++
To maintain thermal balance, heat gained or lost by the body must equal heat dissipated from, or produced by the body. This concept can be mathematically expressed as:
++
++
+
- ΔS = change in heat storage by the body
- M = metabolic heat production and is defined as the rate of transformation of chemical energy into heat and mechanical work
- E = evaporative heat loss and is defined as the rate of heat loss by evaporation of water from skin and surfaces of the respiratory tract. E is dependant on (1) rate of sweat secretion, (2) water vapor pressure of the environment, and (3) the area of evaporative surface.
- R = radiant heat gain or loss and is defined as heat exchange by emission and absorption of electromagnetic (infrared) radiation. This component accounts for 50%–60% of the heat loss in a thermally comfortable (thermoneutral) individual, but can easily become a net heat gain as when one is in direct sunlight.
- C = convective heat gain or loss and is defined as heat exchange due to forced movement of a fluid, either liquid or gas. This component is responsible for the transfer of heat to the skin through skin blood flow and transfer from the skin to the environment by air or water movement. Within the body, the cardiovascular system is the major mediator of convective heat transfer. C is dependent on (1) body surface area, (2) temperature differences, and (3) fluid (or air) movement.
- K = conductive heat gain or loss and is defined as heat transfer by flow down a temperature gradient, as between tissues and blood, between blood and skin, and between skin and the environment. This is usually combined with convective heat transfer.
- W = useful mechanical work
++
The sum of R, C, and K is determined by the temperature gradient between the skin and the environment. If ΔS is zero, the body is in heat balance. This “thermoneutral” condition is characterized as having low skin blood flow of approximately 5% of cardiac output. Sweating does not occur during thermoneutrality.
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If ΔS < 0, the body is losing heat, core temperature is falling, and thermoregulatory reflexes are evoked to conserve heat. Thermal stability is maintained through reduction of skin blood flow that can approach zero during maximal vasoconstriction. Reduction in skin blood flow increases the thermal insulation between the body and the environment by minimizing losses through conductive (K), convective (C), and radiant (R) mechanisms. If heat loss continues despite low skin blood flow, metabolic generation of heat (M) through the shivering of skeletal muscle is initiated to restore and maintain core temperature. Brown adipose tissue can also be a source of metabolic heat generation through nonshivering thermogenesis.1 Although originally thought to be important only in human neonates where brown adipose tissue is 2%–5% of body weight, some brown adipocytes persist into adulthood.2 These adipocytes can directly generate heat (M) to maintain core temperature.3 If, despite all these mechanisms, ΔS remains negative, core temperature will fall and life-threatening hypothermia may result.
++
If ΔS > 0, the body is gaining heat and core temperature is rising. Under this circumstance of heat stress, thermal stability is maintained by increases in skin blood flow to facilitate heat loss through K, C, and even R losses. If heat gain continues despite these mechanisms, sweating is evoked to increase heat loss through evaporation (E) of perspiration. If ΔS remains positive, blood flow is diverted from skeletal muscle and gastrointestinal beds, providing for dramatic increases in skin blood flow. Sweat rate will also increase until maximal levels are achieved. If, despite maximal skin blood flow and maximal stimulation of sweating, ΔS remains positive, core temperature will rise and life-threatening hyperthermia, i.e., heat stroke, will occur.
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Anatomic Considerations
++
The critical role of the skin in human thermoregulation is well understood: thermoregulation is achieved through variations in blood flow and sweat production so as to maintain thermal stability.4 Without these variations, thermal stability cannot be maintained resulting in risk of hypothermia or hyperthermia (see Chapters 94 and 95). Under normothermic conditions, skin blood flow ranges from 30–40 mL/min/100 g of skin in resting humans. However, the cutaneous vasculature is exceedingly compliant so that skin blood flow can vary from nearly zero during cold stress periods with maximal vasoconstriction to 8 L/min over the body's surface during maximal vasodilation in heat stress.5
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Blood vessels in the skin are arranged in several plexuses in superficial and deep layers parallel to the skin surface. Most vessels are in the superficial layer and consist of high-resistance terminal arterioles, papillary loops, and postcapillary venules. Papillary loops are true capillaries. Blood flow through the loops is controlled by highly innervated arterioles. The loops are located near the dermal–epidermal junction, a region characterized by a maximal thermal gradient because of its proximity to the skin surface. Since the papillary loops also have a large surface area, blood flow through these vessels is a major determinant of heat exchange through vasodilation during heat stress and vasoconstriction during cold stress.
++
While papillary loops are found in both glabrous (palms, plantar aspect of feet, and lips) and nonglabrous skin (most of the body's surface, including the limbs, head, and trunk), arteriovenous anastomoses (AVAs) are found mainly in glabrous skin. They represent direct connections between arterioles and venules that bypass the high-resistance arterioles and capillaries of the papillary loops. AVAs have thick muscular walls with rich noradrenergic innervation and lie deep to papillary loops.6 Because of their deeper location in the dermis and smaller surface area, AVAs are less efficient in heat transfer than papillary loops. While AVAs dilate in response to heat stress and constrict during mild-to-moderate cold stress, their major role is to mediate local vasodilation during prolonged cold exposure. AVA vasodilation delivers warm blood to maintain tissue temperature and thus tissue viability through “cold-induced vasodilation.”7
++
