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

Skin tattooing impairs sweating during passive whole body heating

Tattooing of the skin involves repeated needle insertions to deposit ink into the dermal layer of the skin, potentially damaging eccrine sweat glands and the cutaneous vasculature. This study tested the hypothesis that reflex increases in sweat rate (SR) and cutaneous vasodilation are blunted in tattooed skin (TAT) compared with adjacent healthy skin (CON) during a passive whole body heat stress (WBH). Ten individuals (5 males and 5 females) with a sufficient area of tattooed skin participated in the study. Intestinal temperature (T_int), skin temperature (T_skin), skin blood flow (laser Doppler flux; LDF), and SR were continuously measured during normothermic baseline (34°C water perfusing a tube-lined suit) and WBH (increased T_int 1.0°C via 48°C water perfusing suit). SR throughout WBH was lower for TAT compared with CON (P = 0.033). Accumulated sweating responses during WBH (area under curve) were attenuated in TAT relative to CON (23.1 ± 12.9, 26.9 ± 14.5 mg/cm², P = 0.043). Sweating threshold, expressed as the onset of sweating in time or T_int from the initiation of WBH, was not different between TAT and CON. Tattooing impeded the ability to obtain LDF measurements. These data suggest that tattooing functionally damages secretion mechanisms, affecting the reflex capacity of the gland to produce sweat, but does not appear to affect neural signaling to initiate sweating. Decreased sweating could impact heat dissipation especially when tattooing covers a higher percentage of body surface area and could be considered a potential long-term clinical side effect of tattooing.NEW & NOTEWORTHY This study is the first to assess the reflex control of sweating in tattooed skin. The novel findings are twofold. First, attenuated increases in sweat rate were observed in tattooed skin compared with adjacent healthy non-tattooed skin in response to a moderate increase (1.0°C) in internal temperature during a passive whole body heat stress. Second, reduced sweating in tattooed skin is likely related to functional damage to the secretory mechanisms of eccrine sweat glands, rendering it less responsive to cholinergic stimulation.

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

NEW FINDINGS

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

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

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

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Effect of hypohydration on thermoregulatory responses in men with low and high body fat exercising in the heat

It is unclear whether men with low body fat (LO-BF) have impaired thermoregulation during exercise heat stress compared with those with high body fat (HI-BF) when euhydration (EU) is maintained. Furthermore, in LO-BF individuals, hypohydration (HY) impairs thermoregulatory responses during exercise heat stress, but it is unknown whether this occurs in HI-BF counterparts. The purpose of this study was to test the hypotheses that men with HI-BF have impaired thermoregulatory responses to exercise heat stress and that HY further exacerbates these impairments vs. LO-BF. Men with LO-BF [ n = 11, body mass (BM) 73.9 ± 8.5 kg, BF% 13.6 ± 3.8] and HI-BF ( n = 9, BM 89.6 ± 6.9 kg, BF% 30.2 ± 4.1), in a randomized crossover design, performed 60 min of upright cycling in a hot environment (40.3 ± 0.4°C, relative humidity 32.5 ± 1.9%) at a metabolic heat production rate of 6 W/kg BM and finished exercise either euhydrated (EU; 0.3 ± 1.2 vs. 0.3 ± 0.9% BM loss) or HY (−2.5 ± 1.1 vs. −1.7 ± 1.5% BM loss). Changes in rectal temperature (ΔTrec), local sweat rate (ΔLSR), and cutaneous vascular conductance (ΔCVC; %max) were measured throughout. When EU, LO-BF and HI-BF had similar CVC and LSR responses ( P > 0.05); however, LO-BF had a lower ΔTrec vs. HI-BF (0.92 ± 0.35 vs. 1.31 ± 0.32°C, P = 0.021). Compared with EU, HY increased end-exercise ΔTrec in LO-BF (0.47 ± 0.37°C, P < 0.01) but not in HI-BF (−0.06 ± 0.29°C, P > 0.05). HY, compared with EU, did not affect ΔLSR and ΔCVC in either group ( P > 0.05). We conclude that, when euhydrated, men with HI-BF have a greater increase in Trec vs. LO-BF but similar CVC and LSR. HY exacerbates increases in Trec in LO-BF but not HI-BF.

NEW & NOTEWORTHY This is the first known investigation to compare thermoregulatory responses to exercise heat stress between men with high and low body fat (BF) in a physiologically uncompensable environment while simultaneously examining the confounding influence of hydration status. Both groups demonstrated similar sweating and cutaneous vasodilatory responses when euhydrated, despite vast differences in rectal temperature. Furthermore, in contrast to low BF, individuals with high BF demonstrated similar increases in core body temperature when either euhydrated or hypohydrated.

