Skeletal muscle interstitial Po2 kinetics during recovery from contractions

The effect of sepsis and reactive oxygen species on skeletal muscle interstitial oxygen pressure during contractions

OBJECTIVE

This study aims to examine the effect of sepsis on the dynamics of skeletal muscle partial oxygen pressure during muscle contractions as well as the effect of reactive oxygen species (ROS) scavenger (ascorbic acid, Asc).

METHODS

Twenty-seven male Sprague-Dawley rats (2-3 months old) were randomly assigned to three groups; sham, cecal ligation and puncture (CLP), or CLP plus ascorbic acid treatment group (CLP + Asc). Electrical stimuli-induced muscle contractions and partial oxygen pressure measurements were performed at 3 h after CLP. The interstitial oxygen pressure (PO2 is) in the spinotrapezius muscle was measured by the phosphorescence quenching method.

RESULTS

The PO2 is at rest was not different between the three groups. The PO2 is decreased from rest to contraction in all groups. Compared to the sham, the time to decrease PO2 is was significantly faster in CLP but not in CLP + Asc (p < .05). Compared to the sham, the PO2 is during muscle contractions was significantly lower in both CLP and CLP + Asc (p < .05, respectively).

CONCLUSIONS

Our results suggest that CLP-induced sepsis accelerated the decay of PO2 is at the onset of muscle contractions and maintained a low level of PO2 is during muscle contractions.

Effects of acute phosphodiesterase type 5 inhibition on skeletal muscle interstitial PO2 during contractions and recovery

The oxygen partial pressure within the interstitial space (PO2is; mmHg) provides the driving force for oxygen diffusion into the myocyte thereby supporting oxidative phosphorylation. We tested the hypothesis that potentiation of the nitric oxide pathway with sildenafil (phosphodiesterase type 5 inhibitor) would enhance PO2is during muscle metabolic transitions, thereby slowing PO2is on- and accelerating PO2is off-kinetics. The rat spinotrapezius muscle (n = 17) was exposed for PO2is measurements via phosphorescence quenching under control (CON), low-dose sildenafil (1 mg/kg i.a., SIL1) and high-dose sildenafil (7 mg/kg i.a., SIL7). Data were collected at rest and during submaximal twitch contractions (1 Hz, 4-6 V, 3 min) and recovery (3 min). Mean arterial blood pressure (MAP; mmHg) was reduced with both SIL1 (pre:132 ± 5; post:99 ± 5) and SIL7 (pre:111 ± 6; post:99 ± 4) (p < 0.05). SIL7 elevated resting PO2is (18.4 ± 1.1) relative to both CON (15.7 ± 0.7) and SIL1 (15.2 ± 0.7) (p < 0.05). In addition, SIL7 increased end-recovery PO2is (17.7 ± 1.6) compared to CON (12.8 ± 0.9) and SIL1 (13.4 ± 0.8) (p < 0.05). The overall PO2is response during recovery (i.e., area under the PO2is curve) was greater in SIL7 (4107 ± 444) compared to CON (3493 ± 222) and SIL1 (3114 ± 205 mmHg s) (p < 0.05). Contrary to our hypothesis, there was no impact of acute SIL (1 or 7 mg/kg) on the speed of the PO2is response during contractions or recovery (p > 0.05). However, sildenafil lowered MAP and improved skeletal muscle interstitial oxygenation in healthy rats. Specifically, SIL7 enhanced PO2is at rest and during recovery from submaximal muscle contractions. Potentiation of the nitric oxide pathway with sildenafil enhances microvascular blood-myocyte O2 transport and is expected to improve repeated bouts of contractile activity.

How Priming Exercise Affects Oxygen Uptake Kinetics: From Underpinning Mechanisms to Endurance Performance

