Oxygen exchange in muscle of young and old rats: muscle-vascular-pulmonary coupling

Spike of interstitial PO2 produced by a twitch in rhythmically contracted muscle

Oxygen (O2 ) exchange between capillaries and muscle cells in exercising muscles is of great interest for physiology and kinesiology. However, methodical limitations prevent O2 measurements on the millisecond scale. To bypass the constraints of quasi-continuous recording, progressive measurements of O2 partial pressure (PO2 ) in rhythmically contracting skeletal muscle were compiled to describe the O2 kinetics surrounding and including a single muscle contraction. Phosphorescence quenching microscopy measured PO2 in the interstitium of the rat spinotrapezius muscle. Measurements were triggered by contraction-inducing electrical pulses. For the first 60 seconds, measurement preceeded stimulation. After 60, measurement followed with a progressive 20 ms increment. Thus, the first 60 measurements describe the overall PO2 response to electrical stimulation initiated after a 10 second rest period, while 61-100 (stroboscopic mode) were compiled into a single 800 ms profile of the PO2 transient surrounding muscle contraction. Thirty seconds of stimulated contractions decreased interstitial PO2 from a baseline of 71 ± 1.4 mmHg to an "active" steady-state of 43 ± 1.5 mmHg. The stroboscopic mode compilation revealed an unexpected post-contractile rise in PO2 as a 205 ms spike with a maximum amplitude of 58 ± 3.8 mmHg at 68 ms, which restored 58% of the PO2 drop from baseline. Interpretation of this phenomenon is based on classical experiments by G.V. Anrep (1935), who discovered the rapid thrust of blood flow associated with muscle contraction. In addition to the metabolic implications during exercise, the physiological impact of these PO2 spikes may grow with an increased rate of rhythmical contractions in muscle or heart. NEW&NOTEWORTHY: The principal finding is a spike of interstitial PO2 , produced by a twitch in a rhythmically contracting muscle. A possible mechanism is flushing capillaries with arterial blood by mechanical forces. A technical novelty is the PO2 measurement with a "stroboscopic mode" and progressively increasing delay between stimulator pulse and PO2 measuring. That permitted a 20 ms time resolution for a 205 ms spike duration, using an excitation flash rate one per second.

Daily muscle stretching enhances blood flow, endothelial function, capillarity, vascular volume and connectivity in aged skeletal muscle

KEY POINTS

In aged rats, daily muscle stretching increases blood flow to skeletal muscle during exercise. Daily muscle stretching enhanced endothelium-dependent vasodilatation of skeletal muscle resistance arterioles of aged rats. Angiogenic markers and capillarity increased in response to daily stretching in muscles of aged rats. Muscle stretching performed with a splint could provide a feasible means of improving muscle blood flow and function in elderly patients who cannot perform regular aerobic exercise.

ABSTRACT

Mechanical stretch stimuli alter the morphology and function of cultured endothelial cells; however, little is known about the effects of daily muscle stretching on adaptations of endothelial function and muscle blood flow. The present study aimed to determine the effects of daily muscle stretching on endothelium-dependent vasodilatation and muscle blood flow in aged rats. The lower hindlimb muscles of aged Fischer rats were passively stretched by placing an ankle dorsiflexion splint for 30 min day^-1 , 5 days week^-1 , for 4 weeks. Blood flow to the stretched limb and the non-stretched contralateral limb was determined at rest and during treadmill exercise. Endothelium-dependent/independent vasodilatation was evaluated in soleus muscle arterioles. Levels of hypoxia-induced factor-1α, vascular endothelial growth factor A and neuronal nitric oxide synthase were determined in soleus muscle fibres. Levels of endothelial nitric oxide synthase and superoxide dismutase were determined in soleus muscle arterioles, and microvascular volume and capillarity were evaluated by microcomputed tomography and lectin staining, respectively. During exercise, blood flow to plantar flexor muscles was significantly higher in the stretched limb. Endothelium-dependent vasodilatation was enhanced in arterioles from the soleus muscle from the stretched limb. Microvascular volume, number of capillaries per muscle fibre, and levels of hypoxia-induced factor-1α, vascular endothelial growth factor and endothelial nitric oxide synthase were significantly higher in the stretched limb. These results indicate that daily passive stretching of muscle enhances endothelium-dependent vasodilatation and induces angiogenesis. These microvascular adaptations may contribute to increased muscle blood flow during exercise in muscles that have undergone daily passive stretch.

