Faster aging is predicted in more active tissues and animals because of greater reactive oxygen species generation. Yet age-related cell loss is greater in less active cell types, such as type II muscle fibers. Mitochondrial uncoupling has been proposed as a mechanism that reduces reactive oxygen species production and could account for this paradox between longevity and activity. We distinguished these hypotheses by using innovative optical and magnetic resonance spectroscopic methods applied to noninvasively measured ATP synthesis and O 2 uptake in vivo in human muscle. Here we show that mitochondrial function is unchanged with age in mildly uncoupled tibialis anterior muscle (75% type I) despite a high respiratory rate in adults. In contrast, substantial uncoupling and loss of cellular [ATP] indicative of mitochondrial dysfunction with age was found in the lower respiring and well coupled first dorsal interosseus (43-50% type II) of the same subjects. These results reject respiration rate as the sole factor impacting the tempo of cellular aging. Instead, they support mild uncoupling as a mechanism protecting mitochondrial function and contributing to the paradoxical longevity of the most active muscle fibers. magnetic resonance spectroscopy ͉ optical spectroscopy ͉ oxidative phosphorylation T he rate-of-living hypothesis proposes that higher rates of oxidative metabolism cause an increased production of reactive oxygen species (ROS) (1), leading to oxidative damage and mitochondrial dysfunction with age. However, mice with the lowest resting respiration rate have been shown to have the shortest longevity (2). Similarly, isolated muscle fibers show greater generation of ROS in type II fibers (3), which have the lowest oxidative capacity and chronic activity levels. This fiber type also has the shortest longevity and is the first to be lost with age (4). Thus, the tempo of aging appears to vary among mice and muscle fiber types but in an opposite manner than predicted by the rate-of-living hypothesis, with the least active having the shortest longevity.A physiological mechanism that could account for this paradox is mild mitochondrial uncoupling, which has been proposed to ameliorate ROS production by reducing reverse electron flow and superoxide generation (5). Consistent with this prediction are findings in mice with high respiration rates that showed elevated proton leak in isolated muscle mitochondria as a result of activation of the adenine nucleotide translocator and uncoupling protein 3. Mild uncoupling resulting from activation of these mitochondrial factors in type I muscle fibers could reduce ROS production, protect mitochondria from damage, and account for the longevity of this fiber type with age. A number of studies have provided evidence of mild uncoupling in human muscles (6, 7) but have not tested whether this uncoupling protects against mitochondrial dysfunction and cellular aging.New methods permit measurement of mitochondrial coupling in vivo by using a combination of noninvasive spectroscopi...

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Mitochondrial changes are at the centre of a wide range of maladies, including diabetes, neurodegeneration and ageing-related dysfunctions. Here we describe innovative optical and magnetic resonance spectroscopic methods that non-invasively measure key mitochondrial fluxes, ATP synthesis and O 2 uptake, to permit the determination of mitochondrial coupling efficiency in vivo (P/O: half the ratio of ATP flux to O 2 uptake). Three new insights result. First, mitochondrial coupling can be measured in vivo with the rigor of a biochemical determination and provides a gold standard to define well-coupled mitochondria (P/O ≈ 2.5). Second, mitochondrial coupling differs substantially among muscles in healthy adults, from values reflective of well-coupled oxidative phosphorylation in a hand muscle (P/O = 2.7) to mild uncoupling in a leg muscle (P/O = 2.0). Third, these coupling differences have an important impact on cell ageing. We found substantial uncoupling and loss of cellular [ATP] in a hand muscle indicative of mitochondrial dysfunction with age. In contrast, stable mitochondrial function was found in a leg muscle, which supports the notion that mild uncoupling is protective against mitochondrial damage with age. Thus, greater mitochondrial dysfunction is evident in muscles with higher type II muscle fibre content, which may be at the root of the preferential loss of type II fibres found in the elderly. Our results demonstrate that mitochondrial function and the tempo of ageing varies among human muscles in the same individual. These technical advances, in combination with the range of mitochondrial properties available in human muscles, provide an ideal system for studying mitochondrial function in normal tissue and the link between mitochondrial defects and cell pathology in disease. Coupling and pathologyMitochondria are central to the conversion of energy by oxidizing substrates and generating the cell fuel, ATP. Defects in this conversion process are emerging as central to cell pathology and may be a critical part of cellular ageing. Despite the extensive research into mitochondrial pathology (over 7500 hits for 'mitochondrial dysfunction' in PubMed), we know little about the nature and extent of mitochondrial dysfunction in vivo, especially in human tissues, and we are still unclear as to the underlying cause of the defects that lead to dysfunction. One reason for this has been the lack of experimental tools to study mitochondrial function in vivo. Here we show that new technical advances make these measurements possible and highlight human muscle as an ideal system to evaluate the factors responsible for mitochondrial dysfunction. New spectroscopic innovations from our laboratory permit quantitative measurement of mitochondrial O 2 and ATP fluxes to yield mitochondrial capacity and coupling efficiency in vivo [half the ratio of ATP flux to O 2 uptake (P/O: half the ratio of ATP flux to O 2 uptake); Marcinek, 2004]. These innovations permit us to extend our studies beyond the traditional focus of human stud...