Sweat glands also play a major role in human thermoregulation (see Chapter 83). The critical thermoregulatory role of the eccrine sweat glands that are found over most of the body surface is well known. Clearly, the main function of eccrine sweat glands is to increase heat loss through the evaporation of sweat. The density of these glands varies from 700 glands per cm2 in planar and plantar skin to 64 glands per cm2 on the back8; these glands may hypertrophy with repeated heat exposure.9 Each gland is made up of a secretory coil found in the dermis with a duct that extends through the dermis and epidermis to the surface of the skin. Sweat is secreted as an isotonic fluid by the coils. NaCl is reabsorbed within the ducts so sweat that is finally delivered to the surface is hypotonic.10 Each liter of sweat evaporated is capable of removing 580 kcal from the body. Although apocrine sweat glands have been dismissed as “atavistic scent glands,” this has recently been questioned.11 Apocrine glands are usually associated with hair follicles and are most developed on the scalp, face, upper back, and chest. It has been proposed that sebum from apocrine glands acts as a surfactant at high temperatures and, thus, facilitates dispersion of eccrine sweat over the skin's surface. At low temperatures, sebum may function to repel water from the skin and, thus, reduce heat loss.
+++
Cutaneous Thermoregulatory Mechanisms
++
Cutaneous circulation is a major effector of human thermoregulation.4 During heat stress, elevated internal temperature and skin temperature lead to cutaneous vasodilation through neural mechanisms and the local effect of higher temperatures on the skin vessels themselves. During the periods of cold stress, reduced temperatures mediate a cutaneous vasoconstriction through neural as well as local vascular effects. Under normothermic conditions, skin blood flow averages approximately 5% of cardiac output; however, the absolute amount of blood in the skin can vary from nearly zero during periods of maximal vasoconstriction in severe cold stress to as much as 60% of cardiac output in severe heat stress.5
++
Heat dissipation through the secretion and evaporation of eccrine sweat is critical to maintaining thermal stability in hot environments or during heat stress induced by strenuous dynamic exercise. Indeed, when environmental temperature exceeds blood temperature, the evaporation of sweat is the sole mechanism for heat dispersal. Sweat secretion is controlled primarily by sympathetic cholinergic nerves that release acetylcholine (Ach) to activate muscarinic receptors on the glands. Sweat secretion can be augmented by local production of nitric oxide near sweat glands.12 Stimulated glands produce an isotonic fluid that becomes progressively hypotonic as the Na+ is reabsorbed in the sweat gland duct by active ion transfer (see Chapter 84).
+++
Neural Control Mechanisms of the Cutaneous Vasculature
++
In glabrous skin, cutaneous arterioles are innervated by sympathetic vasoconstrictor nerves that release norepinephrine and other cotransmitters.7,13–16 All thermoregulatory reflex changes in blood flow in these areas are caused by changes in noradrenergic vasoconstrictor activity and the effects of local temperature on the skin blood vessels themselves (Fig. 93-1).4,7,17
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++
In nonglabrous skin, changes in skin blood flow are mediated by two branches of the sympathetic nervous system: (1) noradrenergic vasoconstrictor nerves as found in glabrous skin and (2) a cholinergic active vasodilator system.4,7,17 These dual sympathetic neural control mechanisms are the major effectors of thermoregulatory responses. Vessels in nonglabrous skin also respond to the effects of local temperature changes (Fig. 93-2).4,7,17
++
++
In normothermia, cutaneous arterioles are under little neural tone. During cold stress, reduction of skin temperature and/or internal temperature cause a thermoregulatory reflex-mediated reduction in skin blood flow to conserve body heat. Enhanced noradrenergic vasoconstrictor tone mediates an arteriolar vasoconstriction and, thus, decreases skin blood flow.
++
Conversely, during heat stress, thermoregulatory reflexes that facilitate body cooling are affected. As internal temperature continues to rise over a threshold value of approximately 37°C (98.6°F), a cutaneous vasodilation begins. At this threshold, active vasodilator tone to the cutaneous arterioles is enhanced. At rest, sweating also begins at the same internal temperature threshold. Vasodilator tone increases as internal temperature increases. Enhanced vasodilator activity decreases smooth muscle tone, leading to an arteriolar vasodilation, and, thus, an increase in skin blood flow, especially through the papillary loops. High skin blood flow delivers heat to the body surface where it is dissipated to the environment in conjunction with the evaporation of sweat. Overall, the active vasodilator system is responsible for 80%–95% of the elevation in skin blood flow that accompanies heat stress. A small, but significant portion of the vasodilation is mediated by the direct vasodilator effects of local heat on the cutaneous vessels.18
++
Dual vasoconstrictor nerves and vasodilator nerves in skin were first suggested in 1931 by Lewis and Pickering19 and confirmed by Grant and Holling.20 They measured skin temperature as an index of blood flow in the human forearm and found that large increases in response to heat stress could be abolished by sympathectomy or nerve blockade. They noted that while sympathectomy or nerve blockade caused only a slight cutaneous vasodilation during normothermia, heat stress elicited a much greater increase in skin blood flow. In addition, nerve blockade during established heat stress abolished any cutaneous vasodilation. These results suggested that cutaneous vessels in nonglabrous skin are innervated by sympathetic active vasodilator as well as sympathetic vasoconstrictor nerves. In the 1950s, their findings were confirmed by Edholm et al21 and by Roddie et al.22 In addition, it has been shown that bretylium tosylate (a prejunctional noradrenergic neuronal blocking agent) abolishes the cutaneous vasoconstriction induced by cold stress, but does not alter the vasodilator responses induced by heat stress.23 This confirmed that dual efferent neural systems control the cutaneous arterioles: a noradrenergic vasoconstrictor system and a nonadrenergic active vasodilator system.
+++
Cutaneous Active Vasodilator Mechanisms
++

