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

Does attenuated skin blood flow lower sweat rate and the critical environmental limit for heat balance during severe heat exposure?

NEW FINDINGS

What is the central question of this study? Does attenuated skin blood flow diminish sweating and reduce the critical environmental limit for heat balance, which indicates maximal heat loss potential, during severe heat stress? What is the main finding and its importance? Isosmotic hypovolaemia attenuated skin blood flow by ∼20% but did not result in different sweating rates, mean skin temperatures or critical environmental limits for heat balance compared with control and volume-infusion treatments, suggesting that the lower levels of skin blood flow commonly observed in aged and diseased populations may not diminish maximal whole-body heat dissipation. Attenuated skin blood flow (SkBF) is often assumed to impair core temperature (Tc ) regulation. Profound pharmacologically induced reductions in SkBF (∼85%) lead to impaired sweating, but whether the smaller attenuations in SkBF (∼20%) more often associated with ageing and certain diseases lead to decrements in sweating and maximal heat loss potential is unknown. Seven healthy men (28 ± 4 years old) completed a 30 min equilibration period at 41°C and a vapour pressure (Pa ) of 2.57 kPa followed by incremental steps in Pa of 0.17 kPa every 6 min to 5.95 kPa. Differences in heat loss potential were assessed by identifying the critical vapour pressure (Pcrit ) at which an upward inflection in Tc occurred. The following three separate treatments elicited changes in plasma volume to achieve three distinct levels of SkBF: control (CON); diuretic-induced isosmotic dehydration to lower SkBF (DEH); and continuous saline infusion to maintain SkBF (SAL). The Tc , mean skin temperature (Tsk ), heart rate, mean laser-Doppler flux (forearm and thigh; LDFmean ), mean local sweat rate (forearm and thigh; LSRmean ) and metabolic rate were measured. In DEH, a 14.2 ± 5.7% lower plasma volume resulted in a ∼20% lower LDFmean in perfusion units (PU) (DEH, 139 ± 23 PU; CON, 176 ± 22 PU; and SAL, 186 ± 22 PU; P = 0.034). However, LSRmean and whole-body sweat losses were unaffected by treatment throughout (P > 0.482). The Pcrit for Tc was similar between treatments (CON, 5.05 ± 0.30 kPa; DEH, 4.93 ± 0.16 kPa; and SAL, 5.12 ± 0.10 kPa; P = 0.166). Furthermore, no differences were observed in the skin-air temperature gradient, metabolic rate or changes in Tc (P > 0.197). In conclusion, a ∼20% reduction in SkBF alters neither sweat rate nor the upper limit for heat loss from the skin during non-encapsulated passive heat stress.

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.

β-Adrenergic blockade does not impair the skin blood flow sensitivity to local heating in burned and nonburned skin under neutral and hot environments in children

OBJECTIVE

We tested the hypothesis that propranolol, a drug given to burn patients to reduce hypermetabolism/cardiac stress, may inhibit heat dissipation by changing the sensitivity of skin blood flow (SkBF) to local heating under neutral and hot conditions.

METHODS

In a randomized double-blind study, a placebo was given to eight burned children, while propranolol was given to 13 burned children with similar characteristics (mean±SD: 11.9±3 years, 147±20 cm, 45±23 kg, 56±12% Total body surface area burned). Nonburned children (n=13, 11.4±3 years, 152±15 cm, 52±13 kg) served as healthy controls. A progressive local heating protocol characterized SkBF responses in burned and unburned skin and nonburned control skin under the two environmental conditions (23 and 34°C) via laser Doppler flowmetry.

RESULTS

Resting SkBF was greater in burned and unburned skin compared to the nonburned control (main effect: skin, P<.0001; 57±32 burned; 38±36 unburned vs 9±8 control %SkBF_max ). No difference was found for maximal SkBF capacity to local heating between groups. Additionally, dose-response curves for the sensitivity of SkBF to local heating were not different among burned or unburned skin, and nonburned control skin (EC₅₀ , P>.05) under either condition.

CONCLUSION

Therapeutic propranolol does not negatively affect SkBF under neutral or hot environmental conditions and further compromise temperature regulation in burned children.

MDMA, Methylone, and MDPV: Drug-Induced Brain Hyperthermia and Its Modulation by Activity State and Environment

Psychomotor stimulants are frequently used by humans to intensify the subjective experience of different types of social interactions. Since psychomotor stimulants enhance metabolism and increase body temperatures, their use under conditions of physiological activation and in warm humid environments could result in pathological hyperthermia, a life-threatening symptom of acute drug intoxication. Here, we will describe the brain hyperthermic effects of MDMA, MDPV, and methylone, three structurally related recreational drugs commonly used by young adults during raves and other forms of social gatherings. After a short introduction on brain temperature and basic mechanisms underlying its physiological fluctuations, we will consider how MDMA, MDPV, and methylone affect brain and body temperatures in awake freely moving rats. Here, we will discuss the role of drug-induced heat production in the brain due to metabolic brain activation and diminished heat dissipation due to peripheral vasoconstriction as two primary contributors to the hyperthermic effects of these drugs. Then, we will consider how the hyperthermic effects of these drugs are modulated under conditions that model human drug use (social interaction and warm ambient temperature). Since social interaction results in brain and body heat production, coupled with skin vasoconstriction that impairs heat loss to the external environment, these physiological changes interact with drug-induced changes in heat production and loss, resulting in distinct changes in the hyperthermic effects of each tested drug. Finally, we present our recent data, in which we compared the efficacy of different pharmacological strategies for reversing MDMA-induced hyperthermia in both the brain and body. Specifically, we demonstrate increased efficacy of the centrally acting atypical neuroleptic compound clozapine over the peripherally acting vasodilator drug, carvedilol. These data could be important for understanding the potential dangers of MDMA in humans and the development of pharmacological tools to alleviate drug-induced hyperthermia - potentially saving the lives of highly intoxicated individuals.

