Control of cellular respiration in vivo by mitochondrial outer membrane and by creatine kinase. A new speculative hypothesis: possible involvement of mitochondrial-cytoskeleton interactions.
Résumé
The current problems of regulation of myocardial energy metabolism and oxidative phosphorylation in vivo are considered. With this purpose, retarded diffusion of ADP in cardiomyocytes was studied by analysis of elevated apparent Km for this substrate in regulation of respiration of saponin-skinned cardiac fibers, as compared to isolated mitochondria. Recently published data showing the importance of the outer mitochondrial membrane were compared with new experimental results on the proteolysis of skinned fibers and tissue homogenates. In both cases 10 min incubation and 0.125 mg/ml of trypsin resulted in a decrease of apparent Km for ADP from 297 +/- 35 and 228 +/- 16 to 109 +/- 2 and 36 +/- 16, respectively. Thus, the permeability of the outer mitochondrial membrane for ADP may be controlled by some unknown cytoplasmic protein(s), probably related to the cytoskeleton, which are separated from mitochondria during their isolation. The extent of expression of this protein(s) depends on the energy state and type of muscle. Activation of mitochondrial creatine kinase reaction coupled to oxidative phosphorylation overcomes the diffusion difficulties of ADP by amplifying the stimulatory effect of ADP on respiration. It is concluded that both cytoplasmic and mitochondrial creatine kinases, adenylate kinase and cytoplasmic factor controlling outer membrane permeability may participate in metabolic feedback regulation of respiration in muscle cells.