Regulation of feed intake in sheep : the role of hormones and metabolites

In the search for factors involved in the regulation of feed intake, many experiments have been performed in various species. In ruminants, very little is known about the physiological background of the mechanisms involved in feed intake regulation. In earlier experiments, much attention was paid to physical regulation suggesting that the capacity of the digestive tract is the most important limiting factor in feeding.Since ruminants are capable of meeting their energy requirements under a wide range of circumstances and feedstuffs, the concept of physiological regulation was introduced. Physiological regulation (or metabolic regulation) can be defined as feed-back signals arising from sensors in the periphery, which inform the central nervous system about the metabolic status of the individual. In the brain, presumably in the hypothalamus, these signals are integrated and decisions are made whether or not to eat. This thesis focuses on blood-borne factors related to feed intake in sheep. This includes the major energy providing components, metabolic hormones and gastrointestinal hormones.The aim of this thesis is:<UL><LI>To gain insight in the changes in nutrient and hormone concentrations following meals of different feed qualities.<LI>To investigate blood borne nutrients/hormones which may be involved in the regulation of feed intake.</UL>To study this, experiments were performed in wether sheep provided with mesenteric, portal and jugular catheters.To address the effect of feeding and feed quality on nutrients and hormones, animals were fed during 90 minutes. Before, during and after feeding, blood samples were withdrawn from jugular and portal catheters in order to identify candidates for intake regulation. Two experimental pelleted grass diets, qualified as High Quality (HQ) and Low Quality (LQ) based on crude protein and fibre contents were fed according to a cross-over design.In chapter 2, short-term effects of feed intake on jugular and portal concentrations of metabolites such as Volatile Fatty Acids (VFA), Beta-hydroxy-butyrate (BHB) and glucose were studied. Rapid changes were observed in both jugular and portal veins due to feeding. Portal vein (PV) concentrations of acetate, propionate, butyrate, iso-butyrate and BHB were increased post-prandially in HQ-fed sheep. Due to consumption of LQ feed, bi-phasic patterns were found in acetate, propionate, butyrate levels, measured in the jugular vein (JV).The effect of feeding on nutrient concentration may largely differ as a result of feed quality. The PV-JV difference was used to estimate the release of nutrients into the portal vein. Differences in peripheral concentration of a blood component did not necessarily result from a difference in the release of the component but could also have resulted from a changed uptake by peripheral tissues. In most cases, the observed early changes (until 30 minutes past meal start) were probably due to changes in uptake rather than alterations in release. The changes observed later then 30 minutes were likely due to changes in release.In chapter 3, the response of metabolic and gastrointestinal hormones to feeding is described. Rapid fluctuations were shown for insulin, glucagon, Pancreatic Polypeptide (PP) and Cholecystokinin (CCK) levels in HQ-fed sheep. Sustained changes were observed for insulin, glucagon and gastrin levels in HQ-fed sheep. LQ-fed sheep showed rapid alterations in Growth Hormone (GH), gastrin, PP and CCK levels. Sustained changes were observed for insulin, GH, gastrin, PP and CCK levels.The rapid changes in hormone concentration may be due to decreased para-sympathetic activity and/or increased sympathetic activity. More sustained changes were likely nutrient induced. Feed quality mainly affected the magnitude of the meal induced changes in hormone levels, with the HQ-fed sheep showing more pronounced differences.In chapter 4, the hypothesis that propionate is a short-term feed intake regulating agent is discussed. In the first experiment, sheep were infused over 20 min with Na-propionate into the mesenteric vein, while monitoring feed intake and feeding pattern over 1.5 hours. Feed intake was reduced by infusions at 2 mmol/min which were associated with marked increases in jugular as well as portal concentrations of insulin, glucose and propionate.In a second experiment, animals were infused with 2 mmol/min Na-propionate into the portal vein. No decrease in feed intake was observed, though similar increases in insulin, glucose and propionate as found in mesenteric vein infused animals were observed. It was concluded that mesenteric propionate in high doses acted as a satiety factor. Possible explanations for the difference between site of infusion may be a different distribution of the infusate over the liver, and/or the presence of propionate sensitive receptors in the mesenteric/portal vein region.In a more extensive experiment, described in chapter 5, sheep were infused via the mesenteric catheter with 0, 1.5 or 6 mmol/min Na-propionate for 20 minutes. Infusion of 6 mmol/min Na-propionate decreased feed intake but also induced discomfort. Portal levels of propionate, glucose and insulin were increased while decreased levels of butyrate, BHB, gastrin, PP and CCK were observed. Jugular levels generally showed similar patterns as portal levels except for butyrate. Jugular butyrate was immediately increased after start of the