Oxygen dependency of respiration in the rat spinotrapezius muscle contracting at 0.5 to 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-2 consumption (VO2) was measured with phosphorescence quenching microscopy (PQM) as the rate of O-2 disappearance in a muscle with rapid flow arrest. The phosphorescent oxygen probe was loaded into the interstitial space of the muscle to give O-2 tension (PO2) in the interstitium. A set of sigmoid curves relating the PO2 dependence of VO2 was obtained with a O-2-dependent region below a characteristic PO2 (similar to 30 mmHg) and a PO2-independent region above this PO2. The VO2 (PO2) plots were fit by the Hill equation containing O-2 demand (rest to 8 Hz: 216 +/- 26 to 636 +/- 77 nl O-2/cm(3) s) and the PO2 value corresponding to O-2 demand/2 (rest to 8 Hz: 22 +/- 4 to 11 +/- 1 mmHg). The initial PO2 and VO2 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-2 supply chain. For each level of tissue blood flow the set of possible values of PO2 and VO2 consists of the intersection points between this O-2 supply line and the set of VO2 curves. An electrical analogy for the intraorgan O-2 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-2 supply depends on extracellular O-2 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.