Cutaneous vasodilator and vasoconstrictor mechanisms in temperature regulation

Activation of protease-activated receptor 2 mediates cutaneous vasodilatation but not sweating: roles of nitric oxide synthase and cyclo-oxygenase

NEW FINDINGS

What is the central question of this study? Protease-activated receptor 2 (PAR2) is located in the endothelial cells of skin vessels and eccrine sweat glands. However, a functional role of PAR2 in the control of cutaneous blood flow and sweating remains to be assessed in humans in vivo. What is the main finding and its importance? Our results demonstrate that in normothermic resting humans in vivo, activation of PAR2 elicits cutaneous vasodilatation partly through nitric oxide synthase-dependent mechanisms, but does not mediate sweating. These results provide important new insights into the physiological significance of PAR2 in human skin. Protease-activated receptor 2 (PAR2) is present in human skin, including keratinocytes, endothelial cells of skin microvessels and eccrine sweat glands. However, whether PAR2 contributes functionally to the regulation of cutaneous blood flow and sweating remains entirely unclear in humans in vivo. We hypothesized that activation of PAR2 directly stimulates cutaneous vasodilatation and sweating via actions of nitric oxide synthase (NOS) and cyclo-oxygenase (COX). In 12 physically active young men (29 ± 5 years old), cutaneous vascular conductance (CVC) and sweat rate were measured at four intradermal microdialysis forearm skin sites that were treated with the following: (i) lactated Ringer's solution (control); (ii) 10 mm N^G -nitro-l-arginine (NOS inhibitor); (iii) 10 mm ketorolac (COX inhibitor); or (iv) a combination of both inhibitors. At all sites, a PAR2 agonist (SLIGKV-NH₂ ) was co-administered in a dose-dependent fashion (0.06, 0.18, 0.55, 1.66 and 5 mm, each for 25 min). The highest dose of SLIGKV-NH₂ (5 mm) increased CVC from baseline at the control site (P ≤ 0.05). This increase in CVC associated with PAR2 activation was attenuated by NOS inhibition regardless of the presence or absence of simultaneous COX inhibition (both P ≤ 0.05). However, COX inhibition alone did not affect the PAR2-mediated increase in CVC (P > 0.05). No increase in sweat rate was measured at any administered dose of SLIGKV-NH₂ (all P > 0.05). We show that in normothermic resting humans in vivo, PAR2 activation does not increase sweat rate, whereas it does modulate cutaneous vasodilatation through NOS-dependent mechanisms.

Efferent thermoregulatory pathways regulating cutaneous blood flow and sweating

Cutaneous vasoconstrictor nerves regulate heat retention, and are activated by falls in skin or core temperature. The efferent pathways controlling this process originate within the preoptic area. A descending GABAergic pathway, activated by warm skin or core, indirectly inhibits sympathetic premotor neurons in the medullary raphé. Those premotor neurons drive cutaneous vasoconstriction via excitatory glutamatergic and serotonergic connections to spinal preganglionic neurons. Cold skin and/or cold core temperatures activate a direct preoptic-to-raphé excitatory pathway. The balance of inhibitory and excitatory influences reaching the medullary raphé determines cutaneous blood flow. During fever, prostaglandin E₂ inhibits preoptic GABAergic neurons, resulting in disinhibition of the excitatory preoptic-to-raphé pathway, and hence, cutaneous vasoconstriction. A weaker, parallel source of descending excitatory drive reaches cutaneous preganglionic neurons from the rostral ventrolateral medulla. Sweating follows local heating of the preoptic area in cats and monkeys, and heated humans show sweating-related activation of this same region in functional magnetic resonance imaging (fMRI) studies. A descending pathway that drives sweating has been traced in cats from the hypothalamus to putative premotor neurons in the parafacial region at the pontomedullary junction. The homologous parafacial region in humans also shows sweating-related activation in fMRI studies. The central pathways that drive active vasodilatation in human nonacral skin remain unknown.

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.

Voltage-gated potassium channels and NOS contribute to a sustained cutaneous vasodilation elicited by local heating in an interactive manner in young adults

Local skin heating to 42°C causes rapid increases in cutaneous perfusion (initial peak), followed by a brief nadir and subsequent sustained elevation (plateau). Several studies have demonstrated that nitric oxide synthase (NOS) largely contributes to the plateau response during local heating. In this study, we tested the hypothesis that voltage-gated potassium (Kv) channels contribute to the plateau of the cutaneous vasodilation during local heating through NOS-dependent mechanisms. Eleven young males (25±4years) participated in this study wherein cutaneous vascular conductance (CVC) was measured at four intradermal microdialysis sites that were continuously perfused with either 1) lactated Ringer (Control), 2) 10mM 4-aminopyridine (Kv channel blocker), 3) 10mM Nω-Nitro-L-arginine (NOS inhibitor), or 4) a combination of 4-aminopyridine and Nω-Nitro-L-arginine. In comparison to the Control site, the inhibition of Kv channels alone attenuated the increase in CVC observed at the initial peak, nadir, and plateau phases measured during local heating; in contrast, the inhibition of NOS alone attenuated the increase in CVC at the nadir and plateau phases only (e.g., plateau response: Control site: 59±5%max, Kv channel blockade site: 49±8%max, NOS inhibition site: 35±11%max, combined inhibition site: 40±12%max). Further, no effect of Kv channel blockade on CVC was measured at any phase of the local heating response when the modulating influence of NOS was simultaneously removed. We show that Kv channels and NOS contribute to the local heating mediated sustained increase (i.e., plateau) in cutaneous vasodilation in an interactive manner. (243/250 words).

In Patients with Severe Peripheral Arterial Disease, Revascularization-Induced Improvement in Lower Extremity Ischemia Can Be Detected by Laser Speckle Contrast Imaging of the Fluctuation in Blood Perfusion after Local Heating

BACKGROUND

We previously reported the utility of the perfusion value (PV) fluctuation slope for detecting severe ischemia in the lower limb. Our approach was based on a thermal load test mimicking the well-known physiological reaction termed "cold-induced vasodilation," which is known to occur as a 3-phase phenomenon. The slope parameter quantifies the decrease in PVs accompanying the relative cooling (third phase) following the transient increase in blood flow (second phase) induced by the applied thermal load. This phenomenon of "relative" cold-induced vasodilation (rCIVD) can be monitored using laser speckle contrast imaging (LSCI) after applying the thermal load (LTL test). Here, we aimed to determine whether the slope parameter obtained via the LTL test also reflects the improvement in hemodynamics after revascularization.

METHODS

The study enrolled 16 patients (18 limbs), who underwent revascularization for peripheral arterial disease (PAD). The measurements were performed at 2 sites in each limb (in total, 34 sites; 2 sites in one patient were excluded because of significant movement during the measurement). For each site, we recorded the slope describing the behavior of PVs (decrease or plateau) in the third phase of rCIVD, following the initial, heating-induced increase in perfusion (second phase of rCIVD). The plateau group (group P), which included patients with an abnormal rCIVD, and the decrease group (group D), which included patients with a normal rCIVD, were defined based on perfusion slope values of <0.20 and ≥ 0.20 perfusion units/min, respectively. We also quantified the transient increase in perfusion (from baseline to peak) as a descriptor of perfusion behavior during the second phase of rCIVD.

RESULTS

In group P, the change in median values (25-75%) of the slope, transcutaneous oxygen tension, and ankle-brachial index (ABI) from before to after operation was (-0.02 [-0.04 to 0.02]; 4 [1-11]; and 0.08 [0-0.27]) to (0.39 [0.32-0.59]; 46 [37-54]; and 0.81 [0.72-0.90]). Conversely, in group D, the change in the median values of the slope, transcutaneous oxygen tension, and ABI between before and after operation was (0.38 [0.32-0.49]; 40.5 [35-45]; and0.58 [0.57-0.65]) to (0.44 [0.30-0.64]; 52 [43-56]; and 0.92 [0.81-0.99]). Sites exhibiting perfusion pattern of group D in the third phase of rCIVD showed no significant change in slope after revascularization (P = 0.21), whereas the slope in group P increased significantly after revascularization, becoming similar to the postoperative slopes in group D (P = 0.81). The amount of transient increase in perfusion, which quantified the behavior in the second phase of rCIVD, showed a similar behavior. Preoperatively, all patients in group P had rest pain and/or ulcer of the foot, whereas only few patients in group D had such symptoms.

