Cortistatin-8, a synthetic cortistatin-derived ghrelin receptor ligand, does not modify the endocrine responses to acylated ghrelin or hexarelin in humans.

Cortistatin (CST), a neuropeptide with high structural homology with somatostatin (SST), binds all SST receptor (SST-R) subtypes but, unlike SST, also shows high binding affinity to ghrelin receptor (GHS-R1a). CST exerts the same endocrine activities of SST in humans, suggesting that the activation of the SST-R might mask the potential interaction with ghrelin system. CST-8, a synthetic CST-analogue devoid of any binding affinity to SST-R but capable to bind the GHS-R1a, has been reported able to exert antagonistic effects on ghrelin actions either in vitro or in vivo in animals. We studied the effects of CST-8 (2.0 microg/kg i.v. as a bolus or 2.0 microg/kg/h i.v. as infusion) on both spontaneous and ghrelin- or hexarelin- (1.0 microg/kg i.v. as bolus) stimulated GH, PRL, ACTH and cortisol secretion in 6 normal volunteers. During saline, no change occurred in GH and PRL levels while a spontaneous ACTH and cortisol decrease was observed. As expected, both ghrelin and hexarelin stimulated GH, PRL, ACTH and cortisol secretion (p<0.05). CST-8, administered either as bolus or as continuous infusion, did not modify both spontaneous and ghrelin- or hexarelin-stimulated GH, PRL, ACTH and cortisol secretion. In conclusion, CST-8 seems devoid of any modulatory action on either spontaneous or ghrelin-stimulated somatotroph, lactotroph and corticotroph secretion in humans in vivo. These negative results do not per se exclude that, even at these doses, CST-8 might have some neuroendocrine effects after prolonged treatment or that, at higher doses, may be able to effectively antagonize ghrelin action in humans. However, these data strongly suggest that CST-8 is not a promising candidate as GHS-R1a antagonist for human studies to explore the functional interaction between ghrelin and cortistatin systems.

Evidence for a role of the GHS-R1a receptors in ghrelin inhibition of gastric acid secretion in the rat.

Ghrelin, the endogenous ligand of the GH secretagogue receptor (GHS-R) has been previously shown to inhibit gastric acid secretion in pylorus-ligated rats. Two isoforms of GHS-R have been identified: GHS-R(1a) and GHS-R(1b). The present study aimed: (i) to characterise the type of GHS-R involved in the central gastric inhibitory activity of ghrelin by using des-octanoyl ghrelin, and synthetic GHS-R(1a) agonist (EP1572) and antagonist (D-Lys(3)-GHRP-6) and (ii) to investigate the relationship between ghrelin and cortistatin (CST) in the control of gastric acid secretion by using the natural neuropeptide CST-14 and the synthetic octapeptide CST-8. The specific interactions of all the compounds with GHS-R(1a) were determined by comparing their ability to displace labelled ghrelin or somatostatin from its receptors on rat hypothalamic membranes or on rat cardiomyocyte, respectively. Intracerebroventricular administration of 0.01 and 1 nmol/rat des-octanoyl ghrelin did not affect gastric acid secretion in pylorus-ligated rats, whereas EP1572 either i.c.v. (0.01-1 nmol/rat) or i.p. (10 and 20 nmol/kg) inhibited acid gastric secretion. Preteatment with D-Lys(3)GHRP-6 (3 nmol/rat, i.c.v.) was able to remove the inhibitory action of ghrelin (0.01 nmol/rat, i.c.v.) on gastric acid volume and acid output, thus indicating that the type 1a GHS-R likely mediates the gastric inhibitory action of ghrelin. This is supported by binding data showing that D-Lys(3)GHRP-6, but not des-octanoyl ghrelin, binds to hypothalamic GHS-R. CST-14 (1 nmol/rat, i.c.v.) did not affect either basal or ghrelin inhibition of gastric acid secretion. CST-8 (1 nmol/rat, i.c.v.) was able to counteract the gastric ghrelin response. The observation that CST-14 binds both GHR-S and somatostatin receptors, whereas CST-8 specifically displaces only ghrelin binding, indicates that CST-8 behaves as a GHS-R(1a) antagonist.