Tissue Blood Volume Parameters Measured by Continuous-Wave and Spatially Resolved NIRS Show Different Changes During Prolonged Cycling Exercise

Near-infrared spectroscopy (NIRS) shows two types of tissue blood volume (BV) parameters: the total hemoglobin concentration (cHb; assessed by continuous-wave NIRS) and the normalized tissue hemoglobin index (nTHI; evaluated by spatially resolved NIRS). This study was performed to investigate the differences between cHb and nTHI, estimated by calibration using acute reduction of BV at the onset of exercise. Seven active male volunteers (age: 22  ±  4 years) performed a prolonged (60-min) cycling exercise test at 65% or 70% of the peak oxygen uptake. During the tests, cHb and nTHI from the vastus lateralis muscle were monitored. The two parameters were expressed relative to the resting value of 100% and the minimum value of 0% at the onset of exercise. The value of cHb started to increase after arriving at the minimum value and continued until the end of exercise. Similar to cHb, nTHI increased acutely from the lowest level, but the parameter remained almost unchanged from ~5 min throughout the test. Therefore, the two parameters interacted significantly. Moreover, in comparison with the same exercise duration, cHb was significantly higher than nTHI from the 28th min to the end of the test. These results suggest that cHb and nTHI in working muscle are not synchronized during prolonged exercise, especially after ~30 min. The differences between the two BV parameters are likely partly attributable to other factors, such as the increase in skin blood flow.

Early differential diagnosis of the severity of acute pancreatitis

There is a great need for early verification of the severity of acute pancreatitis (AP). The early stage of pathogenesis of AP is characterized by endothelial dysfunction which could be determined by wavelet analysis of skin temperature (WAST) technique. The aim is to investigate whether the dysregulation of microvascular tone caused by endothelial dysfunction and detected by WAST can be a significant indicator in early differential diagnosis of AP severity. The WAST performed in the frequency range of 0.0095-0.02 Hz during the contralateral cold test. Forty nine patients with AP aged 19-65 participated in this study. The control group included 12 healthy volunteers aged 20-65. Dysregulation of vascular tone during the contralateral cold test was observed in all patients with AP. The basal amplitudes of skin temperature oscillations in patients with AP were much lower than in healthy volunteers and progressively decreased as the disease severity increased. In patients with mild and moderate AP only the vasodilator component is destroyed, but vasoconstriction still operates. In patients with severe AP both mechanisms of endothelial vascular tone regulation are destroyed. Patients with AP have abnormal microvascular reactions related to the endothelial mechanism of vascular tone regulation. Based on the initial values of amplitudes and the indices of vasoconstriction and postcold vasodilatation, the WAST method makes it possible to evaluate two related but different characteristics of the endothelial dysfunction in patients with AP on admission which can be a significant indicator in early differential diagnosis of AP severity.

Raynaud's Phenomenon: A Brief Review of the Underlying Mechanisms

Raynaud’s phenomenon (RP) is characterized by exaggerated cold-induced vasoconstriction. This augmented vasoconstriction occurs by virtue of a reflex response to cooling via the sympathetic nervous system as well as by local activation of α_2C adrenoceptors (α_2C-AR). In a cold-initiated, mitochondrion-mediated mechanism involving reactive oxygen species and the Rho/ROCK pathway, cytoskeletal rearrangement in vascular smooth muscle cells orchestrates the translocation of α_2C-AR to the cell membrane, where this receptor readily interacts with its ligand. Different parameters are involved in this spatial and functional rescue of α_2C-AR. Of notable relevance is the female hormone, 17β-estradiol, or estrogen. This is consistent with the high prevalence of RP in premenopausal women compared to age-matched males. In addition to dissecting the role of these various players, the contribution of pollution as well as genetic background to the onset and prevalence of RP are also discussed. Different therapeutic approaches employed as treatment modalities for this disease are also highlighted and analyzed. The lack of an appropriate animal model for RP mandates that more efforts be undertaken in order to better understand and eventually treat this disease. Although several lines of treatment are utilized, it is important to note that precaution is often effective in reducing severity or frequency of RP attacks.

Altered thermoregulatory responses in heart failure patients exercising in the heat

Heart failure (HF) patients appear to exhibit impaired thermoregulatory capacity during passive heating, as evidenced by diminished vascular conductance. Although some preliminary studies have described the thermoregulatory response to passive heating in HF, responses during exercise in the heat remain to be described. Therefore, the aim of this study was to compare thermoregulatory responses in HF and controls (CON) during exercise in the heat. Ten HF (NYHA classes I-II) and eight CON were included. Core temperature (Tc), skin temperature (Tsk), and cutaneous vascular conductance (CVC) were assessed at rest and during 1 h of exercise at 60% of maximal oxygen uptake. Metabolic heat production (Hprod) and the evaporative requirements for heat balance (Ereq) were also calculated. Whole-body sweat rate was determined from pre-post nude body mass corrected for fluid intake. While Hprod (HF: 3.9 ± 0.9; CON: 6.4 ± 1.5 W/kg) and Ereq (HF: 3.3 ± 0.9; CON: 5.6 ± 1.4 W/kg) were lower (P < 0.01) for HF compared to CON, both groups demonstrated a similar rise in Tc (HF: 0.9 ± 0.4; CON: 1.0 ± 0.3°C). Despite this similar rise in Tc, Tsk (HF: 1.6 ± 0.7; CON: 2.7 ± 1.2°C), and the elevation in CVC (HF: 1.4 ± 1.0; CON: 3.0 ± 1.2 au/mmHg) was lower (P < 0.05) in HF compared to CON Additionally, whole-body sweat rate (HF: 0.36 ± 0.15; CON: 0.81 ± 0.39 L/h) was lower (P = 0.02) in HF compared to CON Patients with HF appear to be limited in their ability to manage a thermal load and distribute heat content to the body surface (i.e., skin), secondary to impaired circulation to the periphery.

Regulation of exercise blood flow: Role of free radicals

During exercise, oxygen and nutrient rich blood must be delivered to the active skeletal muscle, heart, skin, and brain through the complex and highly regulated integration of central and peripheral hemodynamic factors. Indeed, even minor alterations in blood flow to these organs have profound consequences on exercise capacity by modifying the development of fatigue. Therefore, the fine-tuning of blood flow is critical for optimal physical performance. At the level of the peripheral circulation, blood flow is regulated by a balance between the mechanisms responsible for vasodilation and vasoconstriction. Once thought of as toxic by-products of in vivo chemistry, free radicals are now recognized as important signaling molecules that exert potent vasoactive responses that are dependent upon the underlying balance between oxidation-reduction reactions or redox balance. Under normal healthy conditions with low levels of oxidative stress, free radicals promote vasodilation, which is attenuated with exogenous antioxidant administration. Conversely, with advancing age and disease where background oxidative stress is elevated, an exercise-induced increase in free radicals can further shift the redox balance to a pro-oxidant state, impairing vasodilation and attenuating blood flow. Under these conditions, exogenous antioxidants improve vasodilatory capacity and augment blood flow by restoring an "optimal" redox balance. Interestingly, while the active skeletal muscle, heart, skin, and brain all have unique functions during exercise, the mechanisms by which free radicals contribute to the regulation of blood flow is remarkably preserved across each of these varied target organs.