CONCLUSIONS

Normal rCIVD response in the LTL test indicates less-than-severe ischemia, while abnormal rCIVD response measured via the LTL test indicates severe ischemic symptoms, such as critical limb ischemia. Notably, patients with an abnormal rCIVD response can develop a normal rCIVD response following revascularization, thereby reflecting an improvement in blood flow. The LTL test assessing rCIVD response can be useful for detecting severe limb ischemia, such as critical limb ischemia (CLI), and determining the departure from severe limb ischemia by revascularization.

Fluid replacement modulates oxidative stress- but not nitric oxide-mediated cutaneous vasodilation and sweating during prolonged exercise in the heat

The roles of nitric oxide synthase (NOS), reactive oxygen species (ROS), and angiotensin II type 1 receptor (AT1R) activation in regulating cutaneous vasodilation and sweating during prolonged (≥60 min) exercise are currently unclear. Moreover, it remains to be determined whether fluid replacement (FR) modulates the above thermoeffector responses. To investigate, 11 young men completed 90 min of continuous moderate intensity (46% V̇o2peak) cycling performed at a fixed rate of metabolic heat production of 600 W (No FR condition). On a separate day, participants completed a second session of the same protocol while receiving FR to offset sweat losses (FR condition). Cutaneous vascular conductance (CVC) and local sweat rate (LSR) were measured at four intradermal microdialysis forearm sites perfused with: 1) lactated Ringer (Control); 2) 10 mM NG-nitro-l-arginine methyl ester (l-NAME, NOS inhibition); 3) 10 mM ascorbate (nonselective antioxidant); or 4) 4.34 nM losartan (AT1R inhibition). Relative to Control (71% CVCmax at both time points), CVC with ascorbate (80% and 83% CVCmax) was elevated at 60 and 90 min of exercise during FR (both P < 0.02) but not at any time during No FR (all P > 0.31). In both conditions, CVC was reduced at end exercise with l-NAME (60% CVCmax; both P < 0.02) but was not different relative to Control at the losartan site (76% CVCmax; both P > 0.19). LSR did not differ between sites in either condition (all P > 0.10). We conclude that NOS regulates cutaneous vasodilation, but not sweating, irrespective of FR, and that ROS influence cutaneous vasodilation during prolonged exercise with FR.

The roles of the Na+/K+-ATPase, NKCC, and K+ channels in regulating local sweating and cutaneous blood flow during exercise in humans in vivo

Na⁺/K⁺‐ATPase has been shown to regulate the sweating and cutaneous vascular responses during exercise; however, similar studies have not been conducted to assess the roles of the Na‐K‐2Cl co‐transporter (NKCC) and K⁺ channels. Additionally, it remains to be determined if these mechanisms underpinning the heat loss responses differ with exercise intensity. Eleven young (24 ± 4 years) males performed three 30‐min semirecumbent cycling bouts at low (30% VO_2peak), moderate (50% VO_2peak), and high (70% VO_2peak) intensity, respectively, each separated by 20‐min recovery periods. Using intradermal microdialysis, four forearm skin sites were continuously perfused with either: (1) lactated Ringer solution (Control); (2) 6 mmol·L⁻¹ ouabain (Na⁺/K⁺‐ATPase inhibitor); (3) 10 mmol·L⁻¹ bumetanide (NKCC inhibitor); or (4) 50 mmol·L⁻¹ BaCl₂ (nonspecific K⁺ channel inhibitor); sites at which we assessed local sweat rate (LSR) and cutaneous vascular conductance (CVC). Inhibition of Na⁺/K⁺‐ATPase attenuated LSR compared to Control during the moderate and high‐intensity exercise bouts (both P ˂ 0.01), whereas attenuations with NKCC and K⁺ channel inhibition were only apparent during the high‐intensity exercise bout (both P ≤ 0.05). Na⁺/K⁺‐ATPase inhibition augmented CVC during all exercise intensities (all P ˂ 0.01), whereas CVC was greater with NKCC inhibition during the low‐intensity exercise only (P ˂ 0.01) and attenuated with K⁺ channel inhibition during the moderate and high‐intensity exercise conditions (both P ˂ 0.01). We show that Na⁺/K⁺‐ATPase, NKCC and K⁺ channels all contribute to the regulation of sweating and cutaneous blood flow but their influence is dependent on the intensity of dynamic exercise.

Body temperature and cold sensation during and following exercise under temperate room conditions in cold-sensitive young trained females

We evaluated cold sensation at rest and in response to exercise-induced changes in core and skin temperatures in cold-sensitive exercise trained females. Fifty-eight trained young females were screened by a questionnaire, selecting cold-sensitive (Cold-sensitive, n = 7) and non-cold-sensitive (Control, n = 7) individuals. Participants rested in a room at 29.5°C for ~100 min after which ambient temperature was reduced to 23.5°C where they remained resting for 60 min. Participants then performed 30-min of moderate intensity cycling (50% peak oxygen uptake) followed by a 60-min recovery. Core and mean skin temperatures and cold sensation over the whole-body and extremities (fingers and toes) were assessed throughout. Resting core temperature was lower in the Cold-sensitive relative to Control group (36.4 ± 0.3 vs. 36.7 ± 0.2°C). Core temperature increased to similar levels at end-exercise (~37.2°C) and gradually returned to near preexercise rest levels at the end of recovery (>36.6°C). Whole-body cold sensation was greater in the Cold-sensitive relative to Control group during resting at a room temperature of 23.5°C only without a difference in mean skin temperature between groups. In contrast, cold sensation of the extremities was greater in the Cold-sensitive group prior to, during and following exercise albeit this was not paralleled by differences in mean extremity skin temperature. We show that young trained females who are sensitive to cold exhibit augmented whole-body cold sensation during rest under temperate ambient conditions. However, this response is diminished during and following exercise. In contrast, cold sensation of extremities is augmented during resting that persists during and following exercise.

The role of shear stress on cutaneous microvascular endothelial function in humans

PURPOSE

Previous studies suggest that exercise and heat stress improve cutaneous endothelial function, caused by increases in shear stress. However, as vasodilatation in the skin is primarily a thermogenic phenomenon, we investigated if shear stress alone without increases in skin temperature that occur with exercise and heat stress increases endothelial function. We examined the hypothesis that repeated bouts of brief occlusion would improve cutaneous endothelial function via shear stress-dependent mechanisms.

METHODS

Eleven males underwent a shear stress intervention (forearm occlusion 5 s rest 10 s) for 30 min, five times·week^-1 for 6 weeks on one arm, the other was an untreated control. Skin blood flow was measured using laser-Doppler flowmetry, and endothelial function was assessed with and without NOS-inhibition with L-NAME in response to three levels of local heating (39, 42, and 44 °C), ACh administration, and reactive hyperaemia. Data are cutaneous vascular conductance (CVC, laser-Doppler/blood pressure).

RESULTS

There were no changes in the control arm (all d ≤ 0.2, p > 0.05). In the experimental arm, CVC to 39 °C was increased after 3 and 6 weeks (d = 0.6; p ≤ 0.01). Nitric oxide contribution was increased after 6 weeks compared to baseline (d = 0.85, p < 0.001). Following skin heating to 42 °C and 44 °C, CVC was not different at weeks 3 or 6 (d ≤ 0.8, p > 0.05). For both 42 and 44 °C, nitric oxide contribution was increased after weeks 3 and 6 (d ≥ 0.4, p < 0.03). Peak and area-under-the-curve responses to ACh increased following 6 weeks (p < 0.001).

CONCLUSIONS

Episodic increases in shear stress, without changes in skin or core temperature, elicit an increase in cutaneous microvascular reactivity and endothelial function.

