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. Author manuscript; available in PMC: 2016 Jun 1.
Published in final edited form as: Exp Physiol. 2015 May 13;100(6):589–602. doi: 10.1113/EP085076

‘Fine-tuning’ blood flow to the exercising muscle with advancing age: an update

D Walter Wray 1,2,3,4, Russell S Richardson 1,2,3,4
PMCID: PMC4806672  NIHMSID: NIHMS767680  PMID: 25858164

Abstract

During dynamic exercise, oxygen demand from the exercising muscle is dramatically elevated, requiring a marked increase in skeletal muscle blood flow that is accomplished through a combination of systemic sympathoexcitation and local metabolic vasodilatation. With advancing age, the balance between these factors appears to be disrupted in favour of vasoconstriction, leading to an impairment in exercising skeletal muscle blood flow in the elderly. This ‘hot topic’ review aims to provide an update to our current knowledge of age-related changes in the neural and local mechanisms that contribute to this ‘fine-tuning’ of blood flow during exercise. The focus is on results from recent human studies that have adopted a reductionist approach to explore how age-related changes in both vasodilators (nitric oxide) and vasoconstrictors (endothelin-1, α-adrenergic agonists and angiotensin II) interact and how these changes impact blood flow to the exercising skeletal muscle with advancing age.

Overview

During dynamic exercise, blood flow to the exercising skeletal muscle is capable of increasing up to 100-fold, with peak blood flows that reach 300–400 ml min−1 (100 g tissue)−1 in young, healthy individuals (Andersen & Saltin, 1985; Richardson et al. 1993). Considering this large vasodilatory capacity, it is no surprise that a host of vasoconstrictor and vasodilatory pathways exist to ‘fine-tune’ skeletal muscle hyperaemia during exercise. With advancing age, however, there appears to be a shift in the contribution of these pathways in favour of vasoconstriction, leading to an overall increase in vascular tone that contributes to the attenuated skeletal muscle blood flow typically observed both at rest (Dinenno et al. 1999; Lawrenson et al. 2003, 2004; Poole et al. 2003; Parker et al. 2008; Wray et al. 2008) and during exercise in the elderly (Proctor et al. 1998; Beere et al. 1999; Lawrenson et al. 2003; Poole et al. 2003).

The underlying mechanisms contributing to this age-related increase in vascular tone and the accompanying alteration in exercising skeletal muscle blood flow have long been a topic of investigation, as outlined in several noteworthy review articles (Lakatta, 1990; Degens, 1998; Proctor & Parker, 2006). For the purposes of the present review, the focus is on the role of the vascular endothelium, with particular attention on endothelium-derived nitric oxide (NO) and on the surrounding vascular smooth muscle, with an emphasis on endothelin-1 (ET-1), α-adrenergic and renin–angiotensin–aldosterone system (RAAS) signalling (Fig. 1). To limit the breadth of the discussion, this ‘hot topic’ review will highlight recent in vivo human studies from our group and others that have examined the regulatory pathways that exist at the unique interface between the vascular smooth muscle and the endothelium of the skeletal muscle vasculature and how these change with advancing age.

Figure 1. Schematic diagram of the interface between the endothelium and the vascular smooth muscle, emphasizing changes in several key pathways with advancing age.

Figure 1

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Arrows indicate age-related changes in the contribution of each pathway to the regulation of skeletal muscle blood flow during exercise. Abbreviations: Ang II, angiotensin II; AT1, angiotensin subtype 1 receptor; ETA, endothelin subtype A receptor; ET-1, endothelin-1; NA, noradrenaline; and NO, nitric oxide.

Nitric oxide

Once thought to be an inert layer of cells lining the lumen of blood vessels, the vascular endothelium is now well recognized as a critical site of cellular signalling, perhaps most notably due to the discovery in 1980 of endothelium-derived relaxing factor (Furchgott & Zawadzki, 1980), which was later determined to be the labile free radical nitric oxide (Palmer et al. 1987). In the endothelium, NO is synthesized from the substrate l-arginine via endothelial nitric oxide synthase (eNOS), and subsequently, diffuses into smooth muscle cells to promote vascular smooth muscle relaxation via a guanylate cyclase-dependent mechanism (Fig. 1).

