Potassium channels modulate hypoxic pulmonary vasoconstriction
Scott A.
Barman
Department of Pharmacology and Toxicology, Medical College of
Georgia, Augusta, Georgia 30912
 |
ABSTRACT |
The role of
Ca2+-activated
K+-channel, ATP-sensitive
K+-channel, and delayed rectifier
K+-channel modulation in the
canine pulmonary vascular response to hypoxia was determined in the
isolated blood-perfused dog lung. Pulmonary vascular resistances
and compliances were measured with vascular occlusion techniques. Under
normoxia, the Ca2+-activated
K+-channel blocker
tetraethylammonium (1 mM), the ATP-sensitive K+-channel inhibitor glibenclamide
(10
5 M), and the delayed
rectifier K+-channel blocker
4-aminopyridine (10
4 M)
elicited a small but significant increase in pulmonary arterial pressure. Hypoxia significantly increased pulmonary arterial and venous
resistances and pulmonary capillary pressure and decreased total
vascular compliance by decreasing both microvascular and large-vessel
compliances. Tetraethylammonium, glibenclamide, and 4-aminopyridine
potentiated the response to hypoxia on the arterial segments but not on
the venous segments and also further decreased pulmonary vascular
compliance. In contrast, the ATP-sensitive K+-channel opener cromakalim and
the L-type voltage-dependent
Ca2+-channel blocker verapamil
(10
5 M) inhibited the
vasoconstrictor effect of hypoxia on both the arterial and venous
vessels. These results indicate that closure of the
Ca2+-activated
K+ channels, ATP-sensitive
K+ channels, and delayed rectifier
K+ channels potentiate the canine
pulmonary arterial response under hypoxic conditions and that L-type
voltage-dependent Ca2+ channels
modulate hypoxic vasoconstriction. Therefore, the possibility exists
that K+-channel inhibition is a
key event that links hypoxia to pulmonary vasoconstriction by eliciting
membrane depolarization and subsequent Ca2+-channel activation, leading
to Ca2+ influx.
hypoxia; pulmonary vascular resistance; pulmonary vascular
compliance; verapamil; tetraethylammonium; cromakalim; glibenclamide
 |
INTRODUCTION |
THE MECHANISM of hypoxic pulmonary vasoconstriction
(HPV) first observed by Euler and Liljestrand (18) remains unclear. Recent evidence (24, 34, 41) strongly suggests that HPV is
elicited through direct modulation of
K+ channels, leading to membrane
depolarization. K+ channels have
been identified in pulmonary vascular smooth muscle cells (13, 30, 37)
and are reported to be involved in the regulation of vascular tone (15,
21). Activation of these channels causes an increase in
K+ efflux, membrane
hyperpolarization, inhibition of
Ca2+ influx, and subsequent
vascular smooth muscle relaxation. Recently, K+-channel inhibition has been
implicated as a critical event in the initiation of HPV (33, 34, 41).
Studies (34, 41) provided evidence that hypoxia inhibits whole cell
macroscopic K+ currents that are
sensitive to 4-aminopyridine (4-AP), causing depolarization of the
resting membrane potential in isolated pulmonary vascular smooth muscle
cells. Post et al. (33) reported that hypoxia inhibited the delayed
rectifier K+ channel, and Yuan et
al. (41) found that the ATP-sensitive K+-channel opener cromaklim
inhibited hypoxic vasoconstriction in pulmonary arterial rings, an
effect reversed by the ATP-sensitive K+-channel blocker glibenclamide
(Glib). Wiener et al. (40) reported that ATP-sensitive
K+ channels modulated pulmonary
vasoconstriction to hypoxia in isolated ferret lungs, and Cornfield et
al. (15) concluded that ATP-dependent K+ channels are present but not
operative in the fetal pulmonary circulation, which is a low oxygen
tension environment. Post et al. (34) observed that hypoxia inhibited
the Ca2+-activated
K+ channel in canine pulmonary
vascular smooth muscle and suggested that
K+-channel inhibition is a key
event linking hypoxia to pulmonary vasoconstriction by causing membrane
depolarization and subsequent Ca2+
entry. In addition, Albarwani and Nye (2) suggested that hypoxia inhibited both Ca2+-activated and
ATP-sensitive K+ channels.