Relationship of oscillating and average components of laser Doppler flowmetry signal

Signals from laser Doppler flowmeters widely used in intravital studies of skin blood flow include, along with a slowly varying average component, an oscillating part. However, in most clinical studies, pulsations are usually smoothed by data preprocessing and only the mean blood flow is analyzed. To reveal the relationship between average and oscillating perfusion components measured by a laser Doppler flowmeter, we examined the microvascular response to the contralateral cold pressor test recorded at two different sites of the hand: dorsal part of the arm and finger pad. Such a protocol makes it possible to provide a wide range of perfusion. The average perfusion always decreases during cooling, while the oscillating component demonstrates a differently directed response. The wavelet analysis of laser Doppler flowmetry (LDF) signals shows that the pulsatile component is nonlinearly related to the average perfusion. Under low perfusion, the amplitude of pulsations is proportional to its mean value, but, as perfusion increases, the amplitude of pulsations becomes lower. The type of response is defined by the basal perfusion and the degree of vasoconstriction caused by cooling. Interpretation of the results is complicated by the nonlinear transfer function of the LDF device, the contribution of which is studied using artificial examples.

A new method of infrared thermography for quantification of brown adipose tissue activation in healthy adults (TACTICAL): a randomized trial

The ability to alter the amount and activity of brown adipose tissue (BAT) in human adults is a potential strategy to manage obesity and related metabolic disorders associated with food, drug, and environmental stimuli with BAT activating/recruiting capacity. Infrared thermography (IRT) provides a non-invasive and inexpensive alternative to the current methods (e.g. ¹⁸F-FDG PET) used to assess BAT. We have quantified BAT activation in the cervical-supraclavicular (C-SCV) region using IRT video imaging and a novel image computational algorithm by studying C-SCV heat production in healthy young men after cold stimulation and the ingestion of capsinoids in a prospective double-blind placebo-controlled randomized trial. Subjects were divided into low-BAT and high-BAT groups based on changes in IR emissions in the C-SCV region induced by cold. The high-BAT group showed significant increases in energy expenditure, fat oxidation, and heat output in the C-SCV region post-capsinoid ingestion compared to post-placebo ingestion, but the low-BAT group did not. Based on these results, we conclude that IRT is a promising tool for quantifying BAT activity.

Effects of Panax ginseng extracts prepared at different steaming times on thermogenesis in rats

BACKGROUND

Panax ginseng (PG) has a long history of use in Asian medicine because of its multiple pharmacological activities. It has been considered that PG in a type of white ginseng may induce undesirable thermogenic effects, but not in a type of red ginseng. However, there is a lack of evidence about the correlation between ginsenoside and thermogenesis.

METHODS

We investigated the effects of PG with different ginsenoside compositions on body temperature, blood pressure, and thermogenesis-related factors in rats.

RESULTS

With increasing steaming time (0 h, 3 h, 6 h, and 9 h), the production of protopanaxadiol ginsenosides increased, whereas protopanaxatriol ginsenosides decreased in white ginseng. In both short- and long-term studies, administration of four ginseng extracts prepared at different steaming times did not induce significant changes in body temperature (skin, tail, and rectum) and blood pressure of rats compared to saline control. In addition, there were no significant differences in the molecular markers related to thermogenesis (p > 0.05), mRNA expressions of peroxisome proliferator-activated receptor-gamma coactivator-1α and uncoupling protein 1 in brown adipose tissue, as well as the serum levels of interleukin-6, inducible nitric oxide synthase, and nitrite among the treatment groups.

CONCLUSION

These observations indicate that the potential undesirable effects of PG on body temperature could not be explained by the difference in ginsenoside composition.

Heat, heat waves, and out-of-hospital cardiac arrest

OBJECTIVE

Cardiac arrest is one of the common presentations of cardiovascular disorders and a leading cause of death. There are limited data on the relationship between out-of-hospital cardiac arrest (OHCA) and ambient temperatures, specifically extreme heat. This study investigated how heat and heat waves affect the occurrence of OHCA.

METHODS

Seven major cities in Korea with more than 1 million residents were included in this study. A heat wave was defined as a daily mean temperature above the 98th percentile of the yearly distribution for at least two consecutive days.

RESULTS

A total of 50,318 OHCAs of presumed cardiac origin were identified from the nationwide emergency medical service database between 2006 and 2013. Ambient temperature and OHCA had a J-shaped relationship with a trough at 28°C. Heat waves were shown to be associated with a 14-% increase in the risk of OHCA. Adverse effects were apparent from the beginning of each heat wave period and slightly increased during its continuation. Excess OHCA events during heat waves occurred between 3PM and 5PM. Subgroup analysis showed that those 65years or older were significantly more susceptible to heat waves.

CONCLUSIONS

Ambient temperature and OHCA had a J-shaped relationship. The risk of OHCA was significantly increased with heat waves. Excess OHCA events primarily occurred during the afternoon when the temperature was high. We found that the elderly were more susceptible to the deleterious effects of heat waves.

