Mitochondrial Physiology Network 10.9: 45-46 (2005) - download pdf

Modulation of energy transfer between mitochondria and myofibrils by changes of the cardiac work.

Marco Vendelin, P Mateo, J Hoerter, J-L Mazet

U-446 INSERM, Faculté de Pharmacie, Université Paris-Sud, Châtenay-Malabry, France. - markov@ioc.ee

  In the heart, the energy supplied by the mitochondria to the myofibrils is  continuously adjusted to the contraction requirement for a wide range of cardiac loads. This process, finely tuned over a broad range of mechanical requirement, is mediated  by a complex specialized enzyme system. The contribution of creatine kinase (CK) shuttle in transferring the energy between mitochondria and myofibrils may vary with the changes in cardiac work. For example, we have observed that the export of ATP from  mitochondria to myofibrils is bypassing CK shuttle if ATP synthesis is partially inhibited [1,2].

    The aim of this study was to determine the dependence of the energy transfer pathways between mitochondria and myofibrils on cardiac workload. We studied the energy transfer pathways by analyzing the 31P-NMR magnetization transfer data using Bloch-McConnell equations [3]. The data was acquired on isovolumetric Langendorff  perfused hearts. The work was varied by changing the external calcium concentration [Ca] (from 0.5 mM to 4.0 mM) and/or by beta adrenergic stimulation. In our analysis, we computed the share of energy transfer through different pathways, consistent with the observed magnetization changes. For this, mathematical models were composed  taking into account the forward and reverse CK reactions, ATPase activity as well ATP synthesis in mitochondria. Several possible energy transfer pathways were considered with different levels of intracellular compartmentation. The simplest model considered in our study was the non-compartmentalized model exchanging magnetization between PCr, ATP, and Pi (three-site model). The most complex one consisted of three compartments for ATP (mitochondrial matrix, cytoplasm, and myofibrillar compartment), three isoforms of CK, ATPase and ATP synthase connecting compartmentalized ATP with PCr and Pi. 

    At low work, ([Ca]=0.5 mM), all considered energy transfer schemes were able to reproduce the measured magnetization transfer within the experimental errors. This includes the simplest considered scheme (the three-site model) as well. However, when the beta adrenergic stimulation was used with high [Ca]=4.0 mM, the simple non-compartmentalized model was not able to fit the data: ATP compartmentalization must be taken into account. When compartmentalized models were used, it was possible to separate the fluxes through two isoforms of CK - the mitochondrial and the cytoplasmic one. According to the experimental data, the total forward rate of CK reaction is almost constant if the cardiac work is changed by increasing [Ca] from 0.5 mM to 4.0 mM. However, the forward and the backward rates of subcellular CK isoenzymes are changing with cardiac work. Namely, a rise in cardiac work increased both mitochondrial PCr production in mitochondria and myofibrillar PCr utilization at constant global CK flux. 

    In addition, the compartmentalized models allow one to find the fraction of ATP which is transported as PCr by the CK shuttle and directly as ATP from mitochondrial matrix to myofibrils. Our analysis of the data suggests  that both pathways (direct export of ATP from the matrix to myofibrils and the CK shuttle) are probably used, but depending on the work their proportion might vary. In extreme conditions of a work demand exceeding  mitochondrial ATP synthesis capacity (beta stimulation and high calcium): global CK flux decrease and ATP is mainly exported directly.

    These findings suggest that the CK shuttle is able to support the energy transfer, except in extreme conditions. This may have implications in understanding the process of cardiac pathology.

1.     Joubert F, Mazet J-L, Mateo P, Hoerter JA (2002) 31P NMR detection of subcellular creatine kinase fluxes in the perfused rat heart: contractility modifies energy transfer pathways. J. Biol. Chem. 277: 18469-18476.

2.     Joubert F, Mateo P, Gillet B, Beloeil J-C, Mazet J-L, Hoerter JA (2004) CK flux or direct ATP transfer: versatility of energy transfer pathways evidenced by NMR in the perfused heart. Mol. Cell. Biochem. 256/257: 43-58.

3.     McConnell HM (1958) Reaction rates by nuclear magnetic resonance. J. Chem. Phys. 28: 430-431.