Cold temperature can constrain the rate of oxygen movement through muscle cells of ectothermic animals because the kinetic energy of the solvent-solute system decreases and the viscosity of the aqueous cytoplasm increases during cooling within the physiological range of body temperatures. These factors affect the movement of both dissolved oxygen and oxymyoglobin, the two predominant routes of intracellular oxygen diffusion in vertebrate oxidative muscles. In addition, reductions in temperature have been shown to increase the affinity of myoglobin for oxygen and to slow the rate of Mb O2-dissociation, compromising the ability of this oxygen-binding protein to facilitate intracellular oxygen diffusion. Experiments with both seasonally cold-bodied fishes and polar fish species suggest that several factors combine to overcome these limitations in delivery of oxygen from the blood to the mitochondria. First, reductions in body temperature induce increases in mitochondrial density of oxidative muscle cells, reducing the mean diffusional pathlength for oxygen between capillaries and mitochondria. Second, cold body temperature in both temperate-zone and polar fishes is frequently correlated with a high content of neutral lipid in oxidative muscles, providing an enhanced diffusional pathway for oxygen through the tissue. Third, recent data indicate that myoglobins from fish species bind and release oxygen more rapidly at cold temperature than do those from mammals. Data from both oxidative skeletal muscle and cardiac muscle of fishes suggest that these factors in various combinations contribute to enhance the aerobically supported mechanical performance of the tissues at cold cellular temperatures.

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