Modulation of cutaneous vasodilation by reactive oxygen species during local and whole-body heating in young and older adults.

OBJECTIVE: To evaluate reactive oxygen species (ROS) modulation of cutaneous vasodilation during local and whole-body passive heating in young and older adults. METHODS: Cutaneous vascular conductance normalized to maximum vasodilation (%CVCmax) was assessed in young and older adults (10 per group) using laser-Doppler flowmetry at 4 dorsal forearm sites treated with 1) Ringer's solution (control), 2) 100 µM apocynin (NADPH oxidase inhibitor), 3) 10 µM allopurinol (xanthine oxidase inhibitor), or 4) 10 µM tempol (superoxide dismutase mimetic), via intradermal microdialysis during local (protocol-1) and whole-body heating (protocol-2). Protocol-1: forearm skin sites were set at 33°C during baseline and then progressively increased to 39°C and 42°C (30 min each). Protocol-2: participants were immersed in warm water (35°C, mid-sternum) with the experimental forearm above water level and local skin sites maintained at 34°C. Bath temperature was increased (~40°C) to clamp core temperature at 38.5°C for 60 min. RESULTS: Protocol-1: there were significant treatment site by age interactions for the 39°C (P=0.015) and 42°C (P=0.004) plateaus. Although, no significant effects were observed after post-hoc adjustment. Protocol-2: there was a significant treatment site by age interaction (P<0.001) whereby %CVCmax in older adults was 11.0% [7.4,14.6] higher for apocynin (P<0.001), 8.9% [5.3,12.5] higher for allopurinol (P<0.001) and 4.8% [1.3,8.4] higher for tempol (P=0.016) sites relative to the control site. CONCLUSION: ROS derived from NADPH oxidase and xanthine oxidase attenuate cutaneous vasodilation in older adults during passive whole-body heating, but not during local skin heating, with negligible effects on their young counterparts for either heating modality.

The effect of seasonal heat acclimatization on cool-seeking behaviour during passive heat stress in young adults.

Seasonal heat acclimatization is known to enhance autonomic thermoeffector responses, whereas the behavioural response following seasonal heat acclimatization remains unknown. We investigated whether seasonal heat acclimatization would alter autonomic and behavioural thermoregulatory responses. Sixteen healthy participants (eight males and eight females) underwent two trials involving 50 min of lower-leg passive heating (lower-leg submersion in 42°C water) with (Fan trial) and without (No fan trial) the voluntary use of a fan in a moderate thermal environment (27°C, 50% relative humidity) across winter and summer months. In Fan trials, participants were allowed to use a fan to maintain thermal comfort, but this was not allowed in the No fan trials. Cool-seeking behaviour was initiated at a lower change in rectal temperature [mean (SD): 0.21 (0.18)°C vs. 0.11 (0.13)°C, P = 0.0327] and change in mean skin temperature [2.34 (0.56)°C vs. 1.81 (0.32)°C, P < 0.0001], and cooling time was longer [16.46 (5.62) vs. 20.40 (4.87) min, P = 0.0224] in summer compared with winter. However, thermal perception was not modified by season during lower-leg passive heating (all P > 0.0864). Furthermore, rectal temperature was higher in summer (P = 0.0433), whereas mean body temperature and skin temperature were not different (all P > 0.0631) between the two seasons in Fan trials. In conclusion, seasonal heat acclimatization enhanced the cool-seeking behaviour from winter to summer.

GH and IGF-1 in skin interstitial fluid and blood are associated with heat loss responses in exercising young adults.

