Exposure to 20 J cm
2 of UV-A light, a dose equivalent to approximately 30 min of Mediterranean summer sunlight (Diffey
2002), has previously been shown to increase plasma [NO
2−] and lead to a sustained reduction in BP (Opländer et al.
2009; Liu et al.
2014). The present study explored the effects of different doses of UV-A exposure and the effects on circulating NO metabolites, BP, and resting
\({{\dot{V}}}{\rm O}_{2}\). The principal findings were that 20 J cm
2 of UV-A exposure resulted in a brief, but significant increase in plasma [NO
2−] whereas 10 J cm
2 was insufficient to alter the concentration of this NO metabolite. In contrast to previous findings, exposure to UV-A light in either dose did not alter BP. However, we here demonstrate for the first time that exposure to UV-A light reduces resting
\({{\dot{V}}}{\rm O}_{2}\) and RMR. While these data suggest that a minimum dose of 20 J cm
2 is necessary to augment plasma NO availability, further work is required to better understand the therapeutic effects of UV-A light on cardiovascular and metabolic health.
Dose-dependent effects of UV-A light on NO metabolites
The observed increase in plasma [NO
2−] following 20 J cm
2 but not 10 J cm
2 of UV-A light supports our original hypothesis that the UV-induced release of NO metabolites from the skin is dose dependent. The higher dose of UV-A light is proportional to approximately 30 min of Mediterranean summer sunlight (Liu et al.
2014). We demonstrate that 20 J cm
2 of UV-A light increased plasma [NO
2−] by 75% immediately following cessation of exposure, which is higher than the proportional increases of 45% (Opländer et al.
2009) and 40% (Liu et al.
2014) that have been previously reported. On the other hand, the absolute increase in plasma [NO
2−] was higher (~ 200 nM) in the study by Liu and colleagues (
2014) than in the present study (123 nM). Collectively, the present study and other studies in the area (Opländer et al.
2009; Liu et al.
2014; Muggeridge et al.
2015) suggest that UV-A induced NO production may alter the overall circulating pool of NO. However, it is clear that there is profound inter-individual variability of NO
2− following UV-A challenge (Opländer et al.
2009). This may be explained by recent data from Holliman and colleagues (
2017) who demonstrated diverging baseline levels of skin NO content in their skin donors and a variable magnitude of NO release from isolated keratinocytes in response to UV-A exposure. Both age and body composition are known to influence the storage and release of NO metabolites from the skin (Ma et al.
2015). These variables, along with skin type and habitual sunlight exposure may help explain the divergent response to UV-A between individuals and study cohorts. The total storage of NO metabolites in the skin is likely to be important as it has recently been demonstrated in vitro that NO
2− is converted to NO upon UV-A exposure (Holliman et al.
2017).
In contrast to previous findings (Opländer et al.
2009; Liu et al.
2014), the observed increase in plasma [NO
2−] immediately following exposure with 20 J cm
2, was not sustained 30 min post-exposure. One possible explanation is that the source and delivery method for the UV-A light differed between all of these studies involving human volunteers. We speculate that the overall ‘dose’ or intensity of light may contribute to diverging NO
2− release and overall NO kinetics. Previous studies have utilized the same dose of 20 J cm
2 of UV-A light (Opländer et al.
2009; Liu et al.
2014; Muggeridge et al.
2015), however, over different time periods and intensities. In the present study, the intensity of light was the same in UVA10 and UVA20, where exposure time was manipulated to alter the overall dose. In vitro experiments highlight the relevance of the UV-A source showing that overall NO kinetics are altered depending on the distance from the UV-A light source (Dejam et al.
2003) and potentially the specific wavelength of the light. The middle to long UV-A wavelength range of 340–400 nm has been shown to be the major contributor to overall NO production in isolated keratinocytes in response to UV-A challenge (Holliman et al.
2017). In the present study, our UV-A light source emitted its maximum intensity at 351 nm. It should be highlighted, however, that natural sunlight contains light in the UV-B wavelength which will increase production of vitamin D
3, and potentially cause erythema and DNA damage (Marionnet et al.
2015). A key question remains, therefore, as to whether the release of NO metabolites differs in response to natural or artificial sources of light.
