Gary J. Hodges

The involvement of norepinephrine, neuropeptide Y, and nitric oxide in the cutaneous vasodilator response to local heating in humans

Presynaptic blockade of cutaneous vasoconstrictor nerves (VCN) abolishes the axon reflex (AR) during slow local heating (SLH) and reduces the vasodilator response. In a two-part study, forearm sites were instrumented with microdialysis fibers, local heaters, and laser-Doppler flow probes. Sites were locally heated from 33 to 40 degrees C over 70 min. In part 1, we tested whether this effect of VCN acted via nitric oxide synthase (NOS). In five subjects, treatments were as follows: 1) untreated; 2) bretylium, preventing neurotransmitter release; 3) N(G)-nitro-L-arginine methyl ester (L-NAME) to inhibit NOS; and 4) combined bretylium + L-NAME. At treated sites, the AR was absent, and there was an attenuation of the ultimate vasodilation (P < 0.05), which was not different among those sites (P > 0.05). In part 2, we tested whether norepinephrine and/or neuropeptide Y is involved in the cutaneous vasodilator response to SLH. In seven subjects, treatments were as follows: 1) untreated; 2) propranolol and yohimbine to antagonize alpha- and beta-receptors; 3) BIBP-3226 to antagonize Y(1) receptors; and 4) combined propranolol + yohimbine + BIBP-3226. Treatment with propranolol + yohimbine or BIBP-3226 significantly increased the temperature at which AR occurred (n = 4) or abolished it (n = 3). The combination treatment consistently eliminated it. Importantly, ultimate vasodilation with SLH at the treated sites was significantly (P < 0.05) less than at the control. These data suggest that norepinephrine and neuropeptide Y are important in the initiation of the AR and for achieving a complete vasodilator response. Since VCN and NOS blockade in combination do not have an inhibition greater than either alone, these data suggest that VCN promote heat-induced vasodilation via a nitric oxide-dependent mechanism.

The effect of microdialysis needle trauma on cutaneous vascular responses in humans

Microdialysis enables in-depth mechanistic study of the cutaneous circulation in humans. However, whether the insertion or presence of the microdialysis fiber (MDF) affects the skin circulation or its responses is unknown. We tested whether the cutaneous vascular response to whole body heating (WBH) was affected by MDF or by pretreatment with ice (part 1) or local anesthesia (LA; part 2). Eleven subjects participated, 9 in part 1 and 8 in part 2 (5 participated in both). In both parts, four sites on the forearm were selected, providing untreated control, MDF only, ice or LA only, and combined MDF plus ice or LA. A tube-lined suit controlled whole body skin temperature, which was raised to approximately 38 degrees C for WBH. Skin sites were instrumented with laser-Doppler flow probes. Data were expressed as cutaneous vascular conductance (CVC). Baseline levels were not different among sites (P > 0.05). In part 1, the internal temperature for the onset of vasodilation was higher (P > 0.05) with MDF with or without ice pretreatment than at untreated control sites (control 36.6 +/- 0.1 degrees C, Ice 36.5 +/- 0.1, MDF 36.8 +/- 0.1 degrees C, and Ice+MDF 36.8 +/- 0.1 degrees C). Peak CVC during WBH was decreased (P < 0.05) by MDF (control 73 +/- 7 vs. MDF 59 +/- 6% of maximal CVC). Ice (73 +/- 6% of maximal CVC) or Ice+MDF (69 +/- 6% of maximal CVC) did not affect (P > 0.05) peak CVC compared with control. In part 2, the temperature threshold for the onset of vasodilation was increased by MDF with or without LA treatment and by LA alone (P < 0.05; control 36.6 +/- 0.1 degrees C, MDF 36.7 +/- 0.1 degrees C, LA 36.8 +/- 0.1 degrees C, and LA+MDF 36.8 +/- 0.1 degrees C). Peak CVC was decreased by MDF (control 69 +/- 6% of maximal CVC vs. MDF 58 +/- 8% of maximal CVC; P < 0.05). LA only (65 +/- 10% of maximal CVC) or MDF in the presence of LA (73 +/- 12% of maximal CVC) did not affect (P > 0.05) peak CVC compared with control. Thus LA or MDF increases the temperature threshold for the onset of vasodilation. MDF alone decreases the peak vasodilator response in CVC to WBH; however, this attenuation did not occur if ice or LA is used before MDF placement. Ice or LA alone do not affect the peak response in CVC to WBH. How those treatments prevent or reverse the effect of MDF placement is presently unclear.

