Metabolic and vascular support for the role of myoglobin in humans: a multiparametric NMR study

In human muscle the role of myoglobin (Mb) and its relationship to factors such as muscle perfusion and metabolic capacity are not well understood. We utilized nuclear magnetic resonance (NMR) to simultaneously study the Mb concentration ([Mb]), perfusion, and metabolic characteristics in calf muscles of athletes trained long term for either sprint or endurance running after plantar flexion exercise and cuff ischemia. The acquisitions for H-1 assessment of Mb desaturation and concentration, arterial spin labeling measurement of muscle perfusion, and P-31 spectroscopy to monitor high-energy phosphate metabolites were interleaved in a 4-T magnet. The endurance-trained runners had a significantly elevated [Mb] (0.28 +/- 0.06 vs. 0.20 +/- 0.03 mmol/kg). The time constant of creatine rephosphorylation (tauPCr), an indicator of oxidative capacity, was both shorter in the endurance-trained group (34 +/- 6 vs. 64 +/- 20 s) and negatively correlated with [Mb] across all subjects (r = 0.58). The time to reach maximal perfusion after cuff release was also both shorter in the endurance-trained group (306 +/- 74 vs. 560 +/- 240 s) and negatively correlated with [Mb] (r = 0.56). Finally, Mb reoxygenation rate tended to be higher in the endurance-trained group and was positively correlated with tauPCr (r = 0.75). In summary, these NMR data reveal that [Mb] is increased in human muscle with a high oxidative capacity and a highly responsive vasculature, and the rate at which Mb resaturates is well correlated with the rephosphorylation rate of Cr, each of which support a teleological role for Mb in O-2 transport within highly oxidative human skeletal muscle.