Nitric oxide and airways *P.J. Barnes

Eur Respir J
1993, 6, 163-165
EDITORIAL
Nitric oxide and airways
*P.J. Barnes
For over 20 yrs it has been recognized that the
vasodilator responses to many agents are mediated by
the release of a vasodilator substance from endothelial cells [1]. The identity of endothelium-derived
relaxant factor (EDRF) remained elusive, largely because of its short half-life, until 1987 when PALMER
and eo-workers [2] were able to show that EDRF was
likely to be nitric oxide (NO). Many were surprised
that such a simple molecule could account for all of
the actions of EDRF, but extensive investigations, in
many species, have now provided supportive evidence
[3]. One of the most important advances has been the
discovery of substances which block the production of
endogenous NO. Nitric oxide is formed from the
semi-essential amino acid L-arginine via the action of
an enzyme NO synthase [4]. NO synthase exists in
constitutive forms (requiring Ca2 + for activation) and
inducible forms (which are independent of Ca2+), and
several NO synthase genes have recently been cloned
[3]. Analogues of L-arginine were found , which acted
as false substrates for the enzyme and, therefore,
blocked the formation of endogenous NO. This blockade can be overcome by adding back L-arginine, but
not by adding D-arginine, which is not a substrate for
the enzyme. Several arginine analogues have been
developed, including N°-monomethyl- L-arginine
(L-NMMA) and N°-nitro-L-arginine methyl ester
(L-NAME), which have proved to be extremely useful in revealing the role of endogenous NO in a whole
variety of processes [5]. Nitric oxide relaxes vascular smooth muscle by activation of soluble guanylyl
cyclase, with an increase in concentration of cyclic
guanosine 3'5'monophosphate (cGMP) . It had long
been recognized that directly acting vasodilators, such
as glyceryl trinitrate and sodium nitroprusside act as
NO donors [6].
The observation that NO is a vasodilator, immediately suggested that it may play a role in the regulation of the pulmonary circulation, and this has been
extensively investigated. Nitric oxide mediates the
vasodilator action of acetylcholine in animal and
human pulmonary vessels [7 , 8], and appears to act
as a braking mechanism against pulmonary vasoconstriction [8]. Release of NO from endothelial cells in
the pulmonary circulation appears to counteract hypoxic vasoconstriction [9, 10], and NO release is
apparently decreased in hypoxia [11] . There is circumstantial evidence that NO release from pulmonary
vessels may be impaired in patients with chronic obstructive pulmonary disease (COPD) [12]. Since NO
is a potent pulmonary vasodilator, inhalation of NO
*Dept of Thoracic Medicin e, National Heart and Lung Institute,
Dovehouse St. London SW3 6L Y, UK.
might be effective as a selective pulmonary vasodilator, in view of its short half-life. Inhaled NO has been
shown to dose-dependently inhibit pulmonary vasoconstriction induced by an infusion of a thromboxane
analogue in lambs [13]. Inhalation of NO has also
been shown to cause selective pulmonary vasodilatation in patients with pulmonary hypertension [14], and
COPD [15].
Nitrovasodilators, such as glyceryl trinitrate and sodium nitroprusside, also relax airway smooth muscle
in vitro, resulting from an increase in soluble guanylyl
cyclase activity, and an increase in cGMP [16]. It is ,
therefore, to be expected that NO may act as a bronchodilator and this has been demonstrated in canine
airways in vitro [ 17]. In the present issue of the
journal H6GMAN et al. [18] have demonstrated that a
relatively high concentration of inhaled NO (80 parts
per million) reduced the bronchoconstrictor effect of
nebulized methacholine in anaesthetized rabbits. A
more detailed study in anaesthetized guinea-pigs has
recently demonstrated a concentration-dependent, but
transient, reversal of methacholine-induced bronchoconstriction from 5-300 ppm [19]. In addition, a high
concentration of NO (300 ppm) caused a baseline
bronchodilatation. There was no evidence of tolerance
after prolonged administration, and the bronchodilator
effect of NO was additive with an ~-adrenoceptor
agonist. This raises the possibility that NO inhalation,
or NO releasing compounds, might have some therapeutic potential as alternative bronchodilators. An
advantage of inhaled NO would be its lack of systemic
effects, since NO would be rapidly inactivated by
haemoglobin. However, there are potential dangers of
inhaling NO [20], since in the presence of oxygen it
is oxidised to N0 2 and, thence, to nitrous and nitric
acids , which may increase airway responsiveness, and
in high concentration might cause pulmonary oedema
[21 , 22]. The interaction between NO and superoxide
anions may lead to the formation of peroxynitrite,
which may generate tissue damaging hydroxyl radicals
[23]. There is also some evidence that high concentrations of NO may have effects on deoxyribonucleic
acid (DNA) and be both genotoxic and cytotoxic [24].
