Effect of extracellular PO2 on the fall in intracellular PO2 in contracting single myocytes

The purpose of this investigation was to study the effects of altered extracellular PO2 (PEO2) on the intracellular PO2 (PIO2) response to contractions in single skeletal muscle cells. Single myocytes (n = 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 PIO2 via phosphorescence quenching. At a PEO2 of similar to20 (low), similar to40 (moderate), and similar to60 (high) Torr, tetanic contractions were induced at a frequency of 0.67 Hz for similar to2 min with a 5-min recovery between bouts ( blocked order design). The PIO2 response to contractions was characterized by a time delay followed by a monoexponential decline to steady-state ( SS) values. The fall in PIO2 to SS values was significantly greater at each progressively greater PEO2 (all P < 0.05). The mean response time (time delay + time constant) was significantly faster in the low (35.2 +/- 5.1 s; P < 0.05 vs. high) and moderate (43.3 +/- 6.4 s; P < 0.05 vs. high) compared with high PEO2 (61.8 +/- 9.4 s) and was correlated positively (r = 0.965) with the net fall in PIO2. However, the initial rate of change of PIO2 (calculated as net fall in PIO2/ time constant) was not different (P > 0.05) among PEO2 trials. These latter data suggest that, over the range of 20-60 Torr, PEO2 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 PEO2 at values nearing 60 Torr.