Bone effects of hexarelin,a GH-releasing peptide,in female rats: influence of estrogen milieu

OBJECTIVE: The present study was performed to evaluate the potential influence of the estrogen milieu in modulating the effects of GH/IGF stimulation by a GH-releasing peptide, hexarelin (HEXA), on bone metabolism and mineral density in middle-aged female rats. METHODS: HEXA was administered for 60 days (50 microg/kg s.c. twice a day) to intact and ovariectomized (OVX) 11-month-old female rats and changes in bone parameters were evaluated with respect to those of the same rats under baseline conditions and with those of control rats (intact and OVX) administered isovolumetric amounts of physiological saline. Serum total alkaline phosphatase (ALP) and urinary deoxypyridinoline (Dpd) were measured before and at various times during HEXA treatment. Bone mineral content (BMC) and density of lumbar vertebrae and femoral mid-diaphyses were measured by dual energy X-ray absorptiometry before and after treatment. In all groups, serum IGF-I levels were determined before and during treatment and the GH secretory response to HEXA was assessed at the end of the experiment. RESULTS: In intact rats, HEXA did not modify Dpd urinary excretion, induced a trend toward an increase of serum ALP activity and significantly increased BMC (+6.5%) and bone area (+4.1%) only at lumbar vertebrae. In OVX rats, HEXA did not modify the OVX-induced increase in bone turnover markers (Dpd and ALP) and did not affect the OVX-induced vertebral bone loss, but significantly increased BMC (+7.2%) and bone area (+5.3%) at femoral mid-diaphyses. HEXA significantly increased serum IGF-I levels at day 14, but not at day 60, in both intact and OVX rats, whereas the GH secretory response to HEXA was higher in the former than in the latter. CONCLUSIONS: Overall, the present data demonstrate that chronic HEXA treatment increases BMC and bone area at lumbar vertebrae in intact rats and at femoral diaphyses in OVX rats. The different sensitivity to HEXA of the skeletal districts examined is related to the estrogen milieu and may reflect a complex interplay between estrogens and GH/IGF function.     

Selective lack of growth hormone (GH) response to the GH-releasing peptide hexarelin in patients with GH-releasing hormone receptor deficiency

The mechanism of the synergistic relationship between GH-releasing peptide (GHRP) and GHRH with respect to GH secretion is poorly understood. We report the response to hexarelin, a potent GHRP, in patients affected with a homozygous mutation in the GHRH receptor gene, with consequent GHRH resistance and GH-deficient dwarfism. This newly described syndrome is the human homolog of the little (lit/lit) mouse. Intravenous administration of hexarelin (2 microg/kg) to four male adult patients (dwarfs of Sindh) resulted in a complete lack of elevation in plasma GH levels (< 1 ng/mL), an at least 50- to 100-fold deviation from the normal response. In contrast, plasma PRL, ACTH, and cortisol levels rose in a normal manner in response to hexarelin. We conclude that an intact GHRH signaling system is critical for GHRPs to exert their effect on GH release, but that the GHRH system is not necessary for the effect of GHRP on PRL and ACTH secretion. Hexarelin (and probably other GHRPs) are not effective agents for the treatment of patients with GHRH resistance due to GHRH receptor deficiency.     

Acute growth hormone (GH) response to GH-releasing hexapeptide in humans is independent of endogenous GH-releasing hormone.

The synthetic GH-releasing hexapeptide (GHRP: His-DTrp-Ala-Trp-DPhe-Lys-NH2) releases GH in man by an undetermined mechanism. To investigate whether acute GH response to GHRP is mediated by endogenous GHRH, we examined the effect of GHRP on GH release during pituitary desensitization to GHRH induced by short-term GHRH infusion. In five healthy men on six occasions, we infused saline (sal) or 1 microgram/kg.h GHRH-44 for 6 h. After 4 h, a bolus of sal, GHRH-44 1 microgram/kg body weight, or GHRP 1 microgram/kg body weight was given iv. GH concentration, measured by RIA, was analyzed by mean area under the curve (AUC) of GH released over the 2 h immediately after bolus injection. Infusion of GHRH had a biphasic effect on GH release; plasma GH increased to 12.7 +/- 3.3 micrograms/L within the first hour, with subsequent decrease to 2.9 +/- 0.3 micrograms/L during the last 2 h of infusion. GH AUC (hours 4-6 of infusion) microgram/L.2 h [table: see text] GH response to bolus GHRH was abolished by GHRH infusion, whereas GH response to GHRP persisted under the same conditions. Thus, we conclude that acute GH response to GHRP in humans is not mediated by endogenous GHRH.     

