Ruminants
There are tension receptors with vagal afferent fibres in the duodenum of the sheep that also respond to chemicals (Cottrell and Iggo, 1984). In view of the evidence of important roles for intestinal mechano- and chemoreceptors in the control of food intake in other classes of animal, it is likely that such receptors are also important in the ruminant. There are two different types of duodenal chemoreceptor: one is excited by potassium chloride solutions, the response increasing with the concentration of the salt (12.5–450.0 mmolar), while the other is insensitive to potassium chloride but excited by acetic, butyric or propionic acids (10–150 mmolar). The responses elicited were directly related to molecular weight but not to pH or osmolality; both were excited by sodium hydroxide solutions but not usually by sodium bicarbonate. Considerable quantities of potassium ions and VFAs leave the rumen and might, therefore, be expected to stimulate the abomasal and duodenal chemoreceptors and act in the negative feedback control of feeding.
Dynes (1993) studied the effect of duodenal osmolality on feeding in sheep by injecting 5 ml/kg of 6.5% NaCl or mannitol into the duodenum 5 min before feeding and without access to drinking water. There was almost complete inhibition of feeding in the first 15 min, an effect not alleviated by local anaesthetic. When water was made available 30 min after feeding had commenced, the sheep immediately drank a large volume and between 1.5 and 3 h food intake was significantly higher than when water was not available.
The amounts of salt, mannitol and anaesthetics were similar to those used by Houpt et al. (1983), who did find that anaesthetic reversed the intake depression by mannitol in pigs. It seems unlikely that there would be basic differences between the intestinal physiology of pigs and sheep, but no explanation has been proposed for these different experimental results.
In adult ruminants offered dry foods, osmolality can be up to 585 mosmol/kg in the duodenum, but not so high further along the intestines. The osmotic effect of digesta in the duodenum could be an important contributor to satiety under many circumstances, but little relevant research has been carried out with ruminants.
Although there have been suggestions on several occasions that flow through the intestines limits voluntary intake by ruminants, this is not a generally important factor. Suspensions of sawdust or methylcellulose infused into the abomasum at rates which doubled the volume of faecal production had no effect on voluntary food intake and it should be concluded, therefore, that intestinal capacity is not a limiting factor in feeding.
The role of gut hormonessuch as cholecystokinin is covered in Chapter 4.
therefore the stimulation of mechanoreceptors by local contractions so that, for example, loss of reticulo-ruminal motility occurs when epithelial receptors in the rumen become stimulated by high levels of VFAs. Lactic acid may facilitate the inhibitory effects of VFAs, as it has been reported that activation of reticulo- ruminal epithelial receptors by low levels of undissociated VFAs occurs after exposure of the luminal surface to dl-lactic acid. Thus, the intake-depressing effects of lactic acid on food intake may be indirectly related to the inhibition of reticulo-ruminal motility.
There is presumably a concentration gradient in digesta for such things as VFAs, with the highest levels in the centre of the rumen and lower levels at the ruminal wall, from where they are absorbed. Such a gradient is difficult to confirm experimentally in view of the contractions that move the tips of sampling tubes, and may be quite small in magnitude. However, the increase in frequency of ruminal contractions during feeding, speeding up the mixing of ruminal contents, may significantly increase the level of intake-depressing activity at the ruminal wall.
Leek (1977) points out that one stimulus excites several types of receptor and one type of receptor is excited by several types of stimulus. Thus, no single stimulus/receptor combination is likely to explain the effects of the physical and chemical properties of digesta in the gastrointestinal tract on motility and food intake. The experimenter measures fullness of a part of the tract as the volume of its contents, whereas to the CNS this may be:
a composite quality depending not on the absolute volume of the contents but on the rate of change of volume, the tonic state of the visceral muscle, the texture and chemistry of the contents and the extent to which the contents displace the viscus on its mesenteric attachment; such possibilities illustrate the difficulty of defining and quantifying a visceral stimulus.
(Leek, 1977) If these are the difficulties involved in understanding receptor involvement in signalling to the CNS, how much more difficult will it be to understand the control of voluntary food intake, in which many more factors must be taken into account?
The great complexity of the signals reaching the CNS from the digestive tract makes it necessary to aggregate the effects of different types of food according to measurable features such as fermentability, nitrogen content, fibre characteristics and energy yield in order to describe and predict their effects on voluntary food intake. Modelling of the convergence of such aggregated signals can then be attempted, for example by the theories discussed in Chapter 10.