Heat shock protein 90 contributes to cutaneous vasodilation through activating nitric oxide synthase in young male adults exercising in the heat

While the mechanisms underlying the control of cutaneous vasodilation have been extensively studied, there remains a lack of understanding of the different factors that may modulate cutaneous perfusion during an exercise-induced heat stress. We evaluated the hypothesis that heat shock protein 90 (HSP90) contributes to the heat loss response of cutaneous vasodilation via the activation of nitric oxide synthase (NOS) during exercise in the heat. In 11 young males (25 ± 5 yr), cutaneous vascular conductance (CVC) was measured at four forearm skin sites that were continuously treated with 1) lactated Ringer solution (control), 2) NOS inhibition with 10 mM NG-nitro-l-arginine methyl ester (l-NAME), 3) HSP90 inhibition with 178 μM geldanamycin, or 4) a combination of 10 mM l-NAME and 178 μM geldanamycin. Participants rested in a moderate heat stress (35°C) condition for 70 min. Thereafter, they performed a 50-min bout of moderate-intensity cycling (~52% V̇o2peak) followed by a 30-min recovery period. We showed that NOS inhibition attenuated CVC (~40-50%) relative to the control site during pre- and postexercise rest in the heat (P ≤ 0.05); however, no effect of HSP90 inhibition was observed (P > 0.05). During exercise, we observed an attenuation of CVC with the separate inhibition of NOS (~40-50%) and HSP90 (~15-20%) compared with control (both P ≤ 0.05). However, the effect of HSP90 inhibition was absent in the presence of the coinhibition of NOS (P > 0.05). We show that HSP90 contributes to cutaneous vasodilation in young men exposed to the heat albeit during exercise only. We also show that the HSP90 contribution is due to NOS-dependent mechanisms.NEW & NOTEWORTHY We show that heat shock protein 90 functionally contributes to the heat loss response of cutaneous vasodilation during exercise in the heat, and this response is mediated through the activation of nitric oxide synthase. Therefore, interventions that may activate heat shock protein 90 may facilitate an increase in heat dissipation through an augmentation of cutaneous perfusion. In turn, this may attenuate or reduce the increase in core temperature and therefore the level of heat strain.

Physiological and psychological correlates of attention-related body sensations (tingling and warmth)

Body sensations play an essential role in the subjective evaluation of our physical health, illness, and healing. They are impacted by peripheral somatic and external processes, but they are also heavily modulated by mental processes, e.g., attention, motor control, and emotion. Body sensations, such as tingling, numbness, pulse, and warmth, can emerge due to simply focusing attention on a body part. It is however an open question, if these sensations are connected with actual peripheral changes or happen “only in the mind.” Here, we first tested whether the intensity of such attention-related body sensations is related to autonomic and somatomotor physiological processes and to psychological traits. In this study, attention-related body sensations were not significantly connected to changes in physiology, except warmth sensation, which was linked to decrease in muscle tension. Overall intensity of tingling significantly correlated with body awareness and tendentiously with body–mind practice. This strengthened the hypothesis that attention-related body sensations are more the result of top–down functions, and the connection with peripheral processes is weak. Here, we suggested a novel protocol to examine the effect of manipulating attention on body sensations, which together with our results and discussion can inspire future researches.

Facial thermal variations: A new marker of emotional arousal

Functional infrared thermal imaging (fITI) is considered a promising method to measure emotional autonomic responses through facial cutaneous thermal variations. However, the facial thermal response to emotions still needs to be investigated within the framework of the dimensional approach to emotions. The main aim of this study was to assess how the facial thermal variations index the emotional arousal and valence dimensions of visual stimuli. Twenty-four participants were presented with three groups of standardized emotional pictures (unpleasant, neutral and pleasant) from the International Affective Picture System. Facial temperature was recorded at the nose tip, an important region of interest for facial thermal variations, and compared to electrodermal responses, a robust index of emotional arousal. Both types of responses were also compared to subjective ratings of pictures. An emotional arousal effect was found on the amplitude and latency of thermal responses and on the amplitude and frequency of electrodermal responses. The participants showed greater thermal and dermal responses to emotional than to neutral pictures with no difference between pleasant and unpleasant ones. Thermal responses correlated and the dermal ones tended to correlate with subjective ratings. Finally, in the emotional conditions compared to the neutral one, the frequency of simultaneous thermal and dermal responses increased while both thermal or dermal isolated responses decreased. Overall, this study brings convergent arguments to consider fITI as a promising method reflecting the arousal dimension of emotional stimulation and, consequently, as a credible alternative to the classical recording of electrodermal activity. The present research provides an original way to unveil autonomic implication in emotional processes and opens new perspectives to measure them in touchless conditions.

Sympathetic function during whole body cooling is altered in hypertensive adults

During moderate cold exposure, cardiovascular-related morbidity and mortality increase disproportionately in hypertensive adults (HTN); however, the mechanisms underlying this association are not well defined. We hypothesized that whole body cold stress would evoke exaggerated increases in blood pressure (BP) and muscle sympathetic nerve activity (MSNA) in HTN compared with normotensive adults (NTN) and that sympathetic baroreflex function would be altered during cooling in HTN. MSNA (peroneal microneurography) and beat-to-beat BP (Finometer) were measured continuously in 10 NTN (6 men/4 women; age 53 ± 3 yr; resting BP 125 ± 3/79 ± 1 mmHg) and 13 HTN (7 men/6 women; age 58 ± 2 yr; resting BP 146 ± 5/88 ± 2 mmHg) during whole body cooling-induced reductions in mean skin temperature (T_sk; water-perfused suit) from 34.0 to 30.5°C. During cooling, the increase in mean arterial pressure was greater in HTN (NTN: Δ6 ± 2 vs. HTN: Δ11 ± 1 mmHg; P = 0.02) and accompanied by exaggerated increases in MSNA (NTN: Δ8 ± 3 vs. HTN: Δ20 ± 3 bursts/100 heart beats; P < 0.01). The slope of the relation between MSNA and diastolic BP did not change during cooling in NTN (T_sk 34.0°C: -4.4 ± 0.8 vs. T_sk 30.5°C: -5.0 ± 0.3 bursts·100 heart beats^-1·mmHg^-1; P = 0.47) but increased in HTN (T_sk 34.0°C: -3.6 ± 0.4 vs. T_sk 30.5°C: -5.4 ± 0.4 bursts·100 heart beats)^-1·mmHg^-1; P = 0.02). These findings demonstrate that the cooling-induced increases in BP and MSNA are exaggerated in HTN. Furthermore, during cooling, sympathetic baroreflex sensitivity increases in HTN, but not NTN, presumably to allow for baroreflex-mediated buffering of excessive cooling-induced increases in BP. Collectively, these findings suggest that sympathetic function is altered during whole body cooling in hypertension. NEW & NOTEWORTHY These novel findings demonstrate that whole body cooling-induced reductions in mean skin temperature elicited greater increases in blood pressure and muscle sympathetic nerve activity in hypertensive adults. In addition, during moderate cold exposure, sympathetic baroreflex sensitivity increased in hypertensive, but not normotensive, adults.

Effect of a patent foramen ovale in humans on thermal responses to passive cooling and heating

Humans with a patent foramen ovale (PFO) have a higher esophageal temperature (T_esoph) than humans without a PFO (PFO-). Thus the presence of a PFO might also be associated with differences in thermal responsiveness to passive cooling and heating such as shivering and hyperpnea, respectively. The purpose of this study was to determine whether thermal responses to passive cooling and heating are different between PFO- subjects and subjects with a PFO (PFO+). We hypothesized that compared with PFO- subjects PFO+ subjects would cool down more rapidly and heat up slower and that PFO+ subjects who experienced thermal hyperpnea would have a blunted increase in ventilation. Twenty-seven men (13 PFO+) completed two trials separated by >48 h: 1) 60 min of cold water immersion (19.5 ± 0.9°C) and 2) 30 min of hot water immersion (40.5 ± 0.2°C). PFO+ subjects had a higher T_esoph before and during cold water and hot water immersion (P < 0.05). However, the rate of temperature change was similar between groups for each condition. Within a subset of 18 subjects (8 PFO+) who experienced thermal hyperpnea, PFO+ subjects experienced thermal hyperpnea at a higher absolute T_esoph but with a blunted magnitude compared with PFO- subjects. These data suggest that PFO+ subjects have a higher T_esoph at rest and have blunted thermal hyperpnea during passive heating.NEW & NOTEWORTHY Patent foramen ovale (PFO) is found in ~25-40% of the population. The presence of a PFO appears to be associated with a greater core body temperature and blunted ventilatory responses during passive heating. The reason for this blunted ventilatory response to passive heating is unknown but may suggest differences in thermal sensitivity in PFO+ subjects compared with PFO- subjects.

