In vivo assessment of muscle mitochondrial function in healthy, young males in relation to parameters of aerobic fitness

European Journal of Applied Physiology https://doi.org/10.1007/s00421-019-04169-8 ORIGINAL ARTICLE In vivo assessment of muscle mitochondrial function in healthy, young males in relation to parameters of aerobic fitness Bart Lagerwaard1 · Jaap Keijer1 · Kevin K. McCully2 · Vincent C. J. de Boer1 · Arie G. Nieuwenhuizen1 Received: 21 January 2019 / Accepted: 28 May 2019 © The Author(s) 2019 Abstract Purpose The recovery of muscle oxygen consumption (mV̇ O2) after exercise provides a measure of skeletal muscle mitochondrial capacity, as more and better-functioning mitochondria will be able to restore mV̇ O2 faster to the pre-exercise state. The aim was to measure muscle mitochondrial capacity using near-infrared spectroscopy (NIRS) within a healthy, normally active population and relate this to parameters of aerobic fitness, investigating the applicability and relevance of using NIRS to assess muscle mitochondrial capacity non-invasively. Methods Mitochondrial capacity was analysed in the gastrocnemius and flexor digitorum superficialis (FDS) muscles of eight relatively high-aerobic fitness (V̇ O2peak ≥ 57 mL/kg/min) and eight relatively low-aerobic fitness male subjects (V̇ O2peak ≤ 47 mL/kg/min). Recovery of whole body V̇ O2, i.e. excess post-exercise oxygen consumption (EPOC) was analysed after a cycling protocol. Results Mitochondrial capacity, as analysed using NIRS, was significantly higher in high-fitness individuals compared to low-fitness individuals in the gastrocnemius, but not in the FDS (p = 0.0036 and p = 0.20, respectively). Mitochondrial capacity in the gastrocnemius was significantly correlated with V̇ O2peak (R2 = 0.57, p = 0.0019). Whole body V̇ O2 recovery was significantly faster in the high-fitness individuals (p = 0.0048), and correlated significantly with mitochondrial capacity in the gastrocnemius (R2 = 0.34, p = 0.028). Conclusion NIRS measurements can be used to assess differences in mitochondrial muscle oxygen consumption within a relatively normal, healthy population. Furthermore, mitochondrial capacity correlated with parameters of aerobic fitness (V̇ O2peak and EPOC), emphasising the physiological relevance of the NIRS measurements. Keywords Mitochondrial capacity · NIRS · EPOC · Oxidative metabolism · Muscle mitochondria Abbreviations EPOC Excess post-exercise oxygen consumption FDS Flexor digitorum superficialis HHb Deoxyhaemoglobin mVO2 Muscle oxygen consumption NIRS Near-infrared spectroscopy O2Hb Oxyhaemoglobin 31 P-MRS Magnetic resonance spectroscopy Communicated by Toshio Moritani. * Arie G. Nieuwenhuizen 1 Human and Animal Physiology, Wageningen University and Research, PO Box 338, 6700AH Wageningen, The Netherlands 2 Department of Kinesiology, University of Georgia, Athens, USA RPM Revolutions per minute V̇ O2max Maximal oxidative capacity V̇ O2peak Whole body peak oxygen uptake Introduction Muscle mitochondrial mass and function are positively affected by regular endurance exercise (Tonkonogi and Sahlin 2002). Due to the pivotal role of mitochondria in determining endurance capacity, there is a need for robust and non-invasive measurements of muscle mitochondrial function (Lanza and Nair 2009). Mitochondrial function in skeletal muscle is classically analysed ex vivo by measuring oxygen consumption in muscle biopsies. Less-invasive techniques have emerged over the last quarter century, allowing the measurement of mitochondrial function in vivo. These techniques are both based on the recovery of muscle 13 Vol.:(0123456789) European Journal of Applied Physiology homeostasis after exercise (Meyer 1988), assessed by measuring either the regeneration of phosphocreatine (PCr) using magnetic resonance spectroscopy (31P-MRS) or by the return of muscle oxygen consumption (mV̇ O2) to basal levels using near-infrared spectroscopy (NIRS). Mitochondrial function analysed by both techniques have been shown to be in good agreement with each other (Ryan et al. 2013), but NIRS offers advantages over 31P-MRS due to its higher portability and relatively low costs, making it more suitable for on-site and routine measurements. NIRS uses the difference in light absorption of oxygenated and deoxygenated haemoglobin and myoglobin in the nearinfrared region (Grassi and Quaresima 2016). By emitting light at different wavelengths, it is possible to differentiate between the oxygenated and deoxygenated states. When used on muscle and combined with arterial occlusions, it allows for measurement of mV̇ O2, as the change from oxygenated to deoxygenated haemoglobin and myoglobin reflects the use of oxygen in the tissue underneath the NIRS probe when blood flow is occluded (Van Beekvelt et al. 2001). Multiple, transient arterial occlusions after a short bout of exercise allows for the measurement of post-exercise recovery of mV̇ O2, a procedure used to assess mitochondrial capacity (Motobe et al. 2004). The underlying assumption is that post-exercise regeneration of readily available energy carriers (i.e. ATP and PCr) is directly linked to aerobic metabolism and, therefore, a higher mitochondrial capacity will be associated with a faster return of mV̇ O2 to the pre-exercise state (McMahon and Jenkins 2002). Indeed, the NIRS procedure to assess recovery kinetics of mV̇ O2 in vivo showed a strong correlation with maximal ADP-stimulated respiration of permeabilised muscle fibres in situ (Ryan et al. 2014). On a whole body level, the regeneration of readily available energy carriers is assumed to contribute to the transient elevation of whole body oxygen consumption (mV̇ O2) above resting values in the immediate post-exercise period, also known as excess post-exercise oxygen consumption (EPOC). EPOC can be divided into a rapid and a prolonged phase, in which the mechanisms that contribute to the elevated mV̇ O2 are different (Gaesser and Brooks 1984). In particular, the rapid phase is defined to reflect the myofibrillar consumption of the readily available energy substrates in the beginning of exercise, such as PCr and ATP, as well as the replenishment of tissue and haemoprotein oxygen stores and lactate removal (Chance et al. 1992; Børsheim and Bahr 2003). In accordance with an important role for PCr regeneration in EPOC, Rossiter et al. showed that whole body V̇ O2 is related to muscle PCr kinetics in the recovery phase (Rossiter et al. 2002). As the latter may be related to skeletal muscle mitochondrial capacity, an inverse relationship between EPOC and NIRS assessment of mV̇ O2, reflecting skeletal muscle mitochondrial function, can be hypothesised (Kemp et al. 2015). However, it should be noted that despite clear effects on mitochondrial capacity 13 (Lanza and Nair 2009), the effect of endurance training status on EPOC is controversial, most likely as a result of methodological difficulties (Børsheim and Bahr 2003). When comparing low- with high-endurance capacity subjects, no research design can control for relative exercise intensity (...truncated)