PURPOSE: Sweat glands and cutaneous vessels possess growth hormone (GH) and insulin-like growth factor 1 (IGF-1) receptors. Here, we assessed if exercise increases GH and IGF-1 in skin interstitial fluid, and whether baseline and exercise-induced increases in GH and IGF-1 concentrations in skin interstitial fluid/blood are associated with heat loss responses of sweating and cutaneous vasodilation. METHODS: Sixteen young adults (7 women) performed a 50-min moderate-intensity exercise bout (50% VO2peak) during which skin dialysate and blood samples were collected. In a sub-study (n = 7, 4 women), we administered varying concentrations of GH (0.025-4000 ng/mL) and IGF-1 (0.000256-100 µg/mL) into skin interstitial fluid via intradermal microdialysis. Sweat rate (ventilated capsule) and cutaneous vascular conductance (CVC) were measured continuously for both studies. RESULTS: Exercise increased sweating and CVC (both P < 0.001), paralleled by increases of serum GH and skin dialysate GH and IGF-1 (all P ≤ 0.041) without changes in serum IGF-1. Sweating was positively correlated with baseline dialysate and serum GH levels, as well as exercise-induced increases in serum GH and IGF-1 (all P ≤ 0.044). Increases in CVC were not correlated with any GH and IGF-1 variables. Exogenous administration of GH and IGF-1 did not modulate resting sweat rate and CVC. CONCLUSION: (1) Exercise increases GH and IGF-1 levels in the skin interstitial fluid, (2) exercise-induced sweating is associated with baseline GH in skin interstitial fluid and blood, as well as exercise-induced increases in blood GH and IGF-1, and (3) cutaneous vasodilation during exercise is not associated with GH and IGF-1 in skin interstitial fluid and blood.

Comparing thermoregulatory responses between short and long moderate intensity intermittent exercise protocols with the same duty cycle.

To date, the thermoregulatory response between continuous and intermittent exercises has been investigated whilst limited studies are available to examine the thermoregulatory responses between different modes of intermittent exercises. We sought to determine the effect of two patterns of short duration intermittent exercises (180:180 (3-min) and 30:30 s (30-s) work: rest) on thermoregulatory responses in a temperate environment (25 °C, 50% RH, vapor pressure: 1.6 kPa) with low airflow (0.2 m/s). Twelve male participants (Age:24.0(5.0) year; VO2max: 53(8) mL.kg-1.min-1; BSA:1.7(0.1) m2) cycled at 50% VO2max for 60 min in 3-min and 30-s intervals to result in the same 30-min net exercise duration. Core and skin temperatures, the percent increase of skin blood flow (forearm and chest) from baseline and local sweat rate (forearm and chest) were not different between 3-min and 30-s (all P > 0.35) from the onset of exercise to the end of the exercise. Similarly, the mean body temperature onsets of skin blood flow (forearm and chest) and local sweat rates (forearm and chest) were not different between different mode of intermittent exercises (all P > 0.1). Furthermore, thermal sensitivities of skin blood flow (forearm and chest) and local sweat rate (forearm and chest) with increasing mean body temperature were not different between different mode of intermittent exercises (all P > 0.1). We conclude that intermittent exercises with different work periods at moderate exercise intensity did not alter core temperature and thermoeffector responses in a temperate environment. (241/250).

Induction and decay of seasonal acclimatization on whole body heat loss responses during exercise in a hot humid environment with different air velocities.

Whether whole body heat loss and thermoregulatory function (local sweat rate and skin blood flow) are different between summer and autumn and between autumn and winter seasons during exercise with different air flow in humid heat remain unknown. We therefore tested the hypotheses that whole body sweat rate (WBSR), evaporated sweat rate, and thermoregulatory function during cycling exercise in autumn would be higher than in winter but would be lower than in summer under hot-humid environment (32 C, 75% RH). We also tested the hypothesis that the increase of air velocity would enhance evaporated sweat rate and sweating efficiency across winter, summer, and autumn seasons. Eight males cycled for 1 h at 40% V̇o2max in winter, summer, and autumn seasons. Using an electric fan, air velocity increased from 0.2 m/s to 1.1 m/s during the final 20 min of cycling. The autumn season resulted in a lower WBSR, unevaporated sweat rate, and a higher sweating efficiency compared with summer (all P ≤ 0.05) but WBSR and unevaporated sweat rate in autumn were higher than in winter and thus sweating efficiency was lower when compared with winter only at the air velocity of 0.2 m/s (All P ≤ 0.05). Furthermore, evaporated sweat rate and core temperature (Tcore) were not different among winter, summer, and autumn seasons (All P > 0.19). In conclusion, changes in WBSR across different seasons do not alter Tcore during exercise in a hot humid environment. Furthermore, increasing air velocity enhances evaporated sweat rate and sweating efficiency across all seasons.

Transdermal iontophoretic application of l-NAME is available in sweating research induced by heat stress in young healthy adults.

