Neurokinin-1 receptor desensitization attenuates cutaneous active vasodilatation in humans

Chronic low-dose aspirin therapy attenuates reflex cutaneous vasodilation in middle-aged humans

Full expression of reflex cutaneous vasodilation is dependent on cyclooxygenase- (COX) and nitric oxide synthase- (NOS) dependent mechanisms. Low-dose aspirin therapy is widely prescribed to inhibit COX-1 in platelets for atherothrombotic prevention. We hypothesized that chronic COX inhibition with daily low-dose aspirin therapy (81 mg) would attenuate reflex vasodilation in healthy human skin. Two microdialysis fibers were placed in forearm skin of seven middle-aged (57 +/- 3 yr), normotensive, healthy humans with no preexisting cardiovascular disease, taking daily low-dose aspirin therapy (aspirin: 81 mg), and seven unmedicated, healthy, age-matched control (no aspirin, 55 +/- 3 yr) subjects, with one site serving as a control (Ringer) and the other NOS inhibited (NOS inhibited: 10 mM N(G)-nitro-l-arginine methyl ester). Red cell flux was measured over each site by laser-Doppler flowmetry, as reflex vasodilation was induced by increasing core temperature (oral temperature) 1.0 degrees C using a water-perfused suit. Cutaneous vascular conductance (CVC) was calculated (CVC = flux/mean arterial pressure) and normalized to maximal CVC (CVC(max); 28 mM sodium nitroprusside). CVC(max) was not affected by either aspirin or NOS inhibition. The plateau in cutaneous vasodilation during heating (change in oral temperature = 1.0 degrees C) was significantly attenuated in the aspirin group (aspirin: 25 +/- 3% CVC(max) vs. no aspirin: 50 +/- 7% CVC(max), P < 0.001 between groups). NOS inhibition significantly attenuated %CVC(max) in both groups (aspirin: 17 +/- 2% CVC(max), no aspirin: 23 +/- 3% CVC(max); P < 0.001 vs. control), but this attenuation was less in the no-aspirin treatment group (P < 0.001). This is the first observation that chronic low-dose aspirin therapy attenuates reflex cutaneous vasodilation through both COX- and NOS-dependent mechanisms.

Endothelial nitric oxide synthase control mechanisms in the cutaneous vasculature of humans in vivo

Nitric oxide (NO) participates in locally mediated vasodilation induced by increased local skin temperature (T(loc)) and in sympathetically mediated vasodilation during whole body heat stress. We hypothesized that endothelial NOS (eNOS) participates in the former, but not the latter, response. We tested this hypothesis by examining the effects of the eNOS antagonist N(G)-amino-l-arginine (l-NAA) on skin blood flow (SkBF) responses to increased T(loc) and whole body heat stress. Microdialysis probes were inserted into forearm skin for drug delivery. One microdialysis site was perfused with l-NAA in Ringer solution and a second site with Ringer solution alone. SkBF [laser-Doppler flowmetry (LDF)] and blood pressure [mean arterial pressure (MAP)] were monitored, and cutaneous vascular conductance (CVC) was calculated (CVC = LDF / MAP). In protocol 1, T(loc) was controlled with LDF/local heating units. T(loc) initially was held at 34 degrees C and then increased to 41.5 degrees C. In protocol 2, after a normothermic period, whole body heat stress was induced (water-perfused suits). At the end of both protocols, 58 mM sodium nitroprusside was perfused at both microdialysis sites to cause maximal vasodilation for data normalization. In protocol 1, CVC at 34 degrees C T(loc) did not differ between l-NAA-treated and untreated sites (P > 0.05). Local skin warming to 41.5 degrees C T(loc) increased CVC at both sites. This response was attenuated at l-NAA-treated sites (P < 0.05). In protocol 2, during normothermia, CVC did not differ between l-NAA-treated and untreated sites (P > 0.05). During heat stress, CVC rose to similar levels at l-NAA-treated and untreated sites (P > 0.05). We conclude that eNOS is predominantly responsible for NO generation in skin during responses to increased T(loc), but not during reflex responses to whole body heat stress.

Altered mechanisms of vasodilation in aged human skin

Human skin blood flow increases in response to increased body core and local skin temperature via distinct reflex and local mechanisms requiring functional nitric oxide (NO) for full expression. The mechanisms mediating cutaneous vasodilation are impaired with primary aging, resulting in attenuated vasodilation. This article highlights recent findings of how age-related vascular impairments in NO signaling contribute to attenuated cutaneous vasodilation.

