Cutaneous Active Vasodilation Research Articles

Elucidating mechanisms of attenuated skin vasodilation during passive heat stress in persons with spinal cord injury

Objective Persons with spinal cord injury (SCI) are unable to efficiently dissipate heat via thermoregulatory vasodilation as efficiently as able-bodied persons during whole body passive heat stress (PHS). Skin blood flow (SkBF) is controlled by dual sympathetic vasomotor systems: noradrenergic vasoconstrictor (VC) nerves and cholinergic vasodilator (VD) nerves. Thus, impaired vasodilation could result from inappropriate increases in noradrenergic VC tone that compete with cholinergic vasodilation or diminished cholinergic tone. To address this issue, we used bretylium (BR) which selectively blocks neural release of norepinephrine, thereby reducing noradrenergic VC tone. If impaired vasodilation during PHS is due to inappropriate increase in VC tone, BR treatment will improve SkBF responses during PHS. Design Prospective interventional trial. Setting laboratory. Participants 22 veterans with SCI. Interventions Skin surface areas with previously defined intact vs. impaired thermoregulatory vasodilation were treated with BR iontophoresis with a nearby untreated site serving as control/CON. Participants underwent PHS until core temperature rose 1°C. Outcome Measures Laser doppler flowmeters measured SkBF over BR and CON sites in areas with impaired and intact thermoregulatory vasodilation. Cutaneous vascular conductance (CVC) was calculated for all sites. Peak-PHS CVC was normalized to baseline (BL): (CVC peak-PHS/CVC BL) to quantify SkBF change. Results CVC rise in BR sites was significantly less than CON sites in areas with intact (P = 0.03) and impaired (P = 0.04) thermoregulatory vasodilation. Conclusion Cutaneous blockade of neural release of noradrenergic neurotransmitters affecting vasoconstriction did not enhance thermoregulatory vasodilation during PHS in persons with SCI; rather BR attenuated the response. Cutaneous blockade of neural release of noradrenergic neurotransmitters affecting vasoconstriction did not restore cutaneous active vasodilation during PHS in persons with SCI.

Cholinergic nerve contribution to cutaneous active vasodilation during exercise is similar to whole body passive heating.

Sympathetic cholinergic nerve cotransmission is widely accepted as the mechanism of cutaneous active vasodilation (CAVD) during whole body passive heating (passive heating). However, recent research suggests that there may be mechanistic differences in CAVD to heating, depending on the modality of thermal loading. It is unknown whether sympathetic cholinergic cotransmission explains CAVD during exercise. This study sought to confirm the role of cholinergic nerves in CAVD during passive heating and expand these findings to exercise. It was hypothesized that CAVD during both exercise and passive heating would be abolished by cholinergic nerve blockade. Eight young (18-30 yr) recreationally active individuals exercised (1 h seated cycling at 60% V̇o2peak) and were passively heated (∼1 h seated passive heating with mean skin temperature clamped at 39°C by water-perfused suit), in randomized order on separate days. Cholinergic nerves were blocked via Botox ∼2 wk prior to the study. Skin blood flow was assessed using laser Doppler flowmetry and expressed as percent of maximum cutaneous vascular conductance (%CVCmax). At the end of exercise/passive heating, internal temperature had increased by ∼0.7°C. The %CVCmax at the Botox-treated sites (exercise: 19 ± 6 and passive heating: 15 ± 14%CVCmax) was significantly less (P < 0.001) than at the untreated sites (exercise: 35 ± 11 and passive heating: 38 ± 6%CVCmax), but there were no differences between exercise and passive heating (modality, P = 0.909; modality-Botox interaction, P = 0.230). We conclude that CAVD during both exercise and passive heating is mediated by sympathetic cholinergic nerves, a critical thermoregulatory mechanism that appears to be independent of the thermal loading modality.NEW & NOTEWORTHY Our study establishes the primacy of cholinergic nerves to cutaneous active vasodilation during exercise and confirms this model during passive heating using a crossover study design. In addition, the mode of heating, whether passive or exercise induced, did not change the sensitivity of the cholinergic component of the thermoeffector response to increased internal temperature. Thus, cutaneous active vasodilator nerves are responsible for similar skin blood flow responses regardless of how thermal loading is accomplished.

