Effect of D-Ala-D-beta Nal-Ala-Trp-D-Phe-Lys-NH2 (KP-102) on GH secretion in urethan-anesthetized rats

GHRP-2, a GHS-R agonist, directly acts on myocytes to attenuate the dexamethasone-induced expressions of muscle-specific ubiquitin ligases, Atrogin-1 and MuRF1

Recent reports suggest that Atrogin-1 and MuRF1, E3 ubiquitin ligases, play a pivotal role in muscle atrophy. In the present study, effect of Growth Hormone Releasing Peptide-2 (GHRP-2), a GH secretagogue receptor (GHS-R) agonist, on the expressions of Atrogin-1 and MuRF1 in vivo rat muscles was examined. Dexamethasone administration increased Atrogin-1 mRNA level in rat soleus muscle. The increased mRNA level of Atrogin-1 was significantly attenuated by GHRP-2. In addition, GHRP-2 decreased MuRF1 mRNA level irrespective of the presence of dexamethasone. Although IGF-I is a well-known protective factor for muscle atrophy, GHRP-2 did not influence plasma IGF-I levels and IGF-I mRNA levels in muscles. To clarify a direct effect of GHRP-2, differentiated C2C12 myocytes were used. Ten micrometer dexamethasone increased both Atrogin-1 and MuRF1 mRNA levels in C2C12 cells. GHRP-2 attenuated dexamethasone-induced expression of them dose-dependently and decreased the basal level of MuRF1 mRNA. The suppressive effect on the expressions of Atrogin-1 and MuRF1 by GHRP-2 was blocked by [D-Lys(3)]-GHRP-6, a GHS-R1a blocker, suggesting the effect of GHRP-2 was mediated through GHS-R1a. Taken together, GHRP-2 directly attenuates Atrogin-1 and MuRF1 mRNA levels through ghrelin receptors in myocytes.

Insulin-like growth factor-I down-regulates ghrelin receptor (growth hormone secretagogue receptor) expression in the rat pituitary

The effects of insulin-like growth factor-I (IGF-I) on the ghrelin receptor [growth hormone secretagogue receptor (GHS-R)] gene expression and on the GH response to GHS in rat pituitary cell cultures were examined. Pituitary GHS-R mRNA levels were decreased in a dose (0.01-10 nM)- and time (4-12 h)-dependent manner by IGF-I as measured with reverse transcriptase (RT)-PCR. The basal GH secretion was not influenced by the pretreatment with IGF-I (1 nM for 8 h); however, the GH response to the receptor ligand, a synthetic GHS, KP-102 (100 nM, 15 min), was significantly reduced by pretreatment with IGF-I. Thus, the present studies indicate that IGF-I could inhibit GH secretion at least in part by regulating the expression of the GHS-R.

Biological, physiological, pathophysiological, and pharmacological aspects of ghrelin

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

Effects of the administration of growth hormone-releasing peptide-2 (GHRP-2) orally by gavage and in feed on growth hormone release in swine

The experiments were conducted to determine the effects of the administration of growth hormone-releasing peptide-2 (GHRP-2, also named KP102), both orally by gavage and in feed, on the release of growth hormone (GH) in swine and to investigate whether attenuation of the GH response occurs after short-term treatment with the peptide in feed. In the first experiment, saline or GHRP-2 at doses of 1, 4.5 and 9 mg/kg body weight (BW) was dissolved in 15 ml saline and administered orally as a bolus by gavage to cross-bred castrated male swine (n = 6). Orally administered GHRP-2 stimulated dose-related increases in peak concentrations of GH, with a return to basal by 120 min. After administering GHRP-2 orally, peak concentrations of GH and areas under the GH response curves (GH AUCs) for 180 min were higher (P < 0.05) than those in saline controls. In Experiment 2, GHRP-2 at doses of 0 (served as control), 1, 4.5 and 9 mg/kg BW was mixed in 150 g of feed and offered to cross-bred castrated male swine (n = 6) at 0900 hr and 1700 hr daily for a 3-d period. Administration of 1 mg/kg BW GHRP-2 to swine in feed failed to stimulate the release of GH, but GHRP-2 at doses of 4.5 and 9 mg/kg BW significantly (P < 0.05) increased plasma concentrations of GH after initial and final treatments at 0900 hr on Days 1 and 3 of treatment, respectively. Peak concentrations of GH and GH AUCs for 180 min after the initial and final treatments in the 4.5 and 9 mg/kg BW GHRP-2-treated swine were higher (P < 0.05) than those in controls. After 3 d of treatment with GHRP-2 in feed at doses of 4.5 and 9 mg/kg BW, GH responses to the peptide were maintained. The results of the present study indicate that the administration of GHRP-2 orally by gavage and in feed stimulates the release of GH in swine, and that the GH-releasing effect of the peptide does not become desensitized after short-term administration in feed.

