Sympathetic nonnoradrenergic cutaneous vasoconstriction in women is associated with reproductive hormone status

Modulation of cold-induced shivering activity by intermittent and continuous voluntary suppression

This investigation assessed the physiological effects of voluntary suppression of shivering thermogenesis in response to whole body cooling. Eleven healthy volunteers underwent passive air cooling (10°C), across three visits: NO_SUP, where participants allowed their body to freely regulate against the cold; FULL_SUP, where participants constantly suppressed shivering; INT_SUP, where participants intermittently suppressed shivering (5 min phases), interspersed with 5 min free regulation. Shivering was assessed via electromyography (EMG), mechanomyography (MMG), and whole body oxygen uptake (V̇o₂), whereas body temperature and heat exchange were assessed via skin temperature, rectal temperature, and heat flux sensors. A 29% increase was observed in shivering onset time in the FULL_SUP trial compared with NO_SUP (P = 0.032). Assessing shivering intensity, EMG activity decreased by 29% (P = 0.034), MMG activity decreased by 35% (P = 0.031), whereas no difference was observed in V̇o₂ (P = 0.091) in the FULL_SUP trial compared with NO_SUP. Partitioning the no-suppression and suppression phases of the INT_SUP trial, acute voluntary suppression significantly decreased V̇o₂ (P = 0.001), EMG (P < 0.001), and MMG (P = 0.012) activity compared with the no-suppression phases. Shivering activity was restored in the no-suppression phases, equivalent to that in the NO_SUP trial (P > 0.3). No difference was observed in thermal metrics between conditions up to 60 min (P > 0.4). Humans can both constantly and periodically suppress shivering activity, leading to a delay in shivering onset and a reduction in shivering intensity. Following suppression, regular shivering is resumed.

Independent and combined impact of hypoxia and acute inorganic nitrate ingestion on thermoregulatory responses to the cold

Purpose

This study assessed the impact of normobaric hypoxia and acute nitrate ingestion on shivering thermogenesis, cutaneous vascular control, and thermometrics in response to cold stress.

Method

Eleven male volunteers underwent passive cooling at 10 °C air temperature across four conditions: (1) normoxia with placebo ingestion, (2) hypoxia (0.130 F_iO₂) with placebo ingestion, (3) normoxia with 13 mmol nitrate ingestion, and (4) hypoxia with nitrate ingestion. Physiological metrics were assessed as a rate of change over 45 min to determine heat loss, and at the point of shivering onset to determine the thermogenic thermoeffector threshold.

Result

Independently, hypoxia expedited shivering onset time (p = 0.05) due to a faster cooling rate as opposed to a change in central thermoeffector thresholds. Specifically, compared to normoxia, hypoxia increased skin blood flow (p = 0.02), leading to an increased core-cooling rate (p = 0.04) and delta change in rectal temperature (p = 0.03) over 45 min, yet the same rectal temperature at shivering onset (p = 0.9). Independently, nitrate ingestion delayed shivering onset time (p = 0.01), mediated by a change in central thermoeffector thresholds, independent of changes in peripheral heat exchange. Specifically, compared to placebo ingestion, no difference was observed in skin blood flow (p = 0.5), core-cooling rate (p = 0.5), or delta change in rectal temperature (p = 0.7) over 45 min, while nitrate reduced rectal temperature at shivering onset (p = 0.04). No interaction was observed between hypoxia and nitrate ingestion.

Conclusion

These data improve our understanding of how hypoxia and nitric oxide modulate cold thermoregulation.

The nitric oxide dependence of cutaneous microvascular function to independent and combined hypoxic cold exposure

When separated from local cooling, whole body cooling elicited cutaneous reflex vasoconstriction via mechanisms independent of nitric oxide removal. Hypoxia elicited cutaneous vasodilatation via mechanisms mediated primarily by nitric oxide synthase, rather than xanthine oxidase-mediated nitrite reduction. Cold-induced vasoconstriction was blunted by the opposing effect of hypoxic vasodilatation, whereas the underpinning mechanisms did not interrelate in the absence of local cooling. Full vasoconstriction was restored with nitric oxide synthase inhibition.

Historical reviews of the assessment of human cardiovascular function: interrogation and understanding of the control of skin blood flow

Several techniques exist for the determination of skin blood flow that have historically been used in the investigation of thermoregulatory control of skin blood flow, and more recently, in clinical assessments or as an index of global vascular function. Skin blood flow measurement techniques differ in their methodology and their strengths and limitations. To examine the historical development of techniques for assessing skin blood flow by describing the origin, basic principles, and important aspects of each procedure and to provide recommendations for best practise. Venous occlusion plethysmography was one of the earliest techniques to intermittently index a limb’s skin blood flow under conditions in which local muscle blood flow does not change. The introduction of laser Doppler flowmetry provided a method that continuously records an index of skin blood flow (red cell flux) (albeit from a relatively small skin area) that requires normalisation due to high site-to-site variability. The subsequent development of laser Doppler and laser speckle imaging techniques allows the mapping of skin blood flow from larger surface areas and the visualisation of capillary filling from the dermal plexus in two dimensions. The use of iontophoresis or intradermal microdialysis in conjunction with laser Doppler methods allows for the local delivery of pharmacological agents to interrogate the local and neural control of skin blood flow. The recent development of optical coherence tomography promises further advances in assessment of the skin circulation via three-dimensional imaging of the skin microvasculature for quantification of vessel diameter and vessel recruitment.

Skin vasoconstriction as a heat conservation thermoeffector

Cold exposure stimulates heat production and conservation to protect internal temperature. Heat conservation is brought about via reductions in skin blood flow. The focus, here, is an exploration of the mechanisms, particularly in humans, leading to that cutaneous vasoconstriction. Local skin cooling has several effects: (1) reduction of tonic nitric oxide formation by inhibiting nitric oxide synthase and element(s) downstream of the enzyme, which removes tonic vasodilator effects, yielding a relative vasoconstriction; (2) translocation of intracellular alpha-2c adrenoceptors to the vascular smooth-muscle cell membrane, enhancing adrenergic vasoconstriction; (3) increased norepinephrine release from vasoconstrictor nerves; and (4) cold-induced vasodilation, seen more clearly in anastomoses-rich glabrous skin. Cold-induced vasodilation occurs in nonglabrous skin when nitric oxide synthase or sympathetic function is blocked. Reflex responses to general body cooling complement these local effects. Sympathetic excitation leads to the increased release of norepinephrine and its cotransmitter neuropeptide Y, each of which contributes significantly to the vasoconstriction. The contributions of these two transmitters vary with aging, disease and, in women, reproductive hormone status. Interaction between local and reflex mechanisms is in part through effects on baseline and in part through removal of the inhibitory effects of nitric oxide on adrenergic vasoconstriction.

