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1 April 2007 Effects of Homologous Ghrelins on the Growth Hormone/Insulin-like Growth Factor-I Axis in the Tilapia, Oreochromis mossambicus
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Abstract

Ghrelin is a gut-brain peptide synthesized mainly in the oxyntic mucosal cells of the stomach, and has potent growth hormone (GH)-releasing and orexigenic activities. Recently, two forms of ghrelin, ghrelin-C8 and -C10, were identified in the Mozambique tilapia (Oreochromis mossambicus). The present study describes in vitro and in vivo effects of these endogenous ghrelins on the GH/insulin-like growth factor-I (IGF-I) axis. Ghrelin-C8 (100 nM) stimulated GH release from primary cultures of pituitary cells after 4 and 8 h of incubation, whereas no effect was seen on prolactin (PRL) release. Stimulatory effects of ghrelin-C8 and -C10 (100 nM) on GH release during 6 h of incubation were blocked by pre-incubation with GHS receptor antagonist, [D-Lys3]-GHRP-6 (10 μM). Intraperitoneal injection of ghrelin-C8 (1 ng/g body weight) and -C10 (0.1 and 1 ng/g body weight) significantly increased plasma GH levels after 5 h. Significant increases were observed also in hepatic expression of IGF-I and GH receptor (GHR) mRNA following injections of both forms of ghrelin (0.1 and 1 ng/g body weight), although there was no effect on plasma levels of IGF-I. In the next experiment, both forms of ghrelin (1 ng/g body weight) significantly increased plasma IGF-I levels 10 h after the injection. No significant effect of either ghrelin was observed on plasma PRL levels. Both forms of GHS receptor (GHSR-1a and -1b) were found in the pituitary, clearly indicating that tilapia ghrelins stimulate primarily GH release through the GHS receptor. Stimulation of hepatic expression of IGF-I and GHR suggests metabolic roles of ghrelin in tilapia.

Bradley K. Fox, Larry G. Riley, Casey Dorough, Hiroyuki Kaiya, Tetsuya Hirano, and E. Gordon Grau "Effects of Homologous Ghrelins on the Growth Hormone/Insulin-like Growth Factor-I Axis in the Tilapia, Oreochromis mossambicus," Zoological Science 24(4), 391-400, (1 April 2007). https://doi.org/10.2108/zsj.24.391
Received: 22 April 2006; Accepted: 1 October 2006; Published: 1 April 2007
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