Deep Phenotyping of Systemic Arterial Hemodynamics in HFpEF (Part 2): Clinical and Therapeutic Considerations

Multiple phase III trials over the last few decades have failed to demonstrate a clear benefit of various pharmacologic interventions in heart failure with a preserved left ventricular (LV) ejection fraction (HFpEF). Therefore, a better understanding of its pathophysiology is important. An accompanying review describes key technical and physiologic aspects regarding the deep phenotyping of arterial hemodynamics in HFpEF. This review deals with the potential of this approach to enhance our clinical, translational, and therapeutic approach to HFpEF. Specifically, the role of arterial hemodynamics is discussed in relation to (1) the pathophysiology of left ventricular diastolic dysfunction, remodeling, and fibrosis, (2) impaired oxygen delivery to peripheral skeletal muscle, which affects peripheral oxygen extraction, (3) the frequent presence of comorbidities, such as renal failure and dementia in this population, and (4) the potential to enhance precision medicine approaches. A therapeutic approach to target arterial hemodynamic abnormalities that are prevalent in this population (particularly, with inorganic nitrate/nitrite) is also discussed.

Barometric calibration of a luminescent oxygen probe

The invention of the phosphorescence quenching method for the measurement of oxygen concentration in blood and tissue revolutionized physiological studies of oxygen transport in living organisms. Since the pioneering publication by Vanderkooi and Wilson in 1987, many researchers have contributed to the measurement of oxygen in the microcirculation, to oxygen imaging in tissues and microvessels, and to the development of new extracellular and intracellular phosphorescent probes. However, there is a problem of congruency in data from different laboratories, because of interlaboratory variability of the calibration coefficients in the Stern-Volmer equation. Published calibrations for a common oxygen probe, Pd-porphyrin + bovine serum albumin (BSA), vary because of differences in the techniques used. These methods are used for the formation of oxygen standards: chemical titration, calibrated gas mixtures, and an oxygen electrode. Each method in turn also needs calibration. We have designed a barometric method for the calibration of oxygen probes by using a regulated vacuum to set multiple PO2 standards. The method is fast and accurate and can be applied to biological fluids obtained during or after an experiment. Calibration over the full physiological PO2 range (1-120 mmHg) takes ∼15 min and requires 1-2 mg of probe.

Developmental effects of wheel running on hippocampal glutamate receptor expression in young and mature adult rats

Recent evidence suggests that the behavioral benefits associated with voluntary wheel running in rodents may be due to modulation of glutamatergic transmission in the hippocampus, a brain region implicated in learning and memory. However, the expression of the glutamatergic ionotropic N-methyl-d-aspartate receptor (GluN) in the hippocampus in response to chronic sustained voluntary wheel running has not yet been investigated. Further, the developmental effects during young and mature adulthood on wheel running output and GluN expression in hippocampal subregions has not been determined, and therefore is the main focus of this investigation. Eight-week-old and 16-week-old male Wistar rats were housed in home cages with free access to running wheels and running output was monitored for 4weeks. Wheel access was terminated and tissues from the dorsal and ventral hippocampi were processed for Western blot analysis of GluN subunit expression. Young adult runners demonstrated an escalation in running output but this behavior was not evident in mature adult runners. In parallel, young adult runners demonstrated a significant increase in total GluN (1 and 2A) subunit expression in the dorsal hippocampus (DH), and an opposing effect in the ventral hippocampus (VH) compared to age-matched sedentary controls; these changes in total protein expression were not associated with significant alterations in the phosphorylation of the GluN subunits. In contrast, mature adult runners demonstrated a reduction in total GluN2A expression in the DH, without producing alterations in the VH compared to age-matched sedentary controls. In conclusion, differential running activity-mediated modulation of GluN subunit expression in the hippocampal subregions was revealed to be associated with developmental effects on running activity, which may contribute to altered hippocampal synaptic activity and behavioral outcomes in young and mature adult subjects.

