Near-infrared Spectroscopy Estimation Of Combined Muscle Oxidative Capacity And Oxygen Diffusion Capacity In Humans

Abstract

PURPOSE: Muscle oxygen uptake (mV̇O2) depends on both O2 supply (convective and diffusive O2 delivery) and O2 demand (ATP utilization rate and mitochondrial function). The mV̇O2 recovery rate constant (k) measured by near-infrared spectroscopy (NIRS), in the presence of non-limiting O2 availability, is associated with muscle oxidative capacity in vivo. However, when O2 availability is limiting, k may be limited by muscle O2 diffusing capacity (DmO2). We aimed to determine associations of: i) k obtained in different O2 availability conditions with ex vivo mitochondrial function and ii) the difference in k (∆k) between conditions of non-limiting and limiting O2 availability with capillary-to-fiber ratio (C:F). METHODS: 12 healthy participants volunteered (28 ± 5 yrs; V̇O2peak 37.1 ± 8.0 ml*kg-1*min-1). On visits 1 and 2, k of the vastus lateralis (VL) was measured by NIRS during 10-15 intermittent arterial occlusions performed immediately after moderate intensity constant work-rate exercise. The duration and timing of occlusions were manipulated to maintain TSI in a 10% range change either below (LOW) or above (HIGH) the mid-point of the functional range from a prolonged occlusion. ∆k is the difference between LOW and HIGH k measurements. On visit 3, the VL was biopsied to measure C:F and maximal O2 flux using saturating substrates for complexes I + II by high-resolution respirometry (HRR). RESULTS: O2 flux was 37.7 ± 10.6 and 56.8 ± 19.8 pmol*s-1*mg-1 wet weight in maximal ADP-stimulated phosphorylating state and maximal noncoupled state, respectively. C:F ratio ranged 2.15 to 2.49. k was greater in HIGH (3.15 ± 0.45 min-1) than in LOW (1.56 ± 0.79 min-1, p < 0.0001). k was significantly correlated with both HRR measurements in HIGH (r = 0.72 - 0.69), but not in LOW (r = -0.24 - -0.28). ∆k ranged -0.19 to -3.19 min-1, and was inversely correlated with C:F ratio (r = -0.68, p = 0.01). CONCLUSION: These data show that mV̇O2 recovery rate constant (k) reflected muscle oxidative capacity only under conditions of non-limiting O2 availability. Moreover, ∆k, obtained under different O2 availability conditions, was associated with C:F ratio, a mediator of DmO2. Thus, assessment of muscle k and ∆k using NIRS under HIGH and LOW TSI conditions provides a non-invasive window on both muscle oxidative capacity and muscle O2 diffusive capacity.