Effect of Heat Exposure on Cognition in Persons with Tetraplegia

Individuals with cervical spinal cord injury (SCI) have impaired thermoregulatory mechanisms attributed to interruption of motor, sensory, and autonomic neuropathways. To determine the effects of heat exposure on core body temperature (Tcore) and cognitive performance in persons with tetraplegia, 8 individuals with chronic tetraplegia (C3-C7, American Spinal Cord Injury Association Impairment Scale A-B) and 9 able-bodied controls were acclimated to 27°C at baseline (BL) before being exposed to 35°C for up to 120 min (Heat Challenge). Rectal temperature (Tcore), distal skin temperatures (Tsk_avg), sweat rate (QS_avg), microvascular skin perfusion (LDF_avg), and plasma norepinephrine (NE) were measured. Cognitive performance was assessed using Stroop Color and Word and Wechsler Adult Intelligence Scale-Fourth Edition Digit Span tests at BL and at the end of Heat Challenge. After Heat Challenge, Tcore increased 0.78 ± 0.18°C (p < 0.001) in tetraplegics after an average of 118 ± 5 min. Tcore did not change in controls after 120 min. The increase in QS_avg was larger in controls than in tetraplegics (946 ± 672% vs. 51 ± 12%; p = 0.007, respectively). LDF_avg increased only in controls (109 ± 93%; p = 0.008). Tsk_avg appeared to increase less in tetraplegics than in controls. Plasma NE levels remained lower in tetraplegics compared to controls after Heat Challenge (86 ± 64 vs. 297 ± 84 pg/mL, respectively; p < 0.001). Stroop Color, Interference, and WAIS-IV Sequence scores increased only in tetraplegics (19.4 ± 17.2%; p < 0.05, 8.3 ± 5.9%; p < 0.05, 29.1 ± 27.4%; p < 0.05, respectively). Dysfunctional thermoregulatory mechanisms in the tetraplegic group allowed Tcore to rise from subnormal levels to normothermia during heat exposure. Normothermia was associated with improvements in attention, working memory, and executive function.

Sustained increases in skin blood flow are not a prerequisite to initiate sweating during passive heat exposure

Some studies have observed a functional relationship between sweating and skin blood flow. However, the implications of this relationship during physiologically relevant conditions remain unclear. We manipulated sudomotor activity through changes in sweating efficiency to determine if parallel changes in vasomotor activity are observed. Eight young men completed two trials at 36°C and two trials at 42°C. During these trials, air temperature remained constant while ambient vapor pressure increased from 1.6 to 5.6 kPa over 2 h. Forced airflow across the skin was used to create conditions of high (HiS_eff) or low (LoS_eff) sweating efficiency. Local sweat rate (LSR), local skin blood flow (SkBF), as well as mean skin and esophageal temperatures were measured continuously. It took longer for LSR to increase during HiS_eff at 36°C (HiS_eff: 99 ± 11 vs. LoS_eff: 77 ± 11 min, P < 0.01) and 42°C (HiS_eff: 72 ± 16 vs. LoS_eff: 51 ± 15 min, P < 0.01). In general, an increase in LSR preceded the increase in SkBF when expressed as ambient vapor pressure and time for all conditions (P < 0.05). However, both responses were activated at a similar change in mean body temperature (average across all trials, LSR: 0.26 ± 0.15 vs. SkBF: 0.30 ± 0.18°C, P = 0.26). These results demonstrate that altering the point at which LSR is initiated during heat exposure is paralleled by similar shifts for the increase in SkBF. However, local sweat production occurs before an increase in SkBF, suggesting that SkBF is not necessarily a prerequisite for sweating.

Diabetes and impaired glucose metabolism is associated with more cold-related cardiorespiratory symptoms

AIMS

Diabetes and impaired glucose metabolism cause metabolic, neural and circulatory disturbances that may predispose to adverse cooling and related symptoms during the cold season. This study assessed the prevalence of cold-related cardiorespiratory symptoms in the general population according to glycaemic status.

METHODS

The study population consisted of 2436 men and 2708 women aged 45-74years who participated in the National FINRISK cold sub-studies in 2002 and 2007. A questionnaire assessed cold-related symptoms (respiratory, cardiac, peripheral circulation). Glycaemic status was determined based on fasting blood glucose, oral glucose tolerance tests or reported diagnosis of diabetes and categorized into normal glucose metabolism, impaired fasting blood glucose, impaired glucose tolerance, screening-detected type 2 diabetes and type 2 diabetes.

RESULTS

Type 2 diabetes was associated with increased odds for cold-related dyspnoea [Adjusted OR 1.72 (95% CI, 1.28-2.30)], chest pain [2.10 (1.32-3.34)] and respiratory symptoms [1.85 (1.44-2.38)] compared with normal glucose metabolism. Screened type 2 diabetes showed increased OR for cold-related dyspnoea [1.36 (1.04-1.77)], cough [1.41 (1.06-1.87)] and cardiac symptoms [1.51 (1.04-2.20)]. Worsening of glycaemic status was associated with increased odds for cold-related dyspnoea (from 1.16 in impaired fasting glucose to 1.72 in type 2 diabetes, P=0.000), cough (1.02-1.27, P=0.032), chest pain (1.28-2.10, P=0.006), arrhythmias (0.87-1.74, P=0.020), cardiac (1.11-1.99, P=0.000), respiratory (1.14-1.84, P=0.000) and all symptoms (1.05-1.66, P=0.003).

CONCLUSIONS

Subjects with diabetes and pre-diabetes experience more cold-related cardiorespiratory symptoms and need instructions for proper protection from cold weather to reduce adverse health effects.

Role of cyclooxygenase in the vascular responses to extremity cooling in Caucasian and African males

NEW FINDINGS

What is the central question of this study? Compared with Caucasians, African individuals are more susceptible to non-freezing cold injury and experience greater cutaneous vasoconstriction and cooler finger skin temperatures upon hand cooling. We investigated whether the enzyme cyclooxygenase is, in part, responsible for the exaggerated response to local cooling. What is the main finding and its importance? During local hand cooling, individuals of African descent experienced significantly lower finger skin blood flow and skin temperature compared with Caucasians irrespective of cyclooxygenase inhibition. These data suggest that in young African males the cyclooxygenase pathway appears not to be the primary reason for the increased susceptibility to non-freezing cold injury. Individuals of African descent (AFD) are more susceptible to non-freezing cold injury (NFCI) and experience an exaggerated cutaneous vasoconstrictor response to hand cooling compared with Caucasians (CAU). Using a placebo-controlled, cross-over design, this study tested the hypothesis that cyclooxygenase (COX) may, in part, be responsible for the exaggerated vasoconstrictor response to local cooling in AFD. Twelve AFD and 12 CAU young healthy men completed foot cooling and hand cooling (separately, in 8°C water for 30 min) with spontaneous rewarming in 30°C air after placebo or aspirin (COX inhibition) treatment. Skin blood flow, expressed as cutaneous vascular conductance (as flux per millimetre of mercury), and skin temperature were measured throughout. Irrespective of COX inhibition, the responses to foot cooling, but not hand cooling, were similar between ethnicities. Specifically, during hand cooling after placebo, AFD experienced a lower minimal skin blood flow [mean (SD): 0.5 (0.1) versus 0.8 (0.2) flux mmHg^-1 , P < 0.001] and a lower minimal finger skin temperature [9.5 (1.4) versus 10.7 (1.3)°C, P = 0.039] compared with CAU. During spontaneous rewarming, average skin blood flow was also lower in AFD than in CAU [2.8 (1.6) versus 4.3 (1.0) flux mmHg^-1 , P < 0.001]. These data provide further support that AFD experience an exaggerated response to hand cooling on reflection this appears to overstate findings; however, the results demonstrate that the COX pathway is not the primary reason for the exaggerated responses in AFD and increased susceptibility to NFCI.