Acute inhibition of ATP-sensitive K+ channels impairs skeletal muscle vascular control in rats during treadmill exercise

The ATP-sensitive K(+) (KATP) channel is part of a class of inward rectifier K(+) channels that can link local O2 availability to vasomotor tone across exercise-induced metabolic transients. The present investigation tested the hypothesis that if KATP channels are crucial to exercise hyperemia, then inhibition via glibenclamide (GLI) would lower hindlimb skeletal muscle blood flow (BF) and vascular conductance during treadmill exercise. In 27 adult male Sprague-Dawley rats, mean arterial pressure, blood lactate concentration, and hindlimb muscle BF (radiolabeled microspheres) were determined at rest (n = 6) and during exercise (n = 6-8, 20, 40, and 60 m/min, 5% incline, i.e., ~60-100% maximal O2 uptake) under control and GLI conditions (5 mg/kg intra-arterial). At rest and during exercise, mean arterial pressure was higher (rest: 17 ± 3%, 20 m/min: 5 ± 1%, 40 m/min: 5 ± 2%, and 60 m/min: 5 ± 1%, P < 0.05) with GLI. Hindlimb muscle BF (20 m/min: 16 ± 7%, 40 m/min: 30 ± 9%, and 60 m/min: 20 ± 8%) and vascular conductance (20 m/min: 20 ± 7%, 40 m/min: 33 ± 8%, and 60 m/min: 24 ± 8%) were lower with GLI during exercise at 20, 40, and 60 m/min, respectively (P < 0.05 for all) but not at rest. Within locomotory muscles, there was a greater fractional reduction present in muscles comprised predominantly of type I and type IIa fibers at all exercise speeds (P < 0.05). Additionally, blood lactate concentration was 106 ± 29% and 44 ± 15% higher during exercise with GLI at 20 and 40 m/min, respectively (P < 0.05). That KATP channel inhibition reduces hindlimb muscle BF during exercise in rats supports the obligatory contribution of KATP channels in large muscle mass exercise-induced hyperemia.

Experimental studies of the molecular pathways regulated by exercise and resveratrol in heart, skeletal muscle and the vasculature

Regular exercise contributes to healthy aging and the prevention of chronic disease. Recent research has focused on the development of molecules, such as resveratrol, that activate similar metabolic and stress response pathways as exercise training. In this review, we describe the effects of exercise training and resveratrol on some of the organs and tissues that act in concert to transport oxygen throughout the body. In particular, we focus on animal studies that investigate the molecular signaling pathways induced by these interventions. We also compare and contrast the effects of exercise and resveratrol in diseased states.

Causes of exercise intolerance in heart failure with preserved ejection fraction: searching for consensus

Exercise intolerance is one of the cardinal symptoms of heart failure with preserved ejection fraction (HFpEF). We review its mechanistic basis using evidence from exercise studies. One barrier to a consensus understanding of the pathophysiology is heterogeneity of the patient population. Therefore, we pay special attention to varying study definitions of the disease and their possible impact on the causal factors that are implicated. We then discuss the role of exercise testing and its potential to subtype HFpEF in to more homogeneous mechanism-based subclasses.

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Exercise: Kinetic considerations for gas exchange

The activities of daily living typically occur at metabolic rates below the maximum rate of aerobic energy production. Such activity is characteristic of the nonsteady state, where energy demands, and consequential physiological responses, are in constant flux. The dynamics of the integrated physiological processes during these activities determine the degree to which exercise can be supported through rates of O₂ utilization and CO₂ clearance appropriate for their demands and, as such, provide a physiological framework for the notion of exercise intensity. The rate at which O₂ exchange responds to meet the changing energy demands of exercise--its kinetics--is dependent on the ability of the pulmonary, circulatory, and muscle bioenergetic systems to respond appropriately. Slow response kinetics in pulmonary O₂ uptake predispose toward a greater necessity for substrate-level energy supply, processes that are limited in their capacity, challenge system homeostasis and hence contribute to exercise intolerance. This review provides a physiological systems perspective of pulmonary gas exchange kinetics: from an integrative view on the control of muscle oxygen consumption kinetics to the dissociation of cellular respiration from its pulmonary expression by the circulatory dynamics and the gas capacitance of the lungs, blood, and tissues. The intensity dependence of gas exchange kinetics is discussed in relation to constant, intermittent, and ramped work rate changes. The influence of heterogeneity in the kinetic matching of O₂ delivery to utilization is presented in reference to exercise tolerance in endurance-trained athletes, the elderly, and patients with chronic heart or lung disease.

