Dynamics of PO(2) and VO(2) in resting and contracting rat spinotrapezius muscle

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