Studies on novel growth hormone (GH) secretory mechanisms by ghrelin in ruminants

点击翻译按钮获取中文摘要

Ghrelin, a novel endogenous growth hormone (GH) secretagogue, has been shown to exert very potent and specific GH-releasing activity in rats and humans. However, little is known about its GH-releasing activity and endocrine effects in domestic animals. To clarify the effects of ghrelin on GH secretion in ruminants, GH-releasing effects of ghrelin were examined in vitro and in vivo in goats and cattle. GH-releasing effects of human ghrelin (hGhrelin) and rat ghrelin (rGhrelin) on bovine anterior pituitary (AP) cells were compared with those of GH-releasing hormone (GHRH) in vitro. hGhrelin and rGhrelin significantly stimulated GH release from the cultured cells at doses from 10^<-10> to 10^<-7> M and from 10^<-9> to 10^<-7> M, respectively (P<0.05). However, GH-releasing effects of ghrelin were weaker than that of GHRH. When hGhrelin (10^<-8>M) and GHRH (10^<-8>M) were added together, the release of GH induced by both peptides was significantly greater than that by hGhrelin alone (P<0.0 … More 5), and tended to be greater than that by GHRH alone. Somatostatin (SS, 10^<-7> M) significantly blunted GH release induced by hGhrelin (10^<-8>M) and GHRH (10^<-8>M) (P<0.05). In the presence of SS, additive effects of hGhrelin and GHRH on the release of GH were observed (P<0.05). These in vitro results in cattle were similar to those in pigs. To clarify the effect of ghrelin on GH secretion in vivo in ruminants, plasma GH responses to intra-arterial and intra-hypothalamic injections of rGhrelin were examined in goats and cattle. The infra-arterial injection of 3 μg/kg BW of rGhrelin in ovariectomized goats significantly increased plasma GH concentrations (P<0.05). However, the secretory response to 3 μg/kg BW of rGhrelin was weaker than that of GHRH (0.25μg/kg BW). An infusion of 10 nmol of ghrelin into the medial basal hypothalamus (arcuate nucleus) significantly stimulated the release of GH in male calves (P<0.05). These results show that ghrelin is one of potent physiological GH secretagogues in ruminants. Less