Although it has been established that hypoxia induces a rise in
pulmonary vascular resistance in hypoxic regions of the lung, controversy exists regarding which vascular segments constrict to
hypoxia. Hakim (19) reported that hypoxia constricted primarily the
arterial segments, with a lesser effect on the venous segments, whereas
Hillier et al. (22) observed that hypoxic venoconstriction occurred in
the small postcapillary venules. Shirai et al. (36) concluded that
alveolar hypoxia induced vasoconstriction in small pulmonary arteries
and veins, with the larger effect occurring in the arteries, and
Nagasaka et al. (27) suggested that hypoxia elicited constriction in
small-artery segments. The importance in the identification of the
specific vascular segments responding to hypoxia relates to
1) the effect on pulmonary capillary
pressure (Ppc), which is
determined by the distribution of vascular resistance in the pulmonary
arteries and veins and 2) the
maintenance of ventilation-perfusion matching by constriction of
pulmonary arteries that direct blood flow away from hypoxic regions
(22).
In light of these previous investigations that appear to establish a
relationship between K+ channels
and HPV, the present study was done to determine the role of
K+-channel modulation on the
effect of hypoxia on pulmonary vascular resistance and compliance in
isolated blood-perfused dog lungs. The vascular occlusion technique was
used to partition the pulmonary circulation into segmental resistances
and compliances. Measurements generated by these occlusion techniques
are based on the assumption that the pulmonary circulation is
represented by a resistance-compliance circuit. In the present study,
the compartmental model of pulmonary vascular resistance and compliance
by Audi et al. (5) was used to determine the effect of hypoxia on
segmental vascular resistance and compliance in the canine pulmonary
circulation. In this particular model, which is relatively simple and
more robust than other models (5), the pulmonary circulation is
represented as a 3C2R circuit composed of precapillary
(R1) and
postcapillary
(R2)
resistances and arterial (C1),
middle-compartment (C2), and
venous (C3) compliances. This
model is based on the assumption that after arterial oclusion (AO) and
before the arterial pressure curve
[Pa(t)]
falls below the equilibrium pressure obtained by simultaneous occlusion
of both the arterial inflow and venous outflow cannulas
[double-occlusion pressure
(Pdo)], the
Pa(t)
curve is determined primarily by the product of
C1 and
R1 (5). With the
use of this assumption, equations were derived from the data obtained
from all three occlusions (AO, venous occlusion, and double occlusion)
that were used to calculate the pulmonary vascular resistances and
compliances represented in the model (5). These occlusion techniques
have previously been used to measure the pulmonary vascular
resistance-compliance profile in normal lungs and in lungs challenged
with hypoxia (25). Specifically, the role of ATP-sensitive
K+ channels was investigated with
cromakalim (Crom; an activator of ATP-sensitive
K+ channels) and Glib
(a blocker of ATP-sensitive K+
channels) to determine if ATP-sensitive
K+ channels modulate the
vasoconstrictor response to hypoxia. In addition, the
Ca2+-activated
K+-channel blocker
tetraethylammonium (TEA) and the delayed rectifier K+-channel inhibitor 4-AP were
used to determine whether these specific K+ channels also potentiate the
pulmonary vascular response to hypoxia.
 |
METHODS |
Adult, heartworm-negative mongrel dogs of either sex (15-19 kg)
were anesthetized with pentobarbital sodium (30 mg/kg), intubated, and
ventilated with a Harvard respirator with room air at a tidal volume of
15 ml/kg. A left thoracotomy was performed through the fifth
intercostal space. The left upper and middle lobes of the lung were
removed, and the lower left lobe was prepared for isolation by placing
loose ligatures around the left main pulmonary artery and lower left
bronchus. Each animal was then heparinized (10,000 U iv) and after
5-10 min was rapidly bled through a carotid arterial cannula.