Contrasting effects of nitric oxide on food intake and GH secretion stimulated by a GH-releasing peptide

OBJECTIVE: Among the many actions of nitric oxide (NO) are those on endocrine and feeding behaviour. Based on NO involvement in the GH-releasing effect of the GH-releasing peptides (GHRPs) and the reported orexigenic activity of these compounds, we sought to evaluate the effect of the combined administration of a long-acting NO donor, molsidomine, and the newly synthesized GHRP EP92632 on food intake and GH secretion in rats. Moreover, to verify the specificity of a potential NO involvement, we evaluated whether or not the effects of GHRPs were abolished by a pre-treatment with an inhibitor of NO synthase, N-nitro-arginine-methyl-ester (NAME). METHODS: In the food intake experiments, adult Sprague-Dawley male rats underwent acute administration of: (1) EP92632 (160 microg/kg, s.c.); (2) molsidomine (100 mg/kg, i.p.); (3) EP92632+molsidomine; (4) l-NAME (40 and 60 mg/kg, i.p.); (5) EP92632+l-NAME (60 mg/kg, i.p.); (6) EP92632+molsidomine+l-NAME (60 mg/kg, i.p.); and (7) 0.9% saline (0.1 ml/kg, i.p.). After treatments, the cumulative food intake in the 6 post-treatment hours was carefully evaluated. In the neuroendocrine experiments, rats were given the same compounds according to the above reported schedule, except for the use of one dose of NAME (60 mg/kg, i.p.) and a lower EP92632 dose (80 microg/kg, s.c.), and were sampled via atrial cannula. RESULTS: EP92632 significantly stimulated food intake, an effect which was further enhanced by molsidomine, though the latter did not elicit per se any orexigenic effect. l-NAME given alone significantly decreased food intake and abolished the orexigenic effect of the GHRP and the enhancing effect of molsidomine. Plasma GH levels increased significantly following administration of EP92632 but, in contrast to the food intake experiments, molsidomine significantly inhibited both basal and EP92632-stimulated GH secretion; moreover, NAME had a biphasic effect on the EP92632-stimulated GH release: initially inhibitory and then, from 45 min on, stimulatory. NAME did not affect basal GH levels but, surprisingly, combined administration of molsidomine and NAME induced a striking inhibition of both basal and the peptide-stimulated GH release. CONCLUSIONS: In summary, these data indicate that NO in the rat is physiologically involved in a stimulatory way in the GHRP-mediated effect on food intake, but exerts a dual action, probably stimulatory at hypothalamic and inhibitory at pituitary levels, on basal and GHRP-stimulated GH secretion.     

Interaction between glucagon and hexarelin, a peptidyl GH secretagogue, on somatotroph and corticotroph secretion in humans

OBJECTIVE: Glucagon administration stimulates both somatotroph and corticotroph secretion in humans, although this happens only if glucagon is administered by the intramuscular route and not by the intravenous route. On the other hand, GH secretagogues (GHS) strongly stimulate GH and also possess ACTH-releasing activity. DESIGN AND METHODS: To clarify the mechanisms underlying the stimulatory effects of both glucagon and GHS on somatotroph and corticotroph secretion, we studied the GH, ACTH and cortisol responses to glucagon (GLU, 0.017 mg/kg i.m.) and Hexarelin, a peptidyl GHS (HEX, 2.0 microg/kg i.v.) given alone or in combination in 6 normal young volunteers (females, aged 26-32 years, body mass index 19.7-22.5 kg/m). RESULTS: GLU administration elicited a clear increase in GH (peak vs baseline, mean+/-S.E.M.: 11.6+/-3.4 vs 3. 3+/-0.7 microg/l, P<0.02), ACTH (11.6+/-3.3 vs 4.1+/-0.3 pmol/l, P<0. 02) and cortisol (613.5+/-65.6 vs 436.9+/-19.3 nmol/l, P<0.05) levels. HEX induced a marked increase in GH levels (55.7+/-19.8 vs 3. 7+/-1.9 microg/l, P<0.005) and also significant ACTH (5.7+/-1.1 vs 3. 4+/-0.6 pmol/l, P<0.01) and cortisol (400.2+/-31.4 vs 363.4+/-32.2 nmol/l, P<0.05) responses. The GH area under the curve (AUC) after HEX was clearly higher than after GLU (1637.3+/-494.0 vs 479.1+/-115. 7 microg/l/120 min, P<0.04) while HEX and GLU coadministration had a true synergistic effect on GH release (3243.8+/-687.5 microg/l/120 min, P<0.02). The ACTH and cortisol AUCs after HEX were lower (P<0. 02) than those after GLU (208.3+/-41.3 vs 426.3+/-80.9 pmol/l/120 min and 18 874.5+/-1626.1 vs 28 338.5+/-2430.7 nmol/l/120 min respectively). The combined administration of HEX and GLU had an effect which was less than additive on both ACTH (564.02+/-76.5 pmol/l/120 min) and cortisol (35 424.6+/-5548.1 nmol/l/120 min) secretion. CONCLUSIONS: These results show that the intramuscular administration of glucagon releases less GH but more ACTH and cortisol than Hexarelin. The combined administration of glucagon and Hexarelin has a true synergistic effect on somatotroph secretion but a less than additive effect on corticotroph secretion; these findings suggest that these stimuli act via different mechanisms to stimulate somatotrophs while they could have a common action on the hypothalamo-pituitary-adrenal axis.     