Comparison of laser speckle contrast imaging with laser Doppler for assessing microvascular function

OBJECTIVE To compare the inter-day reproducibility of post-occlusive reactive hyperaemia (PORH) and sympathetic vasomotor reflexes assessed by single-point laser Doppler flowmetry (SP-LDF), integrating-probe LDF (IP-LDF) and laser speckle contrast imaging (LSCI), and the spatial variability of PORH assessed by IP-LDF and LSCI. We also evaluated the relationship between IP-LDF and LSCI perfusion values across a broad range of skin blood flows. METHODS Eighteen healthy adults (50% male, age 27 ± 4 years) participated in this study. Using SP-LDF, IP-LDF and LSCI, indices of skin blood flow were measured on the forearm during PORH (1-, 5- and 10-min occlusions) and on the finger pad during inspiratory gasp and cold pressor tests. These tests were repeated 3-7 days later. Data were converted to cutaneous vascular conductance (CVC; laser Doppler flow/mean arterial pressure) and expressed as absolute and relative changes from pre-stimulus CVC (ΔCVC(ABS) and ΔCVC(REL), respectively), as well as normalised to peak CVC for the PORH tests. Reproducibility was expressed as within-subjects coefficients of variation (CV, in %) and intraclass correlation coefficients. RESULTS The reproducibility of PORH on the forearm was poorer when assessed with SP-LDF and IP-LDF compared to LSCI (e.g., CV for 5-min PORH ΔCVC(ABS)=35%, 27% and 19%, respectively), with no superior method of data expression. In contrast, the reproducibility of the inspiratory gasp and cold pressor test responses on the finger pad were better with SP-LDF and IP-LDF compared to LSCI (e.g., CV for inspiratory gasp ΔCVC(REL)=13%, 7% and 19%, respectively). The spatial variability of PORH responses was poorer with IP-LDF compared to LSCI (e.g., CV ranging 11-35% versus 3-16%, respectively). The association between simultaneous LSCI and IP-LDF perfusion values was non-linear. CONCLUSION The reproducibility of cutaneous PORH was better when assessed with LSCI compared to SP-LDF and IP-LDF; probably due to measuring larger skin areas (lower inter-site variability). However, when measuring sympathetic vasomotor reflexes on the finger pad, reproducibility was better with SP-LDF and IP-LDF, perhaps due to the high sensitivity of LSCI to changes in skin blood flow at low levels.

Adrenergic control of the human cutaneous circulation

The cutaneous circulation is influenced by a variety of thermoregulatory (skin and internal temperature-driven) and nonthermoregulatory (e.g., baroreflex, exercise-associated reflexes) challenges. The responses to these stimuli are brought about through vasoconstrictor nerves, vasodilator nerves, and changes in the local temperature of the vessels themselves. In this review, we examine how thermoregulatory influences mediate changes in skin blood flow through the sympathetic nervous system. We discuss cutaneous vascular responses to both local and whole-body heating and cooling and the mechanisms underlying these responses, with the overarching conclusion that sympathetic function plays significant roles in reflex vasoconstriction and vasodilatation and in the responses to both local cooling and local heating of the skin.