Cholinergic mechanisms of cutaneous active vasodilation during heat stress in cystic fibrosis

To test the hypothesis that cutaneous active vasodilation in heat stress is mediated by a redundant cholinergic cotransmitter system, we examined the effects of atropine on skin blood flow (SkBF) increases during heat stress in persons with (CF) and without cystic fibrosis (non-CF). Vasoactive intestinal peptide (VIP) has been implicated as a mediator of cutaneous vasodilation in heat stress. VIP-containing cutaneous neurons are sparse in CF, yet SkBF increases during heat stress are normal. In CF, augmented ACh release or muscarinic receptor sensitivity could compensate for decreased VIP; if so, active vasodilation would be attenuated by atropine in CF relative to non-CF. Atropine was administered into skin by iontophoresis in seven CF and seven matched non-CF subjects. SkBF was monitored by laser-Doppler flowmetry (LDF) at atropine treated and untreated sites. Blood pressure [mean arterial pressure (MAP)] was monitored (Finapres), and cutaneous vascular conductance was calculated (CVC = LDF/MAP). The protocol began with a normothermic period followed by a 3-min cold stress and 30-45 min of heat stress. Finally, LDF sites were warmed to 42 degrees C to effect maximal vasodilation. CVC was normalized to its site-specific maximum. During heat stress, CVC increased in both CF and non-CF (P < 0.01). CVC increases were attenuated by atropine in both groups (P < 0.01); however, the responses did not differ between groups (P = 0.99). We conclude that in CF there is not greater dependence on redundant cholinergic mechanisms for cutaneous active vasodilation than in non-CF.

Local ascorbate administration augments NO- and non-NO-dependent reflex cutaneous vasodilation in hypertensive humans

Full expression of reflex cutaneous vasodilation (VD) is dependent on nitric oxide (NO) and is attenuated with essential hypertension. Decreased NO-dependent VD may be due to 1) increased oxidant stress and/or 2) decreased L-arginine availability through upregulated arginase activity, potentially leading to increased superoxide production through uncoupled NO synthase (NOS). The purpose of this study was to determine the effect of antioxidant supplementation (alone and combined with arginase inhibition) on attenuated NO-dependent reflex cutaneous VD in hypertensive subjects. Nine unmedicated hypertensive [HT; mean arterial pressure (MAP) = 112 +/- 1 mmHg] and nine age-matched normotensive (NT; MAP = 81 +/- 10 mmHg) men and women were instrumented with four intradermal microdialysis (MD) fibers: control (Ringer), NOS inhibited (NOS-I; 10 mM N(G)-nitro-L-arginine), L-ascorbate supplemented (Asc; 10 mM L-ascorbate), and Asc + arginase inhibited [Asc+A-I; 10 mM L-ascorbate + 5 mM (S)-(2-boronoethyl)-L-cysteine-HCl + 5 mM N(omega)-hydroxy-nor-L-arginine]. Oral temperature was increased by 0.8 degrees C via a water-perfused suit. N(G)-nitro-L-arginine was then ultimately perfused through all MD sites to quantify the change in VD due to NO. Red blood cell flux was measured by laser-Doppler flowmetry over each skin MD site, and cutaneous vascular conductance (CVC) was calculated (CVC = flux/MAP) and normalized to maximal CVC (%CVC(max); 28 mM sodium nitroprusside + local heating to 43 degrees C). During the plateau in skin blood flow (Delta T(or) = 0.8 degrees C), cutaneous VD was attenuated in HT skin (NT: 42 +/- 4, HT: 35 +/- 3 %CVC(max); P < 0.05). Asc and Asc+A-I augmented cutaneous VD in HT (Asc: 57 +/- 5, Asc+A-I: 53 +/- 6 %CVC(max); P < 0.05 vs. control) but not in NT. %CVC(max) after NOS-I in the Asc- and Asc+A-I-treated sites was increased in HT (Asc: 41 +/- 4, Asc+A-I: 40 +/- 4, control: 29 +/- 4; P < 0.05). Compared with the control site, the change in %CVC(max) within each site after NOS-I was greater in HT (Asc: -19 +/- 4, Asc+A-I: -17 +/- 4, control: -9 +/- 2; P < 0.05) than in NT. Antioxidant supplementation alone or combined with arginase inhibition augments attenuated reflex cutaneous VD in hypertensive skin through NO- and non-NO-dependent mechanisms.