In the context of exercise, NO may be released from the endothelial cells by several mechanisms, including laminar shear stress (Rubanyi et al. 1986), mechanical distortion of blood vessels (Clifford et al. 2006) and a host of local factors emanating from the exercising skeletal muscle, motor neuron terminals and red blood cells (Michel & Vanhoutte, 2010). Nitric oxide-mediated vasodilatation has thus emerged as a potential contributor to vasodilatation within the exercising skeletal muscle vasculature, though significant controversy still exists in the literature regarding the overall contribution of the NO pathway to the hyperaemia in exercising muscle. In humans, a host of studies using NOS inhibition during hand-grip exercise (Endo et al. 1994; Gilligan et al. 1994; Shoemaker et al. 1997; Schrage et al. 2004; Green et al. 2005) have demonstrated a 10–20% reduction in exercising skeletal muscle blood flow in the presence of NOS blockade with NG-monomethyl-l-arginine (l-NMMA). Findings from studies using knee-extensor exercise are more equivocal, with evidence for (Rådegran & Saltin, 1999; Frandsen et al. 2001) and against changes in exercising limb blood flow following l-NMMA infusion (Heinonen et al. 2011). When viewed in light of the reduction in resting limb blood flow produced by NOS inhibition, findings from these studies have led to the prevailing view that this reduced hyperaemic response simply represents a ‘downward shift’ in the blood flow response. Specifically, although attenuated at any given work rate, there is a similar overall change in skeletal muscle blood flow from rest to maximal exercise, such that NO-mediated vasodilatation could be argued to be of little functional consequence to exercise hyperaemia (Tschakovsky & Joyner, 2008).

Recent work from our group has challenged this assertion regarding the minimal importance of NO in the exercising muscle vasculature. In young, healthy individuals, high intra-arterial doses of l-NMMA provoked a 15–25% reduction in brachial artery blood flow across a very broad range of hand-grip exercise intensities (Wray et al. 2011; Fig. 2, top panel). To our knowledge, this was the first study to implicate NO as a significant factor governing exercising limb blood flow. The divergence between this recent finding and former work examining the role of NO during exercise may be due, at least in part, to differences in the exercise protocol. Indeed, the majority of previous studies evaluating the importance of NO in exercise hyperaemia have not examined the intensity-dependent nature of the response, relying instead on a singular exercise intensity that is often only 10% of maximal voluntary contraction (MVC; Shoemaker et al. 1997; Schrage et al. 2004). In the few studies where multiple levels of hand-grip exercise were performed before and after NOS inhibition, forearm blood flow was still unaffected at higher exercise intensities (Dinenno & Joyner, 2003; Green et al. 2005). However, even in these studies which used a more comprehensive exercise protocol, only two or three submaximal exercise levels were performed, leaving uncertainty regarding the role of NO across a wide range of exercise efforts. Thus, with inclusion of an array of blood flow measurements across a wide range of workloads, this recently published study (Wray et al. 2011) differs significantly from previous work, identifying a 15–25% reduction in brachial artery blood flow in the presence of l-NMMA, implicating NO as an important governor of exercising skeletal muscle blood flow in young, healthy adults. However, whether these observations during static intermittent hand-grip exercise can be extended to leg exercise across a similar range of exercise intensities remains to be seen and is worthy of further investigation.

Figure 2. Changes in brachial artery blood flow during progressive hand-grip exercise in young and older healthy subjects after nitric oxide synthase inhibition via NG-monomethyl-l-arginine (l-NMMA).

Figure 2

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The l-NMMA produced a 15–25% reduction in forearm blood flow in the young (top panel) but did not appear to contribute significantly to blood flow regulation during hand-grip exercise in the elderly (bottom panel). Modified from Wray et al. (2011) and Trinity et al. (2013). *Significant difference between control and l-NMMA, P < 0.05. Abbreviations: BA, brachial artery; and MVC, maximal voluntary contraction.

Nitric oxide and ageing

Our previous work supporting the role of NO in governing muscle blood flow during exercise in the young (Wray et al. 2011), coupled with the recognized loss of NO bioavailability in the elderly (Taddei et al. 1995), raises the possibility that this pathway may be highly relevant to age-related changes in the regulation of exercise hyperaemia. Indeed, in one of the first studies to examine the role of NO-mediated vasodilatation during exercise in the elderly, Schrage et al. (2007) assessed changes in forearm blood flow when the NOS inhibitor NG-nitro-l-arginine methyl ester (l-NAME) was administered acutely during dynamic hand-grip exercise at 10% of MVC. This study revealed that the administration of l-NAME reduced forearm blood flow by only ∼ 12% in the elderly cohort, compared with ∼22% when the same dose was administered in young, healthy adults at the same relative exercise intensity, leading the authors to conclude that the relative contribution of NO to exercise hyperaemia is reduced by ∼45% in the elderly. Subsequent to this study, Crecelius et al. (2010) reported no change in forearm blood flow during 10% MVC hand-grip exercise in the presence of l-NMMA in the elderly. Together, these previous studies developed the concept that NO-mediated vasodilatation is impaired during hand-grip exercise in ageing humans.

