Do nitric oxide synthase and cyclooxygenase contribute to the heat loss responses in older males exercising in the heat?

Experimental research in environmentally induced hyperthermic older persons: A systematic quantitative literature review mapping the available evidence

The heat-related health burden is expected to persist and worsen in the coming years due to an aging global population and climate change. Defining the breadth and depth of our understanding of age-related changes in thermoregulation can identify underlying causes and strategies to protect vulnerable individuals from heat. We conducted the first systematic quantitative literature review to provide context to the historical experimental research of healthy older adults - compared to younger adults or unhealthy age matched cases - during exogenous heat strain, focusing on factors that influence thermoregulatory function (e.g. co-morbidities). We identified 4,455 articles, with 147 meeting eligibility criteria. Most studies were conducted in the US (39%), Canada (29%), or Japan (12%), with 71% of the 3,411 participants being male. About 71% of the studies compared younger and older adults, while 34% compared two groups of older adults with and without factors influencing thermoregulation. Key factors included age combined with another factor (23%), underlying biological mechanisms (18%), age independently (15%), influencing health conditions (15%), adaptation potential (12%), environmental conditions (9%), and therapeutic/pharmacological interventions (7%). Our results suggest that controlled experimental research should focus on the age-related changes in thermoregulation in the very old, females, those with overlooked chronic heat-sensitive health conditions (e.g. pulmonary, renal, mental disorders), the impact of multimorbidity, prolonged and cumulative effects of extreme heat, evidence-based policy of control measures (e.g. personal cooling strategies), pharmaceutical interactions, and interventions stimulating protective physiological adaptation. These controlled studies will inform the directions and use of limited resources in ecologically valid fieldwork studies.

Effects of Medications on Heat Loss Capacity in Chronic Disease Patients: Health Implications Amidst Global Warming

Pharmacological agents used to treat or manage diseases can modify the level of heat strain experienced by chronically ill and elderly patients via different mechanistic pathways. Human thermoregulation is a crucial homeostatic process that maintains body temperature within a narrow range during heat stress through dry (i.e., increasing skin blood flow) and evaporative (i.e., sweating) heat loss, as well as active inhibition of thermogenesis, which is crucial to avoid overheating. Medications can independently and synergistically interact with aging and chronic disease to alter homeostatic responses to rising body temperature during heat stress. This review focuses on the physiologic changes, with specific emphasis on thermolytic processes, associated with medication use during heat stress. The review begins by providing readers with a background of the global chronic disease burden. Human thermoregulation and aging effects are then summarized to give an understanding of the unique physiologic changes faced by older adults. The effects of common chronic diseases on temperature regulation are outlined in the main sections. Physiologic impacts of common medications used to treat these diseases are reviewed in detail, with emphasis on the mechanisms by which these medications alter thermolysis during heat stress. The review concludes by providing perspectives on the need to understand the effects of medication use in hot environments, as well as a summary table of all clinical considerations and research needs of the medications included in this review. SIGNIFICANCE STATEMENT: Long-term medications modulate thermoregulatory function, resulting in excess physiological strain and predisposing patients to adverse health outcomes during prolonged exposures to extreme heat during rest and physical work (e.g., exercise). Understanding the medication-specific mechanisms of altered thermoregulation has importance in both clinical and research settings, paving the way for work toward refining current medication prescription recommendations and formulating mitigation strategies for adverse drug effects in the heat in chronically ill patients.

Human temperature regulation under heat stress in health, disease, and injury

The human body constantly exchanges heat with the environment. Temperature regulation is a homeostatic feedback control system that ensures deep body temperature is maintained within narrow limits despite wide variations in environmental conditions and activity-related elevations in metabolic heat production. Extensive research has been performed to study the physiological regulation of deep body temperature. This review focuses on healthy and disordered human temperature regulation during heat stress. Central to this discussion is the notion that various morphological features, intrinsic factors, diseases, and injuries independently and interactively influence deep body temperature during exercise and/or exposure to hot ambient temperatures. The first sections review fundamental aspects of the human heat stress response, including the biophysical principles governing heat balance and the autonomic control of heat loss thermoeffectors. Next, we discuss the effects of different intrinsic factors (morphology, heat adaptation, biological sex, and age), diseases (neurological, cardiovascular, metabolic, and genetic), and injuries (spinal cord injury, deep burns, and heat stroke), with emphasis on the mechanisms by which these factors enhance or disturb the regulation of deep body temperature during heat stress. We conclude with key unanswered questions in this field of research.

The effect of acute intradermal administration of ascorbate on heat loss responses in older adults with uncomplicated controlled hypertension

NEW FINDINGS

What is the central question of this study? Does acute intradermal administration of the antioxidant ascorbate augment local forearm cutaneous vasodilatation and sweating via nitric oxide synthase (NOS)-dependent mechanisms during exercise-heat stress in older adults with uncomplicated controlled hypertension? What is the main finding and its importance? Relative to control site, ascorbate had no effect on forearm cutaneous vascular conductance (CVC) and sweat rate, although CVC was reduced with NOS-inhibition in older adults with hypertension. We showed that acute local administration of ascorbate to forearm skin does not modulate heat loss responses during exercise-heat stress in older adults with hypertension.

