Rat muscle microvascular PO2 kinetics during the exercise off-transient

Dependent upon the relative speed of pulmonary oxygen consumption ((V) over dot o(2)) and blood now ((Q) over dot 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 (V) over dot o(2)/(Q) over dot relationship within the muscle at the onset of the work/recovery transition. To address this issue, microvascular Po-2 < Po-2,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) over dot relative to o(2), post exercise. The exercise blood flow response (radioactive microspheres: muscle (Q) over dot increased similar to 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 Po-2,m (i.e. off-transient) rose progressively until baseline values were achieved (Delta end-recovery exercise Po-2,m 14.0 +/- 1.9 Torr) and at no time fell below exercising PO?,m The off-transient Po-2,m was well fitted by a dual exponential model with both fast (tau = 25.4 +/- 5.1 s) and slow (tau = 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 (V) over dot O-2, falls with faster kinetics than muscle (Q) over dot during the off-transient, such that Po-2,m increases systematically, though biphasically, during recovery.