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 O-2 uptake (V-O2) and, consequently, the O-2 extraction (a-v(O2) diff.)-to-V-O2 relationship. Q was measured with radiolabelled microspheres, while V-O2 Was calculated by the Fick principle using measurements of microvascular O-2 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 (Delta Q/Delta V-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-v(O2) (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-v(O2 diff). differences result from the lower resting in FT muscles. (c) 2005 Elsevier B.V. All rights reserved.