Departments of Physiology and Biochemistry and Molecular Biology, Colorado State University, Fort Collins, Colorado 80523
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ABSTRACT |
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The
hypoxia-induced membrane depolarization and subsequent constriction of
small resistance pulmonary arteries occurs, in part, via inhibition of
vascular smooth muscle cell voltage-gated K+
(KV) channels open at the resting membrane potential.
Pulmonary arterial smooth muscle cell KV channel
expression, antibody-based dissection of the pulmonary arterial smooth
muscle cell K+ current, and the O2 sensitivity
of cloned KV channels expressed in heterologous
expression systems have all been examined to identify the molecular
components of the pulmonary arterial O2-sensitive KV current. Likely components include Kv2.1/Kv9.3 and
Kv1.2/Kv1.5 heteromeric channels and the Kv3.1b -subunit. Although
the mechanism of KV channel inhibition by hypoxia is
unknown, it appears that KV
-subunits do not sense
O2 directly. Rather, they are most likely inhibited through
interaction with an unidentified O2 sensor and/or
-subunit. This review summarizes the role of KV channels in hypoxic pulmonary vasoconstriction, the recent progress toward the
identification of KV channel subunits involved in this
response, and the possible mechanisms of KV channel
regulation by hypoxia.
voltage-gated potassium channel; pulmonary artery; oxygen sensor; smooth muscle
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INTRODUCTION |
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MEMBRANE POTASSIUM CHANNELS play an essential role in cellular homeostasis and nerve and muscle excitability. In vascular smooth muscle, voltage-gated K+ (KV) channels are integral in the regulation of membrane potential and, therefore, vascular tone (42, 60). Closure of vascular smooth muscle cell (VSMC) K+ channels, open at the resting membrane potential, causes membrane depolarization. This change in membrane potential activates voltage-gated Ca2+ channels, leading to an increase in intracellular Ca2+ concentration ([Ca2+]i), and vasoconstriction (60). Activation of VSMC K+ channels leads to hyperpolarization, inhibition of voltage-gated Ca2+ channels, and vasodilation (60). VSMCs have a high input resistance; therefore, even a small change in K+ channel activity can have a significant effect on membrane potential and, consequently, vascular tone (60, 92). Indeed, many factors that modulate vessel tone do so through activating or inhibiting VSMC K+ channels (42, 60, 92). This review focuses on the pulmonary arterial smooth muscle cell (PASMC) K+ channels thought to be inhibited by hypoxia, leading to pulmonary vasoconstriction.
In the systemic circulation, small arteries dilate in response to low levels of O2 (hypoxia) to increase O2 delivery to the tissues. In contrast, small resistance arteries in the pulmonary circulation constrict in response to alveolar hypoxia, a process known as hypoxic pulmonary vasoconstriction (HPV) (95). In the fetal circulation, HPV contributes to the high pulmonary arterial resistance, diverting blood through the ductus arteriosus (55). In the adult, HPV reduces the flow of blood through atelectatic or underventilated areas of the lung where ventilation is not adequate for oxygenation (55). In this manner, acute HPV is a mechanism that helps to match perfusion to ventilation, diverting blood flow away from poorly ventilated portions of the lung to maximize arterial saturation (107). When only a small region of the lung is hypoxic, HPV can occur without a significant effect on pulmonary arterial pressure (59). However, if hypoxia is generalized, as seen with many lung diseases and high-altitude exposure, the subsequent pulmonary vasoconstriction causes an increase in pulmonary arterial pressure, which can potentially lead to pulmonary hypertension, heart failure, and death (34).
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BACKGROUND |
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HPV
Since first described by von Euler and Liljestrand (103) in 1946, HPV has been extensively studied. However, the exact mechanisms underlying HPV remain unknown. Although endothelium-derived vasoactive factors are clearly important and appear to be necessary for the full expression of HPV in vivo [for a review, see Ward and Aaronson (105)], it seems likely that HPV is initiated, at least partially, through a mechanism intrinsic to PASMCs. Responsiveness to acute hypoxia has been demonstrated not only in isolated lungs (38, 54, 73, 102) but also in pulmonary arterial rings denuded of endothelium (3, 37, 49, 113) and in single PASMCs (3, 16, 27, 50, 63, 69, 72, 73, 112).It is currently hypothesized that alveolar hypoxia acts to directly depolarize the PASMC membrane, thus causing an influx of Ca2+ through L-type voltage-gated Ca2+ channels and subsequent contraction. Indeed, in isolated PASMCs, acute hypoxia has been shown to significantly depolarize the membrane potential by ~15-20 mV (3, 31, 49, 112), leading to contraction of individual PASMCs (50). In 1976, McMurtry et al. (54) were the first to demonstrate that HPV is inhibited by the voltage-gated Ca2+ channel antagonists verapamil and SKF-525 in the isolated perfused lung. McMurtry (53) and others (97) also demonstrated that HPV is potentiated by BAY K 8644, a Ca2+ channel agonist. In 1985, Harder et al. (37) were the first to demonstrate that the hypoxia-induced constriction of small pulmonary arteries (<300 µm) was associated with membrane depolarization that could be inhibited by verapamil. More recently, Salvaterra and Goldman (83), using isolated adult PASMCs, and Cornfield et al. (16), using isolated fetal PASMCs, demonstrated that the hypoxia-induced increase in intracellular Ca2+ was inhibited by L-type Ca2+ channel blockers. These studies clearly illustrate the importance of Ca2+ influx through membrane voltage-gated Ca2+ channels in HPV. Indeed, Franco-Obregon and Lopez-Barneo (27) have shown that hypoxia causes a potentiation of the Ca2+ current in the majority of cells isolated from rabbit resistance pulmonary arteries. However, because these channels are generally closed at the resting membrane potential of PASMCs (27), it is likely that hypoxia acts first to depolarize the membrane by inhibiting the K+ channels involved in setting the resting membrane potential (107). Hypoxic inhibition of K+ channels was first demonstrated in 1988 by Lopez-Barneo et al. (47) in carotid body type I cells. Later, Post et al. (73) showed that hypoxia causes both an inhibition of whole cell K+ current and membrane depolarization in isolated PASMCs. An alternative, or perhaps an additional, hypothesis is that hypoxia causes the release of Ca2+ from intracellular stores independent of Ca2+ influx and that this rise in intracellular Ca2+ leads to the inhibition of membrane KV channels (31, 41, 72). The relative importance of these mechanisms is not clear; however, the aforementioned studies suggest that hypoxia acts through multiple pathways to induce pulmonary vasoconstriction.
Hypoxic Vasoconstriction Is Unique to Small Pulmonary Arteries
VSMCs show different responses to hypoxia according to the size and type of vessel from which they were isolated. Small resistance pulmonary arteries (third intrapulmonary artery or greater) constrict in response to hypoxia (3, 49, 72, 113), whereas large conduit pulmonary arteries (main pulmonary artery and right and left branches) usually do not respond or dilate slightly (3, 49, 72), although hypoxic responses have been described in some preparations (39, 113). In addition, the response of small pulmonary arteries is in marked contrast to that of small arteries isolated from the systemic circulation, which dilate in response to hypoxia (113). At the single-cell level, hypoxia causes small contractions (49, 88) and increased intracellular Ca2+ (88) in resistance PASMCs but has little or no effect in conduit PASMCs or VSMCs isolated from small systemic arteries (49, 88). Furthermore, hypoxia causes a significant depolarization of the membrane potential from approximatelyK+ Channels in the Pulmonary Vasculature
Four main classes of K+ channels have been described in vascular smooth muscle based on their biophysical and pharmacological properties: ATP sensitive (KATP), inward rectifier (KIR), large-conductance Ca2+-activated K+ (BKCa), and KV (42, 92). Of these, the KV channels are most likely to control the PASMC membrane potential and thus regulate vessel tone (23, 111).KATP channels are sensitive to changes in the level of intracellular ATP and are therefore regulated by cellular metabolic status (43, 60). KATP currents are enhanced by ADP and inhibited by ATP. Antidiabetic sulfonylurea drugs such as glibenclamide inhibit this channel, whereas antihypertensive drugs such as minoxidil activate this channel (60). These channels probably do not contribute to HPV because glibenclamide has no effect on pulmonary perfusion pressure in normoxia or moderate hypoxia (38). However, these channels may serve as an important negative feedback mechanism, preventing excessive HPV under conditions of severe hypoxia when ATP levels are reduced (109). Furthermore, blockade of the endothelin subtype A receptor leads to activation of KATP channels and subsequent inhibition of HPV (84).
