Effect of whole body heat stress on peripheral vasoconstriction during leg dependency

Effect of reflex and mechanical decreases in skin perfusion on thermal- and agonist-induced eccrine sweating in humans

In humans, skin blood flux (SkBF) and eccrine sweating are tightly coupled, suggesting common neural control and regulation. This study was designed to separate these two sympathetic nervous system end-organ responses via nonadrenergic SkBF-decreasing mechanical perturbations during heightened sudomotor drive. We induced sweating physiologically via whole body heat stress using a high-density tube-lined suit (protocol 1; 2 women, 4 men), and pharmacologically via forearm intradermal microdialysis of two steady-state doses of a cholinergic agonist, pilocarpine (protocol 2; 4 women, 3 men). During sweating induction, we decreased SkBF via three mechanical perturbations: arm and leg dependency to engage the cutaneous venoarteriolar response (CVAR), limb venous occlusion to engage the CVAR and decrease perfusion pressure, and limb arterial occlusion to cause ischemia. In protocol 1, heat stress increased arm cutaneous vascular conductance and forearm sweat rate (capacitance hygrometry). During heat stress, despite decreases in SkBF during each of the acute (3 min) mechanical perturbations, eccrine sweat rate was unaffected. During heat stress with extended (10 min) ischemia, sweat rate decreased. In protocol 2, both pilocarpine doses (ED50 and EMAX) increased SkBF and sweat rate. Each mechanical perturbation resulted in decreased SkBF but minimal changes in eccrine sweat rate. Taken together, these data indicate that a wide range of acute decreases in SkBF do not appear to proportionally decrease either physiologically- or pharmacologically induced eccrine sweating in peripheral skin. This preservation of evaporative cooling despite acutely decreased SkBF could have consequential impacts for heat storage and balance during changes in body posture, limb position, or blood flow restrictive conditions.

Ingestion of carbonated water increases middle cerebral artery blood velocity and improves mood states in resting humans exposed to ambient heat stress

Sugar-free carbonated water is consumed worldwide. The consumption of carbonated water is high in summer, when the heat loss responses of sweating and skin vasodilation are activated, and thermal perceptions (thermal sensation and comfort) and mood states are negatively modulated. However, whether ingesting carbonated water under ambient heat exposure modulates cerebral blood flow index, heat loss responses, thermal perceptions, and mood states remains to be determined. In this study, 17 healthy, habitually active, young adults (eight women) ingested 4 °C noncarbonated or carbonated water under 37 °C ambient heat-stressed resting conditions. Both drinks increased the middle cerebral artery mean blood velocity, an index of cerebral blood flow, and mean arterial pressure, with carbonated water exhibiting higher elevations than noncarbonated water (P < 0.05). However, the heart rate, sweat rate, and skin blood flow during and after drinking remained unchanged between the two conditions (P > 0.05). The thermal sensation and comfort after drinking remained unchanged between the two conditions (P > 0.05); but, a drink-induced reduction in sleepiness was higher, and drink-induced elevations in motivation and exhilaration were higher after ingesting carbonated water than those after ingesting noncarbonated water (P < 0.05). The analyses suggest that in humans under ambient heat-stressed resting conditions, ingestion of cold carbonated water increases the cerebral blood flow index, blood pressure, motivation, and exhilaration, whereas it decreases sleepiness relative to ingestion of noncarbonated cold water. However, ingestion of cold carbonated water fails to modulate thermoregulatory responses and thermal perception as opposed to noncarbonated cold water.

Variability of peripheral pulse wave velocity in patients with diabetes mellitus type 2 during orthostatic challenge