Effect of hypohydration on postsynaptic cutaneous vasodilation and sweating in healthy men

Hypohydration decreases cutaneous vasodilation and sweating during heat stress, but it is unknown if these decrements are from postsynaptic (i.e., sweat gland/blood vessel) alterations. The purpose of this study was to determine if hypohydration affects postsynaptic cutaneous vasodilation and sweating responses. Twelve healthy men participated in euhydrated (EU) and hypohydrated (HY) trials, with hypohydration induced via fluid restriction and passive heat stress. Changes in cutaneous vascular conductance (CVC; %max) in response to incremental intradermal infusion of the endothelium-independent vasodilator sodium nitroprusside (SNP) and the endothelium-dependent vasodilator methacholine chloride (MCh) were assessed by laser Doppler flowmetry. Local sweat rate (LSR) was simultaneously assessed at the MCh site via ventilated capsule. At the end of the last dose, maximal CVC was elicited by delivering a maximal dose of SNP (5 × 10−2 M) for 30 min to both sites with simultaneous local heating (~44°C) at the SNP site. The concentration of drug needed to elicit 50% of the maximal response (log EC50) was compared between hydration conditions. The percent body mass loss was greater with HY vs. EU (−2.2 ± 0.7 vs. −0.1 ± 0.7%, P < 0.001). Log EC50 of endothelium-dependent CVC was lower with EU (−3.62 ± 0.22) vs. HY (−2.93 ± 0.08; P = 0.044). Hypohydration did not significantly alter endothelium-independent CVC or LSR (both P > 0.05). In conclusion, hypohydration attenuated endothelium-dependent CVC but did not affect endothelium-independent CVC or LSR responses. These data suggest that reductions in skin blood flow accompanying hypohydration can be partially attributed to altered postsynaptic function.

Evidence for a functional vasoconstrictor role for ATP in the human cutaneous microvasculature

NEW FINDINGS

What is the central question of this study? In young adults, about half of the cold-related reduction in skin blood flow during cold exposure is mediated by noradrenaline, while the remainder is attributable to other substances co-released with noradrenaline that have yet to be identified. What is the main finding and its importance? Purinergic receptor blockade blunted the vasoconstriction response to whole-body cooling and to intradermal administration of tyramine. These results indicate that ATP is necessary to vasoconstrict blood vessels in the skin adequately and prevent heat loss in a cold environment. Noradrenaline is responsible for eliciting ∼60% of the reflex cutaneous vasoconstriction (VC) response in young adults, while the remainder is attributable to one or more unidentified co-released sympathetic adrenergic neurotransmitter(s). Inconsistent evidence has placed neuropeptide Y in this role; however, other putative cotransmitters have yet to be tested. We hypothesize that ATP contributes to the reflex cutaneous VC response. Two protocols were conducted in young adults (n = 10); both involved the placement of three microdialysis probes in forearm skin and whole-body cooling (skin temperature = 30.5°C). In protocol 1, the following solutions were infused: (i) lactated Ringer solution (control); (ii) 10 mm l-NAME; and (iii) purinergic receptor blockade with 1 mm suramin plus l-NAME. In protocol 2, the following solutions were infused: (i) lactated Ringer solution; (ii) suramin plus l-NAME; and (iii) suramin plus l-NAME plus adrenoreceptor blockade with 5 mm yohimbine plus 1 mm propranolol. Laser Doppler flux (LDF) was measured over each microdialysis site, and cutaneous vascular conductance (CVC) was calculated (CVC = LDF/MAP) and expressed as percentage changes from baseline (%ΔCVC_BASELINE ). l-NAME was used to block the vasodilatory influence of ATP and unmask the P₂ X-mediated VC response to exogenous ATP infusion (-21 ± 6%ΔCVC_BASELINE ). During cooling, the VC response (control, -39 ± 8%ΔCVC_BASELINE ) was attenuated at the suramin site (-21 ± 4%ΔCVC_BASELINE ) and further blunted with combined adrenoreceptor blockade (-9 ± 3%ΔCVC_BASELINE ; P < 0.05). Compared with the control site (-22 ± 5%ΔCVC_BASELINE ), suramin inhibited pharmacologically induced VC to tyramine (-12 ± 6%ΔCVC_BASELINE ; P < 0.05), which displaces adrenergic neurotransmitters from axon terminals. These data indicate that ATP contributes to the cutaneous VC response in humans.

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.

Novel assessment tool based on laser speckle contrast imaging to diagnose severe ischemia in the lower limb for patients with peripheral arterial disease

Objective

We propose a new assessment tool to diagnose severe ischemia of the lower limb in peripheral arterial disease, using laser speckle contrast imaging to evaluate heating‐induced microcirculatory fluctuations in the proximal and distal sites of the dorsal foot.

Study Design

A cross‐sectional study.

Methods

We recorded the slope describing the behavior of perfusion values (decrease or plateau) following the initial, heating‐induced increase in perfusion in 63 feet of patients with clinical signs of peripheral arterial disease.

Results

The plateau and decrease groups were defined as having perfusion slopes of <0.20 and ≥0.20 PU/min, respectively. Transcutaneous oxygen tension was significantly lower (P < 0.001) in the plateau than in the decrease group (8 vs. 45 mmHg), indicating more severe ischemia. The laser speckle contrast imaging thermal load test discriminated transcutaneous oxygen tension <30 mmHg with good sensitivity (78.7%) and specificity (96.2%), and an area under the curve of 0.908.

Conclusions

Both transcutaneous oxygen tension and the laser speckle contrast imaging thermal load test are useful in diagnosing severe ischemia in the foot. Lasers Surg. Med. 49:645–651, 2017. © 2017. The Authors. Lasers in Surgery and Medicine Published by Wiley Periodicals, Inc.

Prostacyclin does not affect sweating but induces skin vasodilatation to a greater extent in older versus younger women: roles of NO and KCa channels

NEW FINDINGS

What is the central question of this study? It remains unknown whether ageing modulates prostacyclin-induced cutaneous vasodilatation in women. What is the main finding and its importance? Prostacyclin induced cutaneous vasodilatation, albeit the magnitude of increase at lower concentrations of prostacyclin was greater in older relative to young women. This response was associated with greater contributions of nitric oxide synthase and calcium-activated potassium channels. Our results suggest that administration of prostacyclin might be an effective therapy to reverse microvascular hypoperfusion, especially in older women. We previously reported that prostacyclin induces cutaneous vasodilatation but not sweating in younger and older men. Furthermore, we demonstrated that nitric oxide synthase and calcium-activated potassium (K_Ca ) channels contribute to the prostacyclin-induced cutaneous vasodilatation in younger men, although these contributions are diminished in older men. Given that the effects of ageing might differ between men and women, the above results cannot simply be applied to women. In this study, cutaneous vascular conductance and sweat rate were evaluated in younger (mean ± SD, 22 ± 3 years old) and older (55 ± 7 years old) women (10 per group) at four intradermal forearm skin sites treated as follows: (i) lactated Ringer solution without any drug (control); (ii) 10 mm N^G -nitro-l-arginine (l-NNA), a non-specific nitric oxide synthase inhibitor; (iii) 50 mm tetraethylammonium (TEA), a non-specific K_Ca channel blocker; or (iv) 10 mm l-NNA plus 50 mm TEA. All four sites were co-administered with prostacyclin in an incremental manner (0.04, 0.4, 4, 40 and 400 μm, each for 25 min). Surprisingly, increases in cutaneous vascular conductance in response to 0.04-4 μm prostacyclin were greater in older relative to younger women (all P ≤ 0.05), and these age-related differences were diminished when both l-NNA and TEA were administered simultaneously (all P > 0.05). No effect on sweat rate was observed in either group (all concentrations, P > 0.05). We show that although prostacyclin does not mediate sweating, it induces cutaneous vasodilatation, and this response elicited by lower concentrations of prostacyclin is greater in older relative to younger women. This greater cutaneous vasodilatation in older women is likely to be attributable to nitric oxide synthase- and K_Ca channel-dependent mechanisms.

Reproducibility of four frequently used local heating protocols to assess cutaneous microvascular function

BACKGROUND

Skin microvascular responses to local heating are frequently used to assess microvascular function. Several local heating protocols have been developed, all varying slightly in execution. The aim of this study was to determine the inter-day reproducibility of the four most commonly used local heating protocols in healthy young subjects.