Despite the fact that the cutaneous active vasodilator system has been studied for many decades, the specific mechanisms by which the cutaneous active vasodilator system functions are only partly understood.
19,20,24 Several hypotheses have been proposed for how active vasodilation in skin works, but none has been completely proven.
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Sudomotor Activity and Active Vasodilation
++

In the initial descriptions of cutaneous active vasodilation, it was noted that sweating and active vasodilation began at about the same time in a resting person subjected to heat stress.
20 This observation led to the hypothesis that the mechanism of cutaneous active vasodilation involved cholinergic sudomotor nerve activity.
20,25–27 Additional evidence that favored a close mechanistic relationship between active vasodilation and sweating comes from observations of persons with anhidrotic ectodermal dysplasia and Ross Syndrome. Persons with this congenital anhidrotic ectodermal dysplasia lack sweat glands (see
Chapter 83), do not sweat nor do they have cutaneous active vasodilator responses to heat stress.
27 Persons with Ross syndrome have an acquired loss of cutaneous cholinergic neurons and, hence, do not sweat or vasodilate skin during heat stress.
28 Patients with either of these rare conditions are extremely heat intolerant. Despite observations such as this, the precise relationship between sweat glands and active vasodilation remains poorly defined. While it is clear that cholinergic sudomotor nerves control sweat glands, whether sudomotor and vasodilator nerves are one and the same or separate systems is unknown.
29,30
+++
Cotransmission and Cutaneous Active Vasodilation
++

An alternative hypothesis with strong support proposes that cutaneous active vasodilation is mediated by a complex cotransmitter system (
eFig. 93-2.1). This proposal is based on studies of an atropine-resistant, cholinergic cotransmitter system in the cat paw that relies on corelease of
acetylcholine (Ach) and vasoactive intestinal peptide (
VIP) to accomplish vasodilation.
31 Based on these observations, it was proposed that a single set of neurons in human skin could control both active vasodilation of skin arterioles and sweating by releasing both Ach and the neuropeptide cotransmitter
VIP. Ach would cause sweating and
VIP would cause active vasodilation.
31 This atropine-resistant cotransmitter mechanism could explain why
atropine abolishes sweating, but not active vasodilation during heat stress in humans.
4,32
++
++