Cholinergic nerve contribution to cutaneous active vasodilation in response to exercise heat‐loading is similar to passive whole‐body heat‐loading

Introduction Twenty-five years ago, Dean Kellogg and colleagues established the sympathetic cholinergic nerve co-transmitter theory for cutaneous active vasodilation (CAVD) during passive heating in humans. In this seminal study, pretreatment of the skin with botulinum toxin type-A (BOTOX), a presynaptic cholinergic nerve blocker, abolished CAVD during passive whole-body heat stress in six subjects. To date, these findings have not been confirmed. More recently, a clear role for NO in CAVD has been established, where NO accounts for up to 50% of CAVD. The specific isoform of NO-synthase has been reported to differ between CAVD in response to different thermal loading modalities (passive heating vs. exercise). These findings suggest that there may be mechanistic differences in the skin blood flow responses (SkBF) to hyperthermic challenge that are dependent on how thermal loading is accomplished. With this in mind, it is unknown if the cholinergic co-transmitter theory of CAVD applies to other thermal loading modalities such as exercise heat-loading. Therefore, this study sought to confirm the role of cholinergic nerve activation in CAVD during passive whole-body heat-loading and expand these findings to exercise heat-loading using a cross-over study design. It was hypothesized that CAVD during exercise and passive whole-body heating would both be abolished by BOTOX administration. Methods Eight young (18-30 years) recreationally-active individuals participated in both an exercise heat-loading study day (40% VO2peak, 1 hour; seated cycle ergometer) and a passive whole-body heat-loading study day (mean skin temperature clamp @ 39°C, ~1 hour; water-perfused suit, seated), in a randomized order. Cholinergic nerve blockade was achieved through BOTOX administration ~2 weeks prior to study days. Throughout heat-loading trials, SkBF was assessed at BOTOX treated and untreated sites using laser-doppler flowmetry. Results At rest, there were no differences in SkBF expressed as % CVCmax by site (BOTOX treated vs. untreated) or by modality (Exercise heat-loading vs. passive whole body heat-loading). The SkBF response, as a function of increasing internal temperature, was similar between heat-loading modalities at BOTOX treated and untreated sites. At the end of heat-loading (@ 0.7 °C increase in internal temperature), % CVCmax at the BOTOX treated sites (Exercise: 19 ± 2 and Passive whole-body heating: 15 ± 5 % CVCmax), were significantly less (P<0.001) than at the untreated sites (Exercise: 35 ± 4 and Passive whole-body heating: 38 ± 2 % CVCmax). Additionally, there were no differences in %CVCMax between thermal loading modalities (modality, P = 0.909; modality-by-microdialysis site interaction, P=0.230) at the end of heat-loading. Conclusions We conclude that CAVD in response to exercise heat-loading and passive whole-body heat-loading are both mediated by cholinergic nerve transmission, a critical thermoregulatory mechanism whose function appears to be independent of the thermal loading modality.

Counterpoint: Investigators should not control for menstrual cycle phase when performing studies of vascular control that include women.

Observed sex differences in the epidemiology of cardiovascular disease incidence and the efficacy of clinical intervention strategies, coupled with recent mandates from the National Institutes of Health (NIH), and its international counterparts to consider sex as a biological variable in all preclinical and clinical work (15), have led to increased enrollment of women in human physiology studies of vascular function with the goal of improving external validity and clinical applicability of the findings. However, standard practice in these studies is to test women during the early follicular phase (~days 1-5) of the menstrual cycle, during which estrogen and progesterone concentrations are lowest, and either exclude or limit testing of women who use hormonal contraceptives to the placebo phase. This approach seeks to control for, or limit, differences in circulating hormone concentrations between men and women and is based on the dogmatic theory that cyclic changes in estrogen and progesterone concentrations lead to greater variability in study outcomes and uninterpretable datasets. Contrary to this common approach, we propose that limiting testing of women to this single phase 1) decreases external validity of the findings, 2) obfuscates potential physiological differences contributable to differences in sex hormone concentrations between men and women, and 3) is, in many instances, contradicted by literature that demonstrates limited or no variability in vascular measurements across the menstrual cycle.