ABSTRACT

Nitric oxide synthase (NOS) contributes to the heat loss responses of cutaneous vasodilatation and sweating during exercise. However, the contribution of NOS may be attenuated in individuals with uncomplicated, controlled hypertension due to elevated oxidative stress, which can reduce NO bioavailability. We evaluated the hypothesis that the acute local intradermal administration of the antioxidant ascorbate would enhance cutaneous vasodilatation and sweating via NOS-dependent mechanisms during an exercise-heat stress in adults with hypertension. Habitually active adults who were normotensive (n = 14, 7 females, 62 ± 4 years) or had uncomplicated, controlled hypertension (n = 13, 6 females, 62 ± 5 years) performed 30-min of moderate-intensity (50% of their pre-determine peak oxygen uptake) semi-recumbent cycling in the heat (35°C, 20% relative humidity). Cutaneous vascular conductance and sweat rate were assessed at four forearm skin sites continuously perfused with either: 1) lactated Ringer (Control), 2) 10 mM antioxidant ascorbate, 3) 10 mM L-NAME, a non-selective NOS inhibitor, or 4) a combination of ascorbate and L-NAME. Relative to Control, no effect of ascorbate was observed on cutaneous vascular conductance or sweating in either group (P = 0.619). However, L-NAME reduced cutaneous vascular conductance relative to Control in both groups (P ≤ 0.038). No effect of any treatment on sweating was observed (P ≥ 0.306). Thus, acute local administration of ascorbate to forearm skin does not enhance the activation of heat loss responses of cutaneous vasodilatation and sweating in older adults, and those with hypertension during an exercise-heat stress (236/250 words) This article is protected by copyright. All rights reserved.

Non-steroidal anti-inflammatory drugs on core body temperature during exercise: A systematic review

BACKGROUND

Because of their anti-pyretic effects, some individuals prophylactically use non-steroidal anti-inflammatory drugs (NSAIDs) to blunt core temperature (Tc) increases during exercise, thus, potentially improving performance by preventing hyperthermia and/or exertional heat illness. However, NSAIDs induce gastrointestinal damage, alter renal function, and decrease cardiovascular function, which could compromise thermoregulation and increase Tc. The aim of this systematic review was to evaluate the effects of NSAIDs on Tc in exercising, adult humans.

METHODS

We conducted searches in MEDLINE, PubMed, Cochrane Reviews, and Google Scholar for literature published up to November 2020. We conducted a quality assessment review using the Physiotherapy Evidence Database scale. Nine articles achieved a score ≥ seven to be included in the review.

RESULTS

Seven studies found aspirin, ibuprofen, and naproxen had no effect (p > .05) on Tc during walking, running, or cycling for ≤ 90 min in moderate to hot environments. Two studies found significant Tc changes. In one investigation, 81 mg of aspirin for 7-10 days prior to exercise significantly increased Tc during cycling (p < .001); final Tc at the end of exercise = 38.3 ± 0.1 °C vs. control = 38.1 ± 0.1 °C. In contrast, participants administered 50 mg rofecoxib for 6 days experienced significantly lower Tc during 45 min of cycling compared to placebo (NSAID Tc range ≈ 36.7-37.2 °C vs control ≈ 37.3-37.8 °C, p < 0.05).

CONCLUSIONS

There are limited quality studies examining NSAID effects on Tc during exercise in humans. The majority suggest taking non-selective NSAIDs (e.g., aspirin) 1-14 days before exercise does not significantly affect Tc during exercise. However, it remains unclear whether Tc increases, decreases, or does not change during exercise with other NSAID drug types (e.g., naproxen), higher dosages, chronic use, greater exercise intensity, and/or greater environmental temperatures.

Effects of short-term heat acclimation on whole-body heat exchange and local nitric oxide synthase- and cyclooxygenase-dependent heat loss responses in exercising older men

NEW FINDINGS

What is the central question of this study? Does short-term heat acclimation enhance whole-body evaporative heat loss and augment nitric oxide synthase (NOS)-dependent cutaneous vasodilatation and NOS- and cyclooxygenase (COX)-dependent sweating, in exercising older men? What is the main finding and its importance? Our preliminary data (n = 8) demonstrated that short-term heat acclimation improved whole-body evaporative heat loss, but it did not influence the effects of NOS and/or COX inhibition on cutaneous vasodilatation or sweating in older men during an exercise-heat stress. These outcomes might imply that although short-term heat acclimation enhances heat dissipation in older men, it does not modulate NOS- and COX-dependent control of cutaneous vasodilatation or sweating on the forearm.

ABSTRACT

Ageing is associated with decrements in whole-body heat loss (evaporative + dry heat exchange), which might stem from alterations in nitric oxide synthase (NOS)- and cyclooxygenase (COX)-dependent cutaneous vasodilatation and sweating. We evaluated whether short-term heat acclimation would (i) enhance whole-body heat loss primarily by increasing evaporative heat loss, and (ii) augment NOS-dependent cutaneous vasodilatation and NOS- and COX-dependent sweating, in exercising older men. Eight older men [mean (SD) age, 59 (8) years] completed a calorimetry and microdialysis trial before and after 7 days of exercise-heat acclimation. For the calorimetry trials, whole-body evaporative and dry heat exchange were assessed using direct calorimetry during 30 min bouts of cycling at light, moderate and vigorous metabolic heat productions (150, 200 and 250 W/m² , respectively) in dry heat (40°C, 20% relative humidity). For the microdialysis trials, local cutaneous vascular conductance and sweat rate were assessed during 60 min exercise in the heat (35°C, 20% relative humidity) at four dorsal forearm skin sites treated with lactated Ringer solution (control), NOS inhibitor, COX inhibitor or combined NOS and COX inhibitors, via microdialysis. Evaporative heat loss during moderate (P = 0.036) and vigorous (P = 0.021) exercise increased after acclimation. Inhibition of NOS alone reduced cutaneous vascular conductance to a similar extent before and after acclimation (P < 0.040), whereas separate and combined NOS and COX inhibition had no significant effects on sweating relative to the control site (P = 0.745). Our preliminary results might suggest that short-term heat acclimation improves evaporative heat loss, but does not significantly modulate the contributions of NOS or COX to cutaneous vasodilatation or sweating on the forearm in older men during an exercise-heat stress.

