Gap junction-mediated contraction of myoepithelial cells induces the peristaltic transport of sweat in human eccrine glands.

Eccrine sweat glands play an essential role in regulating body temperature. Sweat is produced in the coiled secretory portion of the gland, which is surrounded by obliquely aligned myoepithelial cells; the sweat is then peristaltically transported to the skin surface. Myoepithelial cells are contractile and have been implicated in sweat transport, but how myoepithelial cells contract and transport sweat remains unexplored. Here, we perform ex vivo live imaging of an isolated human eccrine gland and demonstrate that cholinergic stimulation induces dynamic contractile motion of the coiled secretory duct that is driven by gap junction-mediated contraction of myoepithelial cells. The contraction of the secretory duct occurs segmentally, and it is most prominent in the region surrounded by nerve fibers, followed by distension-contraction sequences of the excretory duct. Overall, our ex vivo live imaging approach provides evidence of the contractile function of myoepithelial cells in peristaltic sweat secretion from human eccrine glands.

Sweat as a diagnostic biofluid.

Skin-interfaced microfluidic systems help assess health status and chemical exposure.

Repeated mutation of a developmental enhancer contributed to human thermoregulatory evolution.

Humans sweat to cool their bodies and have by far the highest eccrine sweat gland density among primates. Humans' high eccrine gland density has long been recognized as a hallmark human evolutionary adaptation, but its genetic basis has been unknown. In humans, expression of the Engrailed 1 (EN1) transcription factor correlates with the onset of eccrine gland formation. In mice, regulation of ectodermal En1 expression is a major determinant of natural variation in eccrine gland density between strains, and increased En1 expression promotes the specification of more eccrine glands. Here, we show that regulation of EN1 has evolved specifically on the human lineage to promote eccrine gland formation. Using comparative genomics and validation of ectodermal enhancer activity in mice, we identified a human EN1 skin enhancer, hECE18. We showed that multiple epistatically interacting derived substitutions in the human ECE18 enhancer increased its activity compared with nonhuman ape orthologs in cultured keratinocytes. Repression of hECE18 in human cultured keratinocytes specifically attenuated EN1 expression, indicating this element positively regulates EN1 in this context. In a humanized enhancer knock-in mouse, hECE18 increased developmental En1 expression in the skin to induce the formation of more eccrine glands. Our study uncovers a genetic basis contributing to the evolution of one of the most singular human adaptations and implicates multiple interacting mutations in a single enhancer as a mechanism for human evolutionary change.

The protease corin regulates electrolyte homeostasis in eccrine sweat glands.

Sweating is a basic skin function in body temperature control. In sweat glands, salt excretion and reabsorption are regulated to avoid electrolyte imbalance. To date, the mechanism underlying such regulation is not fully understood. Corin is a transmembrane protease that activates atrial natriuretic peptide (ANP), a cardiac hormone essential for normal blood volume and pressure. Here, we report an unexpected role of corin in sweat glands to promote sweat and salt excretion in regulating electrolyte homeostasis. In human and mouse eccrine sweat glands, corin and ANP are expressed in the luminal epithelial cells. In corin-deficient mice on normal- and high-salt diets, sweat and salt excretion is reduced. This phenotype is associated with enhanced epithelial sodium channel (ENaC) activity that mediates Na+ and water reabsorption. Treatment of amiloride, an ENaC inhibitor, normalizes sweat and salt excretion in corin-deficient mice. Moreover, treatment of aldosterone decreases sweat and salt excretion in wild-type (WT), but not corin-deficient, mice. These results reveal an important regulatory function of corin in eccrine sweat glands to promote sweat and salt excretion.

The sweat glands' maximum ion reabsorption rates following heat acclimation in healthy older adults.