Body Temperature, Cutaneous Heat Loss, and Skin Blood Flow During Epidural Anaesthesia for Emergency Cesarean Section

(Anaesthesia. 2018;73:1500–15006) During the peripartum period, hypothermia and hyperthermia are both associated with adverse outcomes. Neuraxial anesthesia hinders a patient’s ability to regulate body temperature by interrupting the sympathetic supply to the cutaneous vasculature. It is unclear how body temperature is affected by epidural administration of a concentrated local anesthetic for emergency cesarean section. This study aimed to determine how dosing of a labor epidural to provide anesthesia for cesarean section would affect mean body temperature and cutaneous vasomotor tone, hypothesizing that active cutaneous vasodilation would be blocked and therefore mean body temperature would increase.

Body temperature, cutaneous heat loss and skin blood flow during epidural anaesthesia for emergency caesarean section.

It is not clear how converting epidural analgesia for labour to epidural anaesthesia for emergency caesarean section affects either cutaneous vasomotor tone or mean body temperature. We hypothesised that topping up a labour epidural blocks active cutaneous vasodilation (cutaneous heat loss and skin blood flow decrease), and that as a result mean body temperature increases. Twenty women in established labour had body temperature, cutaneous heat loss and skin blood flow recorded before and after epidural top-up for emergency caesarean section. Changes over time were analysed with repeated measures ANOVA. Mean (SD) mean body temperature was 36.8 (0.5)°C at epidural top-up and 36.9 (0.6)°C at delivery. Between epidural top-up and delivery, the mean (SD) rate of increase in mean body temperature was 0.5 (0.5) °C.h-1 . Following epidural top-up, chest (p < 0.001) and forearm (p = 0.004) heat loss decreased, but head (p = 0.05), thigh (p = 0.79) and calf (p = 1.00) heat loss did not change. The mean (SD) decrease in heat loss was 15 (19) % (p < 0.001). Neither arm (p = 0.06) nor thigh (p = 0.10) skin blood flow changed following epidural top-up. Despite the lack of change in skin blood flow, the most plausible explanation for the reduction in heat loss and the increase in mean body temperature is blockade of active cutaneous vasodilation. It is possible that a similar mechanism is responsible for the hyperthermia associated with labour epidural analgesia.

Chapter 12 - Cutaneous active vasodilation as a heat loss thermoeffector

Human skin is the interface between the human body and the environment. As such, human temperature regulation relies largely on cutaneous vasomotor and sudomotor adjustments to appropriately thermoregulate. In particular, changes in skin blood flow can increase or decrease the convective heat transfer from internal tissues to the periphery where it can increase or prevent heat loss to the environment. Thermoregulatory control of the cutaneous vasculature is largely due to cutaneous sympathetic nerves. Sympathetic adrenergic nerves mediate vasoconstriction of the skin, similar to other vascular beds, whereas active vasodilator nerves in nonglabrous skin respond to changes in internal and peripheral temperatures and can profoundly increase skin blood flow. Activation of these vasodilator nerves is known as cutaneous active vasodilation and has been the subject of much recent research. This research has uncovered a highly complex system that involves the activation of multiple receptors and vasodilator pathways in a synergistic and sometimes redundant manner. This complexity and redundancy has left our understanding of cutaneous active vasodilation incomplete; however, the employment of new techniques and use of new pharmacologic agents have introduced many new insights into cutaneous active vasodilation.

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.

Effect of short-term exercise-heat acclimation on ventilatory and cerebral blood flow responses to passive heating at rest in humans.

