Reduced blood flow to contracting skeletal muscle in ageing humans: is it all an effect of sand through the hourglass?

Matching of O2 Utilization and O2 Delivery in Contracting Skeletal Muscle in Health, Aging, and Heart Failure

Skeletal muscle is one of the most dynamic metabolic organs as evidenced by increases in metabolic rate of >150-fold from rest to maximal contractile activity. Because of limited intracellular stores of ATP, activation of metabolic pathways is required to maintain the necessary rates of ATP re-synthesis during sustained contractions. During the very early phase, phosphocreatine hydrolysis and anaerobic glycolysis prevails but as activity extends beyond ∼1 min, oxidative phosphorylation becomes the major ATP-generating pathway. Oxidative metabolism of macronutrients is highly dependent on the cardiovascular system to deliver O₂ to the contracting muscle fibres, which is ensured through a tight coupling between skeletal muscle O₂ utilization and O₂ delivery. However, to what extent O₂ delivery is ideal in terms of enabling optimal metabolic and contractile function is context-dependent and determined by a complex interaction of several regulatory systems. The first part of the review focuses on local and systemic mechanisms involved in the regulation of O₂ delivery and how integration of these influences the matching of skeletal muscle O₂ demand and O₂ delivery. In the second part, alterations in cardiovascular function and structure associated with aging and heart failure, and how these impact metabolic and contractile function, will be addressed. Where applicable, the potential of exercise training to offset/reverse age- and disease-related cardiovascular declines will be highlighted in the context of skeletal muscle metabolic function. The review focuses on human data but also covers animal observations.

Functional sympatholysis in mouse skeletal muscle involves sarcoplasmic reticulum swelling in arterial smooth muscle cells

The vasoconstrictive effect of sympathetic activity is attenuated in contracting skeletal muscle (functional sympatholysis), allowing increased blood supply to the working muscle but the underlying mechanisms are incompletely understood. The purpose of this study was to examine α-adrenergic receptor responsiveness in isolated artery segments from non-exercised and exercised mice, using wire myography. Isometric tension recordings performed on femoral artery segments from exercised mice showed decreased α-adrenergic receptor responsiveness compared to non-exercised mice (logEC50 -5.2 ± 0.04 M vs. -5.7 ± 0.08 M, respectively). In contrast, mesenteric artery segments from exercised mice displayed similar α-adrenergic receptor responses compared to non-exercised mice. Responses to the vasoconstrictor serotonin (5-HT) and vasodilator isoprenaline, were similar in femoral artery segments from non-exercised and exercised mice. To study sarcoplasmic reticulum (SR) function, we examined arterial contractions induced by caffeine, which depletes SR Ca2+ and thapsigargin, which inhibits SR Ca2+ -ATPase (SERCA) and SR Ca2+ uptake. Arterial contractions to both caffeine and thapsigargin were increased in femoral artery segment from exercised compared to non-exercised mice. Furthermore, 3D electron microscopy imaging of the arterial wall showed SR volume/length ratio increased 157% in smooth muscle cells of the femoral artery from the exercised mice, whereas there was no difference in SR volume/length ratio in mesenteric artery segments. These results show that in arteries surrounding exercising muscle, the α-adrenergic receptor constrictions are blunted, which can be attributed to swollen smooth muscle cell SR's, likely due to increased Ca2+ content that is possibly reducing free intracellular Ca2+ available for contraction. Overall, this study uncovers a previously unknown mechanism underlying functional sympatholysis.

Exercise stress leads to an acute loss of mitochondrial proteins and disruption of redox control in skeletal muscle of older subjects: An underlying decrease in resilience with aging?

