MYOGLOBIN SATURATION AND INTRACELLULAR PO2 DURING EXERCISE

Abstract

A very low intracellular PO2 is reported in human quadriceps and gastrocnemius muscle at moderate and higher exercise levels. This low PO2 means that the gradient for O2 diffusion in these muscles is very high during exercise. PURPOSE This study tested whether low PO2 and high O2 gradient is characteristic of muscles in general during exercise. METHODS We collected 1H and 31P magnetic resonance spectra from hand (first dorsal interosseus, FDI) and leg (anterior compartment, LEG) muscles in 5 subjects. Myoglobin desaturation during sustained exercise as compared to full desaturation during muscle anoxia was determined from 1H spectra. We measured oxidative flux and estimated oxidative capacity from the kinetics of changes in [PCr] during and after sustained isometric exercise. RESULTS At 50% and 75% of oxidative capacity, LEG myoglobin was 37% and 53% desaturated, respectively. This represents an intracellular PO2 of about 3–5 torr. The FDI showed very little myoglobin desaturation (about 10%; PO2 = 28 torr) even at 85% of its oxidative capacity. CONCLUSION The LEG approaches the maximum gradient for O2 diffusion during exercise, similar to the quadriceps and gastrocnemius muscles. In contrast, the FDI has a low O2 gradient due to a high intracellular PO2 even when functioning very near its oxidative capacity. These results suggest different mechanisms governing O2 transport in human muscle during exercise. Supported by NIH grants AR-41928 and AR-45184.