Naoto Fujii

Evidence for cyclooxygenase‐dependent sweating in young males during intermittent exercise in the heat

Previous studies implicate nitric oxide (NO) in the control of sweating during exercise in the heat; however, it is unclear whether cyclooxygenase (COX) is also involved. We demonstrated that exercise‐induced sweating at a moderate heat production (400 W, ∼40% V̇O2 max ) was similarly reduced when COX and NO synthase were inhibited separately and in combination. Alternatively, inhibiting COX and/or NO synthase did not influence exercise‐induced sweating at a high heat production (700 W, ∼70% V̇O2 max ). We show that both COX and NO are involved in sweating during exercise at moderate heat production and that the effects may not be independent. However, roles for COX and NO are less evident when heat production is elevated. The results lead to better understanding of the mechanisms of sweating and indicate that COX inhibitors (e.g. aspirin) may impair core body temperature regulation and thereby increase the risk of heat‐related illness.

Diminished nitric oxide-dependent sweating in older males during intermittent exercise in the heat.

What is the central question of this study? Sweating during exercise in the heat is, in part, mediated through nitric oxide‐dependent mechanisms. It is unclear whether ageing reduces nitric oxide‐dependent sweating during exercise in the heat. What is the main finding and its importance? Nitric oxide‐dependent sweating during short bouts of exercise in the heat was observed in young men, but not in older adults. These findings show that age‐related impairment in sweating may be associated with age‐related reductions in nitric oxide‐mediated sweating.

Cyclooxygenase inhibition does not alter methacholine-induced sweating

Cholinergic agents (e.g., methacholine) induce cutaneous vasodilation and sweating. Reports indicate that either nitric oxide (NO), cyclooxygenase (COX), or both can contribute to cholinergic cutaneous vasodilation. Also, NO is reportedly involved in cholinergic sweating; however, whether COX contributes to cholinergic sweating is unclear. Forearm sweat rate (ventilated capsule) and cutaneous vascular conductance (CVC, laser-Doppler perfusion units/mean arterial pressure) were evaluated in 10 healthy young (24 ± 4 yr) adults (7 men, 3 women) at four skin sites that were continuously perfused via intradermal microdialysis with 1) lactated Ringer (control), 2) 10 mM ketorolac (a nonselective COX inhibitor), 3) 10 mM N(G)-nitro-l-arginine methyl ester (l-NAME, a nonselective NO synthase inhibitor), or 4) a combination of 10 mM ketorolac + 10 mM l-NAME. At the four skin sites, methacholine was simultaneously infused in a dose-dependent manner (1, 10, 100, 1,000, 2,000 mM). Relative to the control site, forearm CVC was not influenced by ketorolac throughout the protocol (all P > 0.05), whereas l-NAME and ketorolac + l-NAME reduced forearm CVC at and above 10 mM methacholine (all P < 0.05). Conversely, there was no main effect of treatment site (P = 0.488) and no interaction of methacholine dose and treatment site (P = 0.711) on forearm sweating. Thus forearm sweating (in mg·min(-1)·cm(-2)) from baseline up to the maximal dose of methacholine was not different between the four sites (at 2,000 mM, control 0.50 ± 0.23, ketorolac 0.44 ± 0.23, l-NAME 0.51 ± 0.22, and ketorolac + l-NAME 0.51 ± 0.23). We show that both NO synthase and COX inhibition do not influence cholinergic sweating induced by 1-2,000 mM methacholine.

Age-related differences in postsynaptic increases in sweating and skin blood flow postexercise

The influence of peripheral factors on the control of heat loss responses (i.e., sweating and skin blood flow) in the postexercise period remains unknown in young and older adults. Therefore, in eight young (22 ± 3 years) and eight older (65 ± 3 years) males, we examined dose‐dependent responses to the administration of acetylcholine (ACh) and methacholine (MCh) for sweating (ventilated capsule), as well as to ACh and sodium nitroprusside (SNP) for cutaneous vascular conductance (CVC, laser‐Doppler flowmetry, % of max). In order to assess if peripheral factors are involved in the modulation of thermoeffector activity postexercise, pharmacological agonists were perfused via intradermal microdialysis on two separate days: (1) at rest (DOSE) and (2) following a 30‐min bout of exercise (Ex+DOSE). No differences in sweat rate between the DOSE and Ex+DOSE conditions at either ACh or MCh were observed for the young (ACh: P = 0.992 and MCh: P = 0.710) or older (ACh: P = 0.775 and MCh: P = 0.738) adults. Similarly, CVC was not different between the DOSE and Ex+DOSE conditions for the young (ACh: P = 0.123 and SNP: P = 0.893) or older (ACh: P = 0.113 and SNP: P = 0.068) adults. Older adults had a lower sweating response for both the DOSE (ACh: P = 0.049 and MCh: P = 0.006) and Ex+DOSE (ACh: P = 0.050 and MCh: P = 0.029) conditions compared to their younger counterparts. These findings suggest that peripheral factors do not modulate postexercise sweating and skin blood flow in both young and older adults. Additionally, sweat gland function is impaired in older adults, albeit the impairments were not exacerbated during postexercise recovery.