METHODS

Fifteen, healthy males (28±5yrs, BMI 25±2kg/m²) attended two experimental trials 2-7days apart. During each trial, baseline and maximal thermally stimulated forearm skin responses were examined simultaneously at four sites on the dominant forearm using laser Doppler flowmetry (LDF). The following heating protocols were adopted: 1. Rapid 39°C (0.5°C/5s), 2. Rapid 42°C (0.5°C/5s) 3. Gradual 42°C (0.5°C/2min 30s) and 4. Slow 42°C (0.5°C/5min). The coefficient of variation (CV) was calculated for absolute flux, cutaneous vascular conductance (CVC; flux/mean arterial pressure, MAP) and CVC expressed as a percentage of maximal CVC at 44°C (%CVC_max) at three different time points; baseline (33°C), plateau (39/42°C) and maximal (44°C).

RESULTS

Reproducibility of baseline flux, CVC and %CVC_max was 17-29% across all protocols. During the plateau, Rapid, Gradual and Slow 42°C demonstrated a reproducibility of 13-18% for flux and CVC and 5-11% for %CVC_max. However, Rapid 39°C demonstrated a lower reproducibility for flux, CVC and %CVC_max (all 21%). Reproducibility at 44°C was 12-15% for flux and CVC across all protocols.

CONCLUSION

This is the first study examining inter-day reproducibility across four local heating protocols. The good-to-moderate reproducibility of the Rapid, Gradual and Slow 42°C protocols support their (simultaneous) use to assess microvascular function. Using Rapid 39°C may require a greater number of subjects to detect differences within subjects.

Mechanisms of nicotine-induced cutaneous vasodilation and sweating in young adults: roles for KCa, KATP, and KV channels, nitric oxide, and prostanoids

We evaluated the influence of K⁺ channels (i.e., Ca²⁺-activated K⁺ (K_Ca), ATP-sensitive K⁺ (K_ATP), and voltage-gated K⁺ (K_V) channels) and key enzymes (nitric oxide synthase (NOS) and cyclooxygenase (COX)) on nicotine-induced cutaneous vasodilation and sweating. Using intradermal microdialysis, we evaluated forearm cutaneous vascular conductance (CVC) and sweat rate in 2 separate protocols. In protocol 1 (n = 10), 4 separate sites were infused with (i) lactated Ringer (Control), (ii) 50 mmol·L^-1 tetraethylammonium (K_Ca channel blocker), (iii) 5 mmol·L^-1 glybenclamide (K_ATP channel blocker), and (iv) 10 mmol·L^-1 4-aminopyridine (K_V channel blocker). In protocol 2 (n = 10), 4 sites were infused with (i) lactated Ringer (Control), (ii) 10 mmol·L^-1 N^ω-nitro-l-arginine (NOS inhibitor), (iii) 10 mmol·L^-1 ketorolac (COX inhibitor), or (iv) a combination of NOS+COX inhibitors. At all sites, nicotine was infused in a dose-dependent manner (1.2, 3.6, 11, 33, and 100 mmol·L^-1; each for 25 min). Nicotine-induced increase in CVC was attenuated by the K_Ca, K_ATP, and K_V channel blockers, whereas nicotine-induced increase in sweat rate was reduced by the K_Ca and K_V channel blockers (P ≤ 0.05). COX inhibitor augmented nicotine-induced increase in CVC (P ≤ 0.05), which was absent when NOS inhibitor was co-administered (P > 0.05). In addition, our secondrary experiment (n = 7) demonstrated that muscarinic receptor blockade with 58 μmol·L^-1 atropine sulfate salt monohydrate abolished nicotine-induced increases in CVC (1.2-11 mmol·L^-1) and sweating (all doses). We show that under a normothermic resting state: (i) K_Ca, K_ATP, and K_V channels contribute to nicotinic cutaneous vasodilation, (ii) inhibition of COX augments nicotinic cutaneous vasodilation likely through NOS-dependent mechanism(s), and (iii) K_Ca and K_V channels contribute to nicotinic sweating.

Impaired sweating responses to a passive whole body heat stress in individuals with multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease that affects the central nervous system (CNS), disrupting autonomic function. The aim of this study was to test the hypothesis that individuals with MS have blunted control of thermoregulatory reflex increases in sweat rate (SR) and cutaneous vasodilation compared with controls during a passive whole body heat stress (WBH). Eighteen individuals with relapsing-remitting MS and 18 healthy controls (Con) participated in the study. Core temperature (T_core), skin temperature, heart rate, arterial blood pressure (10-min intervals), skin blood flow (laser-Doppler flux, LDF), and SR were continuously measured during normothermic baseline (34°C water perfusing a tube-lined suit) and WBH (increased T_core 0.8°C via 48°C water perfusing the suit). Following WBH, local heaters were warmed to 42°C, inducing peak cutaneous vasodilation at the site of LDF collection. Cutaneous vascular conductance (CVC) was calculated as the ratio of LDF to mean arterial pressure and expressed as a percentage of peak achieved during local heating. Individuals with MS had attenuated SR responses to WBH (ΔSR from baseline: Con, 0.65 ± 0.27; MS, 0.42 ± 0.17 mg·cm^-2·min^-1, P = 0.003), whereas Δ%CVC_42C from baseline was similar between groups (Con, 42 ± 16%; MS, 38 ± 12%, P = 0.39). SR responses were blunted as a function of T_core in MS (interaction: group × T_core, P = 0.03), of which differences were evident at ΔT_core 0.7°C and 0.8°C (P < 0.05). No interaction was observed in Δ%CVC_42C Taken together, the findings show MS blunts sweating responses, whereas control of the cutaneous vasculature is preserved, in response to WBH.NEW & NOTEWORTHY This study is the first to assess the reflex control of the thermoregulatory system in individuals living with multiple sclerosis (MS). The novel findings are twofold. First, attenuated increases in sweat rate in subjects with MS compared with healthy controls were observed in response to a moderate increase (0.8°C) in core temperature via passive whole body heat stress. Second, it appears the reflex control of the cutaneous vasculature is preserved in MS.

Physiological and perceptual effects of a cooling garment during simulated industrial work in the heat

OBJECTIVE

Evaluate physiological and perceptual responses using a phase change cooling (PCC) garment during simulated work in the heat.

METHODS

Twenty males wearing compression undergarments, coverall suit, gloves, and hard-hat, completed two randomly assigned trials (with PCC inserts or control, CON) of simulated industrial tasks in the heat (34.2 ± 0.05 °C, 54.7 ± 0.3%RH). Trials consisted of two 20 min work bouts, a maximum performance bout, and 10 min of recovery.

RESULTS

Physiological strain index (PSI) was lower during PCC after the second work bout and during recovery (all P < 0.05). PCC reduced heat storage (27.0 ± 7.6 W m^-2) compared to CON (42.7 ± 9.9 W m^-2, P < 0.001). Perceptual strain index (PeSI) was reduced with PCC compared to CON (P < 0.001), however performance outcomes were not different between trials (P = 0.10).

CONCLUSIONS

PCC during work in the heat attenuated thermal, physiological, and perceptual strain. This PCC garment could increase safety and reduce occupational heat illness risk.

Thermal stress, human performance, and physical employment standards

Many physically demanding occupations in both developed and developing economies involve exposure to extreme thermal environments that can affect work capacity and ultimately health. Thermal extremes may be present in either an outdoor or an indoor work environment, and can be due to a combination of the natural or artificial ambient environment, the rate of metabolic heat generation from physical work, processes specific to the workplace (e.g., steel manufacturing), or through the requirement for protective clothing impairing heat dissipation. Together, thermal exposure can elicit acute impairment of work capacity and also chronic effects on health, greatly contributing to worker health risk and reduced productivity. Surprisingly, in most occupations even in developed economies, there are rarely any standards regarding enforced heat or cold safety for workers. Furthermore, specific physical employment standards or accommodations for thermal stressors are rare, with workers commonly tested under near-perfect conditions. This review surveys the major occupational impact of thermal extremes and existing employment standards, proposing guidelines for improvement and areas for future research.

Neurovascular mechanisms underlying augmented cold-induced reflex cutaneous vasoconstriction in human hypertension

KEY POINTS

In hypertensive adults (HTN), cardiovascular risk increases disproportionately during environmental cold exposure. Despite ample evidence of dysregulated sympathetic control of the peripheral vasculature in hypertension, no studies have examined integrated neurovascular function during cold stress in HTN. The findings of the present study show that whole-body cold stress elicits greater increases in sympathetic outflow directed to the cutaneous vasculature and, correspondingly, greater reductions in skin blood flow in HTN. We further demonstrate an important role for non-adrenergic sympathetic co-transmitters in mediating the vasoconstrictor response to cold stress in hypertension. In the context of thermoregulation and the maintenance of core temperature, sympathetically-mediated control of the cutaneous vasculature is not only preserved, but also exaggerated in hypertension. Given the increasing prevalence of hypertension, clarifying the mechanistic underpinnings of hypertension-induced alterations in neurovascular function during cold exposure is clinically relevant.