The cotransmission hypothesis is attractive for several reasons: (1)
VIP is a vasodilator (via cyclic
adenosine monophosphate [cAMP]); (2)
VIP is found in human nerve endings associated with sweat glands
33 and blood vessels
34; and, (3)
VIP is colocalized with Ach in cholinergic nerve terminals.
33 VIP has also been implicated in the control of sweat glands; exogenous
VIP increases muscarinic receptor affinity for
methacholine (a muscarinic receptor agonist) and may thus promote sweat production as well as active vasodilation.
35,36
++

The cotransmission hypothesis was tested in humans in a series of three studies designed to examine the involvement of cotransmission in the cutaneous active vasodilator system.
37 The first of these studies confirmed the classic results of Roddie et al
22 that local application of
atropine to skin abolished sweating completely, but only slightly attenuated and delayed active vasodilation during heat stress. A second study showed that iontophoretic pretreatment of skin with
atropine blocked
all vasodilatory responses to exogenously applied Ach, demonstrating that all cutaneous vascular responses to Ach are mediated by muscarinic receptors. These two studies demonstrated that Ach could not be the only neurotransmitter that mediated active vasodilation.
37 In a third study, botulinum toxin was injected into discrete areas of skin and the effect on subsequent skin blood flow responses to heat stress was evaluated. Botulinum toxin is taken up specifically by cholinergic nerve terminals and interrupts the release of
all neurotransmitters from those terminals. Treatment of skin with this agent completely abolished both cutaneous active vasodilation and sweating in the treated area of skin during heat stress. This series of three studies led to the following conclusions: (1) the only functionally important cholinergic receptors on skin blood vessels are muscarinic; (2) active vasodilation and sweating are mediated by cholinergic nerves; (3) the substances causing vasodilation must include at least one neurotransmitter coreleased with Ach from cholinergic nerves.
37
++

One of the current areas of interest in thermoregulatory research is the nature of the cotransmitter (or cotransmitters) that affect active vasodilation. Recent work supports the involvement of
VIP as a cotransmitter in active vasodilation.
38,39 This work tested whether active vasodilation is mediated, in part, by
VIP coreleased from cholinergic nerves with Ach. The neuropeptide fragment,
VIP10-28, was administered by microdialysis to block the effects of
VIP at VPAC1 and VPAC2 receptors. This agent was chosen because it not only blocks the two major receptors for
VIP, it also blocks the effects of peptide histidine methionine (PHM). These two neuropeptides share a close structural relationship, are formed from the same prepropeptide, and are both reported to be present in human skin.
40 VIP10-28 attenuated (but did not abolish) the increase in skin blood flow during heat stress. The combination of
VIP10-28 with
atropine did not enhance the degree of attenuation achieved with
VIP10-28 alone. This finding showed a role for
VIP in active vasodilation; however, since the combination of muscarinic receptor with
VIP receptor blockade did not differ from
VIP receptor blockade only, it was postulated that additional cotransmitters may well be involved in the process.
38,39
++

Recent evidence suggests a role for the neurokinin Substance P in cutaneous active vasodilation in heat stress.
41 Neurokinin receptors can be desensitized by repeated exposures to Substance P so that less vasodilation in response to subsequent exposures to Substance P.
42 It has been shown that repeated exposure to Substance P reduced the subsequent vasodilator response to heat stress, in support of a role for neurokinin receptors and perhaps Substance P as a mediator of cutaneous active vasodilation.
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Nitric Oxide and Active Vasodilation
++