Acute ascorbic acid and hindlimb skeletal muscle blood flow distribution in old rats: rest and exercise

Excess reactive oxygen species are implicated in the impaired peripheral vascular function evident during exercise in older individuals. We tested the hypothesis that an acute infusion of the antioxidant ascorbic acid (AA) in old rats would improve antioxidant capacity and reduce oxidative stress and, therefore, elevate hindlimb muscle blood flow at rest and during treadmill exercise in muscles containing principally type I and IIa muscle fibers. Total and individual hindlimb skeletal muscle blood flow was measured (radiolabeled microspheres) in old rats (26–28 months) at rest (n = 8) and during treadmill exercise (n = 8; 20 m·min–1, 5% grade) before and after AA treatment (76 mg·(kg body mass)–1 intra-arterial (i.a.) injection). AA elevated total antioxidant capacity (rest, ∼37%; and exercise, 31%) and reduced oxidative stress (∼26%, exercise only). AA reduced resting total hindlimb muscle blood flow (control, 25 ± 3; AA, 16 ± 2 mL·min–1·(100 g)–1; p < 0.05) and blood flow to 8 of 28 individual muscles with no fiber-type correlation (p > 0.05). During exercise there was no effect of AA on total hindlimb muscle blood flow (control, 154 ± 14; AA, 162 ± 13 mL·min–1·(100 g)–1; p > 0.05) or blood flow to any individual muscle. This disconnect between whole-body antioxidant status and skeletal muscle blood flow in old rats mandates consideration when pursuing antioxidant treatments experimentally or clinically in older populations.

Effects of neuronal nitric oxide synthase inhibition on microvascular and contractile function in skeletal muscle of aged rats

Advanced age is associated with derangements in skeletal muscle microvascular function during the transition from rest to contractions. We tested the hypothesis that, contrary to what was reported previously in young rats, selective neuronal nitric oxide (NO) synthase (nNOS) inhibition would result in attenuated or absent alterations in skeletal muscle microvascular oxygenation (Po(2)(mv)), which reflects the matching between muscle O(2) delivery and utilization, following the onset of contractions in old rats. Spinotrapezius muscle blood flow (radiolabeled microspheres), Po(2)(mv) (phosphorescence quenching), O(2) utilization (Vo(2); Fick calculation), and submaximal force production were measured at rest and following the onset of contractions in anesthetized old male Fischer 344 × Brown Norway rats (27 to 28 mo) pre- and postselective nNOS inhibition (2.1 μmol/kg S-methyl-l-thiocitrulline; SMTC). At rest, SMTC had no effects on muscle blood flow (P > 0.05) but reduced Vo(2) by ∼23% (P < 0.05), which elevated basal Po(2)(mv) by ∼18% (P < 0.05). During contractions, steady-state muscle blood flow, Vo(2), Po(2)(mv), and force production were not altered after SMTC (P > 0.05 for all). The overall Po(2)(mv) dynamics following onset of contractions was also unaffected by SMTC (mean response time: pre, 19.7 ± 1.5; and post, 20.0 ± 2.0 s; P > 0.05). These results indicate that the locus of nNOS-derived NO control in skeletal muscle depends on age and metabolic rate (i.e., rest vs. contractions). Alterations in nNOS-mediated regulation of contracting skeletal muscle microvascular function with aging may contribute to poor exercise capacity in this population.

