Effects of intense exercise on intracellular glycogen and fat in pigeon pectoralis

The pigeon pectoralis muscle was electrically stimulated in vivo at a high intensity of 25 V and the intracellular glycogen and fat reserves of the muscle examined histochemically to determine the pattern of substrate utilization in the component broad white and narrow red fibre types during the intense flight-simulated exercise. 4 h of continuous stimulation resulted in the depletion of intracellular glycogen from both fibre types and a simultaneous reduction in the intracellular fat stores of the red fibres. While glycogen provides the only source of energy for the white fibres, glycogen as well as fat serves as fuel for the red fibres. It is postulated that, though fat is the major fuel for the red fibres, their glycogen reserves are utilized to ensure a steady supply of oxaloacetate essential in the oxidation of fatty acids via the Krebs cycle, and that fatty acid catabolism helps to maintain gluconeogenesis.