NEW FINDINGS: What is the central question to this study? Do the sweat glands' maximum ion reabsorption rates increase following heat acclimation in healthy older individuals and is this associated with elevated aldosterone concentrations? What is the main finding and its importance? Sweat gland maximum ion reabsorption rates improved heterogeneously across body sites, which occurred without any changes in aldosterone concentration following a controlled hyperthermic heat acclimation protocol in healthy older individuals. ABSTRACT: We examined whether the eccrine sweat glands' ion reabsorption rates improved following heat acclimation (HA) in older individuals. Ten healthy older adults (>65 years) completed a controlled hyperthermic (+0.9°C rectal temperature, Tre ) HA protocol for nine non-consecutive days. Participants completed a passive heat stress test (lower leg 42°C water submersion) pre-HA and post-HA to assess physiological regulation of sweat gland ion reabsorption at the chest, forearm and thigh. The maximum ion reabsorption rate was defined as the inflection point in the slope of the relation between galvanic skin conductance and sweat rate (SR). We explored the responses again after a 7-day decay. During passive heating, the Tb thresholds for sweat onset on the chest and forearm were lowered after HA (P < 0.05). However, sweat sensitivity (i.e. the slope), the SR at a given Tre and gross sweat loss did not improve after HA (P > 0.05). Any changes observed were lost during the decay. Pilocarpine-induced sudomotor responses to iontophoresis did not change after HA (P ≥ 0.801). Maximum ion reabsorption rate was only enhanced at the chest (P = 0.001) despite unaltered aldosterone concentration after HA. The data suggest that this adaptation is lost after 7 days' decay. The HA protocol employed in the present study induced partial adaptive sudomotor responses. Eccrine sweat gland ion reabsorption rates improved heterogeneously across the skin sites. It is likely that aldosterone secretion did not alter the chest sweat ion reabsorption rates observed in the older adults.

Does the iontophoretic application of bretylium tosylate modulate sweating during exercise in the heat in habitually trained and untrained men?

NEW FINDINGS: What is the central question of this study? Does the administration of the adrenergic presynaptic release inhibitor bretylium tosylate modulate sweating during exercise in the heat, and does this response differ between habitually trained and untrained men? What is the main finding and its importance? Iontophoretic administration of bretylium tosylate attenuates sweating during exercise in the heat in habitually trained and untrained men. However, a greater reduction occurred in trained men. The findings demonstrate a role for cutaneous adrenergic nerves in the regulation of eccrine sweating during exercise in the heat and highlight a need to advance our understanding of neural control of human eccrine sweat gland activity. ABSTRACT: We recently reported an influence of cutaneous adrenergic nerves on eccrine sweat production in habitually trained men performing an incremental exercise bout in non-heat stress conditions. Based on an assumption that increasing heat stress induces cholinergic modulation of sweating, we evaluated the hypothesis that the contribution of cutaneous adrenergic nerves on sweating would be attenuated during exercise in the heat. Twenty young habitually trained and untrained men (n = 10/group) underwent three successive bouts of 15 min of light-, moderate- and vigorous-intensity cycling (equivalent to 30, 50, and 70% of peak oxygen uptake ( V̇O2peak ) respectively), each separated by a 15 min recovery while wearing a perfusion suit perfused with warm water (43°C). Sweat rate (ventilated capsule) was measured continuously at two bilateral forearm skin sites treated with 10 mm bretylium tosylate (an inhibitor of neurotransmitter release from adrenergic nerve terminals) and saline (control) via transdermal iontophoresis. A greater sweat rate was measured during vigorous exercise only in trained as compared to untrained men (P = 0.014). In both groups, sweating was reduced at the bretylium tosylate versus control sites, albeit the magnitude of reduction was greater in the trained men (P ≤ 0.024). These results suggest that cutaneous adrenergic nerves modulate sweating during exercise performed under a whole-body heat stress, albeit a more robust response occurs in trained men. While it is accepted that a cholinergic mechanism plays a primary role in the regulation of sweating during an exercise-heat stress, our findings highlight the need for additional studies aimed at understanding the neural control of human eccrine sweating.

Does α1-adrenergic receptor blockade modulate sweating during incremental exercise in young endurance-trained men?

