The results of a comparative study of the respiration rates of mitochondria in saponin-skinned rat cardiac fibers (SF) and in fibers treated with saponin and collagenase (SCF) suggest that only about half of the whole population of mitochondria manifest their activity in SF, in contrast to SCF, in response to extracellular substrates of oxidative phosphorylation. The apparent Km value for ADP with succinate as substrate, which was as high as 330±32 μM in SF in SF at 20 °C, decreased about 2-fold in SCF at the same temperature and in SF at 37 °C, and decreased further to 67±8 μM in SCF at 37 °C. Thus, weakening or breaking of cellular contacts by collagenase and the temperature-dependence of diffusion of substrates such as ADP, seem to be important factors that determine the respiratory activity and regulatory parameters of mitochondria in saponin-permeabilized cardiomyocytes.

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