In contrast with previous research, plasma [NO
3−] was not altered by UV-A exposure. Liu and colleagues (
2014) have previously demonstrated that 20 J cm
2 of UV-A light reduced [NO
3−] by ~ 3 µM. The authors speculated that UV-A exposure may result in direct photolysis of NO
3− to yield NO. Alternatively, it was suggested that UV-A exposure may release NO
3− from skins stores which in turn would enhance the reduction of NO
3− to NO
2− via facultative bacteria in the oral cavity (Duncan et al.
1995) and gut (Tiso and Schechter
2015) or by XOR (Lundberg et al.
2009). However, the authors noted that the decline in plasma [NO
3−] was over ten times the increase in [NO
2−] which suggests the reduction in BP following UV-A exposure resulted from bioactivation of cutaneous rather than circulating NO stores. More recent data now suggest that photolysis of NO
3− to NO by UV-A light seems unlikely (Holliman et al.
2017). These authors demonstrated that irradiation of a solution of sodium NO
2− released NO in a dose-dependent manner. Conversely, irradiation of sodium NO
3− did not yield NO. While these contrasting data between the present research and that of Liu and colleagues (
2014) are not readily explainable, it is conceivable that the aforementioned differences in UV-A delivery methods may be important. Alternatively, inter-individual variability in the release of NO metabolites following UV-A exposure (Oplander et al.
2009; Holliman et al.
2017) and the multiple biological fates of these molecules may help to explain these notable differences. While future research is warranted to elucidate the role of skin and circulating stores of NO
3−, it seems likely that the consistently reported bioactivation of NO
2− is responsible for the myriad of physiological effects that occur in response to UV-A exposure.
Blood pressure is not altered by UV-A light
Our study demonstrated that UV-A light did not significantly alter BP. This finding contrasted our study hypothesis and previous research which has demonstrated that 20 J cm
2 effectively reduces BP in a healthy cohort (Opländer et al.
2009; Liu et al.
2014). Nevertheless, there was a moderate reduction in MAP (
d = 0.5) 30 min after exposure in the UV20 condition with a concomitant moderate reduction in heart rate (
d = 0.5). While the clinical and biological significances of the small reduction in MAP (3 mmHg) in this study are unclear, a reduction of DBP by only 5 mmHg decreases risk for stroke by 34% (MacMahon et al.
1990) and any amount of BP reduction is protective against cardiovascular mortality (Lawes et al.
2004). It should be highlighted that the small sample of participants in this study were all young, healthy, and normotensive males. The most likely explanation for the absence of a significant reduction in BP was that the elevation in plasma [NO
2−] was not sustained long enough to elicit a pronounced biological effect.
The experimental procedures may also have limited the extent to which UV-A light may have reduced BP. Specifically, the measurement of BP following the treatments in each condition was preceded by a 1-h period of lying supine. The consequence is that this sustained period of lying supine likely induced postural venodilation (Gemignani et al.
2008) and lowered BP prior to the experimental intervention. Indeed, DBP was 70 ± 7 mmHg and MAP was 85 ± 6 mmHg at baseline across all three conditions. This may have limited any further biologically significant reductions in BP following UV-A exposure. Recently, we have shown that posture and the period of time of lying supine can alter both BP and plasma [NO
2−] (Liddle et al.
2018), which emphasizes that posture should be carefully considered when conducting future research in this area.
UV-A light reduces \({{\dot{V}}}{\rm O}_{2}\) and RMR
A notable finding in this study is that
\({{\dot{V}}}{\rm O}_{2}\) and RMR fell during both light exposures. To our knowledge, this study is the first to explore the effects of UV-A light on resting metabolism in humans. A significant reduction in resting
\({{\dot{V}}}{\rm O}_{2}\) was observed despite no elevation in plasma [NO
2−] during UVA10, whereas a trend for a reduction was found in the presence of a significant elevation in plasma [NO
2−] in UVA20. The reduction in
\({{\dot{V}}}{\rm O}_{2}\) following UV-A exposure appears to be transient, however, as values returned to baseline 30 min after exposure in both light exposure conditions. While the mechanisms accounting for the reduction in
\({{\dot{V}}}{\rm O}_{2}\) could not be ascertained in the present study, we speculate that the complexity of NO conversion and appearance from NO
2− or other nitrogen oxides following UV-A challenge may have accounted for this finding. We hypothesized that resting
\({{\dot{V}}}{\rm O}_{2}\) would be reduced in the presence of elevated NO
2− as others have shown that NO
2− inhibited respiration by ~ 60% when applied to primary skeletal myotubes, in vitro (Larsen et al.
