EDITORIAL FOCUS
Hypoxic pulmonary vasoconstriction: a multifactorial response?

Norbert Weissmann1, Friedrich Grimminger1, Andrea Olschewski2, and Werner Seeger1

1 Department of Internal Medicine and 2 Department of Anesthesiology and Intensive Care Medicine, Justus-Liebig-University Giessen, 35392 Giessen, Germany


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HYPOXIC PULMONARY VASOCONSTRICTION (HPV) is an essential mechanism that matches lung perfusion to ventilation to optimize pulmonary gas exchange (for reviews, see Refs. 9, 28, 31). Even if early reports observed that alveolar hypoxia induces pulmonary arterial hypertension, the modern era of HPV research started with the description of this mechanism by von Euler and Liljestrand (32) in 1946. Since then, considerable effort has been spent to identify the cell(s) responsible for O2 sensing, the sensor mechanism(s), and the pathway(s) of signal transduction leading to contraction of the vascular smooth muscle cells in the precapillary resistance vessels, which are suggested to be the predominant site of HPV (12, 28). However, we are still far from a complete understanding of the HPV mechanism.

From the ontogenetic point of view, HPV may better be designated as normoxic pulmonary vasodilatation. Before birth, HPV helps to divert blood flow away from the fetal lung through the ductus arteriosus and thus supports the fetal circulation. Postpartum, in concert with inflation of the lungs, the pulmonary vasculature relaxes and allows perfusion of the pulmonary vessels.

The size of the hypoxic lung area and the duration of hypoxia are important determinants for the characteristics of the lung vascular response. The following categories may be differentiated: 1) the acute hypoxic response (occurring within seconds), which matches lung perfusion to ventilation and optimizes pulmonary gas exchange, and 2) the response to sustained or prolonged alveolar hypoxia (developing within minutes to hours), also playing a role in ventilation-perfusion matching and possibly leading to 3) the vascular response to generalized chronic hypoxia, which is believed to result in extensive vascular remodeling, pulmonary hypertension, and cor pulmonale. For all three categories, the underlying biochemical mechanisms remain obscure.

In recent years, evidence has been provided that pulmonary arterial smooth muscle cells themselves respond to hypoxia, thus representing both sensor and effector cells with respect to this fundamental stimulus. Exposure of pulmonary arterial smooth muscle cells to hypoxia in vitro decreases K+ current through voltage-gated K+ (KV) channels (1, 24, 43) and membrane potential (10, 11, 24) and increases intracellular Ca2+ concentration and myosin light chain phosphorylation, causing contraction. (5, 6, 10, 16, 20, 24, 30, 45).

Although numerous investigations focused on the acute hypoxic response in intact animals, isolated perfused lung models, and isolated pulmonary arteries, comparatively few studies addressed the prolonged (>10-15 min) vasoconstrictor response to hypoxia. Typically, sustained hypoxia provokes an initial rapid vasoconstriction followed by a (partial) vasodilatation and a secondary more or less pronounced sustained pressor response. Such reactions have been reported from studies on isolated vessels, isolated perfused lungs, and intact or open chests of animals of several species (e.g., Refs. 3, 8, 14, 15, 22, 25, 26, 39, 41, 46). However, the magnitude and the time course of the secondary vasoconstrictor response vary in different investigations and may also be completely absent or reversed into vasodilatation depending on the pretone of the vessels investigated (21). Moreover, there is still debate whether the first and second vasoconstrictor responses to sustained alveolar hypoxia are regulated by identical or independent mechanisms. This is of interest because the sustained vasoconstrictor response may lead to the vascular remodeling process characterizing chronic hypoxia such as that occurring in obstructive and restrictive lung disease and at high altitude. From a therapeutic point of view, it would be of major interest to block such processes without loss of ventilation-perfusion matching.

