Sympathetic, sensory, and nonneuronal contributions to the cutaneous vasoconstrictor response to local cooling

Rho kinase-mediated local cold-induced cutaneous vasoconstriction is augmented in aged human skin

Cutaneous vasoconstriction (VC), a critical thermoregulatory response to cold, is generally impaired with aging. However, the effects of aging on local cooling-induced VC and its underlying mechanisms are poorly understood. We tested whether aged skin exhibits attenuated localized cold-induced VC and whether Rho kinase-mediated cold-induced VC is augmented with age. Skin blood flow was monitored with laser Doppler flowmetry (LDF) on seven young and seven older subjects. Cutaneous vascular conductance (CVC; LDF/mean arterial pressure) was expressed as percentage change from baseline (%DeltaCVC(base)). In protocol 1, two forearm skin sites were cooled to six temperatures (31.5-19 degrees C) for 10 min each or two temperatures (29 degrees C, 24 degrees C) for 30 min each, with no age differences in the magnitude of VC. In protocol 2, three forearm skin sites were instrumented for intradermal microdialysis and cooled to 24 degrees C for 40 min. During minutes 1-5, there was no age difference in CVC responses at control sites (young: -45 +/- 6% vs. older: -46 +/- 3%, P > 0.9). Adrenoceptor antagonism (yohimbine + propranolol) abolished VC in young (to +15 +/- 13%, P < 0.05) but only partially inhibited VC in older subjects (to -23 +/- 6%, P < 0.05). Rho kinase inhibition plus adrenoceptor antagonism (yohimbine + propranolol + fasudil) abolished VC in both groups. During minutes 35-40, there was no age difference in control (young: -77 +/- 4% vs. older: -70 +/- 2%, P > 0.3) or adrenoceptor-antagonized responses (young: -61 +/- 3% vs. older: -55 +/- 2%, P > 0.3); however, Rho kinase inhibition plus adrenoceptor antagonism blocked more VC in older compared with young subjects (-19 +/- 11% vs. -35 +/- 3%, P < 0.05). Although its magnitude remains unaffected, cold-induced VC becomes less dependent on adrenergic and more dependent on Rho kinase signaling with advancing age.

Cold-induced cutaneous vasoconstriction is mediated by Rho kinase in vivo in human skin

Cutaneous vasoconstriction (VC) is the initial thermoregulatory response to cold exposure and can be elicited through either whole body or localized skin cooling. However, the mechanisms governing local cold-induced VC are not well understood. We tested the hypothesis that Rho kinase participates in local cold-induced cutaneous VC. In seven men and women (20-27 yr of age), up to four ventral forearm skin sites were instrumented with intradermal microdialysis fibers for localized drug delivery during cooling. Skin blood flow was monitored at each site with laser-Doppler flowmetry while local skin temperature was decreased and maintained at 24 degrees C for 40 min. Cutaneous vascular conductance (CVC; laser-Doppler flowmetry/mean arterial pressure) was expressed as percent change from 34 degrees C baseline. During the first 5 min of cooling, CVC decreased at control sites (lactated Ringer solution) to -45 +/- 6% (P < 0.001), increased at adrenoceptor-antagonized sites (yohimbine + propranolol) to 15 +/- 14% (P = 0.002), and remained unchanged at both Rho kinase-inhibited (fasudil) and adrenoceptor-antagonized + Rho kinase-inhibited sites (yohimbine + propranolol + fasudil) (-9 +/- 1%, P = 0.4 and -6 +/- 2%, P = 0.4, respectively). During the last 5 min of cooling, CVC further decreased at all sites when compared with baseline values (control, -77 +/- 4%, P < 0.001; adrenoceptor antagonized, -61 +/- 3%, P < 0.001; Rho kinase inhibited, -34 +/- 7%, P < 0.001; and adrenoceptor antagonized + Rho kinase inhibited sites, -35 +/- 3%, P < 0.001). Rho kinase-inhibited and combined treatment sites were significantly attenuated when compared with both adrenoceptor-antagonized (P < 0.01) and control sites (P < 0.0001). Rho kinase mediates both early- and late-phase cold-induced VC, supporting in vitro findings and providing a putative mechanism through which both adrenergic and nonadrenergic cold-induced VC occurs in an in vivo human thermoregulatory model.

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.

In vivo mechanisms of cutaneous vasodilation and vasoconstriction in humans during thermoregulatory challenges

This review focuses on the neural and local mechanisms that have been demonstrated to effect cutaneous vasodilation and vasoconstriction in response to heat and cold stress in vivo in humans. First, our present understanding of the mechanisms by which sympathetic cholinergic nerves mediate cutaneous active vasodilation during reflex responses to whole body heating is discussed. These mechanisms include roles for cotransmission as well as nitric oxide (NO). Next, the mechanisms by which sympathetic noradrenergic nerves mediate cutaneous active vasoconstriction during whole body cooling are reviewed, including cotransmission by neuropeptide Y (NPY) acting through NPY Y1 receptors. Subsequently, current concepts for the mechanisms that effect local cutaneous vascular responses to direct skin warming are examined. These mechanisms include the roles of temperature-sensitive afferent neurons as well as NO in causing vasodilation during local heating of skin. This section is followed by a review of the mechanisms that cause local cutaneous vasoconstriction in response to direct cooling of the skin, including the dependence of these responses on intact sensory and sympathetic, noradrenergic innervation as well as roles for nonneural mechanisms. Finally, unresolved issues that warrant further research on mechanisms that control cutaneous vascular responses to heating and cooling are discussed.

