Vasoactive intestinal peptide (VIP) and peptide histidine methionine (PHM) in human eccrine sweat glands: demonstration of innervation, specific binding sites and presence in secretions

Electron transfer studies of a conventional redox probe in human sweat and saliva bio-mimicking conditions

Modern day hospital treatments aim at developing electrochemical biosensors for early diagnosis of diseases using unconventional human bio-fluids like sweat and saliva by monitoring the electron transfer reactions of target analytes. Such kinds of health care diagnostics primarily avoid the usage of human blood and urine samples. In this context, here we have investigated the electron transfer reaction of a well-known and commonly used redox probe namely, potassium ferro/ferri cyanide by employing artificially simulated bio-mimics of human sweat and saliva as unconventional electrolytes. Typically, electron transfer characteristics of the redox couple, [Fe(CN)₆]^3−/4− are investigated using electrochemical techniques like cyclic voltammetry and electrochemical impedance spectroscopy. Many different kinetic parameters are determined and compared with the conventional system. In addition, such electron transfer reactions have also been studied using a lyotropic liquid crystalline phase comprising of Triton X-100 and water in which the aqueous phase is replaced with either human sweat or saliva bio-mimics. From these studies, we find out the electron transfer reaction of [Fe(CN)₆]^3−/4− redox couple is completely diffusion controlled on both Au and Pt disc shaped electrodes in presence of sweat and saliva bio-mimic solutions. Moreover, the reaction is partially blocked by the presence of lyotropic liquid crystalline phase consisting of sweat and saliva bio-mimics indicating the predominant charge transfer controlled process for the redox probe. However, the rate constant values associated with the electron transfer reaction are drastically reduced in presence of liquid crystalline phase. These studies are essentially carried out to assess the effect of sweat and saliva on the electrochemistry of Fe^2+/3+ redox couple.

Sweat gland regeneration: Current strategies and future opportunities

For patients with extensive skin defects, loss of sweat glands (SwGs) greatly decreases their quality of life. Indeed, difficulties in thermoregulation, ion reabsorption, and maintaining fluid balance might render them susceptible to hyperthermia, heatstroke, or even death. Despite extensive studies on the stem cell biology of the skin in recent years, in-situ regeneration of SwGs with both structural and functional fidelity is still challenging because of the limited regenerative capacity and cell fate control of resident progenitors. To overcome these challenges, one must consider both the intrinsic factors relevant to genetic and epigenetic regulation and cues from the cellular microenvironment. Here, we describe recent progress in molecular biology, developmental pathways, and cellular evolution associated with SwGdevelopment and maturation. This is followed by a summary of the current strategies used for cell-fate modulation, transmembrane drug delivery, and scaffold design associated with SwGregeneration. Finally, we offer perspectives for creating more sophisticated systems to accelerate patients' innate healing capacity and developing engineered skin constructs to treat or replace damaged tissues structurally and functionally.

Mechanisms of Autonomic Disturbance in the Face During and Between Attacks of Cluster Headache

Lacrimation and nasal secretion during attacks of cluster headache appear to be due to massive trigeminal-parasympathetic discharge. In addition, the presence of oculo-sympathetic deficit and loss of thermoregulatory sweating and flushing on the symptomatic side of the forehead indicate that the cervical sympathetic pathway to the face is injured in a subgroup of cluster headache patients. In this review, it is argued that a peripheral rather than a central lesion produces signs of cervical sympathetic deficit, probably resulting from compression of the sympathetic plexus around the internal carotid artery. Although trigeminal-parasympathetic discharge appears to be the main trigger for vasodilation during attacks, supersensitivity to neurotransmitters such as vasoactive intestinal polypeptide, together with release of sympathetic vasoconstrictor tone, may boost facial blood flow in patients with cervical sympathetic deficit. In addition, parasympathetic neural discharge may provoke aberrant facial sweating during attacks in patients with cervical sympathetic deficit. Although neither trigeminal-parasympathetic discharge nor cervical sympathetic deficit appears to be the primary trigger for attacks of cluster headache, these autonomic disturbances could contribute to the rapid escalation of pain once the attack begins. For example, a pericarotid inflammatory process that excites trigeminal nociceptors might initiate neurogenic inflammation and trigeminal-parasympathetic vasodilation. To complete the loop, neurogenic inflammation and trigeminal-parasympathetic vasodilation could provoke the release of mast cell products, which aggravate inflammation and intensify trigeminal discharge.

