Intramyocellular oxygenation during ischemic muscle contractions in vivo

Abstract

There is some evidence that the fall in intramyocellular oxygen content during ischemic contractions is less than during ischemia alone. We used proton magnetic resonance spectroscopy to determine whether peak deoxy-myoglobin (dMb) obtained during ischemic ankle dorsiflexion contractions attained the maximal dMb level observed during a separate trial of ischemia alone (resting max). In six healthy young men, the rate of myoglobin desaturation was rapid at the onset of ischemic contractions and then slowed as contractions continued, attaining only 75 +/- 3.3% (mean +/- SE) of resting max dMb by the end of contractions (p = 0.03). Myoglobin continued to desaturate while ischemia was maintained following contractions, reaching 98 +/- 1.8% of resting max within 10 min (p = 0.03 vs. end of contractions). Notably, contractions performed after 10 min of ischemia did not affect dMb (dMb = 100 +/- 1.5% of resting max, p > 0.99), suggesting that full desaturation had already been achieved. The blunting of desaturation during ischemic contractions is likely a result of slowed mitochondrial oxygen consumption due to limited oxygen availability.

Figure 1. Myoglobin desaturation in a single subject

Representative stackplots of ¹H-MRS spectra during A) EC and B) LC in a single subject are shown in the left panels. The dashed line indicates ischemia alone and the solid line denotes contractions + ischemia. For each panel, temporal resolution is 60 s for the first spectrum and 12 s in all subsequent spectra. In the right panels, dMb is expressed relative to maximal dMb obtained during 10 min of resting ischemia, in the same subject. Contractions are indicated by the hatched bar. Sigmoid and bilinear fits of the dMb data are shown. A) r² = 0.91 and 0.90, respectively. B) r² = 0.98 and 0.96, respectively.

Figure 2. Changes in dMb and force during EC protocol

Deoxy-myoglobin (dMb), expressed relative to maximal dMb obtained during 10 min of resting cuff ischemia. Contractions are indicated by the hatched bar. Closed symbols indicate ischemia, open symbols indicate free-flow recovery period. In the inset, force is expressed relative to baseline maximum. Each contraction lasted 12 s, followed by 12 s of rest.

Figure 3. Changes in dMb and force during LC Protocol

Deoxy-myoglobin (dMb), expressed relative to maximal dMb, as in Fig. 2. In the inset, force is expressed relative to baseline maximum. Each contraction lasted 1.2 s, followed by 1.8 s of rest.

Figure 4. Deoxymyoglobin and PCr a single subject

Data were acquired in an interleaved manner during A) the EC protocol, and (B) 10-min resting ischemia during LC protocol. Observed dMb, along with the bilinear fit, are presented in the top panel. Phosphocreatine (PCr) is shown in the second panel, with inorganic phosphate (Pi) and intracellular pH below. PCr and Pi are expressed relative to resting levels. Contractions during the EC protocol are indicated by the hatched bar.