Response of women using oral contraception to exercise in the heat

PURPOSE

To compare the response of long-term oral contraceptive users (WomenOC; n = 8) to naturally menstruating women (WomenNM; n = 8) at rest and during exercise in temperate (TEMP; 22 °C) and hot (HEAT; 35 °C) conditions.

METHODS

Participants performed a three-stage cycling trial in each condition at 90, 135, and 180% of lactate threshold 1 (total = 52.5 min). Heart rate (HR) and core temperature (T _c) were recorded continuously, whereas blood pressure (BP), ratings of perceived exertion (RPE), blood lactate [La^-], and skin blood flow (BF_sk) were recorded every 7.5 min.

RESULTS

Baseline T _c was higher in WomenOC (37.5 ± 0.2, 37.6 ± 0.3 °C) than WomenNM (37.2 ± 0.2, 37.0 ± 0.4 °C) before the TEMP (p = 0.03) and HEAT (p < 0.01) trials, respectively. This difference remained for 22.5 min into both trials (p ≤ 0.05), after which time no between-group differences were found (p > 0.05). BF_sk measured in WomenNM plateaued from 7.5 min in the HEAT, whereas BF_sk measured in WomenOC increased for 15.0 min (p = 0.02) before plateauing. There were no between-group differences in HR, BP, or blood [La^-] before or throughout either trial (p > 0.05). WomenOC had higher (p ≤ 0.04) RPE values than WomenNM in the HEAT, reporting 8 ± 1 and 6 ± 2 at the end of Stage 3, respectively.

CONCLUSIONS

WomenOC concluded both trials with a comparable T _c to WomenNM, but had a prolonged BF_sk response and elevated RPE in the HEAT. Changes to BF_sk and RPE observed in women using OC may have implications for exercise tolerance in hot conditions.

Neuropeptide Y not involved in cutaneous vascular control in young human females taking oral contraceptive hormones

We previously reported that the cutaneous vasodilator response to local warming in males required noradrenaline (NA) and neuropeptide-Y (NPY). Animal work has shown no role for NPY in female vascular control. We investigated the contribution of NA and NPY in human female cutaneous vascular control. Nine female and nine male participants volunteered. To elucidate whether synthetic oestrogen and progesterone altered cutaneous vascular responses, females were tested in high-hormone (HH) and low-hormone (LH) phases of oral contraceptive pill (OCP). Skin blood flow was assessed by laser-Doppler flowmetry and expressed as cutaneous vascular conductance (CVC). Treatments were: control, combined yohimbine and propranolol (YP), BIBP-3226, and bretylium tosylate (BT). YP and BT increased basal CVC (p<0.05) relative to control sites in both HH and LH phases; though, BIBP-3226 had no effect in either phase (both p>0.05). Males basal CVC was increased at all treated sites compared to control sites (all p<0.05). YP and BT treated sites were higher in HH compared to LH (p<0.05). YP and BT treatment reduced the local warming-induced vasodilatation compared to control sites (p>0.05) in both HH and LH phases; whereas, BIBP-3226 treatment had no effect (p>0.05). In males, the vasodilatation achieved at all treated sites was reduced compared to the untreated control site (p<0.05). Data indicate that NA, not NPY, regulates basal skin blood flow and contributes to the vasodilator response to local warming in young females; however, both NA and NPY play a role in both basal and heat-induced cutaneous responses in males.

The effect of heating rate on the cutaneous vasomotion responses of forearm and leg skin in humans

We examined skin blood flow (SkBF) and vasomotion in the forearm and leg using laser-Doppler fluxmetry (LDF) and spectral analysis to investigate endothelial, sympathetic, and myogenic activities in response to slow (0.1 °C·10 s(-1)) and fast (0.5 °C·10 s(-1)) local heating. At 33 °C (thermoneutral) endothelial activity was higher in the legs than the forearms (P ≤ 0.02). Fast-heating increased SkBF more than slow heating (P=0.037 forearm; P=0.002 leg). At onset of 42 °C, endothelial (P=0.043 forearm; P=0.48 leg) activity increased in both regions during the fast-heating protocol. Following prolonged heating (42 °C) endothelial activity was higher in both the forearm (P=0.002) and leg (P<0.001) following fast-heating. These results confirm regional differences in the response to local heating and suggest that the greater increase in SkBF in response to fast local heating is initially due to increased endothelial and sympathetic activity. Furthermore, with sustained local skin heating, greater vasodilatation was observed with fast heating compared to slow heating. These data indicate that this difference is due to greater endothelial activity following fast heating compared to slow heating, suggesting that the rate of skin heating may alter the mechanisms contributing to cutaneous vasodilatation.

The cutaneous vasoconstrictor response in lower extremities during whole-body and local skin cooling in young women with a cold constitution

To clarify the characteristics of the thermal control of skin blood flow (SkBF) in individuals with a cold constitution, we examined the cutaneous vasoconstrictor responses in the calf and dorsal foot during whole-body and local skin cooling in young women complaining of chilliness (C group) and young women not suffering from it (N group). During whole-body skin cooling, the vasoconstrictor sensitivity in the dorsal foot, but not in the calf, was greater in the C group than in the N group. The C group also showed greater vasoconstrictor responses in the dorsal foot, but not in the calf, during local skin cooling and the iontophoretic application of norepinephrine. These findings suggest that the C group possesses a specific SkBF controlling system that is characterized by higher adrenergic sensitivity for greater cutaneous vasoconstriction in the distal portion of the lower extremities during cold exposure.

Sympathetic control of reflex cutaneous vasoconstriction in human aging

This Synthesis highlights a series of recent studies that has systematically interrogated age-related deficits in cold-induced skin vasoconstriction. In response to cold stress, a reflex increase in sympathetic nervous system activity mediates reductions in skin blood flow. Reflex vasoconstriction during cold exposure is markedly impaired in aged skin, contributing to the relative inability of healthy older adults to maintain core temperature during mild cold stress in the absence of appropriate behavioral thermoregulation. This compromised reflex cutaneous vasoconstriction in healthy aging can occur as a result of functional deficits at multiple points along the efferent sympathetic reflex axis, including blunted sympathetic outflow directed to the skin vasculature, reduced presynaptic neurotransmitter synthesis and/or release, and altered end-organ responsiveness at several loci, in addition to potential alterations in afferent thermoreceptor function. Arguments have been made that the relative inability of aged skin to appropriately constrict is due to the aging cutaneous arterioles themselves, whereas other data point to the neural circuitry controlling those vessels. The argument presented herein provides strong evidence for impaired efferent sympathetic control of the peripheral cutaneous vasculature during whole body cold exposure as the primary mechanism responsible for attenuated vasoconstriction.