There are no acute cardiac effects of a single iv dose of human ghrelin in severe growth hormone deficient patients.

It has previously been suggested that ghrelin mediates GH-independent biologic activities on the heart. We investigated the acute effects on cardiac contraction of a single iv administration of human ghrelin (in a dose of 1 microg/kg) in severe untreated GH deficient subjects. Prior to the ghrelin infusion, an echocardiographic examination was performed at rest (baseline), after physiologic saline and during dobutamine stress echocardiography (DSE) to exclude a preexisting (subclinical) myocardial dysfunction. To evaluate the acute cardiac effect of infusion and during DSE the velocity of left ventricular (LV) wall contraction was measured continuously by echocardiography. Despite severe GH deficiency we observed in all subjects a normal cardiac function at rest after physiologic saline and during DSE. No acute changes in cardiac performance or cardiac parameters could be observed after a single iv dose of ghrelin. Also, no important increase in GH secretion was detected after ghrelin administration. Our study suggests that, in contrast to hexarelin, a single iv administration of ghrelin in a physiological dose has no acute effects on cardiac function in severe GH deficiency. This suggests that GH-independent effects of ghrelin play no important role in the acute regulation of cardiac function in man.

Presence of cortistatin in the human pancreas.

Cortistatin (CST), a 17-amino acid peptide partially homologous to somatostatin (SRIF), has been originally isolated from the cerebral cortex and recently found in monocytes and macrophages of the immune system. CST binds all 5 SRIF receptors, as well the GH secretagogue (GHS)/ghrelin receptors. CST exerts sleep promoting activities, acts on animal motility and behavior and inhibits GH and insulin secretion. To investigate the possible occurrence and activities in peripheral tissues, expression of CST at the mRNA and peptide level was analyzed in the human pancreas by means of RT-PCR, in situ hybridization and immunohistochemistry. The specific CST mRNA was found in 3 of 4 pancreatic RNA extracts and in the control cerebral cortex. By in situ hybridization, CST mRNA was localized in the pancreatic islets, but not in the exocrine pancreas. This finding was confirmed by immunostaining with a specific antibody to CST-17 which detected CST in single islet cells. These cells also expressed SRIF receptors types 2, 3 and 5, ghrelin and GHS receptors. Thus, our findings show the presence of CST in the human endocrine pancreas. Local autocrine or paracrine circuits, only in part overlapped with those of SRIF, may be active to modulate insulin and/or glucagon levels.

Effects of cortistatin-14 and somatostatin-14 on the endocrine response to hexarelin in humans.

Cortistatin (CST)-14, a neuropeptide with high structural homology with somatostatine (SS)-14, binds all SS receptor subtypes but also shows activities not shared by SS. CST and SS are often co-expressed in the same neurons but are regulated by different stimuli. Moreover, CST, but not SS, also binds the GH secretagogue (GHS) receptor. We compared the effects of CST-14 and SS-14 (2.0 microg/kg/h i.v. from -30 to +90 min) on the endocrine response to hexarelin (HEX, 1.0 microg/kg i.v. at 0 min), a synthetic GHS, in 6 normal volunteers [age (mean+/-SEM): 28.7+/-2.9 yr; body mass index: 23.4+/-0.8 kg/m2]. GH, PRL, ACTH, cortisol, insulin and glucose levels were measured at each time point. CST-14 inhibited spontaneous GH secretion [delta-areas under curves (-AUC): -83.57+/-44.8 vs 2.3+/-2.7 microg/l/h, p<0.01] to the same extent of SS-14 (-186.1+/-162.9 microg/l/h, p<0.01). CST-14 as well as SS-14 also inhibited insulin secretion (p<0.05). The GH response to HEX was similarly inhibited by either CST-14 (AUC: 3814.1+/-924.2 vs 1212.9+/-379.8 microg/l/h, p<0.05) or SS-14 (720.9+/-158.6 microg/l/h, p<0.05). HEX significantly increased PRL, ACTH and cortisol levels but these responses were not modified by either CST-14 or SS-14. The effects of CST-14 and SS-14 on insulin and glucose levels were not modified by HEX. In conclusion, this study shows that CST-14 inhibits the GH response to HEX to the same extent of SS-14. Like SS-14, CST-14 also inhibits insulin secretion but both do not modify the stimulatory effects of HEX on lactotroph and corticotroph secretion. Thus, CST-14 exerts full SS-14 activity in humans.