Three hundred milliliters of blood were used to prime the perfusion
apparatus. After bleeding was completed, the pulmonary artery was
ligated, and with the heart still beating, the lower left lobe with the
attached left atrial appendage was rapidly excised and weighed. Plastic
cannulas were secured in the lobar artery, lobar vein, and bronchus,
and blood perfusion was started within 30 min of lung excision.
The isolated lung circuit has been previously described in detail
(7-9, 17, 31) and is well established in this laboratory. Briefly,
the lung was perfused at a constant flow by a roller pump (Master Flex,
Cole-Parmer) that pumped blood from a venous reservoir through a
heating coil encased in a water jacket (37.5 ± 0.5°C) to the
rest of the closed circuit. The blood was continuously bubbled with a
gas mixture of 95% O2-5%
CO2 to maintain blood gases in a
normal range (arterial PO2 = 100-110 Torr and arterial PCO2 = 30-40 Torr) as well as maintain a normal pH. After initial
hyperinflation, airway pressure
(Paw) was set at 3 cmH2O.
The perfused lobe was placed on a weighing pan that was counterbalanced
by a strain-gauge transducer (Grass FT-10). Pulmonary arterial
(Ppa) and venous
(Ppv) pressures were measured by
inserting catheters into the lobar artery and vein and connecting them
to pressure transducers (Statham 23BC) positioned at the openings of
the inflow and outflow cannulas. Pressures were zeroed at the level of
the lung hilus. Blood flow (Q) was measured by an electromagnetic flow
probe (Carolina Medical SF 300A) positioned in the venous outflow line
that was connected to a digital flowmeter (Carolina Medical 701D).
Ppa,
Ppv, and lung weight were recorded
on a Grass polygraph (model 7F).
Ppa and
Ppv were initially adjusted so
that the lung lobe became isogravimetric, i.e., neither gaining nor losing weight in zone III conditions
(Ppa > Ppv > Paw).
Ppc.
Ppc was determined with the
double-occlusion technique (39). When both arterial and venous cannulas
are simultaneously occluded, Ppa
and Ppv quickly equilibrate to the
same pressure (Ppc). If Ppa and
Ppv did not equilibrate exactly to
the same pressure on double occlusion, then the mean of both pressures
was determined and defined as Ppc.
The occlusion pressures were consistently within 1 cmH2O of each other, and it has
been shown that Pdo is an
excellent estimate of Ppc (39).
Pulmonary vascular resistance. Total
pulmonary vascular resistance
(RT) was
calculated by dividing the measured hydrostatic pressure difference
across the isolated lung by the existing Q
|
(1)
|
The
pulmonary circulation can be represented by a simple linear model
whereby Ppa is separated from
Ppc by
R1 and
Ppc is separated from
Ppv by
R2.
R1 and
R2 were
calculated with the following equations
|
(2)
|
|
(3)
|
All
pulmonary vascular resistances are reported in units of centimeters of
water per liter per minute per 100 g.
Determination of segmental vascular
compliance. Total pulmonary vascular compliance
(CT) was calculated with
Eq. 4 and the slope of the venous
pressure-time transient (
P/
t)
measured after venous occlusion with the existing Q at the time of
occlusion
|
(4)
|
C2
was calculated with the equation derived by Audi et al. (5)
|
(5)
|
C1
was determined with the following equation (2)
|
(6)
|
where
A2 is the area
bounded by
Pa(t)
after AO and Pdo was calculated by
numerical integration. C3 was then
calculated with the following relationship using
CT,
C1, and
C2 obtained from Eqs.
4-6
|
(7)
|
Experimental protocols.
