Intracellular PO2 decreases with increasing stimulation frequency in contracting single Xenopus muscle fibers

There is currently some controversy regarding the manner in which skeletal muscle intracellular PO2 changes with work intensity. Therefore, this study investigated the relationship between intracellular PO2 and stimulation frequency in intact, isolated, single skeletal muscle fibers. Single, living muscle fibers (n = 7) were microdissected from the lumbrical muscles of Xenopus and injected with the oxygen-sensitive probe palladium-meso-tetra(4- carboxyphenyl) porphine (0.5 mM). Fibers were mounted with platinum clips to a force transducer in a chamber, which was continuously perfused with Ringer solution (pH = 7.0) at a PO2 of similar to 30 Torr. Fibers were then stimulated sequentially for 3 min, followed by a 3-min rest, at each of five contraction frequencies (0.15, 0.2, 0.25, 0.33, and 0.5 Hz), in a random order, using tetanic contractions. Resting intracellular PO2 averaged 31.2 +/- 0.9 Torr. During steady-state stimulation, intracellular PO2 declined to 21.2 +/- 2.3, 17.1 +/- 2.4, 15.3 +/- 1.9, 9.8 +/- 2.0, and 5.8 +/- 1.4 Torr for 0.15, 0.2, 0.25, 0.33, and 0.5-Hz stimulation, respectively. Significant fatigue, as defined by a decrease in force to <50% of the initial force, occurred only at the highest (0.5 Hz) stimulation frequency in five of the cells and at 0.33 Hz in the other two. Regression analysis demonstrated that there was a significant (P < 0.0001, r = 0.82) negative correlation between intracellular PO2 and contraction frequency in these isolated, single cells. The linear decrease in intracellular PO2 with stimulation frequency, and thus energy demand, suggests that a fall in intracellular PO2 correlates with increased oxygen uptake in these single contracting cells.