Aging and aerobic fitness affect the contribution of noradrenergic sympathetic nerves to the rapid cutaneous vasodilator response to local heating

Garry A. Tew, John M. Saxton, Markos Klonizakis, James Moss, Alan D. Ruddock, Gary J. Hodges

Abstract

Sedentary aging results in a diminished rapid cutaneous vasodilator response to local heating. We investigated whether this diminished response was due to altered contributions of noradrenergic sympathetic nerves by assessing 1) the age-related decline and 2) the effect of aerobic fitness. Using laser-Doppler flowmetry, we measured skin blood flow (SkBF) in young (24 ± 1 yr) and older (64 ± 1 yr) endurance-trained and sedentary men (n = 7 per group) at baseline and during 35 min of local skin heating to 42°C at 1) untreated forearm sites, 2) forearm sites treated with bretylium tosylate (BT), which prevents neurotransmitter release from noradrenergic sympathetic nerves, and 3) forearm sites treated with yohimbine + propranolol (YP), which antagonizes α- and β-adrenergic receptors. SkBF was converted to cutaneous vascular conductance (CVC = SkBF/mean arterial pressure) and normalized to maximal CVC (%CVCmax) achieved by skin heating to 44°C. Pharmacological agents were administered using microdialysis. In the young trained group, the rapid vasodilator response was reduced at BT and YP sites (P < 0.05); by contrast, in the young sedentary and older trained groups, YP had no effect (P > 0.05), but BT did (P > 0.05). Neither BT nor YP affected the rapid vasodilator response in the older sedentary group (P > 0.05). These data suggest that the age-related reduction in the rapid vasodilator response is due to an impairment of sympathetic-dependent mechanisms, which can be partly attenuated with habitual aerobic exercise. Rapid vasodilation involves noradrenergic neurotransmitters in young trained men and nonadrenergic sympathetic cotransmitters (e.g., neuropeptide Y) in young sedentary and older trained men, possibly as a compensatory mechanism. Finally, in older sedentary men, the rapid vasodilation appears not to involve the sympathetic system.

  • skin blood flow
  • age
  • heat

in humans, the cutaneous circulation has a major role in the control of body temperature through the level of its perfusion. During heat stress, skin blood flow (SkBF) can increase to >6 l/min (30). In contrast, during exposure to extreme cold, SkBF can fall to almost zero (17). In nonglabrous (hairy) skin, the SkBF response to thermal stimuli local to the site of measurement appears to be achieved via a sympathetic noradrenergic system releasing norepinephrine (NE) and the cotransmitter neuropeptide Y (NPY) and a nonadrenergic system that is heavily dependent on nitric oxide (NO) (12, 13, 15).

The skin hyperemic response to a nonpainful, rapid heat stimulus is commonly used as a test of microvascular and endothelial function (6, 22) and involves at least two independent phases: an initial, rapid transitory rise followed by a nadir and, ultimately, a secondary rise and prolonged plateau (20, 23). The specific mechanisms underpinning these phases are complex and not completely understood. The rapid initial peak of the vasodilator response is thought to be primarily mediated by an axon reflex via activation of transient receptor potential vanilloid-1 receptors in C-fiber afferent nociceptive neurons (41). These sensory neurons might increase SkBF through the release of neuropeptides such as calcitonin gene-related peptide and/or substance P (4); however, these theories have yet to be directly tested. Additionally, NO has been shown to contribute modestly to the initial peak of the vasodilator response to rapid local heating (20, 23). The secondary rise and plateau in SkBF, in contrast, are heavily dependent on NO synthesis, as inhibition of NO synthase (NOS) reduces this phase by ∼70% (20, 23).

