Figure 1.

A schematic of adenine nucleotide transfer pathways in the heart. This figure shows simple diffusion of ATP, ADP, and inorganic phosphate (P_i), and CK-facilitated transport operating in the ATP-generating direction (rate constant k_f) and PCr-generating direction (rate constant k_r). In oxidative muscle under normoxic conditions, cytosolic CK isoforms (CK-MM, -MB, -BB) typically work in the ATP-generating direction, whereas mitochondrial CK works in the PCr-generating direction. Forward CK flux is defined by k_f×[PCr], where k_f is the pseudo–first-order unidirectional rate constant in the ATP-generating direction measured using saturation transfer ³¹P magnetic resonance spectroscopy. This should be distinguished from CK total activity (V_Max), which represents maximal velocity in the presence of saturating amounts of substrate and requires destructive freeze-extraction chemicals. ANT indicates adenine nucleotide transfer; CK, creatine kinase; PCr, phosphocreatine; and VDAC, voltage-dependent anion channel;