In the 1990s, a series of studies were done that investigated the control of skin blood flow in the rabbit ear. Rabbits (and other lagomorphs) thermoregulate by actively dilating and constricting the blood vessels in their ears. Rabbits thus provided a possible animal model for study of the dual vasomotor controls of human nonglabrous skin. The studies in the rabbit ear showed a role for nitric oxide (NO) in thermoregulatory reflex-mediated active vasodilation.
43,44 This work provided the rationale to study and clarify roles for the NO system in cutaneous active vasodilation in humans.
45–47 Although initial work based on intra-arterial infusions of the nitric oxide synthase (NOS) inhibitor NG-monomethyl-
l-arginine (
l-NMMA) was unable to establish such a role,
47 later work
45,48 used intradermal microdialysis to deliver NOS inhibitors NG-nitro-
l-arginine-methyl ester (
l-NAME) and
l-NMMA into small areas of skin. These studies found that increases in skin blood flow caused by active vasodilation during heat stress were significantly attenuated but not abolished by NOS inhibition.
45,46,48 The results of these studies show that active vasodilation in skin requires functional NOS to achieve full expression.
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The foregoing discussion of NO might lead one to believe that since “functional” NOS is required for active vasodilation, NO levels in skin must increase during heat stress to cause active cutaneous vasodilation; however, based on studies of how NO functions as a vasodilator in the rabbit ear during heat stress a novel theory was proposed.
49 It had been noted that NOS antagonists abolished heat stress-induced active vasodilation in skin of the rabbit ear. Administration of low doses of the NO donor,
nitroprusside, could restore ear skin vasodilation during heat stress despite NOS blockade; however, the same dose of
nitroprusside infused into the ear circulation in normothermia did not raise ear blood flow at all. The implication was that active vasodilation in the rabbit ear required the presence of NO and activation of vasodilator nerves, but that these two elements were not arranged in series. This lead to the idea that vasodilator nerve activity did not increase NO production, but rather that NO needed to be present to permit another neurotransmitter to effect increases in ear skin blood flow. It was thus proposed that NO served a “permissive” role in the active vasodilation rather than as an actual effector of the process; i.e., NO had to be present for vasodilation to be effected by another neurotransmitter, but that the absolute level of NO did not increase in heat stress.
++

An initial test of this proposal in human skin examined whether increased levels of NO breakdown products could be found in skin during hyperthermia, but no such increases were found.
50 This suggested that NO acted as a “permissive factor” rather than as an effector of cutaneous active vasodilation; however, the study of NO breakdown products is fraught with problems. Subsequent examinations of NO changes in heat stress were done by measuring bioavailable NO by NO-selective amperometric electrodes in vivo.
38 This technique measures bioavailable NO directly from the tissue of interest. In contrast to the initial study based on NO breakdown products, it was found that during heat stress both skin blood flow and bioavailable NO concentrations increased during heat stress. This proved that NO does increase in skin during heat stress in humans, attendant to active vasodilation. This result suggests that NO has a role beyond that of a “permissive factor” in cutaneous active vasodilation; rather, since NO increases in heat stress, it could well be an effector of cutaneous vasodilation during heat stress.
38,48,51
++

Additional evidence in favor of NO as an active effector of cutaneous active vasodilation came from recent work using an “NO-clamp” technique.
52 Intradermal microdialysis was used to deliver a combination of NOS inhibition by
l-NAME with the NO donor
nitroprusside into skin. The effect of this combination was to “clamp” NO at a constant level, i.e., NO was present but did not increase because of NOS blockade. When NO levels were “clamped” and not allowed to increase, active vasodilation during heat stress was attenuated. This showed that NO did not act “permissively,” but rather was an active effector of vasodilation. It is thus clear that NO production increases during heat stress and that NO acts as an actual effector of cutaneous active vasodilation.
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It should be noted that all of the foregoing work on NO in human skin was done in nonglabrous skin. This type of skin has a paucity of AVAs. In contrast, the skin of the rabbit ear is rich in AVAs. It may well be that NO plays a “permissive” role in AVA dilatation, but not in the papillary loops of nonglabrous skin.
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Another question about the role of NO in active vasodilation involves the factors that mediate increased production of NO during heat stress. One hypothesis was that Ach from cholinergic nerves increased NO via muscarinic receptor stimulation.
53 This was investigated by examining the effects of combined muscarinic receptor blockade by
atropine with NOS blockade by
l-NAME on vasodilation during heat stress. These agents were given
after activation of the active vasodilator system in prolonged heat stress, when skin blood flow had already risen significantly. While
atropine was found to have little effect on skin blood flow,
l-NAME was found to significantly reduce skin blood flow during established active vasodilation in prolonged heat stress. This showed that although production of NO was required for active vasodilation, muscarinic receptor-mediated NO production was not needed to
sustain active vasodilation during the
late, established phase of heat stress.
53
++