Lifelong physical activity prevents an age-related reduction in arterial and skeletal muscle nitric oxide bioavailability in humans

Ageing has been proposed to be associated with increased levels of reactive oxygen species (ROS) that scavenge nitric oxide (NO). In eight young sedentary (23 ± 1 years; Y), eight older lifelong sedentary (66 ± 2 years; OS) and eight older lifelong physically active subjects (62 ± 2 years; OA), we studied the effect of ROS on systemic and skeletal muscle NO bioavailability and leg blood flow by infusion of the antioxidant N-acetylcysteine (NAC). Infusion of NAC increased the bioavailability of NO in OS, as evidenced by an increased concentration of stable metabolites of NO (NOx) in the arterial and venous circulation and in the muscle interstitium. In OA, infusion of NAC only increased NOx concentrations in venous plasma whereas in Y, infusion of NAC did not affect NOx concentrations. Skeletal muscle protein levels of endothelial and neuronal NO synthase were 32% and 24% higher, respectively, in OA than in OS. Exercise at 12 W elicited a lower leg blood flow response that was associated with a lower leg oxygen uptake in OS than in Y. The improved bioavailability of NO in OS did not increase blood flow during exercise. These data demonstrate that NO bioavailability is compromised in the systemic circulation and in the musculature of sedentary ageing humans due to increased oxidative stress. Lifelong physical activity opposes this effect within the trained musculature and in the arterial circulation. The lower blood flow response to leg exercise in ageing humans is not associated with a reduced NO bioavailability.

Rapid onset vasodilatation is blunted in obese humans

AIM

Conduit artery function in obese humans is frequently assessed at rest, but very little is known about resistance artery function in response to muscle contraction. We tested the hypothesis that obese adults will exhibit reduced contraction-induced rapid onset vasodilatation. Single and brief forearm contractions were used to isolate the local effects of muscle contraction on the forearm vasodilatory response, independent of systemic haemodynamic and sympathetic neural influence.

METHODS

We measured forearm blood flow (Doppler ultrasound), blood pressure (finger photoplethysmography) and heart rate (electrocardiogram) on a beat-by-beat basis in 14 obese (body mass index = 36.2 ± 1.7 kg m(-2)) and 14 lean (body mass index = 21.6 ± 0.7 kg m(-2)) young (18-40 years) adults. Percent changes from baseline in forearm vascular conductance (FVC(%) ) were calculated in response to single, brief forearm contractions performed in random order at 15, 20, 25, 30, 40 and 50% of maximal voluntary contraction (MVC).

RESULTS

In both groups, each single contraction evoked a significant (P < 0.05), immediate (within one cardiac cycle) and graded FVC(%) increase from one up to six cardiac cycles post-contraction. Immediate (20-50% MVC), peak (15-50% MVC) and total (area under the curve, 20-50% MVC) vasodilatory responses were reduced with obesity. The degree of impaired vasodilatation increased with increasing workloads.

CONCLUSIONS

These novel findings demonstrate a blunted contraction-induced rapid onset vasodilatation with obesity that is exercise intensity dependent. Impaired rapid onset vasodilatation may negatively impact haemodynamic responses to everyday intermittent activities performed by obese humans.

Control of microvascular PO₂ kinetics following onset of muscle contractions: role for AMPK

The microvascular partial pressure of oxygen (Pmv(o(2))) kinetics following the onset of exercise reflects the relationship between muscle O(2) delivery and uptake (Vo(2)). Although AMP-activated protein kinase (AMPK) is known as a regulator of mitochondria and nitric oxide metabolism, it is unclear whether the dynamic balance of O(2) delivery and Vo(2) at exercise onset is dependent on AMPK activation level. We used transgenic mice with muscle-specific AMPK dominant-negative (AMPK-DN) to investigate a role for skeletal muscle AMPK on Pmv(o(2)) kinetics following onset of muscle contractions. Phosphorescence quenching techniques were used to measure Pmv(o(2)) at rest and across the transition to twitch (1 Hz) and tetanic (100 Hz, 3-5 V, 4-ms pulse duration, stimulus duration of 100 ms every 1 s for 1 min) contractions in gastrocnemius muscles (each group n = 6) of AMPK-DN mice and wild-type littermates (WT) under isoflurane anesthesia with 100% inspired O(2) to avoid hypoxemia. Baseline Pmv(o(2)) before contractions was not different between groups (P > 0.05). Both muscle contraction conditions exhibited a delay followed by an exponential decrease in Pmv(o(2)). However, compared with WT, AMPK-DN demonstrated 1) prolongation of the time delay before Pmv(o(2)) began to decline (1 Hz: WT, 3.2 ± 0.5 s; AMPK-DN, 6.5 ± 0.4 s; 100 Hz: WT, 4.4 ± 1.0 s; AMPK-DN, 6.5 ± 1.4 s; P < 0.05), 2) a faster response time (i.e., time constant; 1 Hz: WT, 19.4 ± 3.9 s; AMPK-DN, 12.4 ± 2.6 s; 100 Hz: WT, 15.1 ± 2.2 s; AMPK-DN, 9.0 ± 1.7 s; P < 0.05). These findings are consistent with the presence of substantial mitochondrial and microvascular dysfunction in AMPK-DN mice, which likely slows O(2) consumption kinetics (i.e., oxidative phosphorylation response) and impairs the hyperemic response at the onset of contractions thereby sowing the seeds for exercise intolerance.