The involvement of nitric oxide in the cutaneous vasoconstrictor response to local cooling in humans

Cutaneous vascular conductance (CVC) declines in response to local cooling (LC). Previous work indicates that at least part of the vasoconstrictor response to LC may be through an inhibitory effect on nitric oxide synthase (NOS) activity. In this study we further tested that notion. A total of eight (6 male, 2 female) subjects participated (Part 1 n = 7; Part 2 n = 5, 4 of whom participated in Part 1). Skin blood flow was monitored by laser-Doppler flowmetry. Control of local skin and body temperatures was achieved with Peltier cooler/heater probe holders and water perfused suits, respectively. Microdialysis fibres were inserted aseptically. Saline, L-NAME (20 mM; to inhibit NOS activity) and sodium nitroprusside (SNP 10 microM) were infused by microdialysis. Bretylium tosylate (BT), to block adrenergic function, was administered by iontophoresis. CVC was calculated from blood flow and blood pressure. Part 1 was designed to determine the relative roles of the NO and the adrenergic systems. The infusion of L-NAME elicited a 35 +/- 4% decrease in CVC at the L-NAME and BT + L-NAME sites (P < 0.05); subsequent slow LC (34-24 degrees C) for 35 min caused a significant (P < 0.05) decrease in CVC at control sites (68 +/- 4%) and at the BT treated sites (39 +/- 5%). LC caused a further 23 +/- 5% of initial baseline decrease in CVC at the L-NAME treated sites (P < 0.05). Importantly, CVC at the BT + L-NAME sites was unaffected by LC (P > 0.05). Part 2 was designed to test whether LC influences were specific to the NOS enzymes. Two sites were pretreated with both BT and L-NAME. After 50 min, SNP was added as an NO donor to restore baseline CVC at one site. The same LC process as in Part 1 was applied. There was a 24 +/- 10% decrease (P < 0.05) in CVC at sites with baseline CVC restored, while, as in Part 1, there was no change (P > 0.05) at sites treated with BT + L-NAME only. These data suggest that the vasoconstriction with slow LC is due to a combination of increased noradrenaline release and decreased activity of both NOS per se and of process(es) downstream of NOS.

Local regulation of skin blood flow during cooling involving presynaptic P2 purinoceptors in rats

1. This study investigated a local effect of cooling on the plantar skin blood flow (PSBF) of tetrodotoxin-treated rats by laser-Doppler flowmetry. 2. When the air temperature around the left foot was locally cooled from 25 to 10 degrees C, the PSBF of the left foot decreased. 3. The response was inhibited by the alpha-adrenoceptor antagonist phentolamine, the alpha1-adrenoceptor antagonist bunazosin, the alpha2-adrenoceptor antagonist RS79948, and bretylium and guanethidine that inhibit noradrenaline release from sympathetic nerves. Adrenalectomy of the rats did not affect the cooling-induced response. 4. The P2 purinoceptor antagonists suramin and PPADS also significantly suppressed the cooling-induced reduction of PSBF. However, the inhibitory effect of PPADS on the cooling-induced response was abolished after the treatment with phentolamine. Intra-arterial injections of ATPgammaS, a stable P2 purinoceptor agonist, at 25 degrees C caused a transient decrease in PSBF in a dose-dependent manner, which was significantly inhibited by phentolamine and guanethidine. 5. These results suggest a novel mechanism for local cooling-induced reduction of skin blood flow in vivo; moderate cooling of the skin induces the release of ATP, which stimulates presynaptic P2 purinoceptors on sympathetic nerve terminals and facilitates the release of noradrenaline, thereby causing contractions of skin blood vessels via the activation of alpha1-and alpha2-adrenoceptors.