More recently, our group extended these findings by examining the impact of sustained NOS blockade on forearm blood flow at rest and during four intensities of hand-grip exercise in older adults (Trinity et al. 2013). The magnitude of l-NMMA-induced reductions in forearm blood flow at rest was similar to our previous data in young subjects (Wray et al. 2011), which agrees with the reductions reported by Taddei et al. (2001). Thus, NO appears to be an important contributor to the regulation of resting skeletal muscle blood flow across the typical lifespan. However, this similar contribution of NO across age did not persist during exercise, because NOS inhibition failed to alter skeletal muscle blood flow substantially in the elderly during moderate intensity hand-grip exercise (Fig. 2, bottom panel). Although a reduction in forearm blood flow was documented at the lowest level of hand-grip exercise, this is likely to represent a carry-over effect from the reduced blood flow at rest. Indeed, normalizing the change in forearm blood flow for the l-NMMA-induced reduction at rest in the elderly eliminated this difference between the control and l-NMMA conditions, further supporting the conclusion of reduced NO bioavailability with age (Trinity et al. 2013).

Thus, with the inclusion of multiple exercise intensities and a sustained drug administration protocol, our observation that NOS inhibition did not significantly alter exercising limb blood flow in older individuals (Trinity et al. 2013) both extended and confirmed the work of Schrage et al. (2007), indicating that age and exercise intensity are important factors that must be considered when studying the regulation of muscle blood flow by NO. Indeed, it appears that NO acts as an important regulator of vasodilatation during exercise in the young by ensuring appropriate matching of tissue perfusion and metabolism (Kingwell, 2000); however, a reduction in NO bioavailability and a redundancy of mechanisms that are not yet fully understood appear to govern exercise hyperaemia in the old, such that NO does not significantly contribute to regulation of blood flow during hand-grip exercise with advancing age (Laughlin & Korzick, 2001; Schrage et al. 2007).

Endothelin-1

While NO-mediated vasodilatation is perhaps the most prominent and widely studied pathway within the vascular endothelium, age-related changes in secondary endothelial pathways should not be overlooked. One such pathway of particular interest in this regard is ET-1, the most potent endogenous vasoconstrictor (Yanagisawa et al. 1988). In the periphery, ET-1 provokes a sustained vasoconstrictor response by binding to endothelin subtype A (ETA) receptors located primarily on the vascular smooth muscle (Yanagisawa et al. 1988; Arai et al. 1990; Fig. 1). This endothelium-derived peptide is released in response to a variety of stimuli, including increases in pulsatile stretch (Macarthur et al. 1994), shear stress (Malek & Izumo, 1992) and hypoxia (Kourembanas et al. 1991) and a reduction in pH. Given that these physical and chemical stimuli are among the host of changes that take place within the skeletal muscle during exercise (Lüscher & Barton, 2000), ET-1 has been implicated as an important signalling molecule in the response to physical activity. Indeed, circulating concentrations of ET-1 have been documented to increase in an intensity-dependent manner during exercise (Maeda et al. 1997), suggesting a potential role of ET-1 in blood flow distribution and the support of arterial blood pressure. Moreover, previous work in animals has evaluated the tonic ET-1-mediated restraint on vascular tone via ETA and concomitant ETA/ETB receptor blockade during exercise, and reported that ETA receptor inhibition increased muscle blood flow during exercise (Merkus et al. 2003).

Considering the potential impact that this potent vasoconstrictor may have on vascular tone during exercise, it is somewhat surprising how few studies have explored the possible role of this pathway in the exercise-induced hyperaemic response. Therefore, our group undertook one of the first human studies on this topic, administering ET-1 in the femoral artery at rest and during knee-extensor exercise in young, healthy subjects to examine the possible role of ETA/ETB receptors in the regulation muscle blood flow during exercise (Wray et al. 2007). It was anticipated that exogenous ET-1 administration would evoke vasoconstriction at rest and during exercise, with no exercise-induced attenuation in the vasoconstrictor efficacy of the drug. We identified a significant (∼35%) reduction in resting leg blood flow in response to ET-1 and, somewhat surprisingly, observed an intensity-dependent reduction in ET-1-mediated vasoconstriction during knee-extensor exercise at 20, 40 and 60% of maximal knee-extensor work rate. This significant attenuation of ET-1-mediated vasoconstriction during leg exercise was interpreted as evidence for a high sensitivity of vascular ETA/ETB receptors to metabolic inhibition, which may contribute to the requisite hyperaemia during intense leg exercise, but begged the question whether, during exercise, endogenous ET-1 increases to a level of physiological significance and could therefore play a role in the regulation of blood flow in an exercising limb.

