ON-OFF ASYMMETRY OF LOCOMOTORY MUSCLE MICROVASCULAR PO2 KINETICS

Abstract

A profound asymmetry has been noted between the on- and off-transients for microvascular Po2 (Po2m; McDonough et al., Exp. Physiol., 86: 349, 2001), such that the recovery Po2m kinetics are prolonged compared to the on-transient during moderate-intensity contractions. Exercise intensity itself has important implications for metabolic control during and following contractions, however, the effect of exercise intensity upon the kinetic profile of Po2m during the on- and off-transient is not currently known. PURPOSE To test the hypothesis that increased exercise intensity (moderate: Mod) to heavy/severe: H/S) will speed on-transient, and slow off-transient dynamics in slow-twitch, oxidative (Soleus (S): predominantly type I fibers; citrate synthase activity (CS): ∼21 μmol/min/g), fast-twitch, oxidative (Mixed gastrocnemius (MG): type II; CS: ∼26) and fast-twitch, glycolytic muscle (White gastrocnemius (WG): type II; CS: ∼8). METHODS In female Sprague-Dawley rats (n=20; wt = ∼320g), the S, MG or WG was electrically stimulated (1 Hz) for 180 s at two different stimulation intensities; 2.5 V (Mod) and 4.5 V (H/S). Po2m dynamics were measured using phosphorescence quenching techniques. RESULTS The mean response time (MRT; i.e. time to 63% of steady state Po2m) for the on-transient was consistently faster during H/S compared to Mod (S: 52.0±6.2 and 39.1±5.7; MG: 21.8±4.4 and 12.3±1.5 and WG: 31.6±3.5 and 16.2±2.8 s). However, only the WG displayed an intensity-dependent off-transient MRT (98.9±20.2 and 154.0±23.2 s). CONCLUSIONS The intensity dependent decrease in the on-transient Po2m MRT indicates an accentuated sluggishness of the muscle blood flow response compared with that for Vo2 during H/S vs. Mod. The prolonged Po2m response observed in WG during the off-transient suggests that an O2 delivery limitation constrains recovery in muscles composed primarily of fast-twitch glycolytic fibers. Supported by: NIH HL-50306 and HL-67619