Reactive oxygen species (ROS) are recognized as important signaling molecules in healthy skeletal muscle. Redox sensitive proteins can respond to intracellular changes in ROS by oxidation of reactive thiol groups on cysteine (Cys) residues. Exercise is known to induce the generation of superoxide and nitric oxide, resulting in the activation of several adaptive signaling pathways; however, it has been suggested that aging attenuates these redox-regulated adaptations to acute exercise. In the present study, we used redox proteomics to study the vastus lateralis muscles of Adult (n = 6 male, 6 female; 18-30 yrs) and Old (n = 6 male, 6 female; 64-79 yrs) adults. Participants completed a bout of high intensity cycling exercise consisting of five sets of 2-min intervals performed at 80% maximal aerobic power output (PPO), with 2 min recovery cycling at 40% PPO between sets. Muscle biopsies were collected prior to exercise, and immediately following the first, second, and fifth high intensity interval. Global proteomic analysis indicated differences in abundance of a number of individual proteins between skeletal muscles of Adult and Old subjects at rest with a significant exacerbation of these differences induced by the acute exercise. In particular, we observed an exercise-induced decrease in abundance of mitochondrial proteins in muscles from older subjects only. Redox proteome analysis revealed cysteines from five cytosolic proteins in older subjects with lower oxidation (i.e. greater reduction) than was seen in muscle from the young adults at rest. Redox homeostasis was well maintained in Adult subjects following exercise, but there was significant increase in oxidation of multiple mitochondrial and cytosolic protein cysteines in Old subjects. We also observed that oxidation of peroxiredoxin 3 occurred following exercise in both Adult and Old groups, supporting the possibility that this is a key effector protein for mitochondrial redox signaling. Thus, we show, for the first time that exercise reveals a lack of resilience in muscle of older human participants, that is apparent as a loss of mitochondrial proteins and oxidation of multiple protein cysteines that are not seen in younger subjects. The precise consequences of this redox disruption are unclear, but this likely play a role in the attenuation of multiple adaptations to exercise that are classically seen with aging. Such changes were only seen following the acute stress of exercise., highlighting the need to consider not only basal differences seen during aging but also the difference following physiological challenge.

Chemotherapy acutely impairs neurovascular and hemodynamic responses in women with breast cancer

The purpose of the present study was to test the hypothesis that doxorubicin (DX) and cyclophosphamide (CY) adjuvant chemotherapy (CHT) acutely impairs neurovascular and hemodynamic responses in women with breast cancer. Sixteen women (age: 47.0 ± 2.0 yr; body mass index: 24.2 ± 1.5 kg/m) with stage II-III breast cancer and indication for adjuvant CHT underwent two experimental sessions, saline (SL) and CHT. In the CHT session, DX (60 mg/m2) and CY (600 mg/m2) were administered over 45 min. In the SL session, a matching SL volume was infused in 45 min. Muscle sympathetic nerve activity (MSNA) from peroneal nerve (microneurography), calf blood flow (CBF; plethysmography) and calf vascular conductance (CVC), heart rate (HR; electrocardiography), and beat-to-beat blood pressure (BP; finger plethysmography) were measured at rest before, during, and after each session. Venous blood samples (5 ml) were collected before and after both sessions for assessment of circulating endothelial microparticles (EMPs; flow cytometry), a surrogate marker for endothelial damage. MSNA and BP responses were increased ( P < 0.001), whereas CBF and CVC responses were decreased ( P < 0.001), during and after CHT session when compared with SL session. Interestingly, the vascular alterations were also observed at the molecular level through an increased EMP response to CHT ( P = 0.03, CHT vs. SL session). No difference in HR response was observed ( P > 0.05). Adjuvant CHT with DX and CY in patients treated for breast cancer increases sympathetic nerve activity and circulating EMP levels and, in addition, reduces muscle vascular conductance and elevates systemic BP. These responses may be early signs of CHT-induced cardiovascular alterations and may represent potential targets for preventive interventions.

NEW & NOTEWORTHY It is known that chemotherapy regimens increase the risk of cardiovascular events in patients treated for cancer. Here, we identified that a single cycle of adjuvant chemotherapy with doxorubicin and cyclophosphamide in women treated for breast cancer dramatically increases sympathetic nerve activity and circulating endothelial microparticle levels, reduces the muscle vascular conductance, and elevates systemic blood pressure.

Impaired Muscle Efficiency but Preserved Peripheral Hemodynamics and Mitochondrial Function With Advancing Age: Evidence From Exercise in the Young, Old, and Oldest-Old

Muscle weakness in the elderly has been linked to recurrent falls and morbidity; therefore, elucidating the mechanisms contributing to the loss of muscle function and mobility with advancing age is critical. To this aim, we comprehensively examined skeletal muscle metabolic function and hemodynamics in 11 young (23 ± 2 years), 11 old (68 ± 2 years), and 10 oldest-old (84 ± 2 years) physical activity-matched participants. Specifically, oxidative stress markers, mitochondrial function, and the ATP cost of contraction as well as peripheral hemodynamics were assessed during dynamic plantar flexion exercise at 40 per cent of maximal work rate (WRmax). Both the PCr recovery time constant and the peak rate of mitochondrial ATP synthesis were not significantly different between groups. In contrast, the ATP cost of dynamic contractions (young: 1.5 ± 1.0, old: 3.4 ± 2.1, oldest-old: 6.1 ± 3.6 mM min-1 W-1) and systemic markers of oxidative stress were signficantly increased with age, with the ATP cost of contraction being negatively correlated with WRmax (r = .59, p < .05). End-of-exercise blood flow per Watt rose significantly with increasing age (young: 37 ± 20, old: 82 ± 68, oldest-old: 154 ± 93 mL min-1 W-1). These findings suggest that the progressive deterioration of muscle contractile efficiency with advancing age may play an important role in the decline in skeletal muscle functional capacity in the elderly.