Impact of two or three daily subcutaneous injections of hexarelin, a synthetic growth hormone (GH) secretagogue, on 24-h GH, prolactin, adrenocorticotropin and cortisol secretion in humans

OBJECTIVE: To extend the insights on the action of GH secretagogues (GHS) on pituitary function, we studied the impact of intermittent daily s.c. administration of a peptidyl GHS, hexarelin (HEX), on 24-h GH, PRL, ACTH and cortisol release in healthy volunteers. DESIGN: We investigated the impact of two or three times daily s.c. administration of a short-acting peptidyl GHS, the hexapeptide HEX (1.5 microg/kg) on 24-h GH, PRL, ACTH and cortisol secretion (sampling every 20 min) in six normal young men. To monitor possible down-regulation, the effect of 1 microg/kg i.v. HEX at the end of each 24-h sampling period was studied. METHODS: Multi-parameter deconvolution analysis was used to quantitate pulsatile GH, PRL, ACTH and cortisol secretion and estimate the corresponding hormone half-lives. Complementary to deconvolution analysis, approximate entropy was used as a scale- and model-independent statistic to quantify the serial orderliness or pattern regularity of hormone measurements. RESULTS: Mean and integrated (24-h) serum GH concentrations were increased from baseline values to the same extent by two and three HEX injections. Both HEX schedules equally increased GH secretory burst mass (but not burst frequency), mean daily GH production rate, GH half-life and irregularity of GH release patterns. No change occurred in the secretion of IGF-I, PRL, ACTH and cortisol. Intravenous HEX at the end of each spontaneous 24-h profile induced a significant rise in GH, PRL, ACTH and cortisol. Prior HEX administration blunted the GH response, abolished that of ACTH and cortisol and did not modify the PRL increase. CONCLUSIONS: The study showed that two or three daily s.c. injections of HEX augmented 24-h GH secretion equally, amplifying selectively GH secretory pulse mass without altering lactotroph and corticotroph secretion. IGF-I levels were not modified by these 1-day HEX treatment schedules.     

Acute administration of hexarelin stimulates GH secretion during day and night in normal men

OBJECTIVE Hexarelin is a synthetic hexapeptide with potent GH-releasing activity in both animals and men. The aim of this study was to investigate the effect of a bolus injection of hexarelin given in the morning during wakefulness and during nocturnal sleep in a group of normal adult men. DESIGN AND SUBJECTS Eight normal men, aged 21–33 years, of normal height and within 10% of ideal body weight were studied. All subjects received in random order saline or hexarelin (2 μg/kg) in the morning between 0800 and 0900 h after they had fasted overnight. The same experiments were performed during nocturnal sleep in the same subjects. Saline or hexarelin were injected within 30 minutes after the onset of sleep between 2300 and 2400 h. Sleep was recorded by visual inspection. MEASUREMENTS In all four test sessions blood samples were taken 30, 15 minutes and immediately before the injection of saline or hexarelin and then every 15 minutes for 2 hours. GH was measured by an immunoradiometric assay. All values are expressed as peak GH levels or as area under the curve (AUC) calculated by trapezoidal integration. RESULTS Mean peak GH concentrations after hexarelin during the morning (58.2 ± 4.7 μg/l) (GH μg/l × 2 = mU/l) were not different from those observed during sleep (61.2 ±4.3 μg/l). The rate of disappearance of GH from plasma was slower during sleep (t_1/2 =64.9 ±14.8 min) than during morning hours (t_1/2 = 24.9 ±1.4 min, P <0.01). Mean AUC responses to hexarelin during sleep (1466 ± 145 μg.min/l) were significantly higher than during morning hours (903 ± 94 μg.min/l, P < 0.001). CONCLUSIONS These results show that GH responsiveness to a growth hormone releasing peptide is preserved during the night. This could be exploited for diagnostic and/or therapeutic purposes.     

Effects of the combined administration of hexarelin, a synthetic peptidyl GH secretagogue, and hCRH on ACTH, cortisol and GH secretion in patients with Cushing's disease

