TEA-sensitive K+ channels and human eccrine sweat gland output

Intradermal electrical stimulation of sudomotor nerves and local sweat rate

The local sweat rate (LSR) response to intradermal electrical stimulation generates a sigmodal stimulus-response curve with a peak sweat rate generated during a 30-s period of continuous stimuli at a frequency of 16-32 Hz. However, the in vivo firing pattern of the sudomotor nerve resembles more of a bursting pattern. We tested the hypothesis that a bursting pattern during intradermal electrical stimulation would result in a greater sweating response than the regular continuous stimulus pattern. Fifteen subjects were studied in a temperature-controlled room at 27.6 ± 0.2°C. The LSR was measured with a miniature sweat capsule with guide sleeves for holding the intradermal stimulating electrodes. The nine continuous stimulus frequencies (0.2, 1, 2, 4, 8, 12, 16, 32, and 64 Hz) were compared to a bursting pattern with a similar total number of stimuli. The sweating response was determined as the area under the ∆LSR-time curve. Peak ∆LSR was slightly higher for the continuous stimuli (0.396 ± 0.242 mg·min-1·cm-2, P = 0.023) than for the bursting stimuli (0.356 ± 0.244 mg·min-1·cm-2). The sigmoidal-shaped stimulus-response curves, however, were significantly different (P = 0.0007). The stimulus frequency producing 50% of peak LSR (EC50, P = 0.0029) was higher during continuous stimulation and the Hill slope was lower (P < 0.0001) during bursting stimuli. These data do not support the concept that a bursting stimulus pattern during intradermal electrical stimulation evokes a greater ∆LSR.NEW & NOTEWORTHY Neuron discharge variability can offer some advantages to a downstream physiological response. We examined this possibility with respect to sudomotor nerve activity and local sweat rate. Variable neuron discharge activity, induced by intradermal electrical stimulation, did not have an impact on the peak local sweat rate but did reduce the time to sweating onset and the stimulus intensity required to reach 50% of peak sweating (EC50).

TMEM16A blockers T16Ainh-A01 and benzbromarone do not modulate the regulation of sweating and cutaneous vasodilatation in humans in vivo

NEW FINDINGS

What is the central question of this study? Do transmembrane member 16A (TMEM16A) blockers modulate the activation of heat loss responses of sweating and cutaneous vasodilatation? What are the main finding and its importance? Relative to the vehicle control site, TMEM16A blockers T16Ainh-A01 and benzbromarone had no effect on sweat rate or cutaneous vascular conductance during whole-body heating inducing a 1.1 ± 0.1°C increase in core temperature above baseline resting levels. These results suggest that TMEM16A blockers T16Ainh-A01 and benzbromarone do not modulate the regulation of sweating and cutaneous vasodilatation during whole-body heat stress.

ABSTRACT

Animal and in vitro studies suggest that transmembrane member 16A (TMEM16A), a Ca²⁺ -activated Cl^- channel, contributes to regulating eccrine sweating. However, direct evidence supporting this possibility in humans is lacking. We assessed the hypothesis that TMEM16A blockers attenuate sweating during whole-body heating in humans. Additionally, we assessed the associated changes in the heat loss response of cutaneous vasodilatation to determine if a functional role of TMEM16A may exist. Twelve young (24 ± 2 years) adults (six females) underwent whole-body heating using a water-perfused suit to raise core temperature 1.1 ± 0.1°C above baseline. Sweat rate and cutaneous vascular conductance (normalized to maximal conductance via administration of sodium nitroprusside) were evaluated continuously at four forearm skin sites treated continuously by intradermal microdialysis with (1) lactated Ringer's solution (control), (2) 5% dimethyl sulfoxide (DMSO) serving as a vehicle control, or (3) TMEM16A blockers 1 mM T16Ainh-A01 or 2 mM benzbromarone dissolved in 5% DMSO solution. All drugs were administered continuously via intradermal microdialysis. Whole-body heating increased core temperature progressively and this was paralleled by an increase in sweat rate and cutaneous vascular conductance at all skin sites. However, sweat rate (all P > 0.318) and cutaneous vascular conductance (all P ≥ 0.073) did not differ between the vehicle control site relative to the TMEM16A blocker-treated sites. Collectively, our findings indicate that TMEM16A blockers T16Ainh-A01 and benzbromarone do not modulate the regulation of sweating and cutaneous vasodilatation during whole-body heating in young adults in vivo.

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

NEW & NOTEWORTHY

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 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 mediate the exercise-training-induced augmentation in 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, a low (0.001%) and high (1%) doses of pilocarpine was 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 (T_co ) 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 trained relative to the untrained men (all P≤0.033). Thermal sweating reached ∼90% of the response at a T_co elevation of 0.8°C during 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. This article is protected by copyright. All rights reserved.

