Elena Giaccone e Gabriele Aschiero
UVA Irradiation of Human Skin Vasodilates Arterial Vasculature and Lowers Blood
Pressure Independently of Nitric Oxide Synthase
The incidence of hypertension and cardiovascular disease (CVD) correlates with latitude and rises in winter. As nitric oxide (NO) metabolites are abundant in human skin, we hypothesized that exposure to UVA may mobilize NO bioactivity into the circulation to exert beneficial cardiovascular effects independently of vitamin D.
In 24 healthy volunteers, irradiation of the skin with two standard erythemal doses of UVA lowered blood pressure (BP), with concomitant decreases in circulating nitrate and rises in nitrite concentrations. Unexpectedly, acute dietary intervention aimed at modulating systemic nitrate availability had no effect on UV-induced hemodynamic changes, indicating that cardiovascular effects were not mediated via direct utilization of circulating nitrate.
UVA irradiation of the forearm caused increased blood flow independently of NO synthase (NOS) activity , suggesting involvement of pre-formed cutaneous NO stores.
Confocal fluorescence microscopy studies of human skin pre-labeled with the NO-imaging probe diaminofluorescein 2 diacetate revealed that UVA-induced NO release occurs in a NOS-independent, dose-dependent manner, with the majority of the light-sensitive NO pool in the upper epidermis. Collectively, our data provide mechanistic insights into an important function of the skin in modulating systemic NO bioavailability, which may account for the latitudinal and seasonal variations of BP and CVD.
Physiologic mechanism of NO in smooth muscle
Nitric oxide, known as the 'endothelium-derived relaxing factor', or 'EDRF', is biosynthesized endogenously from L-arginine, oxygen, and NADPH by various nitric oxide synthase (NOS) enzymes.
Reduction of inorganic nitrate may also serve to make nitric oxide. The endothelium (inner lining) of blood vessels uses nitric oxide to signal the surrounding smooth muscle to relax, thus resulting in vasodilation and increasing blood flow. Nitric oxide is highly reactive (having a lifetime of a few seconds), yet diffuses freely across membranes. These attributes make nitric oxide ideal for a transient paracrine (between adjacent cells) and autocrine (within a single cell) signaling molecule.
It was found that NO acts through the stimulation of the soluble guanylate cyclase, which is a heterodimeric enzyme with subsequent formation of cyclic-GMP. Cyclic-GMP activates protein kinase G, which causes reuptake of Ca2+ and the opening of calcium-activated potassium channels. The fall in concentration of Ca2+ ensures that the myosin light-chain kinase (MLCK) can no longer phosphorylate the myosin molecule, thereby stopping the crossbridge cycle and leading to relaxation of the smooth muscle cell.
Cardiovascular disease (CVD) accounts for 30% of deaths globally each year.
Hypertension is a major risk factor for stroke, peripheral vascular disease, and myocardial infarction. Systolic and diastolic blood pressure (DBP) in mild hypertensives are lower in summer than winter, and mean population blood pressure (BP) and hypertension prevalence increase with distance from the equator. The same seasonal and latitude variation is seen in the incidence of acute coronary syndrome, stroke, and cerebrovascular disease.
Incident UVR from the sun decreases in intensity during winter and with increasing latitude. UVB wavelengths of sunlight support the synthesis of vitamin D, which is essential to human health.
Observational studies show an inverse relationship between serum vitamin D levels and BP and ischemic heart disease ; but interventional studies show no effect of vitamin D supplementation on BP, the incidence of stroke, or ischemic heart disease. Although vitamin D is a marker for sunlight exposure, the factors that lower BP and cardiovascular mortality thus appear to be independent of it.
We have previously identified a rich store of nitric oxide (NO) metabolites in human skin, yet its biological role remains unknown.
Nitrite and nitrate are present in significantly higher quantities in skin compared with the circulation. Nitrate is the major nitrogen oxide in the dermis and epidermis where it is present at around 80 μmol l–1, which is 2–3 times higher than the circulating concentration. It is the ultimate oxidation product of NO and has long been considered biologically inert.
We have proposed that solar UVA may mobilize nitrogen oxides from cutaneous stores to the circulation to lower BP and cardiovascular mortality and here demonstrate that UVA irradiation of the skin of normotensive human volunteers lowers BP and causes arterial vasodilatation in a NO synthase (NOS)-independent manner. Consistent with the presence of NO storage forms in the epidermis and liberation of NO from these pools, we observed a rise in circulating nitrite contemporaneously with BP reduction and a fall in plasma nitrate.