PURPOSE: Human eccrine sweat glands respond to α1-adrenergic receptor agonists. We recently reported that adrenergic mechanisms contribute to sweating in endurance-trained men during an incremental exercise to volitional fatigue. However, it remains unclear if this response is mediated by α1-adrenergic receptor activation. METHODS: Twelve endurance-trained men performed an incremental cycling bout until exhaustion while wearing a water-perfused suit to clamp skin temperature at ~ 34 °C. Bilateral forearm sweat rates were measured wherein the distal area was treated with either 1% terazosin (α1-adrenergic receptor antagonist) or saline solution on the opposite limb (Control) via transdermal iontophoresis. We also measured proximal bilateral forearm sweat rate in untreated sites to confirm that no between-limb differences in forearm sweat rate occurred. Once sweat rate returned to pre-exercise resting levels at ~ 20 min postexercise, 0.25% phenylephrine (α1-adrenergic receptor agonist) was iontophoretically administered to skin to verify α1-adrenergic receptor blockade. RESULTS: Sweat rates at the proximal untreated right and left forearm sites were similar during exercise (interaction, P = 0.581). Similarly, no effect of terazosin on sweat rate was measured relative to control site (interaction, P = 0.848). Postexercise administration of phenylephrine increased sweat rate at the control site (0.08 ± 0.09 mg cm-2 min-1), which was suppressed by ~ 90% at the terazosin-treated site (0.01 ± 0.02 mg cm-2 min-1) (P = 0.026), confirming that α1-adrenergic receptor blockade was intact. CONCLUSION: Our findings demonstrate that α1-adrenergic receptors located at eccrine sweat glands do not contribute to eccrine sweating during incremental exercise in young endurance-trained men.

Diversity and evolution of human eccrine sweat gland density.

The human eccrine sweat gland is central to the evolution of the human genus, permitting an enormous thermoregulatory sweating capacity that was essential to the human niche of high physical activity in open, hot, semi-arid environments. Despite a century of research inventorying the structure and function of eccrine glands and the physiological responses of human heat acclimation, we do not have a clear understanding of how intraspecific differences in eccrine density affect thermoregulation. Similarly, existing data does not comprehensively catalogue modern human diversity in this trait, nor do we understand the relative influences of evolutionary forces and phenotypic plasticity in shaping this diversity.

Thermogenic and psychogenic sweating in humans: Identifying eccrine glandular recruitment patterns from glabrous and non-glabrous skin surfaces.

In this experiment, psychogenic (mental arithmetic), thermogenic (mean body temperature elevation of 0.6 °C) and combined thermo-psychogenic treatments were used to explore eccrine sweat-gland recruitment from glabrous (volar hand and forehead) and non-glabrous skin surfaces (chest). It was hypothesised that each treatment would activate the same glands, and that glandular activity would be intermittent. Nine individuals participated in a single trial with normothermic and mildly hyperthermic phases. When normothermic, a 10-min arithmetical challenge was administered, during which sudomotor activity was recorded. Following passive heating and thermal clamping, sweating responses were again evaluated (10 min). A second arithmetical challenge (10 min) was administered during clamped hyperthermia, with its sudorific impact recorded. The activity of individual sweat glands was recorded at 60-s intervals, using precisely positioned, and uniformly applied, starch-iodide papers. Those imprints were digitised and analysed. Peak activity typically occurred during the thermo-psychogenic treatment, revealing physiologically active densities of 128 (volar hand), 165 (forehead) and 77 glands.cm-2 (chest). Except for the hand (46%), glands uniquely activated by one treatment were consistently <10% of the total glands identified. Glandular activations were most commonly of an intermittent nature, particularly during the thermogenic treatment. Accordingly, we accepted the hypothesis that psychogenic, thermogenic and thermo-psychogenic stimuli activate the same sweat glands in both the glabrous and non-glabrous regions. In addition, this investigation has provided detailed descriptions of the intermittent nature of sweat-gland activity, revealing that a consistent proportion of the physiologically active glands are recruited during these thermal and non-thermal stimuli.

Revisiting the dermatomal recruitment of, and pressure-dependent influences on, human eccrine sweating.