KIR channels conduct maximum current at membrane potentials negative to the K+ equilibrium potential and smaller outward currents at membrane potentials positive to the K+ equilibrium potential (60). This channel is very sensitive to inhibition by extracellular Ba2+. The presence of these channels in the pulmonary vasculature has not as yet been confirmed.
BKCa channels are sensitive to both membrane depolarization and increases in intracellular Ca2+, with increasing Ca2+ levels essentially shifting the activation curve in the hyperpolarizing direction. BKCa channels have a single-channel conductance of 250 pS in symmetrical K+ and are sensitive to blocking by tetraethylammonium (TEA), charybdotoxin (CTX), and iberiotoxin (60). BKCa channels play a role in control of the arterial smooth muscle membrane potential by serving as a negative feedback mechanism, regulating the degree of membrane depolarization and the constriction induced by increased cytoplasmic Ca2+ (14, 60). PASMC BKCa channels are activated at membrane potentials positive to the resting membrane potential [which explains why CTX does not depolarize PASMCs (3, 63, 111)]; thus under conditions of hypoxia, it is likely that the membrane depolarization and increased [Ca2+]i activate BKCa channels, leading to membrane hyperpolarization (75). Therefore, it is more likely that BKCa channels prevent excessive HPV rather than initiate it.
All VSMCs examined to date have at least one component of KV current, and many studies report more than one type of current (60). These currents include "delayed rectifier" (IK,DR) and "transient outward" (IK,TO) types (14). Recently, a third noninactivating KV current (IK,N), which is activated at the resting membrane potential, has also been described in rabbit, rat, and pig PASMCs (24, 62, 63). These channels provide an important K+ channel conductance in the physiological membrane potential range of pulmonary arteries (60). Indeed, several groups of investigators (3, 61, 63, 91, 115) have shown that the membrane potential in PASMCs is controlled by one or more subtypes of the delayed rectifier K+ channel. Pharmacologically, most KV currents can be isolated by selective inhibition to 4-aminopyridine (4-AP). The outward K+ current in PASMCs is inhibited by low doses of 4-AP (3, 61) but not by TEA (3) or CTX (3). In addition, 4-AP (3, 63, 111, 115) but not TEA (3, 63), CTX (3, 63, 111), iberiotoxin (63), or glibenclamide (111) depolarizes PASMCs. Furthermore, exposure to 4-AP but not to glibenclamide or CTX leads to an increase in [Ca2+]i (19). Taken together, these studies clearly emphasize the importance of KV channels in the regulation of PASMC membrane potential and pulmonary arterial tone.
K+ Channels in HPV
Agents that block K+ channels, such as 4-AP and TEA, depolarize the membrane (3, 72, 111), increase tension in pulmonary arterial rings (3, 73), and increase pulmonary arterial pressure in isolated perfused lungs (38). Hypoxia acts in a similar manner to inhibit whole cell outward K+ current (3, 72, 73) and cause membrane depolarization (3, 73, 112), constriction of small pulmonary arteries (3), and an increase in pulmonary arterial pressure (38). The fact that PASMC membrane depolarization and pulmonary arterial constriction can be mimicked by K+ channel blockers suggests that the depolarization induced by hypoxia is due to the inhibition of K+ channels open at the resting membrane potential.Two types of PASMC K+ channels have been suggested to be
modulated by hypoxia leading to HPV, BKCa (68,
73) and KV (which includes
IK,DR and IK,N);
however, the majority of electrophysiological and pharmacological
evidence points to members of the KV family as the main
PASMC O2 sensors [see McCulloch et al. (52)
for further review]. The O2-sensitive K+
current described in most rat PASMC preparations is a delayed rectifier, which is sensitive to 4-AP and insensitive to CTX
(6). Indeed, hypoxic inhibition of PASMC
IK,DR has been shown in whole cell and
single-channel studies in the presence of BKCa channel inhibitors (3, 64). Furthermore, hypoxia and 4-AP
constrict resistance arteries, whereas CTX has no effect
(3). In addition, pretreatment of isolated dog PASMCs with
4-AP (1 mM) but not with TEA (1 mM) prevents the hypoxic effect on
outward K+ current (72). Taken together, these
data suggest that KV channels that exhibit
IK,DR and are sensitive to 4-AP but insensitive
to CTX are responsible for the O2-sensitive
K+ current in PASMCs. However, pretreatment with 4-AP
does not always prevent, and may even potentiate, hypoxic constriction
as demonstrated in rat and dog lungs (7, 38), rat
resistance pulmonary arteries (3), and isolated pig PASMCs
(88). Although these studies seem to argue against a role
for KV channels in HPV, several points need to be
considered. For instance, increased sensitivity to hypoxia has been
demonstrated after an initial priming depolarization (98)
and in the presence of agonist-induced pretone (106). Therefore, in some cases, pretreatment with 4-AP may act as a priming
agent, causing decreased background K+ permeability and a
potentiation of the hypoxic response. Additionally, because 4-AP
sensitivity varies between KV channel subtypes and between
species (17), the use of 4-AP as a pharmacological tool for determining the O2-sensitive K+ current is
limited. For example, the heteromeric Kv2.1/Kv9.3 channel, which is a
good candidate for at least one O2-sensitive K+
current, is insensitive to 4-AP at 1-10 mmol/l (69),
the concentration used in the above-mentioned studies where 4-AP did
not attenuate hypoxic constriction. Therefore, it is likely that the
pulmonary arterial O2-sensitive K+ current
represents an ensemble of current from distinct KV channel isoforms, some of which may be 4-AP insensitive. Additionally, the
variability in the 4-AP sensitivity of the hypoxic response may be due
to species-specific -subunit combinations responsible for the
hypoxia-sensitive current. Thus the variable response to hypoxia in the
presence of 4-AP does not exclude a role for KV channels in
HPV. Given that the majority of published studies suggest the
involvement of a KV current in PASMC O2
sensing, the remainder of this review focuses on the role of
KV channels in HPV.
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IDENTIFICATION OF KV CHANNEL GENES INVOLVED IN PULMONARY ARTERIAL O2 SENSING |
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Complexity and Diversity of KV Channels
The first complete nucleotide sequence encoding a KV channel was reported in 1987 with the cloning of the Drosophila Shaker channel (65). Low-stringency screening of Drosophila cDNA libraries with the Shaker cDNA led to isolation of the K+ channel cDNAs Shab, Shal, and Shaw that are derived from three distinct Drosophila genes (10). The sequences are homologous to Shaker, having ~40% identity. Each Drosophila gene has been shown to have one or more mammalian homologs (82), currently grouped into the Kv1, Kv2, Kv3, and Kv4 subfamilies. The Kv1 family, which has >60% homology to Shaker in the core region, is the largest, with at least seven members (12). In addition to the four mammalian subfamilies relating to Shaker, Shab, Shal, and Shaw, five additional subfamilies (Kv5-9) have also been described (81). Currently, over 30 KV channels have been cloned and expressed in heterologous expression systems (12). These channels often display differences in voltage sensitivity, current kinetics, and steady-state activation and inactivation (82).KV channels exist as tetramers formed by 4 six-transmembrane-spanning -subunits combining to form a functional
channel (48). Not only can identical
-subunits combine
to form a functional channel, but distinct
-subunits can also
combine to form functional heteromeric channels both in vitro and in
vivo (13, 81, 89). These heteromeric channels have unique
properties that often represent a blend of the observed properties of
the corresponding homomeric channels. Furthermore, several
KV
-subunits are nonfunctional when expressed alone. For
example, the Kv9.3 subunit, the most recently identified member of the
mammalian KV family, does not form a functional homomeric
channel itself but rather functions only in heteromeric complexes where
it confers altered voltage sensitivity and activation kinetics
(69).
Accessory -subunits can combine with KV
-subunits to
add even more diversity to KV channel function
(36). Currently, four KV
-subunit gene
families have been described (21, 22, 26, 56, 77). All are
cytoplasmic proteins, ~40 kDa in mass, with a conserved core sequence
and variable NH2 termini. KV
-subunits have
been shown to confer functional effects onto
-subunits, including
both fast and slow inactivation, altered voltage sensitivity, and
slowed deactivation (57, 101). Additionally, the
-subunit may play a role as a cellular redox sensor because it
appears to confer O2 sensitivity on the Kv4.2 channel in
heterologous expression systems (71). Thus the roles of
KV
-subunits are many, and, indeed, they may be
important not only in the functional modulation of KV
-subunits but also in the PASMC response to hypoxia.
Recent Progress Toward Identification of
KV Channel -Subunits Likely to Contribute
to Pulmonary Arterial O2-Sensitive
K+ Current
KV channel expression in the pulmonary
vasculature.
