Rat Muscle Microvascular P O2 Kinetics During the Exercise Off‐Transient

Dependent upon the relative speed of pulmonary oxygen consumption (V̇ O2 ) 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 V̇ O2 / Q ˙ relationship within the muscle at the onset of the work/recovery transition. To address this issue, microvascular P O2 ( P O2,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 V̇ O2 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 P O2,m (i.e. off‐transient) rose progressively until baseline values were achieved (Δend‐recovery exercise P O2,m , 14.0 ± 1.9 Torr) and at no time fell below exercising P O2,m . The off‐transient P O2,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 V̇ O2 falls with faster kinetics than muscle Q ˙ during the off‐transient, such that P O2,m increases systematically, though biphasically, during recovery.