The observation that prior heavy or severe-intensity exercise speeds overall oxygen uptake ([Formula: see text]O₂) kinetics, termed the "priming effect", has garnered significant research attention and its underpinning mechanisms have been hotly debated. In the first part of this review, the evidence for and against (1) lactic acidosis, (2) increased muscle temperature, (3) O₂ delivery, (4) altered motor unit recruitment patterns and (5) enhanced intracellular O₂ utilisation in underpinning the priming effect is discussed. Lactic acidosis and increased muscle temperature are most likely not key determinants of the priming effect. Whilst priming increases muscle O₂ delivery, many studies have demonstrated that an increased muscle O₂ delivery is not a prerequisite for the priming effect. Motor unit recruitment patterns are altered by prior exercise, and these alterations are consistent with some of the observed changes in [Formula: see text]O₂ kinetics in humans. Enhancements in intracellular O₂ utilisation likely play a central role in mediating the priming effect, probably related to elevated mitochondrial calcium levels and parallel activation of mitochondrial enzymes at the onset of the second bout. In the latter portion of the review, the implications of priming on the parameters of the power-duration relationship are discussed. The effect of priming on subsequent endurance performance depends critically upon which phases of the [Formula: see text]O₂ response are altered. A reduced [Formula: see text]O₂ slow component or increased fundamental phase amplitude tend to increase the work performable above critical power (i.e. W´), whereas a reduction in the fundamental phase time constant following priming results in an increased critical power.

Aging alters gastrocnemius muscle hemoglobin oxygen saturation (StO2) characteristics in healthy individuals

PURPOSE

Functional limitations during exercise from alterations in the balance of oxygen supply and demand-as reported by lower tissue oxygen saturation and longer recovery time-are well documented in clinical populations. We aimed to assess changes in skeletal muscle hemoglobin oxygen saturation (StO₂) characteristics during exercise as a result of aging in otherwise healthy individuals.

METHODS

We recruited healthy male and female participants (n = 101) from three age ranges-young (18-39 years), middle age (40-65 years), and older (> 65 years)-to complete exercise tests commonly used in clinical populations. Using near-infrared spectroscopy (NIRS) we assessed StO₂ in the medial gastrocnemius during the Gardner Treadmill Protocol and 6 min walk test (6MWT).

RESULTS

Minimum StO₂ (%) during the treadmill test was significantly lower for both middle-age (36.1 ± 20.6) and older (27.3 ± 19.4) participants compared to young (46.8 ± 14.8) (p < 0.05 and p < 0.01 respectively), and recovery time (minutes) was significantly prolonged (young = 0.22 ± 0.34; middle age = 0.66 ± 0.52; older = 1.04 ± 1.00) (p < 0.001 for both middle age and older compared to young). Similar results were shown during the 6MWT, as minimum StO₂ (%) was lower in middle-age (41.7 ± 17.2) and older (40.0 ± 25.9) participants compared to young (53.6 ± 14.5) (p < 0.05), and recovery times (minutes) were prolonged (young: 0.11 ± 0.17; middle age: 0.46 ± 0.42; older: 0.93 ± 0.43) (p < 0.001 for both middle age and older compared to young). Simple linear regression analyses demonstrated that age predicted treadmill recovery and 6MWT recovery.

CONCLUSION

Our study provides evidence that aging, even in otherwise healthy individuals, negatively impacts muscle StO₂ characteristics. In older individuals, working muscle tissue may reach lower oxygen saturation during exercise and take longer to return to baseline oxygen saturation post-exercise.

Proportional Assist Ventilation Improves Leg Muscle Reoxygenation After Exercise in Heart Failure With Reduced Ejection Fraction

Background

Respiratory muscle unloading through proportional assist ventilation (PAV) may enhance leg oxygen delivery, thereby speeding off-exercise oxygen uptake ( V . ⁢ O 2 ) kinetics in patients with heart failure with reduced left ventricular ejection fraction (HFrEF).

Methods

Ten male patients (HFrEF = 26 ± 9%, age 50 ± 13 years, and body mass index 25 ± 3 kg m²) underwent two constant work rate tests at 80% peak of maximal cardiopulmonary exercise test to tolerance under PAV and sham ventilation. Post-exercise kinetics of V . ⁢ O 2 , vastus lateralis deoxyhemoglobin ([deoxy-Hb + Mb]) by near-infrared spectroscopy, and cardiac output (Q _T ) by impedance cardiography were assessed.

Results

PAV prolonged exercise tolerance compared with sham (587 ± 390 s vs. 444 ± 296 s, respectively; p = 0.01). PAV significantly accelerated V . ⁢ O 2 recovery (τ = 56 ± 22 s vs. 77 ± 42 s; p < 0.05), being associated with a faster decline in Δ[deoxy-Hb + Mb] and Q _T compared with sham (τ = 31 ± 19 s vs. 42 ± 22 s and 39 ± 22 s vs. 78 ± 46 s, p < 0.05). Faster off-exercise decrease in Q _T with PAV was related to longer exercise duration (r = -0.76; p < 0.05).

