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

Abstract

In response to an elevated metabolic rate (V̇O2), increased microvascular blood-muscle O2 flux is the product of both augmented O2 delivery (Q̇O2) and fractional O2 extraction. Whole body and exercising limb measurements demonstrate that Q̇O2 and fractional O2 extraction increase as linear and hyperbolic functions, respectively, of V̇O2. Given the presence of disparate vascular control mechanisms among different muscle fibre types, we tested the hypothesis that, in response to muscle contractions, Q̇ O2 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 ΘO2 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 V̇O2 and Q̇O2 in MG and WG under each set of conditions. These data suggest that during submaximal metabolic rates, the relationship between Q̇ O2 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 maybe important for ensuring high blood-myocyte O2 flux and therefore a greater oxidative contribution to energetic requirements. © The Physiological Society 2005.