DESIGN OF THE OXYGEN AND SUBSTRATE PATHWAYS .6. STRUCTURAL BASIS OF INTRACELLULAR SUBSTRATE SUPPLY TO MITOCHONDRIA IN MUSCLE-CELLS

This paper quantifies the structures involved in the transport and oxi dation of carbohydrates and fatty acids within the muscle cell, The st ructural capacity is measured on whole-body random samples of the musc ulature of dogs and pygmy goats and compared with maximal rates of oxy gen consumption and substrate oxidation, Comparing dogs and goats of t he same body size provided a 1.55-fold difference in the maximal rate of oxidation when related to muscle mass. As in previous studies, we f ound that the volume of mitochondria was approximately proportional to aerobic capacity, The maximal glucose flux from intracellular stores to mitochondria is 1.6 times greater in the dog than in the goat; we f ind that the amount of glycogen stored in the muscle cells is 4.2 time s as great in the dog, but part of the intracellular glycogen pool is used for anaerobic rather than for oxidative metabolism, The maximal f atty acid flux from intracellular stores to mitochondria is 1.5 times larger in the dog, and the amount of lipid stored is 2.3 times as grea t in the dog, Every lipid droplet is in direct contact with the outer membrane of a mitochondrion and the contact surface area is 3.6 times greater in the dog than in the goat, Additional measurements are neede d to investigate the role of structural limitation at this step, The a mount of substrates stored intracellularly in the muscle cells of the dog is about twice as much as would match the differences in the maxim al rates of utilization, This allows the endurance-specialized dogs to run for longer periods at higher rates of oxidation.