Herein we describe two experiments in which the recruitment and pressure-induced modifications of human eccrine sweating were investigated. In one experiment, the longstanding belief that glandular recruitment follows a gradual, caudal-to-rostral (dermatomal) recruitment pattern was re-evaluated. The onset of sweating was simultaneously determined (ventilated capsules) from four spinal (dermatomal) segments (forehead, dorsal hand, lower chest and dorsal foot) during the passive heating of supine participants (N = 8). No evidence was found to support either dermatomal or simultaneous glandular recruitment patterns. Instead, the results were more consistent with individualised (random) patterns of regional activation (P > 0.05), with significant time delays among sites. Such delays in the appearance of discharged sweat may reflect differences in neurotransmitter sensitivity, precursor sweat production or ductal reabsorption. In the second experiment, the pressure-induced hemihidrotic reflex (contralateral sudomotor enhancement) was revisited, using pressures applied over 10 cm2 areas of the chest (left side: 6 N cm-2) and left heel (3 N cm-2) during both supine and seated postures (N = 12). Participants were passively heated and thermally clamped before pressure application. Hemihidrosis was not observed from the contralateral surfaces within the same (chest) or lower spinal segments (abdomen; both P > 0.05) during chest pressure, but a generalised enhancement followed heel pressure when supine. We suggest that previous observations of hemihidrosis possibly resulted from elevated heat storage, rather than a neural reflex. Chest pressure significantly inhibited ipsilateral sweating (forehead, hand, chest; all P < 0.05), and that influence is hypothesised to result from interactions between ascending mechanoreceptor afferents and the descending sudomotor pathways.

Comparative evidence for the independent evolution of hair and sweat gland traits in primates.

Humans differ in many respects from other primates, but perhaps no derived human feature is more striking than our naked skin. Long purported to be adaptive, humans' unique external appearance is characterized by changes in both the patterning of hair follicles and eccrine sweat glands, producing decreased hair cover and increased sweat gland density. Despite the conspicuousness of these features and their potential evolutionary importance, there is a lack of clarity regarding how they evolved within the primate lineage. We thus collected and quantified the density of hair follicles and eccrine sweat glands from five regions of the skin in three species of primates: macaque, chimpanzee and human. Although human hair cover is greatly attenuated relative to that of our close relatives, we find that humans have a chimpanzee-like hair density that is significantly lower than that of macaques. In contrast, eccrine gland density is on average 10-fold higher in humans compared to chimpanzees and macaques, whose density is strikingly similar. Our findings suggest that a decrease in hair density in the ancestors of humans and apes was followed by an increase in eccrine gland density and a reduction in fur cover in humans. This work answers long-standing questions about the traits that make human skin unique and substantiates a model in which the evolution of expanded eccrine gland density was exclusive to the human lineage.

The evolution of eccrine sweat glands in human and nonhuman primates.

Sweating is an unusual thermoregulatory strategy for most mammals, yet is critical for humans. This trait is commonly hypothesized to result from human ancestors moving from a forest to a warmer and drier open environment. As soft tissue traits do not typically fossilize, this idea has been difficult to test. Therefore, we used a comparative approach to examine 15 eccrine gland traits from 35 primate species. For each trait we measured phylogenetic signal, tested three evolutionary models to explain trait variation, and used phylogenetic models to examine how traits varied in response to climate variables. Phylogenetic signal in traits varied substantially, with the two traits exhibiting the highest values being gland distribution on the body and percent eccrine vs. apocrine glands on the body. Variation in most traits was best explained by an Ornstein-Uhlenbeck model suggesting the importance of natural selection. Two traits were strongly predicted by climate. First, species with high eccrine gland glycogen content were associated with habitats exhibiting warm temperatures and low rainfall. Second, species with increased capillarization were associated with high temperature. Glycogen is a primary energy substrate powering sweat production and sodium reabsorption in the eccrine gland, and increased capillarization permits greater oxygen, glucose and electrolyte delivery. Thus, our results are evidence of natural selection for increased sweating capacity in primate species with body surface eccrine glands living in hot and dry climates. We suggest that selection for increased glycogen content and capillarization may have been part of initial increases in hominin thermoregulatory sweating capacity.

Soft, skin-mounted microfluidic systems for measuring secretory fluidic pressures generated at the surface of the skin by eccrine sweat glands.