Hexarelin (HEX) is a peptidyl GH secretagogue (GHS) which markedly stimulates GH release but, like other GHS, possesses also CNS-mediated ACTH- and cortisol-releasing activity. Interestingly, the stimulatory effect of HEX on ACTH and cortisol release is exaggerated and higher than that of hCRH in patients with Cushing's disease (CD). To further clarify the mechanisms by which HEX stimulates the activity of hypothalamo-pituitary-adrenal (HPA) axis in man, in 6 patients with CD (6 women, 38-68 yr old) and in 7 control subjects (CS, 7 women, 22-29 yr old) we studied the effects of HEX (2.0 microg/kg i.v.) and/or hCRH (2.0 microg/kg i.v.) on ACTH and cortisol (F) secretion. The GH responses to HEX alone and combined with hCRH were also studied in all subjects. Basal ACTH and F levels in CD were higher than in CS (66.3+/-5.1 vs 16.5+/-0.6 pg/ml and 217.8+/-18.5 vs 134.4+/-4.6 microg/l, respectively; p<0.02). In CS, the ACTH and F responses to HEX, evaluated as deltaAUC (mean+/-SE: 128.7+/-39.2 pg x min/ml and 328.5+/-93.2 microg x min/l, respectively) were lower, though not significantly, than those after hCRH (375.8+/-128.4 pg x min/ml and 1714.2+/-598.0 microg x min/l, respectively), though the peak ACTH and F responses to both stimuli were similar. The co-administration of HEX and hCRH had an additive effect on both ACTH (1189.6+/-237.2 pg x min/ml) and F secretion (3452.9+/-648.6 microg x min/l). In fact, the ACTH and F responses to HEX+/-hCRH were significantly higher (p<0.01) than those elicited by single stimuli. In CD, HEX induced ACTH and F responses (3603.8+/-970.7 pg x min/ml and 10955.9+/-6184.6 microg x min/l, respectively) clearly higher (p<0.002) than those in CS. The HEX-induced ACTH and F responses in CD were higher, though not significantly, than those recorded after hCRH (1432.7+/-793.5 pg x min/ml and 4832.7+/-2146.5 microg x min/l, respectively). On the other hand, the hCRH-induced ACTH and F responses in CD were similar to those in CS. In CD, the coadministration of HEX and hCRH had an additive effect on ACTH (8035.7+/-1191.1 pg x min/ml) but not on F (10985.4+/-3900.8 microg x min/l) secretion. In fact, the ACTH, but not the F response to HEX+hCRH was significantly higher (p<0.02) than that elicited by single stimuli. In conclusion, the present study demonstrates that in patients with Cushing's disease as well as in subjects control Hexarelin and hCRH have an additive effect on ACTH secretion. Considering that, at least in humans, differently from hCRH, GHS have no interaction with AVP, our present findings further agree with the hypothesis that the ACTH-releasing activity of GHS is, at least partially, independent of CRH-mediated mechanisms.     

Recombinant human IGF-I does not modify the ACTH and cortisol responses to hCRH and hexarelin, a peptidyl GH secretagogue, in humans

An inhibitory influence of insulin-like growth factor-I (IGF-I) on hypothalamus-pituitary-adrenal (HPA) axis has been hypothesized. In fact, it has been reported that the rhGH (recombinant human GH)-induced IGF-I increase inhibits both cortisol and GH response to MK-0677, a non-peptidyl GH secretagogue in animals. The aim of this study was to further clarify the inhibitory role, if any, of IGF-I on corticotroph function. We studied the effect of rhIGF-I (recombinant human IGF-I; 20 microg/kg s.c. at -180 min) or placebo on the ACTH and cortisol responses to hCRH (human CRH; 2.0 microg/kg i.v. at 0 min) or hexarelin (HEX; 2.0 microg/kg i.v. at 0 min), a peptidyl GHS, in normal young women. The effect of rhIGF-I on the GH response to HEX was also studied. The subjects were six normal young women [age: 26-35 yr; body mass index (BMI): 19-23 kg/m2] in their early follicular phase. The results showed that after s.c. rhIGF-I administration, circulating IGF-I levels increased approximately 77%, peaking at -60 min and persisting similar up to +120 min. The mean ACTH, cortisol and GH concentrations did not change from -180 to 0 min when evaluated after both placebo or rhIGF-I. CRH and HEX induced similar ACTH (peak vs baseline, mean+/-SE: 47.5+/-10.9 vs 21.3+/-3.0 pg/ml and 30.3+/-6.9 vs 19.2+/-3.8 pg/ml, respectively; p<0.04) and cortisol responses (177.5+/-5.4 vs 109.3+/-10.3 microg/l and 149.4+/-12.3 vs 119.8+/-16.4 microg/l, respectively, p<0.04). RhIGF-I pretreatment did not modify the ACTH and cortisol responses to hCRH (46.0+/-13.8 pg/ml and 181.1+/-16.9 microg/l, respectively) as well as those to HEX (28.8+/-5.0 pg/ml and 144.1+/-16.2 microg/l, respectively). On the other hand, the GH response to HEX was clearly reduced by rhIGF-I (23.9+/-4.7 vs 64.7+/-14.8 microg/l, p<0.05). Our findings show that rhIGF-I-induced increase of circulating IGF-I levels exerts negative feedback action on somatotroph secretion, while it does not modify the corticotroph and the adrenal responsiveness to CRH or hexarelin.     

Growth hormone (GH) rebound rise following somatostatin infusion withdrawal: studies in dogs with the use of GH-releasing hormone and a GH-releasing peptide.