There is increasing evidence that NO may function
as a neurotransmitter of nonadrenergic noncholinergic
(NANC) nerves, and nitrergic neurotransmission has
been demonstrated in the gut, bladder and reproductive organs [25]. There is convincing evidence that
NO is released from nerves themselves, since a particular form of NO synthase has been localized to
peripheral nerves [26], and is activated by calcium entry when the nerve is depolarized . Nitric oxide accounts for approximately half of the inhibitory
(bronchodilator) NANC response in guinea-pig trachea
164
P.J. BARNES
in vitro [27, 28], and modulates neural bronchoconstriction in vivo [29]. Nitric oxide appears to account for most of the bronchodilator NANC response
in human airways in vitro [30, 31], and in contrast to
guinea-pig trachea the neuropeptide vasoactive intestinal peptide appears to play little or no role in this
response. Endogenous NO appears to modulate
cholinergic neurotransmission in both guinea-pig and
human airways, by acting as a functional antagonist to
acetylcholine at airway smooth muscle [32, 33], but
whether it is released from cholinergic nerves in the
airways is not yet clear. Since bronchodilator NANC
nerves are the only neural bronchodilator pathway in
human airways [34], it is possible that there may be
a defect in function of these nerves in asthmatic airways. Airway inflammation may be associated with
release of superoxide anions from activated inflammatory cells, resulting in increased breakdown of NO
[35]. Augmentation of NO release from airway nerves
may, therefore, be of benefit in asthmatic patients.
NO is also a neurotransmitter of vasodilator NANC
responses in pulmonary vessels and, therefore, may be
involved in neural regulation of pulmonary blood flow
[36].
Endothelial cells and nerves are not the only source
of NO in airways. There is convincing evidence that
macrophages, including alveolar macrophages, may
synthesize NO after exposure to various cytokines
[37], and to endotoxin [38], and that NO is important
in host defence [3]. Macrophages express an inducible form of NO synthase, which has recently been
cloned [39]. Other cells also have an inducible form
of NO synthase, including endothelial cells, neutrophils
and vascular smooth muscle cells [3, 40]. Exposure
to cytokines such as tumour necrosis factor-a (TNFa) may result in induction of NO synthase, which may
lead to the formation of large amounts of NO. Indeed there is compelling evidence that NO induced in
septic shock is a major contributor to the cardiovascular collapse [3]. The amount of NO produced by
the inducible enzyme is very much greater than that
produced by the constitutive enzyme in endothelial
cells and nerves. It is likely that airway epithelial
cells may also be a source of NO, and this could be
induced by exposure to cytokines such as TNF-a in
the airway, although there is no evidence that NO is
"epithelium-derived relaxing factor" [41].
Endogenous NO may be a double-edged sword.
Nitric oxide may be beneficial in relaxing airway
smooth muscle in airways, but may have deleterious
effects when produced in high concentrations. It is a
potent vasodilator and might contribute to the hyperaemia of asthmatic airways. This may also increase
exudation of plasma from leaky post-capillary venules
in the airways. Indeed inhibition of endogenous NO
production significantly reduces plasma exudation and
inflammation, both in skin [42], and in airways [43].
Corticosteroids inhibit the expression of the inducible, but not the constitutive form of NO synthase [38,
44], and this may contribute to their anti-inflammatory
action, since massive NO formation may be detri-
mental, as in the case of endotoxic shock. If NO synthase is induced in airway epithelial cells in asthma,
as a result of exposure to cytokines released from inflammatory cells, then inhaled steroids may act to reduce the formation of NO and, thus, to down-regulate
the vascular components of the inflammatory response.
Steroids would not be expected to affect the release
of NO from bronchodilator nerves, since the neural
constitutive form of the enzyme is not steroid sensitive.
It is clear that NO may have a very important regulatory role in airway function, and may be implicated
in the pathophysiology of airway disease. Interest in
NO has revived interest in nitrovasodilators as alternative bronchodilators, although previous studies of
such drugs in asthma have not been impressive [45].
New NO donors, such as S-nitrothiols may have advantages [46]. It is possible, that this may lead to new
treatment approaches, both by enhancing the release of
neuronal NO and, possibly, by inhibiting the formation of NO by the inducible enzyme. While inhibition of endogenous NO, using arginine analogues
which block all forms of the enzyme, is likely to lead
to problems, such as hypertension, it is possible that
selective inhibitors of the inducible enzyme may be
developed in the future. This is an exciting new area
of research which has applicability to every branch of
pulmonary medicine
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