Against this background, the current paper by Dipp et al. (7) in this issue of the American Journal of Physiology-Lung Cellular and Molecular Physiology addresses the role of intracellular Ca2+ stores for the regulation of sustained HPV. They investigated small intrapulmonary arteries of the rabbit challenged with hypoxia for a period of 30 min. HPV in these vessels is composed of an initial transient vasoconstrictor phase followed by a slowly developing increase in vascular tone (second phase). The increase during the second phase was noted to be dependent on the presence of endothelium and extracellular Ca2+. However, extracellular Ca2+ was not needed for phase 1 constriction and the basal constriction during phase 2. Application of ryanodine and caffeine completely blocked the biphasic hypoxic response. This inhibition was specific for HPV because the vasoconstrictive reaction to KCl was still present under such conditions. These general features were independent from precontraction of the vessels by PGF2alpha . Dipp et al. conclude 1) that hypoxia releases Ca2+ from the sarcoplasmic reticulum (SR) and that this is the underlying mechanism of HPV, 2) that Ca2+ flux across the plasmalemma is not essential for HPV, and 3) that the endothelium is not needed for basal Ca2+ release from the SR during hypoxia but may sensitize the contractile response to Ca2+.

Reviewing the literature shows that parts of previous investigations (10, 13) addressing the underlying idea of this paper are in accordance with the results of Dipp et al. (7) in this issue. However, none of the hitherto existing data are fully in line with their findings (e.g., Refs. 3, 14, 26, 41, 46). In particular, it is not entirely clear to date why the first and second phases of sustained HPV are differentially affected by experimental modulation of Ca2+ and the presence or absence of endothelium (for a review, see Ref. 33). Obviously, there are great variations between different investigations as to this issue (10, 33) . What is the reason for such different results? As also discussed in the current paper by Dipp et al. (7), variations in the experimental protocol or the impact of the anesthetic drugs used during explantation of the lungs may partly be responsible. For example, it has been convincingly shown that the biphasic response is largely dependent on the degree of vessel wall pretone (21). In addition to these suggestions, differences in species and gender may play a major role (22, 35), which is also true for the different experimental setups (e.g., comparison of investigations in intact animals, isolated blood- or buffer-perfused lungs, and isolated arteries of conductance or resistance vessel origin). Moreover, many of the conclusions forwarded as the mechanism(s) of HPV are dependent on inhibitor studies, with tools that may not be specific for the hypoxic response. Thus although the paper of Dipp et al. (7) provides an elegant concept for the regulation of HPV, many questions remain unresolved.

Even if release of Ca2+ from intracellular stores and, in particular, the SR is the basic mechanism of HPV, it is not clear how oxygen partial pressure is sensed. Are the stores themselves the oxygen sensor or is there a more remote sensor, triggering Ca2+ release by some intermediate signaling sequence? The same group from which the paper by Dipp et al. (7) in this issue of the American Journal of Physiology-Lung Cellular and Molecular Physiology originates very recently provided evidence for a new concept of oxygen sensing (40). They propose that cADP-ribosyl cyclase and cyclic ADP-ribose (cADPR) hydrolase work as redox sensors. According to this concept, hypoxia increases beta -NADH levels, which then increase the net amount of cADPR synthesized from beta -NAD+ by cADP-ribosyl cyclase, and simultaneously inhibit cADPR degradation by cADPR hydrolase. cADPR promotes Ca2+ release from ryanodine-sensitive SR stores and elicits vasoconstriction. This concept is suggested to be responsible for the sustained vasoconstrictive phase of HPV, whereas phase 1 of HPV may result from an initial fall in ATP levels and inhibition of SR Ca2+-ATPase by hypoxia. However, this concept has to be settled in further investigations.

Moreover, there is a body of literature favoring different sensor and signaling mechanisms in HPV, and data supporting these alternative concepts are to be reconciled with the concept forwarded by Dipp et al. (7).