Wang et al. (104) used RT-PCR and immunoblotting to show
that Kv1.5 and Kv1.2 mRNA and protein were expressed in primary cultures of rat PASMCs. Patel et al. (69), using
degenerate RT-PCR, detected mRNA expression of Kv1.2, Kv1.3, Kv2.1, and
a novel KV channel -subunit, Kv9.3, in rat PASMCs.
Despite several approaches, these investigators (69)
failed to detect Kv1.5 and thus concluded that it was not expressed in
the rat pulmonary artery. Archer et al. (5), using
immunohistochemistry and immunoblotting of dissociated rat resistance
PASMCs, reported expression of Kv1.1, Kv1.2, Kv1.3, Kv1.5, and Kv2.1
but not of Kv1.4. About the same time, Yuan et al. (116),
using RT-PCR of primary cultures of rat PASMCs, confirmed expression of
Kv1.1, Kv1.2, Kv1.5, Kv1.6, Kv2.1, and Kv9.3 mRNAs and reported mRNA
expression of Kv1.4. Using immunoblotting, they detected Kv1.2, Kv1.4,
and Kv1.5 but not Kv1.3. Hulme et al. (40) detected
expression of Kv1.2 and Kv1.5 in PASMCs from rat lung tissue sections.
Most recently, Osipenko et al. (64) reported expression of
Kv3.1b protein in freshly dissociated PASMCs from rats and rabbits
using immunocytochemistry. Additionally, our group has data suggesting
that Kv1.2, Kv1.5, Kv2.1, Kv9.3, and Kv3.1b subunits are expressed in
bovine resistance PASMCs (Coppock and Tamkun, unpublished
observations). Taken together, these results strongly suggest
the expression of the following
-subunits in resistance PASMCs:
Kv1.2, Kv1.5, Kv2.1, Kv3.1b, and Kv9.3 (see Table
1). Not as convincingly, the presence of Kv1.1, Kv1.4, and Kv1.6 is also suggested.
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Antibody dissection of the O2-sensitive K+ current. Recently, antibodies against KV channels have been utilized in the intracellular patch pipette solution in an attempt to dissect the O2-sensitive K+ current in PASMCs during whole cell recordings under normoxic and hypoxic conditions. When Archer et al. (5) added an anti-Kv2.1 antibody to the patch pipette, they found inhibition of the outward K+ current and membrane depolarization in rat resistance PASMCs, suggesting that Kv2.1 plays a role in setting the resting membrane potential. When an anti-Kv1.5 antibody was added to the patch pipette, whole cell K+ current was significantly decreased, and both the hypoxia- and 4-AP-induced increase in [Ca2+]i in isolated PASMCs was attenuated (5). However, the anti-Kv1.5 antibody did not consistently depolarize the membrane in isolated PASMCs. Based on their findings, Archer et al. (5) hypothesized that hypoxic inhibition of Kv2.1 leads to membrane depolarization that shifts the resting membrane potential into a range where Kv1.5 is active and can thus be inhibited by hypoxia, leading to further inhibition of whole cell K+ current. Somewhat contrary to these results, others (31) found that application of anti-Kv1.5 in the patch pipette increased [Ca2+]i and caused membrane depolarization in rat PASMCs. Additionally, subsequent hypoxic challenge resulted in a further increase in intracellular Ca2+, with no effect on membrane potential. In related studies, Conforti et al. (15) used an antibody against Kv1.2 in the patch pipette to specifically inhibit Kv1.2 current from the whole cell current in rat pheochromocytoma (PC12) cells (a model for O2 sensing in the carotid body). When exposed to hypoxia, they found no additional K+ current inhibition, suggesting that the Kv1.2 current was responsible for the hypoxic effect in PC12 cells.
O2 sensitivity of cloned channels. The O2 sensitivity of several cloned KV channels has recently been studied in heterologous expression systems (15, 40, 64, 69). Investigators have primarily focused on those channels that, when expressed in heterologous expression systems, meet the profile of the PASMC O2-sensitive K+ current: delayed rectifier, sensitive to 4-AP, and insensitive to CTX.
KV2.1. Patel et al. (69) examined the O2 sensitivity of Kv2.1 expressed in COS-7 cells. In a subset of COS cells (21% of cells), Kv2.1 was reversibly inhibited by hypoxia by 34% (69). Hulme et al. (40) found similar results with Kv2.1 in mouse L cells. Hypoxia significantly inhibited Kv2.1 current by 23% at 60 mV in nearly all of the cells studied. In both studies, only the Kv2.1 currents that were activated at more positive potentials were significantly inhibited by hypoxia. Furthermore, little Kv2.1 current was detected at potentials more negative thanSummary. The results from the aforementioned studies are summarized in Table 1. Kv1.1, Kv1.3, and Kv1.5 currents were not O2 sensitive in all cell systems studied (40, 64). The O2 sensitivity of Kv1.4 has not been examined; however, as seen in Table 1, there is some controversy as to whether it is even expressed in PASMCs, and because it produces an A-type current, it does not meet the biophysical profile of the O2-sensitive K+ current in PASMCs. Several investigators (5, 40, 69, 116) have detected Kv1.2 expression; however, its O2 sensitivity seems to vary markedly with expression systems (15, 40, 64, 69). Kv2.1 is expressed in PASMCs (5, 69, 116) and is found to be significantly inhibited by hypoxia in COS cells (69) and L cells (40) but not in Xenopus oocytes (15). In L cells, whole cell current from the Kv1.2/Kv1.5 heteromeric channel is significantly inhibited by hypoxia in the voltage range of the PASMC resting membrane potential (40). Although there is no direct evidence that this heteromeric channel is expressed in vivo, both individual subunits have been detected in PASMCs (5, 40, 116). The Kv2.1/Kv9.3 heteromeric channel is sensitive to hypoxia in the voltage range of the PASMC resting membrane potential in COS cells (69) and L cells (40). Both individual subunits are expressed in PASMCs (69, 116), and evidence for assembly in vitro has been demonstrated (69); however, there is no direct evidence that this heteromeric channel is expressed in vivo. Furthermore, when expressed in Xenopus oocytes, the Kv2.1/Kv9.3 current is significantly less sensitive to 4-AP than the Kv2.1 current alone (IC50, 4.5 mM for Kv2.1 vs. 31.6 mM for Kv2.1/9.3) (69), making the Kv2.1/Kv9.3 heteromeric channel much less sensitive to 4-AP than the O2-sensitive K+ current described in PASMCs. Kv3.1b is expressed in PASMCs and is O2 sensitive when expressed in L929 cells (64). In support of a role for Kv3.1b in O2 sensing in the pulmonary artery, data from our laboratory in the bovine pulmonary artery indicate that the protein expression of Kv3.1b markedly increases from the main conduit pulmonary artery to the third and fourth intrapulmonary resistance arteries (Coppock and Tamkun, unpublished observations). Because small resistance pulmonary arteries constrict in response to hypoxia, whereas large conduit pulmonary arteries do not (3, 49, 72), these findings strongly suggest a role for Kv3.1b in HPV.
Taken together, these studies support a potential role for Kv2.1/Kv9.3 and Kv1.2/Kv1.5 heteromeric channels in the physiological response of PASMC K+ channels to hypoxia. The Kv3.1bRole of the KV -Subunit in
O2 Sensing
Multiple KV -subunits have been detected in PASMCs. Yuan
et al. (116), using RT-PCR of rat primary cultured
resistance PASMCs, detected mRNA expression of Kv
1.1, Kv
1.2, and
Kv
2.1. Data from our laboratory indicate that Kv
1.1, Kv
1.2,
and Kv
1.3 proteins are expressed in bovine PASMCs (Coppock and
Tamkun, unpublished observations). Furthermore, our data also suggest
that protein expression of these
-subunits dramatically increases
from the main conduit pulmonary artery to the third and fourth
intrapulmonary resistance arteries. This is interesting in light of the
fact that resistance pulmonary arteries constrict in response to
hypoxia, whereas conduit pulmonary arteries do not (113),
suggesting that KV
-subunit expression may be partially
responsible for the differential effects of hypoxia on conduit and
resistance pulmonary arteries.
The KV -subunit has been shown to confer
O2 sensitivity to certain KV
-subunits.
Indeed, in HEK-293 cells, Kv4.2 alone was unaffected by hypoxia, but
when coexpressed with Kv
1.2, the current was significantly reduced
(15.5% inhibition at 40 mV) (70). Recently, we compared
the effects of hypoxia on Kv2.1 current in L cells and HEK-293 cells.
