Dynamics of PO₂ and VO₂ in resting and contracting rat spinotrapezius muscle

This study examined changes in interstitial PO₂, which allowed calculation of VO₂ during periods of rest, muscle contraction and recovery using an in situ rat spinotrapezius muscle preparation. The PO₂ was measured using phosphorescence quenching microscopy and the muscle VO₂ was calculated as the rate of O₂ disappearance during brief periods of muscle compression to stop blood flow with a supra-systolic pressure. The PO₂ and VO₂ measurements were made during "5 s compression and 15 s recovery" (CR) cycles. With all three stimulation frequencies, 1, 2 and 4 Hz, the fall in interstitial PO₂ and rise in VO₂ from resting values occurred within the first 20 s of contraction. The PO₂ during contraction became lower as stimulation frequency increased from 1 to 4 Hz. VO₂ was higher at 2 Hz than at 1 Hz contraction. With cessation of stimulation, PO₂ began increasing exponentially towards baseline values. After 1 and 2 Hz contraction, the fall in muscle VO₂ was delayed by one CR cycle and then exponentially decreased towards resting values. After 4 Hz stimulation, VO₂ increased for 2 cycles and then decreased. The post-contraction transients of PO₂ and VO₂ were not synchronous and had different time constants. With further analysis two distinct functional responses were identified across all stimulation frequencies having PO₂ during contraction above or below 30 mmHg. The corresponding VO₂ responses were different - for "high" PO₂, muscle VO₂ reached high levels, while for the "low" PO₂ data set muscle VO₂ remained low. Recovery patterns were similar to those described above. In summary, local microscopic PO₂ and VO₂ were measured in resting and contracting muscle in situ and the post-contraction transients of PO₂ and VO₂ were all much slower than the onset transients.