Subsequent work has examined whether muscarinic receptor activation leads to NO production
early in heat stress.
54 Based on earlier studies that showed
atropine delayed the onset of active vasodilation during heat stress, it was postulated that Ach contributed to active vasodilation through muscarinic receptor mediated NO production early in the process. To test this hypothesis, the effect of a combination of acetylcholinesterase inhibition with
neostigmine (to magnify the agonist effects of Ach) and NOS blockade with
l-NAME (to abolish NO effects) on active vasodilation was examined.
Neostigmine and
l-NAME were delivered
prior to initiation of body heating, when the active vasodilator system was quiescent, and continued throughout the early and late periods of heat stress. The results showed that
early in body heating (when skin temperature was increased but internal temperature was not), skin blood flow increased sooner at sites treated with
neostigmine (with presumably augmented Ach levels). The augmenting effects of acetylcholinesterase inhibition were abolished by NOS inhibition.
Late in heat stress, when active vasodilation was well established, the effect of
neostigmine was lost, but skin blood flow at
l-NAME treated sites was attenuated. These results suggested that Ach mediated the increase in NO production
early in heat stress, but not after substantial cutaneous vasodilation had occurred.
54
++

H1 receptors for histamine may play a role in the generation of NO during cutaneous active vasodilation in heat stress.
55 Administration of the first-generation antihistamine pyrilamine attenuated, but did not abolish, the rise of skin blood flow during heat stress. In addition, it was found that the NO generated during active vasodilation was mediated by H1 receptor activation. Thus, there appear to be several pathways that may generate NO in the skin during heat stress.
++

The recent development of antagonists for the different NOS isozymes has allowed investigations into the roles of these isoforms in active cutaneous vasodilation. These investigations found that inhibition of NOS I (neuronal NOS, nNOS) reduced the cutaneous vasodilator response to heat stress.
56 In contrast, inhibition of NOS III (endothelial NOS, eNOS) had no significant effect on the vasodilator response to body heating.
57 These results are also consistent with NOS I having an important role in thermoregulatory reflex vasodilation.
58
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Local Warming of the Skin and Vasodilation
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The foregoing section outlined thermoregulatory reflex responses to heat that are mediated primarily by the sympathetic nervous system. Skin vessels themselves also respond directly to changes in local temperature. For example, in response to increases in local skin temperature, cutaneous blood vessels dilate. This dilation is accomplished by purely local temperature-dependent mechanisms, the effects of which are in addition to the previously discussed neural effects.
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With local warming of skin, local skin blood flow increases in direct proportion to the temperature achieved. If heating is continued at a maximally tolerable level of 42°C–44°C for 35–55 minutes, maximal local skin blood flow can be achieved.
59 The local vasodilation is biphasic with an initial brisk vasodilation. This initial dilation rapidly reaches a peak and is followed by a slight fall in flow. With continued heat application, a second dilation occurs that can reach maximal levels in a prolonged plateau phase. For unknown reasons, the plateau phase declines after approximately 1 hour despite continued heating
60 (
eFig. 93-2.2).
++
++

The mechanisms that mediate the local, temperature-dependent cutaneous vasodilation involves local sensory nerve, axon reflex mechanisms and the local generation of NO. These two mechanisms appear to be independent of each other.
61 The previously discussed neural cutaneous active vasodilator system does not appear to be involved in this local response as neither botulinum toxin induced abolition of active vasodilation nor muscarinic receptor blockade with
atropine alter the skin blood flow response to local skin heating.
37 Prostanoids do not appear to be involved either, as cyclooxygenase inhibition has no effect on the vasodilation induced by local skin heating.
62
++

The initial phase of the local warming response produces a brief peak value and is mediated by local activation of afferent cutaneous sensory nerves. This initial phase of the vasodilation can be greatly attenuated by topical anesthesia directly applied to the heated site. Cutaneous nerve blockade at points distant from the heated site that interrupt sensory nerve function at the heated site have no effect.
61,63 This demonstrates that the initial effects of local skin heating are mediated by a neural axon-reflex mechanism. The specific neurotransmitters involved in local heating responses are uncertain, however, based on studies done with topical application of the vanilloid receptor (VR-1) activator
capsaicin. It has been proposed that local increases in skin temperature stimulate heat-sensitive VR-1 receptors on afferent nerves. This activates the local axon reflex to cause the antidromic release of a vasodilatory neurotransmitter (or neurotransmitters) that mediates local vasodilation.
64
++

