The stimulatory effect of the novel gastric-derived hormone, ghrelin, on growth hormone (GH) secretion has been reported in domestic animals as well as in humans and rats. The octanoyl modification on the Ser3 residue of ghrelin appears to be essential for its endocrine activity. A major portion of circulatory ghrelin lacks acylation but possesses some biological activities other than GH stimulation; therefore, both types of acylated and des-acyl ghrelin are supposed to be important for energy homeostasis. The effects of pharmacological doses of rat and/or human ghrelin on GH secretion have been reported recently in ruminants; however, the physiological effect of exogenous bovine ghrelin on its own plasma level and on GH secretion is still unknown. Moreover, the RIA systems for the measurement of bovine active ghrelin and for bovine total ghrelin including acylated ghrelin, des-acyl ghrelin and all ghrelin peptides with an intact bovine C-terminal have not yet been validated. In this study, we established the RIA system for bovine ghrelin, and the dose-dependent effects of synthesized acylated bovine ghrelin(1–27) on plasma active and total ghrelin, GH, insulin and metabolites were measured in Holstein heifers. Six animals were intravenously injected with synthesized acylated bovine ghrelin (0, 0.1, 0.5, 1.0, 5.0, 10.0 μg/kg body weight (BW)) and plasma hormone concentrations were measured from serially collected samples. Bovine ghrelin RIA showed that the basal level of total ghrelin is approximately 16 times higher than that of active ghrelin in bovine plasma. Both forms of ghrelin were increased in a dose-dependent manner in response to bovine ghrelin injections, peak values were reached at 5 min after administration and returned to pre-injected values within 15 min. Plasma GH was responsive to all doses of bovine ghrelin in a dose-dependent manner, peaked as early as at 5–10 min after injection and returned to the basal value within 60 min. The GH area under curve 1 h after injection of the smallest dose of ghrelin used in this experiment (0.1 μg/kg BW) was significantly higher than that of the vehicle (0.1% BSA saline)-injected control group (P<0.05). The GH response to the highest dose of ghrelin (10.0 μg/kg BW) was greater than the response to 5.0 μg/kg BW ghrelin (P<0.001). Plasma glucose concentrations were not significantly altered by the administration of bovine ghrelin while plasma insulin levels were transiently stimulated by the higher doses of ghrelin (1.0, 5.0, 10.0 μg/kg BW). Plasma non-esterified fatty acid levels also increased following ghrelin administration. Our study indicates that a considerable quantity of both acylated and des-acyl ghrelin is circulating in the bloodstream, and also confirms that ghrelin is not only a potent stimulator of GH secretion but also plays a considerable role in energy homeostasis in Holstein heifers.
Hnin ThidarMyint, Hiroko Yoshida, Tetsuya Ito, and Hideto Kuwayama
Tetsuya Ito, Hnin ThidarMyint, Takahiro Murata, Hiroaki Inoue, Roel Mata Neyra, and Hideto Kuwayama
These studies were designed to investigate the effects of i.v. administration of peptide YY3–36 (PYY3–36) on feed intake, acylghrelin, and GH levels in castrated male pigs. Feed intake levels were evaluated during both ad libitum and fast-refed conditions, and plasma hormone responses were evaluated during fasting. During ad libitum feeding, i.v. injection of PYY3–36 (30 μg/kg body weight, BW) significantly reduced feed intake levels within 3 h post-treatment. In the fast-refed condition, both single bolus injection (30 μg/kg BW) and i.v. infusion (0.25 μg/kg BW per min) of PYY3–36 suppressed feed intake levels 1 h post-treatment. Duration of the elevation of plasma PYY levels induced by i.v. injection of porcine PYY3–36 in ad libitum-fed pigs was longer compared with the values of fasted or fast-refed pigs. In the infusion study, the elevation of plasma PYY levels was maintained throughout the infusion period and values were reduced less than half at 15 min after termination of infusion. These results showed that the anorexigenic short-term effect of PYY3–36 treatment corresponds to its half-life. However, i.v. PYY3–36 injection did not influence plasma acyl-ghrelin levels. On the other hand, single bolus injection of PYY3–36 increased plasma GH levels 30 min after treatment. Similar to previous findings in other mammalian species, the results of these studies show that PYY3–36 can reduce feed intake levels; in particular, the effect is potent and acute in pigs. Furthermore, basal plasma PYY levels were higher in ad libitum-fed pigs than in fasted pigs suggesting that circulating PYY3–36 levels influence satiety and contribute to the termination of feed intake in pigs.
Sabina Börner, Michael Derno, Sandra Hacke, Ulrike Kautzsch, Christine Schäff, Sint ThanThan, Hideto Kuwayama, Harald M Hammon, Monika Röntgen, Rosemarie Weikard, Christa Kühn, Armin Tuchscherer, and Björn Kuhla
Ghrelin is a gastrointestinal peptide hormone that is present in blood mostly in a non-posttranslationally modified form, with a minor proportion acylated at Ser3. Both ghrelin forms were initially assigned a role in the control of food intake but there is accumulating evidence for their involvement in fat allocation and utilization. We investigated changes in the ghrelin system in dairy cows, exhibiting differences in body fat mobilization and fatty liver, from late pregnancy to early lactation. Sixteen dairy cows underwent liver biopsy and were retrospectively grouped based on high (H) or low (L) liver fat content post-partum. Both groups had a comparable feed intake in week −6 (before parturition) and week 2 (after parturition). Only before parturition was preprandial total ghrelin concentration higher in L than in H cows and only after parturition was the basal plasma concentration of non-esterified fatty acids higher in H than in L cows. Both before and after parturition, H cows had higher preprandial plasma concentrations of acyl ghrelin, a higher acyl:total ghrelin ratio, lower plasma triacylglyceride concentrations and a lower respiratory quotient compared with L cows. These group differences could not be attributed to an allelic variant of the acyl ghrelin receptor. Rather, the ratio of acyl:total ghrelin correlated with several aspects of fat metabolism and with respiratory quotient but not with feed intake. These results show that endogenous ghrelin forms are associated with fat allocation, fatty liver, and utilization of fat during the periparturient period.