Hemodynamic Stability to Surface Warming and Cooling During Sustained and Continuous Simulated Hemorrhage in Humans

One in 10 deaths worldwide is caused by traumatic injury, and 30% to 40% of those trauma-related deaths are due to hemorrhage. Currently, warming a bleeding victim is the standard of care due to the adverse effects of combined hemorrhage and hypothermia on survival. We tested the hypothesis that heating is detrimental to the maintenance of arterial pressure and cerebral perfusion during hemorrhage, while cooling is beneficial to victims who are otherwise normothermic. Twenty-one men (31 ± 9 y) were examined under two separate protocols designed to produce central hypovolemia similar to hemorrhage. Following 15 min of supine rest, 10 min of 30 mm Hg of lower body negative pressure (LBNP) was applied. On separate randomized days, subjects were then exposed to skin surface cooling (COOL), warming (WARM), or remained thermoneutral (NEUT), while LBNP continued. Subjects remained in these thermal conditions for either 40 min of 30 mm Hg LBNP (N = 9), or underwent a continuous LBNP ramp until hemodynamic decompensation (N = 12). Arterial blood pressure during LBNP was dependent on the thermal perturbation as blood pressure was greater during COOL (P >0.001) relative to NEUT and WARM for both protocols. Middle cerebral artery blood velocity decreased (P <0.001) from baseline throughout sustained and continuous LBNP, but the magnitude of reduction did not differ between thermal conditions. Contrary to our hypothesis, WARM did not reduce cerebral blood velocity or LBNP tolerance relative to COOL and NEUT in normothermic individuals. While COOL increased blood pressure, cerebral perfusion and time to presyncope were not different relative to NEUT or WARM during sustained or continuous LBNP. Warming an otherwise normothermic hemorrhaging victim is not detrimental to hemodynamic stability, nor is this stability improved with cooling.

Cerebral Vascular Control and Metabolism in Heat Stress

This review provides an in-depth update on the impact of heat stress on cerebrovascular functioning. The regulation of cerebral temperature, blood flow, and metabolism are discussed. We further provide an overview of vascular permeability, the neurocognitive changes, and the key clinical implications and pathologies known to confound cerebral functioning during hyperthermia. A reduction in cerebral blood flow (CBF), derived primarily from a respiratory-induced alkalosis, underscores the cerebrovascular changes to hyperthermia. Arterial pressures may also become compromised because of reduced peripheral resistance secondary to skin vasodilatation. Therefore, when hyperthermia is combined with conditions that increase cardiovascular strain, for example, orthostasis or dehydration, the inability to preserve cerebral perfusion pressure further reduces CBF. A reduced cerebral perfusion pressure is in turn the primary mechanism for impaired tolerance to orthostatic challenges. Any reduction in CBF attenuates the brain's convective heat loss, while the hyperthermic-induced increase in metabolic rate increases the cerebral heat gain. This paradoxical uncoupling of CBF to metabolism increases brain temperature, and potentiates a condition whereby cerebral oxygenation may be compromised. With levels of experimentally viable passive hyperthermia (up to 39.5-40.0 °C core temperature), the associated reduction in CBF (∼ 30%) and increase in cerebral metabolic demand (∼ 10%) is likely compensated by increases in cerebral oxygen extraction. However, severe increases in whole-body and brain temperature may increase blood-brain barrier permeability, potentially leading to cerebral vasogenic edema. The cerebrovascular challenges associated with hyperthermia are of paramount importance for populations with compromised thermoregulatory control--for example, spinal cord injury, elderly, and those with preexisting cardiovascular diseases.

Regional Skin Temperature Response to Moderate Aerobic Exercise Measured by Infrared Thermography

Background:

Infrared thermography (IRT) does not require contact with the skin, and it is a convenient, reliable and non-invasive technique that can be used for monitoring the skin temperature (T_SK).

Objectives:

The aim of this study was to monitor the variations in the regional T_SK during exercise on 28 regions of interest (ROIs) (forehead, face, chest, abdomen, back, lumbar, anterior and posterior neck, and posterior and anterior views of the right and left hands, forearms, upper arms, thighs, and legs) with IRT.

Patients and Methods:

12 physically active young males were monitored with IRT during the following three phases: a) 30 minutes before exercise b) while performing one hour of moderate intensity exercise on a treadmill at 60% of the VO_2max, and c) 60 minutes after exercise.

Results:

During pre-exercise, all T_SK reached a steady-state (P ≤ 0.05), which ensured adequate thermal stabilisation. At the beginning of exercise, there was a significant reduction in the T_SK in most ROIs after 10 minutes of activity, except for the lower limbs (legs and thighs). After one hour of recovery, in the anterior view of the hands and thighs and in the posterior view of the legs, there were significant increases in the T_SK compared to pre-exercise.

Conclusions:

There were significant distinctions in the skin temperature distribution during exercise according to the activity of the area under consideration during exercise, which may be important in the development of physiological models and heat flux analyses for different purposes.

Keystone Island Flap: Effects of Islanding on Vascularity

Based on his clinical observations the “red dot sign” and hyperemic flare, Behan has advocated the superior vascularity of the island flap design for at least 2 decades. The aim of this study was to determine whether (1) surgical islanding of a flap alters the vascularity or blood supply of the flap and (2) these changes in blood supply explain Behan’s clinical observations of “red dot sign” and hyperemic flare.

Relieving thermal discomfort: Effects of sprayed L-menthol on perception, performance, and time trial cycling in the heat

L-menthol stimulates cutaneous thermoreceptors and induces cool sensations improving thermal comfort, but has been linked to heat storage responses; this could increase risk of heat illness during self-paced exercise in the heat. Therefore, L-menthol application could lead to a discrepancy between behavioral and autonomic thermoregulatory drivers. Eight male participants volunteered. They were familiarized and then completed two trials in hot conditions (33.5 °C, 33% relative humidity) where their t-shirt was sprayed with CONTROL-SPRAY or MENTHOL-SPRAY after 10 km (i.e., when they were hot and uncomfortable) of a 16.1-km cycling time trial (TT). Thermal perception [thermal sensation (TS) and comfort (TC)], thermal responses [rectal temperature (Trec ), skin temperature (Tskin )], perceived exertion (RPE), heart rate, pacing (power output), and TT completion time were measured. MENTHOL-SPRAY made participants feel cooler and more comfortable and resulted in lower RPE (i.e., less exertion) yet performance was unchanged [TT completion: CONTROL-SPRAY 32.4 (2.9) and MENTHOL-SPRAY 32.7 (3.0) min]. Trec rate of increase was 1.40 (0.60) and 1.45 (0.40) °C/h after CONTROL-SPRAY and MENTHOL-SPRAY application, which were not different. Spraying L-menthol toward the end of self-paced exercise in the heat improved perception, but did not alter performance and did not increase heat illness risk.

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.

