Skeletal muscle interstitial PO2 kinetics during recovery from contractions

The oxygen partial pressure in the interstitial space (Po-2 is) drives O-2 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-2 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 Po-2 (Po-2 mv) was also evaluated to test the hypothesis that a significant transcapillary gradient (Delta Po-2 = 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 Delta Po-2 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 Delta Po-2 during recovery supports that the microvascular-interstitium interface provides considerable resistance to O-2 transport. As dictated by Fick's law (Vo(2) = Do(2) x Delta Po-2), modulation of O-2 flux (Vo(2)) during recovery must be achieved via corresponding changes in effective diffusing capacity (Do(2); mainly capillary red blood cell hemodynamics and distribution) in the face of unaltered Delta Po-2. NEW & NOTEWORTHY Capillary blood-myocyte O-2 flux (Vo(2)) is determined by effective diffusing capacity (Do(2); mainly erythrocyte hemodynamics and distribution) and microvascular-interstitial Po-2 gradients (Delta Po-2 = 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 Delta Po-2 was preserved during recovery from contractions, indicative of considerable resistance to O-2 diffusion at the microvascular-interstitium interface. This reveals that effective Do(2) declines in step with Vo(2) during recovery, as per Fick's law.