Endothelin-1 and ageing

In the context of exercise hyperaemia in the elderly, the ET-1 pathway is another attractive candidate that may contribute to blood flow dysregulation. Indeed, there is evidence from both animal and human studies for an increase in the activity of the ET-1 pathway with advancing age (Ishihata et al. 1991; Goettsch et al. 2001; Donato et al. 2009), which has been linked to the age-associated reduction in resting skeletal muscle blood flow (Thijssen et al. 2007; Van Guilder et al. 2007) via the ETA receptor pathway (Krum et al. 1998; Van Guilder et al. 2007). Thus, based on our previous finding with ET-1 infusions during exercise in the young (Wray et al. 2007), we sought to compare the cardiovascular responses to knee-extensor exercise before and after inhibition of the ETA receptor via intra-arterial administration of BQ-123, an ETA antagonist, in an exercising limb of young and older, healthy individuals. It was hypothesized that ETA receptor blockade during exercise would enhance skeletal muscle blood flow and reduce mean arterial blood pressure to a greater extent in old individuals, compared with their younger counterparts. As anticipated, this study identified a progressively greater increase in leg blood flow with increasing absolute exercise intensities following ETA receptor blockade in the old compared with the young (Barrett-O'Keefe et al. 2015; Fig. 3). The attenuation in the exercise-induced increase in mean arterial blood pressure with BQ-123 also displayed the expected age-associated difference, with the old exhibiting greater reductions in mean arterial blood pressure following ETA receptor blockade across increasing relative work rates. Together, these findings identified an exaggerated age-related increase in ETA-mediated vasoconstrictor activity that persists during exercise, suggesting an important role for this pathway in the regulation of exercising skeletal muscle blood flow and maintenance of arterial blood pressure in the elderly.

Figure 3. Endothelin subtype A (ETA) receptor inhibition (BQ-123)-induced changes in leg blood flow during progressive knee-extensor exercise in young and older healthy subjects.

Figure 3

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The slope of the relationship between changes in leg blood flow and work rate was significantly greater in the older subjects compared with the young. Data at the highest work rate (80% of maximum) represents the leg blood flow response in those participants whose maximal work rate was >15 W (n = 8, young; n = 5, older). ¥Significant difference from BQ-123-induced changes at rest; *significant difference from saline in the old; ‡significant difference from young. Abbreviations: c, y-intercept of linear regression; and m, slope of linear regression. Adapted from Barrett-O'Keefe et al. (2015).

These age-specific cardiovascular responses following BQ-123 contrast with the only other published study examining the effect of ETA receptor blockade on limb blood flow during exercise. Specifically, McEniery et al. (2002) evaluated forearm blood flow during static intermittent hand-grip exercise in middle-aged, hypertensive and normotensive individuals before and after intra-arterial (brachial) BQ-123 administration and reported no difference between the blocked and control trials in the normotensive group, suggesting that the ETA receptor did not contribute significantly to the modulation of vascular tone during hand-grip exercise. However, while this previous study included a range of ages (30–50 years), it certainly was not designed to examine age-related changes in limb blood flow in response to BQ-123. Thus, to our knowledge, our study was the first to perform ETA receptor inhibition during exercise in the elderly and to report a greater augmentation in leg blood flow in the old compared with the young in response to ETA blockade. These findings revealing an age-associated increase in ETA-mediated restraint of skeletal muscle blood flow during exercise add significantly to our understanding of how this pathway contributes to the sequalae of vascular dysfunction in the elderly.

While there are several factors that may modulate ETA-mediated vasoconstriction during exercise in the elderly, one noteworthy candidate is NO. Indeed, it is now well established that NO may limit the production and action of ET-1 (Goligorsky et al. 1994; Bender & Klabunde, 2007), a reciprocal relationship that may be particularly relevant to age-related changes in the regulation of skeletal muscle blood flow during exercise. As the role of NO in governing hyperaemia is less apparent in the elderly than in their younger counterparts (Fig. 2), it is tempting to speculate that one secondary consequence of the well-described decline in NO bioavailability with age (Taddei et al. 1995) may be less opposition to vasoconstrictor pathways, such as ET-1. Thus, it is possible that an age-related ‘imbalance’ between the NO and ET-1 pathways may be responsible, in part, for the exaggerated increase in leg blood flow following ETA receptor inhibition in the elderly (Fig. 3), although further studies using both NO and ETA antagonism are required to explore this interesting possibility further.

The sympathetic nervous system

While the vascular endothelium is clearly a significant contributor to fine-tuning of blood flow in the exercising muscle, perhaps equally important are the adjustments in vascular tone mediated though the sympathetic nervous system (SNS). Indeed, physical activity provokes a marked increase in SNS activity mediated by parallel activation of central somatosensory pathways and by reflexes arising from stimulation of the metabolically sensitive afferent nerve fibres in the exercising muscle (Smith et al. 2006). Together, these signalling pathways evoke regional changes in sympathetic outflow to the heart, skin and tissue vascular beds that serve to redirect blood flow away from less metabolically active tissue and towards the vasculature of the exercising muscle. It is important to note that while this exercise-induced increase in SNS activity is generated globally, differential end-organ expression is a key factor in optimizing blood flow to meet the metabolic demand of the exercising muscle. Indeed, many studies in both animals and humans have described a reduced sympathetic vasoconstriction in the exercising muscle compared with rest, an event that has been termed ‘functional sympatholysis’ (Remensnyder et al. 1962). As functional sympatholysis is confined to the vasculature of the exercising muscle and exhibits an intensity-dependent response, it is likely to be mediated by local factors related to muscle metabolism, though no one single metabolic factor has been identified as decisive in this process (Thomas & Segal, 2004).