We found that although the Kv2.1 current was significantly suppressed
by hypoxia in L cells, it was not significantly inhibited in HEK-293
cells (Sakamoto N and Tamkun M, unpublished observations). Uebele et
al. (100) have previously shown that mouse L
cells contain an endogenously expressed
-subunit, Kv
2.1.
Therefore, we determined whether this difference (the presence of
Kv
2.1) could account for the different O2 sensitivities of the Kv2.1 current expressed in L cells versus that in HEK-293 cells.
We found that when Kv2.1 was coexpressed with Kv
2.1 in HEK-293
cells, the whole cell K+ current was significantly
inhibited by hypoxia (Sakamoto N and Tamkun M, unpublished
observations). The results from this experiment are rather puzzling,
however, because Kv
2.1 does not immunoprecipitate with Kv2.1
(58). Although the mechanism by which KV
-subunits confer O2 sensitivity onto some KV
-subunits in heterologous expression systems is unknown, much
evidence points to a functional role for KV
-subunits in
PASMC O2 sensing.
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POTENTIAL MECHANISMS OF KV CHANNEL INHIBITION BY HYPOXIA |
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There have been many hypotheses put forth to explain the hypoxic inhibition of KV channels in PASMCs, but the mechanism by which these channels sense low levels of O2 is still unclear. KV channels could be the O2 sensors themselves through the direct interaction of O2 with the channel, or they may be an effector, responding through some indirect mechanism (85). The indirect hypothesis seems more likely, however, because cloned KV channels are O2 sensitive in some heterologous expression systems but not in others. Perhaps they may be coexpressed with some endogenous O2 sensor that confers O2 sensitivity to the channel. As previously mentioned, Kv2.1 and Kv2.1/Kv9.3 channels were inhibited by hypoxia in only a subset of COS cells (69). Variable expression of and/or channel colocalization with such an endogenous O2 sensor would explain some of the apparent differences in the O2 sensitivities of cloned channels expressed in heterologous expression systems.
Membrane-Bound Heme-Linked Protein and/or NADPH Oxidase Mechanisms of O2 Sensing
One possible explanation is the existence of a membrane-bound heme-linked protein closely associated with the channel. In this scenario, two hypotheses have been put forth. The binding of O2 to the sensor could affect its conformation, which, in turn, affects the conformation of the KV channel and thus the K+ current. Alternatively, hypoxia could cause a decrease in the production of reactive oxygen species (ROS) generated by a membrane-bound NADPH oxidase, modulating the channel through a redox mechanism (see Refs. 6, 11, 86 for further reviews). In support of a membrane-bound O2 sensor, KV channel inhibition has been clearly demonstrated in excised patches in the absence of intracellular mediators in type I cells of the carotid body (30) as well as in heterologous expression systems (64, 70). Furthermore, carbon monoxide, which in biological systems is known to only react with heme proteins, significantly reverses the hypoxic inhibition of KV channels expressed in HEK cells (70) and in rabbit carotid body type I cells (46), suggesting that the sensor is a heme protein.In the second hypothesis, the heme protein complex also contains an NADPH oxidase that rapidly generates superoxide and H2O2 under normoxic conditions, creating a relatively oxidized redox state (11). During hypoxia, however, there is decreased substrate for the oxidase leading to decreased production of ROS, a more reduced state, and inactivation of redox-dependent KV channels (11). Indeed, it is well known that K+ channel activity can be regulated by redox modulation in vitro (20, 79). In support of the NADPH oxidase hypothesis, reducing agents such as dithiothreitol, reduced glutathione (GSH), and NADH mimic the effects of hypoxia on PASMCs by decreasing K+ current (66, 67, 114), whereas oxidizing agents such as diamide and oxidized glutathione (GSSG) have the opposite effect, increasing K+ current (67, 76). NADPH oxidase is expressed in a variety of O2-sensitive tissues including pulmonary airway chemoreceptor cells [neuroepithelial body (NEB) cells] (110), carotid bodies (45), and PASMCs (51). Diphenyleneiodonium (DPI), an inhibitor of NADPH oxidase, has been shown to significantly inhibit HPV in isolated rat lungs (94), rabbit lungs (32), and rat pulmonary arteries (96). However, because DPI has been shown to inhibit both K+ and Ca2+ currents (108), it has been criticized as being a useful tool in evaluating the importance of NADPH oxidase in O2 sensing (107). Furthermore, although DPI inhibits HPV, it does not cause sustained vasoconstriction under normoxic conditions (11).
Recently, two independent groups of investigators used a NADPH oxidase-deficient mouse model to examine the role of NADPH oxidase in O2 sensing. These groups studied the effects of hypoxia on K+ current in NEB cells (29) and PASMCs (4) from these mice and found very different results. Hypoxia had no effect on the K+ current in NEB cells from oxidase-deficient mice, whereas it significantly inhibited the K+ current in NEB cells from wild-type mice (29). Additionally, DPI significantly reduced the K+ current by 30% in wild-type NEB cells but had no effect in oxidase-deficient NEB cells. These results clearly support a role for NADPH oxidase as an O2 sensor in pulmonary airway chemoreceptors. In contrast to these results are the results of Archer et al. (4) in PASMCs using the same mouse model. They found the production of ROS to be significantly lower in the lungs of oxidase-deficient mice than in the lungs of wild-type mice. However, there was no significant difference in the hypoxic inhibition of K+ current between PASMCs from wild-type and oxidase-deficient animals, suggesting that NADPH oxidase is not necessary for O2 sensing in PASMCs. However, the NADPH gp91phox knockout mouse experiments do not exclude a role for a "low-output" NADPH oxidase isoform (32). These seemingly contradictory results emphasize the complexity of O2 sensing and strongly suggest a diversity of O2-sensing mechanisms between tissues.
Mitochondria as O2 Sensors
Multiple hypotheses relating to mitochondria as O2 sensors have been proposed (6, 11, 85). These include depletion of high-energy phosphates, a shift toward the reduced forms of redox couples such as NADH/NAD and GSH/GSSG, and an increase in mitochondrial ROS production. A logical potential mechanism is the depletion of intracellular ATP. In isolated rat lungs, inhibitors of oxidative ATP production caused a transient pressor response and a loss of reactivity to hypoxia, suggesting that depression of oxidative ATP elicits pulmonary vasoconstriction (78). Additionally, inhibitors of glycolysis, such as 2-deoxyglucose, and inhibitors of oxidative phosphorylation, such as rotenone, significantly inhibit outward K+ current in PASMCs (2, 114). However, these responses may not be due to simple depletion of ATP because the patch pipettes contained 5-10 mM ATP (2, 114). Additionally, HPV has been shown to occur very rapidly before any changes in ATP levels (8). Severe hypoxia has been shown to decrease ATP concentration; however, the decreased ATP levels result in the opening of KATP channels, increased K+ efflux, and vasodilation (109).An additional hypothesis is that hypoxia inhibits the activity of cytochrome oxidase, which leads to an altered mitochondrial redox state and the generation of ROS (11, 85). In support of this hypothesis, hypoxia has been shown to cause a decrease in the maximum velocity of cytochrome oxidase (11) and an increase in the production of ROS (44, 51). However, in one report (51), the hypoxic generation of superoxide was attributed to a membrane-bound NADPH oxidase because DPI but not the mitochondrial inhibitor myxathiazole significantly attenuated this response.
Alternatively, hypoxia may cause an accumulation of redox couples such as NADH/NAD and GSH/GSSG and/or a decreased production of ROS (6). Mitochondria in the lung have been shown to make ROS in proportion to PO2 (99). Furthermore, rotenone, an inhibitor of the mitochondrial electron transport chain, mimics the effect of hypoxia by decreasing whole cell K+ current (2, 114), reducing lung ROS production (2, 114), and causing pulmonary artery constriction (3, 78). However, there is still much controversy regarding the different responses to metabolic inhibitors in different O2-responsive tissues (11) and some question as to whether hypoxia causes an increase (44, 51) or a decrease (6) in the production of ROS. Further studies are needed to examine these questions.
