Department of Neurology, The Medical College of Wisconsin, Milwaukee 53226; and Research Service, Zablocki Veterans Affairs Medical Center, Milwaukee, Wisconsin 53295
PROBABLY MANY OF US INVOLVED in
pulmonary vascular research have, at some time, begun a grant
application or a manuscript with a variation on the phrase "despite
considerable research, the mechanism(s) responsible for hypoxic
pulmonary vasoconstriction is still unknown." However, even as we
lament that the answer to the hypoxic response still eludes us, the
quest remains fascinating. With each new work, another clue emerges,
and both the questions answered and the questions raised by that work
encourage us to continue. This is the case with the current article
(14) in this issue of the American Journal of
Physiology-Lung Cellular and Molecular Physiology. In this, the
most recent of that laboratory's series of studies examining the role
of endothelin (ET)-1 in the pulmonary vasculature (14,
20-24), Liu et al. (14) propose that hypoxic
pulmonary vasoconstriction depends heavily on the contribution of ET-1,
the potent vasoconstrictor released by the vascular endothelium.
Evidence for and against endothelium dependence of the hypoxic response
in pulmonary arteries can be found in numerous studies performed in a
variety of species. For example, in pigs, the constriction appears to
be, at least to some degree, endothelium dependent (10,
14), although smooth muscle cells isolated from distal pulmonary
arteries have been shown to constrict slightly to hypoxia (20). In rats, there is evidence both for
(13) and against (27) endothelium dependence,
and in cats (15, 16), there does not appear to be any
dependency at all.
It has been hypothesized that the hypoxic response is due to diminished
production of an endothelium-derived dilator substance(s) and/or the
elaboration of a constrictor substance(s). Endothelium-derived vasodilators such as prostacyclin, nitric oxide, and possibly an
endothelium-derived hyperpolarizing factor have been shown to modulate
the hypoxic response, but it now seems likely that hypoxic
vasoconstriction is not due to their reduced synthesis. Nevertheless,
the question of whether an endothelium-derived constrictor substance
such as ET-1 is responsible for hypoxic pulmonary vasoconstriction remains relevant, not only in light of the work by Liu et al. (14) but also by the demonstration of hypoxia-induced ET-1
synthesis by isolated lungs (9) and pulmonary vascular
endothelial cells (11) and strong evidence for ET-1
involvement in the pathogenesis of pulmonary hypertension
(2).
Liu et al. (14) used isolated, cannulated 100- to
150-µm-diameter pig pulmonary arteries. When these arteries were
exposed to hypoxia (PO2 = 30 Torr), their
diameter decreased if the endothelium was intact. This decrease was
somewhat enhanced by inhibiting vasodilator prostaglandins, and it was
greatly enhanced by inhibiting nitric oxide. On the other hand, there
was no change in arterial diameter during hypoxia if the vessels were
treated with the ETA receptor blocker BQ-123 or if the
endothelium was removed. Pretreating the endothelium-denuded vessels
with 10 On the basis of the above results, it would seem logical, then, to
conclude that ET-1 directly mediates hypoxic pulmonary vasoconstriction. But as ET-1 enters its 13th year in the scientific literature, its actions, like those of many teenagers, are "like totally unpredictable." Thus one can find other studies [many cited
by Liu et al. (14)] that just as convincingly argue that ET-1 is not a mediator of the hypoxic response. For example, in contrast to the results of Liu et al., Lazor et al. (12)
found that inhibiting the ETA receptor had no effect on the
hypoxic response in rat pulmonary arteries and Hasanuma et al.
(8) found that ET-1 caused pulmonary vasodilation. It has
also been pointed out (2) that the relatively rapid
response to hypoxia and the relatively slow synthesis of ET-1 make it
difficult to state that ET-1 is responsible for hypoxic pulmonary
vasoconstriction. To further complicate the picture, it has also been
reported (6) that although bovine pulmonary microvessels
produce a contractile factor in vitro, it is not ET-1.