Effects of ghrelin on the insulin and glycemic responses to glucose, arginine, or free fatty acids load in humans.

Ghrelin possesses central and peripheral endocrine actions including influence on the endocrine pancreatic function. To clarify this latter ghrelin action, in seven normal young subjects [age (mean +/- SEM), 28.3 +/- 3.1 yr; body mass index, 21.9 +/- 0.9 kg/m(2)), we studied insulin and glucose levels after acute ghrelin administration (1.0 microg/kg i.v.) alone or combined with glucose [oral glucose tolerance test (OGTT), 100 g orally], arginine (ARG, 0.5 g/kg i.v.) or free fatty acid (FFA, Intralipid 10%, 250 ml). Ghrelin inhibited (P < 0.05) insulin and increased (P < 0.05) glucose levels. OGTT increased (P < 0.01) glucose and insulin levels. FFA increased (P < 0.05) glucose but did not modify insulin levels. ARG increased (P < 0.05) both insulin and glucose levels. Ghrelin did not modify both glucose and insulin responses to OGTT as well as the FFA-induced increase in glucose levels; however, ghrelin administration was followed by transient insulin decrease also during FFA. Ghrelin blunted (P < 0.05) the insulin response to ARG and enhanced (P < 0.05) the ARG-induced increase in glucose levels. In all, ghrelin induces transient decrease of spontaneous insulin secretion and selectively blunts the insulin response to ARG but not to oral glucose load. On the other hand, ghrelin raises basal glucose levels and enhances the hyperglycemic effect of ARG but not that of OGTT. These findings support the hypothesis that ghrelin exerts modulatory action of insulin secretion and glucose metabolism in humans.

Suppression and recovery of LH secretion by a potent and selective GnRH-receptor antagonist peptide in healthy early follicular-phase women are mediated via selective control of LH secretory burst mass.

AIM: GnRH antagonists are competitive inhibitors of GnRH receptors. Their administration induces prompt suppression of the gonadal axis. In animals, GnRH antagonists upregulate the activity of GnRH-secreting neurones, which could cause gonadotrophin rebound following inhibition. The aim of this study was to evaluate the effects of a potent GnRH antagonist, Teverelix (TEV), on the gonadal axis in healthy young women. SUBJECTS AND MEASUREMENTS: In nine women [20-35 years old, body mass index (BMI) 19-25 kg/m2] in the early follicular phase, serum LH and FSH levels were evaluated every 10 min from 08.00 to 12.00 h before, and 24 h and 96 h after TEV injection (2.5 mg in 1 ml subcutaneously on day 0). Serum gonadotrophin and oestradiol levels were also evaluated at baseline and at 6, 8, 12, 48, 72 h after TEV. RESULTS: The antagonist reduced both serum LH and FSH concentrations; LH levels were significantly and promptly reduced at +6 h (nadir at +8 h) until +48 h and recovered at +72 h, while FSH levels were reduced (P<0.05) 24 h after the antagonist and normalized at +48 h. LH (but not FSH) concentrations at +96 h exceeded baseline (P<0.05). TEV suppressed oestradiol concentrations (P<0.05) with a nadir at +24 h, comparable reduction at +48 h and recovery to baseline at +72 h. Deconvolution analysis showed that the antagonist peptide suppressed (P<0.02) the pulsatile production rate, burst mass and amplitude of LH on day 1. Pulsatile FSH secretion also fell at this time (P<0.05). LH and FSH pulse frequency were not modified by TEV. At +96 h, LH pulsatility did not significantly differ from that at baseline. Suppression of mean LH or FSH concentrations did not affect the relative pattern regularity (approximate entropy) of LH and FSH secretion. CONCLUSIONS: This study demonstrates that the acute administration of a potent GnRH antagonist induces prompt inhibition of the gonadal axis lasting for 2 days in women due to mechanistically specific suppression of LH secretory burst mass and the mean FSH secretion rate. The trend toward rebound release of LH following the end of the pharmacological effect of the antagonist could reflect a rise in endogenous GnRH activity.

