Skeletal muscle interstitial Po kinetics during recovery from contractions.

The oxygen partial pressure in the interstitial space (Po) drives O into the myocyte via diffusion, thus supporting oxidative phosphorylation. Although crucial for metabolic recovery and the capacity to perform repetitive tasks, the time course of skeletal muscle Po during recovery from contractions remains unknown. We tested the hypothesis that Po would recover to resting values and display considerable on-off asymmetry (fast on-, slow off-kinetics), reflective of asymmetric capillary hemodynamics. Microvascular Po (Po) was also evaluated to test the hypothesis that a significant transcapillary gradient (ΔPo = Po - Po) would be sustained during recovery. Po and Po (expressed in mmHg) were determined via phosphorescence quenching in the exposed rat spinotrapezius muscle during and after submaximal twitch contractions ( = 12). Po rose exponentially ( < 0.05) from end-contraction (11.1 ± 5.1), such that the end-recovery value (17.9 ± 7.9) was not different from resting Po (18.5 ± 8.1; > 0.05). Po off-kinetics were slower than on-kinetics (mean response time: 53.1 ± 38.3 versus 18.5 ± 7.3 s; < 0.05). A significant transcapillary ΔPo observed at end-contraction (16.6 ± 7.4) was maintained throughout recovery (end-recovery: 18.8 ± 9.6; > 0.05). Consistent with our hypotheses, muscle Po recovered to resting values with slower off-kinetics compared with the on-transient in line with the on-off asymmetry for capillary hemodynamics. Maintenance of a substantial transcapillary ΔPo during recovery supports that the microvascular-interstitium interface provides considerable resistance to O transport. As dictated by Fick's law (V̇o = Do × ΔPo), modulation of O flux (V̇o) during recovery must be achieved via corresponding changes in effective diffusing capacity (Do; mainly capillary red blood cell hemodynamics and distribution) in the face of unaltered ΔPo. Capillary blood-myocyte O flux (V̇o) is determined by effective diffusing capacity (Do; mainly erythrocyte hemodynamics and distribution) and microvascular-interstitial Po gradients (ΔPo = Po - Po). We show that Po demonstrates on-off asymmetry consistent with Po and erythrocyte kinetics during metabolic transitions. A substantial transcapillary ΔPo was preserved during recovery from contractions, indicative of considerable resistance to O diffusion at the microvascular-interstitium interface. This reveals that effective Do declines in step with V̇o during recovery, as per Fick's law.