OBJECTIVE: Evidence has been presented that in both animals and humans the rebound secretion of growth hormone (GH) following withdrawal of an infusion of somatostatin (SS) is due to the functional activation of the hypothalamic GH-releasing hormone (GHRH) neurons of the recipient organism. Based on this premise, this study has sought to assess the existence of functional interactions between endogenous GHRH released by a SS infusion withdrawal (SSIW) and growth hormone-releasing peptides (GHRPs), a class of compounds allegedly acting via GHRH. METHODS: Five young dogs (3 to 4 years old, 2 male and 3 female) were administered, on different occasions, three consecutive intravenous boli of physiological saline (0.1 ml/kg), or GHRH (2 microg/kg), or EP92632 (125 microg/kg), a GHRP compound, or GHRH plus EP92632 at the end of three cycles of 1-h SS infusions (8 microg/(kg x h)) or during a 6-h infusion of saline. RESULTS: Under saline infusion (SALI), plasma GH levels were unaltered, whereas each SSIW cycle was followed by similar GH secretory episodes. Administration of the first GHRH bolus under SALI induced a rise in plasma GH concentrations slightly higher than that induced by the first cycle of SSIW, but the GH response to the second and third GHRH boli was similar to that after SSIW. Following SSIW, the response to the first bolus of GHRH was higher than that during SALI, but the second and third cycles of SSIW induced GH responses similar to those evoked by the GHRH bolus. During SALI, administration of the first bolus of EP92632 induced a rise in plasma GH which was higher than that induced by the first GHRH bolus, the second bolus elicited a GH peak of lesser amplitude and there was a partial restoration of the GH response to the third peptide bolus. SSIW strikingly enhanced the GH release to the first EP92632 bolus, a pattern also present, although to a lesser extent, with the second and third cycles of SSIW. Under SALI, combined administration of GHRH and EP92632 had a synergistic effect on GH release, but a progressive reduction was present in the GH response to the second and third GHRH plus EP92632 boli. SSIW increased only weakly the GH response to the first co-administration of the peptides over that present after administration of EP92632 alone, and did not induce a GH response higher than that present during SALI when the second bolus of the peptides was administered; after the third SSIW a GH rise higher than that present during SALI was elicited by the combined administration of the peptides. CONCLUSIONS: (i) the uniformity of the GH rebound responses to multiple cycles of SSIW may indicate that the latter activate a physiological mechanism which mimics that normally controlling GH pulse generation; (ii) EP92632 elicits, under our experimental conditions, a plasma GH rise higher than that induced by GHRH; (iii) SSIW enhances the GH response to EP92639 alone, to an extent reminiscent of that following combined administration of GHRH and EP92632. This pattern reinforces the view that SSIW elicits release of endogenous GHRH, and infers that the GHRP challenge after SSIW may be exploited in humans to distinguish between healthy and GH-deficient adults.     

Endocrine activities of ghrelin, a natural growth hormone secretagogue (GHS), in humans: comparison and interactions with hexarelin, a nonnatural peptidyl GHS, and GH-releasing hormone

An endogenous ligand for the GH secretagogue-receptor (GHS-receptor) has recently been isolated, from both the rat and the human stomach, and named ghrelin. It is a 28-amino-acid peptide showing a unique structure with an n-octanoyl ester at its third serine residue, which is essential for its potent stimulatory activity on somatotroph secretion. In fact, it has been demonstrated that ghrelin specifically stimulates GH secretion from both rat pituitary cells in culture and rats in vivo. The aim of the present study was to test the GH-releasing activity of ghrelin in humans and to compare it with that of GHRH and hexarelin (HEX), a nonnatural peptidyl GHS, which possesses strong GH-releasing activity but also significantly stimulates PRL, ACTH, and cortisol secretion. To clarify the mechanisms of action underlying the GH-releasing activity of ghrelin in humans, its interaction with GHRH and HEX was also studied. Seven normal young volunteers (7 men; 24-32 yr old; body mass index, 20-24 kg/m(2)) were studied. All subjects underwent the administration of ghrelin, HEX, and GHRH-29 (1.0 microg/kg i.v. at 0 min) as well as placebo (2 mL isotonic saline i.v. at 0 min). Six subjects also underwent the combined administration of ghrelin and GHRH or HEX. Blood samples were taken every 15 min from -15 up to +180 min. GH levels were assayed at each time point in all sessions; PRL, ACTH, cortisol, and aldosterone levels were also assayed after administration of ghrelin and/or HEX. Ghrelin administration induced a prompt and marked increase in circulating GH levels (Cmax, mean +/- SEM, 92.1 +/- 16.7 microg/L; area under the curve, 1894.9 +/- 347.8 microg/L.h). The GH response to ghrelin was clearly higher (P < 0.01) than the one recorded after GHRH (26.7 +/- 8.7 microg/L; 619.6 +/- 174.4 microg/L.h) and even significantly higher (P < 0.05) than after HEX (68.4 +/- 14.7 microg/L; 1546.9 +/- 380.0 microg/L x h). Ghrelin administration also induced an increase in PRL, ACTH, and cortisol levels; these responses were higher (P < 0.05) than those elicited by HEX. A significant increase in aldosterone levels was recorded after ghrelin but not after HEX. The endocrine responses to ghrelin were not modified by the coadministration of HEX. On the other hand, the coadministration of ghrelin and GHRH had a real synergistical effect (P < 0.05) on GH secretion (133.6 +/- 22.5 microg/L; 3374.3 +/- 617.3 microg/L x h). In conclusion, ghrelin, a natural ligand of GHS-receptor, exerts a strong stimulatory effect on GH secretion in humans, releasing more GH than GHRH and even more than a nonnatural GHS such as HEX. Ghrelin, as well as HEX, also stimulates lactotroph and corticotroph secretion. Ghrelin shows no interaction with HEX, whereas it has a synergistical effect with GHRH on GH secretion. Thus, ghrelin is a new hormone playing a major role in the control of somatotroph secretion in humans, and its effects are imitated by nonnatural GHS.     

