Control of microvascular oxygen pressures in rat muscles comprised of different fibre types

In response to an elevated metabolic rate ((V) over dot O-2), increased microvascular blood-muscle O-2 flux is the product of both augmented O-2 delivery ((Q) over dot O-2) and fractional O-2 extraction. Whole body and exercising limb measurements demonstrate that QO(2) and fractional O-2 extraction increase as linear and hyperbolic functions, respectively, of (V) over dot O-2. Given the presence of disparate vascular control mechanisms among different muscle fibre types, we tested the hypothesis that, in response to muscle contractions, (Q) over dot O-2 would be lower and fractional O-2 extraction (as assessed via microvascular O-2 pressure, P-mvO2) higher in fast- versus slow-twitch muscles. Radiolabelled microsphere and phosphorescence quenching techniques were used to measure (Q) over dot O-2 and P-mvO2 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 1), 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, P-mvO2 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 (V) over dot O-2 and (Q) over dot O-2 in MG and WG under each set of conditions. These data suggest that during submaximal metabolic rates, the relationship between (Q) over dot O-2, and O-2 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 O-2 extraction (i.e. lower P-mvO2) than their slow-twitch counterparts. These results also indicate that the greater sustained P-mvO2 in Sol maybe important for ensuring high blood-myocyte O-2 flux and therefore a greater oxidative contribution to energetic requirements.