Responses to hyperthermia. Optimizing heat dissipation by convection and evaporation: Neural control of skin blood flow and sweating in humans

Under normothermic, resting conditions, humans dissipate heat from the body at a rate approximately equal to heat production. Small discrepancies between heat production and heat elimination would, over time, lead to significant changes in heat storage and body temperature. When heat production or environmental temperature is high the challenge of maintaining heat balance is much greater. This matching of heat elimination with heat production is a function of the skin circulation facilitating heat transport to the body surface and sweating, enabling evaporative heat loss. These processes are manifestations of the autonomic control of cutaneous vasomotor and sudomotor functions and form the basis of this review. We focus on these systems in the responses to hyperthermia. In particular, the cutaneous vascular responses to heat stress and the current understanding of the neurovascular mechanisms involved. The available research regarding cutaneous active vasodilation and vasoconstriction is highlighted, with emphasis on active vasodilation as a major responder to heat stress. Involvement of the vasoconstrictor and active vasodilator controls of the skin circulation in the context of heat stress and nonthermoregulatory reflexes (blood pressure, exercise) are also considered. Autonomic involvement in the cutaneous vascular responses to direct heating and cooling of the skin are also discussed. We examine the autonomic control of sweating, including cholinergic and noncholinergic mechanisms, the local control of sweating, thermoregulatory and nonthermoregulatory reflex control and the possible relationship between sudomotor and cutaneous vasodilator function. Finally, we comment on the clinical relevance of these control schemes in conditions of autonomic dysfunction.

Cutaneous vasodilator and vasoconstrictor mechanisms in temperature regulation

In this review, we focus on significant developments in our understanding of the mechanisms that control the cutaneous vasculature in humans, with emphasis on the literature of the last half-century. To provide a background for subsequent sections, we review methods of measurement and techniques of importance in elucidating control mechanisms for studying skin blood flow. In addition, the anatomy of the skin relevant to its thermoregulatory function is outlined. The mechanisms by which sympathetic nerves mediate cutaneous active vasodilation during whole body heating and cutaneous vasoconstriction during whole body cooling are reviewed, including discussions of mechanisms involving cotransmission, NO, and other effectors. Current concepts for the mechanisms that effect local cutaneous vascular responses to local skin warming and cooling are examined, including the roles of temperature sensitive afferent neurons as well as NO and other mediators. Factors that can modulate control mechanisms of the cutaneous vasculature, such as gender, aging, and clinical conditions, are discussed, as are nonthermoregulatory reflex modifiers of thermoregulatory cutaneous vascular responses.

Role of neuropeptides, neurotrophins, and neurohormones in skin wound healing

Due to the close interactions between the skin and peripheral nervous system, there is increasing evidence that the cutaneous innervation is an important modulator of the normal wound healing process. The communication between sensory neurons and skin cells involves a variety of molecules (neuropeptides, neurohormones, and neurotrophins) and their specific receptors expressed by both neuronal and nonneuronal skin cells. It is well established that neurotransmitters and nerve growth factors released in skin have immunoregulatory roles and can exert mitogenic actions; they could also influence the functions of the different skin cell types during the wound healing process.

VIP/PACAP receptor mediation of cutaneous active vasodilation during heat stress in humans

