The physiology and biochemistry of isolated skeletal muscle mitochondria : a comparative study

Abstract

The physiological limit to maximum aerobic capacity ('v02max) in vertebrates has been attributed to cardiovascular oxygen delivery, to the ability of the muscle cells to consume oxygen, or to a fine-tuned development of all components of the respiratory system such that no single component can be shown to limit \702max. The above hypotheses have each been developed using different experiments with different animals. The comparative studies uniting these animals and methods are limited. In order to further our knowledge of the cellular limit to \;t02max, skeletal muscle mitochondria were isolated from species representing four vertebrate classes, and endothermic and ectothermic physiology.Mitochondrial '702 was measured at 15, 25 and 35°C and the results were compared between species and endothermic and ectothermic groups. Mitochondrial enzyme activities were measured at the three treatment temperatures to ascertain which enzyme activity best represents ~02max for all vertebrates. Cytochrome difference spectra were measured to determine the concentrations of mitochondrial cytochromes c+c1 . The results show that mitochondria are unique in all species tested. Each species has its own response to changing temperature and its own mitochondrial enzyme activity profile. In addition, in vitro measurements of mitochondrial 'V02 for all species show rates significantly higher than those estimated from whole organism measurements of 2 ~02max, suggesting that mitochondrial oxygen uptake is not a factor limiting organismal V02max. The 01 o for mitochondrial "02 differed significantly between groups, indicating that differences in ~02max between endotherms and ectotherms cannot be explained solely on the basis of temperature. The activation energy ( Ea) of mitochondrial ~02 was significantly higher in endotherms compared to ectotherms. Mitochondrial enzyme activities did not show the same 01 o and Ea differences as the intact mitochondria. Since enzyme activities were measured on mitochondria disrupted with either detergent or sonication, physical properties of the mitochondrial inner membrane are suggested as being responsible for these differences.