These observations suggest a mechanism for the modulation of systemic NO bioactivity by the skin
UVA irradiation of human skin has opposite effects on circulating nitrite and nitrate
In preliminary studies, plasma nitrite levels rose following UVA irradiation. To minimize fluctuations in circulating NO metabolite levels by dietary intake, variation study subjects consumed a low nitrite/nitrate diet for 2 days before further mechanistic study. Under these conditions, nitrite increased from 0.50±0.04 μm pre-UVA to 0.72±0.04 μm immediately after irradiation and remained elevated at 0.72±0.03 μm for 40 minutes after irradiation. A nonsignificant trend toward lower circulatory nitrite levels was observed during sham irradiation (Figure 2a). These changes seemed to occur at the expense of nitrate (Figure 2b). Circulatory nitrate fell significantly from 11.79±0.64 to 8.99±0.40 μm and remained below baseline at 9.34±0.56 μm 40 minutes after active irradiation only. No significant changes were observed in plasma S-nitrosothiol and vitamin D levels following active or sham irradiation (Figure 2c and d); the delayed minor rise in the latter may be due to the small UVB contribution of our UVA source.
This study demonstrates that irradiation with UVA, corresponding to natural sunlight exposure for 30 minutes at noon on a sunny day, vasodilates the arterial vasculature in NOS-independent manner and reduces BP independently of changes in skin temperature.
The time course of plasma nitrite/nitrate changes and concomitant cardiovascular effects observed are all consistent with the photolytic release of NO from a pre-formed store and NO translocation from the skin to the circulation, but not with vitamin D production.
These observations support a mechanism for the modulation of systemic NO bioactivity and a possible role of the skin in cardiovascular homeostasis.
As with a previous investigation, our study is limited in as much as all interventions were acute and carried out in healthy normotensive young individuals.
We do not know whether UVA maintains its efficiency on repeated exposure and whether or not its BP response varies with age, gender, or disease. Moreover, nothing is known yet about the exact nature of the cutaneous NO pool, its source of formation and depletion–repletion kinetics.
However, if confirmed and found to also occur at lower intensity in a more chronic setting, these results will have significant implications for public health advice and change our risk–benefit assessment for sun exposure.
We have shown that UVA exposure with two standard erythemal doses (SEDs) to one side of the body (equating to a skin surface area exposed when wearing a short-sleeved shirt and shorts) leads to a small but significant fall in mean arterial BP.
Mean arterial and DBP fell following irradiation, but systolic BP did not change. DBP is a function of total peripheral resistance, and its marked drop in actively irradiated subjects, despite the compensatory rise in heart rate suggests that UVA vasodilates the peripheral vasculature.
The relatively small BP changes we observe may appear to be of minor significance, but a 10 mm Hg change in DBP is responsible for a 2-fold change in cardiovascular or cerebrovascular mortality, and this risk alteration is linear.
A reduction of DBP by 5 mm Hg decreases risk for stroke by 34% and coronary heart disease by 21% . In fact, any amount of BP reduction is protective against stroke and cardiovascular mortality, and the magnitude of changes observed in this study would appear to be large enough to account for the standardized mortality differences in populations living at different latitudes.
Dietary nitrate supplementation can lower BP but we found no augmentation of UVA-induced hemodynamic effects in the normal diet group compared with the low nitrate group.
We have carefully controlled for temperature using light-impermeable aluminum foil.
The sham-irradiated subjects had a slightly higher rise in core and skin temperature, but a smaller fall in BP than the actively irradiated group. Moreover, sham irradiation produced no rise in circulating nitrite, which we believe contributes to the UVA-induced fall in BP.
The skin contains large stores of nitrogen oxides, particularly nitrate.
Our in vivo data showing concomitant reductions in circulating nitrate and rises in nitrite upon UVA irradiation suggest nitrate to nitrite conversion is involved in light-induced BP reduction. Not with standing the possibility that enzymatic nitrate to nitrite reduction may be enhanced by light, the rapid onset of biochemical and hemodynamic changes strongly suggests involvement of a photolytic process.
UVA will penetrate 500 μm deep into the skin, so these reactions can occur in the epidermis, dermis, or even the dermal vasculature. These superficial sites receive the greatest UV energy, and we can speculate on the origin of these cutaneous NO stores.
Thus, the skin has the potential to contribute not only to cardiovascular homeostasis but also to distal organ protection by increasing the circulating NO metabolite pool.
The spectrum of incident UV at the earth’s surface varies with latitude and season. Shorter wavelength radiation is preferentially filtered out by the atmosphere. As the azimuth angle decreases with increased latitude and seasonal change to winter, the thickness of the atmosphere through which solar radiation passes increases, with a consequent reduction in the UVB:UVA ratio, and overall incident energy at the earth’s surface. Although the exact chemical composition of the cutaneous NO pool is currently unknown, nitrite is a likely constituent. Nitrite photolysis is wavelength and energy dependent, and the product of its action spectrum with geographic and seasonal incidence data may give an indication of the rate of NO release from cutaneous stores by latitude and season (Figure 5) UVB has a higher energy per photon compared with UVA, but only penetrates to the epidermis. Our calculations show that UVA is responsible for the majority of the NO release, with significant seasonal variation at latitudes between 40° and 60° (Figure 5b), which correlates with the known seasonal variation in BP and cardiovascular mortality at these temperate latitudes. Less NO is produced further from the equator, which mirrors the relationship of latitude with BP and hypertension prevalence.