Vasoactive intestinal peptide (VIP) is implicated in cutaneous active vasodilation in humans. VIP and the closely related pituitary adenylate cyclase activating peptide (PACAP) act through several receptor types: VIP through VPAC1 and VPAC2 receptors and PACAP through VPAC1, VPAC2, and PAC1 receptors. We examined participation of VPAC2 and/or PAC1 receptors in cutaneous vasodilation during heat stress by testing the effects of their specific blockade with PACAP6-38. PACAP6-38 dissolved in Ringer's was administered by intradermal microdialysis at one forearm site while a control site received Ringer's solution. Skin blood flow was monitored by laser-Doppler flowmetry (LDF). Blood pressure was monitored noninvasively and cutaneous vascular conductance (CVC) calculated. A 5- to 10-min baseline period was followed by approximately 70 min of PACAP6-38 (100 microM) perfusion at one site in normothermia and a 3-min period of body cooling. Whole body heating was then performed to engage cutaneous active vasodilation and was maintained until CVC had plateaued at an elevated level at all sites for 5-10 min. Finally, 58 mM sodium nitroprusside was perfused through both microdialysis sites to effect maximal vasodilation. No CVC differences were found between control and PACAP6-38-treated sites during normothermia (19 +/- 3%max untreated vs. 20 +/- 3%max, PACAP6-38 treated; P > 0.05 between sites) or cold stress (11 +/- 2%max untreated vs. 10 +/- 2%max, PACAP6-38 treated, P > 0.05 between sites). PACAP6-38 attenuated the increase in CVC during whole body heating when compared with untreated sites (59 +/- 3%max untreated vs. 46 +/- 3%max, PACAP6-38 treated, P < 0.05). We conclude that VPAC2 and/or PAC1 receptor activation is involved in cutaneous active vasodilation in humans.

Mechanisms and controllers of eccrine sweating in humans

Human body temperature is regulated within a very narrow range. When exposed to hyperthermic conditions, via environmental factors and/or increased metabolism, heat dissipation becomes vital for survival. In humans, the primary mechanism of heat dissipation, particularly when ambient temperature is higher than skin temperature, is evaporative heat loss secondary to sweat secretion from eccrine glands. While the primary controller of sweating is the integration between internal and skin temperatures, a number of non-thermal factors modulate the sweating response. In addition to summarizing the current understanding of the neural pathways from the brain to the sweat gland, as well as responses at the sweat gland, this review will highlight findings pertaining to studies of proposed non-thermal modifiers of sweating, namely, exercise, baroreceptor loading state, and body fluid status. Information from these studies not only provides important insight pertaining to the basic mechanisms of sweating, but also perhaps could be useful towards a greater understanding of potential mechanisms and consequences of disease states as well as aging in altering sweating responses and thus temperature regulation.

Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

This review focuses on the role of the peripheral nervous system in cutaneous biology and disease. During the last few years, a modern concept of an interactive network between cutaneous nerves, the neuroendocrine axis, and the immune system has been established. We learned that neurocutaneous interactions influence a variety of physiological and pathophysiological functions, including cell growth, immunity, inflammation, pruritus, and wound healing. This interaction is mediated by primary afferent as well as autonomic nerves, which release neuromediators and activate specific receptors on many target cells in the skin. A dense network of sensory nerves releases neuropeptides, thereby modulating inflammation, cell growth, and the immune responses in the skin. Neurotrophic factors, in addition to regulating nerve growth, participate in many properties of skin function. The skin expresses a variety of neurohormone receptors coupled to heterotrimeric G proteins that are tightly involved in skin homeostasis and inflammation. This neurohormone-receptor interaction is modulated by endopeptidases, which are able to terminate neuropeptide-induced inflammatory or immune responses. Neuronal proteinase-activated receptors or transient receptor potential ion channels are recently described receptors that may have been important in regulating neurogenic inflammation, pain, and pruritus. Together, a close multidirectional interaction between neuromediators, high-affinity receptors, and regulatory proteases is critically involved to maintain tissue integrity and regulate inflammatory responses in the skin. A deeper understanding of cutaneous neuroimmunoendocrinology may help to develop new strategies for the treatment of several skin diseases.

Neuroimmunological findings in allergic skin diseases

PURPOSE OF REVIEW

Recent studies have gained widespread information about the complex regulation of genetic, environmental, immunologic, and pharmacologic factors that contribute to the development of allergic inflammatory skin diseases such as atopic dermatitis. Neuroimmune mechanisms, however, still remain to be elucidated. This review will focus on the interaction between the cutaneous immune and peripheral nervous system in allergic inflammatory skin such as atopic dermatitis.

RECENT FINDINGS

Neuropeptides and neuropeptide-positive nerve fibres are prominently increased in lesions of atopic dermatitis. The density of nerve fibres is increased while peripheral nerve endings are in an active state of excitation. In this regard, neurotrophins particularly described for their functional role on nerve cells are also expressed in atopic dermatitis skin. In addition, neurotrophins modulate the functional role of eosinophils as main target effector cells in atopic dermatitis, as described recently. Interestingly, eosinophils are capable of neurotrophin as well as neuropeptide production itself, pointing to a bidirectional communication between neuronal cell populations and main target effector cells.

