Hormonal control of glucose homoeostasis in ruminants

Little, if any, glucose is absorbed from the alimentary tract of the grazing ruminant and it appears that significant absorption of glucose only occurs in ruminants consuming relatively large amounts of grain (Bergman, 1973 ;Lindsay, 1978).In spite of this, ruminants have an absolute requirement for glucose which is similar to that of nonruminants.Certainly glucose is an essential metabolite for the brain as there is no oxidation of ketones in the brain of the ruminant (Lindsay, 1980).Moreover, glucose is required for turnover and synthesis of fat, as a precursor of muscle glycogen and in pregnant and lactating ruminants, glucose is an essential metabolite.Indeed the glucose requirements of late-pregnant and lactating ruminants increase dramatically beyond that required for maintenance (Bergman, 1973, see also Table 2It is not surprising that volatile fatty acids derived from rumen fermentation of carbohydrate provide some 70% of the energy requirements of the ruminant (Bergman, 1973).Even so, gluconeogenesis and the maintenance of glucose homoeostasis are critical processes in view of the absolute requirements for Unlike the situation in monogastric species, gluconeogenesis is maximal after feed ingestion and decreases during food restriction.The major factor influencing the rate of gluconeogenesis is the availability of substrates (Lindsay, 1978).In fed ruminants the principal precursors are propionate and amino acids, with lactate and glycerol making minor contributions to glucose production.During fasting, propionate supply is reduced to negligible quantities such that glycerol and amino acids from adipose and muscle tissue respectively are major precursors of glucose (Bergman, 1973).Some 8 5 3 0 % of glucose production occurs in the liver and the remainder is synthesized in the kidney (Bergman, 1973 ;Lindsay, 1978).Ruminants are able to spare glucose by utilizing alternative substrates for oxidative purposes.Under normal conditions, acetate is an important oxidative fuel (Lindsay, 1959) and the contribution of acetate to oxidative metabolism increases markedly under conditions where energy requirements increase (Bird et al. 1981).Similarly, glucose sparing may be effected by utilizing free fatty acids (FFA) as oxidative substrates when energy requirements increase (Bird et al. 1981 ; Pethick & Lindsay, 1981).In considering the control of glucose homoeostasis in the ruminant it is necessary to consider the control of glucose production and its utilization in the body as well as the control of the supply of alternative energy substrates.During the last decade or so an increasing body of information on the means by which glucose.