Intracellular Ca2+ Release as a Mechanism of KV Channel Inhibition by Hypoxia
There is accumulating evidence (31, 41, 72) that Ca2+ release from intracellular stores may initiate HPV. Hypoxia has been shown to cause a rapid, transient increase in [Ca2+]i in PASMCs that is unaffected by extracellular Ca2+ removal or blockade of voltage-gated Ca2+ channels but is significantly decreased by caffeine depletion of Ca2+ from the sarcoplasmic reticulum (83). Furthermore, ryanodine plus caffeine significantly inhibits HPV in dog resistance pulmonary arteries (41), and thapsigargin, cyclopiazonic acid, and ryanodine (used to deplete Ca2+ from intracellular Ca2+ stores) prevent HPV in rat pulmonary arterial resistance vessels (72). In PASMCs loaded with fura 2, hypoxic challenge, thapsigargin, cyclopiazonic acid, and ryanodine all resulted in a significant increase in [Ca2+]i and membrane depolarization (31). However, pretreatment with thapsigargin, cyclopiazonic acid, or ryanodine prevented a hypoxic challenge-induced increase in [Ca2+]i. Taken together, these studies suggest a predominant role for Ca2+ release from the intracellular stores in HPV. Delayed rectifier K+ channels are inhibited by [Ca2+]i in dog PASMCs (72). Additionally, application of an anti-Kv1.5 antibody within the patch pipette solution causes an increase in [Ca2+]i and membrane depolarization, whereas subsequent hypoxic challenge results in a further increase in [Ca2+]i, with no effect on membrane potential (31). These results led the researchers to hypothesize that hypoxia acts to activate Ca2+ release from the sarcoplasmic reticulum, leading to a rise in [Ca2+]i, which, in turn, causes the inhibition of KV channels and membrane depolarization (31, 72). Although this hypothesis is fairly well supported, it is conceivable that hypoxia acts through multiple pathways and multiple ion channels to produce HPV. ![]() |
KV CHANNEL REGULATION BY CHRONIC HYPOXIA |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Long-term hypoxic exposure leads to decreased PASMC KV
channel current (62, 90, 91) and membrane depolarization
(18, 74, 90, 91). Although acute hypoxia modulates
KV channel current by affecting channel function, chronic
hypoxic exposure is thought to modulate KV channel current
by affecting channel expression. Indeed, Wang et al. (104)
showed that prolonged exposure (24-72 h) of rat primary cultured
PASMCs to hypoxia is associated with a downregulation of Kv1.2 and
Kv1.5 -subunit mRNA and protein. Therefore, they concluded that the
diminished transcription and expression of KV
-subunits
most likely results in fewer functional KV channels, a
decrease in outward KV current, and subsequent membrane depolarization.
Chronic hypoxic exposure is associated with the regulation of a number of genes in a wide variety of cell types. Protein phosphorylation and/or redox modulation of transcription factors are thought to be part of the signaling pathway that translates a hypoxic stimulus into the regulation of gene expression (9). The mechanism by which KV channels are downregulated by chronic hypoxia is unknown but is likely to involve modulation of a hypoxia-inducible transcription factor. For instance, hypoxia-inducible factor-1 has been implicated in the regulation of an ever-growing number of genes, the products of which include endothelin-1, erythropoietin, nitric oxide synthase 2, and vascular endothelial growth factor (87). Additionally, accumulating evidence suggests that the c-fos and c-jun family of genes can also be induced by hypoxia (9). Preliminary data by Sweeney and Yuan (93) suggest that overexpression of c-jun significantly decreases whole cell KV current, making c-jun an attractive candidate for the transcriptional downregulation of KV channel expression during prolonged conditions of hypoxia. Whatever the mechanism for the hypoxic downregulation of KV channel expression, it is likely to be important in the pathogenesis of chronic hypoxic pulmonary hypertension because the consequent reduction in outward K+ current contributes to a sequence of events that ultimately lead to pulmonary hypertension (93).
![]() |
SUMMARY |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Recently, many advances have been made in the study of PASMC
O2 sensing through the use of cloned channels, heterologous
expression systems, and transgenic mice. Through these studies, it has
become evident that KV channels play an important role in
the response of PASMCs to hypoxia. Because the O2
sensitivity of some cloned channels varies with the cell type in which
they are expressed, it not likely that KV -subunits
sense O2 directly; rather, they are most likely inhibited
through some indirect mechanism that involves interaction with an
unidentified O2 sensor and/or
-subunit. In this manner,
it is likely that the varied results represent multiple mechanisms and
multiple effectors through which hypoxia is sensed throughout the body.
Further work is needed to examine this possibility and to elucidate the
many mechanisms by which KV channels "sense" and
respond to acute and chronic hypoxia.
![]() |
ACKNOWLEDGEMENTS |
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We thank Dr. Naoya Sakamoto for technical assistance and Barbara Birks for review of this manuscript.
![]() |
FOOTNOTES |
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This work was supported by National Heart, Lung, and Blood Institute Grant R01-HL-49330 (to M. M. Tamkun).
J. R. Martens was supported by National Institute of Child Health and Human Development Postdoctoral Fellowship 1-F32-HD-08496-01.
Address for reprint requests and other correspondence: M. M. Tamkun, Dept. of Physiology, Colorado State Univ., Fort Collins, CO 80523 (E-mail: tamkunmm{at}lamar.colostate.edu).
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Albarwani, S,
Robertson BE,
Nye PC,
and
Kozlowski RZ.
Biophysical properties of Ca2+- and Mg-ATP-activated K+ channels in pulmonary arterial smooth muscle cells isolated from the rat.
Pflügers Arch
428:
446-454,
1994[ISI][Medline].
2.
Archer, SL,
Huang J,
Henry T,
Peterson D,
and
Weir EK.
A redox-based O2 sensor in rat pulmonary vasculature.
Circ Res
73:
1100-1112,
1993[Abstract].
3.
Archer, SL,
Huang JM,
Reeve HL,
Hampl V,
Tolarova S,
Michelakis E,
and
Weir EK.
Differential distribution of electrophysiologically distinct myocytes in conduit and resistance arteries determines their response to nitric oxide and hypoxia.
Circ Res
78:
431-442,
1996
4.
Archer, SL,
Reeve HL,
Michelakis E,
Puttagunta L,
Waite R,
Nelson DP,
Dinauer MC,
and
Weir EK.
O2 sensing is preserved in mice lacking the gp91 phox subunit of NADPH oxidase.
Proc Natl Acad Sci USA
96:
7944-7949,
1999
5.
Archer, SL,
Souil E,
Dinh-Xuan AT,
Schremmer B,
Mercier JC,
El Yaagoubi A,
Nguyen-Huu L,
Reeve HL,
and
Hampl V.
Molecular identification of the role of voltage-gated K+ channels, Kv1.5 and Kv21, in hypoxic pulmonary vasoconstriction and control of resting membrane potential in rat pulmonary artery myocytes.
J Clin Invest
101:
2319-2330,
1998
6.
Archer, SL,
Weir EK,
Reeve HL,
and
Michelakis E.
Molecular identification of O2 sensors and O2-sensitive potassium channels in the pulmonary circulation.
Adv Exp Med Biol
475:
219-240,
2000[ISI][Medline].
7.
Barman, SA.
Potassium channels modulate hypoxic pulmonary vasoconstriction.
Am J Physiol Lung Cell Mol Physiol
275:
L64-L70,
1998
8.
Buescher, PC,
Pearse DB,
Pillai RP,
Litt MC,
Mitchell MC,
and
Sylvester JT.
Energy state and vasomotor tone in hypoxic pig lungs.
J Appl Physiol
70:
1874-1881,
1991
9.
Bunn, HF,
and
Poyton RO.
Oxygen sensing and molecular adaptation to hypoxia.
Physiol Rev
76:
839-885,
1996
10.
Butler, A,
Wei AG,
Baker K,
and
Salkoff L.
A family of putative potassium channel genes in Drosophila.
Science
243:
943-947,
1989[ISI][Medline].
11.
Chandel, NS,
and
Schumacker PT.
Cellular oxygen sensing by mitochondria: old questions, new insight.
J Appl Physiol
88:
1880-1889,
2000
12.
Chandy, KG,
and
Gutman GA.
Voltage-gated K+ channels.
In: Handbook of Receptors and Channels: Ligand- and Voltage-Gated Ion Channels, edited by North RA.. Boca Raton, FL: CRC, 1995, p. 1-71.
13.
Christie, MJ,
North RA,
Osborne PB,
Douglass J,
and
Adelman JP.
Heteropolymeric potassium channels expressed in Xenopus oocytes from cloned subunits.
Neuron
4:
405-411,
1990[ISI][Medline].
14.
Clapp, LH,
and
Tinker A.
Potassium channels in the vasculature.
Curr Opin Nephrol Hypertens
7:
91-98,
1998[ISI][Medline].
15.
Conforti, L,
Bodi I,
Nisbet JW,
and
Millhorn DE.
O2-sensitive K+ channels: role of the Kv1.2-subunit in mediating the hypoxic response.