Ageing augments β-adrenergic cutaneous vasodilatation differently in men and women, with no effect on β-adrenergic sweating

NEW FINDINGS

What is the central question of this study? β-Adrenergic receptor activation modulates cutaneous vasodilatation and sweating in young adults. In this study, we assessed whether age-related differences in β-adrenergic regulation of these responses exist and whether they differ between men and women. What is the main finding and its importance? We showed that ageing augmented β-adrenergic cutaneous vasodilatation, although the pattern of response differed between men and women. Ageing had no effect on β-adrenergic sweating in men or women. Our findings advance our understanding of age-related changes in the regulation of cutaneous vasodilatation and sweating and provide new directions for research on the significance of enhanced β-adrenergic cutaneous vasodilatation in older adults.

ABSTRACT

β-Adrenergic receptor agonists, such as isoprenaline, can induce cutaneous vasodilatation and sweating in young adults. Given that cutaneous vasodilatation and sweating responses to whole-body heating and to pharmacological agonists, such as acetylcholine, ATP and nicotine, can differ in older adults, we assessed whether ageing also modulates β-adrenergic cutaneous vasodilatation and sweating and whether responses differ between men and women. In the context of the latter, prior reports showed that the effects of ageing on cutaneous vasodilatation (evoked with ATP and nicotine) and sweating (stimulated by acetylcholine) were sex dependent. Thus, in the present study, we assessed the role of β-adrenergic receptor activation on forearm cutaneous vasodilatation and sweating in 11 young men (24 ± 4 years of age), 11 young women (23 ± 5 years of age), 11 older men (61 ± 8 years of age) and 11 older women (60 ± 8 years of age). Initially, a high dose (100 µm) of isoprenaline was administered via intradermal microdialysis for 5 min to induce maximal β-adrenergic sweating. Approximately 60 min after the washout period, three incremental doses of isoprenaline were administered (1, 10 and 100 µm, each for 25 min) to assess dose-dependent cutaneous vasodilatation. Isoprenaline-mediated cutaneous vasodilatation was greater in both older men and older women relative to their young counterparts. Augmented cutaneous vasodilatory responses were observed at 1 and 10 µm in women and at 100 µm in men. Isoprenaline-mediated sweating was unaffected by ageing, regardless of sex. We show that ageing augments β-adrenergic cutaneous vasodilatation differently in men and women, without influencing β-adrenergic sweating.

No thermoregulatory or ergogenic effect of dietary nitrate among physically inactive males, exercising above gas exchange threshold in hot and dry conditions

The aim of this study was to determine the effect of five days dietary nitrate (NO₃^-) consumption on exercise tolerance and thermoregulation during cycling in hot, dry conditions. In a double-blind, randomised crossover design, 11 healthy males participated in an exercise tolerance test (T_lim) in the heat (35°C, 28% relative humidity), cycling above the thermoneutral gas exchange threshold, after five days of dietary supplementation, with either NO₃^-rich beetroot juice (BR; ∼ 9.2 mmol NO₃^-) or placebo (PLA). Changes in plasma [NO₃^-] and nitrite [NO₂^-], core and mean skin temperatures, mean local and whole-body sweat rates, heart rate, perceptual ratings and pulmonary gas exchange were measured during exercise, alongside calorimetric estimations of thermal balance. Mean arterial pressures (MAP) were recorded pre-T_lim. There were no differences in T_lim between conditions (BR = 22.8 ± 8.1 min; Placebo = 20.7 ± 7.9 min) (P = 0.184), despite increases in plasma [NO₃^-] and [NO₂^-] (P < 0.001) and a 3.8% reduction in resting MAP (P = 0.004) in the BR condition. There were no other differences in thermoregulatory, cardio-metabolic, perceptual or calorimetric responses to the T_lim between conditions (P > 0.05). Dietary NO₃^- supplementation had no effect on exercise tolerance or thermoregulation in hot, dry conditions, despite reductions in resting MAP and increases in plasma [NO₃^-] and [NO₂^-]. Healthy, yet physically inactive individuals with no known impairments in vasodilatory and sudomotor function do not appear to require BR for ergogenic or thermolytic effects during exercise in the heat.

Thermoregulation in the Aging Population and Practical Strategies to Overcome a Warmer Tomorrow

As global temperatures continue to rise, improving thermal tolerance in the aged population is crucial to counteract age-associated impairments in thermoregulatory function. Impairments in reflex cutaneous vasodilation and sweating response can augment the vulnerability of older adults to heat-related injuries following exposure to heat stress. Mechanisms underlying a compromised cutaneous vasodilation are suggested to include reduced sympathetic neural drive, diminished cholinergic co-transmitter contribution, and altered second messenger signaling events. On the other hand, impairments in sweating response are ascribed to reduced sweat gland cholinergic sensitivity and altered cyclooxygenase and nitric oxide signaling. Several practical mitigation strategies such as exercise, passive heating, and behavioral adaptations are proposed as means to overcome heat stress and improve thermal tolerance in the aged. Aerobic exercise training is shown to be amongst the most effective ways to enhance thermoregulatory function. However, in elderly with limited exercise capability due to chronic diseases and mobility issues, passive heating can serve as a functional alternative as it has been shown to confer similar benefits to that of exercise training. Supplementary to exercise training and passive heating, behavioral adaptations can be applied to further enhance the heat-preparedness of the aged.

Contribution of nitric oxide synthase to cutaneous vasodilatation and sweating in men of black-African and Caucasian descent during exercise in the heat

NEW FINDINGS

What is the central question of this study? Nitric oxide modulates cutaneous vasodilatation and sweating during exercise-induced heat stress in young men. However, it remains uncertain whether these effects are reduced in black-African descendants, who commonly demonstrate reduced nitric oxide bioavailability. Therefore, we assessed whether black-African descendants display reduced nitric oxide-dependent cutaneous vasodilatation and sweating compared with Caucasians in these conditions. What is the main finding and its importance? Nitric oxide-dependent cutaneous vasodilatation and sweating were similar between groups, indicating that reduced nitric oxide bioavailability in black-African descendants does not attenuate these heat-loss responses during an exercise-induced heat stress.

