Intracellular energetic units in red muscle cells

Abstract

The kinetics of regulation of mitochondrial respiration by endogenous and exogenous ADP in muscle cells in situ was studied in skinned cardiac and skeletal muscle fibres. Endogenous ADP production was initiated by addition of MgATP; under these conditions the respiration rate and ADP concentration in the medium were dependent on the calcium concentration, and 70-80% of maximal rate of respiration was achieved at ADP concentration below 20 microM in the medium. In contrast, when exogenous ADP was added, maximal respiration rate was observed only at millimolar concentrations. An exogenous ADP-consuming system consisting of pyruvate kinase (PK; 20-40 units/ml) and phosphoenolpyruvate (PEP; 5 mM), totally suppressed respiration activated by exogenous ADP, but the respiration maintained by endogenous ADP was not suppressed by more than 20-40%. Creatine (20 mM) further activated respiration in the presence of ATP and PK+PEP. Short treatment with trypsin (50-500 nM for 5 min) decreased the apparent K(m) for exogenous ADP from 300-350 microM to 50-60 microM, increased inhibition of respiration by PK+PEP system up to 70-80%, with no changes in MgATPase activity and maximal respiration rates. Electron-microscopic observations showed detachment of mitochondria and disordering of the regular structure of the sarcomere after trypsin treatment. Two-dimensional electrophoresis revealed a group of at least seven low-molecular-mass proteins in cardiac skinned fibres which were very sensitive to trypsin and not present in glycolytic fibres, which have low apparent K(m) for exogenous ADP. It is concluded that, in oxidative muscle cells, mitochondria are incorporated into functional complexes ('intracellular energetic units') with adjacent ADP-producing systems in myofibrils and in sarcoplasmic reticulum, probably due to specific interaction with cytoskeletal elements responsible for mitochondrial distribution in the cell. It is suggested that these complexes represent the basic pattern of organization of muscle-cell energy metabolism.

Integrated and Organized Cellular Energetic Systems: Theories of Cell Energetics, Compartmentation, and Metabolic Channeling

“…Remarkably, in the heart the intracellular energy transfer networks are structurally organized in the intracellular medium, where macromolecules and organelles surrounding a regular mitochondrial lattice are involved in multiple structural and functional interactions [112][113][114]. Figure 3.5 summarizes the available information about such an organized and compartmentalized energy metabolism in cardiac cells.…”

“…Figure 3.5 summarizes the available information about such an organized and compartmentalized energy metabolism in cardiac cells. This scheme also illustrates the view that mitochondria in muscle cells are structurally organized into functional complexes with myofibrils and sarcoplasmic reticulum [112][113][114]. These complexes are called ''intracellular energetic units'' (ICEUs) and are taken to represent the basic pattern of organization of muscle energy metabolism [112].…”

“…This scheme also illustrates the view that mitochondria in muscle cells are structurally organized into functional complexes with myofibrils and sarcoplasmic reticulum [112][113][114]. These complexes are called ''intracellular energetic units'' (ICEUs) and are taken to represent the basic pattern of organization of muscle energy metabolism [112]. There are no physical barriers between ICEUs; each mitochondrion (or several adjacent mitochondria) can be taken to be in the center of its own ICEU.…”

“…Adenine nucleotides within the ICEU and bulk water phase may be connected by more rapidly diffusing metabolites such as Cr-PCr. (Reproduced from [112] with permission). myocytes have been shown for cAMP in the range of approximately 1 mm exhibiting high local concentrations [53,[116][117][118].…”

Architectural and functional remodeling of cardiac and skeletal muscle cells in mice lacking specific isoenzymes of creatine kinase

“…Mitochondria are organized into functional complexes (intracellular energetic complexes) with adjacent ATP-utilizing systems (Saks et al 2001). ADP is probably directly channeled from mitochondria to other ATPases in the context of reduced diffusion in the cytosol.…”