The sympathetic nervous system and ageing

Considering the evidence for increased resting SNS activity with healthy ageing (Ng et al. 1993), it is not surprising that excessive sympathoexcitation has emerged as a potential explanation for the dysregulation of exercising skeletal muscle blood flow in the elderly. To date, a small number of studies have been undertaken to examine the hypothesis that age-related changes in functional sympatholysis might be responsible for the reduced skeletal muscle blood flow during exercise in this population. In what appears to be the first study to test this hypothesis in humans, Koch et al (2003) used the cold pressor test to provoke reflex sympathetic vasoconstriction during upright cycle exercise at 60% of maximal O2 uptake in young and older men. The authors reported a sizeable reduction (∼13%) in exercising leg vascular conductance in response to the cold pressor test in the older group, while conductance remained unchanged in the young group, presenting the first evidence for an age-related augmentation of vascular responsiveness to sympathoexcitation in the exercising limb. Using a different reflex sympathetic stimulus, Fadel et al. (2004) evaluated changes in muscle oxygenation during hand-grip exercise before and during orthostatic stress in a cohort of younger and older, postmenopausal women. Their study identified a reduced ability of hand-grip exercise to blunt sympathetic vasoconstriction in the exercising forearm of postmenopausal women, a deficit that was reversed following 1 month of transdermal oestrogen replacement therapy.

Building on these studies, Dinenno et al. (2005) evaluated arm vasoconstriction in response to sympathomimetic drug administration in young and older men during moderate-intensity (15% MVC) hand-grip exercise. When responses were evaluated using the magnitude of sympatholysis, i.e. subtraction of the drug-induced changes during exercise from the response observed at rest, the authors observed a blunted effect in the elderly cohort. Most recently, Mortensen et al. (2012) used tyramine, a drug that provokes endogenous noradrenaline (NA) release from the sympathetic bouton, to examine non-selective α-adrenergic vasoconstriction during knee-extensor exercise. A significant reduction in leg blood flow was observed in the older sedentary men when tyramine was infused into the exercising leg, while the tyramine-induced change in these variables was not significant in the young group. Although no between-group differences were observed, these authors concluded that attenuated functional sympatholysis was present in the older, sedentary individuals. Together, these studies have provided a line of evidence of a sustained capacity for α-adrenergic vasoconstriction during exercise with age, findings which have promoted the concept of impaired sympatholysis in the elderly.

Also using a pharmacological approach, we recently sought to determine the effect of age on α1-adrenergic vasoconstriction in the leg at rest and during knee-extensor exercise at a variety of absolute and relative exercise intensities (Wray et al. 2009). Based on the previous studies outlined above, we hypothesized that vasoconstriction in response to phenylephrine (PE, an α1-adrenergic agonist) would be reduced in the older group compared with young subjects at rest and that knee-extensor exercise would blunt PE-mediated vasoconstriction to a greater extent in the young than in their older counterparts. As hypothesized, we observed less vasoconstriction in the older group at rest during intra-arterial PE administration, in agreement with previous findings (Smith et al. 2007). During exercise, PE-induced vasoconstriction that was greater at 40 and 60% of knee-extensor maximal work rate (WRmax) in older subjects compared with their younger counterparts, a response which initially seemed to suggest that this receptor subtype was less susceptible to exercise-induced metabolic inhibition in the leg. However, confounding these initially simple observations was the finding that maximal knee-extensor exercise capacity was significantly lower in the older group (28 ± 5 W) compared with the young (50 ± 5 W), raising the issue of whether the observed differences in leg α1 sensitivity could be attributed to the fact that older subjects did not reach absolute work rates high enough to provoke full attenuation of this receptor group.

While the a priori design of this study was focused on a relative exercise intensity protocol, the inclusion of multiple exercise intensities offered the unique opportunity for post hoc examination of PE-mediated vasoconstriction at both relative and absolute work rates. With this approach, several noteworthy observations were apparent. When PE-induced changes in blood flow between rest and exercise are compared, the ‘magnitude of sympatholysis’ appears to be attenuated in the older group (Fig. 4, top panel), although it should be noted that between-subject differences in work rate preclude a direct statistical comparison. However, given the marked autonomic and haemodynamic changes that take place during exercise, it is conceivable that the age-related changes in α-adrenergic sensitivity observed at rest may differ during exercise. Thus, the response to PE during exercise alone was also evaluated (Fig. 4, bottom panel). Interestingly, at a single work rate achieved by both groups (10 W), there was no difference between groups in the vasoconstrictor response to similar doses of PE (16 ± 4 μg min−1, young; 17 ± 2 μg min−1, older). Although this work rate clearly represents a greater relative effort in the old (40% WRmax) than in the young subjects (20% WRmax), this comparison offers the advantage of viewing α1-receptor sensitivity between age groups in conditions where the leg musculature performs a similar amount of mechanical work and, perhaps, a similar metabolic cost.

