Intramuscular energy sources in dogs during physical work

Three groups of dogs were run under different experimental conditions characterized by varying the work load or the running time. Lipid and glycogen analyses were carried out on biopsy specimens from the biceps femoris muscle before and after exercise. In addition, arterial and venous triglycerides and free fatty acids were determined on plasma samples from one group of dogs that had been previously catheterized. Under the conditions of these experiments, results revealed: (1) plasma triglycerides did not contribute significantly to the energy supply for muscle contraction; (2) plasma free fatty acid efflux into muscle was increased during mild exercise but significantly lowered during heavy exercise; (3) exercise did not affect the phospholipid level or its composition in the muscle; and (4) muscle triglyceride levels may increase, decrease, or remain unchanged, depending upon the work load imposed by the exercise.