31P NMR Detection of Subcellular Creatine Kinase Fluxes in the Perfused Rat Heart

The subcellular fluxes of exchange of ATP and phosphocreatine (PCr) between mitochondria, cytosol, and ATPases were assessed by 31P NMR spectroscopy to investigate the pathways of energy transfer in a steady state beating heart. Using a combined analysis of four protocols of inversion magnetization transfer associated with biochemical data, three different creatine kinase (CK) activities were resolved in the rat heart perfused in isovolumic control conditions: (i) a cytosolic CK functioning at equilibrium (forward, Ff = PCr → ATP, and reverse flux,Fr = ATP → PCr = 3.3 mm·s−1), (ii) a CK localized in the vicinity of ATPases (MM-CK bound isoform) favoring ATP synthesis (Ff = 1.7 × Fr), and (iii) a mitochondrial CK displaced toward PCr synthesis (Ff = 0.3 and Fr = 2.6 mm·s−1). This study thus provides the first experimental evidence that the energy is carried from mitochondria to ATPases by PCr (i.e. CK shuttle) in the whole heart. In contrast, a single CK functioning at equilibrium was sufficient to describe the data when ATP synthesis was partly inhibited by cyanide (0.15 mm). In this case, ATP was directly transferred from mitochondria to cytosol suggesting that cardiac activity modified energy transfer pathways. Bioenergetic implications of the localization and activity of enzymes within myocardial cells are discussed.