ABSTRACT

Men of black-African descent are at an increased risk of heat-related illness relative to their Caucasian counterparts. This might be attributable, in part, to reduced cutaneous nitric oxide (NO) bioavailability in this population, which might alter local cutaneous vasodilatation and sweating. To evaluate this, we compared these heat-loss responses in young men (18-30 years of age) of black-African (n = 10) and Caucasian (n = 10) descent during rest, exercise and recovery in the heat. Participants were matched for physical characteristics and fitness, and they were all born and raised in the same temperate environment (i.e. Canada; second generation and higher). Both groups rested for 10 min and then performed 50 min of moderate-intensity exercise at 200 W m^-2 , followed by 30 min of recovery in hot, dry heat (35°C, 20% relative humidity). Local cutaneous vascular conductance (CVC_%max ) and sweat rate (SR) were measured at two forearm skin sites treated with either lactated Ringer solution (control) or 10 mm N^G -nitro-l-arginine methyl ester (l-NAME, a nitric oxide (NO) synthase inhibitor). l-NAME significantly reduced CVC_%max throughout rest, exercise and recovery in both groups (both P < 0.001). However, there were no significant main effects for the contribution of NO to CVC_%max between groups (all P > 0.500). l-NAME significantly reduced local SR in both groups (both P < 0.050). The contribution of NO to SR was similar between groups such that l-NAME reduced SR relative to control at 40 and 50 min into exercise (both P < 0.05). We demonstrate that ethnicity per se does not influence NO-dependent cutaneous vasodilatation and sweating in healthy young men of black-African and Caucasian descent during exercise in dry heat.

Exogenous Activation of Protease-Activated Receptor 2 Attenuates Cutaneous Vasodilatation and Sweating in Older Men Exercising in the Heat

BACKGROUND

Protease-activated receptor 2 (PAR2) exists in the cutaneous vasculature and eccrine sweat glands. We previously showed that in young habitually active men, exogenous PAR2 activation via the agonist SLIGKV-NH2 had no effect on heat loss responses of cutaneous vasodilatation and sweating during rest or exercise in the heat. However, ageing is associated with altered mechanisms governing these responses. Thus, the effect of exogenous PAR2 activation on cutaneous vasodilatation and sweating in older individuals may differ from that in young adults.

METHODS

Local cutaneous vascular conductance (CVC) and sweat rate were measured in 9 older males (62 ± 4 years) at four forearm skin sites treated with the following: (1) lactated Ringer solution (control), (2) 0.05 mM, (3) 0.5 mM, or (4) 5 mM SLIGKV-NH2. Measurements were performed while participants rested in a non-heat-stress environment (25°C) for ∼60 min and an additional 50 min thereafter in the heat (40°C). Participants then performed 50 min of cycling at a fixed metabolic heat load of 200 W/m2 (to maintain the same thermal drive for heat loss between participants) followed by a 30-min recovery.

RESULTS

CVC during non-heat-stress resting was elevated from the control site with 5 mM SLIGKV-NH2 (p ≤ 0.05), but this response was not observed during ambient heat exposure. By contrast, 5 mM SLIGKV-NH2 lowered CVC during the early stage (10 and 20 min) of exercise compared to the control site (all p ≤ 0.05). Although sweating during non-heat-stressed and heat-stressed resting was not affected by any dose of SLIGKV-NH2, it was reduced with all SLIGKV-NH2 doses relative to the control site during and following exercise (all p ≤ 0.05).

CONCLUSION

We show that while exogenous PAR2 activation induces cutaneous vasodilatation at rest under non-heat-stressed conditions, it attenuates cutaneous vasodilatation and sweating during and following an exercise-induced heat stress in older men.

Separate and combined effects of KCa and KATP channel blockade with NOS inhibition on cutaneous vasodilation and sweating in older men during heat stress

Our objective in this study was to examine the separate and combined effects of potassium (K⁺) channels and nitric oxide synthase (NOS) on cutaneous vasodilation and sweating in older men during rest and exercise in the heat. In 13 habitually active men (61 ± 4 yr), cutaneous vascular conductance and local sweat rate were assessed at six dorsal forearm skin sites continuously perfused with either 1) lactated Ringer (control), 2) 10 mM N^G-nitro-l-arginine methyl ester (l-NAME, NOS inhibitor), 3) 50 mM tetraethylammonium (TEA; Ca²⁺-activated K⁺ channel blocker), 4) 5 mM glybenclamide (GLY; ATP-sensitive K⁺ channel blocker), 5) 50 mM TEA + 10 mM l-NAME, and 6) 5 mM GLY + 10 mM l-NAME via microdialysis. Participants rested in non-heat stress (25°C) and heat stress (35°C) conditions for ∼60 min each, followed by 50 min of moderate-intensity cycling (∼55% V̇o_2peak) and 30 min of recovery in the heat. During rest and exercise in the heat, l-NAME, TEA + l-NAME, and GLY + l-NAME attenuated CVC relative to control (all P ≤ 0.05), although l-NAME was not different from TEA + l-NAME or GLY + l-NAME (all P > 0.05). TEA attenuated CVC during rest, whereas GLY attenuated CVC during exercise (both P ≤ 0.05). Additionally, whereas neither l-NAME nor TEA altered sweating throughout the protocol (all P > 0.05), combined TEA + l-NAME attenuated sweating during exercise in the heat (P ≤ 0.05). We conclude that in habitually active older men blockade of K_Ca and K_ATP channels attenuates cutaneous vasodilation during rest and exercise in the heat, respectively, and these effects are NOS dependent. Furthermore, combined NOS inhibition and K_Ca channel blockade attenuates sweating during exercise in the heat.