J Physiol (Lond)
524:
783-793,
2000
16.
Cornfield, DN,
Stevens T,
McMurtry IF,
Abman SH,
and
Rodman DM.
Acute hypoxia causes membrane depolarization and calcium influx in fetal pulmonary artery smooth muscle cells.
Am J Physiol Lung Cell Mol Physiol
266:
L469-L475,
1994
17.
Deal, KK,
England SK,
and
Tamkun MM.
Molecular physiology of cardiac potassium channels.
Physiol Rev
76:
49-67,
1996
18.
Doggrell, SA,
Wanstall JC,
and
Gambino A.
Functional effects of 4-aminopyridine (4-AP) on pulmonary and systemic vessels from normoxic control and hypoxic pulmonary hypertensive rats.
Naunyn Schmiedebergs Arch Pharmacol
360:
317-323,
1999[ISI][Medline].
19.
Doi, S,
Damron DS,
Ogawa K,
Tanaka S,
Horibe M,
and
Murray PA.
K+ channel inhibition, calcium signaling, and vasomotor tone in canine pulmonary artery smooth muscle.
Am J Physiol Lung Cell Mol Physiol
279:
L242-L251,
2000
20.
Duprat, F,
Guillemare E,
Romey G,
Fink M,
Lesage F,
Lazdunski M,
and
Honore E.
Susceptibility of cloned K+ channels to reactive oxygen species.
Proc Natl Acad Sci USA
92:
11796-11800,
1995[Abstract].
21.
England, SK,
Uebele VN,
Kodali J,
Bennett PB,
and
Tamkun MM.
A novel K+ channel beta-subunit (hKv beta 1.3) is produced via alternative mRNA splicing.
J Biol Chem
270:
28531-28534,
1995
22.
England, SK,
Uebele VN,
Shear H,
Kodali J,
Bennett PB,
and
Tamkun MM.
Characterization of a voltage-gated K+ channel beta subunit expressed in human heart.
Proc Natl Acad Sci USA
92:
6309-6313,
1995[Abstract].
23.
Evans, AM,
Osipenko ON,
and
Gurney AM.
Properties of a novel K+ current that is active at resting potential in rabbit pulmonary artery smooth muscle cells.
J Physiol (Lond)
496:
407-420,
1996[Abstract].
24.
Evans, AM,
Osipenko ON,
Haworth SG,
and
Gurney AM.
Resting potentials and potassium currents during development of pulmonary artery smooth muscle cells.
Am J Physiol Heart Circ Physiol
275:
H887-H899,
1998
25.
Fearon, IM,
Varadi G,
Koch S,
Isaacsohn I,
Ball SG,
and
Peers C.
Splice variants reveal the region involved in oxygen sensing by recombinant human L-type Ca2+ channels.
Circ Res
87:
537-539,
2000
26.
Fink, M,
Duprat F,
Lesage F,
Heurteaux C,
Romey G,
Barhanin J,
and
Lazdunski M.
A new K+ channel beta subunit to specifically enhance Kv2.2 (CDRK) expression.
J Biol Chem
271:
26341-26348,
1996
27.
Franco-Obregon, A,
and
Lopez-Barneo J.
Differential oxygen sensitivity of calcium channels in rabbit smooth muscle cells of conduit and resistance pulmonary arteries.
J Physiol (Lond)
491:
511-518,
1996[Abstract]. [Corrigenda. J Physiol (Lond) 493: June 1996, p. 923.]
28.
Franco-Obregon, A,
Montoro R,
Urena J,
and
Lopez-Barneo J.
Modulation of voltage-gated Ca2+ channels by O2 tension Significance for arterial oxygen chemoreception.
Adv Exp Med Biol
410:
97-103,
1996[Medline].
29.
Fu, XW,
Wang D,
Nurse CA,
Dinauer MC,
and
Cutz E.
NADPH oxidase is an O2 sensor in airway chemoreceptors: evidence from K+ current modulation in wild-type and oxidase-deficient mice.
Proc Natl Acad Sci USA
97:
4374-4379,
2000
30.
Ganfornina, MD,
and
Lopez-Barneo J.
Potassium channel types in arterial chemoreceptor cells and their selective modulation by oxygen.
J Gen Physiol
100:
401-426,
1992[Abstract].
31.
Gelband, CH,
and
Gelband H.
Ca2+ release from intracellular stores is an initial step in hypoxic pulmonary vasoconstriction of rat pulmonary artery resistance vessels.
Circulation
96:
3647-3654,
1997
32.
Grimminger, F,
Weissmann N,
Spriestersbach R,
Becker E,
Rosseau S,
and
Seeger W.
Effects of NADPH oxidase inhibitors on hypoxic vasoconstriction in buffer-perfused rabbit lungs.
Am J Physiol Lung Cell Mol Physiol
268:
L747-L752,
1995
33.
Grissmer, S,
Nguyen AN,
Aiyar J,
Hanson DC,
Mather RJ,
Gutman GA,
Karmilowicz MJ,
Auperin DD,
and
Chandy KG.
Pharmacological characterization of five cloned voltage-gated K+ channels, types Kv1.1, 1.2, 1.3, 1.5, and 3.1, stably expressed in mammalian cell lines.
Mol Pharmacol
45:
1227-1234,
1994[Abstract].
34.
Grover, RF.
Pulmonary circulation in animals and man at high altitude.
Ann NY Acad Sci
127:
632-639,
1965[Medline].
35.
Gulbis, JM,
Mann S,
and
MacKinnon R.
Structure of a voltage-dependent K+ channel beta subunit.
Cell
97:
943-952,
1999[ISI][Medline].
36.
Gulbis, JM,
Zhou M,
Mann S,
and
MacKinnon R.
Structure of the cytoplasmic beta subunit-T1 assembly of voltage-dependent K+ channels.
Science
289:
123-127,
2000
37.
Harder, DR,
Madden JA,
and
Dawson C.
Hypoxic induction of Ca2+-dependent action potentials in small pulmonary arteries of the cat.
J Appl Physiol
59:
1389-1393,
1985
38.
Hasunuma, K,
Rodman DM,
and
McMurtry IF.
Effects of K+ channel blockers on vascular tone in the perfused rat lung.
Am Rev Respir Dis
144:
884-887,
1991[ISI][Medline].
39.
Herold, CJ,
Wetzel RC,
Robotham JL,
Herold SM,
and
Zerhouni EA.
Acute effects of increased intravascular volume and hypoxia on the pulmonary circulation: assessment with high-resolution CT.
Radiology
183:
655-662,
1992[Abstract].
40.
Hulme, JT,
Coppock EA,
Felipe A,
Martens JR,
and
Tamkun MM.
Oxygen sensitivity of cloned voltage-gated K+ channels expressed in the pulmonary vasculature.
Circ Res
85:
489-497,
1999
41.
Jabr, RI,
Toland H,
Gelband CH,
Wang XX,
and
Hume JR.
Prominent role of intracellular Ca2+ release in hypoxic vasoconstriction of canine pulmonary artery.
Br J Pharmacol
122:
21-30,
1997[Abstract].
42.
Jackson, WF.
Ion channels and vascular tone.
Hypertension
35:
173-178,
2000
43.
Jan, LY,
and
Jan YN.
Cloned potassium channels from eukaryotes and prokaryotes.
Annu Rev Neurosci
20:
91-123,
1997[ISI][Medline].
44.
Killilea, DW,
Hester R,
Balczon R,
Babal P,
and
Gillespie MN.
Free radical production in hypoxic pulmonary artery smooth muscle cells.
Am J Physiol Lung Cell Mol Physiol
279:
L408-L412,
2000
45.
Kummer, W,
and
Acker H.
Immunohistochemical demonstration of four subunits of neutrophil NAD(P)H oxidase in type I cells of carotid body.
J Appl Physiol
78:
1904-1909,
1995
46.
Lahiri, S,
and
Acker H.
Redox-dependent binding of CO to heme protein controls P(O2)-sensitive chemoreceptor discharge of the rat carotid body.
Respir Physiol
115:
169-177,
1999[ISI][Medline].
47.
Lopez-Barneo, J,
Lopez-Lopez JR,
Urena J,
and
Gonzalez C.
Chemotransduction in the carotid body: K+ current modulated by PO2 in type I chemoreceptor cells.
Science
241:
580-582,
1988[ISI][Medline].
48.
MacKinnon, R.
Determination of the subunit stoichiometry of a voltage-activated potassium channel.
Nature
350:
232-235,
1991[ISI][Medline].
49.
Madden, JA,
Dawson CA,
and
Harder DR.