Hyperthermia induces hyperventilation and cerebral hypoperfusion in resting humans. We tested the hypothesis that short-term exercise-heat acclimation would alleviate those effects. Twenty healthy male subjects were divided into two groups that performed exercise training in the heat (TR-HEAT, n = 10) or cold (TR-COLD, n = 10). Before and after the training, the subjects in both groups participated in passive-heat tests at rest. Training was performed at 37°C (TR-HEAT) or 10°C (TR-COLD) and entailed four 20-min bouts of cycling at 50% peak oxygen uptake separated by 10-min recoveries daily for 6 consecutive days. After TR-HEAT, esophageal temperature was lowered when measured before and during passive heating, as was the esophageal temperature threshold for cutaneous active vasodilation, whereas plasma volume was increased (all P < 0.05). These traditional indices of successful heat acclimation were not all induced by TR-COLD (all P > 0.05). TR-HEAT had no significant effect on passive heating-induced increases in minute ventilation, even when evaluated as the esophageal temperature threshold for increases in minute ventilation and the slope relating minute ventilation to esophageal temperature (all P > 0.05). By contrast, TR-HEAT attenuated the passive heating-induced reduction in the cerebral vascular conductance index (middle cerebral artery mean blood velocity/mean arterial pressure) (all P < 0.05). TR-COLD did not attenuate the increase in minute ventilation or the decrease in the cerebral vascular conductance index observed during passive heating (all P > 0.05). These data suggest that in resting heated humans, short-term heat acclimation achieved through moderate-intensity exercise training (i.e., 50% peak oxygen uptake) in the heat does not influence hyperthermia-induced hyperventilation, but it does potentially attenuate cerebral hypoperfusion.

Augmented reflex cutaneous vasodilatation following short-term dietary nitrate supplementation in humans.

What is the central question of this study? Nitrate supplementation via beetroot juice has been shown to have several benefits in healthy humans, including reduced blood pressure and increased blood flow to exercising muscle. Whether nitrate supplementation can improve blood flow to the skin in heat-stressed humans has not been investigated. What is the main finding and its importance? Similar to previous studies, we found that nitrate supplementation reduces blood pressure. Nitrate supplementation increased vasodilatation in the skin of heat-stressed humans but did not directly increase skin blood flow. Nitrate supplementation has been shown to increase NO-dependent vasodilatation through both NO synthase (NOS)-dependent and NOS-independent pathways. We hypothesized that nitrate supplementation would augment reflex cutaneous active vasodilatation. Subjects were equipped with two microdialysis fibres on the forearm randomly assigned as control (Ringer solution) or NOS inhibition (20mm l-NAME). Whole-body heating was performed to raise core temperature by 0.8°C above baseline core temperature. Maximal cutaneous vasodilatation was achieved via 54mm sodium nitroprusside and local heating to 43°C. Skin blood flow (measured by laser-Doppler flowmetry) and blood pressure were measured. Cutaneous vascular conductance (CVC) was calculated as skin blood flow divided by mean arterial pressure (MAP) and expressed as a percentage of maximal CVC (%CVCmax ). Subjects underwent heat stress before and after nitrate supplementation (3days of beetroot juice; 5mm, 0.45g nitrates per day). During heat stress, MAP was reduced following nitrate supplementation compared with the control conditions (before 88±3mmHg versus after 78±2mmHg; P<0.05); however, resting MAP was not different between conditions (before 88±3mmHg versus after 83±2mmHg; P=0.117). Nitrate supplementation increased plateau CVC at control sites (before 67±2%CVCmax versus after 80±5%CVCmax ; P=0.01) but not at l-NAME-treated sites (before 45±4%CVCmax versus after 40±5%CVCmax ; P=0.617). There was no change in the calculated percentage of NOS-dependent vasodilatation before and after supplementation (before 59±4%versus after 64±6%; P=0.577). These data suggest that nitrate supplementation augments CVC and reduces MAP during heat stress.

No independent, but an interactive, role of calcium-activated potassium channels in human cutaneous active vasodilation