Clinically Relevant Pharmacological Strategies That Reverse MDMA-Induced Brain Hyperthermia Potentiated by Social Interaction

MDMA-induced hyperthermia is highly variable, unpredictable, and greatly potentiated by the social and environmental conditions of recreational drug use. Current strategies to treat pathological MDMA-induced hyperthermia in humans are palliative and marginally effective, and there are no specific pharmacological treatments to counteract this potentially life-threatening condition. Here, we tested the efficacy of mixed adrenoceptor blockers carvedilol and labetalol, and the atypical antipsychotic clozapine, in reversing MDMA-induced brain and body hyperthermia. We injected rats with a moderate non-toxic dose of MDMA (9 mg/kg) during social interaction, and we administered potential treatment drugs after the development of robust hyperthermia (>2.5 °C), thus mimicking the clinical situation of acute MDMA intoxication. Brain temperature was our primary focus, but we also simultaneously recorded temperatures from the deep temporal muscle and skin, allowing us to determine the basic physiological mechanisms of the treatment drug action. Carvedilol was modestly effective in attenuating MDMA-induced hyperthermia by moderately inhibiting skin vasoconstriction, and labetalol was ineffective. In contrast, clozapine induced a marked and immediate reversal of MDMA-induced hyperthermia via inhibition of brain metabolic activation and blockade of skin vasoconstriction. Our findings suggest that clozapine, and related centrally acting drugs, might be highly effective for reversing MDMA-induced brain and body hyperthermia in emergency clinical situations, with possible life-saving results.

Thermal impact of near-infrared laser in advanced noninvasive optical brain imaging

The propagation of laser light in human tissues is an important issue in functional optical imaging. We modeled the thermal effect of different laser powers with various spot sizes and different head tissue characteristics on neonatal and adult quasirealistic head models. The photothermal effect of near-infrared laser (800 nm) was investigated by numerical simulation using finite-element analysis. Our results demonstrate that the maximum temperature increase on the brain for laser irradiance between 0.127 (1 mW) and [Formula: see text] (100 mW) at a 1 mm spot size, ranged from 0.0025°C to 0.26°C and from 0.03°C to 2.85°C at depths of 15.9 and 4.9 mm in the adult and neonatal brain, respectively. Due to the shorter distance of the head layers from the neonatal head surface, the maximum temperature increase was higher in the neonatal brain than in the adult brain. Our results also show that, at constant power, spot size changes had a lesser heating effect on deeper tissues. While the constraints for safe laser irradiation to the brain are dictated by skin safety, these results can be useful to optimize laser parameters for a variety of laser applications in the brain. Moreover, combining simulation and adequate in vitro experiments could help to develop more effective optical imaging to avoid possible tissue damage.

Sympathetic regulation during thermal stress in human aging and disease

Humans control their core temperature within a narrow range via precise adjustments of the autonomic nervous system. In response to changing core and/or skin temperature, several critical thermoregulatory reflex effector responses are initiated and include shivering, sweating, and changes in cutaneous blood flow. Cutaneous vasomotor adjustments, mediated by modulations in sympathetic nerve activity (SNA), aid in the maintenance of thermal homeostasis during cold and heat stress since (1) they serve as the first line of defense of body temperature and are initiated before other thermoregulatory effectors, and (2) they are on the efferent arm of non-thermoregulatory reflex systems, aiding in the maintenance of blood pressure and organ perfusion. This review article highlights the sympathetic responses of humans to thermal stress, with a specific focus on primary aging as well as impairments that occur in both heart disease and type 2 diabetes mellitus. Age- and pathology-related changes in efferent muscle and skin SNA during cold and heat stress, measured directly in humans using microneurography, are discussed.

Using skin temperature and muscle thickness to assess muscle response to strength training

ABSTRACT Introduction: Several studies already reported the response of many biomarkers after strength training, but studies using low cost diagnostic imaging tools are rare. Objective: To evaluate the usage of skin temperature and muscle thickness (MT) to monitor muscle response (until 96 hours after) to high-intensity strength training. Methods: This is a short-term longitudinal study with 13 trained, healthy male volunteers. Volunteers performed five sets of biceps bi-set exercise with their dominant arm with dumbbells, with load of 70% of one-repetition maximum (1RM). The ultrasound (US) and thermal images were acquired before and immediately after the last set, 24, 48, 72 and 96 hours after exercise. Results: The analysis was divided in two stages: acute muscle response (until 24 hours after training) and delayed muscle response (from 24 to 96 hours after training). The elbow flexors thickness showed the peak value immediately after the last set of training. Skin temperature (on elbow flexors) and the elbow flexors thickness grew continuously from 24 to 96 hours after strength training. There is a high correlation (r=0.941, p=0.017) between skin temperature and muscle thickness from the end of exercise until 96 hours after strength training. Conclusions: The US images showed high sensibility for muscle physiological changes on the first 24 hours after exercise. On the other hand, the thermal images had higher sensibility for muscle physiological changes than US images from 24 to 96 hours after training.

Reference breast temperature: proposal of an equation

OBJECTIVE

To develop an equation to estimate the breast reference temperature according to the variation of room and core body temperatures.

METHODS

Four asymptomatic women were evaluated for three consecutive menstrual cycles. Using thermography, the temperature of breasts and eyes was measured as indirect reference of core body and room temperatures. To analyze the thermal behavior of the breasts during the cycle, the core body and room temperatures were normalized by means of a mathematical equation.

RESULTS

We performed 180 observations and the core temperature had the highest correlation with the breast temperature, followed by room temperature. The proposed prediction model could explain 45.3% of the breast temperature variation, with variable room temperature variable; it can be accepted as a way to estimate the reference breast temperature at different room temperatures.

CONCLUSION

The average breast temperature in healthy women had a direct relation with the core and room temperature and can be estimated mathematically. It is suggested that an equation could be used in clinical practice to estimate the normal breast reference temperature in young women, regardless of the day of the cycle, therefore assisting in evaluation of anatomical studies.

Dehydration accelerates reductions in cerebral blood flow during prolonged exercise in the heat without compromising brain metabolism

Reductions in cerebral blood flow and extracranial perfusion, induced by dehydration during prolonged exercise in the heat, may be coupled to fatigue. However, cerebral metabolism remains stable through enhanced O₂ and glucose extraction. Thus, fatigue developed during prolonged exercise with dehydration is related to reductions in cerebral blood flow rather than to the cerebral metabolic rate for O₂.