Hypoxia-induced activation in small isolated pulmonary arteries from the cat.
J Appl Physiol
59:
113-118,
1985
50.
Madden, JA,
Vadula MS,
and
Kurup VP.
Effects of hypoxia and other vasoactive agents on pulmonary and cerebral artery smooth muscle cells.
Am J Physiol Lung Cell Mol Physiol
263:
L384-L393,
1992
51.
Marshall, C,
Mamary AJ,
Verhoeven AJ,
and
Marshall BE.
Pulmonary artery NADPH-oxidase is activated in hypoxic pulmonary vasoconstriction.
Am J Respir Cell Mol Biol
15:
633-644,
1996[Abstract].
52.
McCulloch, KM,
Osipenko ON,
and
Gurney AM.
Oxygen-sensing potassium currents in pulmonary artery.
Gen Pharmacol
32:
403-411,
1999[Medline].
53.
McMurtry, IF.
BAY K 8644 potentiates and A23187 inhibits hypoxic vasoconstriction in rat lungs.
Am J Physiol Heart Circ Physiol
249:
H741-H746,
1985
54.
McMurtry, IF,
Davidson AB,
Reeves JT,
and
Grover RF.
Inhibition of hypoxic pulmonary vasoconstriction by calcium antagonists in isolated rat lungs.
Circ Res
38:
99-104,
1976[Abstract].
55.
Michelakis, ED,
Archer SL,
and
Weir EK.
Acute hypoxic pulmonary vasoconstriction: a model of oxygen sensing.
Physiol Res
44:
361-367,
1995[ISI][Medline].
56.
Morales, MJ,
Castellino RC,
Crews AL,
Rasmusson RL,
and
Strauss HC.
A novel beta subunit increases rate of inactivation of specific voltage-gated potassium channel alpha subunits.
J Biol Chem
270:
6272-6277,
1995
57.
Morales, MJ,
Wee JO,
Wang S,
Strauss HC,
and
Rasmusson RL.
The N-terminal domain of a K+ channel beta subunit increases the rate of C-type inactivation from the cytoplasmic side of the channel.
Proc Natl Acad Sci USA
93:
15119-15123,
1996
58.
Nakahira, K,
Shi G,
Rhodes KJ,
and
Trimmer JS.
Selective interaction of voltage-gated K+ channel beta-subunits with alpha-subunits.
J Biol Chem
271:
7084-7089,
1996
59.
Nakanishi, K,
Tajima F,
Osada H,
Nakamura A,
Yagura S,
Kawai T,
Suzuki M,
and
Torikata C.
Pulmonary, vascular responses in rats exposed to chronic hypobaric hypoxia at two different altitude levels.
Pathol Res Pract
192:
1057-1067,
1996[ISI][Medline].
60.
Nelson, MT,
and
Quayle JM.
Physiological roles and properties of potassium channels in arterial smooth muscle.
Am J Physiol Cell Physiol
268:
C799-C822,
1995
61.
Okabe, K,
Kitamura K,
and
Kuriyama H.
Features of 4-aminopyridine sensitive outward current observed in single smooth muscle cells from the rabbit pulmonary artery.
Pflügers Arch
409:
561-568,
1987[ISI][Medline].
62.
Osipenko, ON,
Alexander D,
MacLean MR,
and
Gurney AM.
Influence of chronic hypoxia on the contributions of non-inactivating and delayed rectifier K+ currents to the resting potential and tone of rat pulmonary artery smooth muscle.
Br J Pharmacol
124:
1335-1337,
1998[Abstract].
63.
Osipenko, ON,
Evans AM,
and
Gurney AM.
Regulation of the resting potential of rabbit pulmonary artery myocytes by a low threshold, O2-sensing potassium current.
Br J Pharmacol
120:
1461-1470,
1997[Abstract].
64.
Osipenko, ON,
Tate RJ,
and
Gurney AM.
Potential role for kv3.1b channels as oxygen sensors.
Circ Res
86:
534-540,
2000
65.
Papazian, DM,
Schwarz TL,
Tempel BL,
Jan YN,
and
Jan LY.
Cloning of genomic and complementary DNA from Shaker, a putative potassium channel gene from Drosophila.
Science
237:
749-753,
1987[ISI][Medline].
66.
Park, MK,
Bae YM,
Lee SH,
Ho WK,
and
Earm YE.
Modulation of voltage-dependent K+ channel by redox potential in pulmonary and ear arterial smooth muscle cells of the rabbit.
Pflügers Arch
434:
764-771,
1997[ISI][Medline].
67.
Park, MK,
Lee SH,
Ho WK,
and
Earm YE.
Redox agents as a link between hypoxia and the responses of ionic channels in rabbit pulmonary vascular smooth muscle.
Exp Physiol
80:
835-842,
1995[Abstract].
68.
Park, MK,
Lee SH,
Lee SJ,
Ho WK,
and
Earm YE.
Different modulation of Ca2+-activated K+ channels by the intracellular redox potential in pulmonary and ear arterial smooth muscle cells of the rabbit.
Pflügers Arch
430:
308-314,
1995[ISI][Medline].
69.
Patel, AJ,
Lazdunski M,
and
Honore E.
Kv2.1/Kv9.3, a novel ATP-dependent delayed-rectifier K+ channel in oxygen-sensitive pulmonary artery myocytes.
EMBO J
16:
6615-6625,
1997
70.
Perez-Garcia, MT,
Lopez-Lopez JR,
and
Gonzalez C.
Kvbeta1.2 subunit coexpression in HEK293 cells confers O2 sensitivity to kv4.2 but not to Shaker channels.
J Gen Physiol
113:
897-907,
1999
71.
Perez-Garcia, MT,
Lopez-Lopez JR,
Riesco AM,
Hoppe UC,
Marban E,
Gonzalez C,
and
Johns DC.
Viral gene transfer of dominant-negative Kv4 construct suppresses an O2-sensitive K+ current in chemoreceptor cells.
J Neurosci
20:
5689-5695,
2000
72.
Post, JM,
Gelband CH,
and
Hume JR.
[Ca2+]i inhibition of K+ channels in canine pulmonary artery. Novel mechanism for hypoxia-induced membrane depolarization.
Circ Res
77:
131-139,
1995
73.
Post, JM,
Hume JR,
Archer SL,
and
Weir EK.
Direct role for potassium channel inhibition in hypoxic pulmonary vasoconstriction.
Am J Physiol Cell Physiol
262:
C882-C890,
1992
74.
Priest, RM,
Robertson TP,
Leach RM,
and
Ward JP.
Membrane potential-dependent and -independent vasodilation in small pulmonary arteries from chronically hypoxic rats.
J Pharmacol Exp Ther
285:
975-982,
1998
75.
Reeve, HL,
Archer SL,
and
Weir EK.
Ion channels in the pulmonary vasculature.
Pulm Pharmacol Ther
10:
243-252,
1997[ISI][Medline].
76.
Reeve, HL,
Weir EK,
Nelson DP,
Peterson DA,
and
Archer SL.
Opposing effects of oxidants and antioxidants on K+ channel activity and tone in rat vascular tissue.
Exp Physiol
80:
825-834,
1995[Abstract].
77.
Rettig, J,
Heinemann SH,
Wunder F,
Lorra C,
Parcej DN,
Dolly JO,
and
Pongs O.
Inactivation properties of voltage-gated K+ channels altered by presence of beta-subunit.
Nature
369:
289-294,
1994[ISI][Medline].
78.
Rounds, S,
and
McMurtry IF.
Inhibitors of oxidative ATP production cause transient vasoconstriction and block subsequent pressor responses in rat lungs.
Circ Res
48:
393-400,
1981[Abstract].
79.
Ruppersberg, JP,
Stocker M,
Pongs O,
Heinemann SH,
Frank R,
and
Koenen M.
Regulation of fast inactivation of cloned mammalian IK(A) channels by cysteine oxidation.
Nature
352:
711-714,
1991[ISI][Medline].
80.
Russell, SN,
Overturf KE,
and
Horowitz B.
Heterotetramer formation and charybdotoxin sensitivity of two K+ channels cloned from smooth muscle.
Am J Physiol Cell Physiol
267:
C1729-C1733,
1994
81.
Salinas, M,
Duprat F,
Heurteaux C,
Hugnot JP,
and
Lazdunski M.
New modulatory alpha subunits for mammalian Shab K+ channels.
J Biol Chem
272:
24371-24379,
1997
82.
Salkoff, L,
Baker K,
Butler A,
Covarrubias M,
Pak MD,
and
Wei A.
An essential `set' of K+ channels conserved in flies, mice and humans.