The prolonged plateau phase of vasodilation in response to local skin heating is mediated by local generation of NO. The plateau phase can be greatly attenuated by pretreatment of the locally heated area of skin with the NOS inhibitors
48,61; thus, NO generation is clearly necessary for this phase. Recent findings with specific isoforms for endothelial NOS (eNOS, type III NOS) and neuronal NOS (nNOS, type I NOS) suggests that endothelial NOS may mediate increased NO production in forearm skin during local skin heating, but that neuronal NOS may mediate increased NO production in the skin of the leg.
56–58,65 In addition, HSP90 has been shown to bind eNOS, enhance eNOS activation, and thus increase NO generation.
66 Treatment of skin with the heat shock protein 90 (HSP90) inhibitor, geldanamycin, attenuates the plateau phase of local skin heating by approximately 20%. This suggests that eNOS may be the NOS isoform that mediates the prolonged plateau phase of the cutaneous vascular response to local skin heating
67 (see
eFig. 93-2.2).
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Cutaneous Active Vasoconstrictor Mechanisms
++

Initial evidence for the control of skin blood vessels by sympathetic active vasoconstrictor nerves came from observations of the effects of peripheral sympathectomies in humans.
68,69 In these studies, the interruption of sympathetic nerves by sympathectomy led to increases in skin blood flow when the intervention was done in a cool environment. This observation was consistent with the interruption of vasoconstrictor activity leading to subsequent relaxation of cutaneous and, hence, passive increases in blood flow.
++

The sympathetic vasoconstrictor system in skin is well understood and causes vasoconstriction through noradrenergic stimulation of α
1- and α
2-adrenergic receptors.
4,20,21,69–72 This has been clearly demonstrated by studies using prejunctional sympathetic nerve blockade and postjunctional blockade with α-receptor antagonists.
23,71,73,74
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Sympathetic nerves in skin contain several different neurotransmitters colocalized with
norepinephrine, including neuropeptide Y (NPY) and
adenosine triphosphate (ATP). This suggests that the sympathetic vasoconstrictor system could be a cotransmitter system. In studies that compared the effects of differing combinations of α
1 and α
2 on the skin vasoconstriction during cold stress it was noted that the vasoconstriction was attenuated. These results confirmed a role for postjunctional excitation of α
1-and α
2-adrenergic receptors. It was also noted that pretreatment of skin with the β-receptor antagonist
propranolol led to a more profound vasoconstriction during cold stress. This suggested that a β-receptor-mediated vasodilation modulated an α-receptor-mediated vasoconstriction during whole body cooling. It is interesting to observe, however, that simultaneous and complete blockade of α
1-, α
2-, and β-receptors fails to abolish the cutaneous vasoconstriction induced by cold stress although blockade of all neurotransmitter release from cutaneous noradrenergic nerves with
bretylium tosylate does abolish the response. These studies suggest that nonnoradrenergic, cotransmitter mechanisms mediate the cutaneous vasoconstriction during cold stress.
64
++

The observation that cold stress responses in skin are mediated by a cotransmitter system has led to studies designed to identify the cotransmitters of that system. Recently, it was proposed that NPY acted as a cotransmitter along with
norepinephrine to mediate reductions in skin blood flow during cold stress. It was hypothesized that NPY worked through activation of NPY Y1 receptors on cutaneous arterioles. Administration of the NPY Y1 antagonist BIBP-3226 was found to significantly attenuate reductions in skin blood flow during whole body cold stress. Further, the combination of NPY Y1 receptor antagonism with complete α
1-, α
2-, and β-receptor blockade abolished reductions in skin blood flow during cold stress. These results clearly demonstrate that the cutaneous active vasoconstrictor system is a cotransmitter system that works via the release of NPY and
norepinephrine. These neurotransmitters cause the postjunctional activation of NPY Y1, α
1-, α
2-, and perhaps β-receptors, which in turn constrict cutaneous arterioles, reduce skin blood flow, and, thus, conserve body heat during cold stress
75 (
eFig. 93-2.3).
++
+++
Local Cooling of the Skin and Vasoconstriction
++