Local arginase inhibition does not modulate cutaneous vasodilation or sweating in young and older men during exercise

Age-related impairments in cutaneous vascular conductance (CVC) and sweat rate (SR) during exercise may result from increased arginase activity, which can attenuate endogenous nitric oxide (NO) production. We therefore evaluated whether arginase inhibition modulates these heat-loss responses in young (n = 9, 23 ± 3 yr) and older (n = 9, 66 ± 6 yr) men during two 30-min bouts of moderate-intensity cycling (Ex1 and Ex2) in the heat (35°C). CVC and SR were measured at forearm skin sites perfused with 1) lactated Ringer's (control), 2) NG-nitro-L-arginine methyl ester (L-NAME; NO synthase-inhibited), or 3) Nω-hydroxy-nor-arginine and S-(2-boronoethyl)-l-cysteine (Nor-NOHA + BEC; arginase-inhibited). In both groups, CVC was reduced at L-NAME relative to control and Nor-NOHA + BEC (both P < 0.01). Likewise, SR was attenuated with L-NAME compared with control and Nor-NOHA + BEC during each exercise bout in the young men (all P ≤ 0.05); however, no influence of treatment on SR in the older men was observed (P = 0.14). Based on these findings, we then evaluated responses in 7 older men (64 ± 7 yr) during passively induced elevations in esophageal temperature (∆Tes) equal to those in Ex1 (0.6°C) and Ex2 (0.8°C). L-NAME reduced CVC by 18 ± 20% CVCmax at a ∆Tes of 0.8°C (P = 0.03) compared with control, whereas Nor-NOHA + BEC augmented CVC by 20 ± 18% CVCmax, on average, throughout heating (both P ≤ 0.03). SR was not influenced by either treatment (P = 0.80) Thus, arginase inhibition does not modulate CVC or SR during exercise in the heat but, consistent with previous findings, does augment CVC in older men during passive heating. NEW & NOTEWORTHY In the current study, we demonstrate that local arginase inhibition does not influence forearm cutaneous vasodilatory and sweating responses in young or older men during exercise-heat stress. Consistent with previous findings, however, we observed augmented cutaneous blood flow with arginase inhibition during whole-body passive heat stress. Thus, arginase differentially affects cutaneous vasodilation depending on the mode of heat stress but does not influence sweating during exercise or passive heating.

The effect of exogenous activation of protease-activated receptor 2 on cutaneous vasodilatation and sweating in young males during rest and exercise in the heat

Protease-activated receptor 2 (PAR2) exists in the endothelial cells of skin vessels and eccrine sweat glands. We evaluated the hypothesis that exogeneous activation of PAR2 augments cutaneous vasodilatation and sweating during rest and exercise in the heat. In 10 young males (23 ± 5 y), cutaneous vascular conductance (CVC) and sweat rate were measured at four forearm skin sites treated with either 1) lactated Ringer (Control), 2) 0.05 mM, 3) 0.5 mM, or 4) 5 mM SLIGKV-NH2 (PAR2 agonist). Participants initially rested in a semi-recumbent posture under a normothermic ambient condition (25°C) for ~60 min. Thereafter, ambient temperature was increased to 35°C while the participants rested for an additional 60 min. Participants then performed a 50-min bout of cycling (~55% of their pre-determined peak oxygen uptake) followed by a 30-min recovery period. Administration of 5 mM SLIGKV-NH2 increased cutaneous vascular conductance relative to the Control site during normothermic resting (P ≤ 0.05). However, we showed that relative to the Control site, no effect on CVC was observed for any administered dose of SLIGKV-NH2 (0.05-5 mM) during rest (33-39%max CVC), end-exercise (68-70%max CVC), and postexercise recovery (49-53%max CVC) in the heat (all P > 0.05). There were no differences in sweat rate between the Control and all SLIGKV-NH2-treated sites throughout the protocol (0.21-0.23, 1.20-1.27, and 0.32-0.33 mg∙min-1∙cm-2 for rest, end-exercise, and postexercise in the heat, respectively, all P > 0.05). We show that while exogeneous PAR2 activation induces cutaneous vasodilatation during normothermic rest, it does not influence the cutaneous blood flow and sweating responses during rest, exercise or recovery in the heat.

Cyclooxygenase-1 and -2 modulate sweating but not cutaneous vasodilation during exercise in the heat in young men

We recently reported that the nonselective cyclooxygenase (COX) inhibitor ketorolac attenuated sweating but not cutaneous vasodilation during moderate-intensity exercise in the heat. However, the specific contributions of COX-1 and COX-2 to the sweating response remained to be determined. We tested the hypothesis that COX-1 but not COX-2 contributes to sweating with no role for either COX isoform in cutaneous vasodilation during moderate-intensity exercise in the heat. In thirteen young males (22 ± 2 years), sweat rate and cutaneous vascular conductance were measured at three forearm skin sites that were continuously treated with (1) lactated Ringer's solution (Control), (2) 150 μmmol·L-1 celecoxib, a selective COX-2 inhibitor, or (3) 10 mmol L-1 ketorolac, a nonselective COX inhibitor. Participants first rested in a non heat stress condition (≥85 min, 25°C) followed by a further 70-min rest period in the heat (35°C). They then performed 50 min of moderate-intensity cycling (~55% peak oxygen uptake) followed by a 30-min recovery period. At the end of exercise, sweat rate was lower at the 150 μmol·L-1 celecoxib (1.51 ± 0.25 mg·min-1 ·cm-2 ) and 10 mmol·L-1 ketorolac (1.30 ± 0.30 mg·min-1 ·cm-2 ) treated skin sites relative to the Control site (1.89 ± 0.27 mg·min-1 ·cm-2 ) (both P ≤ 0.05). Additionally, sweat rate at the ketorolac site was attenuated relative to the celecoxib site (P ≤ 0.05). Neither celecoxib nor ketorolac influenced cutaneous vascular conductance throughout the experiment (both P > 0.05). We showed that both COX-1 and COX-2 contribute to sweating but not cutaneous vasodilation during moderate-intensity exercise in the heat in young men.

