No abstract

The effects of feeding and intravenous injections of glucose and VFA on blood ghrelin concentrations were investigated in calves before and after weaning. Eight Holstein bull calves were fed whole milk, allowed free access to solid feeds, and weaned at 7 wk. Measurements were carried out at 2, 4, 6, 9, 11, and 13 wk, at which time jugular blood samples were taken from 4 calves through a catheter from 10 min before to 120 min after their morning feed at 10 min intervals (Exp. 1). An additional 4 calves of the same age were intravenously injected with glucose (1.0 mmol·kg BW(-1)) and a solution of VFA (2.4 mmol·kg BW(-1), acetate:propionate:butyrate in a 6:3:1 ratio) using a catheter, and jugular blood samples were taken temporally relative to the injection time (Exp. 2). In Exp. 1, blood ghrelin concentrations decreased (P < 0.05) after feeding at all ages. However, preprandial ghrelin concentrations were less (P = 0.025) and the degree of postprandial depression of ghrelin tended to be greater during the postweaning period (P = 0.084) than during the preweaning period. Blood glucose concentrations increased after feeding during the preweaning period (P < 0.05), whereas blood acetate concentrations increased during the postweaning period (P < 0.05). In Exp. 2, injection of VFA induced a greater decrease in blood ghrelin concentrations than glucose injection throughout the entire period (P < 0.05). These results indicate that weaning reduces the basal concentration of blood ghrelin because the circulating VFA derived from ruminal fermentation may more strongly depress blood ghrelin concentrations than glucose.

Ghrelin action, which stimulates growth hormone (GH) secretion, may alter during the weaning period in calves. Our objective was to compare the effects of intravenous ghrelin injection on plasma GH, insulin and glucose concentrations in calves around the weaning period. Four Holstein bull calves were fed whole milk and allowed free access to solid feeds, and weaned at 7 weeks of age. Measurements were performed at weeks 1, 2, 4, 6, 7, 9, 11 and 13, when calves were intravenously injected with ghrelin (1.0 μg/kg body weight (BW)) through a catheter, and jugular blood samples were obtained temporally relative to the injection time. Estimated digestible energy intake per metabolic BW transiently decreased at week 7 because of low solid intake immediately after weaning, and thereafter gradually increased. Plasma insulin and glucose concentrations were not affected by ghrelin injection at all ages. In contrast, plasma GH concentrations increased with ghrelin injection at all ages. The incremental area of GH at week 7 was greatest and significantly higher compared with weeks 2, 4, 6 and 9. This result suggests that nutrient insufficiency immediately after weaning enhances GH responsiveness to ghrelin.