Decreases in skin temperature with local cooling of the skin cause a local, temperature-dependent vasoconstriction. While the vasodilatory response to local heating of the skin is independent of the cholinergic cutaneous active vasodilator system, the vasoconstriction mediated by local skin cooling is dependent on intact noradrenergic active vasoconstrictor nerves.
63,76,77 Confirmatory studies used
bretylium tosylate to block neurotransmitter release from prejunctional noradrenergic nerve terminals in skin. When noradrenergic nerves are intact, local cooling causes a prompt and progressive reduction of skin blood flow as local temperature falls. Interruption of neurotransmitter release with
bretylium changes the initial portion of the local cooling response from a prompt vasoconstriction into a prompt vasodilation. The vasodilation induced by local cooling is transient and continued local skin cooling leads to the progressive diminution of the vasodilation and eventual replacement by a progressive vasoconstriction.
4
++

Recent work has defined how the cutaneous active vasoconstrictor system participates in the vasoconstriction caused by local skin cooling, and established roles for noradrenergic receptors and afferent sensory nerves
76 (
eFig. 93-2.4).
++
++

It was observed that blockade of α-receptors in skin altered the local cooling vasoconstriction in the same way that
bretylium did, i.e., combined α- and β-receptor blockade reversed the initial phase of vasoconstriction into a vasodilation followed eventually by vasoconstriction with continued local cooling. Topical anesthesia with EMLA (a eutectic mixture of
lidocaine anesthetic) cream also reversed the initial vasoconstriction to a dilation followed again by constriction with prolonged cooling. NPY Y1 receptor blockade with BIBP-3226 had no effect on the response to local cooling. Based on this work, it is clear that local cooling of skin occurs in two phases: (1) an initial rapid phase lasting a few minutes, and (2) a prolonged late phase. The initial phase is caused by activation of cold-sensitive afferent neurons that mediate release of
norepinephrine from sympathetic cutaneous vasoconstrictor nerves. The released
norepinephrine vasoconstricts cooled skin vessels through postjunctional α-receptors. NPY Y1 receptors do not appear to be involved in either phase of the local cooling response. It was been speculated that local cooling stimulated
norepinephrine release from adrenergic nerves through cold receptors acting through axon reflexes; however, this was recently shown to be incorrect leaving the exact mechanism whereby afferent sensory nerves contribute to the vasoconstriction induced by local cooling unclear.
78 The mechanisms for the prolonged, late vasoconstrictor phase of cooling appear to be nonneurogenic as no manipulations alter this portion of the response.
76
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A significant role for α
2-receptors in causing the cutaneous vasoconstriction by local skin cooling has been proposed.
79–83 For example, local skin cooling increases α
2-adrenergic receptor-mediated vasoconstriction. In contrast, local cooling does not augment constriction mediated by α
1-adrenergic receptors.
84 In mouse skin, it has been shown that local cooling causes augmented noradrenergic constriction through α
2C-adrenergic receptors.
82 While these receptors are not directly thermosensitive themselves, they have been found to be translocated from the trans-Golgi apparatus of vascular smooth muscle cells to the plasma membrane through the cold-induced activation of RhoA and Rho kinase.
80,81 In this way, more α
2C-receptors appear on the surface of vascular smooth muscle. In addition, activation of these kinases enhances the calcium sensitivity of the vascular smooth muscle contractile apparatus. These factors explain how local cooling causes vasoconstriction.
80
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The generation of reactive
oxygen species (ROS) from mitochondria in vascular smooth muscle appears have a role in the vasoconstriction induced by local cooling. Cooling of mouse tail skin arteries leads to increased ROS generation in vascular smooth muscle cells. Alteration of ROS levels by application free radical scavengers abolishes the vasoconstrictor response to α
2C-adrenergic receptor stimulation. Manipulation of ROS levels also abolishes the activation of RhoA in cultured human vascular smooth muscle cells. Taken together, these results suggest that the vasoconstriction caused by local cooling is caused by enhanced ROS generation in mitochondria of vascular smooth muscle. Increased ROS then activates RhoA/Rho kinase, which causes a consequent movement of α
2C-adrenergic receptors to the cell membrane. Upon reaching the cell surface, α
2C-adrenergic receptors can be activated by
norepinephrine released by sympathetic neurons and cause vasoconstriction.
81
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The NO system also plays a role in causing vasoconstriction during local skin cooling. Antagonism of NO production during local cooling attenuates the skin vasoconstrictor response.
85 Combined antagonism of both the NO system and adrenergic mechanisms eliminates the vasoconstrictor response to local cooling.
86 These findings show that direct local cooling of skin causes vasoconstriction through both the NO-dependent and adrenergic mechanisms.