Determination of mitochondrial creatine kinase fluxes in intact heart mitochondria using 31P-saturation transfer nuclear magnetic resonance spectroscopy

CK flux or direct ATP transfer: versatility of energy transfer pathways evidenced by NMR in the perfused heart

How the myocardium is able to permanently coordinate its intracellular fluxes of ATP synthesis, transfer and utilization is difficult to investigate in the whole organ due to the cellular complexity. The adult myocardium represents a paradigm of an energetically compartmented cell since 50% of total CK activity is bound in the vicinity of other enzymes (myofibrillar sarcolemmal and sarcoplasmic reticulum ATPases as well as mitochondrial adenine nucleotide translocator, ANT). Such vicinity of enzymes is well known in vitro as well as in preparations of skinned fibers to influence the kinetic properties of these enzymes and thus the functioning of the subcellular organelles. Intracellular compartmentation has often been neglected in the NMR analysis of CK kinetics in the whole organ. It is indeed a methodological challenge to reveal subcellular kinetics in a working organ by a global approach such as NMR. To get insight in the energy transfer pathway in the perfused rat heart, we developed a combined analysis of several protocols of magnetization transfer associated with biochemical data and quantitatively evaluated which scheme of energetic exchange best describes the NMR data. This allows to show the kinetic compartmentation of subcellular CKs and to quantify their fluxes. Interestingly, we could show that the energy transfer pathway shifts from the phosphocreatine shuttle in the oxygenated perfused heart to a direct ATP diffusion from mitochondria to cytosol under moderate inhibition of ATP synthesis. Furthermore using NMR measured fluxes and the known kinetic properties of the enzymes, it is possible to model the system, estimate local ADP concentrations and propose hypothesis for the versatility of energy transfer pathway. In the normoxic heart, a 3-fold ADP gradient was found between mitochondrial intermembrane space, cytosol and ADP in the vicinity of ATPases. The shift from PCr to ATP transport observed when ATP synthesis decreases might result from a balance in the activity of two populations of ANT, either coupled or uncoupled to CK. We believe this NMR approach could be a valuable tool to reinvestigate the control of respiration by ADP in the whole heart reconciling the biochemical knowledge of mitochondrial obtained in vitro or in skinned fibers with data on the whole heart as well as to identify the implication of bioenergetics in the pathological heart.

Cardiac performance and creatine kinase flux during inhibition of ATP synthesis in the perfused rat heart

To study the relation among mitochondrial energy supply, cardiac performance, and energy transfer through creatine kinase (CK), two acute models of inhibition of ATP synthesis were compared in the isovolumic acetate-perfused rat heart. Similar impairments of mechanical performance (rate-pressure product, RPP) were achieved by various stepwise decreases in O(2) supply (PO(2) down to 20% of control) or by infusing CN (0.15-0.25 mM). The forward CK flux measured by saturation-transfer (31)P NMR spectroscopy was 6.1 +/- 0. 4 mM/s in control hearts. Only after severe hypoxia (PO(2) < 40% of control) did CK flux drop (to 1.9 +/- 0.2 mM/s at PO(2) = 25% of control) together with impaired systolic activity and a rise in end-diastolic pressure. In contrast, in mild hypoxia CK flux remained constant and similar to control (5.3 +/- 0.5 mM/s, not significant) despite a twofold reduction in systolic activity. Similarly in all CN groups, constant CK flux was maintained for a threefold reduction in RPP, showing the absence of a relation between cardiac performance and global NMR-measured CK flux during mild ATP synthesis inhibition.