Figure 4. Haemodynamic response to phenylephrine (PE) during exercise, expressed both as the magnitude of sympatholysis (top panel) and as the PE-induced change in leg blood flow (bottom panel).

Figure 4

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Abbreviations: c, y-intercept of linear regression; and m, slope of linear regression. Adapted from Wray et al. (2009).

Amelioration of the age-related difference in sympatholysis when absolute work performed is controlled is in agreement with our previous findings in the exercising leg of young subjects, which revealed a clear progression of postjunctional α-adrenergic inhibition with increasing levels of absolute work (Brothers et al. 2006; Wray et al. 2004). Furthermore, a clear relationship was evident when PE-induced vasoconstriction was viewed in terms of absolute work rate across the complete spectrum assessed in both young and old; indeed, linear regression analysis confirmed that mean slope and y-intercept values were almost identical between young and old groups (Fig. 4, bottom panel). Together, these findings suggest that, when comparing groups with dissimilar exercise capacities, the degree of sympatholysis in the leg should be interpreted with caution. Differences in exercise-induced inhibition of α1-adrenergic vasoconstriction between young and old may therefore not be as pronounced as previously reported, emphasizing the need to consider the metabolic challenge of exercise when comparing these groups and presenting a clear demand for further study to provide better characterization of potential age-related changes in this important regulatory pathway.

Angiotensin II

In addition to the direct effect of sympathoexcitation via α-adrenergic vasoconstriction, a significant secondary effect of sympathetic vasoconstriction is expressed through the RAAS. Indeed, it is now well known that renin is released from kidney afferent arterioles in response to SNS stimulation (Gordon et al. 1967), which in turn catalyses the conversion of angiotensinogen to angiotensin I (Ang I). Following subsequent conversion of Ang I to Ang II via angiotensin-converting enzyme, this potent vasoconstrictor binds to the AT1 receptor on arteriolar vascular smooth muscle (Fig. 1). The potential involvement of endogenous Ang II in the regulation of exercise hyperaemia is supported by the acute increase in circulating Ang II during exercise (Tidgren et al. 1991), as well as the profound haemodynamic effect seen when AT1 receptor blockade is applied during exercise. Indeed, in an animal model, pretreatment with the AT1 antagonist losartan has been documented to decrease systemic vascular resistance and mean arterial blood pressure significantly during exercise comparedwith the unblocked state (Stebbins & Symons, 1995; Symons & Stebbins, 1996), indicating that Ang II is an important component of the systemic cardiovascular response to dynamic exercise. Although this previous work using AT1 receptor blockade identified a significant role of endogenous Ang II during exercise, potential metabolic inhibition of this receptor group was not examined. Members of our group have recently addressed this issue though acute, intra-arterial administration of exogenous Ang II and PE in younger subjects at rest and during light and moderate-intensity exercise (Brothers et al. 2006). In this study, we reported a significant decrease in leg blood flow with Ang II infusion at rest, which was blunted in an intensity-dependent manner during knee-extensor exercise to a similar degree to that observed with the sympathomimetic PE (Fig. 5). This inhibition of Ang II-mediated vasoconstriction was a somewhat unexpected finding and provided new evidence for the presence of a ubiquitous metabolic inhibition in the exercising limb that affects both adrenergic and non-adrenergic vasoconstrictor pathways. However, to our knowledge, studies using AT1 receptor antagonist infusions have not been undertaken in humans, and thus the exact role of endogenous Ang II during exercise remains a somewhat unexplored topic.

Figure 5. Angiotensin II-induced changes in leg blood flow during two levels of knee-extensor exercise in young and older healthy subjects.

Figure 5

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While both young and older subjects experienced an exercise-induced ‘lysing’ of angiotensin II-mediated vasoconstriction, this response was more pronounced in the elderly, particularly when viewed at a similar absolute exercise intensity (∼10 W). Abbreviation: WRmax, maximal knee-extensor work rate. Adapted from Wray et al. (2008).