Do nitric oxide synthase and cyclooxygenase contribute to sweating response during passive heating in endurance-trained athletes?

The aim of our study was to determine if habitual endurance training can influence the relative contribution of nitric oxide synthase (NOS) and cyclooxygenase (COX) in the regulation of sweating during a passive heat stress in young adults. Ten trained athletes and nine untrained counterparts were passively heated until oral temperature (as estimated by sublingual temperature, T_or) increased by 1.5°C above baseline resting. Forearm sweat rate (ventilated capsule) was measured at three skin sites continuously perfused with either lactated Ringer's solution (Control), 10 mmol/L N^G -nitro-_L-arginine methyl ester (_L-NAME, non-selective NOS inhibitor), or 10 mmol/L ketorolac (Ketorolac, non-selective COX inhibitor) via intradermal microdialysis. Sweat rate was averaged for each 0.3°C increase in T_or Sweat rate at the _L-NAME site was lower than Control following a 0.9 and 1.2°C increase in T_or in both groups (all P ≤ 0.05). Relative to the Control site, NOS-inhibition reduced sweating similarly between the groups (P = 0.51). Sweat rate at the Ketorolac site was not different from the Control at any levels of T_or in both groups (P > 0.05). Nevertheless, a greater sweat rate was measured at the end of heating in the trained as compared to the untrained individuals (P ≤ 0.05). We show that NOS contributes similarly to sweating in both trained and untrained individuals during a passive heat stress. Further, no effect of COX on sweating was measured for either group. The greater sweat production observed in endurance-trained athletes is likely mediated by factors other than NOS- and COX-dependent mechanisms.

No effect of ascorbate on cutaneous vasodilation and sweating in older men and those with type 2 diabetes exercising in the heat

Aging and chronic disease such as type 2 diabetes (T2D) are associated with impairments in the body's ability to dissipate heat. To reduce the risk of heat-related injuries in these heat vulnerable individuals, it is necessary to identify interventions that can attenuate this impairment. We evaluated the hypothesis that intradermal administration of ascorbate improves cutaneous vasodilation and sweating in older adults via nitric oxide synthase (NOS)-dependent mechanisms during exercise in the heat and whether these improvements, if any, are greater in individuals with T2D. Older males with (n = 12, 61 ± 9 years) and without (n = 12, 64 ± 7 years) T2D performed two 30-min bouts of cycling at a fixed rate of metabolic heat production of 500 W (~70% peak oxygen uptake) in the heat (35°C); each followed by a 20- and 40-min recovery, respectively. Cutaneous vascular conductance (CVC) and sweat rate were measured at four intradermal microdialysis sites treated with either (1) lactated Ringer (Control), (2) 10 mmol/L ascorbate (an antioxidant), (3) 10 mmol/L L-NAME (non-selective NOS inhibitor), or (4) a combination of ascorbate + L-NAME. In both groups, ascorbate did not modulate CVC or sweating during exercise relative to Control (all P > 0.05). In comparison to Control, L-NAME alone or combined with ascorbate attenuated CVC during exercise (all P ≤ 0.05) but had no influence on sweating (all P > 0.05). We show that in both healthy and T2D older adults, intradermal administration of ascorbate does not improve cutaneous vasodilation and sweating during exercise in the heat. However, NOS plays an important role in mediating cutaneous vasodilation.

Individual variations in nitric oxide synthase-dependent sweating in young and older males during exercise in the heat: role of aerobic power

We evaluated the association between aerobic power (defined by peak oxygen consumption; VO_2peak) and the contribution of nitric oxide synthase (NOS) to the sweating response in young and older individuals during exercise in the heat. Data from 44 young (24 ± 1 years) and 48 older (61 ± 2 years) males with mean VO_2peak of 47.8 ± 2.4 (range, 28.0–62.3) and 39.1 ± 2.3 (range, 26.4–55.7) mLO₂ kg⁻¹ min⁻¹, respectively, were compiled from our prior studies. Participants performed two 15‐ to 30‐min bouts of exercise at a fixed rate of metabolic heat production of 400 or 500 W, each separated by 15–20 min recovery in the heat (35°C, relative humidity of 20%). Forearm sweat rate (ventilated capsule technique) was measured at two skin sites that were continuously and simultaneously administered with lactated Ringers solution (Control) or 10 mmol/L N^G‐nitro‐_L‐arginine methyl ester (_L‐NAME, nonselective NOS inhibitor) via intradermal microdialysis. Sweat rate during the final 5 min of each exercise bout was lower with _L‐NAME compared to the Control in both groups (all P < 0.05). The magnitude of the attenuation in sweat rate induced by _L‐NAME compared to the Control was not correlated with VO_2peak (all P ≥ 0.46) while this attenuation was negatively correlated with the sweat rate at the Control in both groups and in both exercise bouts (all P < 0.01, R ≤ −0.43). These results suggest that NOS‐dependent sweating is not associated with aerobic power per se, while it becomes evident in individuals who produce larger sweat rates during exercise irrespective of age.