The contribution of sensory nerves to cutaneous vasodilatation of the forearm and leg to local skin heating

PURPOSE

The initial cutaneous vasodilatory response to local skin heating is larger in the forearm than the leg. While the initial vasodilatation of the forearm to local heating is primarily dependent on sensory nerves, their role in the leg is unknown. We compared the contribution of sensory nerves in driving the cutaneous vasodilatory response of the forearm and leg to local heating using local anaesthetic (EMLA) cream.

METHOD

In seven participants, two skin sites were selected on both the dorsal forearm and anterolateral calf; one site on each region received EMLA, with the other an untreated control. All sites were controlled at 33 °C and then locally heated to 42 °C with integrated laser-Doppler local heating probes.

RESULTS

Cutaneous vascular conductance (CVC) during the initial vasodilatation to local heating was smaller in the leg (47 ± 9% max) compared to the forearm (62 ± 7 % max) (P = 0.012). EMLA reduced the initial vasodilatation at both the leg (27 ± 13 % max) (P = 0.02) and forearm (33 ± 14% max) (P < 0.001). The times to onset of vasodilatation, initial vasodilatory peak, and plateau phase were longer in the leg compared to the forearm (all P < 0.05), and EMLA increased these times in both regions (both P < 0.05). CVC during the plateau phase to sustained local skin heating was higher in the leg compared to the forearm at both the untreated (93 ± 6 vs. 85 ± 4% max) (P = 0.33) and EMLA-treated (94 ± 5 vs. 86 ± 6% max) (P = 0.001) sites; EMLA did not affect CVC (P > 0.05).

CONCLUSION

The differences in the initial vasodilatory peak to local skin heating between the forearm and the leg are due to the contribution of sensory nerves.

Effect of skin temperature on cutaneous vasodilator response to the β-adrenergic agonist isoproterenol

The vascular response to local skin cooling is dependent in part on a cold-induced translocation of α2C-receptors and an increased α-adrenoreceptor function. To discover whether β-adrenergic function might contribute, we examined whether β-receptor sensitivity to the β-agonist isoproterenol was affected by local skin temperature. In seven healthy volunteers, skin blood flow was measured from the forearm by laser-Doppler flowmetry and blood pressure was measured by finger photoplethysmography. Data were expressed as cutaneous vascular conductance (CVC; laser-Doppler flux/mean arterial blood pressure). Pharmacological agents were administered via intradermal microdialysis. We prepared four skin sites: one site was maintained at a thermoneutral temperature of 34°C (32 ± 10%CVCmax) one site was heated to 39°C (38 ± 11%CVCmax); and two sites were cooled, one to 29°C (22 ± 7%CVCmax) and the other 24°C (16 ± 4%CVCmax). After 20 min at these temperatures to allow stabilization of skin blood flow, isoproterenol was perfused in concentrations of 10, 30, 100, and 300 μM. Each concentration was perfused for 15 min. Relative to the CVC responses to isoproterenol at the thermoneutral skin temperature (34°C) (+21 ± 10%max), low skin temperatures reduced (at 29°C) (+17 ± 6%max) or abolished (at 24°C) (+1 ± 5%max) the vasodilator response, and warm (39°C) skin temperatures enhanced the vasodilator response (+40 ± 9%max) to isoproterenol. These data indicate that β-adrenergic function was influenced by local skin temperature. This finding raises the possibility that a part of the vasoconstrictor response to direct skin cooling could include reduced background β-receptor mediated vasodilation.

Cutaneous vasodilator and vasoconstrictor mechanisms in temperature regulation

In this review, we focus on significant developments in our understanding of the mechanisms that control the cutaneous vasculature in humans, with emphasis on the literature of the last half-century. To provide a background for subsequent sections, we review methods of measurement and techniques of importance in elucidating control mechanisms for studying skin blood flow. In addition, the anatomy of the skin relevant to its thermoregulatory function is outlined. The mechanisms by which sympathetic nerves mediate cutaneous active vasodilation during whole body heating and cutaneous vasoconstriction during whole body cooling are reviewed, including discussions of mechanisms involving cotransmission, NO, and other effectors. Current concepts for the mechanisms that effect local cutaneous vascular responses to local skin warming and cooling are examined, including the roles of temperature sensitive afferent neurons as well as NO and other mediators. Factors that can modulate control mechanisms of the cutaneous vasculature, such as gender, aging, and clinical conditions, are discussed, as are nonthermoregulatory reflex modifiers of thermoregulatory cutaneous vascular responses.

Sympathetic neural activity to the cardiovascular system: integrator of systemic physiology and interindividual characteristics

The sympathetic nervous system is a ubiquitous, integrating controller of myriad physiological functions. In the present article, we review the physiology of sympathetic neural control of cardiovascular function with a focus on integrative mechanisms in humans. Direct measurement of sympathetic neural activity (SNA) in humans can be accomplished using microneurography, most commonly performed in the peroneal (fibular) nerve. In humans, muscle SNA (MSNA) is composed of vasoconstrictor fibers; its best-recognized characteristic is its participation in transient, moment-to-moment control of arterial blood pressure via the arterial baroreflex. This property of MSNA contributes to its typical "bursting" pattern which is strongly linked to the cardiac cycle. Recent evidence suggests that sympathetic neural mechanisms and the baroreflex have important roles in the long term control of blood pressure as well. One of the striking characteristics of MSNA is its large interindividual variability. However, in young, normotensive humans, higher MSNA is not linked to higher blood pressure due to balancing influences of other cardiovascular variables. In men, an inverse relationship between MSNA and cardiac output is a major factor in this balance, whereas in women, beta-adrenergic vasodilation offsets the vasoconstrictor/pressor effects of higher MSNA. As people get older (and in people with hypertension) higher MSNA is more likely to be linked to higher blood pressure. Skin SNA (SSNA) can also be measured in humans, although interpretation of SSNA signals is complicated by multiple types of neurons involved (vasoconstrictor, vasodilator, sudomotor and pilomotor). In addition to blood pressure regulation, the sympathetic nervous system contributes to cardiovascular regulation during numerous other reflexes, including those involved in exercise, thermoregulation, chemoreflex regulation, and responses to mental stress.

