Oxygen exchange profile in rat muscles of contrasting fibre types

Abstract

To determine whether fibre type affects the O2 exchange characteristics of skeletal muscle at the microcirculatory level we tested the hypothesis that, following the onset of contractions, muscle comprising predominately type I fibres (soleus, Sol, 86 % type I) would, based on demonstrated blood flow responses, exhibit a blunted microvascular PO2 (PO2,m, which is determined by the O2 delivery (QO2) to O2 uptake (VO2) ratio) profile (assessed via phosphorescence quenching) compared to muscle of primarily type II fibres (peroneal, Per, 84 % type II). PO2,m was measured at rest, and following the rest-contractions (twitch, 1 Hz, 2-4 V for 120 s) transition in Sol (n = 6) and Per (n = 6) muscles of Sprague-Dawley rats. Both muscles exhibited a delay followed by a mono-exponential decrease in PO2,m to the steady state. However, compared with Sol, Per demonstrated (1) a larger change in baseline minus steady state contracting PO2,m (DeltaPO2,m) (Per, 13.4 +/- 1.7 mmHg; Sol, 8.6 +/- 0.9 mmHg, P < 0.05); (2) a faster mean response time (i.e. time delay (TD) plus time constant (tau); Per, 23.8 +/- 1.5 s; Sol, 39.6 +/- 4.3 s, P < 0.05); and therefore (3) a greater rate of PO2,m decline (DeltaPO2,m/tau; Per, 0.92 +/- 0.08 mmHg s-1; Sol, 0.42 +/- 0.05 mmHg s-1, P < 0.05). These data demonstrate an increased microvascular pressure head of O2 at any given point after the initial time delay for Sol versus Per following the onset of contractions that is probably due to faster QO2 dynamics relative to those of VO2.

Figure 2. Model parameter differences in P_O2,m dynamics

Individual (filled symbols) and mean (±s.e.m.) (open symbols) P_O2,m values for soleus (circles) and peroneal (triangles) muscles. ^*Significantly different from peroneal response, P < 0.05.

Figure 1. P_O2,m profiles for Sol and Per

P_O2,m responses (dashed lines) and model fits (continuous lines) to 1 Hz electrical stimulation in representative soleus and peroneal muscles. Period before stimulation is resting P_O2,m.

Figure 3. Mean time courses for P_O2,m, _,musc and

Schematic showing mean fit of the measured mean P_O2,m responses (top panels), estimated _,musc responses (middle panels) and resultant response (bottom panels). Note substantially faster response in soleus required to produce the observed P_O2,m profile. Dashed lines represent the theoretical effect of slowing the Sol response to match that of the Per (i.e. changing τ() from 10 to 15 s). The resultant P_O2,m profile is substantially different from that actually observed.

Figure 4. Relationship between P_O2,m and _,musc

Graphical representation of the interaction between convective O₂ delivery and diffusive conductance of O₂. Fick equation lines (=(C_a,O2 - C_v,O2)), where represents blood flow and C_a,O2 and C_v,O2 represent arterial and venous O₂ contents, respectively. Curvilinear lines drawn from oxyhaemoglobin dissociation curve (P₅₀= 38.5 mmHg, ‘n’= 2.4; Altman & Dittmer, 1974). Fick's Law lines, =D_O2,musc×P_O2,m, where D_O2,musc (diffusional coefficient) is the slope of the line from origin to calculated and P_O2,m represents the microvascular O₂ pressure within the capillary bed. Note the reduced (ordinate) and D_O2,musc (slope of lines projecting from origin) in Per compared to Sol. SS-Contractions, steady state contracting values.