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

Abstract

In response to an elevated metabolic rate (V O 2 ), increased microvascular blood-muscle O 2 flux is the product of both augmented O 2 delivery (Q O 2 ) and fractional O 2 extraction. Whole body and exercising limb measurements demonstrate thatQ O 2 and fractional O 2 extraction increase as linear and hyperbolic functions, respectively, ofV 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 O 2 would be lower and fractional O 2 extraction (as assessed via microvascular O 2 pressure, P mvO 2 ) higher in fast-versus slow-twitch muscles. Radiolabelled microsphere and phosphorescence quenching techniques were used to measureQ O 2 and P mvO 2 , 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, P mvO 2 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 lowerV O 2 andQ O 2 in MG and WG under each set of conditions. These data suggest that during submaximal metabolic rates, the relationship betweenQ 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 mvO 2 ) than their slow-twitch counterparts. These results also indicate that the greater sustained P mvO 2 in Sol may be important for ensuring high blood-myocyte O 2 flux and therefore a greater oxidative contribution to energetic requirements. The relationship between blood flow (Q) and metabolic rate (V O 2 ) is strong and has been well characterized for both the whole body and the exercising limbs. In the exercising human, legQ andV O 2 are linearly related by the equationQ = S ×V O 2 + I, where S represents the slope (5.3) and I the intercept (2.8 l min −1 ; data from Knight et al. 1992

Kinetics of O2uptake, leg blood flow, and muscle deoxygenation are slowed in the upper compared with lower region of the moderate-intensity exercise domain

“…Recent studies by Poole and colleagues show that there is a transient undershoot in microvascular PO 2 at the onset of electrically stimulated muscle contractions (reflecting a greater muscle O 2 utilization relative to muscle microvascular blood flow) in exposed muscle from animals with disease (6, 11), muscle from old vs. young animals (5), and in muscles having a higher fast-vs. slow-twitch fiber composition (27). In accordance with the data on microvascular PO 2 profiles, a transient overshoot in the HHb response might be expected whenever the adaptation of muscle O 2 consumption exceeds that of microvascular blood flow.…”

“…1 in Ref. 27) is seldom seen. However, in our analysis, we average data into 5-or 10-s time bins to reduce variability in the profiles that may obscure any overshoot that might exist during the exercise transient.…”

“…However, in our analysis, we average data into 5-or 10-s time bins to reduce variability in the profiles that may obscure any overshoot that might exist during the exercise transient. Also, Poole and colleagues (5,6,11,27) use an exposed, in situ animal muscle preparation to study microvascular PO 2 , whereas we study NIRS-derived HHb changes measured on the skin surface above the quadriceps muscle during human exercise, where the fidelity of the response may not be as great. In general the NIRS-derived HHb signal adapts with an "exponential-like" profile that we find can be fit adequately using a monoexponential model (see Fig.…”

Hypoxia and Resistance Exercise: A Comparison of Localized and Systemic Methods

“…Furthermore, it is likely that type II muscle fibres may be more sensitive to increased perfusion (i.e. via hypoxia-induced vasodilation) than type I fibres [140], owing to their greater fractional oxygen extraction if highly perfused [141]. This increased microvascular oxygen delivery may cause type II fibres to behave more like their oxidatively efficient type I counterparts [142], potentially attenuating contractile fatigue.…”

Dietary nitrate supplementation improves sprint and high-intensity intermittent running performance

“…14 Given that the reduction of NO 2 -to NO is potentiated with decreasing O 2 tension van Faassen et al 2009) and that PO 2 is lower in type II compared to type I muscle (Behnke et al 2003;McDonough et al 2005), increasing plasma [NO 2 -] via NO 3 -supplementation may enhance NO 2 --derived NO synthesis and thus performance during exercise at higher intensities. We have previously reported a fall in plasma [NO 2 -] during high-intensity intermittent exercise following NO 3 -supplementation (Wylie et al 2013a;Thompson et al 2015).…”