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1 Department of Medicine, Physiology Division, University of California-San Diego, La Jolla, CA, USA
* To whom correspondence should be addressed. E-mail: ckindig{at}ucsd.edu.
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 isolated skeletal muscle cells. We hypothesized that as PeO2 increased, thereby increasing the driving force for O2 flux, the fall in PiO2 (proportional to the net increase in VO2) and the speed of the initial metabolic response (calculated as fall in PiO2/
) would be increased. 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. For each cell, at PeO2's of ~20 (low), ~40 (moderate) and ~60 (high) mmHg, tetanic contractions were induced at a frequency of 0.67 Hz for ~2 min with a 5 min recovery between bouts (blocked order design). The PiO2 response to contractions was characterized by a time delay (TD) followed by a mono-exponential 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 (TD + ) 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/) was not different (p>0.05) between PeO2 trials. These latter data suggest that, over the range of 20 - 60 mmHg, PeO2 does not play a deterministic role in setting the initial metabolic response across the rest-to-contractions transition 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 mmHg.
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