In concept 1, KV channels have been proposed as the primary oxygen sensors. These channels are inhibited by hypoxia, which leads to membrane depolarization, activation of L-type Ca2+ channels, Ca2+ influx, an increase in intracellular Ca2+ concentration, and cell contraction. However, it is not clear how the KV channels are gated. It is likely that the K+ current is controlled by changes in the cellular redox status, with mitochondrial- and/or NAD(P)H oxidase-dependent oxygen radical formation or the ratios of redox couples affected by mitochondrial metabolism possibly being involved (2, 34). Alternatively, hypoxia-induced elevation in intracellular Ca2+ levels might secondarily affect the K+ channels, thereby inducing the hypoxia-provoked membrane depolarization (23, 33).

In concept 2, the pulmonary oxygen sensor may be represented by a NADH oxidoreductase, which, via superoxide, generates hydrogen peroxide (H2O2). H2O2 then stimulates guanylate cyclase to release vasodilatating cGMP. The nitric oxide pathway synergistically contributes to cGMP synthesis via guanylate cyclase. According to this concept, there is tonic stimulation of cGMP formation during normoxia. Under hypoxic conditions, release of H2O2 by the oxidoreductase is decreased, and the subsequent loss of vasodilatation contributes to HPV (18, 19).

Concept 3 proposes a nonphagocytic (low-output) NADPH oxidase as the oxygen sensor. The idea of NADPH oxidase as a pulmonary oxygen sensor was initially related to the work of Thomas et al. (29) and Youngson et al. (42). Investigations by Marshall et al. (17) and investigations by our laboratory (36, 37) suggest that this system may paradoxically be activated during hypoxia, with a subsequent increase in superoxide and H2O2 levels during hypoxia. The increase in H2O2 levels then elicits vasoconstriction by a still unidentified mechanism that may involve hydroxyl radicals.

Recently, Chandel and Schumacker (4) proposed mitochondria as cellular oxygen sensors as concept 4. They provided evidence that the hypoxic signal is transduced by a hypoxic blockade of mitochondrial electron transport distal of complex III and a subsequent rise in superoxide levels during hypoxia. Although this concept was primarily forwarded for gene regulation during chronic hypoxia, it also has to be taken into account for acute and sustained oxygen sensing.

As concept 5, it has been suggested that cytochrome P-450-dependent arachidonic acid metabolism with, for example, the appearance of hydroxyeicosatetraenoic acids is involved in HPV (44, 47).

Thus we are apparently far from establishing a unifying hypothesis for the sequelae underlying hypoxic vasoconstriction in the pulmonary vasculature. One common aspect of the various concepts depicted above is the role of changes in the cellular redox potential; however, this may still be part of a multifactorial response. We do not even know whether the acute and sustained responses to alveolar hypoxia as well as the mechanisms leading to the vascular remodeling process during chronic hypoxia are triggered by identical or different (initial) pathways. An interesting aspect in this context is the dependence of the progressive increase in vascular tone on the endothelium (and extracellular Ca2+) during phase 2 that was found in the paper by Dipp et al. (7). This may point to a prominent role of the endothelium for the transition of the increase in vascular tone during sustained hypoxia to the vascular remodeling process occurring during chronic hypoxia (38). However, this finding, too, has to be reconciled with various controversial observations as to the role of the endothelium in HPV (e.g., Ref. 27; for a review, see Ref. 33).

One major reason for the discrepancies in the current concepts of HPV may be related to the use of nonspecific inhibitors. Additionally, there are methodological uncertainties concerning the measurement of reactive oxygen species and their specific role for hypoxia sensing. Thus more than 50 years after the investigations of von Euler and Liljestrand (32), we are still in doubt about the underlying mechanisms of hypoxic vascular responses of the lung. Hopefully, new technologies and investigations in (conditioned) transgenic and gene-deficient mice may clarify the controversies we are currently facing.


    FOOTNOTES

Address for reprint requests and other correspondence: W. Seeger, Dept. of Internal Medicine, Justus-Liebig-Univ. Giessen, Klinikstrasse 36, 35392 Giessen, Germany (E-mail: Werner.Seeger{at}innere.med.uni-giessen.de).


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