Effects of obesity and mild hypohydration on local sweating and cutaneous vascular responses during passive heat stress in females

The purpose of this study was to evaluate the effect of obesity and mild hypohydration on local sweating (LSR) and cutaneous vascular conductance (CVC) responses during passive heat stress in females. Thirteen obese (age, 24 ± 4 years; 45.4% ± 5.2% body fat) and 12 nonobese (age, 22 ± 2 years; 25.1% ± 3.9% body fat) females were passively heated (1.0 °C rectal temperature increase) while either euhydrated (EUHY) or mildly hypohydrated (HYPO; via fluid restriction). Chest and forearm LSR (ventilated capsule) and CVC (Laser Doppler flowmetry) onset, sensitivity, and plateau/steady state were recorded as mean body temperature increased (ΔTb). Participants began trials EUHY (urine specific gravity, Usg = 1.009 ± 0.006) or HYPO (Usg = 1.025 ± 0.004; p < 0.05), and remained EUHY or HYPO. Independent of obesity, HYPO decreased sweat sensitivity at the chest (HYPO = 0.79 ± 0.35, EUHY = 0.95 ± 0.39 Δmg·min(-1)·cm(-2)/°C ΔTb) and forearm (HYPO = 0.82 ± 0.39, EUHY = 1.06 ± 0.34 Δmg·min(-1)·cm(-2)/°C ΔTb); forearm LSR plateau was also decreased (HYPO = 0.66 ± 0.19, EUHY = 0.78 ± 0.23 mg·min(-1)·cm(-2); all p < 0.05). Overall, obese females had lower chest-sweat sensitivity (0.72 ± 0.35 vs. 1.01 ± 0.33 Δmg·min(-1)·cm(-2)/°C ΔTb) and plateau (0.55 ± 0.27 vs. 0.80 ± 0.25 mg·min(-1)·cm(-2); p < 0.05). While hypohydrated, obese females had a lower chest LSR (p < 0.05) versus nonobese females midway (0.45 ± 0.26 vs. 0.73 ± 0.23 mg·min(-1)·cm(-2)) and at the end (0.53 ± 0.27 vs. 0.81 ± 0.24 mg·min(-1)·cm(-2)) of heating. Furthermore, HYPO (relative to the EUHY trials) led to a greater decrease in CVC sensitivity in obese (-28 ± 27 Δ% maximal CVC/°C ΔTb) versus nonobese females (+9.2 ± 33 Δ% maximal CVC/°C ΔTb; p < 0.05). In conclusion, mild hypohydration impairs females' sweating responses during passive heat stress, and this effect is exacerbated when obese.

Intradermal administration of ATP augments methacholine-induced cutaneous vasodilation but not sweating in young males and females

Acetylcholine released from cholinergic nerves is a key neurotransmitter contributing to heat stress-induced cutaneous vasodilation and sweating. Given that sympathetic cholinergic nerves also release ATP, ATP may play an important role in modulating cholinergic cutaneous vasodilation and sweating. However, the pattern of response may differ between males and females given reports of sex-related differences in the peripheral mechanisms governing these heat loss responses. Cutaneous vascular conductance (CVC, laser-Doppler perfusion units/mean arterial pressure) and sweat rate (ventilated capsule) were evaluated in 17 young adults (8 males, 9 females) at four intradermal microdialysis skin sites continuously perfused with: 1) lactated Ringer (Control), 2) 0.3 mM ATP, 3) 3 mM ATP, or 4) 30 mM ATP. At all skin sites, methacholine was coadministered in a concentration-dependent manner (0.0125, 0.25, 5, 100, 2,000 mM, each for 25 min). In both males and females, CVC was elevated with the lone infusion of 30 mM ATP (both P < 0.05), but not with 0.3 and 3 mM ATP compared with control (all P >0.27). However, 0.3 mM ATP induced a greater increase in CVC compared with control in response to 100 mM methacholine infusion in males (P < 0.05). In females, 0.3 mM ATP infusion resulted in a lower concentration of methacholine required to elicit a half-maximal response (EC50) (P < 0.05). In both males and females, methacholine-induced sweating was unaffected by any concentration of ATP (all P > 0.44). We demonstrate that ATP enhances cholinergic cutaneous vasodilation albeit the pattern of response differs between males and females. Furthermore, we show that ATP does not modulate cholinergic sweating.