The mechanisms underlying the muscle metaboreflex modulation of sweating and cutaneous blood flow in passively heated humans

Metaboreceptors can modulate cutaneous blood flow and sweating during heat stress but the mechanisms remain unknown. Fourteen participants (31 ± 13 years) performed 1‐min bout of isometric handgrip (IHG) exercise at 60% of their maximal voluntary contraction followed by a 3‐min occlusion (OCC), each separated by 10 min, initially under low (LHS, to activate sweating without changes in core temperature) and high (HHS, whole‐body heating to a core temperature increase of 1.0°C) heat stress conditions. Cutaneous vascular conductance (CVC) and sweat rate were measured continuously at four forearm skin sites perfused with 1) lactated Ringer's solution (Control), 2) 10 mmol L‐NAME [inhibits nitric oxide synthase (NOS)], 3) 10 mmol Ketorolac [inhibits cyclooxygenase (COX)], or 4) 4 mmol theophylline (THEO; inhibits adenosine receptors). Relative to pre‐IHG levels with Control, NOS inhibition attenuated the metaboreceptor‐mediated increase in sweating under LHS and HHS (P ≤ 0.05), albeit the attenuation was greater under LHS (P ≤ 0.05). In addition, a reduction from baseline was observed with THEO under LHS during OCC (P ≤ 0.05), but not HHS (both P > 0.05). In contrast, CVC was lower than Control with L‐NAME during OCC in HHS (P ≤ 0.05), but not LHS (P > 0.05). We show that metaboreceptor activation modulates CVC via the stimulation of NOS and adenosine receptors, whereas NOS, but not COX or adenosine receptors, contributes to sweating at all levels of heating.

Type 1 diabetes modulates cyclooxygenase- and nitric oxide-dependent mechanisms governing sweating but not cutaneous vasodilation during exercise in the heat

Both cyclooxygenase (COX) and nitric oxide synthase (NOS) contribute to sweating, whereas NOS alone contributes to cutaneous vasodilation during exercise in the heat. Here, we evaluated if Type 1 diabetes mellitus (T1DM) modulates these responses. Adults with (n = 11, 25 ± 5 yr) and without (n = 12, 24 ± 4 yr) T1DM performed two bouts of 30-min cycling at a fixed rate of heat production of 400 W in the heat (35°C); each followed by a 20- and 40-min recovery period, respectively. Sweat rate and cutaneous vascular conductance (CVC) were measured at four intradermal microdialysis sites treated with either 1) lactated Ringer (vehicle control site), 2) 10 mM ketorolac (nonselective COX inhibitor), 3) 10 mM N^G-nitro-l-arginine methyl ester (nonselective NOS inhibitor), or 4) a combination of both inhibitors. In nondiabetic adults, separate and combined inhibition of COX and NOS reduced exercise sweat rate (P ≤ 0.05), and the magnitude of reductions were similar across sites. In individuals with T1DM, inhibition of COX resulted in an increase in sweat rate of 0.10 ± 0.09 and 0.09 ± 0.08 mg ·: min^-1 ·: cm^-2 for the first and second exercise bouts, respectively, relative to vehicle control site (P ≤ 0.05), whereas NOS inhibition had no effect on sweating. In both groups, NOS inhibition reduced CVC during exercise (P ≤ 0.05), although the magnitude of reduction did not differ between the nondiabetic and T1DM groups (exercise 1: -28 ± 10 vs. -23 ± 8% max, P = 0.51; exercise 2: -31 ± 12 vs. -24 ± 10% max, P = 0.38). We show that in individuals with T1DM performing moderate intensity exercise in the heat, NOS-dependent sweating but not cutaneous vasodilation is attenuated, whereas COX inhibition increases sweating.

Administration of prostacyclin modulates cutaneous blood flow but not sweating in young and older males: roles for nitric oxide and calcium-activated potassium channels

KEY POINTS

In young adults, cyclooxygenase (COX) contributes to the heat loss responses of cutaneous vasodilatation and sweating, and this may be mediated by prostacyclin-induced activation of nitric oxide synthase (NOS) and calcium-activated potassium (KCa) channels. This prostacyclin-induced response may be diminished in older relative to young adults because ageing is known to attenuate COX-dependent heat loss responses. We observed that, although prostacyclin does not mediate sweating in young and older males, it does modulate cutaneous vasodilatation, although the magnitude of increase is similar between groups. We also found that, although NOS and KCa channels contribute to prostacyclin-induced cutaneous vasodilatation in young males, these contributions are diminished in older males. Our findings provide new insight into the mechanisms governing heat loss responses and suggest that the age-related diminished COX-dependent heat loss responses reported in previous studies may be a result of the reduced COX-derived production of prostanoids (e.g., prostacyclin) rather than the decreased sensitivity of prostanoid receptors.

ABSTRACT

Cyclooxygenase (COX) contributes to the regulation of cutaneous vasodilatation and sweating; however, the mechanism(s) underpinning this response remain unresolved. We hypothesized that prostacyclin (a COX-derived product) may directly mediate cutaneous vasodilatation and sweating through nitric oxide synthase (NOS) and calcium-activated potassium (KCa) channels in young adults. However, these responses would be diminished in older adults because ageing attenuates COX-dependent cutaneous vasodilatation and sweating. In young (25 ± 4 years) and older (60 ± 6 years) males (nine per group), cutaneous vascular conductance (CVC) and sweat rate were evaluated at four intradermal forearm skin sites: (i) control; (ii) 10 mm N^G -nitro-l-arginine (l-NNA), a non-specific NOS inhibitor; (iii) 50 mm tetraethylammonium (TEA), a non-specific KCa channel blocker; and (iv) 10 mm l-NNA + 50 mm TEA. All four sites were coadministered with prostacyclin in an incremental manner (0.04, 0.4, 4, 40 and 400 μm each for 25 min). Prostacyclin-induced increases in CVC were similar between groups (all concentrations, P > 0.05). l-NNA and TEA, as well as their combination, lowered CVC in young males at all prostacyclin concentrations (P ≤ 0.05), with the exception of l-NNA at 0.04 μm (P > 0.05). In older males, CVC during prostacyclin administration was not influenced by l-NNA (all concentrations), TEA (4-400 μm) or their combination (400 μm) (P > 0.05). No effect on sweat rate was observed in either group (all concentrations, P > 0.05). We conclude that, although prostacyclin does not mediate sweating, it modulates cutaneous vasodilatation to a similar extent in young and older males. Furthermore, although NOS and KCa channels contribute to the prostacyclin-induced cutaneous vasodilatation in young males, these contributions are diminished in older males.

