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Received November 18, 2004
Revised December 6, 2004
Accepted after revision January 5, 2005
1 UT southwestern medical center
2 Texas A&M University
3 Kansas State University
* To whom correspondence should be addressed. E-mail: poole{at}vet.k-state.edu.
In response to an elevated metabolic rate (Vo2), increased microvascular blood-muscle O2 flux is the product of both augmented O2 delivery (Qo2) and fractional O2 extraction. Whole body and exercising limb measurements demonstrate that Qo2 and fractional O2 extraction increase as linear and hyperbolic functions of Vo2, respectively. Given the presence of disparate vascular control mechanisms among different muscle fiber types, we tested the hypothesis that, in response to muscle contractions, Qo2 would be lower and fractional O2 extraction (as assessed via microvascular O2 pressure, PmvO2) higher in fast vs slow-twitch muscles. Radiolabelled microsphere and phosphorescence quenching techniques were utilized to measure Qo2 and PmvO2 at rest and across the transition to 1 Hz twitch contractions at low (LO, 2.5 v) and high (HI, 4.5 v) intensities 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 (rest, 22.5+1.0; LO, 15.3+1.3; HI, 10.2+1.6 mmHg) and WG (rest, 19.0+1.3; LO, 12.2+1.1; HI, 9.9+1.1 mmHg) than in SOL (rest, 33.1+3.2; LO, 19.0+2.3; HI, 18.7+1.8 mmHg), despite lower and in MG and WG under each condition. These data suggest that during submaximal metabolic rates, the relationship between Qo2 and O2 extraction is fiber type dependent (at least in the muscles studied herein), such that muscles comprised of fast-twitch fibers 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 ensuring a greater oxidative contribution to energetic requirements. Key words: muscle contraction; microvascular O2 exchange; muscle fiber type; O2 extraction; phosphorescence quenching
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