Effects of L-type voltage-gated Ca2+ channel blockade on cholinergic and thermal sweating in habitually trained and untrained men

We evaluated the hypothesis that the activation of L-type voltage-gated Ca²⁺ channels contributes to exercise training-induced augmentation in cholinergic sweating. On separate days, 10 habitually trained and 10 untrained men participated in two experimental protocols. Prior to each protocol, we administered 1% verapamil (Verapamil, L-type voltage-gated Ca²⁺ channel blocker) and saline (Control) at forearm skin sites on both arms via transdermal iontophoresis. In protocol 1, we administered low (0.001%) and high (1%) doses of pilocarpine at both the verapamil-treated and verapamil-untreated forearm sites. In protocol 2, participants were passively heated by immersing their limbs in hot water (43°C) until rectal temperature increased by 1.0°C above baseline resting levels. Sweat rate at all forearm sites was continuously measured throughout both protocols. Pilocarpine-induced sweating in Control was higher in trained than in untrained men for both the concentrations of pilocarpine (both P ≤ 0.001). Pilocarpine-induced sweating at the low-dose site was attenuated at the Verapamil versus the Control site in both the groups (both P ≤ 0.004), albeit the reduction was greater in trained as compared with in untrained men (P = 0.005). The verapamil-mediated reduction in sweating remained intact at the high-dose pilocarpine site in the untrained men (P = 0.004) but not the trained men (P = 0.180). Sweating did not differ between Control and Verapamil sites with increases in rectal temperature in both groups (interaction, P = 0.571). We show that activation of L-type voltage-gated Ca²⁺ channels modulates sweat production in habitually trained men induced by a low dose of pilocarpine. However, no effect on sweating was observed during passive heating in either group.

The relative contribution of α- and β-adrenergic sweating during heat exposure and the influence of sex and training status

While human eccrine sweat glands respond to adrenergic agonists, there remains a paucity of information on the factors modulating this response. Thus, we assessed the relative contribution of α- and β-adrenergic sweating during a heat exposure and as a function of individual factors of sex and training status. α- and β-adrenergic sweating was assessed in forty-eight healthy young men (n = 35) and women (n = 13) including endurance-trained (n = 12) and untrained men (n = 12) under non-heat exposure (temperate, 25°C; n = 17) and heat exposure (hot, 35°C; n = 48) conditions using transdermal iontophoresis of phenylephrine (α-adrenergic agonist) and salbutamol (β-adrenergic agonist) on the ventral forearm, respectively. Adrenergic sweating was also measured after iontophoretic administration of atropine (muscarinic receptor antagonist) or saline (control) to evaluate how changes in muscarinic receptor activity modulate the adrenergic response to a heat exposure (n = 12). α- and β-adrenergic sweating was augmented in hot compared with temperate conditions (both P ≤ .014), albeit the relative increase was greater in β (~5.4-fold)- as compared to α (~1.5-fold)-adrenergic-mediated sweating response. However, both α- and β-adrenergic sweating was abolished by atropinization (P = .001). Endurance-trained men showed an augmentation in α- (P = .043) but not β (P = .960)-adrenergic sweating as compared to untrained men. Finally, a greater α- and β-adrenergic sweating response (both P ≤ .001) was measured in habitually active men than in women. We show that heat exposure augments α-and β-adrenergic sweating differently via mechanisms associated with altered muscarinic receptor activity. Sex and training status modulate this response.

Role of nitric oxide synthase in human sweat gland output

The mechanism by which nitric oxide synthase (NOS) inhibition impacts human sweating is unknown. We tested the hypothesis that the activation of NOS and release on nitric oxide acts to open K⁺ channels and enhance sweat gland output. Local sweat rate (LSR) was measured with a small sweat capsule mounted on the skin while sweating was initiated by intradermal electrical stimulation. Sigmoid shape stimulus-response curves were generated by plotting the area under the LSR-time curve (LSR AUC) versus log₁₀ stimulus frequency and normalized to the peak AUC response during control trials. NOS inhibition alone reduced the peak sweat rate response to 81.5 ± 4.5% peak LSR AUC of that seen with lactated Ringer's (P = 0.0004). Fifty mM of tetraethylammonium chloride (TEA) alone reduced peak LSR (0.317 ± 0.060 vs. 0.511 ± 0.104 mg·min^-1·cm^-2, P = 0.03) and the peak LSR AUC response from 0.193 ± 0.170 to 0.158 ± 0.127 mg·cm^-2 (P = 0.004). Delivery of a 20 mM nitro-l-arginine methyl ester (l-NAME) following 50 mM TEA produced a further decrease in the peak LSR AUC response to 0.095 ± 0.064 mg·cm^-2 (≈20% reduction, P = 0.0145). These data support the hypothesis that sudomotor control of sweat gland activity is locally modulated by a functioning NOS system that appears to be additive and independent to the effect of blockade of K⁺ channels with TEA.NEW & NOTEWORTHY The contribution of nitric oxide synthase (NOS) to the process of cholinergic-mediated human eccrine sweat production is unclear. Using a novel model for cholinergic-mediated sweating in humans, I demonstrate that blocking the NOS system led to a reduction in local sweat rate (LSR). This reduction in LSR was maintained in the presence of K⁺ channel blockade with tetraethylammonium.