During periods of activity, sweat glands produce pressures associated with osmotic effects to drive liquid to the surface of the skin. The magnitudes of these pressures may provide insights into physiological health, the intensity of physical exertion, psychological stress factors and/other information of interest, yet they are currently unknown due to absence of means for non-invasive measurement. This paper introduces a thin, soft wearable microfluidic system that mounts onto the surface of the skin to enable precise and routine measurements of secretory fluidic pressures generated at the surface of the skin by eccrine sweat glands (surface SPSG, or s-SPSG) at nearly any location on the body. These platforms incorporate an arrayed collection of unit cells each of which includes an opening to the skin, an inlet through which sweat can flow, a capillary bursting valve (CBV) with a unique bursting pressure (BP), a corresponding microreservoir to receive sweat and an outlet to the surrounding ambient to allow release of backpressure. The BPs systematically span the physiologically relevant range, to enable a measurement precision approximately defined by the ratio of the range to the number of unit cells. Human studies demonstrate measurements of s-SPSG under different conditions, from various regions of the body. Average values in healthy young adults lie between 2.4 and 2.9 kPa. Sweat associated with vigorous exercise have s-SPSGs that are somewhat higher than those associated with sedentary activity. For all conditions, the forearm and lower back tend to yield the highest and lowest s-SPSGs, respectively.

Effects of Brn2 overexpression on eccrine sweat gland development in the mouse paw.

Eccrine sweat glands regulate body temperature by secreting water and electrolytes. In humans, eccrine sweat glands are ubiquitous in the skin, except in the lips and external genitalia. In mice, eccrine sweat glands are present only in the paw pad. Brn2 is a protein belonging to a large family of transcription factors. A few studies have examined Brn2 in melanoma cells and epidermal keratinocytes. This study investigated changes in the skin in the K5-Brn2 transgenic mouse, which overexpresses Brn2 and contains the keratin 5 promotor. Interestingly, the volume of eccrine sweat glands was reduced markedly in the K5-Brn2 transgenic mouse compared with the wild-type, while the expression of aquaporin 5, important molecule in sweat secretion, was increased in each sweat gland cell, probably to compensate for the reduction in gland development. However, sweating response to a pilocarpine injection in the hind paw was significantly decreased in the K5-Brn2 transgenic mouse compared with the wild-type. The paw epidermis was thicker in the K5-Brn2 transgenic mouse compared with the wild-type. Taken together, eccrine sweat gland development and sweat secretion were suppressed markedly in the K5-Brn2 transgenic mouse. These results may be associated with dominant development of the epidermis by Brn2 overexpression in the paw skin.

Thermogenic and psychogenic recruitment of human eccrine sweat glands: Variations between glabrous and non-glabrous skin surfaces.

Human eccrine sweat-gland recruitment and secretion rates were investigated from the glabrous (volar) and non-glabrous hand surfaces during psychogenic (mental arithmetic) and thermogenic stimuli (mild hyperthermia). It was hypothesised that these treatments would activate glands from both skin surfaces, with the non-thermal stimulus increasing secretion rates primarily by recruiting more sweat glands. Ten healthy men participated in two seated, resting trials in temperate conditions (25-26°C). Trials commenced under normothermic conditions during which the first psychogenic stress was applied. That was followed by passive heating (0.5°C mean body temperature elevation) and thermal clamping, with a second cognitive challenge then applied. Sudomotor activity was evaluated from both hands, with colourimetry used to identify activated sweat glands, skin conductance to determine the onset of precursor sweating and ventilated sweat capsules to measure rates of discharged sweating. From glandular activation and sweat rate data, sweat-gland outputs were derived. These psychogenic and thermogenic stimuli activated sweat glands from both the glabrous and non-glabrous skin surfaces, with the former dominating at the glabrous skin and the latter at the non-glabrous surface. Indeed, those stimuli individually accounted for ~90% of the site-specific maximal number of activated sweat glands observed when both stimuli were simultaneously applied. During the normothermic psychological stimulation, sweating from the glabrous surface was elevated via a 185% increase in the number of activated glands within the first 60s. The hypothetical mechanism for this response may involve the serial activation of additional eccrine sweat glands during the progressive evolution of psychogenic sweating.

Nicotinic receptor activation augments muscarinic receptor-mediated eccrine sweating but not cutaneous vasodilatation in young males.