Recent work (12, 13, 15) supports the somewhat counterintuitive concept that cutaneous noradrenergic sympathetic nerves are also involved in the cutaneous vasodilator response to local heating. Indeed, presynaptic blockade of neurotransmitter release from these nerves with bretylium tosylate (BT) abolishes the rapid vasodilator (initial) phase and greatly reduces the overall vasodilator response to slow local heating (+0.1°C/min) (13, 15). By separate postsynaptic antagonism of α- and β-adrenergic receptors and of Y1 receptors, Hodges et al. (13) found evidence of roles for NE and the cotransmitter NPY. Adrenergic involvement in thermal hyperemia also has a rate dependency: the initial peak evoked by slow local heating (+0.1°C/min) is completely abolished by pretreatment with BT; by contrast, the initial peak evoked by rapid local heating (+2°C/min) is only halved under conditions of sympathetic nerve blockade (12).

The initial rapid peak and secondary plateau are diminished with sedentary aging (24, 34). Whereas the diminished secondary plateau of older adults is largely explained by attenuated NO-mediated vasodilation (2, 24), the mechanisms underpinning the decline in the initial rapid vasodilation are less clear. Diminished function of local sensory nerves might be implicated, because Tew et al. (33) found that blockade of sensory nerve function by a topical local anesthetic cream abolished the difference between young and older sedentary adults. They found that the size of the initial peak and the contribution of sensory nerves to the initial peak were similar between older endurance-trained and younger adults, suggesting that regular aerobic exercise can preserve sensory nerve-mediated vasodilator function in older adults. Previous studies demonstrated that aging is associated with decreases in skin sympathetic efferent outflow in response to heat exposure (10) and vasoconstrictor responsiveness to NE (36, 40). Therefore, the diminished initial peak of sedentary older adults might also involve a decreased contribution of noradrenergic sympathetic nerves.

Hence, the primary aim of this study was to investigate the role of cutaneous noradrenergic sympathetic nerves in the age-related decline in the initial rapid vasodilator response to local heating. A secondary aim was to further investigate the effect of regular aerobic exercise (as reflected by a higher aerobic fitness) on the initial peak in young and older adults by assessing whether the effects of habitual exercise on cutaneous vasodilation can be explained by altered contributions of sympathetic neurotransmitters. We hypothesized that the contribution of noradrenergic sympathetic nerves to the initial vasodilator response would be greater in the young and well-trained than the older sedentary individuals.

MATERIALS AND METHODS

Ethics Approval

This study was approved by the Ethics Committee of Sheffield Hallam University and conducted according to the principles of the Declaration of Helsinki. Written, informed consent was obtained before participants entered the study.

Participants

We recruited 28 men who were equally divided among four groups: young endurance-trained (24 ± 1 yr), young sedentary (25 ± 1 yr), older endurance-trained (64 ± 1 yr), and older sedentary (64 ± 1 yr). The trained participants were recruited from running and cycling clubs in and around Sheffield, UK. They had performed vigorous endurance exercise ≥3 times/wk, ≥30 min/session, for ≥5 yr. The sedentary participants reported undertaking no regular exercise. All participants were healthy, nonsmokers; they were free from cardiovascular disease and diabetes and were not taking any medications. The participants attended the testing facility on two separate occasions. For both sessions, they were asked to abstain from caffeine, alcohol, and exercise for 24 h prior to their attendance. The participants are the same as those described in a recently published study by our group (33).

Visit 1: Assessment of Cardiopulmonary Fitness

Participants completed a continuous, incremental cycling test to volitional exhaustion on an electronically braked cycle ergometer (Excalibur Sport, Lode, The Netherlands). Pedaling frequency was self-selected within the range of 60–90 rpm. After a 2-min warm-up against no resistance (0 W), the intensity of exercise was increased by 20–30 W/min. Participants were encouraged to continue cycling to volitional exhaustion or until a plateau in O2 consumption was observed. Heart rate was recorded continuously by electrocardiogram (Cardioperfect, Welch Allyn). The volume of O2 consumed during exercise was calculated from minute ventilation, measured using a pneumotachograph, and simultaneous breath-by-breath analysis of expired gas fractions (Ultima CardiO2, MedGraphics). Gas analyzers and flow probes were calibrated before each test. O2 consumption was expressed relative to body mass (ml·kg−1·min−1). Maximal O2 consumption (V̇o2max) was calculated as the highest consecutive 20-s period of gas exchange data in the last minute before volitional exhaustion, which generally occurred due to leg fatigue and/or breathlessness.