Trends Neurosci
15:
161-166,
1992[ISI][Medline].
83.
Salvaterra, CG,
and
Goldman WF.
Acute hypoxia increases cytosolic calcium in cultured pulmonary arterial myocytes.
Am J Physiol Lung Cell Mol Physiol
264:
L323-L328,
1993
84.
Sato, K,
Morio Y,
Morris KG,
Rodman DM,
and
McMurtry IF.
Mechanism of hypoxic pulmonary vasoconstriction involves ETA receptor-mediated inhibition of KATP channel.
Am J Physiol Lung Cell Mol Physiol
278:
L434-L442,
2000
85.
Semenza, GL.
Perspectives on oxygen sensing.
Cell
98:
281-284,
1999[ISI][Medline].
86.
Semenza, GL.
Chairman's summary: mechanisms of oxygen homeostasis, circa 1999.
Adv Exp Med Biol
475:
303-310,
2000[ISI][Medline].
87.
Semenza, GL.
HIF-1: mediator of physiological and pathophysiological responses to hypoxia.
J Appl Physiol
88:
1474-1480,
2000
88.
Sham, JS,
Crenshaw BR, Jr,
Deng LH,
Shimoda LA,
and
Sylvester JT.
Effects of hypoxia in porcine pulmonary arterial myocytes: roles of KV channel and endothelin-1.
Am J Physiol Lung Cell Mol Physiol
279:
L262-L272,
2000
89.
Sheng, M,
Liao YJ,
Jan YN,
and
Jan LY.
Presynaptic A-current based on heteromultimeric K+ channels detected in vivo.
Nature
365:
72-75,
1993[ISI][Medline].
90.
Shimoda, LA,
Sylvester JT,
and
Sham JS.
Chronic hypoxia alters effects of endothelin and angiotensin on K+ currents in pulmonary arterial myocytes.
Am J Physiol Lung Cell Mol Physiol
277:
L431-L439,
1999
91.
Smirnov, SV,
Robertson TP,
Ward JP,
and
Aaronson PI.
Chronic hypoxia is associated with reduced delayed rectifier K+ current in rat pulmonary artery muscle cells.
Am J Physiol Heart Circ Physiol
266:
H365-H370,
1994
92.
Standen, NB,
and
Quayle JM.
K+ channel modulation in arterial smooth muscle.
Acta Physiol Scand
164:
549-557,
1998[ISI][Medline].
93.
Sweeney, M,
and
Yuan XJ.
Hypoxic pulmonary vasoconstriction: role of voltage-gated potassium channels.
Respir Res
1:
40-48,
2000[Medline].
94.
Thomas, HM, III,
Carson RC,
Fried ED,
and
Novitch RS.
Inhibition of hypoxic pulmonary vasoconstriction by diphenyleneiodonium.
Biochem Pharmacol
42:
R9-R12,
1991[ISI][Medline].
95.
Thompson, BT,
and
Hales CA.
Hypoxic pulmonary hypertension: acute and chronic.
Heart Lung
15:
457-465,
1986[ISI][Medline].
96.
Thompson, JS,
Jones RD,
Rogers TK,
Hancock J,
and
Morice AH.
Inhibition of hypoxic pulmonary vasoconstriction in isolated rat pulmonary arteries by diphenyleneiodonium (DPI).
Pulm Pharmacol Ther
11:
71-75,
1998[ISI][Medline].
97.
Tolins, M,
Weir EK,
Chesler E,
Nelson DP,
and
From AH.
Pulmonary vascular tone is increased by a voltage-dependent calcium channel potentiator.
J Appl Physiol
60:
942-948,
1986
98.
Turner, JL,
and
Kozlowski RZ.
Relationship between membrane potential, delayed rectifier K+ currents and hypoxia in rat pulmonary arterial myocytes.
Exp Physiol
82:
629-645,
1997[Abstract].
99.
Turrens, JF,
Freeman BA,
and
Crapo JD.
Hyperoxia increases H2O2 release by lung mitochondria and microsomes.
Arch Biochem Biophys
217:
411-421,
1982[ISI][Medline].
100.
Uebele, VN,
England SK,
Chaudhary A,
Tamkun MM,
and
Snyders DJ.
Functional differences in Kv1.5 currents expressed in mammalian cell lines are due to the presence of endogenous Kv beta 2.1 subunits.
J Biol Chem
271:
2406-2412,
1996
101.
Uebele, VN,
England SK,
Gallagher DJ,
Snyders DJ,
Bennett PB,
and
Tamkun MM.
Distinct domains of the voltage-gated K+ channel Kv1.3
-subunit affect voltage-dependent gating.
Am J Physiol Cell Physiol
274:
C1485-C1495,
1998
102.
Voelkel, NF,
Morris KG,
McMurtry IF,
and
Reeves JT.
Calcium augments hypoxic vasoconstriction in lungs from high-altitude rats.
J Appl Physiol
49:
450-455,
1980
103.
Von Euler, US,
and
Liljestrand G.
Observations on the pulmonary arterial blood pressure in the cat.
Acta Physiol Scand
12:
301-320,
1946.
104.
Wang, J,
Juhaszova M,
Rubin LJ,
and
Yuan XJ.
Hypoxia inhibits gene expression of voltage-gated K+ channel alpha subunits in pulmonary artery smooth muscle cells.
J Clin Invest
100:
2347-2353,
1997
105.
Ward, JP,
and
Aaronson PI.
Mechanisms of hypoxic pulmonary vasoconstriction: can anyone be right?
Respir Physiol
115:
261-271,
1999[ISI][Medline].
106.
Ward, JP,
and
Robertson TP.
The role of the endothelium in hypoxic pulmonary vasoconstriction.
Exp Physiol
80:
793-801,
1995[Abstract].
107.
Weir, EK,
and
Archer SL.
The mechanism of acute hypoxic pulmonary vasoconstriction: the tale of two channels.
FASEB J
9:
183-189,
1995
108.
Weir, EK,
Wyatt CN,
Reeve HL,
Huang J,
Archer SL,
and
Peers C.
Diphenyleneiodonium inhibits both potassium and calcium currents in isolated pulmonary artery smooth muscle cells.
J Appl Physiol
76:
2611-2615,
1994
109.
Wiener, CM,
Banta MR,
Dowless MS,
Flavahan NA,
and
Sylvester JT.
Mechanisms of hypoxic vasodilation in ferret pulmonary arteries.
Am J Physiol Lung Cell Mol Physiol
269:
L351-L357,
1995
110.
Youngson, C,
Nurse C,
Yeger H,
and
Cutz E.
Oxygen sensing in airway chemoreceptors.
Nature
365:
153-155,
1993[ISI][Medline].
111.
Yuan, XJ.
Voltage-gated K+ currents regulate resting membrane potential and [Ca2+]i in pulmonary arterial myocytes.
Circ Res
77:
370-378,
1995
112.
Yuan, XJ,
Goldman WF,
Tod ML,
Rubin LJ,
and
Blaustein MP.
Hypoxia reduces potassium currents in cultured rat pulmonary but not mesenteric arterial myocytes.
Am J Physiol Lung Cell Mol Physiol
264:
L116-L123,
1993
113.
Yuan, XJ,
Tod ML,
Rubin LJ,
and
Blaustein MP.
Contrasting effects of hypoxia on tension in rat pulmonary and mesenteric arteries.
Am J Physiol Heart Circ Physiol
259:
H281-H289,
1990
114.
Yuan, XJ,
Tod ML,
Rubin LJ,
and
Blaustein MP.
Deoxyglucose and reduced glutathione mimic effects of hypoxia on K+ and Ca2+ conductances in pulmonary artery cells.
Am J Physiol Lung Cell Mol Physiol
267:
L52-L63,
1994
115.
Yuan, XJ,
Tod ML,
Rubin LJ,
and
Blaustein MP.
Hypoxic and metabolic regulation of voltage-gated K+ channels in rat pulmonary artery smooth muscle cells.
Exp Physiol
80:
803-813,
1995[Abstract].
116.
Yuan, XJ,
Wang J,
Juhaszova M,
Golovina VA,
and
Rubin LJ.
Molecular basis and function of voltage-gated K+ channels in pulmonary arterial smooth muscle cells.
Am J Physiol Lung Cell Mol Physiol
274:
L621-L635,
1998
117.
Zhu, WH,
Conforti L,
Czyzyk-Krzeska MF,
and
Millhorn DE.
Membrane depolarization in PC-12 cells during hypoxia is regulated by an O2-sensitive K+ current.
Am J Physiol Cell Physiol
271:
C658-C665,
1996