Pituitary adenylate cyclase activating polypeptide, growth hormone (GH)-releasing peptide and GH-releasing hormone stimulate GH release through distinct pituitary receptors.

GH secretion has been thought traditionally to be regulated by the two hypothalamic hormones, GH-releasing hormone (GHRH) and somatostatin (SRIF). Recent evidence has suggested that other factors may be involved. These factors include the natural ligand for the synthetic hexapeptide GH-releasing peptide (GHRP) and the putative hypophysiotropic factor pituitary adenylate cyclase-activating polypeptide (PA-CAP). Accordingly, we examined the effects of GHRP and PACAP on GH secretion at the single cell level using the reverse hemolytic plaque assay which allows distinction of effects on the number of secreting cells and the amount of hormone each cell secretes. Both factors stimulated GH secretion in a dose-dependent fashion, with PACAP being more effective. PACAP increased both the number of cells secreting and the mean amount of hormone secreted per cell. In contrast, GHRP increased the number of secreting cells, although it had no effect on the amount of secretion per cell. GH secretion induced by GHRH, GHRP, and PACAP was inhibited by SRIF, but the effect was predominantly on the number of cells secreting rather than the amount secreted per cell. Specific antagonists to GHRP and GHRH inhibited GH secretion induced by the respective agonist but not that induced by the other factor nor by PACAP. These findings confirm the complex nature of the regulation of GH secretion at the level of the somatotrope. At least three factors, operating via distinct receptors, are able to increase GH secretion. In addition, they ascribe a potential physiological role for the hitherto putative hypophysiotropic factor PACAP.     

Growth hormone (GH)-releasing peptide stimulates GH release in normal men and acts synergistically with GH-releasing hormone

The acute GH release stimulated by the synthetic hexapeptide, His-DTrp-Ala-Trp-DPhe-Lys-NH2 [GH releasing peptide (GHRP)], was determined in 18 normal men and compared with the effects of GH-releasing hormone, GHRH-(1-44)-NH2. Specificity of effect was assessed by measurement of serum PRL, LH, TSH, and cortisol. GHRP was administered at doses of 0.1, 0.3, and 1.0 microgram/kg by iv bolus. GHRH at a dose of 1.0 microgram/kg was administered alone and together with various does of GHRP. No adverse clinical effects of laboratory abnormalities were observed in response to GHRP. A side-effect of mild facial flushing of 1- to 3-min duration occurred in 16 of the 18 subjects who received GHRH-(1-44)-NH2. Mean (+/- SEM) peak serum GH levels after injection of placebo and 0.1, 0.3, and 1.0 microgram/kg GHRP were 1.2 +/- 0.3, 7.6 +/- 2.5, 16.5 +/- 4.1, and 68.7 +/- 15.5 micrograms/L, respectively. The submaximal dosages of 0.1 and 0.3 microgram/kg GHRP plus 1 microgram/kg GHRH stimulated GH release synergistically. Serum PRL and cortisol levels rose about 2-fold above basal levels only at the 1 microgram/kg dose of GHRP, and there were no changes in serum LH and TSH over the first hour after administration of the peptide(s). GHRP is a potent secretagogue of GH in normal men. Since GHRP and GHRH together stimulate GH release synergistically, these results suggest that GHRP and GHRH act independently. This supports our hypothesis that the GH-releasing activity of GHRP reflects a new physiological system in need of further characterization in animals and man.     

The GH-releasing effect of ghrelin,a natural GH secretagogue,is only blunted by the infusion of exogenous somatostatin in humans

