LAUSR

The oxygen dependence of respiration was obtained in situ in microscopic regions of rat spinotrapezius muscle for different levels of metabolic activity produced by electrical stimulation at rates from 0.5 to 8 Hz. The rate of O2 consumption (V̇o2) was measured with phosphorescence quenching microscopy (PQM) as the rate of O2 disappearance in a muscle with rapid flow arrest. The phosphorescent oxygen probe was loaded into the interstitial space of the muscle to give O2 tension (Po2) in the interstitium. A set of sigmoid curves relating the Po2 dependence of V̇o2 was obtained with a Po2-dependent region below a characteristic Po2 (~30 mmHg) and a Po2-independent region above this Po2. The V̇o2(Po2) plots were fit by the Hill equation containing O2 demand (rest to 8 Hz: 216 ± 26 to 636 ± 77 nl O2/cm3 s) and the Po2 value corresponding to O2 demand/2 (rest to 8 Hz: 22 ± 4 to 11 ± 1 mmHg). The initial Po2 and V̇o2 pairs of values measured at the moment of flow arrest formed a straight line, determining the rate of oxygen supply. This line had a negative slope, equal to the oxygen conductance for the O2 supply chain. For each level of tissue blood flow the set of possible values of Po2 and V̇o2 consists of the intersection points between this O2 supply line and the set of V̇o2 curves. An electrical analogy for the intraorgan O2 supply and consumption is an inverting transistor amplifier, which allows the use of graphic analysis methods for prediction of the behavior of the oxygen processing system in organs. NEW & NOTEWORTHY The sigmoidal shape of curves describing oxygen dependence of muscle respiration varies from basal to maximal workload and characterizes the oxidative metabolism of muscle. The rate of O2 supply depends on extracellular O2 tension and is determined by the overall oxygen conductance in the muscle. The dynamics of oxygen consumption is determined by the supply line that intersects the oxygen demand curves. An electrical analogy for the oxygen supply/consumption system is an inverting transistor amplifier.