Reproducibility of cutaneous thermal hyperaemia assessed by laser Doppler flowmetry in young and older adults

OBJECTIVES The primary objective of this study was to assess the inter-day reproducibility of cutaneous thermal hyperaemia, as assessed using integrating-probe laser Doppler flowmetry (LDF), in young and older men. A secondary objective was to identify the most reproducible form of data expression. METHODS Cutaneous thermal hyperaemia was assessed on the forearm in 14 young (25±1 year) and 14 older (65±1 year) men, using integrating-probe LDF. The test was repeated 7-14 days later. The baseline, initial peak, and plateau phases of the data traces were identified and expressed as raw cutaneous vascular conductance (CVC), CVC normalised to baseline (%CVC(BL)), and CVC normalised to 44°C vasodilatation (%CVC(MAX)). Reproducibility was assessed using the coefficient of variation (CV) and intraclass correlation coefficient (ICC) statistics. RESULTS The inter-day reproducibility was dependent on how the data were expressed. The reproducibility of the initial peak and plateau was equally acceptable in both young and older adults when data were expressed as %CVC(MAX) (e.g., CVs ranging from 4 to 11%). However, the baseline phase was poorly reproducible in both groups irrespective of the data presentation method used (e.g., CVs ranging from 25 to 35%). Furthermore, expressing data as raw CVC or as %CVC(BL) generally showed poor reproducibility for both groups and all phases of the test (e.g., CVs ranging from 15 to 39%). CONCLUSION Integrating-probe LDF is a reproducible technique to assess cutaneous thermal hyperaemia on the forearm when data are expressed as %CVC(MAX) in healthy young and older adults without history of hypertension or taking system drugs.

Role of sensory nerves in the rapid cutaneous vasodilator response to local heating in young and older endurance‐trained and untrained men

The ability to increase skin blood flow (SkBF) rapidly in response to local heating is diminished with advanced age; however, the mechanisms are unclear. The primary aim of this study was to investigate the role of sensory nerves in this age‐related change. A secondary aim was to investigate the effect of aerobic fitness on sensory nerve‐mediated vasodilatation in young and aged skin. We measured SkBF (using laser Doppler flowmetry) in young and older endurance‐trained and untrained men (n = 7 in each group) at baseline and during 35 min of local skin heating to 42°C at two sites on the ventral forearm. One site was pretreated with topical anaesthetic cream to block local sensory nerve function. Cutaneous vascular conductance (CVC) was calculated as SkBF divided by mean arterial pressure and normalized to maximal values (CVCmax) achieved during local heating to 44°C. At the untreated site, the rapid vasodilatation during the first ∼5 min of local heating (initial peak) was lower in the older untrained group (68 ± 3%CVCmax) compared with all other groups (young trained, 76 ± 4%CVCmax; young untrained, 75 ± 5%CVCmax; and older trained, 81 ± 3%CVCmax; P < 0.05). Sensory nerve blockade abolished these differences among the groups (P > 0.05). The contribution of sensory nerve‐mediated vasodilatation was lower in the older untrained group compared with all other groups (P< 0.05). Our results suggest that the age‐related decline in the rapid vasodilator response to local heating in human skin is explained by diminished sensory nerve‐mediated vasodilatation. These findings also indicate that this age‐related change can be prevented through participation in regular aerobic exercise.

Neuropeptide Y and neurovascular control in skeletal muscle and skin

Neuropeptide Y (NPY) is a ubiquitous peptide with multiple effects on energy metabolism, reproduction, neurogenesis, and emotion. In addition, NPY is an important sympathetic neurotransmitter involved in neurovascular regulation. Although early studies suggested that the vasoactive effects of NPY were limited to periods of high stress, there is growing evidence for the involvement of NPY on baseline vasomotor tone and sympathetically evoked vasoconstriction in vivo in both skeletal muscle and the cutaneous circulation. In Sprague-Dawley rat skeletal muscle, Y(1)-receptor activation appears to play an important role in the regulation of basal vascular conductance, and this effect is similar in magnitude to the alpha(1)-receptor contribution. Furthermore, under baseline conditions, agonist and receptor-based mechanisms for Y(1)-receptor-dependent control of vascular conductance in skeletal muscle are greater in male than female rats. In skin, there is Y(1)-receptor-mediated vasoconstriction during whole body, but not local, cooling. As with the NPY system in muscle, this neural effect in skin differs between males and females and in addition, declines with aging. Intriguingly, skin vasodilation to local heating also requires NPY and is currently thought to be acting via a nitric oxide pathway. These studies are establishing further interest in the role of NPY as an important vasoactive agent in muscle and skin, adding to the complexity of neurovascular regulation in these tissues. In this review, we focus on the role of NPY on baseline vasomotor tone in skeletal muscle and skin and how NPY modulates vasomotor tone in response to stress, with the aim of compiling what is currently known, while highlighting some of the more pertinent questions yet to be answered.