Visit 2: Assessment of SkBF Responses to Local Heating

Instrumentation.

The microvascular assessments were performed in a temperature-controlled (22–24°C) room, with participants resting supine and the experimental (left) arm positioned at heart level for the entire protocol. Blood pressure was measured automatically on the right arm every 2 min (Dinamap Dash 2500, GE Healthcare).

Two microdialysis fibers (Linear 30, CMA Microdialysis, Stockholm, Sweden) with a membrane length of 10 mm and a 6-kDa cutoff were placed ∼5 cm apart in the dermal layer of skin on the ventral aspect of the left forearm. Before implantation, the skin was temporarily anesthetized by application of an ice pack for 5 min (11). A 21-gauge needle was introduced aseptically into the dermis along a length of ∼2.5 cm before exiting. A microdialysis fiber was threaded through the lumen of the needle, before removal of the needle, to leave the fiber in place. All microdialysis fibers were placed in this manner. To allow for the effects of the insertion trauma to subside, we waited 1.5–2 h before beginning the protocol (13).

To obtain an index of SkBF, cutaneous red blood cell flux was measured using laser-Doppler flowmetry (Periflux 5000 System, Perimed, Järfälla, Sweden) at the two microdialysis sites and at a third “no-fiber” control site. Local heater disks (model 455, Perimed) were used to control local skin temperature, and integrating laser-Doppler probes (model 413, Perimed) were placed in the center of each local heating disk.

Drugs.

Blockade of neurotransmitter release from sympathetic adrenergic nerves was achieved at one of the microdialysis sites by administration of a 20 mM solution of BT (US Pharmacopeia, Rockville, MD). Administration of BT causes a selective and localized blockade of neurotransmitter release from cutaneous sympathetic adrenergic nerves lasting several hours (19).

Blockade of the α- and β-adrenergic receptors was achieved by administration of a combination of 5 mM yohimbine (Sigma Aldrich, St. Louis, MO) and 1 mM propranolol (Sigma Aldrich) to antagonize those receptors. We will refer to these skin sites as the YP sites. Yohimbine is traditionally regarded as an α2-adrenergic antagonist; however, this combination and concentration of adrenergic antagonists have been shown to be effective in inhibiting the cutaneous vascular responses to exogenous NE (18, 31), suggesting that all α- and β-adrenergic receptors are blocked.

As in previous studies investigating the role of sympathetic-dependent mechanisms in cutaneous vasodilation, all drugs were infused at a rate of 4 μl/min (12, 13).

Protocol.

Data collection began after the trauma resolution period. Baseline data were recorded for 5 min with the local heating disk at 33°C. The temperature of the disks was then increased at a rate of 1°C every 10 s to 42°C (34) and held constant at this temperature for 35 min (32). Then local heating temperature was increased to 44°C for 10 min to induce maximal SkBF (37). No participants experienced pain or discomfort during the local heating protocol.

Data Collection and Analysis

SkBF data were divided by mean arterial pressure for calculation of cutaneous vascular conductance (CVC). CVC data were expressed as raw values [arbitrary units (AU)/mmHg] and as a percentage of maximal vasodilation recorded during local heating to 44°C (%CVCmax). Because of the rapid and transient nature of the initial peak responses, stable 30-s periods of SkBF were used for analysis. For the secondary plateau and maximal SkBF phases, stable 2-min periods of SkBF were used for analysis.