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 cm2) and compared the sweating response with a classic capsule (2.8 cm2). In Protocol 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. In Protocol 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 cm2). In Protocol 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.

Mechanisms underlying the postexercise baroreceptor‐mediated suppression of heat loss

Reports indicate that postexercise heat loss is modulated by baroreceptor input; however, the mechanisms remain unknown. We examined the time‐dependent involvement of adenosine receptors, noradrenergic transmitters, and nitric oxide (NO) in modulating baroreceptor‐mediated changes in postexercise heat loss. Eight males performed two 15‐min cycling bouts (85% VO2max) each followed by a 45‐min recovery in the heat (35°C). Lower body positive (LBPP), negative (LBNP), or no (Control) pressure were applied in three separate sessions during the final 30‐min of each recovery. Four microdialysis fibres in the forearm skin were perfused with: (1) lactated Ringers (Ringers); (2) 4 mmol·L−1 Theophylline (inhibits adenosine receptors); (3) 10 mmol·L−1 Bretylium (inhibits noradrenergic transmitter release); or (4) 10 mmol·L−1 l‐NAME (inhibits NO synthase). We measured cutaneous vascular conductance (CVC; percentage of maximum) calculated as perfusion units divided by mean arterial pressure, and local sweat rate. Compared to Control, LBPP did not influence CVC at l‐NAME, Theophylline or Bretylium during either recovery (P > 0.07); however, CVC at Ringers was increased by ~5‐8% throughout 30 min of LBPP during Recovery 1 (all P < 0.02). In fact, CVC at Ringers was similar to Theophylline and Bretylium during LBPP. Conversely, LBNP reduced CVC at all microdialysis sites by ~7–10% in the last 15 min of Recovery 2 (all P < 0.05). Local sweat rate was similar at all treatment sites as a function of pressure condition (P > 0.10). We show that baroreceptor input modulates postexercise CVC to some extent via adenosine receptors, noradrenergic vasoconstriction, and NO whereas no influence was observed for postexercise sweating.

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.

Local infusion of ascorbate augments NO‐dependent cutaneous vasodilatation during intense exercise in the heat

Recent work demonstrates that nitric oxide (NO) contributes to cutaneous vasodilatation during moderate (400 W of metabolic heat production) but not high (700 W of metabolic heat production) intensity exercise bouts performed in the heat (35°C). The present study evaluated whether the impairment in NO‐dependent cutaneous vasodilatation was the result of a greater accumulation of reactive oxygen species during high (700 W of metabolic heat production) relative to moderate (500 W of metabolic heat production) intensity exercise. It was shown that local infusion of ascorbate (an anti‐oxidant) improves NO‐dependent forearm cutaneous vasodilatation during high intensity exercise in the heat. These findings provide novel insight into the physiological mechanisms governing cutaneous blood flow during exercise‐induced heat stress and provide direction for future research exploring whether oxidative stress underlies the impairments in heat dissipation that may occur in older adults, as well as in individuals with pathophysiological conditions such as type 2 diabetes.

Cutaneous vascular and sweating responses to intradermal administration of ATP: a role for nitric oxide synthase and cyclooxygenase?

In humans in vivo, the mechanisms behind ATP‐mediated cutaneous vasodilatation along with whether and how ATP increases sweating remains uncertain. Recent work has implicated nitric oxide synthase (NOS), cyclooxygenase (COX) and/or adenosine in the modulation of cutaneous vasodilatation and sweat production during both local (i.e. localized heating) and whole‐body heat stress (i.e. exercise‐induced heat stress). We evaluated whether ATP‐mediated cutaneous vasodilatation and sweating is mediated via NOS, COX and/or adenosine. We show that in humans in vivo, intradermal administration of ATP induces pronounced vasodilatation which is partially mediated by NOS, but neither COX nor adenosine influences ATP‐mediated vasodilatation, and ATP alone does not induce an increase in sweating. These findings advance our basic physiological knowledge regarding control of skin blood flow and sweating, and provide insight into the mechanisms governing thermoeffector activity, which has major implications for whole‐body heat exchange and therefore core temperature regulation in humans during heat stress.

Adenosine receptor inhibition attenuates the suppression of postexercise cutaneous blood flow

Skin blood flow (SkBF) is an important avenue for heat loss; however, it is rapidly suppressed after exercise despite persistently high core and muscle temperatures. This has been ascribed to altered active vasodilation; however, recent work has identified a role for adenosine receptors in the decrease in SkBF following passive heating. In this study, we examined whether adenosine receptors are involved in the postexercise regulation of SkBF by infusion of 4 mm theophylline (a non‐selective adenosine receptor antagonist) via microdialysis. We show that adenosine receptors have a major role in modulating postexercise SkBF, as evidenced by a marked elevation during theophylline infusion compared to a control site. These results help us to better understand the mechanisms underlying the postexercise reduction in SkBF and subsequently heat loss which is associated with heat‐related illness and/or injury.