Non-acylated ghrelin does not possess the pituitaric and pancreatic endocrine activity of acylated ghrelin in humans.

Ghrelin, a 28-amino acid peptide predominantly produced by the stomach, displays strong GH-releasing activity mediated by the GH secretagogue (GHS)-receptor (GHS-R) type 1a at the hypothalamus-pituitary level. Ghrelin and synthetic GHS also possess other GH-independent peripheral endocrine and non-endocrine activities via the activation of peripheral GHS-R subtypes. In rats in vivo non-acylated ghrelin has been reported devoid of any endocrine activity; however, in vitro, it has been shown as effective as ghrelin in exerting anti-proliferative activity on tumor cell lines. The aim of the present study was to clarify whether non-acylated human ghrelin shares some of the endocrine activities of its acylated form in humans. To this goal, the effects of acylated or non-acylated ghrelin (1.0 microg/kg i.v. at 0 min) on GH, PRL, ACTH, F, insulin and glucose levels were studied in two different testing sessions in 7 normal young volunteers (age [mean +/- SE]: 24.3 +/- 1.7 yr; BMI: 21.5 +/- 0.9 kg/m2). The effects of placebo administration were also studied. The administration of acylated ghrelin induced prompt and marked increase in circulating GH levels (AUC: 5452.4 +/- 904.9 microg*min/l; p < 0.01 vs placebo) and significant increase in PRL (1273.5 +/- 199.7 microg*min/l; p < 0.01 vs placebo), ACTH (4482.7 +/- 954.4 pg*min/ml; p < 0.01 vs placebo) and F levels (15985.0 +/- 1141.9 microg*min/l; p < 0.01 vs placebo). Its administration was also followed by decrease in insulin levels (1448.67 +/- 137.9 mU*min/l; p < 0.05 vs placebo) that was coupled with an increase in plasma glucose levels (10974.2 +/- 852.5 mg*min/dl; p < 0.05 vs placebo). The administration of non-acylated ghrelin and that of placebo did not induce any change in the hormonal parameters or in glucose levels. In conclusion, this study shows that in humans nonacylated ghrelin does not possess the pituitaric and pancreatic endocrine activities of human ghrelin octanoylated in Serine 3.

Natural and synthetic growth hormone secretagogues: do they have therapeutic potential?

Ghrelin, a 28 amino acid-acylated peptide predominantly produced by the stomach, displays strong growth hormone (GH)-releasing activity. It is mediated by the hypothalamic-pituitary GH secretagogue (GHS) receptors, which are specific to a family of synthetic, orally active molecules known as GHSs. However, despite their potent and reproducible GH-releasing activity, the potential clinical use of GHSs as orally active growth-promoting agents or anabolic anti-aging drugs has not been confirmed. Ghrelin and GHSs also exert other actions mediated through central and peripheral receptors, including stimulation of adrenocorticotrophic hormone and prolactin secretion, influence on insulin secretion and glucose metabolism, orexigenic effects and modulatory activity on the neuroendocrine and metabolic response to starvation, influence on exocrine gastro-entero-pancreatic functions, cardiovascular effects and modulation of cell proliferation and apoptosis. The discovery of ghrelin and the characterization of these GH-independent biological activities has widened the knowledge of some critical aspects of neuroendocrinology and suggests possible roles for GHSs and ghrelin in the treatment of pathophysiological conditions, including those unrelated to disorders of GH secretion.

Ghrelin protects against ethanol-induced gastric ulcers in rats: studies on the mechanisms of action.