Diabetes mellitus 2 (DM2) is the seventh cause of death worldwide. One of the reasons is late diagnosis of vascular damage. Pulse wave velocity (PWV) has become an independent marker of arterial stiffness and cardiovascular risk. Moreover, the previous studies have shown the importance of beat-to-beat PWV measurement due to its variability among the heart cycle. However, variability of PWV (PWVv) of the whole body hasn't been examined yet. We have studied a group of DM II and heathy volunteers, to investigate the beat-to-beat mean PWV (PWVm) and PWVv in the different body positions. PWV of left lower and upper extremities were measured in DM2 (7 m/8 f, age 68+/-10 years, BP 158/90+/-19/9 mm Hg) and healthy controls (5 m/6 f, age 23+/-2 years, BP 117/76+/-9/5 mm Hg). Volunteers were lying in the resting position and of head-up-tilt in 45° (HUT) for 6 min. PWVv was evaluated as a mean power spectrum in the frequency bands LF and HF (0.04-0.15 Hz, 0.15-0.5 Hz). Resting PWVm of upper extremity was higher in DM2. HUT increased lower extremity PWVm only in DM2. Extremities PWVm ratio was significantly lower in DM2 during HUT compared to controls. LF and HF PWVv had the same response to HUT. Resting PWVv was higher in DM2. Lower extremity PWVv increased during HUT in both groups. PWVm and PWVv in DM2 differed between extremities and were significantly influenced by postural changes due to hydrostatic pressure. Increased resting PWVm and PWVv in DM2 is a marker of increased arterial stiffness.

Intradermal Administration of Atrial Natriuretic Peptide Attenuates Cutaneous Vasodilation but Not Sweating in Young Men during Exercise in the Heat

INTRODUCTION

Prolonged exercise in the heat stimulates plasma release of atrial natriuretic peptide (ANP) in association with dehydration-induced reductions in blood volume. Elevated plasma ANP levels under these conditions may indirectly attenuate cutaneous blood flow and sweating responses due to the effects of this hormone on central blood volume and plasma osmolality and the resulting stimulation of nonthermal reflexes. However, it remains unclear whether cutaneous blood flow and sweating are directly modulated by ANP at the level of the cutaneous end organs (cutaneous microvessels and eccrine sweat glands) during prolonged exercise in the heat.

OBJECTIVE

Therefore, we evaluated the effects of local ANP administration on forearm cutaneous vascular conductance (CVC) and local sweat rate (LSR) during rest and exercise in the heat.

METHODS

In 9 habitually active young men (26 ± 6 years) CVC and LSR were evaluated at 3 intradermal microdialysis sites continuously perfused with lactated Ringer solution (control) or ANP (0.1 or 1.0 μM). Participants rested in a non-heat-stress condition (25°C) for approximately 60 min followed by 70 min in the heat (35°C). They then performed 50 min of moderate-intensity cycling (approx. 55% VO2 peak), with a 30-min recovery. Thereafter, 50 mM sodium nitroprusside was administered at all sites to elicit maximum CVC, which was subsequently used to normalize all values (CVC%max).

RESULTS

No effects of ANP on CVC%max were observed in the non-heat-stress resting condition compared to the untreated control site (both p > 0.05). Conversely during rest in the heat there was an 11% (5-17%) reduction in CVC%max at the 1.0 μM ANP site relative to the untreated control site (p < 0.05). At the end of exercise CVC%max was attenuated by 12% (1-23%) at the 0.1 μM ANP site and by 21% (7-35%) at the 1.0 μM ANP site relative to the untreated control site (all p < 0.05). Conversely, neither concentration of ANP influenced sweating at any time point (all p > 0.05).

CONCLUSION

Intradermal ANP administration directly attenuated cutaneous blood flow, but not sweating, in habitually active young men during rest and exercise in the heat.

KCa and KV channels modulate the venoarteriolar reflex in non-glabrous human skin with no roles of KATP channels, NOS, and COX

