Rest-to-work and work-to-rest transients of interstitial PO2 in spinotrapezius muscle of young and old male rats.

Muscle function declines with age. Since the primary energy source for contraction is aerobic, this study investigated age-related changes in muscle oxygenation dynamics to: characterize PO₂ transients during rest-work transitions, identify age-specific differences in oxygen delivery/utilization balance, and examine the relationship between interstitial and arterial oxygen tension (PO₂). Interstitial PO₂ was measured with a high-resolution stroboscopic phosphorescence quenching technique to map intra-contractile dynamics during changes in muscle activity-rest-to-work (RtW) and work-to-rest (WtR) in rats aged three (young) and 23 (old) months. RtW (τ_w) and WtR (τ_r) PO₂ transitions had lag periods and mono-exponential time constants. In young muscles, lag was 4 s, τ_w = 9.0 ± 3.7 s, and τ_r = 15.4 ± 3.9 s. For old, lag was also 4 s with increases to τ_w = 15.9 ± 3.5 s and τ_r = 41.4 ± 8.3 s. Resting PO₂'s were higher for young than for old (66.7 ± 13.7 vs. 60.2 ± 13.0 mmHg; p < 0.05). Work reduced PO₂ with a greater effect on old (42.5 ± 14.0 vs. 28.3 ± 16.5 mmHg; p < 0.05). Intra-contractile measurements revealed a spike in PO₂ (11 mmHg amplitude for >200 ms), which was absent in old. Further, sustained exercise in young showed a rising trend in PO₂, while old remained at nadir. The missing PO₂ spike in aged muscle contributes to reduced PO₂ during work and may explain age-related loss of endurance.