Cutaneous neuronal nitric oxide is specifically decreased in postural tachycardia syndrome

Exercise prevents age-related decline in nitric-oxide-mediated vasodilator function in cutaneous microvessels

Ageing is associated with impaired endothelium-derived nitric oxide (NO) function in human microvessels. We investigated the impact of cardiorespiratory fitness and exercise training on physiological and pharmacological NO-mediated microvascular responses in older subjects. NO-mediated vasodilatation was examined in young, older sedentary and older fit subjects who had two microdialysis fibres embedded into the skin on the ventral aspect of the forearm and laser Doppler probes placed over these sites. Both sites were then heated to 42 degrees C, with Ringer solution infused in one probe and N-nitro-L-arginine methyl ester (L-NAME) through the second. In another study, three doses of ACh were infused in the presence or absence of L-NAME in similar subjects. The older sedentary subjects then undertook exercise training, with repeat studies at 12 and 24 weeks. The NO component of the heat-induced rise in cutaneous vascular conductance (CVC) was diminished in the older sedentary subjects after 30 min of prolonged heating at 42 degrees C (26.9 +/- 3.9%CVC(max)), compared to older fit (46.2 +/- 7.0%CVC(max), P < 0.05) and young subjects (41.2 +/- 5.2%CVC(max), P < 0.05), whereas exercise training in the older sedentary group enhanced NO-vasodilator function in response to incremental heating (P < 0.05). Similarly, the NO contribution to ACh responses was impaired in the older sedentary versus older fit subjects (low dose 3.2 +/- 1.3 versus 6.6 +/- 1.3%CVC(max); mid dose 11.4 +/- 2.4 versus 21.6 +/- 4.5%CVC(max); high dose 35.2 +/- 6.0 versus 52.6 +/- 7.9%CVC(max), P < 0.05) and training reversed this (12 weeks: 13.7 +/- 3.6, 28.9 +/- 5.3, 56.1 +/- 3.9%CVC(max), P < 0.05). These findings indicate that maintaining a high level of fitness, or undertaking exercise training, prevents age-related decline in indices of physiological and pharmacological microvascular NO-mediated vasodilator function. Since higher levels of NO confer anti-atherogenic benefit, this study has potential implications for the prevention of microvascular dysfunction in humans.

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.

Angiotensin II type 1 receptor blockade corrects cutaneous nitric oxide deficit in postural tachycardia syndrome

Low-flow postural tachycardia syndrome (POTS) is associated with increased plasma angiotensin II (ANG II) and reduced neuronal nitric oxide (NO), which decreases NO-dependent vasodilation. We tested whether the ANG II type 1 receptor (AT(1)R) antagonist losartan would improve NO-dependent vasodilation in POTS patients. Furthermore, if the action of ANG II is dependent on NO, then the NO synthase inhibitor nitro-L-arginine (NLA) would reverse this improvement. We used local heating of the skin of the left calf to 42 degrees C and laser-Doppler flowmetry to assess NO-dependent conductance [percent maximum cutaneous vascular conductance (%CVC(max))] in 12 low-flow POTS patients aged 22.5 +/- 0.8 yr and in 15 control subjects aged 22.0 +/- 1.3 yr. After measuring the baseline local heating response at three separate sites, we perfused individual intradermal microdialysis catheters at those sites with 2 microg/l losartan, 10 mM NLA, or losartan + NLA. The predrug heat response was reduced in POTS, particularly the plateau phase reflecting NO-dependent vasodilation (50 +/- 5 vs. 91 +/- 7 %CVC(max); P < 0.001 vs. control). Losartan increased baseline flow in both POTS and control subjects (from 6 +/- 1 to 21 +/- 3 vs. from 10 +/- 1 to 21 +/- 2 %CVC(max); P < 0.05 compared with predrug). The baseline increase was blunted by NLA. Losartan increased the POTS heat response to equal the control subject response (79 +/- 7 vs. 88 +/- 6 %CVC(max); P = 0.48). NLA decreased both POTS and control subject heat responses to similar conductances (38 +/- 4 vs. 38 +/- 3 %CVC(max); P < 0.05 compared with predrug). The addition of NLA to losartan reduced POTS and control subject conductances compared with losartan alone (48 +/- 3 vs. 53 +/- 2 %CVC(max)). The data suggest that the reduction in cutaneous NO-dependent vasodilation in low-flow POTS is corrected by AT(1)R blockade.