Ghrelin, the endogenous ligand for GH secretagogue receptors, has been reported to influence acid gastric secretion and motility, but its potential gastroprotective effect is unknown. The aims of this study were 1) to examine the effects of central and peripheral administration of ghrelin on ethanol-induced gastric ulcers in conscious rats, and 2) to investigate the possible roles of nitric oxide (NO), vagal nerve, and sensory fibers in the gastric effects of ghrelin. Ghrelin was administered either intracerebroventricularly or sc 30 min before ethanol, and mucosal lesions were examined macroscopically. Additionally, rats were either treated with the inhibitor of NO synthesis N(omega)-nitro-L-arginine methyl ester (L-NAME) or underwent bilateral cervical vagotomy or capsaicin-induced sensory denervation. Conventional histology and immunohistochemistry for ghrelin, gastrin, and somatostatin were performed on gastric specimens from representative rats. Central ghrelin (4-4,000 ng/rat) dose-dependently reduced ethanol-induced gastric ulcers by 39-77%. Subcutaneous ghrelin administration (80 micro g/kg) reduced ulcer depth only. L-NAME and capsaicin, but not vagotomy, prevented the gastroprotective effect of central ghrelin (4000 ng/rat). This is the first evidence that ghrelin exerts a potent central gastroprotective activity against ethanol-induced lesions. The gastroprotective effect of ghrelin is mediated by endogenous NO release and requires the integrity of sensory nerve fibers.

Natural (ghrelin) and synthetic (hexarelin) GH secretagogues stimulate H9c2 cardiomyocyte cell proliferation.

Recent experimental data demonstrate cardiovascular effects of the GH secretagogues (GHSs) hexarelin and ghrelin, the proposed natural ligand for the GHS receptor. Moreover, specific cardiac binding sites for GHSs have been suggested. The aim of the present study was to investigate if the natural ligand ghrelin and synthetic GHS peptide hexarelin and analogues have direct effects on the cardiomyocyte cell line, H9c2. Hexarelin stimulated thymidine incorporation in a dose-dependent manner with significant responses at 3 micro M (147+/-3% of control, P<0.01) and elicited maximal effects at concentrations around 30 micro M. This activity was seen already after 12 h of incubation with a maximal effect after 18 h (176+/-9% of control, P<0.01). Ghrelin also had a significant stimulatory effect on thymidine incorporation (129+/-2% of control at 3 micro M and 18 h, P<0.05). The stimulatory effect on thymidine incorporation of hexarelin, Tyr-Ala-hexarelin, EP80317 and ghrelin was specific and no stimulatory effect was observed with the truncated GH-releasing peptide EP51389 or the non-peptidyl GHS MK-0677. In competitive binding studies, (125)I-labeled Tyr-Ala-hexarelin was used as radioligand and competition curves showed displacement with hexarelin, Tyr-Ala-hexarelin, EP80317 and ghrelin, whereas MK-0677 and EP51389 produced very little displacement at 1 micro M concentration, adding further support for an alternative subtype binding site in the heart compared with the pituitary. In conclusion, we have demonstrated a dose-dependent and specific stimulation of cardiomyocyte thymidine incorporation by natural and synthetic GHS analogues, suggesting increased cell proliferation and binding of GHS to H9c2 cardiomyocyte cell membranes. These findings support potential peripheral effects of GHS on the cardiovascular system independent of an increased GH secretion.

EP1572: a novel peptido-mimetic GH secretagogue with potent and selective GH-releasing activity in man.

EP1572 UMV1843 [Aib-DTrp-DgTrp-CHO]) is a new peptido-mimetic GH secretagogue (GHS) showing binding potency to the GHS-receptor in animal and human tissues similar to that of ghrelin and peptidyl GHS. EP1572 induces marked GH increase after s.c. administration in neonatal rats. Preliminary data in 2 normal young men show that: 1) acute i.v. EP1572 administration (1.0 microg/kg) induces strong and selective increase of GH levels; 2) single oral EP1572 administration strongly and reproducibly increases GH levels even after a dose as low as 0.06 mg/kg. Thus, EP1572 is a new peptido-mimetic GHS with potent and selective GH-releasing activity.