The venoarteriolar reflex is a local mechanism that induces vasoconstriction during venous congestion in various tissues, including skin. This response is thought to play a critical role in minimizing capillary damage or edema resulting from overperfusion, though factors that modulate this response remain largely unknown. Here, we hypothesized that nitric oxide synthase (NOS), cyclooxygenase (COX), and Ca²⁺-activated, ATP-sensitive, and voltage-gated K⁺ channels (K_Ca, K_ATP, and K_V channels, respectively) modulate the venoarteriolar reflex in human skin. Cutaneous blood flow (laser-Doppler flowmetry) was monitored during a 3-min pre-occlusion baseline and following a 3-min venous occlusion of 45 mmHg, the latter maneuver was used to induce the venoarteriolar reflex. The venoarteriolar reflex was assessed at the following forearm skin sites: Experiment 1 (n = 11): 1) lactated Ringer solution (Control), 2) 10 mM N^ω-nitro-_L-arginine (NOS inhibitor), 3) 10 mM ketorolac (COX inhibitor), and 4) combined NOS + COX inhibition; Experiment 2 (n = 15): 1) lactated Ringer solution (Control), 2) 50 mM tetraethylammonium (K_Ca channel blocker), 3) 5 mM glybenclamide (K_ATP channel blocker), and 4) 10 mM 4-aminopyridine (K_V channel blocker). Separate and combined NOS and COX inhibition as well as K_ATP channel blocker had no effect on venoarteriolar reflex. Conversely, venoarteriolar reflex was attenuated by K_Ca channel blockade (36-38%) and augmented by K_V channel blockade (38-55%). We showed that K_Ca and K_V channels modulate the venoarteriolar reflex with minimum roles of NOS, COX, and K_ATP channels in human non-glabrous forearm skin in vivo. Thus, cutaneous venoarteriolar reflex changes could reflect altered K⁺ channel function.

Augmented venoarteriolar response with ageing is associated with morning blood pressure surge

NEW FINDINGS

What is the central question of this study? The venoarteriolar response (VAR) contributes substantially to the maintenance of orthostatic tolerance in humans. Despite its importance in haemodynamic homeostasis, the impact of ageing on the VAR remains understudied. What is the main finding and its importance? Older adults exhibit an augmented VAR in response to leg dependency. The age-related augmentation of the VAR might be linked with progressive increases of peripheral vascular resistance with ageing. We found a modest but significant correlation between the leg VAR and the morning blood pressure surge in older adults. Augmented leg VAR might contribute to the blood pressure elevation in the early morning.

ABSTRACT

The venoarteriolar response (VAR) is a non-adrenergic, non-baroreflex-mediated mechanism of vasoconstriction, which has been proposed to contribute ∼45% of the increase in total peripheral resistance during orthostasis. Despite its importance in human cardiovascular control during orthostatic stress, there is no information available regarding the impact of age and sex on the VAR or its role in diurnal blood pressure (BP) variation. We studied 33 (15 women) young (mean ± SD; 28 ± 4 years old) and 26 (12 women) older (71 ± 3 years old) healthy individuals. Brachial and femoral blood flow were measured using Doppler ultrasound. The percentage reduction in vascular conductance (blood flow/mean BP) during 4 min of limb dependency (35-40 cm below the heart level) was used to assess the VAR. The morning surge in BP was assessed using 24 h ambulatory BP monitoring. Peak VAR in the lower limb, but not in the upper limb, was significantly higher in the older than the younger adults (33 ± 4 versus 26 ± 6%, older versus young; P < 0.05). There was no sex difference in the VAR in either the young or the older group. A greater leg VAR was related to a greater morning surge in BP in older adults (r = -0.4, P = 0.02) but not in the young adults (r = -0.26, P = 0.1). Thus, advancing age enhances the VAR in the lower limb and is associated with the morning blood pressure surge in older adults. Sex does not affect this local axonal reflex in healthy humans.

Prolonged standing increases lower limb arterial stiffness

PURPOSE

Standing workstations have recently been promoted as a healthy alternative to sitting. However, it is unknown how prolonged standing affects arterial stiffness, a prognostic indicator of cardiovascular health. The purpose of this study was twofold: to observe changes in arterial stiffness, as assessed by pulse wave velocity (PWV), with a 2-h bout of standing, and to determine if short, intermittent walking bouts provide a comparative advantage to standing alone.

METHODS

Nineteen adults had arterial stiffness assessed by pulse wave velocity. Central (C_PWV), upper peripheral (U_PWV), and lower peripheral (L_PWV) PWV were assessed before (supine), during standing (min 10, 60, and 120), and after (supine) the 2-h standing bout. In one trial, the participants stood at a standing desk immobile for 2 h. In the other trial, participants performed 5-min walking breaks after every 25 min of standing.