Aging alters the contribution of nitric oxide to regional muscle hemodynamic control at rest and during exercise in rats

Advanced age is associated with altered skeletal muscle hemodynamic control during the transition from rest to exercise. This study investigated the effects of aging on the functional role of nitric oxide (NO) in regulating total, inter-, and intramuscular hindlimb hemodynamic control at rest and during submaximal whole body exercise. We tested the hypothesis that NO synthase inhibition (N(G)-nitro-l-arginine methyl ester, l-NAME; 10 mg/kg) would result in attenuated reductions in vascular conductance (VC) primarily in oxidative muscles in old compared with young rats. Total and regional hindlimb muscle VCs were determined via radiolabeled microspheres at rest and during treadmill running (20 m/min, 5% grade) in nine young (6-8 mo) and seven old (27-29 mo) male Fisher 344 × Brown Norway rats. At rest, l-NAME increased mean arterial pressure (MAP) significantly by ∼17% and 21% in young and old rats, respectively. During exercise, l-NAME increased MAP significantly by ∼13% and 19% in young and old rats, respectively. Compared with young rats, l-NAME administration in old rats evoked attenuated reductions in 1) total hindlimb VC during exercise (i.e., down by ∼23% in old vs. 43% in young rats; P < 0.05), and 2) VC in predominantly oxidative muscles both at rest and during exercise (P < 0.05). Our results indicate that the dependency of highly oxidative muscles on NO-mediated vasodilation is markedly diminished, and therefore mechanisms other than NO-mediated vasodilation control the bulk of the increase in skeletal muscle VC during the transition from rest to exercise in old rats. Reduced NO contribution to vasomotor control with advanced age is associated with blood flow redistribution from highly oxidative to glycolytic muscles during exercise.

Dynamics of muscle microcirculatory and blood-myocyte O(2) flux during contractions

The O(2) requirements of contracting skeletal muscle may increase 100-fold above rest. In 1919, August Krogh's brilliant insights recognized the capillary as the principal site for this increased blood-myocyte O(2) flux. Based on the premise that most capillaries did not sustain RBC flux at rest, Krogh proposed that capillary recruitment [i.e. initiation of red blood cell (RBC) flux in previously non-flowing capillaries] increased the capillary surface area available for O(2) flux and reduced mean capillary-to-mitochondrial diffusion distances. More modern experimental approaches reveal that most muscle capillaries may support RBC flux at rest. Thus, rather than contraction-induced capillary recruitment per se, increased RBC flux and haematocrit within already-flowing capillaries probably elevate perfusive and diffusive O(2) conductances and hence blood-myocyte O(2) flux. Additional surface area for O(2) exchange is recruited but, crucially, this may occur along the length of already-flowing capillaries (i.e. longitudinal recruitment). Today, the capillary is still considered the principal site for O(2) and substrate delivery to contracting skeletal muscle. Indeed, the presence of very low intramyocyte O(2) partial pressures (PO(2)s) and the absence of intramyocyte PO(2) gradients, whilst refuting the relevance of diffusion distances, place an even greater importance on capillary hemodynamics. This emergent picture calls for a paradigm-shift in our understanding of the function of capillaries by de-emphasizing de novo'capillary recruitment'. Diseases such as heart failure impair blood-myocyte O(2) flux, in part, by decreasing the proportion of RBC-flowing capillaries. Knowledge of capillary function in healthy muscle is requisite for identification of pathology and efficient design of therapeutic treatments.