Lack of limb or sex differences in the cutaneous vascular responses to exogenous norepinephrine

The cutaneous circulation is used to examine vascular adrenergic function in clinical populations; however, limited studies have examined whether there are regional limb and sex differences in microvascular adrenergic responsiveness. We hypothesized that cutaneous adrenergic responsiveness would be greater in the leg compared with the arm and that these regional limb differences would be blunted in young women (protocol 1). We further hypothesized that cutaneous vasoconstriction to exogenous norepinephrine (NE) during β-adrenergic receptor antagonism would be augmented in young women (protocol 2). In protocol 1, one microdialysis fiber was placed in the skin of the calf and the ventral forearm in 20 healthy young adults (11 men and 9 women). Laser-Doppler flowmetry was used to measure red blood cell flux in response to graded intradermal microdialysis infusions of NE (10(-12) to 10(-2) M). In protocol 2, three microdialysis fibers were placed in the forearm (6 men and 8 women) for the local perfusion of lactated Ringer (control), 5 mM yohimbine (α-adrenergic receptor antagonist), or 2 mM propranolol (β-adrenergic receptor antagonist) during concurrent infusions of NE (10(-12) to 10(-2) M). There were no limb or sex differences in cutaneous adrenergic responsiveness (logEC50) to exogenous NE. During α-adrenergic receptor blockade, women had greater exogenous NE-induced cutaneous vasodilation at the lowest doses of NE (10(-12) to 10(-10) M). Collectively, these data indicate that there are no limb or sex differences in cutaneous adrenergic responsiveness to exogenous NE; however, young women have a greater β-adrenergic receptor-mediated component of the vascular responsiveness to exogenous NE.

Differences in conductive foot cooling: a comparison between males and females

PURPOSE

This study investigated possible sex-related and intra-menstrual differences in local vascular and skin temperature responses when conductive cooling was applied to the soles of the feet.

METHOD

Twelve females and twelve males exposed the soles of their feet to a cooling plate (which cooled from 35 to 15 °C) on two occasions 12-15 days apart. For females, sessions took place during their inactive and active contraceptive pill phases. Tip of Great toe temperature and Great toe skin blood flow were recorded throughout.

RESULTS

Females' feet cooled to a greater extent than males' (P = 0.001). Sensitivity of toe skin blood flow to changes in skin temperature (1 or 2 °C) was not different between males and females. Dimensions of males' feet were larger than females' (P < 0.05) and correlations between foot dimensions and toe skin cooling were found (r = 0.728, P < 0.001). Analysis of the residual variance showed that foot volume, contact area and skin blood flow correlated with the rate of toe skin cooling (r = 0.812, r (2) = 0.659, P < 0.001). No intra-menstrual differences were found.

CONCLUSION

The feet of females cooled at a faster rate than those of males in response to the same conductive cooling stimulus to the soles of the feet. However, similar reductions in skin blood flow were found for the same change in toe skin temperature. Therefore, sex related differences may be due to the differing dimensions of the feet, but further research including males and females matched for foot dimensions are required to confirm this mechanism.

Adenosine receptor inhibition attenuates the suppression of postexercise cutaneous blood flow

The time-dependent contributions of active vasodilation (e.g. nitric oxide) and noradrenergic vasoconstriction to the postexercise suppression of cutaneous perfusion despite persistent hyperthermia remain unknown. Moreover, adenosine receptors have been shown to mediate the decrease in cutaneous perfusion following passive heating. We examined the time-dependent modulation of nitric oxide synthase, noradrenergic vasoconstriction and adenosine receptors on postexercise cutaneous perfusion. Eight males performed 15 min of high-intensity (85% VO2 max) cycling followed by 60 min of recovery in temperate ambient conditions (25°C). Four microdialysis probes were inserted into the forearm skin and continuously infused with: (1) lactated Ringer solution (Control); (2) 10 mm N(G)-nitro-l-arginine methyl ester (l-NAME; nitric oxide synthase inhibitor); (3) 10 mm bretylium tosylate (BT; inhibitor of noradrenergic vasoconstriction); or (4) 4 mm theophylline (THEO; adenosine receptor inhibitor). Cutaneous vascular conductance (CVC) was expressed as a percentage of maximum and was calculated as perfusion units (laser Doppler) divided by mean arterial pressure. End-exercise CVC was similar in Control, THEO and BT (P > 0.1), but CVC with l-NAME (39 ± 4%) was lower than Control (59 ± 4%, P < 0.01). At 20 min of recovery, Control CVC (22 ± 3%) returned to baseline levels (19 ± 2%, P = 0.11). Relative to Control, CVC was reduced by l-NAME for the first 10 min of recovery whereas CVC was increased with BT for the first 30 min of recovery (P < 0.03). In contrast, CVC with THEO was elevated throughout the 60 min recovery period (P ≤ 0.01) compared to Control. We show that adenosine receptors appear to have a major role in postexercise cutaneous perfusion whereas nitric oxide synthase and noradrenergic vasoconstriction are involved only earlier during recovery.

Cutaneous sympathetic neural responses to body cooling in type 2 diabetes mellitus

In humans, sympathetic vasoconstrictor nerves in the skin contribute to resting vascular tone and mediate reflex vasoconstrictor responses to body cooling. Although it is well recognized that type 2 diabetes mellitus (T2DM) is associated with peripheral neurovascular changes, it is unclear to what extent the thermal responsiveness of the cutaneous vasoconstrictor system is altered in individuals with relatively uncomplicated T2DM. We tested the hypothesis that skin sympathetic nerve activity (SSNA) is decreased at baseline and during body cooling in individuals with T2DM compared to healthy controls (C) of similar age and body size. We measured SSNA (microneurography) and skin blood flow (laser-Doppler flowmetry) in the innervated area in 8 T2DM and 12 C subjects at baseline and during 3-4min of rapid whole body cooling via a water-perfused suit. SSNA (total integrated activity) increased, and cutaneous vascular conductance decreased in both groups during body cooling (P<0.01 for both). However, SSNA was not different between groups during either baseline or body cooling conditions (P=NS). The deltas in SSNA between baseline and body cooling were similar between groups: T2DM: 55±27 and C: 57±12 units (P=NS). We conclude that reflex cutaneous sympathetic and vascular responses to rapid whole body cooling are preserved in relatively healthy individuals with T2DM.