2011). Others have demonstrated that increasing plasma [NO
2−] and NO bioavailability via dietary NO
3− supplementation reduces
\({{\dot{V}}}{\rm O}_{2}\) at rest (Larsen et al.
2014; Whitfield et al.
2016). Larsen and colleagues (
2014) speculated that the reduction in RMR was most likely due to an NO-mediated inhibition of cytochrome
c oxidase (Carr and Ferguson
1990) and found that changes in this parameter following dietary NO
3− were independent of insulin sensitivity and thyroid hormones. However, Whitfield and Colleagues (
2016) observed a similar reduction in whole body
\({{\dot{V}}}{\rm O}_{2}\) at rest, in the absence of change in skeletal muscle mitochondrial respiration. It is conceivable that the changes in resting
\({{\dot{V}}}{\rm O}_{2}\) observed in the present study following UV-A may have occurred via other NO-related mechanisms involving species distinct from those of the canonical NO
3−–NO
2−—NO pathway. It must also be considered that the absence of an elevation in plasma NO
2− following UVA10 and the presence in UVA20 may be indicative of an NO-independent mechanism.
Functional relevance of the findings
Given that NO plays a pivotal role in the regulation of vascular tone (Stamler et al.
1994) and glucose uptake (Balon and Nadler
1997; Bergandi et al.
2003), augmentation of NO bioavailability through environmental exposure to UV-A light has the potential to have a profound impact on human health. The present data suggests that a minimum dose of 20 J cm
2 of UV-A light is required to elicit a significant increase in plasma NO
2−, a known marker of NO availability. The potential total daily exposure to UV-A light during daylight hours is likely to exceed 20 J cm
2 in most countries during the spring and summer months (Marionnet et al.
2015). This suggests that habitual exposure to sunlight may complement endogenous NO production and exogenous NO generation from dietary pathways although the aforementioned differences between artificial and natural light should be reemphasized at this stage. Clothing, working patterns, and leisure behaviors are also likely to have a considerable impact on the exposure of skin to UV-A light. Furthermore, in the winter months, countries further from the equator are likely to see daily UV-A exposure fall well below the minimum threshold required to increase NO
2− (Liu et al.
2014). Indeed, incidences of acute coronary syndrome and stroke are known to be higher in winter (Rosengren et al.
1999; Oberg et al.
2000). Importantly, epidemiological data also suggests that sunlight exposure reduces all-cause and cardiovascular mortality (Yang et al.
2011; Brondum-Jacobsen et al.
2013). There may be value, therefore, in a targeted approach to increase NO availability via the ingestion of NO
3−-rich food and beverages during these periods of reduced UV exposure.
The long term effects of the reduction in resting
\({{\dot{V}}}{\rm O}_{2}\) and RMR following UV-A exposure are unclear but there may be positive effects on cellular function and signaling given that an augmented plasma NO
2− can improve mitochondrial efficiency (Larsen et al.
2011). This may be particularly relevent where tissue oxygen supply is reduced through either clinical (Kenjale et al.
2011) or environmental conditions (Muggeridge et al.
2014). Conversely, a sustained reduction in RMR must be considered as contraindicative for energy balance. For example, Hill et al. (
2003) estimated that, on average, the gain of body weight over time was due to a positive energy balance of 15 kcal/day
−1. This is substantially lower than the reductions in RMR which were induced by exposure to UV-A light in the present study (57–129 kcal/day
−1). This is of relevance given the composition of UV-A in overall sunlight exposure is approximately 90% although the intensity depends on latitude and seasonal variations in the light/dark cycle (Diffey
2002). These current data would seem to conflict with the suggestion that UV exposure is as a potential intervention for obesity (Geldenhuys et al.
2014; Fleury et al.
2016), potentially mediated via vitamin D or NO effects (Gorman et al.
2017). The present study highlights the extent to which sunlight exposure may have a potentially confounding impact on key markers of cardiometabolic health; data which should be carefully considered in epidemiological research.