Conclusions
There are stretch receptors in most or all parts of the gut that relay information on fill to the brain via the nervous system to inhibit feeding. Hypertonic solutions infused into the gut inhibit feeding but it is not certain whether there are osmoreceptors or whether water drawn in by the hypertonic solution
Feedbacks from the Gastrointestinal Tract 67
stimulates the stretch receptors. There is evidence for chemoreceptors in the duodenum, but the relative importance of distension, stretch or chemical effects probably varies depending on the type of food.
The capacity of the rumen to hold digesta clearly sets a limit to how much the ruminant can eat, but rarely is this the only factor controlling food intake.
Those receptors in the ruminal mucosa sensitive to chemical influences cannot be envisaged as ceasing to inform the CNS about pH, VFA concentration and osmolality when the mechanoreceptors are being strongly stimulated. In fact chemo- and mechanoreception are properties of the same neurones and it seems likely that chemical and stretch information are dealt with in an additive manner (see Chapter 10).
The osmolality of ruminal contents has an important influence on feeding when it is raised above the normal range, and this changes the interpretation of results from experiments in which the sodium salts of the VFAs were infused.
The situation is clouded by the difficulty of demonstrating receptors sensitive to osmolality, and new experimental approaches will have to be developed to resolve this difficult area.
We can rarely be sure of the exact mechanism(s) being brought into play when intake is affected by a change of diet or an experimental treatment. It is difficult to design experiments to isolate one factor at a time, but such experiments are necessary if progress is to be made in understanding more fully the control of food intake. It might be technically possible, for example, to manipulate the composition of ruminal fluid while maintaining normal conditions in the rest of the tract by preventing the flow of digesta from the rumen and infusing artificial digesta of normal composition into the abomasum.
However, a multiplicity of cannulae and tubes, even if ethically acceptable, will undoubtedly increase the risk of inappetence in the experimental animals.
4 Metabolites and Hormones
Digesta in the stomach(s) and intestines stimulate mechano- and chemoreceptors (Chapter 3), but it is unlikely that the total of the information from these receptors is sufficient for the CNS to gain a complete picture of the quantities of nutrients ingested in order to balance intake with output. The liver is the first point at which most of the absorbed nutrients can be monitored by a single organ but, even then, lipids are absorbed via the lymphatic system and bypass the liver. The general circulation transports nutrients between organs and is also the medium whereby hormones, secreted by endocrine organs, pass to their target tissues.
Many of the hormones have metabolic functions and have been implicated in the control of food intake (e.g. insulin, leptin), while others have primary roles in other bodily functions but influence intake secondarily (e.g. oestrogens).
Early theories of food intake control gave a prominent place to the monitor- ing of blood metabolite levels such as glucose, suggesting that they were sensed by the CNS. However, many of the functions of the body have evolved to protect the CNS from fluctuations in its supply of nutrients. A parallel can be made with the regulation of body temperature, where it appears that the temperature sensors in the periphery are more important than central receptors under normal conditions, so that the CNS is made aware of potential changes in deep body temperature and can set in motion actions to balance environmental changes before brain temperature itself has changed. So, with nutrient supply, we can envisage a more important role for peripheral receptors, with those in the CNS only being stimulated under severe conditions of nutrient shortage or excess. This will be explored further in Chapter 5, and it is sufficient here to say that blood concentrations of metabolites or hormones are unlikely to be the only factors taken into account by the CNS circuits controlling voluntary intake.
This is not to say that blood-borne nutrients and hormones cannot exert marked influences on feeding, as changes in their concentrations induce changes in the metabolism of tissues and organs. Satiated sheep began to eat soon after the start of blood exchange with hungry donors; the intake of hungry
© J.M. Forbes 2007.Voluntary Food Intake and Diet Selection 69
in Farm Animals 2nd Edition (J.M. Forbes)
sheep was reduced when blood was exchanged with satiated donors (Seoane et al., 1972).
The levels of many metabolites and hormones in the blood have been suggested to inform the brain of the animal’s metabolic state and to be involved in the short-term control of feeding. Of these, insulin and glucagon are the most likely candidates despite the attention that has been directed at other hormones, particularly cholecystokinin. In the longer-term control of intake and body weight, leptin is prime candidate as the link between adipose tissue and the CNS.