Rat Muscle Microvascular PO2 Kinetics During the Exercise Off‐Transient

Abstract
Dependent upon the relative speed of pulmonary oxygen consumption (V̇O2) and blood flow (Q˙) kinetics, the exercise off‐transient may represent a condition of sub‐ or supra‐optimal perfusion. To date, there are no direct measurements of the dynamics of the V̇O2/Q˙ relationship within the muscle at the onset of the work/recovery transition. To address this issue, microvascular PO2 (PO2,m) dynamics were studied in the spinotrapezius muscles of 11 female Sprague‐Dawley rats (weight ∼220 g) during and following electrical stimulation (1 Hz) to assess the adequacy of Q˙ relative to V̇O2 post exercise. The exercise blood flow response (radioactive microspheres: muscle Q˙ increased ∼240%), and post‐exercise arterial blood pH (7.40 ± 0.02) and blood lactate (1.3 ± 0.4 mM l−1) values were consistent with moderate‐intensity exercise. Recovery PO2,m (i.e. off‐transient) rose progressively until baseline values were achieved (Δend‐recovery exercise PO2,m, 14.0 ± 1.9 Torr) and at no time fell below exercising PO2,m. The off‐transient PO2,m was well fitted by a dual exponential model with both fast (τ= 25.4 ± 5.1 s) and slow (τ= 71.2 ± 34.2 s) components. Furthermore, there was a pronounced delay (54.9 ± 10.7 s) before the onset of the slow component. These data, obtained at the muscle microvascular level, support the notion that muscle V̇O2 falls with faster kinetics than muscle Q˙ during the off‐transient, such that PO2,m increases systematically, though biphasically, during recovery.
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