Exercise Training Slows Microvascular PO2 (PO2m) On-Kinetics in Rat Skeletal Muscle

Abstract

1589 In skeletal muscle, the PO2m reflects the dynamic balance between oxygen delivery (QO2) and oxygen uptake and utilization (VO2). Exercise training in known to increase both blood flow capacity and oxidative capacity in skeletal muscle. Thus, we predicted that training would result in a better matching between QO2 and VO2 and the onset of contractions, thereby slowing the fall in PO2m. PURPOSE: The purpose of this study was to test the hypothesis that exercise training will slow PO2m on-kinetics in rat hindlimb skeletal muscle. METHODS: Female Sprague-Dawley rats were randomly divided into sedentary (S, n = 10; 299 ± 6 g) and exercise trained (ET, n = 10; 294 ± 6 g) groups. ET rats performed treadmill exercise 60 min/day, 5 days/wk for 9–12 wks. PO2m was measured in the extensor digitorum longus muscle (EDL) using the phosphorescence quenching technique, at rest and during muscle contractions (1 Hz, 6 V, 2 ms). PO2m on-kinetics were determined across the rest (30 s) to steadystate contractions (150 s) transition using a time delay plus mono-exponential model. RESULTS: At the same arterial PO2 (S, 81 ± 2; ET, 81 ± 3 mmHg), resting PO2m (S, 26 ± 2; ET, 28 ± 2 mmHg) was the same in S and ET rats. The time delay before the fall in PO2m (ΔPO2m) at the onset of contractions was unaffected by training (S, 7 ± 1; ET, 7 ± 1 s), and the fall in PO2m during contractions (S, 16 ± 2; ET, 17 ± 2 mmHg) was the same in S and ET rats. However, the time constant for the fall in PO2m was more than twice as long in ET rats (S, 7 ± 1; ET 16 ± 4 s; P<0.05). Further, the rate of change in PO2m (ΔPO2m/time constant) was slower in ET (S, 2.33 ± 0.27; ET, 1.64 ± 0.35 mmHg/s; P<0.05). CONCLUSIONS: These data support the hypothesis that exercise training results in a slowing of skeletal muscle PO2m on-kinetics at the onset of contractions, leading to a delayed fall in PO2m which would maintain a higher driving pressure for oxygen flux early in the contraction bout. (Supported by the Heartland Affiliate, American Heart Association.)