Oxygen dependence of respiration in rat spinotrapezius muscle contracting at 0.5-8 twitches per second

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.