Blood flow and O2 extraction as a function of O2 uptake in muscles composed of different fiber types

We examined how the greater vasodilatory capacity of slow- (ST) versus fast-twitch (FT) muscles impacts the relationship between blood flow (Q˙) and O2 uptake (V˙O2) and, consequently, the O2 extraction (a-vO2 diff.a-vO2 diff.)-to-V˙O2 relationship. Q˙ was measured with radiolabelled microspheres, while V˙O2 was calculated by the Fick principle using measurements of microvascular O2 pressure (phosphorescence quenching) at rest, low- (2.5 V) and high-intensity contractions (4.5 V) for soleus (Sol; ST, n = 5), mixed-gastrocnemius (MG; FT, n = 7) and white-gastrocnemius (WG; FT, n = 7). The slope of the Q˙-to-V˙O2 relationship (ΔQ˙/ΔV˙O2) was not different among muscles (Sol = 5.5 ± 0.2, MG = 6.0 ± 0.11 and WG = 5.8 ± 0.06; P > 0.05). In contrast, the intercept was greater (P < 0.05) for Sol (16.3 ± 2.7 ml min−1 100 g−1) versus MG and WG (in ml min−1 100 g−1: 1.39 ± 0.26 and 1.45 ± 0.23, respectively; MG and WG, P > 0.05). In addition, the a-vO2 diff.a-vO2 diff.-to-V˙O2 relationship for Sol was shifted rightward compared to MG and WG. These data suggest that the increase in Q˙ for a given change in V˙O2 is similar for slow- and fast-twitch muscles, at least for the range of metabolic rates and muscles studied herein and that a-vO2 diff.a-vO2 diff. differences result from the lower resting Q˙ in FT muscles.