Mechanisms and modifiers of reflex induced cutaneous vasodilation and vasoconstriction in humans

Human skin blood flow responses to body heating and cooling are essential to the normal processes of physiological thermoregulation. Large increases in skin blood flow provide the necessary augmentation of convective heat loss during environmental heat exposure and/or exercise, just as reflex cutaneous vasoconstriction is key to preventing excessive heat dissipation during cold exposure. In humans, reflex sympathetic innervation of the cutaneous circulation has two branches: a sympathetic noradrenergic vasoconstrictor system, and a non-noradrenergic active vasodilator system. Noradrenergic vasoconstrictor nerves are tonically active in normothermic environments and increase their activity during cold exposure, releasing both norepinephrine and cotransmitters (including neuropeptide Y) to decrease skin blood flow. The active vasodilator system in human skin does not exhibit resting tone and is only activated during increases in body temperature, such as those brought about by heat exposure or exercise. Active cutaneous vasodilation occurs via cholinergic nerve cotransmission and has been shown to include potential roles for nitric oxide, vasoactive intestinal peptide, prostaglandins, and substance P (and/or neurokinin-1 receptors). It has proven both interesting and challenging that no one substance has been identified as the sole mediator of active cutaneous vasodilation. The processes of reflex cutaneous vasodilation and vasoconstriction are both modified by acute factors, such as exercise and hydration, and more long-term factors, such as aging, reproductive hormones, and disease. This review will highlight some of the recent findings in these areas, as well as interesting areas of ongoing and future work.

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.

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.

17β-Oestradiol indirectly inhibits thyrotrophin-releasing hormone expression in the hypothalamic paraventricular nucleus of female rats and blunts thyroid axis response to cold exposure

Energy expenditure and thermogenesis are regultated by thyroid and sex hormones. Several parameters of hypothalamic-pituitary-thyroid (HPT) axis function are modulated by 17β-oestradiol (E(2)) but its effects on thyrotrophin-releasing hormone (TRH) mRNA levels remain unknown. We evaluated, by in situ hybridisation and Northern bloting, TRH expression in the paraventricular nucleus of the hypothalamus (PVN) of cycling rats, 2 weeks-ovariectomised (OVX) and OVX animals injected s.c. during 1-4 days with E(2) (5, 50, 100 or 200 μg ⁄ kg) (OVX-E). Serum levels of E(2), thyroid-stimulating hormone (TSH), prolactin, corticosterone and triiodothyronine (T(3)) were quantified by radioimmunoassay. Increased serum E(2) levels were observed after 4 days injection of 50 μg ⁄ kg E(2) (to 68.5 ± 4.8 pg ⁄ ml) in OVX rats. PVN-TRH mRNA levels were slightly higher in OVX than in virgin females at dioestrous 1 or pro-oestrous, decreasing proportionally to increased serum E(2) levels. E(2) injections augmented serum T(3), prolactin, and corticosterone levels. Serum TSH levels augmented with 4 days 50 μg ⁄ kg E(2), but not with the higher doses that enhanced serum T(3) levels. Exposure to cold for 1 h resulted in marked HPT axis activation in OVX rats, increasing the levels of TRH mRNA along the rostro-caudal PVN areas, as well as serum TSH, T(3), corticosterone and prolactin levels. By contrast, no significant changes in any of these parameters were observed in cold-exposed OVX-E (50 μg ⁄ kg E(2)) rats. Very few PVN-TRHergic neurones expressed the oestrogen receptor type-α, suggesting that the effects of E(2) on PVN-TRH expression are indirect, most probably as a result of its multiple modulatory effects on circulating hormones and their receptor sensitivity. The blunted response of OVX-E rats to cold coincides with the effects of E(2) on the autonomic nervous system and increased cold tolerance.

Altered mechanisms of thermoregulatory vasoconstriction in aged human skin

Human exposure to cold stimulates cutaneous vasoconstriction by activating both sympathetic reflex and locally mediated pathways. Older humans are vulnerable to hypothermia because primary aging impairs thermoregulatory cutaneous vasoconstriction. This article highlights recent findings discussing how age-related decrements in sympathetic neurotransmission contribute directly to thermoregulatory impairment, whereas changes in local cold-induced intracellular signaling suggest a more generalized age-associated vascular dysfunction.

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.

Exogenous melatonin administration modifies cutaneous vasoconstrictor response to whole body skin cooling in humans

Humans and other diurnal species experience a fall in internal temperature (T(int)) at night, accompanied by increased melatonin and altered thermoregulatory control of skin blood flow (SkBF). Also, exogenous melatonin induces a fall in T(int), an increase in distal skin temperatures and altered control of the cutaneous active vasodilator system, suggesting an effect of melatonin on the control of SkBF. To test whether exogenous melatonin also affects the more tonically active vasoconstrictor system in glabrous and nonglabrous skin during cooling, healthy males (n = 9) underwent afternoon sessions of whole body skin temperature (T(sk)) cooling (water-perfused suits) after oral melatonin (Mel; 3 mg) or placebo (Cont). Cutaneous vascular conductance (CVC) was calculated from SkBF (laser Doppler flowmetry) and non-invasive blood pressure. Baseline T(int) was lower in Mel than in Cont (P < 0.01). During progressive reduction of T(sk) from 35 degrees C to 32 degrees C, forearm CVC was first significantly reduced at T(sk) of 34.33 +/- 0.01 degrees C (P < 0.05) in Cont. In contrast, CVC in Mel was not significantly reduced until T(sk) reached 33.33 +/- 0.01 degrees C (P < 0.01). The decrease in forearm CVC in Mel was significantly less than in Cont at T(sk) of 32.66 +/- 0.01 degrees C and lower (P < 0.05). In Mel, palmar CVC was significantly higher than in Cont above T(sk) of 33.33 +/- 0.01 degrees C, but not below. Thus exogenous melatonin blunts reflex vasoconstriction in nonglabrous skin and shifts vasoconstrictor system control to lower T(int). It provokes vasodilation in glabrous skin but does not suppress the sensitivity to falling T(sk). These findings suggest that by affecting the vasoconstrictor system, melatonin has a causal role in the nocturnal changes in body temperature and its control.