Dehydration hastens the decline in cerebral blood flow (CBF) during incremental exercise, whereas the cerebral metabolic rate for O₂ (CMR_O2) is preserved. It remains unknown whether CMR_O2 is also maintained during prolonged exercise in the heat and whether an eventual decline in CBF is coupled to fatigue. Two studies were undertaken. In study 1, 10 male cyclists cycled in the heat for ∼2 h with (control) and without fluid replacement (dehydration) while internal and external carotid artery blood flow and core and blood temperature were obtained. Arterial and internal jugular venous blood samples were assessed with dehydration to evaluate CMR_O2. In study 2, in 8 male subjects, middle cerebral artery blood velocity was measured during prolonged exercise to exhaustion in both dehydrated and euhydrated states. After a rise at the onset of exercise, internal carotid artery flow declined to baseline with progressive dehydration (P < 0.05). However, cerebral metabolism remained stable through enhanced O₂ and glucose extraction (P < 0.05). External carotid artery flow increased for 1 h but declined before exhaustion. Fluid ingestion maintained cerebral and extracranial perfusion throughout nonfatiguing exercise. During exhaustive exercise, however, euhydration delayed but did not prevent the decline in cerebral perfusion. In conclusion, during prolonged exercise in the heat, dehydration accelerates the decline in CBF without affecting CMR_O2 and also restricts extracranial perfusion. Thus, fatigue is related to a reduction in CBF and extracranial perfusion rather than CMR_O2.

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.

Peripheral microcirculatory hemodynamic changes in patients with myocardial ischemia

BACKGROUND

Patients with coronary heart disease demonstrate changes in skin microcirculation and a decrease in cutaneous blood mass.

OBJECTIVE

The goal of this study was to assess the feasibility of diagnosing myocardial ischemia based on peripheral microcirculatory variables.

METHODS

The skin microcirculatory measurements were monitored using an LPT system comprising a Laser Doppler Flowmeter (LDF), a photoplethysmograph (PPG) and a transcutaneous oxygen tension device (tc-PO2). Concurrently, heart rate and blood pressure were monitored. Measurements were performed before and after exercise stress test. Subjects were divided into ischemic (20) and nonischemic (27) patients based on myocardial perfusion imaging (MPI).

RESULTS

The results indicate differences in LPT variables between ischemic and nonischemic patients following exercise, while no differences in the central variable values were observed between the two groups.

CONCLUSIONS

Peripheral microcirculatory variables may be useful for non-invasive assessment of myocardial ischemia. The system has clinical potential for sensitive and noninvasive monitoring of vital variables during medical procedures in clinics, as well as in home care for patients who suffer from ischemic cardiac diseases.

Immediate effects of an end-range mobilization technique on shoulder range of motion and skin temperature in individuals with posterior shoulder tightness

[Purpose] This study investigated the effects of an end-range mobilization technique on the range of motion of the glenohumeral internal rotation and the skin temperature of the shoulder in individuals with posterior shoulder tightness. [Subjects] Thirteen subjects with posterior shoulder tightness who had glenohumeral internal rotation deficit ≥ 15° participated. [Methods] All subjects underwent glenohumeral joint end-range mobilization intervention. The internal rotation range of motion of the glenohumeral joint was measured by a goniometer and the shoulder skin temperature was measured by a digital infrared thermographic imaging device before and immediately after the intervention. Paired t-tests were used to analyze the differences in these parameter pre and post-intervention. [Results] The glenohumeral internal rotation range of motion and skin temperature of the posterolateral shoulder in increased significantly post-intervention. [Conclusion] The end-range mobilization technique is effective for increasing the glenohumeral internal rotation range of motion and skin temperature of the shoulder in individuals with posterior shoulder tightness.

Abnormal thermography in Parkinson's disease

BACKGROUND

An autonomic denervation and abnormal vasomotor reflex in the skin have been described in Parkinson's disease (PD) and might be evaluable using thermography with cold stress test.

METHODS

A cross-sectional pilot study was undertaken in 35 adults: 15 patients with PD and abnormal [(123)I]-metaiodobenzylguanidine cardiac scintigraphy and 20 healthy controls. Baseline thermography of both hands was obtained before immersing one in cold water (3 ± 1 °C) for 2 min. Continuous thermography was performed in: non-immersed hand (right or with lesser motor involvement) during immersion of the contralateral hand and for 6 min afterward; and contralateral immersed hand for 6 min post-immersion. The region of interest was the dorsal skin of the third finger, distal phalanx.

RESULTS

PD patients showed a lower mean baseline hand temperature (p = 0.037) and greater thermal difference between dorsum of wrist and third finger (p = 0.036) and between hands (p = 0.0001) versus controls, regardless of the motor laterality. Both tests evidenced an adequate capacity to differentiate between groups: in the non-immersed hand, the PD patients did not show the normal cooling pattern or final thermal overshoot observed in controls (F = 5.29; p = 0.001), and there was an AUC of 0.897 (95%CI 0.796-0.998) for this cooling; in the immersed hand, thermal recovery at 6 min post-immersion was lesser in patients (29 ± 17% vs. 55 ± 28%, p = 0.002), with an AUC of 0.810 (95%CI 0.662-0.958).

CONCLUSIONS

PD patients reveal abnormal skin thermal responses in thermography with cold stress test, suggesting cutaneous autonomic dysfunction. This simple technique may be useful to evaluate autonomic dysfunction in PD.

Sodium channel Nav1.7 in vascular myocytes, endothelium, and innervating axons in human skin

BACKGROUND

The skin is a morphologically complex organ that serves multiple complementary functions, including an important role in thermoregulation, which is mediated by a rich vasculature that is innervated by sympathetic and sensory endings. Two autosomal dominant disorders characterized by episodes of severe pain, inherited erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD) have been directly linked to mutations that enhance the function of sodium channel Nav1.7. Pain attacks are accompanied by reddening of the skin in both disorders. Nav1.7 is known to be expressed at relatively high levels within both dorsal root ganglion (DRG) and sympathetic ganglion neurons, and mutations that enhance the activity of Nav1.7 have been shown to have profound effects on the excitability of both cell-types, suggesting that dysfunction of sympathetic and/or sensory fibers, which release vasoactive peptides at skin vasculature, may contribute to skin reddening in IEM and PEPD.