RESULTS

After 2-h of standing, supine (85.8 ± 90.1 cm/s) and standing (303.4 ± 390.2 cm/s), L_PWV increased independent of trial (i.e., main effect of time; p < 0.001). Walking breaks during 2 h of standing did not significantly attenuate these changes. In addition, standing C_PWV decreased over time (- 38.5 ± 61.5 cm/s; p = 0.04). Yet, U_PWV, standing or supine, did not change over the course of standing (p > 0.05).

CONCLUSIONS

These findings indicate that prolonged standing increases the measures of arterial stiffness and there is no evidence that walk breaks attenuate this response.

Measuring and quantifying skin sympathetic nervous system activity in humans

Development of the technique of microneurography has substantially increased our understanding of the function of the sympathetic nervous system (SNS) in health and in disease. The ability to directly record signals from peripheral autonomic nerves in conscious humans allows for qualitative and quantitative characterization of SNS responses to specific stimuli and over time. Furthermore, distinct neural outflow to muscle (MSNA) and skin (SSNA) can be delineated. However, there are limitations and caveats to the use of microneurography, measurement criteria, and signal analysis and interpretation. MSNA recordings have a longer history and are considered relatively more straightforward from a measurement and analysis perspective. This brief review provides an overview of the development of the technique as used to measure SSNA. The focus is on the utility of measuring sympathetic activity directed to the skin, the unique issues related to analyzing and quantifying multiunit SSNA, and the challenges related to its interpretation.

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.

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.

Mechanisms of orthostatic intolerance during heat stress

Heat stress profoundly and unanimously reduces orthostatic tolerance. This review aims to provide an overview of the numerous and multifactorial mechanisms by which this occurs in humans. Potential causal factors include changes in arterial and venous vascular resistance and blood distribution, and the modulation of cardiac output, all of which contribute to the inability to maintain cerebral perfusion during heat and orthostatic stress. A number of countermeasures have been established to improve orthostatic tolerance during heat stress, which alleviate heat stress induced central hypovolemia (e.g., volume expansion) and/or increase peripheral vascular resistance (e.g., skin cooling). Unfortunately, these countermeasures can often be cumbersome to use with populations prone to syncopal episodes. Identifying the mechanisms of inter-individual differences in orthostatic intolerance during heat stress has proven elusive, but could provide greater insights into the development of novel and personalized countermeasures for maintaining or improving orthostatic tolerance during heat stress. This development will be especially impactful in occuational settings and clinical situations that present with orthostatic intolerance and/or central hypovolemia. Such investigations should be considered of vital importance given the impending increased incidence of heat events, and associated cardiovascular challenges that are predicted to occur with the ensuing changes in climate.

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.

The magnitude of heat stress-induced reductions in cerebral perfusion does not predict heat stress-induced reductions in tolerance to a simulated hemorrhage

The mechanisms responsible for heat stress-induced reductions in tolerance to a simulated hemorrhage are unclear. Although a high degree of variability exists in the level of reduction in tolerance amongst individuals, syncope will always occur when cerebral perfusion is inadequate. This study tested the hypothesis that the magnitude of reduction in cerebral perfusion during heat stress is related to the reduction in tolerance to a lower body negative pressure (LBNP) challenge. On different days (one during normothermia and the other after a 1.5°C rise in internal temperature), 20 individuals were exposed to a LBNP challenge to presyncope. Tolerance was quantified as a cumulative stress index, and the difference in cumulative stress index between thermal conditions was used to categorize individuals most (large difference) and least (small difference) affected by the heat stress. Cerebral perfusion, as indexed by middle cerebral artery blood velocity, was reduced during heat stress compared with normothermia (P < 0.001); however, the magnitude of reduction did not differ between groups (P = 0.51). In the initial stage of LBNP during heat stress (LBNP 20 mmHg), middle cerebral artery blood velocity and end-tidal PCO(2) were lower; whereas, heart rate was higher in the large difference group compared with small difference group (P < 0.05 for all). These data indicate that variability in heat stress-induced reductions in tolerance to a simulated hemorrhage is not related to reductions in cerebral perfusion in this thermal condition. However, responses affecting cerebral perfusion during LBNP may explain the interindividual variability in tolerance to a simulated hemorrhage when heat stressed.

