INTRACELLULAR OXYGEN DIFFUSION - THE ROLES OF MYOGLOBIN AND LIPID AT COLD BODY-TEMPERATURE

Cold temperature can constrain the rate of oxygen movement through mus cle cells of ectothermic animals because the kinetic energy of the sol vent-solute system decreases and the viscosity of the aqueous cytoplas m increases during cooling within the physiological range of body temp eratures. These factors affect the movement of both dissolved oxygen a nd oxymyoglobin, the two predominant routes of intracellular oxygen di ffusion in vertebrate oxidative muscles. In addition, reductions in te mperature have been shown to increase the affinity of myoglobin for ox ygen and to slow the rate of Mb O-2-dissociation, compromising the abi lity of this oxygen-binding protein to facilitate intracellular oxygen diffusion. Experiments with both seasonally cold-bodied fishes and po lar fish species suggest that several factors combine to overcome thes e limitations in delivery of oxygen from the blood to the mitochondria . First, reductions in body temperature induce increases in mitochondr ial density of oxidative muscle cells, reducing the mean diffusional p athlength for oxygen between capillaries and mitochondria, Second, col d body temperature in both temperate-zone and polar fishes is frequent ly correlated with a high content of neutral lipid in oxidative muscle s, providing an enhanced diffusional pathway for oxygen through the ti ssue. Third, recent data indicate that myoglobins from fish species bi nd and release oxygen more rapidly at cold temperature than do those f rom mammals. Data from both oxidative skeletal muscle and cardiac musc le of fishes suggest that these factors in various combinations contri bute to enhance the aerobically supported mechanical performance of th e tissues at cold cellular temperatures.