Concept evaluation: an assay for receptor-mediated and biochemical antiestrogens using pubertal rats.

At present, assessment of chemicals for receptor-mediated antiestrogenic activity involves inhibition of uterine growth stimulated by coadministration of a reference estrogen in either ovariectomized or immature rodents. In the present paper, we describe an alternative assay for both receptor-mediated and biochemical antiestrogens. The assay involves treatment of immature rats from postnatal (pnd) 25 or 26 for either 7 or 14 days and monitors two benchmarks of puberty, the mean day of vaginal opening and the weight of the uterus, that require estrogen activity. The receptor-mediated antiestrogens ZM 189,154 and Faslodex (ICI 182,780), the aromatase inhibitor Arimidex (Anastrozole), and the GnRH inhibitor Antarelix were each effective in preventing uterine growth and in delaying vaginal opening for the course of the experiments. The 5alpha-reductase inhibitor Finasteride was inactive in the assay indicating assay specificity for antiestrogens. Delays in uterine growth were clearly evident in the 7-day experiments, but assessment of vaginal opening required the 14-day protocol. No significant changes in body weight were observed in any of the experiments. It is concluded that the assay holds promise as a simple method of detecting antiestrogens and that it is worthy of further study.

Cortistatin-14 inhibits cell proliferation of human thyroid carcinoma cell lines of both follicular and parafollicular origin.

Cortistatin (CST-14, Pro-c[Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Ser-Ser-Cys]-Lys-NH2), a neuropeptide member of the SRIH family, binds to all 5 SRIH receptor (sst) subtypes, but also possesses a significant binding affinity to GH secretagogue receptors (GHS-R), which have been reported to mediate the antiproliferative activity of GHS on thyroid cancer cells. The effect of CST-14 on cell proliferation was studied in 3 different human thyroid carcinoma cell lines of follicular origin (N-PAP, WRO, ARO) and in one thyroid medullary carcinoma cell line (TT). CST-14 1 pM determined a significant inhibition of cell proliferation in TT, N-PAP and WRO cells and this effect was dose-dependent and more pronounced than that displayed by SRIH-14 (Ala-Gly-c[Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys]-OH) treatment. To a minor extent, CST-14, but not SRIH-14, also temporary inhibited ARO cell proliferation. By immunofluorescence, sst2, sst3 and sst5 have been demonstrated in TT cells, whereas types 3 and 5 only were expressed in N-PAP and WRO cells, and no sst subtype was found in ARO cells. The presence of both GHS-Rla and lb mRNA has been studied and demonstrated in the TT medullary carcinoma cell line, whereas follicular derived cell lines were already known to express GHS binding sites. Addition of EP-80874 (D-Mrp-c[D-Cyspyridilalanyl3-D-Trp-Lys-Val-Cys]-Mrp-NH2), a synthetic peptide that binds to SRIH and GHS-R, completely abolished the antiproliferative effects of CST-14 or SRIH-14 on sst/GHS-R positive thyroid carcinoma cell lines (WRO, N-PAP and TT). EP-80874 was also able to antagonize the inhibitory activity of CST-14 on the growth of cells (ARO) expressing GHS-R but not sst. Taken together, these data firstly demonstrate that EP-80874 has a mixed SRIH/CST antagonist activity and suggest that the oncostatic effect of CST-14 on thyroid cancer cells could be mediated by both sst and/or GHS-R.

Growth hormone response to a novel growth hormone-releasing tripeptide in horses: interaction with gonadotropin-releasing hormone, thyrotropin-releasing hormone, and sulpiride.