SUMMARY

Neurotrophins and neuropeptides modulate both the functional activity of sensory neurons and immune cells. We have therefore developed the concept of a neuroimmune network between target effector cells and sensory nerves that links pathogenic events to dysfunctions of the cutaneous immune and peripheral nervous system in allergic inflammatory skin diseases.

Evidence for a role for vasoactive intestinal peptide in active vasodilatation in the cutaneous vasculature of humans

Active vasodilatation (AVD) in human, non-glabrous skin depends on functional cholinergic fibres but not on acetylcholine (ACh). We tested whether AVD is a redundant system in which ACh and vasoactive intestinal polypeptide (VIP) are co-released from cholinergic nerves. (1) We administered VIP by intradermal microdialysis to four discrete areas of skin in the presence of different levels of the VIP receptor antagonist, VIP(10-28), also delivered by microdialysis. Skin blood flow (SkBF) was continuously monitored by laser Doppler flowmetry (LDF). Mean arterial pressure (MAP) was measured non-invasively and cutaneous vascular conductance (CVC) calculated as LDF/MAP. Subjects were supine and wore water-perfused suits to control whole-body skin temperature (Tsk) at 34 degrees C. Concentrations of 54 microM, 107 microM, or 214 microM VIP(10-28) were perfused via intradermal microdialysis at 2 microl min-1 for approximately 1 h. Then 7.5 microM VIP was added to the perfusate containing VIP(10-28) at the three concentrations or Ringer solution and perfusion was continued for 45-60 min. At the control site, this level of VIP caused approximately the vasodilatation typical of heat stress. All VIP(10-28)-treated sites displayed an attenuated dilatation in response to the VIP. The greatest attenuation was observed at the site that received 214 microM VIP(10-28) (P < 0.01). (2) We used 214 microM VIP(10-28) alone and with the iontophoretically administered muscarinic receptor antagonist atropine (400 microA cm-2, 45 s, 10 mM) in heated subjects to test the roles of VIP and ACh in AVD. Ringer solution and 214 microM VIP(10-28) were each perfused at two sites, one of which in each case was pretreated with atropine. After 1 h of VIP(10-28) treatment, individuals underwent 45-60 min of whole-body heating (Tsk to 38.5 degrees C). VIP(10-28), alone or in combination with atropine, attenuated the increase in CVC during heat stress, suggesting an important role for VIP in AVD.

Neuropeptídeos na pele

Há evidências crescentes de que a inervação cutânea é capaz de modular uma variedade de fenômenos cutâneos agudos e crônicos, interagindo com as células da pele e seus componentes imunes. Essa forma de sinalização local entre tecido nervoso e tecido cutâneo ocorre especialmente por meio dos neuropeptídeos, uma numerosa família de neurotransmissores de natureza química comum e nomenclatura heterogênea presentes em todo o sistema nervoso e secretados pelas fibras nervosas cutâneas. São alvo desta revisão os neuropeptídeos substância P (SP), o peptídeo relacionado ao gene da calcitonina (CGRP), o peptídeo vasoativo intestinal (VIP), o peptídeo ativador da adenilato-ciclase pituitária (PACAP), o neuropeptídeo Y (NPY) e a somatostatina (SOM). Serão discutidas suas ações sobre as células da pele e sistema imune, bem como estudos recentes que sugerem a participação dos neuropeptídeos nas respostas inflamatórias cutâneas, nas reações de hipersensibilidade e em dermatoses humanas, notadamente na psoríase, dermatite atópica, hanseníase e alopecia.

Proposal of sampling process for collecting human sweat and determination of caffeine concentration in it by using GC/MS

Caffeine concentration in human sweat was estimated by measuring separately the amounts of water and caffeine. After washing a finger with tap water for 15 s and waiting 2 min for drying, 70 microL aqueous ethanol solution in a small vial (0.6 mL) was used to sample for several minutes. Then 3 microL of the aliquot was used for GC/MS analysis of caffeine. As a first-order relationship between the sweat amount secreted on the left and right hands was obtained (correlation factor 0.848), the amount of sweat secretion during sampling on one hand was estimated by the value obtained on the other hand. This new indirect evaluation was used for the estimation of the amount of sweat secreted during sampling. Typical variations of caffeine concentration in sweat were demonstrated. Thirty minutes after the intake of caffeine, it was secreted in sweat, and the secretion had continued for more than 4 h.