In human cutaneous microvasculature, endothelium-derived hyperpolarizing factors (EDHFs) account for a large portion of vasodilation associated with local stimuli. Thus we sought to determine the role of EDHFs in active vasodilation (AVD) to passive heating in two protocols. Whole body heating was achieved using water-perfused suits (core temperature increase of 0.8-1.0°C), and skin blood flow was measured using laser-Doppler flowmetry. In the first protocol, four sites were perfused continuously via microdialysis with: 1) control; 2) tetraethylammonium (TEA) to block calcium-activated potassium (KCa) channels, and thus the actions of EDHFs; 3) N-nitro-l-arginine methyl ester (l-NAME) to inhibit nitric oxide synthase (NOS); and 4) TEA + l-NAME (n = 8). Data are presented as percent maximal cutaneous vascular conductance (CVC). TEA had no effect on AVD (CVC during heated plateau: control 57.4 ± 4.9% vs. TEA 63.2 ± 5.2%, P = 0.27), indicating EDHFs are not obligatory. l-NAME attenuated plateau CVC to 33.7 ± 5.4% (P < 0.01 vs. control); while TEA + l-NAME augmented plateau CVC compared with l-NAME alone (49.7 ± 5.3%, P = 0.02). From these data, it appears combined blockade of EDHFs and NOS necessitates dilation through other means, possibly through inward rectifier (KIR) and/or ATP-sensitive (KATP) potassium channels. To test this second hypothesis, we measured AVD at the following sites (n = 8): 1) control, 2) l-NAME, 3) l-NAME + TEA, and 4) l-NAME + TEA + barium chloride (BaCl2; KIR and KATP blocker). The addition of BaCl2 to l-NAME + TEA reduced plateau CVC to 32.7 ± 6.6% (P = 0.02 vs. l-NAME + TEA), which did not differ from the l-NAME site. These data combined demonstrate a complex interplay between vasodilatory pathways, with cross-talk between NO, KCa channels, and KIR and/or KATP channels.

A complex interplay between NO, EDHFs, and KIR channels in cutaneous active vasodilation

EDHFs contribute largely to skin local hyperemia. We sought to determine if they also contribute to active vasodilation (AVD) during whole‐body heating. Skin blood flow was measured at 4 sites perfused via microdialysis with 1) Lactated Ringer's, 2) tetraethylammonium (TEA, calcium‐sensitive potassium channel &amp; EDHF inhibitor), 3) L‐NAME (NOS inhibitor), and 4) TEA + L‐NAME. Data is presented as % of max cutaneous vascular conductance. TEA had no effect on AVD (plateau at the end of heat stress: control=55.2±4.6% vs. TEA=63.2±5.2%, p=0.34), indicating EDHFs are not obligatory. L‐NAME attenuated plateau (33.7±5.4%); while TEA+L‐NAME augmented plateau compared to L‐NAME alone (49.7±5.3%, p=0.02 vs. L‐NAME), and did not differ from control (p=0.72). From these data, it appears that combined blockade of EDHFs and NOS forces dilation to occur through other means, possibly through inward‐rectifying potassium (KIR) channels. To test this second hypothesis, we measured AVD at the following sites: 1) Lactated Ringer's, 2) L‐NAME, 3) L‐NAME + TEA, and 4) L‐NAME + TEA + barium chloride (BaCl2, KIR channel inhibitor). The addition of BaCl2 to L‐NAME + TEA reduced plateau to 25.4±6.2% (p=0.01 vs. L‐NAME + TEA), which did not differ from the L‐NAME site (p=0.59). These data combined demonstrate a complex interplay between vasodilatory pathways, with cross talk between NO, EDHFs, and KIR channels.Supported by NIH Grant HL081671

Dietary nitrate supplementation augments cutaneous reflex vasodilation in healthy humans

Beetroot juice (BRJ) has been shown to increase NO‐dependent dilation through both NOS‐dependent and independent pathways. We hypothesized BRJ supplementation would augment reflex cutaneous active vasodilation. Subjects were equipped with two microdialysis fibers on the forearm and randomly assigned as control (Ringer's) or NOS inhibition (20mM L‐NAME). Whole‐body heating was achieved via water‐perfused suits to raise core temperature (Tc; ingestible telemetric pill) 0.8°C. Maximal cutaneous vasodilation was reached by administering 54mM SNP and local heating to 43°C. Skin blood flow was measured via laser‐Doppler flowmetry and mean arterial pressure was measured; cutaneous vascular conductance (CVC) was calculated and expressed as %CVCmax. Subjects underwent heat stress pre‐ and post‐nitrate supplementation (3 days of BRJ: 5mM, .45g nitrates per day). BRJ increased the plateau CVC at control (pre: 55±3 vs. post: 77±5 %CVCmax) and L‐NAME (pre: 27±3 vs. post: 41±6 %CVCmax; p&lt;0.05 for all conditions) sites. The %NO contribution increased from pre‐ (47±2 %CVCmax) to post‐BRJ (71±4 %CVCmax; p&lt;0.05). These data suggest BRJ augments NO‐dependent component of reflex cutaneous vasodilation.