Preexercise sodium loading aids fluid balance and endurance for women exercising in the heat

This study was conducted during the high-hormone phase of both natural and oral contraceptive pill (OCP)-mediated menstrual cycles to determine whether preexercise ingestion of a concentrated sodium beverage would increase plasma volume (PV), reduce physiological strain, and aid endurance of moderately trained women cycling in warm conditions. Thirteen trained cyclists [peak O2 uptake 52 ml·kg−1·min−1 (SD 2), age 26 yr (SD 6), weight 60.8 kg (SD 5)] who were oral contraceptive users ( n = 6) or not ( n = 7) completed this double-blind, crossover experiment. Cyclists ingested a concentrated-sodium (High Na+: 164 mmol Na+/l) or low-sodium (Low Na+: 10 mmol Na+/l) beverage (10 ml/kg) before cycling to exhaustion at 70% Peak O2 uptake in warm conditions (32°C, 50% relative humidity, air velocity 4.5 m/s). Beverage (∼628 ml) was ingested in seven portions across 60 min beginning 105 min before exercise, with no additional fluid given until the end of the trial. Trials were separated by one to two menstrual cycles. High Na+ increased PV (calculated from hematocrit and hemoglobin concentration) before exercise, whereas Low Na+ did not [−4.4 (SD 1.1) vs. −1.9% (SD 1.3); 95% confidence interval: for the difference 5.20, 6.92; P < 0.0001], and it involved greater time to exhaustion [98.8 (SD 25.6) vs. 78.7 (SD 24.6) min; 95% confidence interval: 13.3, 26.8; P < 0.0001]. Core temperature rose more quickly with Low Na+ [1.6°C/h (SD 0.2)] than High Na+ [1.2°C/h (SD 0.2); P = 0.04]. Plasma [AVP], [Na+] concentration, and osmolality, and urine volume, [Na+], and osmolality decreased with sodium loading ( P < 0.05) independent of pill usage. Thus preexercise ingestion of a concentrated sodium beverage increased PV, reduced thermoregulatory strain, and increased exercise capacity for women in the high-hormone phase of natural and oral contraceptive pill-mediated menstrual cycles, in warm conditions.

Rho kinase-mediated local cold-induced cutaneous vasoconstriction is augmented in aged human skin

Cutaneous vasoconstriction (VC), a critical thermoregulatory response to cold, is generally impaired with aging. However, the effects of aging on local cooling-induced VC and its underlying mechanisms are poorly understood. We tested whether aged skin exhibits attenuated localized cold-induced VC and whether Rho kinase-mediated cold-induced VC is augmented with age. Skin blood flow was monitored with laser Doppler flowmetry (LDF) on seven young and seven older subjects. Cutaneous vascular conductance (CVC; LDF/mean arterial pressure) was expressed as percentage change from baseline (%DeltaCVC(base)). In protocol 1, two forearm skin sites were cooled to six temperatures (31.5-19 degrees C) for 10 min each or two temperatures (29 degrees C, 24 degrees C) for 30 min each, with no age differences in the magnitude of VC. In protocol 2, three forearm skin sites were instrumented for intradermal microdialysis and cooled to 24 degrees C for 40 min. During minutes 1-5, there was no age difference in CVC responses at control sites (young: -45 +/- 6% vs. older: -46 +/- 3%, P > 0.9). Adrenoceptor antagonism (yohimbine + propranolol) abolished VC in young (to +15 +/- 13%, P < 0.05) but only partially inhibited VC in older subjects (to -23 +/- 6%, P < 0.05). Rho kinase inhibition plus adrenoceptor antagonism (yohimbine + propranolol + fasudil) abolished VC in both groups. During minutes 35-40, there was no age difference in control (young: -77 +/- 4% vs. older: -70 +/- 2%, P > 0.3) or adrenoceptor-antagonized responses (young: -61 +/- 3% vs. older: -55 +/- 2%, P > 0.3); however, Rho kinase inhibition plus adrenoceptor antagonism blocked more VC in older compared with young subjects (-19 +/- 11% vs. -35 +/- 3%, P < 0.05). Although its magnitude remains unaffected, cold-induced VC becomes less dependent on adrenergic and more dependent on Rho kinase signaling with advancing age.

Cold-induced cutaneous vasoconstriction is mediated by Rho kinase in vivo in human skin

Cutaneous vasoconstriction (VC) is the initial thermoregulatory response to cold exposure and can be elicited through either whole body or localized skin cooling. However, the mechanisms governing local cold-induced VC are not well understood. We tested the hypothesis that Rho kinase participates in local cold-induced cutaneous VC. In seven men and women (20-27 yr of age), up to four ventral forearm skin sites were instrumented with intradermal microdialysis fibers for localized drug delivery during cooling. Skin blood flow was monitored at each site with laser-Doppler flowmetry while local skin temperature was decreased and maintained at 24 degrees C for 40 min. Cutaneous vascular conductance (CVC; laser-Doppler flowmetry/mean arterial pressure) was expressed as percent change from 34 degrees C baseline. During the first 5 min of cooling, CVC decreased at control sites (lactated Ringer solution) to -45 +/- 6% (P < 0.001), increased at adrenoceptor-antagonized sites (yohimbine + propranolol) to 15 +/- 14% (P = 0.002), and remained unchanged at both Rho kinase-inhibited (fasudil) and adrenoceptor-antagonized + Rho kinase-inhibited sites (yohimbine + propranolol + fasudil) (-9 +/- 1%, P = 0.4 and -6 +/- 2%, P = 0.4, respectively). During the last 5 min of cooling, CVC further decreased at all sites when compared with baseline values (control, -77 +/- 4%, P < 0.001; adrenoceptor antagonized, -61 +/- 3%, P < 0.001; Rho kinase inhibited, -34 +/- 7%, P < 0.001; and adrenoceptor antagonized + Rho kinase inhibited sites, -35 +/- 3%, P < 0.001). Rho kinase-inhibited and combined treatment sites were significantly attenuated when compared with both adrenoceptor-antagonized (P < 0.01) and control sites (P < 0.0001). Rho kinase mediates both early- and late-phase cold-induced VC, supporting in vitro findings and providing a putative mechanism through which both adrenergic and nonadrenergic cold-induced VC occurs in an in vivo human thermoregulatory model.

Cutaneous vasoconstrictor responses to norepinephrine are attenuated in older humans

Cutaneous vasoconstriction (VC) in response to cooling is impaired with human aging. On the basis of previous findings that older humans rely predominantly on norepinephrine (NE) for reflex VC of skin blood vessels, and that the VC effects of NE are blunted with age in many vascular beds, we tested the hypothesis that cutaneous VC responses to exogenous NE are attenuated in aged skin compared with young skin. In 11 young (18–30 yr) and 11 older (62–76 yr) men and women, skin blood flow was monitored at two forearm sites with laser Doppler (LD) flowmetry, while local skin temperature was clamped at 34°C. At one site, five doses of NE (10−10 to 10−2 M) were sequentially infused via intradermal microdialysis while the other site served as control (C; Ringer). Cutaneous vascular conductance (CVC; LD flux/mean arterial pressure) was expressed as percent change from baseline (%ΔCVCbase). At 10−10, 10−8, and 10−6 M NE, older VC responses were attenuated compared with young [10−10: −35 (95% confidence interval: −16, −52) vs. −49 (−40, −58) %ΔCVCbase, P = 0.02; 10−8: −38 (−20, −56) vs. −50 (−40, −61) %ΔCVCbase, P = 0.03; 10−6: −52 (−35, −70) vs. −67 (−60, −74) %ΔCVCbase, P = 0.01]. Older maximal VC responses were also blunted compared with young [−80 (confidence interval: −73,−87) vs. −88 (confidence interval: −87, −90) %ΔCVCbase, P = 0.03]. NE-mediated cutaneous VC is blunted at both physiological and superphysiological doses in older subjects compared with young subjects. Considering that NE is the only functional neurotransmitter mediating reflex VC in aged skin, attenuated NE-mediated VC may further predispose older humans to excess heat loss in the cold.