Neuropeptides in the skin: interactions between the neuroendocrine and the skin immune systems

The interaction between components of the nervous system and multiple target cells in the cutaneous immune system has been receiving increasing attention. It has been observed that certain skin diseases such as psoriasis and atopic dermatitis have a neurogenic component. Neuropeptides released by sensory nerves that innervate the skin and often contact epidermal and dermal cells can directly modulate functions of keratinocytes, Langerhans cells (LC), mast cells, dermal microvascular endothelial cells and infiltrating immune cells. Among these neuropeptides the tachykinins substance P (SP) and neurokinin A (NKA), calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP) and somatostatin (SOM) have been reported to effectively modulate skin and immune cell functions such as cell proliferation, cytokine production or antigen presentation under physiological or pathophysiological conditions. Expression and regulation of their corresponding receptors that are expressed on a variety of skin cells as well as the presence of neuropeptide-specific peptidases such as neutral endopeptidase (NEP) or angiotensin-converting enzyme (ACE) determine the final biological response mediated by these peptides on the target cell or tissue. Likewise, skin cells like keratinocytes or fibroblasts are a source for neurotrophins such as nerve growth factor that are required not only for survival and regeneration of sensory neurons but also to control responsiveness of these neurons to external stimuli. Therefore, neuropeptides, neuropeptide receptors, neuropeptide-degrading enzymes and neurotrophins participate in a complex, interdependent network of mediators that modulate skin inflammation, wound healing and the skin immune system. This review will focus on recent studies demonstrating the role of tachykinins, CGRP, SOM and VIP and their receptors and neuropeptide-degrading enzymes in mediating neurogenic inflammation in the skin.

Neuropeptides in skin

Neuropeptides are a heterogeneous group of more than 50 molecules that play a role in various cutaneous functions and diseases; they act as neuromodulators, neurotransmitters, neurohormones, and hormones. In the skin, neuropeptides are synthesized locally (i.e., in keratinocytes and in endothelial cells) and are transported by nerve fibers or immune cells (i.e., lymphocytes, monocytes, and polymorphonuclear cells). Specific receptors and binding sites for neuropeptides have been described in different cell lines in the skin (keratinocytes, endothelial cells, immune cells, fibroblasts). Many different biologic actions of neuropeptides have been demonstrated. Depletion of cutaneous neuropeptides (i.e., with capsaicin cream) or therapeutic use of neuropeptide agonists and/or antagonists may aid in the treatment of skin diseases.

The effects of vasoactive intestinal polypeptide and substance P on methacholine-induced sweating and vascular flare in diabetic neuropathy

Vasoactive intestinal polypeptide (VIP) and substance P (SP) immunoreactivity are reduced in the cutaneous nerves of diabetic patients with peripheral neuropathy. The functional significance of this finding was studied by measuring the forearm sweat response to intradermal methacholine and the effect of coadministration of VIP and SP in six normal subjects, and in six diabetic patients with neuropathy and eight without. Flare responses to the two peptides were also measured. Methacholine-induced sweat output was significantly greater in neuropathic patients compared with the other groups (p < 0.05), suggesting upper limb denervation supersensitivity. VIP and SP alone did not evoke sweating in any subject. Injection of VIP or SP reduced methacholine-induced sweating to a similar degree in all groups, except that the reduction was smaller in the non-neuropathic group than in the others (p = 0.028 versus normal subjects, p = 0.014 versus neuropathic diabetic patients). Flare responses to the peptides were markedly reduced in the neuropathic patients compared with the other groups (p < 0.01). In neuropathic patients, increased sweat responses and decreased flare coexist with diminished neurophysiological measurements; cutaneous sweating and flare responses provide valuable additional information to conventional methods of neurological assessment in diabetic neuropathy.

