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Energy metabolism in rested, exercised and over-fed sheep

Harman, Nathan Gordon (1991) Energy metabolism in rested, exercised and over-fed sheep. PhD thesis, Murdoch University.

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The aim of this study was to investigate energy metabolism in over-fed and exercised sheep, with an emphasis on nutrient partitioning during both physiological states, and on factors affecting endurance during exercise. The methodology used was isotope dilution with simultaneous determination of arterio-venous differences and blood flow by sampling from chronically indwelling catheters. Blood flow to skeletal muscle was determined in separate experiments. Whole body metabolism was determined together with tissue flux rates for skeletal muscle, the portal-drained viscera (PDV) and the liver.

Over-fed sheep were fed a diet of lucerne pellets and lupin seed for up to 21 days. This diet, which supplied 2.3 times maintenance metabolizeable energy at the maximal intake, resulted in a pattern of nutrient excess characterized by high production and utilization of volatile fatty acids (VFAs) and glucose and by lower contributions by other substrates to whole body and tissue metabolism. There was a 73% increase in acetate production by the PDV, a high whole body entry rate of acetate (225 mmol/h), a 3.1-fold increase in the rate of glucose irreversible loss (GIL) and increases in the oxygen consumption by the PDV, liver and skeletal muscle. The increase in GIL was largely unseen by the extra-splanchnic tissues because of utilization by the PDV. While utilizing more glucose, the net production of carbon by the PDV doubled. Recycling of glucose as determined by lactate, pyruvate or alanine release from skeletal muscle was reduced during over-feeding and the direct oxidation of glucose in skeletal muscle increased from 18% to 57%. The acetate entry rate could not be accounted for by utilization of this substrate by muscle, the PDV and the liver, indicating probable increased utilization by adipose tissue. Finally, the concentration of non-esterified fatty acids (NEFA) and ketone bodies in arterial blood were low indicating a low turnover.

The two levels of exercise used were described as being of a moderate (4.5 km/h, 0° incline; 3.2 W/kg) and a high (4.5 km/h, 9° incline; 10.6 W/kg) intensity. During exercise there was an increased NEFA entry rate and GIL, the magnitude of both increases being dependent upon the level of exercise. Hepatic glycogenolysis and gluconeogenesis were potential significant contributors to the GIL as hepatic glucose production amounted to 65 - 96% of the GIL. The potential production of glucose from gluconeogenic substrates by the liver was doubled and this could have totally sustained hepatic glucose production during moderate exercise and sustained about 60% of glucose production during high intensity exercise. Glycogenolysis could have supplied from 58 - 100% of the hepatic glucose production during moderate exercise and up to 90% during high intensity exercise.

There were substantial increases in both the concentration of lactate in arterial blood and the lactate to pyruvate ratio during high intensity exercise, this level of exercise being above the anaerobic threshold, and in contrast to moderate exercise. Potential changes in the hormonal milieu between rest and exercise and between the two levels of exercise were discussed.

During exercise there was little ketogenesis in comparison to the large increases in both the NEFA entry rate and NEFA utilization by the liver, probably because of inactivation of carnitine acyl transferase I. However, during high intensity exercise the production of acetate from the liver was substantial.

Utilization of oxygen by skeletal muscle which was increased 4- and 8-fold during moderate and high intensity exercise respectively, was largely a reflection of changes in blood flow to this tissue and was obviously a major contributor to the 2.4- and 4.2-fold increase in the whole body production of carbon dioxide. The most important fuels for skeletal muscle during exercise were NEFA and glucose.

The utilization of NEFA was increased 5- to 15-fold and the utilization of glucose increased 2.2- to 4.4-fold that seen in rested animals, the magnitude being dependent upon the work rate. The utilization of both substrates by skeletal muscle during exercise was dependent upon their supply. A large proportion (30-50%) of the glucose utilized by skeletal muscle during exercise was directly oxidized.

The flow of blood to the PDV and to the liver was decreased by approximately 30% during exercise. However, the rates of net utilization or net production of many substrates by these tissues including oxygen, VFAs, alanine, pyruvate and lactate were maintained by increases in the arterio-venous differences.

Sheep were able to sustain exercise at a moderate level for long periods with little compromise. At higher levels of exercise, however, while hepatic glycogen depletion did occur the main contribution to fatigue appeared to be hyperthermia and/or an alkalosis, probably related to temperature regulation.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary Studies
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Pethick, David
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