Sweat loss during heat stress contributes to subsequent reductions in lower-body negative pressure tolerance

The contribution of sweating to heat stress-induced reductions in haemorrhagic tolerance is not known. This study tested the hypothesis that fluid loss due to sweating contributes to reductions in simulated haemorrhagic tolerance in conditions of heat stress. Eight subjects (35 ± 8 years old; 77 ± 5 kg) underwent a normothermic time control and two heat stress trials (randomized). The two heat stress trials were as follows: (i) with slow intravenous infusion of lactated Ringer solution sufficient to offset sweat loss (IV trial); or (ii) without intravenous infusion (dehydration; DEH trial). Haemorrhage was simulated via progressive lower-body negative pressure (LBNP) to presyncope after core body (intestinal) temperature was raised by ~1.5 °C using a water-perfused suit or a normothermic time control period. The LBNP tolerance was quantified via a cumulative stress index. Middle cerebral artery blood velocity (transcranial Doppler) and mean blood pressure (Finometer®) were measured continuously. Relative changes in plasma volume were calculated from haematocrit and haemoglobin. Increases in core body temperature and sweat loss (~1.6% body mass deficit) were similar (P > 0.05) between heat stress trials. Slow intravenous infusion (1.2 ± 0.3 litres) prevented heat-induced reductions in plasma volume (IV trial, -0.6 ± 6.1%; and DEH trial, -6.6 ± 5.1%; P = 0.01). Intravenous infusion improved LBNP tolerance (632 ± 64 mmHg min) by ~20% when compared with the DEH trial (407 ± 117 mmHg min; P = 0.01), yet tolerance remained 44% lower in the IV trial relative to the time control normothermic trial (1138 ± 183 mmHg min; P < 0.01). These data indicate that although sweat-induced dehydration impairs simulated haemorrhagic tolerance, this impairment is secondary to the negative impact of heat stress itself.

Colloid volume loading does not mitigate decreases in central blood volume during simulated haemorrhage while heat stressed

Heat stress results in profound reductions in the capacity to withstand a simulated haemorrhagic challenge; however, this capacity is normalized if the individual is volume loaded prior to the challenge. The present study tested the hypothesis that volume loading during passive heat stress attenuates the reduction in regional blood volumes during a simulated haemorrhagic challenge imposed via lower-body negative pressure (LBNP). Seven subjects underwent 30 mmHg LBNP while normothermic, during passive heat stress (increased internal temperature ∼1◦C), and while continuing to be heated after intravenous colloid volume loading (11 ml kg⁻¹). Relative changes in torso and regional blood volumes were determined by gamma camera imaging with technetium-99m labelled erythrocytes. Heat stress reduced blood volume in all regions (ranging from 7 to 16%), while subsequent volume loading returned those values to normothermic levels. While normothermic,LBNP reduced blood volume in all regions (torso: 22 ± 8%; heart: 18 ± 6%; spleen: 15 ± 8%). During LBNP while heat stressed, the reductions in blood volume in each region were markedly greater when compared to LBNP while normothermic (torso: 73 ± 2%; heart: 72 ± 3%; spleen: 72 ± 5%, all P<0.001 relative to normothermia). Volume loading during heat stress did not alter the extent of the reduction in these blood volumes to LBNP relative to heat stress alone (torso: 73 ± 1%; heart: 72 ± 2%; spleen: 74 ± 3%, all P>0.05 relative to heat stress alone). These data suggest that blood volume loading during passive heat stress (via 11 ml kg⁻¹ of a colloid solution) normalizes regional blood volumes in the torso, but does not mitigate the reduction in central blood volume during a simulated haemorrhagic challenge combined with heat stress.

Human skeletal muscle feed arteries studied in vitro: the effect of temperature on α(1)-adrenergic responsiveness