Preserved reflex cutaneous vasodilation in cystic fibrosis does not include an enhanced nitric oxide-dependent mechanism

In humans, vasoactive intestinal peptide (VIP) may play a role in reflex cutaneous vasodilation during body heating. We tested the hypothesis that the nitric oxide (NO)-dependent contribution to active vasodilation is enhanced in the skin of subjects with cystic fibrosis (CF), compensating for sparse levels of VIP. In 2 parallel protocols, microdialysis fibers were placed in the skin of 11 subjects with CF and 12 controls. Lactated Ringer was perfused at one microdialysis site and NG-nitro-L-arginine methyl ester (2.7 mg/ml) was perfused at a second microdialysis site. Skin blood flow was monitored over each site with laser-Doppler flowmetry. In protocol 1, local skin temperature was increased 0.5 degrees C every 5 s to 42 degrees C, and then it maintained at 42 degrees C for approximately 45 min. In protocol 2, subjects wore a tube-lined suit perfused with water at 50 degrees C, sufficient to increase oral temperature (Tor) 0.8 degrees C. Cutaneous vascular conductance (CVC) was calculated (flux/mean arterial pressure) and scaled as percent maximal CVC (sodium nitroprusside; 8.3 mg/ml). Vasodilation to local heating was similar between groups. The change (Delta%CVCmax) in CVC with NO synthase inhibition on the peak (9+/-3 vs. 12+/-5%CVCmax; P=0.6) and the plateau (45+/-3 vs. 35+/-5%CVCmax; P=0.1) phase of the skin blood flow response to local heating was similar in CF subjects and controls, respectively. Reflex cutaneous vasodilation increased CVC in CF subjects (58+/-4%CVCmax) and controls (53+/-4%CVCmax; P=0.37) and NO synthase inhibition attenuated CVC in subjects with CF (37+/-6%CVCmax) and controls (35+/-5%CVCmax; P=0.8) to a similar degree. Thus the preservation of cutaneous active vasodilation in subjects with CF is not associated with an enhanced NO-dependent vasodilation.

Up-regulation of arginase activity contributes to attenuated reflex cutaneous vasodilatation in hypertensive humans

Reflex cutaneous vasodilatation is dependent on nitric oxide (NO), which is diminished in hypertension (HTN). Arginase may be up-regulated with HTN, which preferentially metabolizes L-arginine (L-arg), competing with NO-synthase (NOS)-mediated pathways and limiting NO synthesis. We hypothesized that NO-dependent vasodilatation would be attenuated in HTN skin, and arginase inhibition (A-I) alone or with concurrent l-arginine supplementation, would augment vasodilatation. Five microdialysis fibres were placed in skin of eight unmedicated subjects with HTN (mean arterial pressure (MAP), 112 +/- 1 mmHg) and nine age-matched normotensive (AMN) (MAP: 87 +/- 1 mmHg) men and women to serve as: control (C, Ringer solution), NOS inhibited (NOS-I, 10 mM L-NAME), A-I (5 mM BEC + 5 mM nor-NOHA), L-arg supplemented (L-arg, 10 mM L-arg), and combined A-I + L-arg. Reflex vasodilatation was induced by using a water-perfused suit to increase oral temperature (T(or)) 1.0 degrees C. Red cell flux was measured by laser-Doppler flowmetry over each site. Cutaneous vascular conductance was calculated (CVC = flux/MAP) and normalized to maximal CVC (28 mM SNP + local heating to 43 degrees C). The Delta%CVC(max) between the control and NOS-I site was calculated as the difference between C and NOS-I sites. Maximal CVC was attenuated in the HTN subjects by approximately 25% compared with AMN subjects (P<0.001). Throughout, whole body heating %CVC(max) was not different between the groups (HTN, 43 +/- 3%CVC(max) versus AMN, 45 +/- 3%CVC(max), P>0.05). NOS-I significantly decreased %CVC(max) in both groups but %CVC(max) was greater in the HTN group (HTN, 32 +/- 4%CVC(max) versus AMN, 23 +/- 3%CVC(max), P<0.05). The Delta%CVC(max) between the control and NOS-I sites was attenuated at DeltaT(or) > 0.5 degrees C in the HTN group (P < 0.001 versus AMN). A-I alone augmented %CVC(max) only in the HTN group (HTN, 65 +/- 5%CVC(max) versus AMN, 48 +/- 3%CVC(max), P<0.05). L-Arg alone did not affect %CVC(max) in either group (HTN, 49 +/- 5%CVC(max) versus AMN, 49 +/- 3%CVC(max), P > 0.05). Combined A-I + L-arg augmented %CVC(max) in both subject groups compared with their respective control sites (HTN, 60 +/- 7%CVC(max) versus AMN, 61 +/- 3%CVC(max), both P<0.05 versus respective control sites). Vasodilatation is attenuated with HTN due to decreased NO-dependent vasodilatation and can be augmented with arginase inhibition but not L-arg supplementation, suggesting that arginase is up-regulated with HTN.