ABDOMINAL CHEMOSENSITIVITY AND MECHANOSENSITIVITY IN RUMINANTS AND ITS ROLE IN THE CONTROL OF FOOD-INTAKE

Ruminant animals have evolved a large and complex set of stomachs which allow fermentation of fibrous food by symbiotic micro-organisms. These stomachs are well innervated and generate signals which are thought to be important in the control of voluntary food intake. Tension receptors in the muscular wall of the rumen and reticulum are slowly adapting and provide a measure of distension while epithelial receptors are rapidly adapting and provide information on the fibrousness of the digesta; they are involved in the control of stomach motility and voluntary food intake in order to prevent excessive distension. The epithelial receptors are also sensitive to the chemical nature of the digesta, particularly acidity. There are mechano- and chemoreceptors in the abomasum (true stomach) and duodenum and chemoreceptors in the liver, all of which have been implicated in the control of intake. It is relatively easy to prepare and maintain ruminants with a rumen fistula and many studies have shown the effects of such manipulations as distension of balloons in the rumen on voluntary intake. With fibrous, slowly digested feeds intake is primarily limited by rumen distension. With more rapidly digested feeds, however, the products of digestion play an important role in controlling intake. Short-chain fatty acids are the main products of fermentation and infusion of their salts into the rumen depresses food intake to a much greater extent than infusion into the general circulation. Acetate or propionate given into the rumen are more effective, mole for mole, than butyrate but must be given at rates exceeding the natural rate of production in order to have a significant effect. It has been suggested that much of the effect of sodium acetate is via the increase in the osmolality of rumen fluid but there is considerable uncertainty as to the physiological significance of osmotic effects, especially when animals have free access to water and can prevent excessive increases in tonicity by increasing their water intake. Other constituents of rumen fluid have been implicated in the control of food intake. particularly lactic acid and nitrogenous compounds, especially as these can be found in fermented feeds such as silage. The omasum controls the flow of digesta to the abomasum; it is therefore well placed to control rumen fill and thus intake but there has been little study in this area and this is also true for the abomasum. Mechano- and chemoreceptors are present in the walls of the duodenum and the rest of the intestinal tract but the role of these in the control of food intake has been little studied in ruminants. Propionate, produced by fermentation in the rumen, is absorbed and taken up from hepatic portal blood for gluconeogenesis in the liver. Some studies have shown that elevation of propionate in the portal vein by infusion depresses food intake and that this can be alleviated by section of some or all of the nerves innervating the liver. However, other studies have failed to show effects of propionate. By analogy with simple-stomached animals, there is probably a mechanism whereby oxidation of substrates in the liver affects the firing rate of liver afferent nerves. Interpretation of many studies is made difficult by interactions between several mechanisms. For example, reducing the pH of rumen contents, as by rapid production of short-chain fatty acids after a large meal of rapidly fermented food, inhibits rumen motility and thus emptying so that it is uncertain whether an observed reduction in subsequent food intake is due to physical or chemical stimuli. Many interpretations of effects of experimental manipulations on feeding have been based on the assumption that only one factor influences feeding, under a given set of circumstances. However, experiments in which two or more stimuli have been given show additive effects and it seems most likely that intake is in fact controlled by a multiplicity of factors, the decision as to whether to eat or not being taken on the total of signals reaching the central nervous system from many types of receptors in many parts of the body.