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

Abstract

Intracellular oxygen (O2) availability and the impact of ambient hypoxia have far reaching ramifications in terms of cell signalling and homeostasis; however, in vivo cellular oxygenation has been an elusive variable to assess. Within skeletal muscle the extent to which myoglobin desaturates (deoxy-Mb) and the extent of this desaturation in relation to O2 availability provide an endogenous probe for intracellular O2 partial pressure (P(iO2)). By combining proton nuclear magnetic resonance spectroscopy (1H NMRS) at a high field strength (4 T), assessing a large muscle volume in a highly efficient coil, and extended signal averaging (30 min) we assessed the level of skeletal muscle deoxy-Mb in 10 healthy men (30 +/- 4 years) at rest in both normoxia and hypoxia (10% O2). In normoxia there was an average deoxy-Mb signal of 9 +/- 1%, which, when converted to P(iO2) using an O2/Mb half-saturation (P50) of 3.2 mmHg, revealed an P(iO2) of 34 +/- 6 mmHg. In ambient hypoxia the deoxy-Mb signal rose to 13 +/- 3% (P(iO2) = 23 +/- 6 mmHg). However, intersubject variation in the defence of arterial oxygenation (S(aO2)) in hypoxia (S(aO2) range: 86-67%) revealed a significant relationship between the changes in S(aO2) and P(iO2)(r2 = 0.5). These data are the first to document resting intracellular oxygenation in human skeletal muscle, highlighting the relatively high P(iO2) values that contrast markedly with those previously recorded during exercise (approximately 2-5 mmHg). Additionally, the impact of ambient hypoxia on P(iO2) and the relationship between changes in S(aO2) and P(iO2) stress the importance of the O2 cascade from air to cell that ultimately effects O2 availability and O2 sensing at the cellular level.

Figure 1

A comparison of the greater signal-to-noise ratio attained in the current 4 T system with a large tissue volume (the entire lower leg) (upper panel) in comparison to our previous data collected with a surface coil (sample volume ∼100 ml) at 2 T (lower panel). Both signals represent the final 2 min of supra-systolic cuff occlusion (8–10 min). au, arbitrary units.

Figure 2

Example spectra of the deoxy-Mb signal collected in the resting lower leg in normoxia (upper panel) and hypoxia (lower panel) in both cases contrasted with the signal attained during minutes 8–10 of supra-systolic cuff occlusion. The somewhat indefinable peak in normoxia contrasts with that attained in hypoxia as a consequence of the 2 min summation of these spectra, highlighting the need for extended averaging in normoxia to reveal a measurable signal.

Figure 3

The deoxy-Mb signal in normoxia and hypoxia with 10, 20 and 30 min of signal averaging, highlighting the need in normoxia for extended averaging to define a clear peak.

Figure 4

The significant relationship between the changes in arterial P_O2 and intracellular P_O2 induced by ambient hypoxia. Although the hypoxic gas breathed (10% O₂) was uniform across all subjects, the response in terms of arterial and intracellular P_O2 was not, presumably the consequence of variations in hypoxic ventilatory response. This relationship is supportive of the intuitive relationship between arterial P_O2 and intracellular P_O2, illustrating the importance of a diffusion gradient in the passive movement of O₂ from air to cell.

Figure 5

A comparison of the O₂ cascade measured at rest in the current study and during maximal knee-extensor exercise with similar methodologies in a previous investigation (Richardson et al. 1995b).