A series of experiments was performed to determine the factor(s) responsible for an apparent inhibition of GH secretion in mares administered the GH secretagogue EP51389 in combination with GnRH, thyrotropin-releasing hormone (TRH), and sulpiride. Experiment 1 tested the repeatability of the original observation: 10 mares received EP51389 at 10 microg/kg BW; five received TRH (10 microg/kg BW), GnRH (1 microg/kg BW), and sulpiride (100 microg/kg BW) immediately before EP51389, and five received saline. The mixture of TRH, GnRH, and sulpiride reduced (P = 0.0034) the GH response to EP51389, confirming the inhibitory effects. Experiment 2 tested the hypothesis that sulpiride, a dopamine antagonist, was the inhibitory agent. Twelve mares received EP51389 as in Exp. 1; six received sulpiride before EP51389 and six received saline. The GH responses in the two groups were similar (P > 0.1), indicating that sulpiride was not the inhibitory factor. Experiment 3 tested the effects of TRH and(or) GnRH in a 2 x 2 factorial arrangement of treatments. Three mares each received saline, TRH, GnRH, or the combination before EP51389 injection. There was a reduction (P < 0.0001) in GH response in mares receiving TRH, whereas GnRH had no effect (P > 0.1). Given those results, Exp. 4 was conducted to confirm that TRH was inhibitory in vivo as opposed to some unknown chemical interaction of the two compounds in the injection solution. Twenty mares received TRH or saline and(or) EP51389 or saline in a 2 x 2 factorial arrangement of treatments. Injections were given separately so that the two secretagogues never came in contact before injection. Again, TRH reduced (P < 0.0001) the GH response to EP51389. In addition, TRH and EP51389 each resulted in a temporary increase in cortisol concentrations. Experiment 5 tested whether TRH would alter the GH response to GHRH itself. Twelve mares received porcine GHRH at 0.4 microg/kg BW; six received TRH prior to GHRH and six received saline. After adjustment for pretreatment differences between groups, the GHRH-induced GH response was completely inhibited (P = 0.068) by TRH. Exp. 6 was a repeat of Exp. 5, except geldings were used (five per group). Again, pretreatment with TRH inhibited (P < 0.0001) the GH response to GHRH. In conclusion, TRH inhibits the GH response not only to EP51389 but also to GHRH in horses, and in addition to its known secretagogue action on prolactin and TSH it may also stimulate ACTH at the dosage used in these experiments.

Evidence for a central inhibitory role of growth hormone secretagogues and ghrelin on gastric acid secretion in conscious rats.

We examined the possible central and peripheral effects of synthetic growth hormone secretagogues (GHS), hexarelin (Hexa) and EP 40737 (D-Thr-D-Trp (2-Me)-Ala- Trp-D-Phe-Lys-NH2), and of their endogenous counterpart, ghrelin, on gastric acid secretion. The compounds were administered intracerebroventricularly (i.c.v.) or subcutaneously (s.c.) in conscious male rats and the volume of gastric secretion and gastric acid output were examined 3 h after pylorus ligation (Shay-test). Central Hexa, EP 40737 and ghrelin administration (from 0.1 pmol to 1 nmol/rat, i.c.v.) significantly inhibited gastric acid secretion. The maximum inhibitory effect on gastric acid output was detected at the dose of 10 pmol/rat, i.c.v. for Hexa (-51.3%), of 100 pmol/rat, i.c.v. for EP 40737 (-70%) and of 1 pmol/rat, i.c.v. for ghrelin (-60%). All peptides were less effective at the highest dose used (1 nmol/rat, i.c.v.). Hexa, EP 40737 and ghrelin injected s.c. did not modify gastric acid secretion. The inhibitory action of Hexa on gastric acid secretion seems to involve brain somatostatinergic system since Hexa (10 pmol/rat, i.c.v.) did not inhibit gastric acid secretion in rats pretreated (4 h before) with cysteamine (300 mg/kg, s.c.), a depletor of endogenous somatostatin. These results show that synthetic GHS and ghrelin exert a central long-lasting inhibitory effect on gastric acid secretion in conscious pylorus-ligated rats. The fact that very low doses of ghrelin and GHS inhibit gastric secretion, provide evidence for a tonic inhibitory role of the peptides in the central control of gastric secretory function.