Sympathetic sudomotor function and aging

Many studies have reported the influence of aging on different portions of the autonomic nervous system components but only partially for the sympathetic cholinergic system. We evaluated postganglionic sudomotor function in 196 healthy subjects, 104 women and 92 men, by determining sweat gland density (SGD) per square centimeter of skin, on the dorsum of the hand and foot, with the impression mold technique. The age range was from 5 to 84 years. A significant decrease of SGD was observed in both hand and foot in relation to age (P < 0.001). The ANOVA analysis of foot data shows that age is the only significant factor for SGD reduction. In the hand, both sex and body surface area are significant covariates with age. The dorsum of the foot is the most appropriate place to examine sweating in studies of aging. The lower normal limits for SGD in the foot are 213/cm2 for subjects younger than 30 years, 199/cm2 for those from 30 to 59 years, and 123/cm2 for subjects over 59 years old.

The regional distribution of neuropeptides in human skin as assessed by radioimmunoassay and high-performance liquid chromatography

In this study radioimmunoassay was used to determine neuropeptide levels in extracts from 17 differing anatomical regions of human skin. Marked regional variations of neuropeptide content for human skin were found and these variations are likely to reflect true physiological functions for the neuropeptides studied. In general the tachykinins, substance P (SP), neurokinin A (NKA) and calcitonin gene-related peptide (CGRP) were found in highest concentrations in regions of skin with the greatest tactile sensation. By contrast, highest concentrations of vasoactive intestinal peptide (VIP) and peptide histidine methionine (PHM) were found in axillary skin, where they probably play a part in axillary eccrine sweat production. Neurotensin was not found in any of the skin areas sampled, suggesting that it is relatively unimportant in human physiological skin control. Reverse-phase high-performance liquid chromatography (rpHPLC) was used to verify the results of radioimmunoassay. Both SP and NKA occurred in several regions in both their reduced and oxidized forms, as well as displaying molecular heterogeneity. CGRP occurred as one molecular species, this being alpha-CGRP, suggesting that this is the predominant molecular form in human skin. Likewise, both VIP and PHM displayed molecular homogeneity in the regions investigated by rpHPLC.

Neuropeptides and a neuronal marker in cutaneous innervation during human foetal development

There is evidence that foetal body movements first occur at 6 weeks gestation, and that the reflex arc is functional at 8 weeks. This correlates with the detection of the sensory neuropeptides calcitonin gene-related peptide (CGRP) and substance P (SP) in spinal cord at 10 weeks gestation. However, the development of cutaneous neuropeptide-containing nerves is not well documented in humans. We have investigated the early appearance and distribution pattern of CGRP, SP, vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY), as well as those of the general neuronal marker protein gene product 9.5 (PGP) in various areas of foetal skin at different gestational ages. PGP-immunoreactive nerves were first seen in the subepidermal plexus at 6 weeks gestational age. Initially, the immunoreactive nerves are thick, club-shaped and distributed in the superficial dermis. Beaded adult-like fibres become more numerous only at later ages (10-12 weeks), and extend from this plexus to penetrate the epidermis. Histologically, the skin of the hand develops faster than that of other body areas and at 9 weeks, more PGP-immunoreactive nerves were seen in the palm than in the dorsum. Primitive sweat glands were first noted in axillary skin at 17 weeks, accompanied by a few PGP-immunoreactive nerves. Occasional, small CGRP-immunoreactive fibres were first noticed in the dermis at 7 weeks, but it was at 17 weeks that the presence of this neuropeptide was unequivocal in the subepidermal plexus. Sparse VIP-, SP- and NPY-immunoreactive fibres were not found until 16-17 weeks gestation, when they were seen in the dermis and around small blood vessels.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptides in skin disease: increased VIP in eczema and psoriasis but not axillary hyperhidrosis

The neuropeptides vasoactive intestinal polypeptide (VIP), substance P and somatostatin were studied in skin biopsies from patients with eczema, psoriasis and axillary hyperhidrosis. VIP concentrations were elevated in skin affected by eczema and psoriasis, whereas substance P and somatostatin levels did not differ from controls. There was a higher concentration of VIP, but not of substance P or somatostatin, in normal axillary skin when compared to adjacent trunk skin, with abundant VIP-containing fibres surrounding eccrine sweat glands. The VIP concentration was unchanged in skin affected by axillary hyperhidrosis. VIP may increase local blood flow in eczema and psoriasis, but does not appear to play a role in axillary hyperhidrosis.