ADP and AMP concentration differences between the IMS and the cytoplasm (myofilaments) are determined by three factors: cytoplasmic ATP hydrolysis rate, the diffusivity of ADP and AMP in cytoplasm, and the permeability of the outer mitochondrial membrane to AMP and ADP (i.e., porin activity). (A) A minimal scheme of nucleotide homeostasis, including ATPase activity, adenylate kinase activities (ADK), diffusion, porins, nucleotide exchange at the inner mitochondrial membrane, and ATP synthesis within the matrix space. (B) Predicted ADP and AMP concentration differences between the IMS and the cytoplasm that are independent of simulation model details. With an ATP hydrolysis rate of 0.5 mmol/liter/s, a fivefold slowing of diffusion across myofilaments (K_diff) causes a 10 μM increase in the total concentration of ADP and AMP. That increase is greater than the known cytoplasmic concentration of ADP in murine myocytes (Dobson and Himmelreich, 2002). It is therefore likely that diffusion (K_diff) is an important determinant of ADP and AMP concentrations in the myocyte cytoplasm, together with the permeability of the outer mitochondrial membrane (K_porin).