Fig. 8: Proposed sequence of events leading to altered mitochondrial metabolism by elevated intracellular Na.

a Elevated cytoplasmic Na leads to activation of mitochondrial NCLX leading to decreased Ca in the mitochondrial matrix and reduced activity of Ca-sensitive mitochondrial enzymes including pyruvate dehydrogenase (PDH), isocitrate dehydrogenase (IDH), α-ketoglutarate dehydrogenase (KDH) and Complex III and V (ATP synthase). Ca-sensitive enzymes are indicated in red while Ca-insensitive enzymes are blue. The normal flow of electrons through the ETC complexes are accepted by CoQ to produce CoQH₂ at Complexes I and II. CoQH₂ is reoxidised back to CoQ at Complex III allowing electron transport to continue at Complexes I and II. b Under conditions of high cytoplasmic Na, matrix Ca will be decreased leading to reduced activity of Complex III, decreased recycling of CoQ and a build-up of CoQH₂. Excess electrons in the CoQH₂ pool can lead to ROS generation via forward electron transport (FET) or reverse electron transport (RET) at Complexes I and II. There are two possible mechanisms for the build-up of succinate, (i) reduced CoQ availability will decrease activity of Complex II resulting in decreased oxidation of succinate to fumarate, or (ii) RET at Complex II could reduce fumarate back to succinate in the reverse direction. The build-up of succinate will inhibit proly hydroxylases (PHDs) causing decreased degradation of HIF1α. c Pre-treatment in vivo with MitoQ loads the mitochondria with an exogenous antioxidant that is an analogue of CoQ. This could buffer the CoQ pool at Complex II by reducing MitoQ to MitoQH₂, limiting the build-up of succinate. MitoQH₂ is recycled to MitoQ by scavenging ROS, preventing the ROS-induced functional deficit on ouabain washout. Note: MitoQ can access the active site of Complex II but not Complexes I and III. Additional abbreviations: aconitase (A); citrate synthase (CS); cytochrome C (Cyt C); ubiquinone (CoQ); ubiquinol (CoQH₂); fumarase (F); malate dehydrogenase (MDH); mitochondrial pyruvate carrier (MPC); mitochondrial Na/Ca exchanger (NCLX); succinyl CoA synthetase (SCS); succinate dehydrogenase (SDH). Basic layout redrawn from Williams et al. (2015). Panel d links the observations made in this study to this model²².