Fig. 9. Scheme illustrating the functional intracellular energetic units (ICEUs) in the cardiac cell. By interaction with cytoskeletal elements, the mitochondria and sarcoplasmic reticulum (SR) are precisely fixed with respect to the structure of the sarcomere of the myofibrils between two Z-lines and, correspondingly, between two T-tubules. Calcium is released from the SR into the space in the ICEU in the vicinity of the mitochondria and sarcomeres to activate contraction and mitochondrial dehydrogenases. Adenine nucleotides within the ICEU do not equilibrate rapidly with adenine nucleotides in the bulk water phase. The mitochondria, SR and MgATPase of myofibrils and ATP-sensitive systems in the sarcolemma are interconnected by metabolic channeling of reaction intermediates and energy transfer within the ICEU by the creatine kinase (CK)–phosphocreatine (PCr) and adenylate kinase (AK) systems. CK_cyt and AK_cyt represent the CK and AK in the cytoplasmic space. F₀F₁ is the mitochondrial ATPase synthase complex. The protein factors (still unknown and marked as `X'), most probably connected to cytoskeleton, fix the position of mitochondria and probably also control the permeabilty of the VDAC channels to ADP and ATP. Adenine nucleotides within the ICEU and bulk water phase may be connected by some more rapidly diffusing metabolites than creatine (Cr)–PCr. Synchronization of functioning of ICEUs within the cell may occur by the same metabolites, for example, inorganic phosphate (P_i) or PCr, and/or synchronized release of calcium during the excitation–contraction coupling process. Adapted from Saks et al., 2001.

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