To assess the contribution of sympathetic adrenergic nerves to the initial peak in each group, we compared the SkBF responses between the control and drug sites. For example, similar responses between all three sites would suggest that NE and NPY do not contribute to the initial peak. Equal depression of the initial peak at the BT and YP sites would indicate that NE contributes to the initial peak, whereas NPY does not. Finally, depression of the initial peak at the BT site, but not the YP site, would indicate that NPY contributes to the initial peak, whereas NE does not.

A one-way independent ANOVA (SAS version 9.1, SAS Institute, Cary, NC) was used for comparison of participant characteristics among groups. The effects of age, training status, and pharmacological manipulations on hemodynamic measures were assessed using a three-way ANOVA. Where significant interaction effects were observed, Tukey's post hoc analyses were used to identify significant differences in the pair-wise comparisons. Statistical significance was set at P < 0.05, and all data are presented as means ± SE.

RESULTS

Participant Characteristics

The characteristics of the participants are reported elsewhere (33). Briefly, the groups did not differ in body mass, stature, or resting systolic or diastolic blood pressure (P > 0.05). All participants were normotensive and achieved V̇o2max according to standard criteria (16). V̇o2max was higher (P < 0.05) for the young trained group (58 ± 3 ml·kg−1·min−1) than the young sedentary, older trained, and older sedentary groups (40 ± 2, 44 ± 2, and 28 ± 2 ml·kg−1·min−1, respectively). V̇o2max was lower for the older sedentary group than all other groups (P < 0.05), and there was no difference between the young sedentary and older trained groups (P > 0.05).

CVC Responses

Local heating resulted in the characteristic biphasic SkBF response previously described (23): an initial rapid increase and peak at the onset of heating, a brief nadir, and then a slower rise and plateau. This pattern was seen in all four groups and at all skin sites.

Normalized Baseline

Baseline CVC did not differ among groups at each skin site (P > 0.05). For example, at the control site, baseline CVC for the young trained, young sedentary, older trained, and older sedentary groups was 7 ± 1, 8 ± 1, 7 ± 1, and 8 ± 1% CVCmax, respectively (P > 0.05). Furthermore, baseline CVC did not differ among skin sites within any of the groups (8 ± 1, 9 ± 1, and 9 ± 1% CVCmax for young sedentary control, BT, and YP sites, respectively, P > 0.05), indicating no effect of pharmacological treatment.

Normalized Initial Peak

Figure 1 shows the normalized initial peak data for all groups at the control, BT, and YP sites. At the control site, the initial peak was higher (P < 0.05) for the young trained and older trained (82 ± 3 and 79 ± 3% CVCmax, respectively) than the young sedentary and older sedentary (74 ± 3 and 66 ± 5% CVCmax, respectively) groups. The initial peak was also lower for the older sedentary than the young sedentary group (P < 0.05), and there was no difference between the young trained and older trained groups (P > 0.05). The difference in the initial peak between trained and sedentary groups was more pronounced in the older than the young men (19 ± 4 vs. 11 ± 2%, P < 0.05).

Fig. 1.

Initial vasodilation to local skin heating. Data are means ± SE for each group and skin site. CT, control; BT, bretylium tosylate; YP, yohimbine + propranolol. Significantly different (P < 0.05) from *CT within group, †young trained, ‡older trained, §young untrained.

The initial peak was lower (P < 0.05) at the BT than the control site in all groups, except the older sedentary group (control − BT = 10 ± 3, 7 ± 2, 9 ± 2, and 2 ± 1% CVCmax for young trained, young sedentary, older trained, and older sedentary groups, respectively; Fig. 1). In addition, the initial peak at the BT site did not differ between the young trained, young sedentary, and older trained groups (P > 0.05), whereas the responses were lower for the older sedentary group than the young trained and older trained groups (P < 0.05).