Angiotensin II and ageing

Subsequent to the study in young, healthy individuals (Brothers et al. 2006), a similar experimental protocol was used by our group to explore whether the observed metabolic inhibition of Ang II-mediated vasoconstriction could be extended to older, healthy individuals (Wray et al. 2008). The hypotheses were as follows: (i) at rest, AT1 receptor sensitivity would be elevated in older individuals compared with their younger counterparts; (ii) knee-extensor exercise would blunt Ang II-mediated vasoconstriction in an intensity-dependent manner in both groups; and (iii) older subjects would remain relatively more sensitive to Ang II than the young group during exercise. As expected, both groups experienced a diminished response to exogenous Ang II administration during exercise. However, it is noteworthy that knee-extensor exercise capacity was attenuated in older subjects, and thus 20 and 40% WRmax represented a lower absolute work in this group. Regardless, between-group differences were amplified when the young and older groups were compared at a single absolute work rate (Fig. 5), confirming the presence of an age-related diminution in Ang II-mediated vasoconstriction during exercise.

This age-dependent increase in AT1 sensitivity was even more pronounced when the change from rest to exercise was compared. Indeed, Ang II infusion decreased resting leg blood flow to a greater extent in the elderly (−61 ±8%) compared with the young (−31 ± 5%), and in view of this enhanced Ang II-mediated vasoconstriction in the elderly at rest, the ‘magnitude of attenuation’ in leg blood flow from rest to exercise at 40% of WRmax was much greater in the elderly (50–60%) compared with the young (10–20%). These data are suggestive of a powerful mechanism within the active muscle tissue, probably multiple byproducts of aerobic and/or anaerobic metabolism, that is capable of overcoming the potent vasoconstrictor effects of Ang II, and again indicate that age-related elevations in resting AT1 receptor sensitivity do not contribute significantly to the blunted exercise hyperaemia often observed in the elderly (Proctor et al. 1998; Beere et al. 1999; Lawrenson et al. 2003; Poole et al. 2003). Indeed, it seems that the profound ‘lysing’ of both RAAS (Wray et al. 2008; Fig. 5) and adrenergic vasoconstriction (Wray et al. 2009; Fig. 4) may be a non-discriminate, yet essential, event in the overall series of reactions that collectively support the necessary increase in skeletal muscle blood flow during exercise in the elderly. Together, these data identify a differential role of the Ang II pathway in the resting and exercising muscle vasculature and indicate that age-related elevations in resting AT1 receptor sensitivity may play a role in the attenuated resting leg blood flow associated with the elderly, but probably do not contribute to the blunted exercise hyperaemia widely observed in this population.

α-Adrenergic and angiotensin II ‘cross-talk’

While the independent contribution of the α-adrenergic and RAAS pathways to excessive vascular tone in the elderly may be substantial, significant cross-talk exists that could produce a potentiating effect. Indeed, Ang II induces a marked vasoconstriction by binding to AT1 receptors located presynaptically on postganglionic sympathetic nerves, which enhances the release of NA, attenuates the reuptake of NA and, ultimately, potentiates α-adrenergic vasoconstriction (Clemson et al. 1994; Lyons et al. 1995). Considering the evidence for an age-related increase in circulating Ang II and AT1 receptor density (Siebers et al. 1990; Duggan et al. 1992), the direct and indirect (α-adrenergically mediated) vasoconstricting effects of Ang II in the elderly may be profound. This age-associated increase in RAAS activity is especially important in the context of underlying changes in autonomic function with age. Indeed, one of the most pronounced and reproducible observations associated with ageing is a progressive increase in SNS activity (Sundlöf & Wallin, 1978; Ng et al. 1993; Davy et al. 1998), which is modulated peripherally by both the α-adrenergic and RAAS pathways. Thus, it follows that in the presence of elevated basal SNS activity, the potentiating effect of circulating Ang II may be even more pronounced. Considering this, presynaptic AT1 receptors may be seen as uniquely positioned to act as a regulatory point for SNS control of skeletal muscle blood flow with advancing age.

In order to attain a better understanding of the importance of this α-adrenergic and Ang II interaction in terms of the regulation of skeletal muscle blood flow with advancing age, our group recently performed a study using intra-arterial (brachial artery) Ang II administration before and after regional sympathetic inhibition via the non-selective α-adrenergic antagonist phentolamine (Barrett-O'Keefe et al. 2013). We hypothesized that Ang II-mediated vasoconstriction would be greater in the old compared with the young, but this age effect would be attenuated in the presence of α-adrenergic antagonism. In this study, muscle sympathetic nerve activity and circulating catecholamines were higher in the old compared with their younger counterparts, confirming an age-associated increase in SNS activity. During maximal doses of Ang II, we observed an increase in venous NA in the old, but not in the young, clearly indicating an exaggerated potentiation of NA release in response to Ang II in the elderly. In support of our hypothesis, the age-associated increase in the Ang II-mediated vasoconstriction was abolished after α-adrenergic blockade, such that the maximal reduction in forearm blood flow in response to Ang II was similar to that observed in the young group (Fig. 6). These data revealed that α-adrenergic vasoconstriction was responsible for nearly 20% of the maximal reduction in blood flow induced by intra-arterial Ang II infusion in the old, approximately double that observed in the young. Taken together, these data reveal that with healthy ageing, an increased Ang II receptor sensitivity may be attributed, at least in part, to a potentiation of the α-adrenergically mediated vasoconstriction and implicate the ‘cross-talk’ between the RAAS and adrenergic systems as an important consideration in therapeutic strategies targeting these two pathways. In the context of an age-related increase in SNS activity, these findings may also represent a potential mechanism contributing to enhanced α-adrenergically mediated vascular tone associated with healthy ageing. Whether this potentiating effect persists during exercise is currently unknown and is certainly a topic worthy of further study.