Acute antioxidant supplementation and skeletal muscle vascular conductance in aged rats: role of exercise and fiber type

Age-related increases in oxidative stress contribute to impaired skeletal muscle vascular control. However, recent evidence indicates that antioxidant treatment with tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) attenuates flow-mediated vasodilation in isolated arterioles from the highly oxidative soleus muscle of aged rats. Whether antioxidant treatment with tempol evokes similar responses in vivo at rest and during exercise in senescent individuals and whether this effect varies based on muscle fiber type composition are unknown. We tested the hypothesis that redox modulation via acute systemic tempol administration decreases vascular conductance (VC) primarily in oxidative hindlimb locomotor muscles at rest and during submaximal whole body exercise (treadmill running at 20 m/min, 5% grade) in aged rats. Eighteen old (25–26 mo) male Fischer 344 x Brown Norway rats were assigned to either rest ( n = 8) or exercise ( n = 10) groups. Regional VC was determined via radiolabeled microspheres before and after intra-arterial administration of tempol (302 μmol/kg). Tempol decreased mean arterial pressure significantly by 9% at rest and 16% during exercise. At rest, similar VC in 26 out of 28 individual hindlimb muscles or muscle parts following tempol administration compared with control resulted in unchanged total hindlimb muscle VC (control: 0.18 ± 0.02; tempol: 0.17 ± 0.05 ml·min−1·100 g−1·mmHg−1; P > 0.05). During exercise, all individual hindlimb muscles or muscle parts irrespective of fiber type composition exhibited either an increase or no change in VC with tempol (i.e., ↑11 and ↔17 muscles or muscle parts), such that total hindlimb VC increased by 25% (control: 0.93 ± 0.04; tempol: 1.15 ± 0.09 ml·min−1·100 g−1·mmHg−1; P ≤ 0.05). These results demonstrate that acute systemic administration of the antioxidant tempol significantly impacts the control of regional vascular tone in vivo presumably via redox modulation and improves skeletal muscle vasodilation independently of fiber type composition during submaximal whole body exercise in aged rats.

Ornithine alpha-ketoglutarate: could it be a new therapeutic option for sarcopenia?

Our current knowledge on the causes of sarcopenia is still fragmentary. One of the most evident candidates to explain muscle loss in elderly includes imbalance in protein turnover, i.e. decreased muscle protein synthesis rate, notably in the post-prandial state. Nutritional strategies such as leucine supplementation, use of fast digested proteins or a pulse protein intake have been show to enhance the synthesis rate of muscle proteins in older individuals. Ornithine alpha-ketoglutarate (OKG) is a precursor of amino acids such as glutamine, arginine and proline, and increases the secretion of anabolic hormones, i.e. insulin and growth hormone. A beneficial anabolic action of OKG has been demonstrate in several pathological conditions associated with muscle loss. Therefore, OKG may be of a potential interest to modulate muscle protein metabolism and to maintain muscle mass during aging.

Rapid vasodilation in isolated skeletal muscle arterioles: impact of branch order

We tested the hypothesis that segmental differences in the responsiveness and time course of vasodilation to metabolic signals putatively involved in rapid onset vasodilation (ROV) at the start of exercise exist within the skeletal muscle vasculature. Cannulated first-order (1As) and third-order arterioles (3As) of the rat gastrocnemius (G) muscle were exposed to cumulative doses of KCl, acetylcholine (Ach), or adenosine (Ado). In addition, time course and magnitude of vasodilation to localized application of these agonists were determined. 1As and 3As dilated similarly to incremental doses of the agonists. Continuous monitoring of internal diameter revealed a fast and transient dilatory response to microinjections of the agonists, with an average time delay (TD) before the onset of vasodilation of 2.8 +/- 0.2 seconds (1As: 3.0 +/- 0.3 seconds and 3As: 2.6 +/- 0.3 seconds) and time-to-peak (TP) of 8.2 +/- 0.7 seconds (1As: 10.3 +/- 1 seconds and 3As:5.7 +/- 0.5 seconds). No significant differences were detected for all parameters between 1As and 3As for KCl or Ado application, while 1As had a significantly longer TP and greater peak dilation than 3As to Ach. These findings demonstrate that 1As and 3As from the rat G muscle appear to have similar responsiveness to vasoactive agonists. Furthermore, the average TD before vasodilation supports a role for metabolic signals as contributors to the ROV.