In the young trained group, the initial peak was lower at the YP than the control site (control − YP = 11 ± 3% CVCmax, P < 0.05); however, the initial peak was similar between the BT and YP sites (72 ± 6 and 72 ± 4% CVCmax, respectively, P > 0.05), suggesting that NE contributes to the initial peak in young trained adults, whereas NPY does not (Fig. 1). In the young sedentary and older trained groups, the initial peak at the YP site (75 ± 2 and 77 ± 3% CVCmax, respectively) did not differ (P > 0.05) from that at the control site (74 ± 3 and 79 ± 4% CVCmax, respectively). The reduced vasodilator response to BT, but not YP, treatment suggests a role for NPY, but not NE, in the initial peak of these groups. As with BT treatment, YP did not affect the initial peak in the older sedentary group (control − YP = 0 ± 1% CVCmax, P > 0.05).

Normalized Plateau

Figure 2 shows the normalized plateau data for all groups at the control, BT, and YP sites. The plateau at the control site did not differ among groups (P > 0.05). The plateau was lower (P < 0.05) at the BT than the control site for the young trained (71 ± 1 vs. 91 ± 2% CVCmax, respectively) and older trained (85 ± 3 vs. 93 ± 2% CVCmax, respectively) groups. In contrast, the plateau was similar (P > 0.05) between the BT and control sites in the young sedentary (85 ± 5 vs. 90 ± 3% CVCmax, respectively) and older sedentary (91 ± 3 vs. 94 ± 1% CVCmax, respectively) groups. The plateau at the YP site in the young trained and older trained groups (90 ± 2 and 92 ± 3% CVCmax, respectively) did not differ from that at the control site (P > 0.05). In these groups, BT, but not YP, reduced vasodilation; this suggests a role for NPY, but not NE, in the plateau of these groups. As with BT treatment, YP did not affect (P > 0.05) the plateau in the young sedentary and older sedentary groups (88 ± 2 and 91 ± 2% CVCmax, respectively).

Fig. 2.

Normalized secondary plateau cutaneous vascular conductance responses to local heating. Data are means ± SE for each group and skin site. *Significantly different (P < 0.05) from CT within group.

Raw CVC Responses

The findings for the raw (nonnormalized) data (Table 1) are similar to those for the normalized data. There was no effect of group or treatment on baseline responses (P > 0.05). At the control site, the initial peak was higher in the young trained group than all other groups (P < 0.05), whereas the response was lower for the older sedentary group than all other groups (P < 0.05). There was no difference between the young sedentary and older trained groups (P > 0.05). At the control site, the plateau was higher in the young trained group than all other groups (P < 0.05), and there were no differences among the remaining three groups (P > 0.05). Treatment with BT and YP reduced the initial peak and plateau in the young trained group only (P < 0.05), and these phases did not differ between groups at the BT and YP sites (P > 0.05). Finally, in the young trained group, the initial peak did not differ between BT and YP sites (P > 0.05), whereas the plateau was lower at the BT than the YP site (P < 0.05).

View this table:
Table 1.

Raw CVC data for each phase of the local heating protocol

DISCUSSION

The main novel findings from this study are as follows. 1) The age-related decline in the rapid skin hyperemic response to localized heating is partially explained by a diminished contribution of noradrenergic sympathetic nerves. 2) The contribution of noradrenergic sympathetic nerves to the initial peak is greater in individuals with a higher level of aerobic fitness. 3) The sympathetic neurotransmitters contributing to the initial peak vary between young trained, young sedentary, and older trained adults. Our data suggest that NE contributes to the initial peak of the young trained group, whereas NPY does not. Conversely, NPY seems to play a role in the initial peak of the young sedentary and older trained groups, whereas NE does not. In the older sedentary group, there is a significant reduction in the initial peak and no involvement of noradrenergic sympathetic nerves.