Relative roles of local and reflex components in cutaneous vasoconstriction during skin cooling in humans

The reduction in skin blood flow (SkBF) with cold exposure is partly due to the reflex vasoconstrictor response from whole body cooling (WBC) and partly to the direct effects of local cooling (LC). Although these have been examined independently, little is known regarding their roles when acting together, as occurs in environmental cooling. We tested the hypothesis that the vasoconstrictor response to combined LC and WBC would be additive, i.e., would equal the sum of their independent effects. We further hypothesized that LC would attenuate the reflex vasoconstrictor response to WBC. We studied 16 (7 women, 9 men) young (30.5+/-2 yr) healthy volunteers. LC and WBC were accomplished with metal Peltier cooler-heater probe holders and water-perfused suits, respectively. Forearm SkBF was monitored by laser-Doppler flowmetry (LDF). Cutaneous vascular conductance (CVC) was calculated as LDF/blood pressure. Subjects underwent 15 min of LC alone or 15 min of WBC with and without simultaneous LC, either at equal levels (34-31 degrees C) or as equipotent stimuli (34-28 degrees C LC; 34-31 degrees C WBC). The fall in CVC with combined WBC and LC was greater (P<0.05) than for either alone (57.0+/-5% combined vs. 39.2+/-6% WBC; 34.4+/-4% LC) with equipotent cooling, but it was only significantly greater than for LC alone with equal levels of cooling (51.3+/-8% combined vs. 29.5+/-4% LC). The sum of the independent effects of WBC and LC was greater than their combined effects (74.9+/-4 vs. 51.3+/-8% equal and 73.6+/-7 vs. 57.0+/-5% equipotent; P<0.05). The fall in CVC with WBC at LC sites was reduced compared with control sites (17.6+/-2 vs. 42.4+/-8%; P<0.05). Hence, LC contributes importantly to the reduction in SkBF with body cooling, but also suppresses the reflex response, resulting in a nonadditive effect of these two components.

Rate dependency and role of nitric oxide in the vascular response to direct cooling in human skin

Local cooling of nonglabrous skin without functional sympathetic nerves causes an initial vasodilation followed by vasoconstriction. To further characterize these responses to local cooling, we examined the importance of the rate of local cooling and the effect of nitric oxide synthase (NOS) inhibition in intact skin and in skin with vasoconstrictor function inhibited. Release of norepinephrine was blocked locally (iontophoresis) with bretylium tosylate (BT). Skin blood flow was monitored from the forearm by laser-Doppler flowmetry (LDF). Cutaneous vascular conductance (CVC) was calculated as the ratio of LDF to blood pressure. Local temperature was controlled over 6.3 cm2 around the sites of LDF measurement. Local cooling was applied at -0.33 or -4 degrees C/min. At -4 degrees C/min, CVC increased (P < 0.05) at BT sites in the early phase. At -0.33 degrees C/min, there was no early vasodilator response, but there was a delay in the onset of vasoconstriction relative to intact skin. The NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) (intradermal microdialysis) decreased (P < 0.05) CVC by 28.3 +/- 3.8% at untreated sites and by 46.9 +/- 6.3% at BT-treated sites from the value before infusion. Rapid local cooling (-4 degrees C/min) to 24 degrees C decreased (P < 0.05) CVC at both untreated (saline) sites and L-NAME only sites from the precooling levels, but it transiently increased (P < 0.05) CVC at both BT + saline sites and BT + L-NAME sites in the early phase. After 35-45 min of local cooling, CVC decreased at BT + saline sites relative to the precooling levels (P < 0.05), but at BT + L-NAME sites CVC was not reduced below the precooling level (P = 0.29). These findings suggest that the rate of local cooling, but not functional NOS, is an important determinant of the early non-adrenergic vasodilator response to local cooling and that functional NOS, adrenergic nerves, as well as other mechanisms play roles in vasoconstriction during prolonged local cooling of skin.

Attenuated noradrenergic sensitivity during local cooling in aged human skin

Reflex-mediated cutaneous vasoconstriction (VC) is impaired in older humans; however, it is unclear whether this blunted VC also occurs during local cooling, which mediates VC through different mechanisms. We tested the hypothesis that the sensitization of cutaneous vessels to noradrenaline (NA) during direct skin cooling seen in young skin is blunted in aged skin. In 11 young (18-30 years) and 11 older (62-76 years) men and women, skin blood flow was monitored at two forearm sites with laser Doppler (LD) flowmetry while local skin temperature was cooled and clamped at 24 degrees C. Cutaneous vascular conductance (CVC; LD flux/mean arterial pressure) was expressed as percentage change from baseline (% DeltaCVC(base)). At one site, five doses of NA (10(-10)-10(-2) m) were sequentially infused via intradermal microdialysis during cooling while the other 24 degrees C site served as control (Ringer solution + cooling). At control sites, VC due to cooling alone was similar in young versus older (-54 +/- 5 versus -56 +/- 3% DeltaCVC(base), P = 0.46). In young, NA infusions induced additional dose-dependent VC (10(-8), 10(-6), 10(-4) and 10(-2) m: -70 +/- 2, -72 +/- 3, -78 +/- 3 and -79 +/- 4% DeltaCVC(base); P < 0.05 versus control). In older subjects, further VC did not occur until the highest infused dose of NA (10(-2) m: -70 +/- 5% DeltaCVC(base); P < 0.05 versus control). When cutaneous arterioles are sensitized to NA by direct cooling, young skin exhibits the capacity to further constrict to NA in a dose-dependent manner. However, older skin does not display enhanced VC capacity until treated with saturating doses of NA, possibly due to age-associated decrements in Ca2+ availability or alpha2C-adrenoceptor function.