Iontophoretic transdermal administration of NG-nitro-l-arginine methyl ester hydrochloride [l-NAME, a nitric oxide synthase (NOS) inhibitor] has been used as a non-invasive evaluation of NOS-dependent mechanisms in human skin. However, the availability has yet to be investigated in sweating research. Prior observations using invasive techniques (e.g., intradermal microdialysis technique) to administer l-NAME have implicated that NOS reduces sweating induced by heat stress but rarely influences the response induced by the administration of cholinergic muscarinic receptor agonists. Therefore, we investigated whether the transdermal iontophoretic administration of l-NAME modulates sweating similar to those prior observations. Twenty young healthy adults (10 males, 10 females) participated in two experimental protocols on separate days. Before each protocol, saline (control) and 1% l-NAME were bilaterally administered to the forearm skin via transdermal iontophoresis. In protocol 1, 0.001% and 1% pilocarpine were iontophoretically administered at l-NAME-treated and untreated sites. In protocol 2, passive heating was applied by immersing the lower limbs in hot water (43 °C) until the rectal temperature increased by 0.8 °C above baseline. The sweat rate was continuously measured throughout both protocols. Pilocarpine-induced sweat rate was not significantly different between the control and l-NAME-treated sites in both pilocarpine concentrations (P ≥ 0.316 for the treatment effect and interaction of treatment and pilocarpine concentration). The sweat rate during passive heating was attenuated at the l-NAME-treated site relative to the control (treatment effect, P = 0.020). Notably, these observations are consistent with prior sweating studies administrating l-NAME into human skin using intradermal microdialysis techniques. Based on the similarity of our results with already known observations, we conclude that transdermal iontophoresis of l-NAME is a valid non-invasive technique for the investigation of the mechanisms of sweating related to NOS during heat stress.

Dietary nitrate supplementation increases nitrate and nitrite concentrations in human skin interstitial fluid.

Acute dietary nitrate (NO3-) supplementation can increase [NO3-], but not nitrite ([NO2-]), in human skeletal muscle, though its effect on [NO3-] and [NO2-] in skin remains unknown. In an independent group design, 11 young adults ingested 140 mL of NO3--rich beetroot juice (BR; 9.6 mmol NO3-), and 6 young adults ingested 140 mL of a NO3--depleted placebo (PL). Skin dialysate, acquired through intradermal microdialysis, and venous blood samples were collected at baseline and every hour post-ingestion up to 4 h to assess dialysate and plasma [NO3-] and [NO2-]. The relative recovery rate of NO3- and NO2- through the microdialysis probe (73.1% and 62.8%), determined in a separate experiment, was used to estimate skin interstitial [NO3-] and [NO2-]. Baseline [NO3-] was lower, whereas baseline [NO2-] was higher in the skin interstitial fluid relative to plasma (both P < 0.001). Acute BR ingestion increased [NO3-] and [NO2-] in the skin interstitial fluid and plasma (all P < 0.001), with the magnitude being smaller in the skin interstitial fluid (e.g., 183 ± 54 vs. 491 ± 62 µM for Δ[NO3-] from baseline and 155 ± 190 vs. 217 ± 204 nM for Δ[NO2-] from baseline at 3 h post BR ingestion, both P ≤ 0.037). However, due to the aforementioned baseline differences, skin interstitial fluid [NO2-] post BR ingestion was higher, whereas [NO3-] was lower relative to plasma (all P < 0.001). These findings extend our understanding of NO3- and NO2- distribution at rest and indicate that acute BR supplementation increases [NO3-] and [NO2-] in human skin interstitial fluid.

Impacts of age, diabetes, and hypertension on serum endothelial monocyte-activating polypeptide-II after prolonged work in the heat.

BACKGROUND: With rising temperature extremes, older workers are becoming increasingly vulnerable to heat-related injuries because of age- and disease-associated decrements in thermoregulatory function. Endothelial monocyte-activating polypeptide-II (EMAP-II) is a proinflammatory cytokine that has not yet been well-characterized during heat stress, and which may mediate the inflammatory response to high levels of physiological strain. METHODS: We evaluated serum EMAP-II concentrations before and after 180 min of moderate-intensity work (200 W/m2 ) in temperate (wet-bulb globe temperature [WBGT] 16°C) and hot (WBGT 32°C) environments in heat-unacclimatized, healthy young (n = 13; mean [SD]; 22 [3] years) and older men (n = 12; 59 [4] years), and unacclimatized older men with hypertension (HTN) (n = 10; 60 [4] years) or type 2 diabetes (T2D) (n = 9; 60 [5] years). Core temperature and heart rate were measured continuously. RESULTS: In the hot environment, work tolerance time was lower in older men with HTN and T2D compared to healthy older men (both p < 0.049). While core temperature and heart rate reserve increased significantly (p < 0.001), they did not differ across groups. End-exercise serum EMAP-II concentrations were higher in young men relative to their older counterparts due to higher baseline levels (both p ≤ 0.02). Elevations in serum EMAP-II concentrations were similar between healthy older men and older men with HTN, while serum EMAP-II concentrations did not change in older men with T2D following prolonged work in the heat. CONCLUSION: Serum EMAP-II concentrations increased following prolonged moderate-intensity work in the heat and this response is influenced by age and the presence of HTN or T2D.