Effects of Aging on the Initial Peak and Potential Mechanisms

The observation that the rapid skin hyperemic response to local heating was higher in both young groups than the older sedentary group is consistent with previous findings (24, 33, 34). It has been suggested that a smaller initial peak might be associated with a greater risk of local tissue damage in response to directly applied heat (24, 41). This seems logical, since a rapid increase in SkBF will minimize the heat transferred to the underlying tissues; however, further research is needed to substantiate this suggestion, especially since the initial peak normally does not usually occur until 3–4 min after the initiation of skin heating. In addition, the age-related decline in the initial peak might be associated with impaired wound healing. Indeed, the magnitude of the initial peak reflects sensory nerve function (33), which is known to be an important contributory factor to wound-healing capacity (29).

Our current findings suggest that the age-related decline is partly due to a diminished contribution of noradrenergic sympathetic nerves. Indeed, sympathetic nerve blockade decreased the initial peak in the young, but not the older sedentary, individuals, such that the between-group difference in the initial peak was smaller at the BT than the control site (Fig. 1). This finding may be due to the generalized decline in skin sympathetic efferent activity that occurs with primary aging (10), although other factors, such as a reduced release and postjunctional binding of sympathetic transmitters, cannot be excluded. Further research is needed to clarify the mechanisms by which noradrenergic sympathetic nerves contribute to local heating-induced cutaneous vasodilation. Previous research suggests that noradrenergic sympathetic nerves may sensitize the vascular responsiveness to local skin heating, which would affect the initial rise (axon reflex) in blood flow. Indeed, Houghton et al. (15) reported that low-dose NE infusion decreased the temperature threshold of the axon reflex response to slow local heating, and Drummond and Lipnicki (7) observed that iontophoresis of NE caused an axon reflex response in immediately adjacent skin that was blocked by pretreatment with a local anesthetic cream. There might also be an important interaction between NE and/or NPY and the production of NO via endothelial NOS (1, 5, 38), which would probably contribute more to the plateau phase of the local heating response; however, this is yet to be directly tested.

Importantly, the age-related decrement in the initial peak was not completely abolished by sympathetic nerve blockade, indicating that other factors are involved. One such factor might be diminished function of heat-sensitive nociceptors, since sensory nerve function blockade has been shown to abolish the difference in the initial peak between young adults and older sedentary adults (33). NO might also be implicated, given that it contributes modestly to the initial peak (20, 23); however, a previous study showed that NOS inhibition did not abolish the difference in the initial peak between young and older adults (24). Nevertheless, NO might be involved via potential interactions with sensory nerves (41) and/or noradrenergic sympathetic nerves (13). Further research is needed to understand how noradrenergic sympathetic nerves, sensory nerves, and NO interact in the rapid vasodilator response to local heating and their respective roles in the age-related decline in the initial peak. The current data would suggest a considerable degree of redundancy among these systems, similar to that known to be involved with reflex vasodilation in response to increases in core temperature (3).

Effects of Aerobic Fitness on the Initial Peak and Potential Mechanisms

The age-related decline in the initial peak was not present in the older trained group. This finding, which is consistent with our previous findings (33, 34), indicates that participation in regular aerobic exercise preserves the capacity to rapidly increase SkBF in response to skin heating into advanced age. Exercise training also appears to be associated with a higher initial peak in young adults; however, the impact of exercise training seems greater in older adults (Fig. 1), perhaps because these individuals have greater potential for improvement than their younger counterparts. This is consistent with what is known about conduit artery and resistance vessel function; exercise training seems to enhance vascular function to a greater extent in those with depressed function at baseline (35).

The difference in SkBF between the control and BT sites was lower in the sedentary groups, indicating that regular aerobic exercise can also increase the contribution of noradrenergic sympathetic nerves to the initial peak in young and older adults. As for the aging data, the underpinning mechanisms of this finding are unclear. Nevertheless, the current study provides novel and important data on the effects of aging and aerobic fitness on local heating-induced cutaneous vasodilation and the contribution of sympathetic neurotransmitters to this response. Further research is needed to identify the acute and chronic effects of exercise on cutaneous neurovascular function, including noradrenergic sympathetic nerve function.