Figure 6. Maximal reductions in brachial artery blood flow induced by intra-arterial infusion of the α-antagonist noradrenaline (NA), angiotensin II (Ang II) alone and Ang II after non-selective α-adrenergic blockade with phentolamine.

Figure 6

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These responses suggest that α-adrenergic vasoconstriction is responsible for nearly 20% of the maximal reduction in blood flow induced by intra-arterial Ang II infusion in the old, which is approximately double that observed in the young. #P < 0.05 compared with Ang II alone. Adapted from Barrett-O';Keefe et al. (2013). †P < 0.05 compared with young.

Age and skeletal muscle blood flow: a maladaptive process?

While the age-related reduction in exercising skeletal muscle blood flow is generally considered to be detrimental, the question of whether this observation represents an adaptive or maladaptive process has been increasingly recognized (Proctor & Newcomer, 2006). This issue was underscored in a recent study by Mortensen et al. (2012) exploring the impact of ageing and physical activity on adrenergic and purinergic signalling during exercise. One of the more interesting conclusions from this study was related to the comparison between older men who were sedentary and their habitually active counterparts. While both older groups demonstrated impaired vasodilatation during exercise compared with the young group, only the sedentary older men experienced a greater lactate release in association with the reduction in leg blood flow during exercise, suggesting an impairment of local aerobic metabolism in the older, sedentary group. The importance of these distinctions regarding the ‘supply-and-demand’ aspect of age-related changes in exercising muscle blood flow were highlighted in an invited editorial on this study (Proctor & Moore, 2012). Additional support for this concept comes from recent work from our own group, where an increase in exercising muscle blood flow induced by ETA receptor inhibition was accompanied by an increase in leg O2 uptake with no change in leg lactate release. This decrease inintramuscular efficiency was interpreted as evidence that the age-related change in the ETA pathway may provide an important governing influence in optimizing O2 supply and demand, and further reinforces the evolving idea that an age-related reduction in exercising limb blood flow, per se, may not represent an entirely maladaptive process.

Summary

Evidence has been presented in support of a general shift towards vasoconstriction with advancing age, although it is clear that no individual pathway is solely responsible for this age-related change (Fig. 1). In terms of the vascular endothelium, the combined effect of reduced NO bioavailability and increased ETA-mediated vasoconstriction, perhaps working in concert, have been documented to contribute significantly to the increase in vascular tone during exercise in the elderly. There is clear evidence for an age-related increase in resting SNS activity with advancing age, and the preponderance of experimental evidence to date supports impairment of exercise-induced ‘lysing’ of sympathetic vasoconstriction in the elderly, although this issue remains open to alternative interpretations. Angiotensin II-mediated vasoconstriction is elevated in resting conditions in the elderly, yet this increase in AT1 receptor sensitivity does not persist during exercise, and thus it appears that the RAAS pathway does not contribute significantly to age-related changes in skeletal muscle hyperaemia during exercise. As with many physiological systems, it seems that the regulation of skeletal muscle blood flow in the elderly involves many interwoven, redundant pathways, each of which may have been uniquely altered, making deciphering roles difficult. It is also noteworthy that these pathways may be altered in a limb- and sex-specific manner, further adding to the challenges of interpreting findings and developing consensus on age-related changes. Future work addressing the overlap and interaction of these pathways appears necessary to elucidate fully the complex manner in which blood flow to the exercising muscle is governed in our growing ageing population.

New Findings.

What is the topic of this review?

This review focuses on age-related changes in the regulatory pathways that exist at the unique interface between the vascular smooth muscle and the endothelium of the skeletal muscle vasculature, and how these changes contribute to impairments in exercising skeletal muscle blood flow in the elderly.

What advances does it highlight?

Several recent in vivo human studies from our group and others are highlighted that have examined age-related changes in nitric oxide, endothelin-1, alpha adrenergic, and renin-angiotensin-aldosterone (RAAS) signaling.

Acknowledgments

Funding: Funded in part by NIH PO1 HL-091830, VA RR&D E6910R, AHA 0835209N, and NIH R01 HL118313.

Footnotes

Competing interests: None declared.

Author contributions: Both authors were involved with conception and design of experiments, collection, analysis, and interpretation of data, and manuscript preparation. Both authors approved the final version of the manuscript, all persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

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