Dietary Protein, Muscle and Physical Function in the Very Old

There is an ongoing debate as to the optimal protein intake in older adults. An increasing body of experimental studies on skeletal muscle protein metabolism as well as epidemiological data suggest that protein requirements with ageing might be greater than many current dietary recommendations. Importantly, none of the intervention studies in this context specifically investigated very old individuals. Data on the fastest growing age group of the oldest old (aged 85 years and older) is very limited. In this review, we examine the current evidence on protein intake for preserving muscle mass, strength and function in older individuals, with emphasis on data in the very old. Available observational data suggest beneficial effects of a higher protein intake with physical function in the oldest old. Whilst, studies estimating protein requirements in old and very old individuals based on whole-body measurements, show no differences between these sub-populations of elderly. However, small sample sizes preclude drawing firm conclusions. Experimental studies that compared muscle protein synthetic (MPS) responses to protein ingestion in young and old adults suggest that a higher relative protein intake is required to maximally stimulate skeletal muscle MPS in the aged. Although, data on MPS responses to protein ingestion in the oldest old are currently lacking. Collectively, the data reviewed for this article support the concept that there is a close interaction of physical activity, diet, function and ageing. An attractive hypothesis is that regular physical activity may preserve and even enhance the responsiveness of ageing skeletal muscle to protein intake, until very advanced age. More research involving study participants particularly aged ≥85 years is warranted to better investigate and determine protein requirements in this specific growing population group.

Effects of aging and exercise training on leg hemodynamics and oxidative metabolism in the transition from rest to steady-state exercise: role of cGMP signaling

Aging is associated with slower skeletal muscle O₂ uptake (V̇o₂) kinetics; however, the mechanisms underlying this effect of age are unclear. Also, the effects of exercise training in elderly on the initial vascular and metabolic response to exercise remain to be elucidated. We measured leg hemodynamics and oxidative metabolism in the transition from rest to steady-state exercise engaging the knee-extensor muscles in young ( n = 15, 25 ± 1 yr) and older ( n = 15, 72 ± 1 yr) subjects before and after a period of aerobic high-intensity exercise training. To enhance cGMP signaling, pharmacological inhibition of phosphodiesterase 5 (PDE5) was performed. Before training, the older group had a slower ( P <0.05) increase in femoral arterial blood flow and leg vascular conductance in the transition from rest to steady-state exercise at low- and moderate-intensity compared with the young group. The rate of increase in leg V̇o₂ was, however, similar in the two groups as a result of higher ( P < 0.05) arteriovenous O₂ difference in the older group. Potentiation of cGMP signaling did not affect the rate of increase in blood flow or V̇o₂ in either group. Exercise training augmented ( P < 0.05) the increase in leg vascular conductance and blood flow during the onset of moderate-intensity exercise in both groups without altering V̇o₂. These findings suggest that an age-related reduction in the initial vascular response to low- and moderate-intensity knee-extensor exercise is not limiting for V̇o₂ in older individuals. A lower blood flow response in aging does not appear to be a result of reduced cGMP signaling.

Probenecid Inhibits α-Adrenergic Receptor-Mediated Vasoconstriction in the Human Leg Vasculature

Coordination of vascular smooth muscle cell tone in resistance arteries plays an essential role in the regulation of peripheral resistance and overall blood pressure. Recent observations in animals have provided evidence for a coupling between adrenoceptors and Panx1 (pannexin-1) channels in the regulation of sympathetic nervous control of peripheral vascular resistance and blood pressure; however, evidence for a functional coupling in humans is lacking. We determined Panx1 expression and effects of treatment with the pharmacological Panx1 channel inhibitor probenecid on the vasoconstrictor response to α1- and α2-adrenergic receptor stimulation in the human forearm and leg vasculature of young healthy male subjects (23±3 years). By use of immunolabeling and confocal microscopy, Panx1 channels were found to be expressed in vascular smooth muscle cells of arterioles in human leg skeletal muscle. Probenecid treatment increased (P<0.05) leg vascular conductance at baseline by ≈15% and attenuated (P<0.05) the leg vasoconstrictor response to arterial infusion of tyramine (α1- and α2-adrenergic receptor stimulation) by ≈15%, whereas the response to the α1-agonist phenylephrine was unchanged. Inhibition of α1-adrenoceptors prevented the probenecid-induced increase in baseline leg vascular conductance, but did not alter the effect of probenecid on the vascular response to tyramine. No differences with probenecid treatment were detected in the forearm. These observations provide the first line of evidence in humans for a functional role of Panx1 channels in setting resting tone via α1-adrenoceptors and in the constrictive effect of noradrenaline via α2-adrenoceptors, thereby contributing to the regulation of peripheral vascular resistance and blood pressure in humans.