Aging impacts microvascular oxygen pressures during recovery from contractions in rat skeletal muscle

Aging-induced alterations in peripheral circulatory control during contractions reduce the microvascular partial pressure of O(2) (P(O)(2)mv; which reflects the dynamic balance in the O(2) delivery-to-O(2) uptake ratio), resulting in exaggerated intramuscular metabolic disturbances and premature fatigue. However, the extent to which this altered P(O)(2)mv during contractions is associated with prolongated muscle metabolic recovery is not known. We tested the hypothesis that the aging-induced speeding of the P(O)(2)mv on-kinetics would presage slowed P(O)(2)mv off-kinetics. The spinotrapezius muscle was exposed in six young (6-8 months) and seven old (26-28 months) male Fischer 344xBrown Norway F1-hybrid rats. The P(O)(2)mv kinetic profile was measured via phosphorescence quenching at rest, during electrically stimulated contractions (1Hz, 7-9V, 2ms pulse duration, 180s), and throughout recovery (180s). Aged rats which evidenced faster P(O)(2)mv on-kinetics (reduced mean response time (MRTon), young: 27.3+/-3.6s, old: 19.2+/-1.6s; P<0.05) exhibited markedly slowed P(O)(2)mv off-kinetics (increased MRToff, young: 46.5+/-5.9s, old: 84.8+/-7.9s; P<0.05). Accordingly, a greater degree of P(O)(2)mv on-off asymmetry (MRToff-MRTon) in the aged muscle was observed (young: 19.1+/-4.5s, old: 65.6+/-8.6s; P<0.01). We conclude that aging-induced speeding of the P(O)(2)mv on-kinetics does indeed presage a slowed P(O)(2)mv off-kinetics, which likely compromises muscle metabolic recovery and may reduce subsequent contractile performance. Moreover, the greater degree of P(O)(2)mv on-off asymmetry in the aged muscle suggests a mechanistic link between impaired microvascular oxygenation and altered muscle metabolic responses during exercise transitions.

Effect of eccentric exercise-induced muscle damage on the dynamics of muscle oxygenation and pulmonary oxygen uptake

Unaccustomed eccentric exercise has a profound impact on muscle structure and function. However, it is not known whether associated microvascular dysfunction disrupts the matching of O2delivery (Q̇o2) to O2utilization (V̇o2). Near-infrared spectroscopy (NIRS) was used to test the hypothesis that eccentric exercise-induced muscle damage would elevate the muscle Q̇o2:V̇o2ratio during severe-intensity exercise while preserving the speed of the V̇o2kinetics at exercise onset. Nine physically active men completed “step” tests to severe-intensity exercise from an unloaded baseline on a cycle ergometer before (Pre) and 48 h after (Post) eccentric exercise (100 squats with a load corresponding to 70% of body mass). NIRS and breath-by-breath pulmonary V̇o2were measured continuously during the exercise tests and subsequently modeled using standard nonlinear regression techniques. There were no changes in phase II pulmonary V̇o2kinetics following the onset of exercise (time constant: Pre, 25 ± 4 s; Post, 24 ± 2 s; amplitude: Pre, 2.36 ± 0.23 l/min; Post, 2.37 ± 0.23 l/min; all P > 0.05). However, the primary (Pre, 14 ± 3 s; Post, 19 ± 3 s) and overall (Pre, 16 ± 4 s; Post, 21 ± 4 s) mean response time of the [HHb] response was significantly slower following eccentric exercise ( P < 0.05). The slower [HHb] kinetics observed following eccentric exercise is consistent with an increased Q̇o2:V̇o2ratio during transitions to severe-intensity exercise. We propose that unchanged primary phase V̇o2kinetics are associated with an elevated Q̇o2:V̇o2ratio that preserves blood-myocyte O2flux.