OBJECTIVE: Ghrelin, a 28-amino-acid peptide purified from the stomach and showing a unique structure with an n-octanoyl ester at the serine 3 residue, is a natural ligand of the GH secretagogue (GHS) receptor (GHS-R). Ghrelin strongly stimulates GH secretion in both animals and humans, showing a synergistic effect with GH-releasing hormone (GHRH) but no interaction with synthetic GHS. However, the activity of ghrelin as well as that of non-natural GHS is not fully specific for GH; ghrelin also induces a stimulatory effect on lactotroph and corticotroph secretion, at least in humans. DESIGN: To further clarify the mechanisms underlying the GH-releasing activity of this natural GHS, we studied the effects of somatostatin (SS, 2.0 microg/kg/h from -30 to +90 min) on the endocrine responses to ghrelin (1.0 microg/kg i.v. at 0 min) in seven normal young male volunteers [age (mean +/- SEM) 28.6 +/- 2.9 years; body mass index (BMI) 22.1 +/- 0.8 kg/m2]. In the same subjects, the effect of SS on the GH response to GHRH (1.0 microm/kg i.v. at 0 min) was also studied. MEASUREMENTS: Blood samples were taken every 15 min from -30 up to +120 min. GH levels were assayed at each time point in all sessions; PRL, ACTH and cortisol levels were assayed after ghrelin administration alone and during SS infusion. RESULTS: The GH response to ghrelin (hAUC0'-->120' 2695.0 +/- 492.6 microg min/l) was higher (P < 0.01) than that after GHRH (757.1 +/- 44.1 microg min/l). SS infusion almost abolished the GH response to GHRH (177.0 +/- 37.7 microg min/l, P < 0.01); the GH response to ghrelin was inhibited by SS (993.8 +/- 248.5 microg min/l, P < 0.01) but GH levels remained higher (P < 0.05) than with GHRH. Ghrelin induced significant increases in PRL, ACTH and cortisol levels and these responses were not modified by SS. CONCLUSIONS: Ghrelin, a natural GHS-R ligand, exerts a strong stimulatory effect on GH secretion in humans and this effect is only blunted by an exogenous somatostatin dose which almost abolishes the GH response to GHRH. The stimulatory effect of ghrelin on lactotroph and corticotroph secretion is refractory to exogenous somatostatin, indicating that these effects occur through pathways independent of somatostatinergic influence.     

Effects of ghrelin and synthetic GH secretagogues on the cardiovascular system.

Ghrelin, a newly discovered endogenous hormone that is produced by the stomach, and synthetic peptides have been identified recently as potent growth-hormone secretagogues. This effect is exerted through interaction with a specific G-protein-coupled receptor, GHS-R1a, which is expressed mainly in the hypothalamus-pituitary complex. A study of the peripheral distribution of GHS receptors has shown that it is also present in cardiovascular tissue, which has led to the exploration of the cardiovascular functions of ghrelin and synthetic, growth-hormone-releasing peptides. These ligands have several cardiovascular activities, including a cardioprotective effect against myocardial ischemia, and vasoactive and cardiotropic effects in both experimental models and humans. These effects are mediated by the interaction of these ligands with binding sites, including GHS-1Ra, for which the signalling pathways are not documented fully. Identification of the cardiac and vascular binding sites for ghrelin and synthetic, growth-hormone-releasing peptides will provide new perspectives for treating cardiovascular diseases with these ligands.     

Endocrine and metabolic effects of growth hormone (GH) compared with GH-releasing peptide, thyrotropin-releasing hormone, and insulin infusion in a rabbit model of prolonged critical illness

Treatment with recombinant human GH (rhGH) increases the mortality of critical illness. We postulated that combined GH-releasing peptide-2 (GHRP-2), TRH, and insulin infusion is a less toxic anabolic strategy through a putative inability to overstimulate the GH axis and a capacity to normalize thyroid hormone concentrations while foregoing excessive hyperglycemia. Burn-injured, parenterally fed, New Zealand White rabbits were randomized to receive 4-d treatment with saline (n=8); 60 microg/kg.h GHRP-2 and 60 microg/kg.h TRH, i.v. (n=9); or 3.5 mg/kg rhGH, s.c. (n=7). In the GHRP-2+TRH group, insulin was adjusted to maintain blood glucose below 180 mg/dl. Endocrine function and biochemical organ system function markers were studied. Animals were killed for assay of deiodinase activity in snap-frozen liver. Mortality, organ system function, hyperglycemia, and insulin requirement were equal in the three groups. GHRP-2+TRH increased pulsatile rabbit GH (rGH) and TSH release on d 1. After 4 d, rGH secretion and T4 and T3 concentrations were elevated, with a significant increase in hepatic activity of type 1 deiodinase and a decrease in type 3 deiodinase. Exogenous rhGH suppressed endogenous rGH secretion and increased IGF-I more than GHRP-2+TRH without altering thyroid hormone levels. Unlike GHRP-2+TRH, rhGH down-regulated liver type 3 deiodinase and did not affect type 1 deiodinase. We conclude that in experimentally induced critical illness, GHRP-2+TRH reactivated the GH and TSH axes and altered liver deiodinase activity, driving T4 to T3 conversion. In contrast to the human model, high dose rhGH was not rapidly lethal in this rabbit model. Whether this is explained by lack of rhGH-induced insulin resistance and hyperglycemia remains unclear.     

