Human skeletal muscle intracellular oxygenation: the impact of ambient oxygen availability

Changes in intracellular oxygen availability have far reaching consequences likely involved in such diverse processes as angiogenesis (Richardson et al. 1999c; Wagner, 2001) and hypoxic pulmonary vasoconstriction (Wang et al. 2005; Wolin et al. 2005). However, data directly assessing intracellular oxygenation are sparse, especially in humans in vivo. Several very different methods have been employed in an attempt to assess local O2 availability including the use of PO2 electrodes (Hofer et al. 1992), O2 phosphorescence quenching (Rumsey et al. 1989), near infrared spectroscopy (Marcinek et al. 2003; Miura et al. 2004), and proton nuclear magnetic resonance spectroscopy (1H NMRS) of myoglobin (Mb) (Richardson et al. 1995b). Unlike the other methods which cause significant tissue damage or raise questions regarding the compartment of measurement, 1H NMRS of Mb is non-invasive and is clearly limited to muscle tissue, the only site in which this endogenous intracellular marker of oxygenation is found. Thus, this NMRS approach is ideally suited for in vivo human investigations. The measurement of deoxy-Mb by NMRS to determine skeletal muscle oxygenation has been utilized in several studies (Richardson et al. 1995b, 1998a, 2001, 2002; Mole et al. 1999; Vanderthommen et al. 2003). However, until now the assessment of deoxy-Mb has been limited to exercise where, due to falling PiO2, there is sufficient signal visible even with a relatively low signal-to-noise ratio. Previously, the lack of a resting deoxy-Mb signal prior to muscular contraction or supra-systolic cuff occlusion has been interpreted as indicative of a relatively high PiO2 in resting skeletal muscle (Richardson et al. 1995b, 2001; Mole et al. 1999; Chung et al. 2005). Therefore, although it is known that muscle PiO2 falls to very low values of 2–5 mmHg during exercising (Mole et al. 1999; Richardson et al. 2001), the starting point for skeletal muscle oxygenation or resting PiO2 is, as of yet, unknown. Hypoxia is both an important stimulus and a constant threat to the human body and its vital organs throughout life. Environmental changes such as exposure to high altitude reduce ambient O2 availability, while lung, vascular, and sleep disorders can result in hypoxia under normoxic conditions. It is known that hypoxia mediates adaptive changes in metabolism, O2 sensing and gene expression. However, although much research has examined the consequences of experimental hypoxic conditions, data documenting hypoxically mediated changes in cellular oxygenation in humans are sparse, if not non-existent. Consequently, we performed 1H NMRS at a high field strength (4 Tesla (T)) with a large volume coil and extended signal averaging (30 min) to assess resting O2 saturation (deoxy-Mb) in muscles of the lower leg in both normoxia and hypoxia (10% O2). This study had three specific goals: (1) to measure the resting intracellular oxygenation level and calculate the PiO2 of human skeletal muscle in vivo, (2) to test the hypothesis that ambient hypoxia will significantly reduce the level of cellular oxygenation and hence lower PiO2 in human skeletal muscle, and (3) to assimilate these findings into the current understanding of skeletal muscle intracellular oxygenation and PiO2 during exercise.