Group-Specific Roles of NE and NPY in the Initial Peak

Our findings also indicate that the sympathetic neurotransmitters contributing to the initial peak vary between young trained, young sedentary, and older trained adults. Indeed, NE seemed to play a role in the initial peak of the young trained group, and NPY did not. By contrast, NPY, and not NE, appeared to play a role in the initial peak of the young sedentary and older trained groups. Neither NE nor NPY appears to be involved in the relatively diminished initial peak of the older sedentary group. Aging and sedentary behavior lead to an increase in sympathetic outflow, and it is under stressful conditions that NPY appears to play a role in sympathetic function (26, 42). We propose that our data indicate a role for NPY only as a compensatory mechanism. NE is the neurotransmitter usually used (young trained group), but with a sedentary lifestyle (young untrained group) or primary aging (older trained group), it would appear that NPY is required to compensate for a loss of adrenergic function. We are unable to speculate as to whether this is due to pre- or postsynaptic alterations, i.e., whether these changes are due to alterations in transmitter synthesis and release or in receptor density or affinity. The combination of a sedentary lifestyle and aging appears to result in a complete loss of sympathetic involvement, such that the initial peak responses under control conditions for the older sedentary group were somewhat similar to the responses achieved at the BT-treated sites for the other three groups. Also note the absence of any change following treatment with BT or YP.

Effects of Age and Aerobic Fitness on the Plateau and Maximum CVC Responses and the Role of Noradrenergic Sympathetic Nerves

The secondary plateau of the SkBF response to local heating did not differ between groups when the data were normalized to maximal CVC (Fig. 2), in contrast to some (8, 9, 14, 21, 24, 27, 28, 39), but not all (2, 25), of the previous studies that have investigated the impact of age and exercise training on local heating-induced SkBF responses. However, this finding might simply reflect our approach of normalizing data to CVC values recorded during local heating at 44°C. Although this method, which is used to account for the wide heterogeneity in capillary density across the forearm, is acceptable in healthy young adults and in mechanistically driven, carefully controlled studies (22), it might be inappropriate for comparing data between young and older adults (24, 25) because of the age-associated decline in the maximal SkBF response to local heating (21). For example, the CVC responses during the plateau phase were similar between groups when expressed as %CVCmax but lower in the older groups than in the young trained group when expressed as raw CVC (Table 1). Therefore, although participants from all groups reached a similar %CVCmax, lower maximal CVC values in aged skin would probably translate to a lower absolute SkBF for a given %CVCmax. For this reason, we chose to present data as raw CVC and %CVCmax. It is reassuring that the interpretation of the initial peak data did not change greatly between these different methods.

Our findings indicate that the contribution of cutaneous sympathetic nerves to the plateau phase of heat-induced vasodilation is dependent on the individual's aerobic fitness. Indeed, sympathetic nerve blockade using BT decreased the plateau in the young and older trained, but not the young and older sedentary, groups (Fig. 2). Three other studies demonstrated that cutaneous noradrenergic sympathetic nerves contribute to the plateau in young healthy adults (12, 13, 15). The findings of these studies are not directly comparable with our data, because two of the three studies used a slow heating protocol (+0.1°C/min) (13, 15), and the fitness/training status of the participants throughout the studies was unclear. Whereas NE and NPY contribute to the plateau response to slow local heating (13, 15), it seems that the sympathetic-related contribution to rapid local heating (in the trained groups) involves NPY only. Further research is needed to help understand this difference and how exercise training alters the contribution of sympathetic neurotransmitters.

A final curious observation that warrants discussion is the significantly greater maximal raw CVC in the young trained group at the control site than at the BT and YP sites (Table 1). No differences were observed among the sites in the other three groups. This may be due to the presence of microdialysis fibers at the BT and YP sites, but not at the control site; however, as this scenario did not occur in any of the other groups, we believe that this is unlikely. This might indicate is that, in young trained adults, either noradrenergic sympathetic nerves contribute to the maximal CVC response to local heating at 44°C (similar to our initial peak and plateau phase data) or local heating to 44°C was an insufficient stimulus to elicit a maximal vasodilator response. As we are unable to definitively state whether “true” maximum CVC was obtained at every skin site in the different groups, inspection of the raw CVC responses (Table 1) is particularly important in the interpretation of our results. It is reassuring that both methods of data presentation support our interpretation of the initial peak results.

