Oxygen dependence of respiration in rat spinotrapezius muscle in situ

Abstract

The oxygen dependence of respiration in striated muscle in situ was studied by measuring the rate of decrease of interstitial Po2[oxygen disappearance curve (ODC)] following rapid arrest of blood flow by pneumatic tissue compression, which ejected red blood cells from the muscle vessels and made the ODC independent from oxygen bound to hemoglobin. After the contribution of photo-consumption of oxygen by the method was evaluated and accounted for, the corrected ODCs were converted into the Po2dependence of oxygen consumption, V̇o2, proportional to the rate of Po2decrease. Fitting equations obtained from a model of heterogeneous intracellular Po2were applied to recover the parameters describing respiration in muscle fibers, with a predicted sigmoidal shape for the dependence of V̇o2on Po2. This curve consists of two regions connected by the point for critical Po2of the cell (i.e., Po2at the sarcolemma when the center of the cell becomes anoxic). The critical Po2was below the Po2for half-maximal respiratory rate ( P50) for the cells. In six muscles at rest, the rate of oxygen consumption was 139 ± 6 nl O2/cm3·s and mitochondrial P50was k = 10.5 ± 0.8 mmHg. The range of Po2values inside the muscle fibers was found to be 4–5 mmHg at the critical Po2. The oxygen dependence of respiration can be studied in thin muscles under different experimental conditions. In resting muscle, the critical Po2was substantially lower than the interstitial Po2of 53 ± 2 mmHg, a finding that indicates that V̇o2under this circumstance is independent of oxygen supply and is discordant with the conventional hypothesis of metabolic regulation of the oxygen supply to tissue.