The effects of growth hormone-releasing peptide-2 (GHRP-2) on the release of growth hormone and growth performance in swine

The effects of GHRP-2 (also named KP102), a new growth hormone-releasing peptide, on the release of growth hormone (GH) and growth performance were examined in swine. The single intravenous (i. v.) injection of GHRP-2 at doses of 2, 10, 30 and 100 microg/kg body weight (BW) to cross-bred castrated male swine stimulated GH release in a dose-dependent manner, with a return to the baseline by 120 min. The peak GH concentrations and GH areas under the response curves (GH AUCs) for 180 min after the injections of GHRP-2 were higher (P < 0.05) than those after the injection of saline. The GH responses to repeated i.v. injections of GHRP-2 (30 microg/kg BW) at 2-h intervals for 6 h were decreased after each injection. The chronic subcutaneous (s.c.) administration of GHRP-2 (30 microg/kg BW) once daily for 30 days consistently stimulated GH release. The GH AUCs for 300 min after the injections on d 1, 10 and 30 of treatment in GHRP-2-treated swine were higher than those in saline-treated swine. However, chronic administration of GHRP-2 caused a partial attenuation of GH response between d 1 and 10 of treatment. The chronic s.c. administration of GHRP-2 also increased average daily gain for the entire treatment period by 22.35% (P < 0.05) and feed efficiency (feed/gain) by 20.64% (P < 0.01) over the saline control values, but did not significantly affect daily feed intake. These results indicate that GHRP-2 stimulates GH release and enhancing growth performance in swine.

Effects of the synthesized growth hormone releasing peptide, KP-102, on growth hormone release in sodium glutamate monohydrate-treated low growth rats

KP-102 (D-Ala-D-beta-Nal-Ala-Trp-D-Phe-Lys-NH2), a new second generation hexapeptide, has a potent growth hormone (GH)-releasing action in vivo and in vitro. Here, we evaluated the GH-releasing action of KP-102 under pentobarbital (PB) anesthesia in neonatally sodium-glutamate-monohydrate-treated low growth (NMSG-LG) rats. The plasma GH level in NMSG-LG rats after i.v. administration of KP-102 at 100 micrograms/kg was 1/6.7 (95% C.L. 1/14.7 - 1/3.0) of that in normal rats given the same dose (p < 0.01). However, the increase was significant compared with that in normal rats after saline administration (p < 0.01). The plasma GH releasing action of KP-102 at 100 micrograms/kg i.v. in rats with lesions in the bilateral hypothalamic arcuate nuclei (ARC), was about 1/6.3 (95% C.L. 1/12.4 - 1/3.2) of that in normal rats under PB anesthesia (p < 0.01). When KP-102 was injected into the ARC at doses of 0.0002, 0.02 and 2 micrograms/rat, GH release was dose-related (p < 0.01) under PB anesthesia. KP-102 at 2 micrograms i.c.v. also increased the plasma GH levels (p < 0.01) to about 1/8.3 (95% C.L. 1/22.7 - 1/3.1) of that by systematic administration, at the same potency as the ARC injection (1/13.7 and 95% C.L. 1/37.2 - 1/5.0). These findings suggest that KP-102 potently stimulates the GH release by a direct or indirect antagonism of somatostatin (SRIF) and growth hormone releasing hormone (GHRH) release in the hypothalamus and by a direct action on the pituitary. Furthermore, the GH-releasing action of KP-102 was similar and additive upon both regions in vivo at the maximum effective dose. Moreover, since the GH-release in response to KP-102 administration differed between NMSG-LG and normal rats, and since KP-102 increased the GH release even in NMSG-LG rats, it should be evaluated in the hypophysial GH secretion tests, and may be used to treat the hypophysial GH secretion insufficiency.