Conclusion

PAV accelerates the recovery of central hemodynamics and muscle oxygenation in HFrEF. These beneficial effects might prove useful to improve the tolerance to repeated exercise during cardiac rehabilitation.

The role of vascular function on exercise capacity in health and disease

Three sentinel parameters of aerobic performance are the maximal oxygen uptake ( V ̇ O 2 max ), critical power (CP) and speed of the V ̇ O 2 kinetics following exercise onset. Of these, the latter is, perhaps, the cardinal test of integrated function along the O2 transport pathway from lungs to skeletal muscle mitochondria. Fast V ̇ O 2 kinetics demands that the cardiovascular system distributes exercise-induced blood flow elevations among and within those vascular beds subserving the contracting muscle(s). Ideally, this process must occur at least as rapidly as mitochondrial metabolism elevates V ̇ O 2 . Chronic disease and ageing create an O2 delivery (i.e. blood flow × arterial [O2 ], Q ̇ O 2 ) dependency that slows V ̇ O 2 kinetics, decreasing CP and V ̇ O 2 max , increasing the O2 deficit and sowing the seeds of exercise intolerance. Exercise training, in contrast, does the opposite. Within the context of these three parameters (see Graphical Abstract), this brief review examines the training-induced plasticity of key elements in the O2 transport pathway. It asks how structural and functional vascular adaptations accelerate and redistribute muscle Q ̇ O 2 and thus defend microvascular O2 partial pressures and capillary blood-myocyte O2 diffusion across a ∼100-fold range of muscle V ̇ O 2 values. Recent discoveries, especially in the muscle microcirculation and Q ̇ O 2 -to- V ̇ O 2 heterogeneity, are integrated with the O2 transport pathway to appreciate how local and systemic vascular control helps defend V ̇ O 2 kinetics and determine CP and V ̇ O 2 max in health and how vascular dysfunction in disease predicates exercise intolerance. Finally, the latest evidence that nitrate supplementation improves vascular and therefore aerobic function in health and disease is presented.

Impact of supine exercise on muscle deoxygenation kinetics heterogeneity: mechanistic insights into slow pulmonary oxygen uptake dynamics

Oxygen uptake (V̇o₂) kinetics are slowed in the supine (S) position purportedly due to impaired muscle O₂ delivery ([Formula: see text]); however, these conclusions are predicated on single-site measurements in superficial muscle using continuous-wave near-infrared spectroscopy (NIRS). This study aimed to determine the impact of body position [i.e., upright (U) versus S] on deep and superficial muscle deoxygenation (deoxy[heme]) using time-resolved (TR-) NIRS, and how these relate to slowed pulmonary V̇o₂ kinetics. Seventeen healthy men completed constant power tests during 1) S heavy-intensity exercise and 2) U exercise at the same absolute work rate, with a subset of 10 completing additional tests at the same relative work rate as S. Pulmonary V̇o₂ was measured breath-by-breath and, deoxy- and total[heme] were resolved via TR-NIRS in the superficial and deep vastus lateralis and superficial rectus femoris. The fundamental phase V̇o₂ time constant was increased during S compared with U (S: 36 ± 10 vs. U: 27 ± 8 s; P < 0.001). The deoxy[heme] amplitude (S: 25-28 vs. U: 13-18 µM; P < 0.05) and total[heme] amplitude (S: 17-20 vs. U: 9-16 µM; P < 0.05) were greater in S compared with U and were consistent for the same absolute (above data) and relative work rates (n = 10, all P < 0.05). The greater deoxy- and total[heme] amplitudes in S vs. U supports that reduced perfusive [Formula: see text] in S, even within deep muscle, necessitated a greater reliance on fractional O₂ extraction and diffusive [Formula: see text]. The slower V̇o₂ kinetics in S versus U demonstrates that, ultimately, these adjustments were insufficient to prevent impairments in whole body oxidative metabolism.NEW & NOTEWORTHY We show that supine exercise causes a greater degree of muscle deoxygenation in both deep and superficial muscle and increases the spatial heterogeneity of muscle deoxygenation. Therefore, this study suggests that any O₂ delivery gradient toward deep versus superficial muscle is insufficient to mitigate impairments in oxidative function in response to reduced whole muscle O₂ delivery. More heterogeneous muscle deoxygenation is associated with slower V̇o₂ kinetics.