ABSTRACT

Despite ample evidence of dysregulated sympathetic control of the peripheral vasculature in hypertension, no studies have examined integrated neurovascular function during cold stress in hypertensive adults (HTN). We hypothesized that (i) whole-body cooling would elicit greater cutaneous vasoconstriction and greater increases in skin sympathetic nervous system activity (SSNA) in HTN (n = 14; 56 ± 2 years) compared to age-matched normotensive adults (NTN; n = 14; 55 ± 2 years) and (ii) augmented reflex vasoconstriction in HTN would be mediated by an increase in cutaneous vascular adrenergic sensitivity and a greater contribution of non-adrenergic sympathetic co-transmitters. SSNA (peroneal microneurography) and red cell flux (laser Doppler flowmetry; dorsum of foot) were measured during whole-body cooling (water-perfused suit). Sympathetic adrenergic- and non-adrenergic-dependent contributions to reflex cutaneous vasoconstriction and vascular adrenergic sensitivity were assessed pharmacologically using intradermal microdialysis. Cooling elicited greater increases in SSNA (NTN: +64 ± 13%_baseline  vs. HTN: +194 ± 26%_baseline ; P < 0.01) and greater reductions in skin blood flow (NTN: -16 ± 2%_baseline  vs. HTN: -28 ± 3%_baseline ; P < 0.01) in HTN compared to NTN, reflecting an increased response range for sympathetic reflex control of cutaneous vasoconstriction in HTN. Norepinephrine dose-response curves showed no HTN-related difference in cutaneous adrenergic sensitivity (logEC₅₀ ; NTN: -7.4 ± 0.3 log M vs. HTN: -7.5 ± 0.3 log M; P = 0.84); however, non-adrenergic sympathetic co-transmitters mediated a significant portion of the vasoconstrictor response to cold stress in HTN. Collectively, these findings indicate that hypertension increases the peripheral cutaneous vasoconstrictor response to cold via greater increases in skin sympathetic outflow coupled with an increased reliance on non-adrenergic neurotransmitters.

Spectral analysis of reflex cutaneous vasodilatation during passive heat stress

Previous work has demonstrated that spectral analysis is a useful tool to non-invasively ascertain the mechanisms of control of the cutaneous circulation. The majority of work using spectral analysis has focused on local control mechanisms, with none examining reflex control. Skin blood flow was analysed using spectral analysis on the dorsal aspect of the forearm of 7 males and 7 females during passive heat stress, with mean forearm and local temperature at the site of measurement maintained at thermoneutral (33°C) to minimize the effect of local control mechanisms. Participants were passively heated to ~1.2±0.1°C above baseline rectal temperature (d=4.0, P<0.001) using a water-perfused, tube lined suit, with skin blood flow assessed using a laser-Doppler probe with an integrated temperature monitor. Spectral analysis was performed using a Morlet wavelet on the entire data set, with median power extracted during 20min of data during baseline (normothermia) and hyperthermia. Passive heat stress significantly increased laser-Doppler flux above baseline (d=4.7, P<0.001). Spectral power of the endothelial nitric oxide-independent (0.005-0.01Hz; d=1.1, P=0.004), neurogenic (0.2-0.05Hz; d=0.6, P=0.025), myogenic (0.05-0.15Hz; d=1.5, P=0.002), respiratory (0.15-0.4Hz; d=1.4 P=0.002), and cardiac (0.4-2.0Hz; d=1.1, P=0.012) frequency intervals increased with passive heat stress. In contrast, the endothelial nitric oxide-dependent frequency interval did not change (0.01-0.02Hz; d=0.3, P=0.09) with passive heat stress. These data suggest that cutaneous reflex vasodilatation is neurogenic in origin and not mediated by endothelial-nitric oxide synthase, and are congruent with invasive examinations of reflex cutaneous vasodilatation.

Acute black tea consumption improves cutaneous vascular function in healthy middle-aged humans

BACKGROUND & AIMS

Dietary flavonoids, such as those present in black tea, are associated with reduced risk of cardiovascular disease (CVD), possibly through improving nitric oxide (NO) mediated vascular function. The aim of this study was to examine the effect of acute black tea ingestion on cutaneous microvascular function.

METHODS

Twenty healthy participants (58 ± 5 y, 9 men) attended two experimental trials (tea, placebo), 7-days apart in a randomised, controlled, double-blind, cross-over design. Participants ingested a single dose of 200 ml black tea or placebo, followed by assessment of forearm cutaneous perfusion using laser-Doppler flowmetry (LDF) using three distinct heating protocols, enabling us to distinguish between axon- and endothelium-dependent vasodilation: 1. rapid 42°C, 2. rapid 39°C and 3. gradual 42°C. On the contralateral arm, full-field laser perfusion imaging (FLPI) was used to assess forearm perfusion during gradual 42°C. Data were presented as cutaneous vascular conductance (CVC; flux/mean arterial pressure, MAP) and CVC expressed as a percentage of maximal CVC (%CVC_max).

RESULTS

Rapid local heating to 39°C or 42°C demonstrated no effect of tea for flux, CVC or %CVC_max (all P > 0.05). Gradual local heating to 42 °C, however, produced a higher skin blood flow following black tea ingestion for absolute CVC (P = 0.04) when measured by LDF, and higher absolute flux (P < 0.001) and CVC (P < 0.001) measured with FLPI. No effect of tea was found for %CVC_max when assessed by either LDF or FLPI.

CONCLUSIONS

Acute tea ingestion enhanced cutaneous vascular responses to gradual local heating to 42 °C in healthy, middle-aged participants, possibly through a mechanism related to activation of endothelium-derived chemical mediators, such as NO. These improvements may contribute to the cardiovascular health benefits of regular tea ingestion.

The effect of endothelin A and B receptor blockade on cutaneous vascular and sweating responses in young men during and following exercise in the heat

During exercise, cutaneous vasodilation and sweating responses occur, whereas these responses rapidly decrease during postexercise recovery. We hypothesized that the activation of endothelin A (ET_A) receptors, but not endothelin B (ET_B) receptors, attenuate cutaneous vasodilation during high-intensity exercise and contribute to the subsequent postexercise suppression of cutaneous vasodilation. We also hypothesized that both receptors increase sweating during and following high-intensity exercise. Eleven men (24 ± 4 yr) performed an intermittent cycling protocol consisting of two 30-min bouts of moderate- (40% V̇o_2peak) and high-intensity (75% V̇o_2peak) exercise in the heat (35°C), each separated by a 20- and 40-min recovery period, respectively. Cutaneous vascular conductance (CVC) and sweat rate were evaluated at four intradermal microdialysis skin sites: 1) lactated Ringer (control), 2) 500 nM BQ123 (a selective ET_A receptor blocker), 3) 300 nM BQ788 (a selective ET_B receptor blocker), or 4) a combination of BQ123 + BQ788. There were no between-site differences in CVC during each exercise bout (all P > 0.05); however, CVC following high-intensity exercise was greater at BQ123 (56 ± 9%max) and BQ123 + BQ788 (55 ± 14%max) sites relative to the control site (43 ± 12%max) (all P ≤ 0.05). Sweat rate did not differ between sites throughout the protocol (all P > 0.05). We show that neither ET_A nor ET_B receptors modulate cutaneous vasodilation and sweating responses during and following moderate- and high-intensity exercise in the heat, with the exception that ET_A receptors may partly contribute to the suppression of cutaneous vasodilation following high-intensity exercise.