RESULTS

In the present study, we demonstrate that smooth muscle cells of cutaneous arterioles and arteriole-venule shunts (AVS) in the skin express sodium channel Nav1.7. Moreover, Nav1.7 is expressed by endothelial cells lining the arterioles and AVS and by sensory and sympathetic fibers innervating these vascular elements.

CONCLUSIONS

These observations suggest that the activity of mutant Nav1.7 channels in smooth muscle cells of skin vasculature and innervating sensory and sympathetic fibers contribute to the skin reddening and/or pain in IEM and PEPD.

Local temperature-sensitive mechanisms are important mediators of limb tissue hyperemia in the heat-stressed human at rest and during small muscle mass exercise

Limb tissue and systemic blood flow increases with heat stress, but the underlying mechanisms remain poorly understood. Here, we tested the hypothesis that heat stress-induced increases in limb tissue perfusion are primarily mediated by local temperature-sensitive mechanisms. Leg and systemic temperatures and hemodynamics were measured at rest and during incremental single-legged knee extensor exercise in 15 males exposed to 1 h of either systemic passive heat-stress with simultaneous cooling of a single leg (n = 8) or isolated leg heating or cooling (n = 7). Systemic heat stress increased core, skin and heated leg blood temperatures (T_b), cardiac output, and heated leg blood flow (LBF; 0.6 ± 0.1 l/min; P < 0.05). In the cooled leg, however, LBF remained unchanged throughout (P > 0.05). Increased heated leg deep tissue blood flow was closely related to T_b (R² = 0.50; P < 0.01), which is partly attributed to increases in tissue V̇O₂ (R² = 0.55; P < 0.01) accompanying elevations in total leg glucose uptake (P < 0.05). During isolated limb heating and cooling, LBFs were equivalent to those found during systemic heat stress (P > 0.05), despite unchanged systemic temperatures and hemodynamics. During incremental exercise, heated LBF was consistently maintained ∼0.6 l/min higher than that in the cooled leg (P < 0.01), with LBF and vascular conductance in both legs showing a strong correlation with their respective local T_b (R² = 0.85 and 0.95, P < 0.05). We conclude that local temperature-sensitive mechanisms are important mediators in limb tissue perfusion regulation both at rest and during small-muscle mass exercise in hyperthermic humans.

Forearm cutaneous vascular and sudomotor responses to whole body passive heat stress in young smokers

The purpose of this study was to compare smokers and nonsmokers' sudomotor and cutaneous vascular responses to whole body passive heat stress. Nine regularly smoking (SMK: 29 ± 9 yr; 10 ± 6 cigarettes/day) and 13 nonsmoking (N-SMK: 27 ± 8 yr) males were passively heated until core temperature (TC) increased 1.5°C from baseline. Forearm local sweat rate (LSR) via ventilated capsule, sweat gland activation (SGA), sweat gland output (SGO), and cutaneous vasomotor activity via laser-Doppler flowmetry (CVC) were measured as mean body temperature increased (ΔTb) during passive heating using a water-perfused suit. Compared with N-SMK, SMK had a smaller ΔTb at the onset of sweating (0.52 ± 0.19 vs. 0.35 ± 0.14°C, respectively; P = 0.03) and cutaneous vasodilation (0.61 ± 0.21 vs. 0.31 ± 0.12°C, respectively; P < 0.01). Increases in LSR and CVC per °C ΔTb (i.e., sensitivity) were similar in N-SMK and SMK (LSR: 0.63 ± 0.21 vs. 0.60 ± 0.40 Δmg/cm(2)/min/°C ΔTb, respectively, P = 0.81; CVC: 82.5 ± 46.2 vs. 58.9 ± 23.3 Δ%max/°C ΔTb, respectively; P = 0.19). However, the plateau in LSR during whole body heating was higher in N-SMK vs. SMK (1.00 ± 0.13 vs. 0.79 ± 0.26 mg·cm(-2)·min(-1); P = 0.03), which was likely a result of higher SGO (8.94 ± 3.99 vs. 5.94 ± 3.49 μg·gland(-1)·min(-1), respectively; P = 0.08) and not number of SGA (104 ± 7 vs. 121 ± 9 glands/cm(2), respectively; P = 0.58). During whole body passive heat stress, smokers had an earlier onset for forearm sweating and cutaneous vasodilation, but a lower local sweat rate that was likely due to lower sweat output per gland. These data provide insight into local (i.e., forearm) thermoregulatory responses of young smokers during uncompensatory whole body passive heat stress.

Elevation in blood flow and shear rate prevents hyperglycemia-induced endothelial dysfunction in healthy subjects and those with type 2 diabetes

Hyperglycemia, commonly present after a meal, causes transient impairment in endothelial function. We examined whether increases in blood flow (BF) protect against the hyperglycemia-mediated decrease in endothelial function in healthy subjects and patients with type 2 diabetes mellitus (T2DM). Ten healthy subjects and 10 age- and sex-matched patients with T2DM underwent simultaneous bilateral assessment of brachial artery endothelial function by means of flow-mediated dilation (FMD) using high-resolution echo-Doppler. FMD was examined before and 60, 120, and 150 min after a 75-g oral glucose challenge. We unilaterally manipulated BF by heating one arm between minute 30 and minute 60. Oral glucose administration caused a statistically significant, transient increase in blood glucose in both groups (P < 0.001). Forearm skin temperature, brachial artery BF, and shear rate significantly increased in the heated arm (P < 0.001), and to a greater extent compared with the nonheated arm in both groups (interaction effect P < 0.001). The glucose load caused a transient decrease in FMD% (P < 0.05), whereas heating significantly prevented the decline (interaction effect P < 0.01). Also, when correcting for changes in diameter and shear rate, we found that the hyperglycemia-induced decrease in FMD can be prevented by local heating (P < 0.05). These effects on FMD were observed in both groups. Our data indicate that nonmetabolically driven elevation in BF and shear rate can similarly prevent the hyperglycemia-induced decline in conduit artery endothelial function in healthy volunteers and in patients with type 2 diabetes. Additional research is warranted to confirm that other interventions that increase BF and shear rate equally protect the endothelium when challenged by hyperglycemia.