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

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 (%CVC(max)) 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.

Noninvasive examination of endothelial, sympathetic, and myogenic contributions to regional differences in the human cutaneous microcirculation.

The aim of this study was to examine whether there are regional differences in the cutaneous microvascular responses of the forearm and the leg. Utilizing a non-invasive measure (spectral analysis),we looked at the influence of the endothelial, sympathetic, and myogenic function between regions at thermoneutral conditions (33 °C) and in response to local skin warming (42 °C) using laser-Doppler flowmetry (LDF). We recruited 18 young, healthy participants, who visited the lab on 2 separate occasions. Participants were instrumented with LDF probes and local skin heater probe-holders, placed on the forearm or the leg. Blood pressure was recorded by oscillometry. At both 33 °C and during local skin warming to 42 °C, skin vasomotion for the forearm and leg were evaluated using spectral analysis of the LDF recordings. There were significant differences among all frequencies of interest between the forearm and the leg. At 33 °C the leg presented with higher (P=0.003) activity for endothelial (0.009-0.021 Hz), sympathetic (P=0.002) (0.021-0.052 Hz), and myogenic (P=0.004) (0.052-0.145 Hz) activity when compared to the forearm. In contrast, following 35 min of local skin warming, the forearm had greater endothelial (P=0.019), sympathetic (P=0.006), and myogenic (P=0.005) vasomotion than the leg. These outcomes indicate regional differences in the cutaneous microcirculation. The current results are similar to our previous work using invasive methods and pharmacological agents, indicating that non-invasive analyses may be useful in the diagnoses and understanding of the mechanisms that control the microvascular function of pathological conditions.

Sensory and sympathetic nerve contributions to the cutaneous vasodilator response from a noxious heat stimulus.

We investigated the roles of sensory and noradrenergic sympathetic nerves on the cutaneous vasodilator response to a localized noxious heating stimulus. In two separate studies, four forearm skin sites were instrumented with microdialysis fibres, local heaters and laser‐Doppler probes. Skin sites were locally heated from 33 to 42°C or rapidly to 44°C (noxious). In the first study, we tested sensory nerve involvement using EMLA cream. Treatments were as follows: (1) control 42°C; (2) EMLA 42°C; (3) control 44°C; and (4) EMLA 44°C. At the EMLA‐treated sites, the axon reflex was reduced compared with the control sites during heating to 42°C (P < 0.05). There were no differences during the plateau phase (P > 0.05). At both the sites heated to 44°C, the initial peak and nadir became indistinguishable, and the EMLA‐treated sites were lower compared with the control sites during the plateau phase (P < 0.05). In the second study, we tested the involvement of noradrenergic sympathetic nerves in response to the noxious heating using bretylium tosylate (BT). Treatments were as follows: (1) control 42°C; (2) BT 42°C; (3) control 44°C; and (4) BT 44°C. Treatment with BT at the 42°C sites resulted in a marked reduction in both the axon reflex and the secondary plateau (P < 0.05). At the 44°C sites, there was no apparent initial peak or nadir, but the plateau phase was reduced at the BT‐treated sites (P < 0.05). These data suggest that both sympathetic nerves and sensory nerves are involved during the vasodilator response to a noxious heat stimulus.