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 O consumption (V̇o) was measured with phosphorescence quenching microscopy (PQM) as the rate of O disappearance in a muscle with rapid flow arrest. The phosphorescent oxygen probe was loaded into the interstitial space of the muscle to give O tension (Po) in the interstitium. A set of sigmoid curves relating the Po dependence of V̇o was obtained with a Po-dependent region below a characteristic Po (~30 mmHg) and a Po-independent region above this Po. The V̇o(Po) plots were fit by the Hill equation containing O demand (rest to 8 Hz: 216 ± 26 to 636 ± 77 nl O/cm s) and the Po value corresponding to O demand/2 (rest to 8 Hz: 22 ± 4 to 11 ± 1 mmHg). The initial Po and V̇o 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 O supply chain. For each level of tissue blood flow the set of possible values of Po and V̇o consists of the intersection points between this O supply line and the set of V̇o curves. An electrical analogy for the intraorgan O 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 O supply depends on extracellular O 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.