A transient ghrelin surge occurs just before feeding in a scheduled meal-fed sheep

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Abstract

Ghrelin, a gastric-derived peptide, has recently been identified as an endogenous natural ligand for the growth hormone (GH) secretagogue receptor. However, secretory characteristics of ghrelin are still obscure in ruminants. To investigate the diurnal rhythm in ghrelin secretion and its relationship to GH secretion, plasma ghrelin and GH concentrations were determined in Suffolk rams fed with a roughage diet once daily (Experiment 1). Abrupt increases (P<0.05) in plasma ghrelin occurred just before a meal-feeding compared with that at 1 h before feeding, then rapidly fell with a minimum during the feeding. A pulsatile surge (P<0.01) in plasma GH concentrations, which seemed to follow a single surge in plasma ghrelin, was observed during the feeding. In Experiment 2, plasma ghrelin and GH were determined in sheep subjected to a pseudo-feeding of 2 h to determine whether feed ingestion itself influences ghrelin and GH secretions. Compared with those at 1 h before feeding, a tendency of increases (P<0.1) in plasma ghrelin and significant increases (P<0.05) in GH occurred just before and during a pseudo-feeding, respectively. Plasma ghrelin temporally declined within 1 h after the start of the pseudo-feeding, and increased again and maintained higher levels during the last period of the pseudo-feeding. These results suggest that the transient surge of ghrelin secretion just before a scheduled meal feeding would not be due to the ingestion of feed, and that a pulsatile increase in plasma GH during the actual- or pseudo-feeding could be induced by the transient ghrelin surge.

Section snippets

Animals and diets

Experiment 1. Four Suffolk rams, aged 1–2 years, weighing approximately 51.3±0.5 kg (mean ± SE) were used. They were fitted with a jugular catheter on the left jugular vein at least one day before the experimental day as described previously [17]. Animals were kept in metabolism crates in a controlled environment chamber at 20 °C under a 12-h light-dark cycle (07:30–19:30, light; 19:30–07:30, dark). The rams were fed a sufficient quantity of alfalfa hay cubes to meet 120–125% of their daily

TR-FIA of ghrelin and GH

The standard curves of rat ghrelin and ovine GH are shown in Fig. 1. The competitive TR-FIA of ghrelin had a standard curve ranging from 0.01 to 10 ng/mL with a sensitivity of 0.025 ng/mL. The intra- and interassay variations of ghrelin were 6.9% and 5.5%, respectively. The median inhibitory concentration (IC50) of ghrelin in TR-FIA system was 0.831 ng/mL. Recovery of ghrelin from ovine plasma was 97.6%. Addition of ovine plasma resulted in displacement of the ghrelin tracer in a dose–response

Experiment 1: Diurnal rhythm of plasma ghrelin and GH concentrations in a scheduled meal-fed sheep

Changes in plasma ghrelin and GH concentrations in sheep fed a diet once daily are presented in Fig. 2. In this experiment blood samples were obtained from animals at 10 min intervals for 3 h period from 1 h before feeding (08:00) to 2 h after feeding (11:00), and at 2 h intervals for 21 h period from 11:00 to 08:00 on the second day, respectively. Ghrelin levels were relatively high 1 h before feeding on the first day. They rose from this time to a peak just before feeding, and then declined to a

Experiment 2: Changes in plasma ghrelin and GH concentrations in the pseudo-feeding sheep

Changes in plasma ghrelin and GH concentrations during the 3 h period of 08:00–11:00 in fed- and pseudo-feeding sheep are presented in Fig. 3. In fed-sheep, plasma ghrelin concentrations increased just before feeding, then decreased during feeding in the same manner as described in Experiment 1 (Fig. 3A). In the pseudo-fed sheep, plasma ghrelin levels just before pseudo-feeding tended to increase (P<0.1) comparing to values at 1 h before pseudo-feeding as seen in the fed sheep and reached the

Discussion

Sheep used in the experiment were trained to the feeding schedule employed, it was noticed, (a) a transient surge of plasma ghrelin occurred in the pre-feeding period, (b) the single surge of ghrelin could increase plasma GH during feeding in scheduled-meal-fed sheep.

In Experiment 1, drastic increases in plasma ghrelin occurred just before feeding in a scheduled-meal feeding. Such a drastic change in blood ghrelin has not been reported. In addition, the plasma ghrelin changed from the maximum

Acknowledgements

We acknowledge the NIDDK's National Hormone and Pituitary Program, and Dr. A.F. Parlow for the ovine GH radioimmunoassay kit.

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