Figure 1.

Schematic representation of the cardiac energy system. Three major components are denoted: substrate supply; Krebs cycle and oxidative phosphorylation (OXPHOS); the energy transfer system. The red oval demarks mitochondrial contained processes discussed in this review. Substrate combinations enter and are processed through the Krebs cycle. Electrons are delivered as NADH & FADH₂ to the electron transport chain (ETC), along which oxidative phosphorylation (OXPHOS) is completed. Newly formed ATP is transferred to cytoplasmic usage sites by phosphocreatine energy transfer system. Measurement of the phosphocreatine/ATP (PCr/ATP) ratio quantifies a cell’s energy state.