Elevated local skin temperature impairs cutaneous vasoconstrictor responses to a simulated haemorrhagic challenge while heat stressed

During a simulated haemorrhagic challenge, syncopal symptoms develop sooner when individuals are hyperthermic relative to normothermic. This is due, in part, to a large displacement of blood to the cutaneous circulation during hyperthermia, coupled with inadequate cutaneous vasoconstriction during the hypotensive challenge. The influence of local skin temperature on these cutaneous vasoconstrictor responses is unclear. This project tested the hypothesis that local skin temperature modulates cutaneous vasoconstriction during simulated haemorrhage in hyperthermic humans. Eight healthy participants (four men and four women; 32 ± 7 years old; 75.2 ± 10.8 kg) underwent lower-body negative pressure to presyncope while heat stressed via a water-perfused suit sufficiently to increase core temperature by 1.2 ± 0.2 °C. At forearm skin sites distal to the water-perfused suit, local skin temperature was either 35.2 ± 0.6 (mild heating) or 38.2 ± 0.2 °C (moderate heating) throughout heat stress and lower-body negative pressure, and remained at these temperatures until presyncope. The reduction in cutaneous vascular conductance during the final 90 s of lower-body negative pressure, relative to heat-stress baseline, was greatest at the mildly heated site (-10 ± 15% reduction) relative to the moderately heated site (-2 ± 12%; P = 0.05 for the magnitude of the reduction in cutaneous vascular conductance between sites), because vasoconstriction at the moderately heated site was either absent or negligible. In hyperthermic individuals, the extent of cutaneous vasoconstriction during a simulated haemorrhage can be modulated by local skin temperature. In situations where skin temperature is at least 38 °C, as is the case in soldiers operating in warm climatic conditions, a haemorrhagic insult is unlikely to be accompanied by cutaneous vasoconstriction.

The influence of internal and skin temperatures on active cutaneous vasodilation under different levels of exercise and ambient temperatures in humans

To clarify the influence of internal and skin temperature on the active cutaneous vasodilation during exercise, the body temperature thresholds for the onset of active vasodilation during light or moderate exercise under different ambient temperature conditions were compared. Seven male subjects performed 30 min of a cycling exercise at 20 % or 50 % of peak oxygen uptake in a room maintained at 20, 24, or 28 °C. Esophageal (Tes) and mean skin temperature (Tsk) as measured by a thermocouple, deep thigh temperature (Tdt) by the zero-heat-flow (ZHF) method, and forearm skin blood flow by laser-Doppler flowmetry (LDF) were monitored. The mean arterial pressure (MAP) was also monitored non-invasively, and the cutaneous vascular conductance (CVC) was calculated as the LDF/MAP. Throughout the experiment, the Tsk at ambient temperatures of 20, 24, and 28 °C were approximately 30, 32, and 34 °C, respectively, for both 20 % and 50 % exercise. During 50 % exercise, the Tes or Tdt thresholds for the onset of the increase in CVC were observed to be similar among the 20, 24, and 28 °C ambient conditions. During 20 % exercise, the increase in Tes and Tdt was significantly lower than those found at 50 %, and the onset of the increase in CVC was only observed at 28 °C. These results suggest that the onset of active vasodilation was affected more strongly by the internal or exercising tissue temperatures than by the skin temperatures during exercise performed at a moderate load in comparison to a light load under Tsk variations ranging from 30 °C to 34 °C. Therefore, the modification by skin temperature of the central control on cutaneous vasomotor tone during exercise may differ between different exercise loads.