meal, presumably due to a smaller liver uptake.Infusion of 1.5 mmol/min Na-propionate resulted in elevated levels of propionate and insulin while gastrin and PP concentrations were decreased.It was concluded that propionate is not a major factor influencing intake, since infusion of a physiological dose did not affect meal size. It is possible that effects found during and after a meal on insulin, gastrin, and PP can be attributed to propionate.Since propionate levels, which affected intake were rather high, and VFA's are reported to stimulate release of insulin, an experiment was performed in which sheep received an infusion with insulin. This study described in chapter 6, was designed to investigate the effect of insulin infusion on feed intake, feeding pattern and blood concentrations of metabolites and hormones related to feeding and feed quality. During a 90 minutes feeding period, sheep provided with jugular, portal and mesenteric catheters were infused via the mesenteric catheter with 6.7 mU/min insulin or saline for 20 minutes. Blood was frequently sampled from jugular and portal veins. The study was performed on two diets differing in feed quality.Infusion of insulin did not decrease feed intake but decreased feeding time. Portal insulin levels of sheep receiving an insulin infusion were increased in animals fed a low quality diet but not in animals fed a high quality diet. Insulin levels in the jugular vein were not influenced by infusion of insulin compared to saline infusion. No differences due to infusion of insulin were shown on glucose, glucagon, gastrin, and PP levels. Effects of diet composition were reflected by glucagon levels but not by other hormones.It was concluded that insulin might be a factor involved in satiety, but not by regulation of meal size. It was also postulated that regulation of endogenous insulin release might be more sensible in animals fed a higher feed quality.In chapters 7 and 8, the results are presented of a combined infusion of CCK (two dosages) and Na-propionate. In chapter 7, the effect of the 20 minutes infusion with 0, 1.2 or 2.4 nmol/min CCK-8 is described. Infusion of CCK-8 increased levels of CCK-8 in the portal vein but not in the jugular vein. A very accurate clearance of CCK-8 from the liver may have attributed to the absence of increased jugular levels. Infusion of both 1.2 and 2.4 nmol/min CCK-8 decreased portal and jugular CCK-33 levels, suggesting a decreased endogenous release of CCK. Portal PP levels were decreased as a result of 2.4 nmol/min CCK-8 infusion. This may be due to a decrease in release or an enhanced portal blood flow.Cortisol concentrations, as an indicator of stress, were decreased during infusion of saline but increased as a result of CCK-8 infusion. It was concluded that CCK-8 might have induced some discomfort. Despite the increased portal CCK-8 levels and the increased cortisol levels no effect was found on feed intake.In chapter 8, the effect of an infusion with 0, or 2.4 nmol/min CCK-8 or 0.5 mmol/min propionate or a combination of CCK and propionate are described.Feed intake was only reduced by combined infusion of CCK-8 and propionate but not by separate infusion of CCK-8 or propionate. Increased levels of propionate and insulin were observed following the propionate infusion. Infusion of CCK decreased propionate, acetate, butyrate, and glucose levels while insulin levels were initially increased followed by decreased levels. Combined infusion of CCK and propionate induced similar blood concentration as infusion of CCK solely on acetate, butyrate, glucose, and insulin, while propionate levels were decreased compared to propionate infused animals but increased compared to CCK infused sheep.It was postulated that decreased levels of VFA's and insulin may be due to increased portal flow and that mechanisms of induction of insulin secretion by propionate and CCK may be different.In chapter 9, some of the observations made in the preceding chapters are analysed using a conceptual model. The model proved very suitable in explaining profiles of hormones and metabolites following feed intake. It also proved useful in interpreting the effects the infusion studies. One of the major observations was that release and uptake are usually very tightly matched even during infusion of large amounts of propionate. It also showed that changed plasma concentration may arise from changed uptake (in case of the early peak in jugular propionate following feeding) or changed release (in case of insulin increase following feeding).Finally, in the general discussion (chapter 10) some remarks are made concerning the possible role of hormones and metabolites in the regulation of feed intake.In conclusion, as a result of feeding, sheep showed both rapid and more sustained changes in plasma concentration of several metabolites and hormones. Furthermore, differences in feed quality may result in differences in hormone and metabolite concentration but also in differences in plasma profiles.The infusion studies with propionate, insulin and CCK indicated that the regulation of intake must be regarded as a multifactorial process. It is therefore necessary to study intake regulation as a multifactorial system bearing in mind that gross manipulations may influence other systems involved in intake regulation. Experiments with low (physiological) dosages of combinations of hormones and metabolites should be performed to elucidate the concerted role of these substances.