Fig. 1. See text. increased lactate/pyruvate ratios which are in equilibrium with NADH/NAD+ via lactate dehydrogenase) is not attributable to hypoxia; it is, instead, the consequence of metabolic imbalances arising in the cytosol. The importance of the NADH/NAD+ redox couple in mitochondrial and cytosolic energy metabolism, and evidence that cytosolic free NADH/NAD+ is modulated by mitochondrial NADH/NAD+ and by intra-and extracellular lactate and pyruvate (via lactate dehydrogenase and plasma membrane transporters of lactate and pyruvate), indicate that cytosolic free NADH/NAD+ is a sensor of the (NADH/NAD+) redox state of mitochondria, cytosol, and the extracellular milieu. It is unlikely to be a coincidence, therefore, that cytosolic NADH/NAD+ also appears to play an important role in regulating tissue blood flow in response to changes in oxygen tension, energy metabolism, and the extracellular lactate/pyruvate ratio. Reductive stress, regardless of the cause, is associated with increased blood flow. In addition to increased oxidation of glucosederived metabolites in diabetes, increased oxidation of ethanol, electrochemical or mechanical work, hyperlactataemia, experimental galactosaemia, cyanide and carbon monoxide poisoning, and fasting all cause reductive stress and are associated with increased blood flow in the affected tissue (s). The importance of cytosolic reductive stress is that it impacts on the activity of many dehydrogenase enzymes, several of which have been implicated in the pathogenesis of diabetic complications, that require NAD+ or NADH as cofactors and are regulated by NADH/NAD+. Several lines of evidence suggest that intracellular production of oxygen reactive species, glycation reactions, and activation of protein kinase C are mediated in large part by a cascade of events initiated by the effects of cytosolic reductive stress on glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and glycerol 3-phosphate dehydrogenase (G3PDH)[1, 4].

Because of the equilibrium between NADH/NAD+ and glyceraldehyde 3-phosphate/1, 3-bisphosphoglycerate established by GAPDH, an increased ratio of NADH/NAD+ favours increased levels of glyceraldehyde 3-phosphate which is in equilibrium with other trioses (dihydroxyacetone phosphate [DHAP], and fructose 1, 6-bisphosphate, referred to collectively as ‘triose phosphates’). Triose phosphates are highly reactive sugars which undergo autoxidation (with production of free radicals including superoxide) resulting in non-enzymatic glycation and oxidative damage to intracellular proteins, DNA, and membrane lipids (Fig. 1). As a result of the corresponding equilibrium between NADH/NAD+ and glycerol 3-phosphate/DHAP established by G3PDH, an increased ratio of NADH/NAD+ favours reduction of DHAP to glycerol 3-phosphate, the first step in one pathway for de