Attenuated noradrenergic sensitivity during local cooling in aged human skin

Reflex-mediated cutaneous vasoconstriction (VC) is impaired in older humans; however, it is unclear whether this blunted VC also occurs during local cooling, which mediates VC through different mechanisms. We tested the hypothesis that the sensitization of cutaneous vessels to noradrenaline (NA) during direct skin cooling seen in young skin is blunted in aged skin. In 11 young (18-30 years) and 11 older (62-76 years) men and women, skin blood flow was monitored at two forearm sites with laser Doppler (LD) flowmetry while local skin temperature was cooled and clamped at 24 degrees C. Cutaneous vascular conductance (CVC; LD flux/mean arterial pressure) was expressed as percentage change from baseline (% DeltaCVC(base)). At one site, five doses of NA (10(-10)-10(-2) m) were sequentially infused via intradermal microdialysis during cooling while the other 24 degrees C site served as control (Ringer solution + cooling). At control sites, VC due to cooling alone was similar in young versus older (-54 +/- 5 versus -56 +/- 3% DeltaCVC(base), P = 0.46). In young, NA infusions induced additional dose-dependent VC (10(-8), 10(-6), 10(-4) and 10(-2) m: -70 +/- 2, -72 +/- 3, -78 +/- 3 and -79 +/- 4% DeltaCVC(base); P < 0.05 versus control). In older subjects, further VC did not occur until the highest infused dose of NA (10(-2) m: -70 +/- 5% DeltaCVC(base); P < 0.05 versus control). When cutaneous arterioles are sensitized to NA by direct cooling, young skin exhibits the capacity to further constrict to NA in a dose-dependent manner. However, older skin does not display enhanced VC capacity until treated with saturating doses of NA, possibly due to age-associated decrements in Ca2+ availability or alpha2C-adrenoceptor function.

Sympathetic, sensory, and nonneuronal contributions to the cutaneous vasoconstrictor response to local cooling

Previous work indicates that sympathetic nerves participate in the vascular responses to direct cooling of the skin in humans. We evaluated this hypothesis further in a four-part series by measuring changes in cutaneous vascular conductance (CVC) from forearm skin locally cooled from 34 to 29 degrees C for 30 min. In part 1, bretylium tosylate reversed the initial vasoconstriction (-14 +/- 6.6% control CVC, first 5 min) to one of vasodilation (+19.7 +/- 7.7%) but did not affect the response at 30 min (-30.6 +/- 9% control, -38.9 +/- 6.9% bretylium; both P < 0.05, P > 0.05 between treatments). In part 2, yohimbine and propranolol (YP) also reversed the initial vasoconstriction (-14.3 +/- 4.2% control) to vasodilation (+26.3 +/- 12.1% YP), without a significant effect on the 30-min response (-26.7 +/- 6.1% YP, -43.2 +/- 6.5% control; both P < 0.05, P > 0.05 between sites). In part 3, the NPY Y1 receptor antagonist BIBP 3226 had no significant effect on either phase of vasoconstriction (P > 0.05 between sites both times). In part 4, sensory nerve blockade by anesthetic cream (Emla) also reversed the initial vasoconstriction (-20.1 +/- 6.4% control) to one of vasodilation (+213.4 +/- 87.0% Emla), whereas the final levels did not differ significantly (-37.7 +/- 10.1% control, -37.2 +/- 8.7% Emla; both P < 0.05, P > 0.05 between treatments). These results indicate that local cooling causes cold-sensitive afferents to activate sympathetic nerves to release norepinephrine, leading to a local cutaneous vasoconstriction that masks a nonneurogenic vasodilation. Later, a vasoconstriction develops with or without functional sensory or sympathetic nerves.

Effect of age on cutaneous vasoconstrictor responses to norepinephrine in humans

To test the hypothesis that cutaneous vasoconstrictor responsiveness to exogenous norepinephrine is reduced in older compared with young subjects, dose-response relations between norepinephrine and skin blood flow were established. Seven doses of norepinephrine (1·10−8 to 10−2 log M) were perfused (2 μl/min) intradermally (4 min/dose) using cutaneous microdialysis (2 probes/subject). To account for possible differences in endogenous norepinephrine between groups, one microdialysis probe was perfused with bretylium tosylate to locally block noradrenergic vesicle release before establishing the norepinephrine dose-response relations. Skin blood flow was indexed via laser-Doppler flowmetry directly over both microdialysis probe sites and is expressed as cutaneous vascular conductance (laser-Doppler flux/mean arterial blood pressure). Local skin temperature was maintained at 34°C at both sites throughout the protocol. Dose-response relation between norepinephrine and cutaneous vascular conductance was similar between control and bretylium-pretreated sites in young subjects (EC50 = −5.18 ± 0.27 and −5.03 ± 0.27 log M, respectively). In contrast, the dose-response relation was significantly shifted to the right (i.e., a higher dose of norepinephrine was needed to produce the same vasoconstrictor response) in the bretylium-pretreated site in older subjects (EC50 = −5.46 ± 0.23 and −4.53 ± 0.23 log M, respectively). Significant increases in EC50 were observed in older compared with young subjects at the bretylium-pretreated but not the control sites. These data indicate that cutaneous vasoconstrictor responsiveness is decreased in older subjects when endogenous release of norepinephrine is antagonized. Furthermore, these findings suggest that differences in presynaptic norepinephrine release between older and younger subjects are profound enough to affect dose-response relations between norepinephrine and cutaneous vascular conductance.

