Glucose and acetate metabolism in sheep at rest and during exercise

Development of a dynamic, mechanistic model of lamb metabolism and growth

A dynamic, mechanistic model of lamb metabolism and growth was developed for the purpose of evaluating hypotheses regarding the mechanisms of action of growth promotants. The model relates tissue growth to DNA accretion and protein turn-over. State variables include circulating amino acids, glucose, lipids and acetate; four protein pools (carcass, viscera, other tissues and wool) and storage triacylglycerol are also included. Equations are mainly of the Michaelis-Menten form, allowing for nutrient utilization patterns to be determined by relative tissue affinities for substrates (ko.5), enzymatic capacities (Vmax) and substrate concentrations ([S]). Protein degradation rates are defined as first-order with respect to protein. The model adequately simulated growth from 20 to 40 kg empty body weight. Simulated changes in nutrient input yielded reasonable energy balance response patterns, although theoretical growth efficiencies were greater than those observed in practice. Variations in volatile fatty acid absorption patterns were accommodated well, with predicted nitrogen retention closely approximating experimental observations. The model also responded appropriately to changes in dietary protein level, with body fat varying inversely with amino acid absorption. In summary, the model was found to perform adequately for the purpose of examining mechanisms responsible for alteration of growth and body composition.

Response of sympatho-adrenal axis and adrenal cortex to short-term restraint stress in sheep

The sympatho-adrenal and pituitary-adrenal cortex axes are the most sensitive, and specific indicators of stress in animals. Increased plasma levels of catecholamines and glucocorticoids are generally considered as the classical response to stress. Most experiments on immobilization have been performed on rats and only a few of them concerned domestic animals. In this experiment we want to learn whether short-term restraint-a stressor most commonly used in animal husbandry-is a stressor for sheep (ewes) like in rats. For this reason we measured adrenaline (A), noradrenaline (NA) (radioenzymatic method), cortisol (RIA method), glucose and free fatty acids (FFA). Unlike in rats, in stressed sheep the peak of A appeared earlier than the NA peak, i.e. at 2 and 5 min. of stress, respectively. In contrast to rats, the basal and stress levels of NA exceeded the corresponding level of A. Cortisol concentration rose 7 fold above baseline and maximal concentration appeared at a time (15-30 min.) observed in other animal species. A similar time-related increase was observed in the plasma FFA concentration. It increased maximally 3.2 fold at 15 min. of stress. A significant correlation coefficient was found between plasma cortisol and FFA (r = 0.91) what may suggest the lipolytic effect of ACTH and/or a positive feedback of FFA on the hypophysis-adrenal axis. The plasma glucose of stressed animals rose only 1.47 fold above the basal level. A significant correlation was found between cortisol and glucose (r = 0.53) whereas no correlations have been obtained between A, NA and glucose or FFA.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose partitioning in the pregnant ewe: effects of undernutrition and exercise

Maternal whole-body glucose entry rate and uterine and umbilical net uptakes of glucose and oxygen were measured in single-pregnant ewes which were either well-fed throughout, or fed at 0.3-0.4 predicted energy requirement for 7-21 d during late pregnancy. All ewes were studied while standing at rest and then while walking on a treadmill at 0.7 m/s on a 10 degrees slope for 60 min. Underfed ewes suffered significant decreases in live weight and had lower fetal, but not placental, weights at 140-144 d gestation. Undernutrition also caused large decreases in maternal glycaemia and glucose entry rate, which were associated with equally large decreases in uterine and umbilical net uptakes and O2 quotients of glucose, and with a decrease in placental glucose transfer capacity. Exercise caused increases in maternal blood concentration, entry rate and uterine net uptake of glucose, the magnitudes of which were not significantly affected by plane of nutrition. Umbilical glucose uptake and placental glucose transfer capacity increased during exercise in underfed but not fed ewes. The fractional distribution of maternal glucose to the pregnant uterus, and of uterine glucose uptake to the fetus, were unaltered by undernutrition; during exercise, a disproportionately small fraction of the increased maternal glucose supply went to the uterus. The results confirm that the ovine conceptus responds to nutritional reduction in maternal glucose availability in a manner similar to non-uterine maternal tissues. Major reductions in glucose supply appear to override putative glucose-sparing mechanisms which may operate to favour the conceptus in better-nourished animals.

Simulation of the metabolism of absorbed energy-yielding nutrients in young sheep

A mathematical model is described, which simulates the metabolism of absorbed nutrients (amino acids, acetic acid, butyric acid, glucose, lipid and propionic acid) in growing sheep. The basic assumption of the model is that each nutrient is partitioned between synthetic, oxidative or intermediate reactions with rates of reaction which are described using enzyme kinetics. These rates depend on the relationship between maximum reaction rates, constants of affinity and inhibition and the concentrations of metabolites as determined by the model. Synthetic reactions calculate fat and protein deposition while intermediate reactions involve the production of ATP and NADPH. There is a total of twelve state variables and the model, programmed in CSMP and ACSL, is solved by integration of twelve differential equations. The model calculates the efficiency of utilization of metabolizable energy for different nutrient inputs and the results may be interpreted in terms of fluxes through the metabolite pools. Simulations using inputs representing forage- and concentrate-based diets indicated decreased efficiency for the forage at high levels of intake and possible reasons for this were further studied in simulations where the inputs of protein and glucose were varied.