Text]
  19. Mahajan T, Lightman SL. 2000 A simple test for growth hormone deficiency in adults. J Clin Endocrinol Metab. 85:1473–1476.[Abstract/Free Full Text]
  20. Casanueva FF, Dieguez C. 1999 Growth hormone secretagogues: physiological role and clinical utility. Trends Endocrinol Metab. 10:30–38.[CrossRef][Medline]
  21. Aimaretti G, Baffoni C, DiVito L, et al. 2000 Comparisons among old and new provocative tests of GH secretion in 178 normal adults. Eur J Endocrinol. 142:347–352.[Medline]
  22. Ong H, Bodart V, McNicoll N, Lamontagne D, Couchard JF. 1998 Identification and characterization of a new GHRP receptor in the heart. GH IGF Res. 8:137–140.
  23. Papotti M, Ghe’ C, Cassoni P, et al. 2000 Growth hormone secretagogue binding sites in peripheral human tissues. J Clin Endocrinol Metab. 85:3803–3807.[Abstract/Free Full Text]
  24. Guan XM, Yu H, Palyha OC, et al. 1997 Distribution of mRNA encoding the growth hormone secretagogue receptor in brain and peripheral tissues. Mol Brain Res. 48:23–29.[CrossRef][Medline]
  25. Tivesten A, Bollano E, Caidahl K, et al. 2000 The growth hormone secretagogue hexarelin improves cardiac function in rats after experimental myocardial infarction. Endocrinology. 141:60–66.[Abstract/Free Full Text]
  26. Bisi G, Podio Valetto MR, et al. 1999 Cardiac effects of hexarelin hypopituitary adults. Eur J Pharmacol. 381:31–38.[CrossRef][Medline]
  27. Weekers F, Van Herck E, Isgaard J, Van Den Berghe G. 2000 Pretreatment with growth hormone-releasing peptide-2 directly protects against the diastolic dysfunction of myocardial stunning in an isolated, blood-perfused rabbit heart model. Endocrinology. 141:3993–3999.[Abstract/Free Full Text]
  28. Pandya N, DeMott-Friberg R, Bowers CY, Barkan AL, Jaffe CA. 1998 Growth hormone (GH) releasing peptide-6 requires endogenous hypothalamic GH-releasing hormone for maximal GH stimulation. J Clin Endocrinol Metab. 83:1186–1189.[Abstract/Free Full Text]
  29. Tannenbaum GS, Lapointe M, Beaudet A, Howard AD. 1998 Expression of growth hormone secretagogue-receptors by growth hormone-releasing hormone neurons in the mediobasal hypothalamus. Endocrinology. 139:4420–4423.[Abstract/Free Full Text]