Leading to deficient OXPHOS. While potentially induced by the extreme pathological environment in which the mitochondria reside in vivo, a mounting collection of evidence suggests a natural history of inherited metabolic impairment alongside dystrophin deficiency. Onopiuk et al. has demonstrated that metabolic dysfunction is present in dystrophic myoblasts prior to the time of dystrophin expression. This suggests that while dystrophin-deficiency induced pathophysiology may exacerbate mitochondrial dysfunction, metabolic impairment exists beforehand. That female carriers of DMD who do not express the disease exhibit abnormal muscle energy metabolism, especially when ATP demand is increased during exercise, lends further credence to this notion. Whilst several groups have demonstrated depressed oxygen consumption rate and isolated mitochondrial enzyme function in dystrophic skeletal muscle, we have highlighted the differential contributions of Complex I and II H+ flux into the ETC and resultant ATP production in dystrophic mouse mitochondria. Importantly, these impairments were shown in the ‘healthy’ mitochondria that survived the isolation process. MAPR was shown in this study to be severely depressed by up to 75% of ASP1517 control levels in both diaphragm and TA mitochondria from the mdx mouse. MAPR depression was more evident when stimulated by substrates that enter the TCA cycle and rely on NADH-mediated shuttling of H+ into the ETC through Complex I. In contrast, the complete inhibition of Complex I with rotenone and stimulation of Complex II-mediated MAPR with succinate partially ameliorated mdx MAPR, albeit depression was still evident. This suggests a problem with NADH flux into the ETC of mdx mitochondria, whereby NADH is either being sequestered away from, or is unable to be efficiently oxidised by, Complex I to establish proton motive force. In this instance, the accumulation of NADH at Complex I would be inhibitory to all dehydrogenases of the Krebs cycle except for succinate dehydrogenase, which would explain why succinate stimulation was able to partially restore MAPR of mdx mitochondria closer to control levels, whereas other Kreb’s substrates had no effect. Recent literature has demonstrated reduced Complex I activity in permeabilised skeletal muscle from mdx mice and in mdx brain, in which dystrophin is normally expressed but is also notably absent in DMD. The expression of various Complex I subunits is also evident in mdx skeletal muscle at the protein level, in human DMD skeletal muscle at the transcript level and in mdx cardiac muscle. Thus our data together with others suggests that a persistent impairment of Complex I function underpins dystrophic pathology, which strongly limits – but does not obliterate – the ATPproducing capacity of mitochondria. While Godin et al. suggests that reduced mitochondrial biomass underscores loss of ETC function rather than specific respiratory impairment, our data would suggest the opposite as the stimulation of respiration with succinate following Complex I inhibition with rotenone partially restored MAPR in mdx mitochondria with equivale.