Effect of extracellular PO2 on the fall in intracellular PO 2 in contracting single myocytes. this investigation was to study the effects of altered extracellular PO2 (PEO 2 ) on the intracellular PO 2 (PIO 2 ) response to contractions in single skeletal muscle cells. Single myocytes (n ϭ 12) were dissected from lumbrical muscles of adult female Xenopus laevis and injected with 0.5 mM Pd-meso-tetra(4-carboxyphenyl)porphine for assessment of PIO 2 via phosphorescence quenching. At a PE O 2 of

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... (low), ϳ40 (moderate), and ϳ60 (high) Torr, tetanic contractions were induced at a frequency of 0.67 Hz for ϳ2 min with a 5-min recovery between bouts (blocked order design). The PIO 2 response to contractions was characterized by a time delay followed by a monoexponential decline to steady-state (SS) values. The fall in PIO 2 to SS values was significantly greater at each progressively greater PEO 2 (all P Ͻ 0.05). The mean response time (time delay ϩ time constant) was significantly faster in the low (35.2 Ϯ 5.1 s; P Ͻ 0.05 vs. high) and moderate (43.3 Ϯ 6.4 s; P Ͻ 0.05 vs. high) compared with high PEO 2 (61.8 Ϯ 9.4 s) and was correlated positively (r ϭ 0.965) with the net fall in PIO 2 . However, the initial rate of change of PIO 2 (calculated as net fall in PIO 2 / time constant) was not different (P Ͼ 0.05) among PEO 2 trials. These latter data suggest that, over the range of 20-60 Torr, PEO 2 does not play a deterministic role in setting the initial metabolic response to contractions in isolated frog myocytes. Additionally, these results suggest that oxidative phosphorylation in these myoglobin-free myocytes may be compromised by PEO 2 at values nearing 60 Torr. Xenopus laevis; oxidative phosphorylation; myoglobin THE REGULATORY EFFECT of O 2 concentration ([O 2 ]) on oxidative phosphorylation during exercise is an issue that presently remains unresolved due, in part, to differences associated with work intensity and the organ/system studied. Within isolated mitochondria, as PO 2 falls across a physiological range, cytochrome c becomes more reduced, whereas O 2 consumption (V O 2 ) remains uniform to levels near 1 Torr (32, 33). In intact, exercising muscle, the microcirculatory PO 2 considered rate limiting at the level of the mitochondria is dependent, in part, on the capillary-to-myocyte interface, fiber type, and mitochondrial capacity. In addition to the complexity of examining the effect of [O 2 ] on Address for reprint requests and other correspondence: C. A.