Experimental Considerations

It might be argued that our assessment of drug effects was clouded by the absence of a microdialysis fiber at the control site. Indeed, it has been demonstrated that fiber placement alone decreases the peak reflex cutaneous vasodilator response to whole body heating (11). However, it has also been shown that this attenuation did not occur if ice was applied for 5 min before fiber placement. Since we used ice in this manner, any between-site differences are probably due to the action of the drugs, and not the absence of a microdialysis fiber at the control site. Furthermore, our pilot tests (n = 5) investigating this issue indicated that the SkBF response to rapid local heating is not affected by ice treatment + fiber placement (e.g., 71 ± 7 and 67 ± 7% CVCmax normalized initial peak for fiber and no fiber, and 2.56 ± 0.22 and 2.36 ± 0.12 AU/mmHg raw maximum for fiber and no fiber, P = 0.13 and P = 0.40, respectively).

Another potential limitation is that we did not use a NPY-specific antagonist such as BIBP-3226 to assess the contribution of NPY to the thermal hyperemic response. Hodges et al. (13) assessed the contribution of NE and NPY to the cutaneous vasodilator response to slow local heating using a 4-site “Latin-square” design: 1) control, 2) α- and β-adrenoceptor antagonism, 3) Y1-receptor antagonism, and 4) a combination of 2 and 3. We could not use this approach, because we have a three-channel laser-Doppler flowmeter. Nevertheless, with only three sites (control, neurotransmitter block, and α- and β-adrenoceptor antagonism), we were essentially able to obtain the effects of NE and, indirectly, NPY.

In summary, we present a comparison of cutaneous microvascular responses to localized heating between young and older endurance-trained and sedentary individuals, with specific focus on the initial vasodilator response and the contribution of noradrenergic sympathetic nerves to this phase. At untreated control sites, the initial vasodilator response to local heating was lower in the older sedentary group than in both young groups. The lower responses of the older sedentary group appeared to be partly explained by diminished contribution of noradrenergic sympathetic nerves. Our findings also indicate that the sympathetic contribution to the initial peak can be preserved into advanced age by maintenance of a high level of aerobic fitness and/or participation in regular aerobic exercise. Finally, the sympathetic neurotransmitters contributing to the initial peak vary between young trained, young sedentary, and older trained men. Specifically, NE seems to play a role in the initial peak of young trained men, whereas NPY does not. Conversely, NPY seems to play a role in the initial peak of young sedentary and older trained men, whereas NE does not. Finally, in older sedentary men, the rapid vasodilation is greatly reduced, with no involvement of the sympathetic system.

GRANTS

This study was funded by Sheffield Hallam University.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

ACKNOWLEDGMENTS

We thank the participants for their time and commitment to this study.

REFERENCES

  1. 1.
  2. 2.
  3. 3.
  4. 4.
  5. 5.
  6. 6.
  7. 7.
  8. 8.
  9. 9.
  10. 10.
  11. 11.
  12. 12.
  13. 13.
  14. 14.
  15. 15.
  16. 16.
  17. 17.
  18. 18.
  19. 19.
  20. 20.
  21. 21.
  22. 22.
  23. 23.
  24. 24.
  25. 25.
  26. 26.
  27. 27.
  28. 28.
  29. 29.
  30. 30.
  31. 31.
  32. 32.
  33. 33.
  34. 34.
  35. 35.
  36. 36.
  37. 37.
  38. 38.
  39. 39.
  40. 40.
  41. 41.
  42. 42.
View Abstract