Nitric oxide and receptors for VIP and PACAP in cutaneous active vasodilation during heat stress in humans

VPAC2 receptors sensitive to vasoactive intestinal polypeptide (VIP) and pituitary adenylyl cyclase activating polypeptide (PACAP), PAC1 receptors sensitive to PACAP, and nitric oxide (NO) generation by NO synthase (NOS) are all implicated in cutaneous active vasodilation (AVD) through incompletely defined mechanisms. We hypothesized that VPAC2/PAC1 receptor activation and NO are synergistic and interdependent in AVD and tested our hypothesis by examining the effects of VPAC2/PAC1 receptor blockade with and without NOS inhibition during heat stress. The VPAC2/PAC1 antagonist, pituitary adenylate cyclase activating peptide 6-38 (PACAP6-38) and the NOS inhibitor, N(G)-nitro-l-arginine methyl ester (l-NAME) were administered by intradermal microdialysis. PACAP6-38, l-NAME, a combination of PACAP6-38 and l-NAME, or Ringer's solution alone were perfused at four separate sites. Skin blood flow was monitored by laser-Doppler flowmetry at each site. Body temperature was controlled with water-perfused suits. Blood pressure was monitored by Finapres, and cutaneous vascular conductance (CVC) calculated (CVC = laser-Doppler flowmetry/mean arterial pressure). The protocol began with a 5- to 10-min baseline period without antagonist perfusion, followed by perfusion of PACAP6-38, l-NAME, or combined PACAP6-38 and l-NAME at the different sites in normothermia (45 min), followed by 3 min of whole body cooling. Whole body heating was then performed to induce heat stress and activate AVD. Finally, 58 mM sodium nitroprusside were perfused at all sites to effect maximal vasodilation for normalization of blood flow data. No significant differences in CVC (normalized to maximum) were found among Ringer's PACAP6-38, l-NAME, or combined antagonist sites during normothermia (P > 0.05 among sites) or cold stress (P > 0.05 among sites). CVC responses at all treated sites were attenuated during AVD (P < 0.05 vs. Ringer's). Attenuation was greater at l-NAME and combined PACAP6-38- and l-NAME-treated sites than at PACAP6-38 sites (P > 0.05). Because responses did not differ between l-NAME and combined treatment sites (P > 0.05), we conclude that VPAC2/PAC1 receptors require NO in series to effect AVD.

No direct role for A1/A2 adenosine receptor activation to reflex cutaneous vasodilatation during whole-body heat stress in humans

The precise mechanisms underlying reflex cutaneous vasodilatation during hyperthermia remain unresolved. The purpose of this study was to investigate a potential contribution of adenosine A1/A2 receptor activation to reflex cutaneous vasodilatation. Eight subjects were equipped with four microdialysis fibres on the left forearm, and each fibre was randomly assigned one of four treatments: (1) lactated Ringer's (control); (2) 4 mm of the non-selective A1/A2 adenosine receptor antagonist theophylline; (3) 10 mm L-NAME to inhibit nitric oxide (NO) synthase; and (4) combined 4 mm theophylline and 10 mm L-NAME. Laser-Doppler flowmetry (LDF) was used as an index of skin blood flow, and blood pressure was measured beat-by-beat via photoplethysmography and verified via brachial auscultation. Whole-body heat stress to raise oral temperature 0.8 °C above baseline was induced via water-perused suits. Cutaneous vascular conductance (CVC) was calculated as LDF/mean arterial pressure and normalized to maximal (%CVC max) via infusion of 28 mm nitroprusside and local heating to 43 °C. There was no difference between control (65 ± 5%CVC max) and theophylline (63 ± 5%CVC max) sites. L-NAME (44 ± 4%CVC max) and theophylline + L-NAME (32 ± 3%CVC max) sites were significantly attenuated compared to both control and theophylline only sites (P<0.05), and combined theophylline + L-NAME sites were significantly reduced compared to L-NAME only sites (P<0.05). These data suggest A1/A2 adenosine receptor activation does not directly contribute to cutaneous active vasodilatation; however, a role for A1/A2 adenosine receptor activation is unmasked when NO synthase is inhibited.