Exercise training improves blood flow to contracting skeletal muscle of older men via enhanced cGMP signaling

Physical activity has the potential to offset age-related impairments in the regulation of blood flow and O₂ delivery to the exercising muscles; however, the mechanisms underlying this effect of physical activity remain poorly understood. The present study examined the role of cGMP in training-induced adaptations in the regulation of skeletal muscle blood flow and oxidative metabolism during exercise in aging humans. We measured leg hemodynamics and oxidative metabolism during exercise engaging the knee extensor muscles in young [ n = 15, 25 ± 1 (SE) yr] and older ( n = 15, 72 ± 1 yr) subjects before and after a period of aerobic high-intensity exercise training. To determine the role of cGMP signaling, pharmacological inhibition of phosphodiesterase 5 (PDE5) was performed. Before training, inhibition of PDE5 increased ( P < 0.05) skeletal muscle blood flow and O₂ uptake during moderate-intensity exercise in the older group; however, these effects of PDE5 inhibition were not detected after training. These findings suggest a role for enhanced cGMP signaling in the training-induced improvement of regulation of blood flow in contracting skeletal muscle of older men. NEW & NOTEWORTHY The present study provides evidence for enhanced cyclic GMP signaling playing an essential role in the improved regulation of blood flow in contracting skeletal muscle of older men with aerobic exercise training.

POTENTIAL EFFECTS OF WHOLE-BODY VIBRATION EXERCISES ON BLOOD FLOW KINETICS OF DIFFERENT POPULATIONS: A SYSTEMATIC REVIEW WITH A SUITABLE APPROACH

BACKGROUND

The ability to control skin blood flow decreases with advancing age and some clinical disorders, as in diabetes and in rheumatologic diseases. Feasible clinical strategies such as whole-body vibration exercise (WBVE) are being used without a clear understanding of its effects. The aim of the present study is to review the effects of the WBVE on blood flow kinetics and its feasibility in different populations.

MATERIAL AND METHODS

The level of evidence (LE) of selected papers in PubMed and/or PEDRo databases was determined. We selected randomized, controlled trials in English to be evaluated.

RESULTS

Six studies had LE II, one had LE III-2 and one III-3 according to the NHMRC. A great variability among the protocols was observed but also in the assessment devices; therefore, more research about this topic is warranted.

CONCLUSION

Despite the limitations, it is can be concluded that the use of WBVE has proven to be a safe and useful strategy to improve blood flow. However, more studies with greater methodological quality are needed to clearly define the more suitable protocols.

Effect of PDE5 inhibition on the modulation of sympathetic α-adrenergic vasoconstriction in contracting skeletal muscle of young and older recreationally active humans

Aging is associated with an altered regulation of blood flow to contracting skeletal muscle; however, the precise mechanisms remain unclear. We recently demonstrated that inhibition of cGMP-binding phosphodiesterase 5 (PDE5) increased blood flow to contracting skeletal muscle of older but not young human subjects. Here we examined whether this effect of PDE5 inhibition was related to an improved ability to blunt α-adrenergic vasoconstriction (functional sympatholysis) and/or improved efficacy of local vasodilator pathways. A group of young (23 ± 1 yr) and a group of older (72 ± 1 yr) male subjects performed knee-extensor exercise in a control setting and following intake of the highly selective PDE5 inhibitor sildenafil. During both conditions, exercise was performed without and with arterial tyramine infusion to evoke endogenous norepinephrine release and consequently stimulation of α1- and α2-adrenergic receptors. The level of the sympatholytic compound ATP was measured in venous plasma by use of the microdialysis technique. Sildenafil increased (P < 0.05) vascular conductance during exercise in the older group, but tyramine infusion reduced (P < 0.05) this effect by 38 ± 9%. Similarly, tyramine reduced (P < 0.05) the vasodilation induced by arterial infusion of a nitric oxide (NO) donor by 54 ± 9% in the older group, and this effect was not altered by sildenafil. Venous plasma [ATP] did not change with PDE5 inhibition in the older subjects during exercise. Collectively, PDE5 inhibition in older humans was not associated with an improved ability for functional sympatholysis. An improved efficacy of the NO system may be one mechanism underlying the effect of PDE5 inhibition on exercise hyperemia in aging.