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Effects of arterial hypotension on microvascular oxygen exchange in contracting skeletal muscle

¹Department of Kinesiology and the Clarenburg Laboratory in the Department of Anatomy and Physiology, Kansas State University, Manhattan, Kansas; and ²Division of Exercise Physiology, West Virginia University School of Medicine, Morgantown, West Virginia

Submitted 5 April 2005 ; accepted in final form 7 November 2005

In healthy animals under normotensive conditions (N), contracting skeletal muscle perfusion is regulated to maintain microvascular O₂ pressures (Pmv) at levels commensurate with O₂ demands. Hypovolemic hypotension (H) impairs muscle contractile function; we tested whether this condition would alter the matching of O₂ delivery (O₂) to O₂ utilization (O₂), as determined by Pmv at the onset ofmuscle contractions. Pmv in the spinotrapezius muscles of seven female Sprague-Dawley rats (280 ± 6 g) was measured every 2 s across the transition from rest to 1-Hz twitch contractions. Measurements were made under N (mean arterial pressure, 97 ± 4 mmHg) and H (induced by arterial section; mean arterial pressure, 58 ± 3 mmHg, P < 0.05) conditions; Pmv profiles were modeled using a multicomponent exponential fitted with independent time delays. Hypotension reduced muscle blood flow at rest (24 ± 8 vs. 6 ± 1 ml^–1·min^–1·100 g^–1 for N and H, respectively; P < 0.05) and during contractions (74 ± 20 vs. 22 ± 4 ml^–1·min^–1·100 g^–1 for N and H, respectively; P < 0.05). H significantly decreased resting Pmv and steady-state contracting Pmv(19.4 ± 2.4 vs. 8.7 ± 1.6 Torr for N and H, respectively, P < 0.05). At the onset of contractions, H reduced the time delay (11.8 ± 1.7 vs. 5.9 ± 0.9 s for N andH, respectively, P < 0.05) before the fall in Pmv and accelerated therate of Pmv decrease (time constant, 12.6 ± 1.4 vs. 7.3 ± 0.9 s for N and H, respectively, P < 0.05). Muscle O₂ was reduced by 71% at rest and 64% with contractions in H vs. N, and O₂ extraction during H averaged 78% at rest and 94% during contractions vs. 51 and 78% in N. These results demonstrate that H constrains the increase of skeletal muscle O₂ relative to that of O₂ at the onset of contractions,leading to a decreased Pmv. According to Fick's law, this scenario will decrease blood-myocyte O₂ flux, thereby slowing O₂ kinetics and exacerbating the O₂ deficit generated at exercise onset.

oxygen delivery; oxygen utilization; rat spinotrapezius muscle; exercise; kinetics

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