(Hypertension. 1998;31:719.)
© 1998 American Heart Association, Inc.
Ultraviolet Light May Contribute to Geographic and Racial Blood Pressure Differences
Christopher Bell
Faculty of Health Sciences,
Department of Physiology,
Trinity College Dublin, Ireland
To the Editor:
Dr Rostand's recent article1 draws attention to
a fascinating but hitherto-neglected result of the INTERSALT study:
that population mean blood pressures and the incidence of hypertension
both appear to be positively correlated with the latitude north or
south of the equator at which the population lives. Dr Rostand notes
that geographical movement away from the equator is associated with a
fall in ambient ultraviolet radiation. Decreased ultraviolet radiation
reduces vitamin D synthesis, and this can elevate parathyroid hormone
levels; furthermore, parathyroid hormone can stimulate vascular wall
growth. Drawing from these data and published evidence for altered
calcium homeostasis in human hypertensive subjects, Rostand postulates
that the variation in blood pressure with latitude may therefore be
linked to the different intensities of ambient ultraviolet light.
Dr Rostand's proposal is an attractive one, serving as it does to pull
together many disparate strands of the complex fabric that seems to
underlie the genesis of essential hypertension. It has important
potential implications for our interpretation of the role of calcium in
hypertension and may offer valuable clues to the understanding of
racial differences in salt sensitivity and renin profiles and of
different racial susceptibilities to high blood pressure.
There is, however, an additional mechanism by which ambient
electromagnetic radiation may reduce blood pressure. Robert Furchgott
and his colleagues noted as long ago as 1961 that exposure to light
relaxed isolated arterial
preparations,2 although other types of smooth
muscle tissue were much less sensitive.3 The
vascular photorelaxation was wavelength dependent, increasing as
wavelength was reduced from the visible into the ultraviolet range and
peaking at around 310 nm. In later experiments, Furchgott's group
found that photorelaxation involved an increase in muscle cell cyclic
GMP and that it was independent of the
endothelium.4
As far as I am aware, Furchgott did not suggest on the basis of his
work that ambient light intensity might affect the vasculature in vivo.
Nonetheless, ultraviolet radiation can penetrate deep enough into skin
to reach the microvessels,5 and the cutaneous
circulation represents the site of a substantial proportion of
total peripheral resistance. Although sympathetic tone has
the most dramatic controlling influence on cutaneous vascular flow, it
is not unreasonable to imagine that resting cutaneous vascular tone may
under some circumstances also be tonically reduced by absorbed
ultraviolet radiation. Withdrawal of this dilator effect when adapted
individuals are translocated to a higher latitude could well increase
total peripheral resistance and predispose to elevated
blood pressure.
If tonic photorelaxation of skin blood vessels does occur, then it
should be altered by changes in ambient light intensity much more
rapidly than are parathyroid function or vascular wall structure. It
would therefore be interesting to know whether readers have any
evidence that exposure of subjects to different climatic conditions
produces short-term changes in peripheral resistance that
can be uncoupled from the concomitant thermoregulatory adjustments.
References
1.
Rostand SG. Ultraviolet light may contribute to
geographic and racial blood pressure differences.
Hypertension. 1997;30:150–156.[Abstract/Free Full Text]
2.
Furchgott RF, Ehrreich SJ, Greenblatt E. The
photoactivated relaxation of smooth muscle of rabbit aorta.
J Gen Physiol. 1961;44:499–519.[Abstract/Free Full Text]
3.
Ehrreich SJ, Kao CY, Furchgott RF. Relaxation of
mammalian smooth muscles by visible and ultraviolet radiation.
Nature. 1968;218:682–684.[Medline]
[Order article via Infotrieve]
4.
Furchgott RF, Martin W, Jothianandan D, Villani GM.
Comparison of endothelium-dependent relaxation by
acetylcholine and endothelium-independent relaxation by
light in the rabbit aorta. In: Paton W, Mitchell J, Turner P, eds.
Proceedings of the IUPHAR 9th International Congress of
Pharmacology. London, UK: Macmillan Press; 1984:149–158.
5.
Holick MF. Photosynthesis of vitamin D in the skin:
effect of environmental and life-style variables. Fed
Proc. 1987;46:1876–1882.[Medline]
[Order article via Infotrieve]
Response
Stephen G. Rostand
Nephrology Research and Training Center,
Division of Nephrology,
University of Alabama at Birmingham,
Birmingham, Ala
I would like to thank Professor Bell for his kind comments
regarding my recently published hypothesis1 on
the possible role of ultraviolet light in explaining some geographic
differences and racial differences in blood pressure and the prevalence
of hypertension. Based on the in vitro studies of Furchgott et al
showing UV light exposure could produce relaxation of isolated rabbit
aorta strips treated with sodium nitrite,2 3 Bell
suggests that some of the inverse association between UV light exposure
and blood pressure discussed in my article could result from direct
photorelaxation of cutaneous arterioles produced by UV penetration of
the skin. It should be noted, however, that not all smooth muscle
responds to photostimulation2 and there have been
no studies performed on small peripheral resistance
vessels. Moreover, because most of the human body is protected from
sunlight by clothing, it seems unlikely that sufficient body surface
area is exposed for direct photorelaxation of vessels to play an
important role is blood pressure regulation. However, there may be
sufficient cutaneous exposure to UV light to modulate vascular tone
through alterations in vitamin D, parathyroid hormone, and calcium
status. Cutaneous vasodilatation in response to sunlight exposure in
humans may be more a thermoregulatory response to heat rather than an
effect of UV light. Nevertheless, the idea presented by Bell is
intriguing and readily testable.
References
1.
Rostand SG. Ultraviolet light may contribute to
geographic and racial blood pressure differences.
Hypertension. 1997;30:150–156.
2.
Furchgott RF. The pharmacology of vascular smooth
muscle. Pharmacol Rev. 1955;7:183–253.[Free Full Text]
3.
Ehrreich SJ, Kao CV, Furchgott RF. Relaxation of
mammalian smooth muscles by visible and ultraviolet radiation.
Nature. 1968;44:499–519.