A vascular mechanistic approach to understanding Raynaud phenomenon

During exposure to cold, our bodies attempt to maintain normal core temperature by restricting heat loss through cutaneous vasoconstriction, and by increasing heat production through shivering and nonshivering thermogenesis. In selected areas of human skin (including on the fingers and toes), the vascular system has specialized structural and functional features that enable it to contribute to thermoregulation. These features include arteriovenous anastomoses, which directly connect the arterial and venous systems and bypass the nutritional capillaries supplying blood to the skin tissue. Of note, Raynaud phenomenon predominantly affects the arterial territories supplying these specialized areas of skin. Indeed, Raynaud phenomenon can be considered a disorder of vascular thermoregulatory control. This Review presents an understanding of Raynaud phenomenon in the context of vascular and thermoregulatory control mechanisms, including the role of unique thermosensitive vascular structural and functional specialization, and describes the potential role of thermogenesis in this disorder. This new approach provides remarkable insight into the disease process and builds a framework to critically appraise the existing knowledge base. This paradigm also explains the deficiencies in some current therapeutic approaches, and highlights new areas of potential relevance to the pathogenesis and treatment of Raynaud phenomenon that should be expanded and explored.

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.

Performance in the heat-physiological factors of importance for hyperthermia-induced fatigue

This article presents a historical overview and an up-to-date review of hyperthermia-induced fatigue during exercise in the heat. Exercise in the heat is associated with a thermoregulatory burden which mediates cardiovascular challenges and influence the cerebral function, increase the pulmonary ventilation, and alter muscle metabolism; which all potentially may contribute to fatigue and impair the ability to sustain power output during aerobic exercise. For maximal intensity exercise, the performance impairment is clearly influenced by cardiovascular limitations to simultaneously support thermoregulation and oxygen delivery to the active skeletal muscle. In contrast, during submaximal intensity exercise at a fixed intensity, muscle blood flow and oxygen consumption remain unchanged and the potential influence from cardiovascular stressing and/or high skin temperature is not related to decreased oxygen delivery to the skeletal muscles. Regardless, performance is markedly deteriorated and exercise-induced hyperthermia is associated with central fatigue as indicated by impaired ability to sustain maximal muscle activation during sustained contractions. The central fatigue appears to be influenced by neurotransmitter activity of the dopaminergic system, but inhibitory signals from thermoreceptors arising secondary to the elevated core, muscle and skin temperatures and augmented afferent feedback from the increased ventilation and the cardiovascular stressing (perhaps baroreceptor sensing of blood pressure stability) and metabolic alterations within the skeletal muscles are likely all factors of importance for afferent feedback to mediate hyperthermia-induced fatigue during submaximal intensity exercise. Taking all the potential factors into account, we propose an integrative model that may help understanding the interplay among factors, but also acknowledging that the influence from a given factor depends on the exercise hyperthermia situation.

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.

Reproductive hormone influences on thermoregulation in women

The present discussion reviews current knowledge regarding influences of the primary reproductive hormones on mechanisms of thermoregulatory control in women. The human body is remarkably capable of maintaining body temperature within a few tenths of a degree of normal (37°C) over a wide range of activity and environmental exposures; this regulation is accomplished via integration of central and peripheral thermal information at the preoptic area of the anterior hypothalamus (PO/AH). We describe both central and peripheral mechanisms involved in controlling thermoregulation in humans, and how these mechanisms are affected by sex and hormone exposure. Estrogens generally promote vasodilation, heat dissipation, and lower body temperature and progesterone or progestins generally have the opposite effect. Estrogens and progesterone/progestins can also interact with androgens; this is an important point because androgens in the body can increase in both older and younger women. The study of reproductive hormone (estrogens, progesterone, luteinizing, and follicle stimulating hormones) effects on body systems is challenging because of the complex and multifaceted influences of these hormones, both individually and in combination. Thus, a number of methods to alter hormone exposure are explained in this article. We conclude that men and women do not exhibit major quantitative differences in physiological thermoregulatory responses to exercise and/or body heating when factors such as fitness and body size are taken into account. However, female and male reproductive hormones have important influences that can significantly alter individual thermoregulatory responses at various points throughout the lifespan.

Brown adipose tissue improves whole-body glucose homeostasis and insulin sensitivity in humans

Brown adipose tissue (BAT) has attracted scientific interest as an antidiabetic tissue owing to its ability to dissipate energy as heat. Despite a plethora of data concerning the role of BAT in glucose metabolism in rodents, the role of BAT (if any) in glucose metabolism in humans remains unclear. To investigate whether BAT activation alters whole-body glucose homeostasis and insulin sensitivity in humans, we studied seven BAT-positive (BAT(+)) men and five BAT-negative (BAT(-)) men under thermoneutral conditions and after prolonged (5-8 h) cold exposure (CE). The two groups were similar in age, BMI, and adiposity. CE significantly increased resting energy expenditure, whole-body glucose disposal, plasma glucose oxidation, and insulin sensitivity in the BAT(+) group only. These results demonstrate a physiologically significant role of BAT in whole-body energy expenditure, glucose homeostasis, and insulin sensitivity in humans, and support the notion that BAT may function as an antidiabetic tissue in humans.

Assessment of skin blood flow following spinal manual therapy: a systematic review

Skin blood flow (SBF) indexes have been used to describe physiological mechanisms associated with spinal manual therapy (SMT). The aims of the current review were to assess methods for data collection, assess how investigators interpreted SBF changes, and formulate recommendations to advance manual medicine research. A database search was performed in PubMed, Cochrane Library, the Physiotherapy Evidence Database, and the Cumulative Index to Nursing and Allied Health Literature through April 2014. Articles were included if at least 1 outcome measure was changes in 1 SBF index following SMT. The database search yielded 344 records. Two independent authors applied the inclusion criteria. Twenty studies met the inclusion criteria. Selected studies used heterogeneous methods to assess short-term post-SMT changes in SBF, usually vasoconstriction, which was interpreted as a general sympathoexcitatory effect through central mechanisms. However, this conclusion might be challenged by the current understanding of skin sympathetic nervous activity over local endothelial mechanisms that are specifically controlling SBF. Evaluation of SBF measurements in peripheral tissues following SMT may document physiological responses that are beyond peripheral sympathetic function. Based on the current use of SBF indexes in clinical and physiological research, 14 recommendations for advancing manual medicine research using laser Doppler flowmetry are presented.