Energy Mobilization in Exercising Dogs

Physical work represents the most common stress to which the mammalian organism is exposed. In "fight or flight" situations it is imperative for survival that the rate of mobilization of energy sources should meet the elevated energy demand for as long as possible; otherwise the blood sugar falls and impairs the function of the central nervous system, leading to premature cessation of work. Most of the energy is supplied from three sources: (a) adipose tissue, (b) muscle glycogen, and (c) liver glycogen. Only this latter source is directly involved in producing glucose for the blood, and it is assisted in this function by gluconeogenesis from C3 units such as lactic acid, alanine, and glycerol. The simplest and probably the most reliable way to measure the rate of mobilization of blood-borne substrates is based on the dilution of radioactive tracers infused intravenously at a constant rate (preceded by a priming dose, if necessary). By using a mixture of [1-C]palmitate and [3-H]glucose, one can simultaneously measure the rate of mobilization (Ra) of both extramuscular fuels. Figure 1 shows such an experiment on a well-trained dog running on a treadmill (slope 15%, speed 100 m/min), with indwelling venous (jugular) and arterial (carotid) catheters. Based on measurements of O2 uptake at this work load, the average energy expenditure was 0.2 ± 0.007 kcal/kg/ min. For the sake of comparison, both rates were expressed in /u,mol/kg/min. Both hepatic glucose output and the rate of release of free fatty acids (FFA) considerably increase during exercise. There is, however, a marked difference in the course of their plasma concentrations. While the FFA level keeps rising during the run, plasma glucose remains relatively unchanged for a period of time, then slowly de-