Acute limb heating improves macro- and microvascular dilator function in the leg of aged humans

Local heating of an extremity increases blood flow and vascular shear stress throughout the arterial tree. Local heating acutely improves macrovascular dilator function in the upper limbs of young healthy adults through a shear stress-dependent mechanism but has no such effect in the lower limbs of this age group. The effect of acute limb heating on dilator function within the atherosclerotic prone vasculature of the lower limbs of aged adults is unknown. Therefore, the purpose of this study was to test the hypothesis that acute lower limb heating improves macro- and microvascular dilator function within the leg vasculature of aged adults. Nine young and nine aged adults immersed their lower limbs at a depth of ~33 cm into a heated (~42°C) circulated water bath for 45 min. Before and 30 min after heating, macro (flow-mediated dilation)- and microvascular (reactive hyperemia) dilator functions were assessed in the lower limb, following 5 min of arterial occlusion, via Doppler ultrasound. Compared with preheat, macrovascular dilator function was unchanged following heating in young adults (P = 0.6) but was improved in aged adults (P = 0.04). Similarly, microvascular dilator function, as assessed by peak reactive hyperemia, was unchanged following heating in young adults (P = 0.1) but was improved in aged adults (P < 0.01). Taken together, these data suggest that acute lower limb heating improves both macro- and microvascular dilator function in an age dependent manner.

NEW & NOTEWORTHY

We demonstrate that lower limb heating acutely improves macro- and microvascular dilator function within the atherosclerotic prone vasculature of the leg in aged adults. These findings provide evidence for a potential therapeutic use of chronic lower limb heating to improve vascular health in primary aging and various disease conditions.

Thermoregulatory responses to combined moderate heat stress and hypoxia

OBJECTIVE

The aim of this study was to examine the cutaneous vascular and sudomotor responses to combined moderate passive heat stress and normobaric hypoxia.

METHOD

Thirteen healthy young males, dressed in a water-perfused suit, underwent passive heating (Δcore temperature ~0.7°C) twice (Normoxia; 20.9% O₂ and Hypoxia; 13% O₂ ). Chest and forearm skin blood flow (SkBF; laser-Doppler flux) and sweat rate (SR; capacitance hygrometry), core (intestinal pill), and skin temperatures, were recorded.

RESULTS

Hypoxia reduced baseline oxygen saturation (98±1 vs 89±6%, P<.001) and elevated chest (P=.03) and forearm SkBF (P=.03), and HR (64±9 vs 69±8 beats.min^-1 , P<.01). During heating, mean body temperature (T¯BODY) thresholds for SkBF (P=.41) and SR (P=.28) elevations were not different between trials. The SkBF: T¯BODY linear sensitivity during the initial phase of heating was lower at the chest (P=.035) but not different at the forearm (P=.17) during hypoxia. With increasing levels of heating chest SkBF was not different (P=.55) but forearm SkBF was lower (P<.01) during hypoxia. Chest (P=.85) and forearm (P=.79) SR: T¯BODY linear sensitivities were not different between trials.

CONCLUSION

While sudomotor responses and the initiation of cutaneous blood flow elevations are unaffected, hypoxia differentially effects regional SkBF responses during moderate passive heating.

Development of a mechanism-based pharmacokinetic/pharmacodynamic model to characterize the thermoregulatory effects of serotonergic drugs in mice

We have shown recently that concurrent harmaline, a monoamine oxidase-A inhibitor (MAOI), potentiates serotonin (5-HT) receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT)-induced hyperthermia. The objective of this study was to develop an integrated pharmacokinetic/pharmacodynamic (PK/PD) model to characterize and predict the thermoregulatory effects of such serotonergic drugs in mice. Physiological thermoregulation was described by a mechanism-based indirect-response model with adaptive feedback control. Harmaline-induced hypothermia and 5-MeO-DMT–elicited hyperthermia were attributable to the loss of heat through the activation of 5-HT_1A receptor and thermogenesis via the stimulation of 5-HT_2A receptor, respectively. Thus serotonergic 5-MeO-DMT–induced hyperthermia was readily distinguished from handling/injection stress-provoked hyperthermic effects. This PK/PD model was able to simultaneously describe all experimental data including the impact of drug-metabolizing enzyme status on 5-MeO-DMT and harmaline PK properties, and drug- and stress-induced simple hypo/hyperthermic and complex biphasic effects. Furthermore, the modeling results revealed a 4-fold decrease of apparent SC₅₀ value (1.88–0.496 µmol/L) for 5-MeO-DMT when harmaline was co-administered, providing a quantitative assessment for the impact of concurrent MAOI harmaline on 5-MeO-DMT–induced hyperthermia. In addition, the hyperpyrexia caused by toxic dose combinations of harmaline and 5-MeO-DMT were linked to the increased systemic exposure to harmaline rather than 5-MeO-DMT, although the body temperature profiles were mispredicted by the model. The results indicate that current PK/PD model may be used as a new conceptual framework to define the impact of serotonergic agents and stress factors on thermoregulation.

K+ channel mechanisms underlying cholinergic cutaneous vasodilation and sweating in young humans: roles of KCa, KATP, and KV channels?

Acetylcholine released from cholinergic nerves is involved in heat loss responses of cutaneous vasodilation and sweating. K(+) channels are thought to play a role in regulating cholinergic cutaneous vasodilation and sweating, though which K(+) channels are involved in their regulation remains unclear. We evaluated the hypotheses that 1) Ca(2+)-activated K(+) (KCa), ATP-sensitive K(+) (KATP), and voltage-gated K(+) (KV) channels all contribute to cholinergic cutaneous vasodilation; and 2) KV channels, but not KCa and KATP channels, contribute to cholinergic sweating. In 13 young adults (24 ± 5 years), cutaneous vascular conductance (CVC) and sweat rate were evaluated at intradermal microdialysis sites that were continuously perfused with: 1) lactated Ringer (Control), 2) 50 mM tetraethylammonium (KCa channel blocker), 3) 5 mM glybenclamide (KATP channel blocker), and 4) 10 mM 4-aminopyridine (KV channel blocker). At all sites, cholinergic cutaneous vasodilation and sweating were induced by coadministration of methacholine (0.0125, 0.25, 5, 100, and 2,000 mM, each for 25 min). The methacholine-induced increase in CVC was lower with the KCa channel blocker relative to Control at 0.0125 (1 ± 1 vs. 9 ± 6%max) and 5 (2 ± 5 vs. 17 ± 14%max) mM methacholine, whereas it was lower in the presence of KATP (69 ± 7%max) and KV (57 ± 14%max) channel blocker compared with Control (79 ± 6%max) at 100 mM methacholine. Furthermore, methacholine-induced sweating was lower at the KV channel blocker site (0.42 ± 0.17 mg·min(-1)·cm(-2)) compared with Control (0.58 ± 0.15 mg·min(-1)·cm(-2)) at 2,000 mM methacholine. In conclusion, we show that KCa, KATP, and KV channels play a role in cholinergic cutaneous vasodilation, whereas only KV channels contribute to cholinergic sweating in normothermic resting humans.

Morphological dependency of cutaneous blood flow and sweating during compensable heat stress when heat-loss requirements are matched across participants

Human heat loss is thought, in part, to be morphologically related. It was therefore hypothesized that when heat-loss requirements and body temperatures were matched, that the mass-specific surface area alone could significantly explain both cutaneous vascular and sudomotor responses during compensable exercise. These thermoeffector responses were examined in 36 men with widely varying mass-specific surface areas (range, 232.3-292.7 cm(2)/kg), but of similar age, aerobic fitness, and adiposity. Subjects completed two trials under compensable conditions (28.1°C, 36.8% relative humidity), each involving rest (20 min) and steady-state cycling (45 min) at two matched metabolic heat-production rates (light, ∼135 W/m(2); moderate, ∼200 W/m(2)). Following equivalent mean body temperature changes, forearm blood flow and vascular conductance (r = 0.63 and r = 0.65) shared significant, positive associations with the mass-specific surface area during light work (P < 0.05), explaining ∼45% of the vasomotor variation. Conversely, during light and moderate work, whole body sweat rate, as well as local sweat rate and sudomotor sensitivity at three of four measured sites, revealed moderate, negative relationships with the mass-specific surface area (correlation coefficient range -0.37 to -0.73, P < 0.05). Moreover, those relationships could uniquely account for between 10 and 53% of those sweating responses (P < 0.05). Therefore, both thermoeffector responses displayed a significant morphological dependency in the presence of equivalent thermoafferent drive. Indeed, up to half of the interindividual variation in these effector responses could now be explained through morphological differences and the first principles governing heat transfer.