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J Physiol Volume 563, Number 3, 903-913, March 15, 2005 DOI: 10.1113/jphysiol.2004.079533
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Control of microvascular oxygen pressures in rat muscles comprised of different fibre types

Paul McDonough1, Brad J Behnke2, Danielle J Padilla3, Timothy I Musch3 and David C Poole3

1 Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX 75309-9034, USA
2 Department of Kinesiology, Texas A & M University, College Station, TX, USA
3 Departments of Anatomy & Physiology and Kinesiology, Kansas State University, Manhattan, KS 66506-5802, USA

In response to an elevated metabolic rate {tjp_751_mu1}, increased microvascular blood–muscle O2 flux is the product of both augmented O2 delivery {tjp_751_mu2} and fractional O2 extraction. Whole body and exercising limb measurements demonstrate that {tjp_751_mu3} and fractional O2 extraction increase as linear and hyperbolic functions, respectively, of {tjp_751_mu4}. Given the presence of disparate vascular control mechanisms among different muscle fibre types, we tested the hypothesis that, in response to muscle contractions, {tjp_751_mu5} would be lower and fractional O2 extraction (as assessed via microvascular O2 pressure, PmvO2) higher in fast- versus slow-twitch muscles. Radiolabelled microsphere and phosphorescence quenching techniques were used to measure {tjp_751_mu6} and PmvO2, respectively at rest and across the transition to 1 Hz twitch contractions at low (Lo, 2.5 V) and high intensities (Hi, 4.5 V) in rat (n = 20) soleus (Sol, slow-twitch, type I), mixed gastrocnemius (MG, fast-twitch, type IIa) and white gastrocnemius (WG, fast-twitch, type IIb) muscle. At rest and for Lo and Hi (steady-state values) transitions, PmvO2 was lower (all P < 0.05) in MG (mmHg: rest, 22.5 ± 1.0; Lo, 15.3 ± 1.3; Hi, 10.2 ± 1.6) and WG (mmHg: rest, 19.0 ± 1.3; Lo, 12.2 ± 1.1; Hi, 9.9 ± 1.1) than in Sol (rest, 33.1 ± 3.2 mmHg; Lo, 19.0 ± 2.3 mmHg; Hi, 18.7 ± 1.8 mmHg), despite lower {tjp_751_mu7} and {tjp_751_mu8} in MG and WG under each set of conditions. These data suggest that during submaximal metabolic rates, the relationship between {tjp_751_mu9} and O2 extraction is dependent on fibre type (at least in the muscles studied herein), such that muscles comprised of fast-twitch fibres display a greater fractional O2 extraction (i.e. lower PmvO2) than their slow-twitch counterparts. These results also indicate that the greater sustained PmvO2 in Sol may be important for ensuring high blood–myocyte O2 flux and therefore a greater oxidative contribution to energetic requirements.

(Received 19 November 2004; accepted after revision 5 January 2005; first published online 6 January 2005)
Corresponding author D. C. Poole: Department of Anatomy and Physiology, College of Veterinary Medicine, Kansas State University, Manhattan, KS 66506-5802, USA. Email: poole{at}vet.k-state.edu




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