Relationship between mean body temperature calculated by two- or three-compartment models and active cutaneous vasodilation in humans: a comparison between cool and warm environments during leg exercise

The aim of this study was to assess whether the three-compartment model of mean body temperature (Tb3) calculated from the esophageal temperature (Tes), temperature in deep tissue of exercising muscle (Tdt), and mean skin temperature (Tsk) has the potential to provide a better match with the thermoregulatory responses than the two-component model of mean body temperature (Tb2) calculated from Tes and Tsk. Seven male subjects performed 40 min of a prolonged cycling exercise at 30% maximal oxygen uptake at 21°C or 31°C (50% relative humidity). Throughout the experiment, Tsk, Tb2, Tb3, and Tdt were significantly (P < 0.01) lower at 21°C than at 31°C temperature conditions, while Tes was similar under both conditions. During exercise, an increase in cutaneous vascular conductance (skin blood flow / mean arterial pressure) over the chest (%CVCc) was observed at both 21°C and 31°C, while no increase was observed at the forearm at 21°C. Furthermore, the Tb3 and Tdt threshold for the onset of the increase in %CVCc was similar, but the Tes and Tb2 threshold differed significantly (P < 0.05) between the conditions tested. These results suggest that active cutaneous vasodilation at the chest is related more closely to Tb3 or Tdt than that measured by Tes or Tb2 calculated by Tes and Tsk during exercise at both 21°C and 31°C.

TRPV‐1 Channels Contribute to Cutaneous Active Vasodilation in Humans

We tested the hypothesis that inhibition of TRPV-1 channels would attenuate cutaneous active vasodilation (AVD). Six subjects were equipped with four microdialysis fibers on the ventral forearm and each site was randomly assigned a drug treatment: 1) 90% propylene glycol + 10% lactated Ringer's (vehicle control); 2)10 mM L-NAME to inhibit NO synthase; 3) 20 mM capsazepine (CPZ) to inhibit TRPV-1 channels; 4) combined L-NAME + CPZ. Whole body heating (WBH) was achieved via water perfused suits to raise oral temperature 0.8°C above baseline. Systemic arterial pressure (SAP) was monitored and skin blood flow was measured via laser-Doppler flowmetry (LDF). Maximal skin blood flow was achieved by local heating to 43°C and infusion of 28mM nitroprusside. Cutaneous vascular conductance (CVC) was calculated as SAP/LDF and normalized to maximal. During WBH, CPZ sites were significantly reduced compared to CON sites (51±2% vs. 65±3% CVCmax, respectively; p < 0.05). L-NAME (32±1%CVCmax) sites were attenuated compared to CPZ. There was no difference between L-NAME and combined L-NAME + CPZ. These data suggest: 1)TRPV-1 receptors directly contribute to AVD; and 2) TRPV-1 channels account for a portion of the NO component.

Using intradermal electrical stimulation to evaluate neurogenic control of skin blood flow

Intradermal microdialysis and laser Doppler velocimetry were combined to evaluate neurogenic control of vasomotor activity of human skin blood vessels during non-painful intradermal electrical stimulation (E-stim). The laser Doppler flow probe was mounted directly over the microdialysis probe and the stimulating electrodes penetrated the skin 2 mm with the electrode tips separated by ≈ 4 mm. E-stim consisted of a constant current (1.8 ± 0.2 mA) pulses delivered at increasing frequency (0.2, 1, 2, 4, 8, and 32 Hz) in one minute steps. Stimulus- response curves of frequency versus skin blood flow (SkBF) were constructed and the area under the SkBF-time response curve was calculated. During saline perfusion increasing stimulus frequencies cause progressively greater reductions in SkBF with significant declines from baseline observed at frequencies of ≥2 Hz (p<0.05). The peak reduction in SkBF at 32 Hz was represented by an AUC of −2566 ± 854 SkBF·sec. In contrast, following adrenergic blockade (phentolamine (0.05 mg/ml), propranolol (1 mM), and BIBP-3226 (10 μM) SkBF failed to decline below baseline and the AUC at 32 Hz averaged only −342 ± 540 SkBF·sec (p<0.05). These data support the hypothesis that non-painful E-stim protocol provided a significant activation of the local sympathetic vasoconstrictor nerves and may provide a novel approach to investigating active cutaneous vasodilation.