Heat and cold exposure can decrease and increase total peripheral resistance, respectively, in humans. With unique access to human skeletal muscle feed arteries, we sought both to characterize these vessels and to determine the interaction between temperature and α(1)-adrenergic receptor responsiveness. We hypothesized that α(1)-mediated vasocontraction of human feed arteries would be attenuated in response to 39 or 35°C. Skeletal muscle feed arteries were harvested from thirty-two human volunteers and studied using isometric techniques. Vessel function was assessed using KCl, sodium nitroprusside (SNP), phenylephrine (PE) and ACh dose-response curves to characterize non-receptor- and receptor-mediated vasocontraction and vasorelaxation. Single doses of PE (1 mm) and KCl (100 mm) were administered at 37°C and then, in a balanced design, repeated at both 35 and 39°C. The KCl and PE dose-response curves elicited significant vasocontraction (2009 ± 407 and 1974 ± 508 mg developed tension, respectively), whereas SNP and ACh induced the expected vasorelaxation (102 ± 6 and 73 ± 10% relaxation, respectively). Altering the temperature had no effect on inherent smooth muscle function (KCl response), but both a reduction (35°C) and an increase in temperature (39°C) decreased the vasocontractile response to 1 mm PE (37°C, 1478 ± 338 mg; 35°C, 546 ± 104 mg; and 39°C, 896 ± 202 mg; P < 0.05) or across PE dose (P < 0.05, 35 and 39 versus 37°C). Despite clear heterogeneity between both the human volunteers and the feed arteries themselves, this novel approach to the procurement of human vessels revealed a robust 'inverted U' response to altered temperature, such that α(1)-mediated vasocontraction was attenuated with either warming or cooling.

Combined heat and mental stress alters neurovascular control in humans

This study examined the effect of combined heat and mental stress on neurovascular control. We hypothesized that muscle sympathetic nerve activity (MSNA) and forearm vascular responses to mental stress would be augmented during heat stress. Thirteen subjects performed 5 min of mental stress during normothermia (Tcore; 37 ± 0°C) and heat stress (38 ± 0°C). Heart rate, mean arterial pressure (MAP), MSNA, forearm vascular conductance (FVC; venous occlusion plethysmography), and forearm skin vascular conductance (SkVCf; via laser-Doppler) were analyzed. Heat stress increased heart rate, MSNA, SkVCf, and FVC at rest but did not change MAP. Mental stress increased MSNA and MAP during both thermal conditions; however, the increase in MAP during heat stress was blunted, whereas the increase in MSNA was accentuated, compared with normothermia (time × condition; P < 0.05 for both). Mental stress decreased SkVCf during heat stress but not during normothermia (time × condition, P < 0.01). Mental stress elicited similar increases in heart rate and FVC during both conditions. In one subject combined heat and mental stress induced presyncope coupled with atypical blood pressure and cutaneous vascular responses. In conclusion, these findings indicate that mental stress elicits a blunted increase of MAP during heat stress, despite greater increases in total MSNA and cutaneous vasoconstriction. The neurovascular responses to combined heat and mental stress may be clinically relevant to individuals frequently exposed to mentally demanding tasks in hyperthermic environmental conditions (i.e., soldiers, firefighters, and athletes).

Obligatory role of hyperaemia and shear stress in microvascular adaptation to repeated heating in humans

The endothelium, a single layer of cells lining the entire circulatory system, plays a key role in maintaining vascular health. Endothelial dysfunction independently predicts cardiovascular events and improvement in endothelial function is associated with decreased vascular risk. Previous studies have suggested that exercise training improves endothelial function in macrovessels, a benefit mediated via repeated episodic increases in shear stress. However, less is known of the effects of shear stress modulation in microvessels. In the present study we examined the hypothesis that repeated skin heating improves cutaneous microvascular vasodilator function via a shear stress-dependent mechanism. We recruited 10 recreationally active males who underwent bilateral forearm immersion in warm water (42 degrees C), 3 times per week for 30 min. During these immersion sessions, shear stress was manipulated in one arm by inflating a pneumatic cuff to 100 mmHg, whilst the other arm remained uncuffed. Vasodilatation to local heating, a NO-dependent response assessed using laser Doppler, improved across the 8 week intervention period in the uncuffed arm (cutaneous vascular conductance week 0 vs. week 4 at 41 degrees C: 1.37 +/- 0.45 vs. 2.0 +/- 0.91 units, P = 0.04; 42 degrees C: 2.06 +/- 0.45 vs. 2.68 +/- 0.83 units; P = 0.04), whereas no significant changes were evident in the cuffed arm. We conclude that increased blood flow, and the likely attendant increase in shear stress, is a key physiological stimulus for enhancing microvascular vasodilator function in humans.