Cutaneous vascular and sweating responses to intradermal administration of prostaglandin E1 and E2 in young and older adults: a role for nitric oxide?

Cyclooxygenase (COX) contributes to cutaneous vasodilation and sweating responses; however, the mechanisms underpinning these responses remain unknown. We hypothesized that prostaglandin E1 (PGE1) and E2 (PGE2) (COX-derived vasodilator products) directly mediate cutaneous vasodilation and sweating through nitric oxide synthase (NOS)-dependent mechanisms in young adults. Furthermore, we hypothesized that this response is diminished in older adults, since aging attenuates COX-dependent cutaneous vasodilation and sweating. In 9 young (22 ± 5 yr) and 10 older (61 ± 6 yr) adults, cutaneous vascular conductance (CVC) and sweat rate were evaluated at four intradermal forearm skin sites receiving incremental doses (0.05, 0.5, 5, 50, 500 μM each for 25 min) of PGE1 or PGE2 with and without coadministration of 10 mM N(ω)-nitro-l-arginine, a nonspecific NOS inhibitor. N(ω)-nitro-l-arginine attenuated PGE1-mediated increases in CVC at all concentrations in young adults, whereas it reduced PGE2-mediated increases in CVC at lower concentrations (0.05-0.5 μM) in older adults (all P < 0.05). However, the magnitude of the PGE1- and PGE2-mediated increases in CVC did not differ between groups (all P > 0.05). Neither PGE1 nor PGE2 increased sweat rate at any of the administered concentrations for either the young or older adults (all P > 0.05). We show that although cutaneous vascular responsiveness to PGE1 and PGE2 is similar between young and older adults, the cutaneous vasodilator response is partially mediated through NOS albeit via low-to-high concentrations of PGE1 in young adults and low concentrations of PGE2 in older adults, respectively. We also show that in both young and older adults, PGE1 and PGE2 do not increase sweat rate under normothermic conditions.

Exploring the mechanisms underpinning sweating: the development of a specialized ventilated capsule for use with intradermal microdialysis

Many studies have aimed to identify the controllers of sweating using ventilated capsules with intradermal microdialysis. It is unclear, however, if the surface area covered by the capsule influences the observed response as a result of differences in the number of sweat glands affected by the infused pharmacological agent relative to the total glands captured by the capsule. We evaluated the area of skin perfused with agents delivered via microdialysis. Thereafter, we developed a specialized sweat capsule (1.1 cm(2)) and compared the sweating response with a classic capsule (2.8 cm(2)). InProtocol 1(n = 6), methacholine was delivered to forearm skin in a dose-dependent manner (1-2000 mmol L(-1)). The area of activated sweat glands was assessed via the modified iodine-paper technique. InProtocol 2(n = 6), the area of inhibited sweat glands induced by ouabain and atropine was assessed during moderate-intensity cycling. Marked variability in the affected skin area was observed (0.9 ± 0.4 to 5.2 ± 1.1 cm(2)). InProtocol 3(n = 6), we compared the attenuation in local sweat rate (LSR) induced by atropine between the new and classic capsule during moderate-intensity cycling. Atropine attenuated sweating as assessed using the new (control: 0.87 ± 0.23 mg min(-1) cm(-2)vs. atropine: 0.54 ± 0.22 mg min(-1) cm(-2);P < 0.01) and classic (control: 0.85 ± 0.33 mg min(-1) cm(-2)vs. atropine: 0.60 ± 0.26 mg min(-1) cm(-2);P = 0.05) capsule designs. Importantly, responses did not differ between capsule designs (P = 0.23). These findings provide critical information regarding the skin surface area perfused by microdialysis and suggest that use of a larger capsule does not alter the mechanistic insight into the sweating response gained when using microdialysis.

iNOS-dependent sweating and eNOS-dependent cutaneous vasodilation are evident in younger adults, but are diminished in older adults exercising in the heat

Nitric oxide synthase (NOS) contributes to sweating and cutaneous vasodilation during exercise in younger adults. We hypothesized that endothelial NOS (eNOS) and neuronal NOS (nNOS) mediate NOS-dependent sweating, whereas eNOS induces NOS-dependent cutaneous vasodilation in younger adults exercising in the heat. Further, aging may upregulate inducible NOS (iNOS), which may attenuate sweating and cutaneous vasodilator responses. We hypothesized that iNOS inhibition would augment sweating and cutaneous vasodilation in exercising older adults. Physically active younger (n = 12, 23 ± 4 yr) and older (n = 12, 60 ± 6 yr) adults performed two 30-min bouts of cycling at a fixed rate of metabolic heat production (400 W) in the heat (35°C). Sweat rate and cutaneous vascular conductance (CVC) were evaluated at four intradermal microdialysis sites with: 1) lactated Ringer (control), 2) nNOS inhibitor (nNOS-I, NPLA), 3) iNOS inhibitor (iNOS-I, 1400W), or 4) eNOS inhibitor (eNOS-I, LNAA). In younger adults during both exercise bouts, all inhibitors decreased sweating relative to control, albeit a lower sweat rate was observed at iNOS-I compared with eNOS-I and nNOS-I sites (all P < 0.05). CVC at the eNOS-I site was lower than control in younger adults throughout the intermittent exercise protocol (all P < 0.05). In older adults, there were no differences between control and iNOS-I sites for sweating and CVC during both exercise bouts (all P > 0.05). We show that iNOS and eNOS are the main contributors to NOS-dependent sweating and cutaneous vasodilation, respectively, in physically active younger adults exercising in the heat, and that iNOS inhibition does not alter sweating or cutaneous vasodilation in exercising physically active older adults.