Altered neurotransmitter control of reflex vasoconstriction in aged human skin

Cutaneous vasoconstriction (VC) in response to cooling is attenuated in older humans; however, mechanisms underlying this functional decline remain unclear. The present study tested the hypothesis that the contributions of noradrenaline (NA) and sympathetic cotransmitters to reflex-mediated cutaneous VC are altered with age. In 11 young (18-26 years) and 11 older (61-77 years) men and women, forearm skin blood flow was monitored at three sites using laser Doppler flowmetry (LDF) while mean skin temperature was lowered from 34 to 30.5 degrees C using a water-perfused suit. Cutaneous vascular conductance (CVC; LDF/mean arterial pressure) was expressed as percentage change from baseline (% DeltaCVC(base)). Solutions of yohimbine + propranolol (Y + P), bretylium tosylate (BT), and lactated Ringer solution were infused via intradermal microdialysis at each LDF site to antagonize alpha- and beta-adrenoceptors, block sympathetic release of NA and cotransmitters, and act as control, respectively. During cooling, VC was attenuated at the control site in older subjects compared to young subjects (-16 +/- 3 versus-34 +/- 4% DeltaCVC(base), P < 0.001). Y + P attenuated VC in young subjects (-13 +/- 8% DeltaCVC(base), P < 0.001 versus control) and abolished VC in older subjects (0 +/- 3% DeltaCVC(base), P > 0.9 versus baseline). BT completely blocked VC in both age groups. Cutaneous VC in young subjects is mediated by both NA and sympathetic cotransmitter(s); however, reflex VC in aged skin is attenuated compared to young and appears to be mediated solely by NA.

Neuropeptide Y antagonism reduces reflex cutaneous vasoconstriction in humans

Previous studies have provided evidence of a non-noradrenergic contributor to reflex cutaneous vasoconstriction in humans but did not identify the transmitter responsible. To test whether neuropeptide Y (NPY) has a role, in two series of experiments we slowly reduced whole body skin temperature (TSK) from 34.5 to 31.7 degrees C. In protocol 1, Ringer solution and the NPY receptor antagonist BIBP-3226 alone were delivered intradermally via microdialysis. In protocol 2, yohimbine plus propranolol (Yoh + Pro), Yoh + Pro in combination with BIBP-3226, and Ringer solution were delivered to antagonize locally the vasomotor effects of NPY and norepinephrine. Blood flow was measured by laser Doppler flowmetry (LDF). Mean arterial blood pressure (MAP) was monitored at the finger (Finapres). In protocol 1, cutaneous vascular conductance (CVC) fell by 45%, to 55.1 +/- 5.6% of baseline at control sites (P < 0.05). At BIBP-3226-treated sites, CVC fell by 34.1% to 65.9 +/- 5.0% (P < 0.05; P < 0.05 between sites). In protocol 2, during body cooling, CVC at control sites fell by 32.6%, to 67.4 +/- 4.3% of baseline; at sites treated with Yoh + Pro, CVC fell by 18.7%, to 81.3 +/- 4.4% of baseline (P < 0.05 vs. baseline; P < 0.05 vs. control) and did not fall significantly at sites treated with BIBP-3226 + Yoh + Pro (P > 0.05; P < 0.05 vs. other sites). After cooling, exogenous norepinephrine induced vasoconstriction at control sites (P < 0.05) but not at sites treated with Yoh + Pro + BIBP-3226 (P > 0.05). These results indicate that NPY participates in sympathetically mediated cutaneous vasoconstriction in humans during whole body cooling.

Skin blood flow in adult human thermoregulation: how it works, when it does not, and why

The thermoregulatory control of human skin blood flow is vital to the maintenance of normal body temperatures during challenges to thermal homeostasis. Sympathetic neural control of skin blood flow includes the noradrenergic vasoconstrictor system and a sympathetic active vasodilator system, the latter of which is responsible for 80% to 90% of the substantial cutaneous vasodilation that occurs with whole body heat stress. With body heating, the magnitude of skin vasodilation is striking: skin blood flow can reach 6 to 8 L/min during hyperthermia. Cutaneous sympathetic vasoconstrictor and vasodilator systems also participate in baroreflex control of blood pressure; this is particularly important during heat stress, when such a large percentage of cardiac output is directed to the skin. Local thermal control of cutaneous blood vessels also contributes importantly--local warming of the skin can cause maximal vasodilation in healthy humans and includes roles for both local sensory nerves and nitric oxide. Local cooling of the skin can decrease skin blood flow to minimal levels. During menopause, changes in reproductive hormone levels substantially alter thermoregulatory control of skin blood flow. This altered control might contribute to the occurrence of hot flashes. In type 2 diabetes mellitus, the ability of skin blood vessels to dilate is impaired. This impaired vasodilation likely contributes to the increased risk of heat illness in this patient population during exposure to elevated ambient temperatures. Raynaud phenomenon and erythromelalgia represent cutaneous microvascular disorders whose pathophysiology appears to relate to disorders of local and/or reflex thermoregulatory control of the skin circulation.

Cutaneous vasoconstrictor response to whole body skin cooling is altered by time of day

To test for a diurnal difference in the vasoconstrictor control of the cutaneous circulation, we performed whole body skin cooling (water-perfused suits) at 0600 (AM) and 1600 (PM). After whole body skin temperature (T(sk)) was controlled at 35 degrees C for 10 min, it was progressively lowered to 32 degrees C over 18-20 min. Skin blood flow (SkBF) was monitored by laser-Doppler flowmetry at three control sites and at a site that had been pretreated with bretylium by iontophoresis to block noradrenergic vasoconstriction. After whole body skin cooling, maximal cutaneous vascular conductance (CVC) was measured by locally warming the sites of SkBF measurement to 42 degrees C for 30 min. Before whole body skin cooling, sublingual temperature (T(or)) in the PM was significantly higher than that in the AM (P < 0.05), but CVC, expressed as a percentage of maximal CVC (%CVC(max)), was not statistically different between AM and PM. During whole body skin cooling, %CVC(max) levels at bretylium-treated sites in AM or PM were not significantly reduced from baseline. In the PM, %CVC(max) at control sites fell significantly at T(sk) of 34.3 +/- 0.01 degrees C and lower (P < 0.05). In contrast, in the AM %CVC(max) at control sites was not significantly reduced from baseline until T(sk) reached 32.3 +/- 0.01 degrees C and lower (P < 0.05). Furthermore, the decrease in %CVC(max) in the PM was significantly greater than that in AM at T(sk) of 33.3 +/- 0.01 degrees C and lower (P < 0.05). Integrative analysis of the CVC response with respect to both T(or) and T(sk) showed that the cutaneous vasoconstrictor response was shifted to higher internal temperatures in the PM. These findings suggest that during whole body skin cooling the reflex control of the cutaneous vasoconstrictor system is shifted to a higher internal temperature in the PM. Furthermore, the slope of the relationship between CVC and T(sk) is steeper in the PM compared with that in the AM.