Localization and growth hormone (GH)-releasing activity of rat testicular GH-releasing hormone-like peptide

The testis contains many peptides originally described as originating in the central nervous system. The physiological function of these factors in the testis is generally unknown. We previously reported that the rat testis contains both a peptide with GH-releasing hormone-like immunoactivity (tGHRH-LI) and a mRNA species that cross-hybridizes with a hypothalamic cDNA for rat GHRH (rGHRH). The current study was designed to further characterize tGHRH-LI by determining its location within rat testis, and to evaluate whether tGHRH-LI and hypothalamic GHRH share similar biological and electrophoretic properties. Partially purified tGHRH is capable of stimulating GH secretion from cultured anterior pituitary cells in a dose-dependent manner. Testicular GHRH and rGHRH have different HPLC retention times and significantly different electrophoretic properties by Western gel analysis. The estimated size of tGHRH-LI is approximately 3.7 times that of synthetic rGHRH. Using immunohistochemistry, tGHRH-LI is localized to mature sperm forms in rat testis. We conclude that rat tGHRH-LI and rGHRH share some structural and functional properties and are probably related peptides. However, the difference in electrophoretic mobility and HPLC retention time indicates that they are not identical. The presence of tGHRH-LI in rat sperm, within the confines of the blood-testis barrier, which is generally impermeable to peptides, leads us to speculate that tGHRH serves a paracrine or autocrine role in testicular physiology.     

Growth hormone (GH) responses to the hexapeptide GH-releasing peptide and GH-releasing hormone (GHRH) in the cynomolgus macaque: evidence for non-GHRH-mediated responses.

GH-releasing peptide (GHRP; His-D-Trp-Ala-Trp-D-Phe-Lys-NH2), a hexapeptide derived from enkephalin, has been shown to have GH-releasing activity in man and several animal species. To characterize the GHRP dose-response curve and compare it with that of GH-releasing hormone [GHRH-(1-44)NH2], six unanesthetized young adult cynomolgus macaques were tested with a range of iv doses of GHRP or GHRH in random order. Animals were fitted with vests and tethers. Blood samples were obtained before and at 15-min intervals after the administration of drugs. Doses ranged from 0.03-3 mg/kg for GHRP and from 1-30 micrograms/kg for GHRH. The dose-response curves for the two peptides were not parallel. GHRP had lower potency, but evoked a much higher peak GH response than GHRH (greater than 55 vs. 12 micrograms/L). Because one of the proposed mechanisms of action of GHRP is the inhibition of somatostatin (SS), we tested the effects of propranolol, which inhibits SS, on the GH responses to GHRH and GHRP. Propranolol was given at a dose of 14 micrograms/kg, iv, 10 min before the injection of saline, GHRH (10 micrograms/kg), or GHRP (1 mg/kg). GH responses to propranolol alone did not differ from those to placebo (peak, 6 +/- 2 vs. 8 +/- 2 micrograms/L). However, propranolol pretreatment doubled the GH responses to both GHRH and GHRP compared with those to GHRH or GHRP alone 28 +/- 5 micrograms/L vs. 14 +/- 5 (P less than 0.05) and 54 +/- 2 vs. 25 +/- 6 micrograms/L (P less than 0.001), respectively]. These results show that GHRP causes a potent dose-dependent release of GH in this primate species. Since GHRP can produce a greater maximal GH response than GHRH, mechanisms other than release of endogenous GHRH must be involved.     

The negative GH auto-feedback in childhood: effects of rhGH and/or GHRH on the somatotroph response to GHRH or hexarelin, a peptidyl GH secretagogue, in children

Aim of the present study was to further clarify the negative GH auto-feedback mechanisms in childhood. To this goal we studied the effects of rhGH and/or GHRH administration on the GH response to GHRH or hexarelin (HEX), a peptidyl GH secretagogue, in normal short children. In 34 prepubertal children (12 girls and 22 boys, age 8.2- 14.2 yr) with normal short stature (normal height velocity and IGF-I levels) the following tests were performed: group A (no.=11): GHRH (GHRH 1 - 29, Geref, Serono; 1 microg/kg iv at 150 min) preceded by saline or GHRH at 0 min; group B (no.=6): GHRH preceded by saline or rhGH (0.005 IU/kg iv at 0 min); group C (no.=6): GHRH preceded by rhGH alone or combined with GHRH; group D (no.=6): HEX (2 microg/kg iv at 150 min) alone or preceded by rhGH. In group A, the GH response to GHRH was not modified by pre-treatment with GHRH (GH peak, mean+/-SEM: 16.7+/-2.9 vs 15.1+/-2.3 microg/l, respectively). In group B, the GH response to GHRH was clearly inhibited by rhGH (8.7+/-2.3 vs 38.8+/-4.5 microg/l, p<0.001); the GH rise after rhGH in group B overlapped with that after GHRH in group A. In group C, the GH response to GHRH after pre-treatment with rhGH (13.2+/-4.0 microg/l) was similar to that in group B and was not significantly modified by pre-treatment with rhGH+ GHRH (6.9+/-2.7 microg/l); the GH rise after rhGH+GHRH was higher (p<0.05) than that after rhGH alone. In group D, the GH response to HEX was significantly blunted by pre-treatment with rhGH (34.1+/-11.7 vs 51.2+/-17.9 microg/l, p<0.05). Our results demonstrate that in childhood the somatotroph response to GHRH is preserved after GHRH while it is inhibited after rhGH administration, which is also able to blunt the GH response to HEX. Thus, the somatostatin-mediated negative GH auto-feedback is already operative in childhood; the reason why the GHRH- induced GH rise is not inhibited by GHRH pre-treatment is unexplained.