NEW FINDINGS: What is the central question of this study? Acetylcholine released from cholinergic nerves can activate both muscarinic and nicotinic receptors. Although each receptor can independently induce cutaneous vasodilatation and eccrine sweating, it remains to be elucidated whether the two receptors interact in order to mediate these responses. What is the main finding and its importance? We show that although nicotinic receptor activation does not modulate muscarinic cutaneous vasodilatation, it lowers the muscarinic receptor agonist threshold at which onset for eccrine sweating (augmentation of muscarinic eccrine sweating) occurs in young men in normothermic resting conditions. These results provide new insights into the physiological significance of nicotinic receptors in the regulation of cutaneous perfusion and eccrine sweating. Acetylcholine released from cholinergic nerves can activate both muscarinic and nicotinic receptors; each is known independently to induce cutaneous vasodilatation and eccrine sweating in humans. However, it is not known whether the two receptors interact in order to mediate cutaneous vasodilatation and eccrine sweating. In 10 young men (27 ± 6 years old), cutaneous vascular conductance and sweat rate were evaluated at intradermal microdialysis sites that were continuously perfused with either lactated Ringer's solution (control) or three different concentrations of nicotine (0.1, 1 and 10 mm), a nicotinic receptor agonist. Co-administration of methacholine, a muscarinic receptor agonist, was performed at all skin sites in a dose-proportional fashion (0.0125, 0.25, 5, 100 and 2000 mm, each for 25 min). Administration of nicotine alone caused dose-dependent transient increases in cutaneous vascular conductance and sweat rate (all P ≤ 0.05), which thereafter returned to pre-nicotine levels, except that a portion of transient responses remained with continuous administration of 10 mm nicotine (both P ≤ 0.05). Cutaneous vascular conductance was increased by administration of ≥0.25 mm methacholine at the control site, and this response was likewise observed in the presence of co-administration of all doses of nicotine used (all P ≤ 0.05). Sweat rate at the control site was increased by administration of ≥0.25 mm methacholine, but the lowest dose of methacholine (0.0125 mm) was able to increase sweat rate in the presence of 10 mm nicotine (P ≤ 0.05). We conclude that nicotinic receptor activation lowers the muscarinic receptor agonist threshold for eccrine sweating (augmentation of muscarinic sweating) but does not affect muscarinic cutaneous vasodilatation in young men in normothermic resting conditions.

Identification of a new sweat gland progenitor population in mice and the role of their niche in tissue development.

Sweat gland cells are responsible for the regulation of body temperature and are critical for wound repair. Furthermore, they have the regenerative potential in response to injury, and show a substantial turnover during both wound healing and homeostasis. However, as a usual research model of sweat gland, mice have not too much glandular cells for experiments. In this study, we identify previously unreported sweat gland progenitor population in mice and characterize them. The progenitor characteristics of sweat gland were confirmed using cellular immunofluorescence assay and quantitative real-time PCR assay. K8 and K18 expression was barely detected in the early stage of skin development (Embryo 17.5d) and increased to a high level at P5d (postnatal 5d), then showed reduction at adult stage (P28d). Further investigation of K8 and K18 positive cells using tissue immunofluorescence revealed the presence of sweat gland progenitors in back epidermis of mice at early stage of sweat gland development and continuous reduction during the developmental process. In vivo transplantation assay with animal models elucidated that sweat gland specific niche in paw pads was critical for the development of sweat gland cells. Although the relationship between new sweat gland progenitors and their niche still needs to be further investigated, the presence of these cells implicates that there is more source ascribed to sweat glands in addition to serving as progenitors in mice.

Dynamic analysis of mental sweating of eccrine sweat gland of human fingertip by time-sequential piled-up en face optical coherence tomography images.

In this paper, we demonstrate dynamic analysis of mental sweating for sound stimulus of a few tens of eccrine sweat glands by the time-sequential piled-up en face optical coherence tomography (OCT) images with the frame spacing of 3.3 sec. In the experiment, the amount of excess sweat can be evaluated simultaneously for a few tens of sweat glands by piling up of all the en face OCT images. Non-uniformity was observed in mental sweating where the amount of sweat in response to sound stimulus is different for each sweat gland. Furthermore, the amount of sweat is significantly increased in proportion to the strength of the stimulus.