Utilization of oxidizable substrates by the sheep hind limb: effects of starvation and exercise

Measurements of substrate uptake by the sheep hind limb show a pattern similar to human and other monogastric animals. Thus free fatty acids (FFA) are the principal substrates at rest and during exercise while beta-hydroxybutyrate and acetoacetate are major nutrients in starved animals. The hind limb has arteriovenous differences for glucose and lactate which indicate that glucose supplies about 27% of the fuel of respiration during exercise, but the hind limb in resting, fed, and starved animals returns essentially all of the glucose carbon to the blood in the form of lactate. This finding is consistent with a conservation of glucose in aminals which obtain very little dietary glucose. Although some acetate is extracted from the blood in fed sheep, the utilization of this nutrient can account for only 2% or less of the oxygen uptake in the hind limb of starved or exercising animals. Thus, while acetate is the major product of the sheep rumen it is not used directly as a major energy source. We propose that most of the actate is converted to FFA which can be stored as triglyceride or oxidized in muscle.