If, however, we acknowledge that ET-1 is probably involved in some
capacity in the hypoxic response, then how might it work? (For a
thorough treatment of the mechanisms involved in the action of ET-1 in
vascular smooth muscle and the pulmonary circulation in particular, see
Refs. 3, 5, 7.) Briefly, though,
ET-1 directly interacts with ETA and ETB
receptor subtypes on vascular smooth muscle and endothelial cells. It
is generally accepted that the ETA receptor is involved in
the constrictor responses and the ETB receptor in the
vasodilatory responses (3, 5, 7). Recent work by Schmeck
et al. (19) raises the intriguing possibility that it is
not the ETA receptor but rather another form of the
ETB receptor that mediates the hypoxic constriction seen
during conditions of elevated pulmonary vascular tone. An increase in
intracellular Ca2+ is one of the integral components of the
ET-1 signaling process (3, 5, 7, 24), but the mechanisms
by which intracellular Ca2+ increases or by which ET-1
might increase the sensitivity of the contractile apparatus to
Ca2+ are still unclear. In addition to its effects on
intracellular Ca2+ release and/or the open probability of
Ca2+ channels, ET-1 may alter the activity of one or more
types of K+ channels (1, 17, 18, 20, 22, 23)
as well as the activity of other ion channels and ion-exchange
processes (3).
If ET-1 does not initiate the hypoxic response, then, as Liu et al.
(14) state, "the role of ET-1 in hypoxic pulmonary
vasoconstriction may be more subtle than direct concentration-dependent
activation of smooth muscle contraction." Liu et al. did not assess
any potential role for K+ channels in the isolated arteries
of this study, nor did Sham et al. (20) measure the effect
of ET-1 on voltage-dependent or other K+ channels in pig
pulmonary artery myocytes during acute hypoxia. Although Sham et al.
suggested that cellular sensitivity to Ca2+ might be
enhanced by ET-1, this was also not determined.
Editorial license allows one to propose all kinds of future studies
without incurring the obligation of actually having to design and do
them. Some of the more intriguing studies would deal with the
differences in the hypoxic responses among the various species. We
often invoke the different species argument to explain discrepancies
between our findings and those of others. But in order for us to truly
understand the role of ET-1 in the pulmonary system, similar types of
studies should be performed in rats, pigs, cats, dogs, rabbits, and,
where possible, humans. Studies should also be done in whole animals as
well as in their isolated lungs, arteries, veins, and smooth muscle and
endothelial cells.
Perhaps even more perplexing than species differences is the question
of the legitimacy and relevance of in vitro responses. When pulmonary
vessels are removed from their sheltered existence in the lung
parenchyma or smooth muscle and endothelial cells are isolated and
grown in culture, are they then predisposed or activated to manifest
behaviors not typically seen in vivo or are they merely providing us
with a realistic view of possible pathological responses?
While we wrestle with those issues, studies should also be conducted to
determine the different types of ET-1 receptor(s) subtypes, their
relative populations in different branches of the vascular tree, and
the differences in their responsiveness to ET-1. It should also be
investigated whether pulmonary vascular smooth muscle and endothelial
cells produce the same types and amounts of ETs and whether cells
within the same tissue differ in their production and/or responsiveness
to them (25). To what extent does the contractile state,
intrinsic tone, and intracellular Ca2+ concentration of the
pulmonary vasculature determine its response to ET-1? With respect to
the mechanisms of action in the pulmonary vasculature, is the effect of
ET-1 similar on all types of K+ channels, and how does this
effect(s) relate to other membrane mechanisms? What are the effects of
acute and prolonged hypoxia on the production of and response to ET-1
(4)? Do the kinetics of conversion from the ET precursor
to the active form differ between normoxic and hypoxic animals?
With apologies to the many authors whose excellent work was not cited,
this editorial ends with the hope that in the near future, we will all
be able to write "due to considerable research, the mechanism(s)
responsible for hypoxic pulmonary vasoconstriction is now known."
ARTICLE
TOP
ARTICLE
REFERENCES
10 M ET-1 restored the hypoxic response close to
its original magnitude. This study extended the results of prior work
from their group (20), which showed that small
contractions to hypoxia by pig pulmonary artery smooth muscle cells
were markedly enhanced by pretreatment with 10
10 M ET-1,
a dose that under normoxic conditions had no effect on either cell
length or intracellular Ca2+ concentration. A similar
priming effect by ET-1 on hypoxic pulmonary vasoconstriction has also
been seen in rat pulmonary artery myocytes (26). Liu et
al. (14) also cited numerous other studies in which ET-1
receptor antagonists blocked hypoxic vasoconstriction and hypoxia
resulted in increased ET-1 production in both intact and isolated lungs
and pulmonary vascular endothelial cells.
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FOOTNOTES |
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Address for reprint requests and other correspondence: J. A. Madden, Neurology Research 151, VAMC, Milwaukee, WI 53295 (E-mail: jmadden{at}mcw.edu).
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REFERENCES |
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