Skeletal muscle interstitial PO2 kinetics during recovery from contractions.

The oxygen partial pressure in the interstitial space (PO₂ is) drives O₂ 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 PO₂ is during recovery from contractions remains unknown. We tested the hypothesis that PO_{2 is} would recover to resting values and display considerable on-off asymmetry (fast on-, slow off-kinetics), reflective of asymmetric capillary hemodynamics. Microvascular PO₂ (PO_{2 mv}) was also evaluated to test the hypothesis that a significant transcapillary gradient (ΔPO₂ = PO_{2 mv} -- PO_{2 is}) would be sustained during recovery. PO_{2 mv} and PO_{2 is} (expressed in mmHg) were determined via phosphorescence quenching in the exposed rat spinotrapezius muscle during and after submaximal twitch contractions (n = 12). PO_{2 is} rose exponentially (P < 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_{2 is} (18.5 ± 8.1; P > 0.05). PO_{2 is} off-kinetics were slower than on-kinetics (mean response time: 53.1 ± 38.3 versus 18.5 ± 7.3 s; P < 0.05). A significant transcapillary ΔPO₂ observed at end-contraction (16.6 ± 7.4) was maintained throughout recovery (end-recovery: 18.8 ± 9.6; P > 0.05). Consistent with our hypotheses, muscle PO_{2 is} 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 ΔPO₂ during recovery supports that the microvascular-interstitium interface provides considerable resistance to O₂ transport. As dictated by Fick's law (VO₂ = DO₂ x ΔPO₂), modulation of O₂ flux (VO₂) during recovery must be achieved via corresponding changes in effective diffusing capacity (DO₂; mainly capillary red blood cell hemodynamics and distribution) in the face of unaltered ΔPO₂. NEW & NOTEWORTHY Capillary blood-myocyte O₂ flux (VO₂) is determined by effective diffusing capacity (DO₂; mainly erythrocyte hemodynamics and distribution) and microvascular-interstitial PO₂ gradients (ΔPO₂ = PO_{2 mv} -- PO_{2 is}). We show that PO_{2 is} demonstrates on-off asymmetry consistent with PO_{2 mv} and erythrocyte kinetics during metabolic transitions. A substantial transcapillary ΔPO₂ was preserved during recovery from contractions, indicative of considerable resistance to O2 diffusion at the microvascular-interstitium interface. This reveals that effective DO₂ declines in step with VO2 during recovery, as per Fick's law. [ABSTRACT FROM AUTHOR]