Does aging alter skin vascular function in humans when spatial variation is considered?

OBJECTIVE: Reports evaluating age-related impairments in cutaneous vascular function assessed by either the venoarteriolar reflex (VAR) induced by venous congestion, or post-occlusive reactive hyperemia (PORH) activated by arterial occlusion, have yielded mixed findings. This may be due to region-specific variability that occurs when assessing local cutaneous vascular responses. We evaluated the hypothesis that aging attenuates VAR and PORH responses in forearm skin assessed across four adjacent sites, each separated by ~4 cm to account for inter-site variability. METHODS: In twenty young (24 ± 4 years, 10 females) and twenty older (60 ± 7 years, 9 females) adults, VAR and PORH were achieved by a 3-min venous occlusion and 5-min arterial occlusion, each induced by inflating a pressure cuff to 45 and 240 mmHg, respectively. Cutaneous blood flow at all skin sites was measured by laser-Doppler flowmetry with the average response from all sites used for between-group comparisons. RESULTS: VAR and PORH responses were similar between groups with the exception that the time required to achieve peak PORH was delayed in older adults (mean difference of 5.5 ± 4.4 s, p = 0.003, Cohen's d = 0.812). CONCLUSIONS: We showed that aging had a negligible influence on VAR and PORH responses in forearm skin even when controlling for region-specific variability.

Effects of tetraethylammonium-sensitive K+ channel blockade on cholinergic and thermal sweating in endurance-trained and untrained men.

NEW FINDINGS: What is the central question of this study? Does inhibition of K+ channels modulate the exercise-training-induced augmentation in cholinergic and thermal sweating? What is the main finding and its importance? Iontophoretic administration of tetraethylammonium, a K+ channel blocker, blunted sweating induced by a low dose (0.001%) of the cholinergic agent pilocarpine, but not heat-induced sweating. However, no differences in the cholinergic sweating were observed between young endurance-trained and untrained men. Thus, while K+ channels play a role in the regulation of eccrine sweating, they do not contribute to the increase in sweating commonly observed in endurance-trained adults. Our findings provide important new insights into the mechanisms underlying the regulation of sweating by endurance conditioning. ABSTRACT: We evaluated the hypothesis that the activation of K+ channels mediates the exercise-training-induced augmentation of cholinergic and thermal sweating. On separate days, 11 endurance-trained and 10 untrained men participated in two experimental protocols. Prior to each protocol, we administered 2% tetraethylammonium (TEA, K+ channels blocker) and saline (Control) at forearm skin sites on both arms via transdermal iontophoresis. In protocol 1, low (0.001%) and high (1%) doses of pilocarpine were administered at the TEA-treated and Control sites over a 60-min period. In protocol 2, participants were passively heated by immersing their lower limbs in hot water (43°C) until core (rectal) temperature (Tc ) increased by 0.8°C above resting levels. Administration of TEA attenuated cholinergic sweating (P = 0.001) during the initial 20 min after the treatment of low dose of pilocarpine only whilst the response was similar between the groups (P = 0.163). Cholinergic and thermal sweating were higher in the trained relative to the untrained men (all P ≤ 0.033). Thermal sweating reached ∼90% of the response at a Tc elevation of 0.8°C during the initial 20 min of passive heating, which corresponds to the period wherein TEA attenuated cholinergic sweating in protocol 1. However, sweating did not differ between the Control and TEA sites in either group (P = 0.704). We showed that activation of K+ channels does not appear to mediate the elevated sweating response induced by a low dose of pilocarpine in trained men. We also demonstrated that K+ channels do not contribute to sweating during heat stress in either group.