Intracellular oxygen tension limits muscle contraction-induced change in muscle oxygen consumption under hypoxic conditions during Hb-free perfusion.

Under acute hypoxic conditions, the muscle oxygen uptake (mV˙O) during exercise is reduced by the restriction in oxygen-supplied volume to the mitochondria within the peripheral tissue. This suggests the existence of a factor restricting the mV˙O under hypoxic conditions at the peripheral tissue level. Therefore, this study set out to test the hypothesis that the restriction in mV˙O is regulated by the net decrease in intracellular oxygen tension equilibrated with myoglobin oxygen saturation (∆PO) during muscle contraction under hypoxic conditions. The hindlimb of male Wistar rats (8 weeks old, n = 5) was perfused with hemoglobin-free Krebs-Henseleit buffer equilibrated with three different fractions of O gas: 95.0%O, 71.3%O, and 47.5%O The deoxygenated myoglobin (Mb) kinetics during muscle contraction were measured under each oxygen condition with a near-infrared spectroscopy. The ∆[deoxy-Mb] kinetics were converted to oxygen saturation of myoglobin (SO), and the PO was then calculated based on the SO and the O dissociation curve of the Mb. The SO and PO at rest decreased with the decrease in O supply, and the muscle contraction caused a further decrease in SO and PO under all O conditions. The net increase in mV˙O from the muscle contraction (∆mV˙O) gradually decreased as the ∆PO decreased during muscle contraction. The results of this study suggest that ΔPO is a key determinant of the ΔmV˙O.