Initially, for all isolated lobes,
Ppv was set at 4-5
cmH2O to provide zone III blood
flow conditions. Ppa was adjusted
(ranging from 15 to 20 cmH2O)
until the lower left lobe attained an isogravimetric state. Blood flow
through the lobe was between 500 and 800 ml · min
1 · 100 g wet wt
1, and during the
control period, the lung was allowed to stabilize for ~30 min. In all
experiments, 10
5 M
indomethacin (cyclooxygenase inhibitor; Sigma) was added during the
stabilization period to prevent the production of vasodilatory prostanoids. A previous study (23) showed that cyclooxygenase inhibition induces a hypoxic pulmonary pressor response in the dog.
After this stabilization period, all vascular occlusions were done and
repeated at least three times to obtain average control values. After
control measurements were done, the lobes were divided into seven
treatment groups.
Group 1 was hypoxic control lungs
(n = 6) that consisted of isolated
lung lobes ventilated with 95%
N2-5%
CO2 for 30 min to achieve a
PO2 < 50 mmHg (23). To study the
effect that Ca2+-activated
K+-channel modulation has on the
vasoactive response to hypoxia, a set of lobes (group
2; n = 5) was
pretreated with 1 mM TEA (Sigma), which is in the concentration range
selective for blockade of Ca2+-activated
K+ channels (10, 11, 29), for 30 min before the onset of hypoxia. For group
3 (n = 5 lungs),
10
5 M Glib (Sigma) was used
as a pretreatment for 15 min before hypoxia was induced to evaluate the
importance of ATP-sensitive K+
channels on the vasoactive response under hypoxic conditions. In
group 4, the lungs
(n = 5) were pretreated with 1 mM TEA
and 10
5 M Glib for 30 min
before the lobes were made hypoxic. In group 5, the lobes (n = 5)
were pretreated with 10
5 M
of the ATP-sensitive K+-channel
opener Crom for 15 min before hypoxia was induced. In group 6, the lobes
(n = 5) were pretreated with
10
4 M 4-AP (Sigma), an
inhibitor of delayed rectifier K+
channels for 15 min before hypoxia was induced. For
group 7, the lobes
(n = 5) were pretreated with the
voltage-dependent Ca2+-channel
blocker verapamil (10
5 M;
Sigma) for 15 min before hypoxic stimulation to determine whether the
hypoxic pressor response was dependent on activation of these specific
Ca2+ channels (34). All drugs were
given as a bolus into the venous reservoir, and all drug concentrations
were calculated on the basis of the final volume of the perfusion
system after the drug(s) was to be given.
Statistical analysis. All values are
expressed as means ± SE. Significance was determined with an
analysis of variance for within-group and between-group comparisons. If
a significant F ratio was found, then
specific statistical comparisons were made with the Bonferroni-Dunn
post hoc test. Statistical significance was accepted when
P < 0.05.
 |
RESULTS |
Figure 1 shows the effect of
Ca2+-activated
K+-channel, ATP-sensitive
K+-channel, and delayed rectifier
K+-channel modulation on the
Ppa response. Under normoxia
(control), TEA, Glib, and 4-AP elicited a small but significant
increase in Ppa. As shown in Fig.
2, hypoxia elicited a significant increase in Ppa, an effect potentiated when
the Ca2+-activated
K+ channels were blocked by TEA,
the ATP-sensitive K+ channels were
blocked by Glib, and the delayed rectifier
K+ channels were inhibited by
4-AP. In contrast, opening the ATP-sensitive K+ channels with Crom inhibited
the pressor response to hypoxia.

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Fig. 1.
Effect of tetraethylammonium (TEA), glibenclamide (Glib), and
4-aminopyridine (4-AP) on pulmonary arterial pressure response ( )
under normoxia. Values are means ± SE;
n = 5 lungs/group.
* Significantly different from zero,
P < 0.05.
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Fig. 2.
Effect of TEA, Glib, 4-AP, and cromakalim (Crom) on pulmonary arterial
pressure response to hypoxia (HYPOX). Values are means ± SE;
n = 5 lungs for all groups except
n = 6 lungs for HYPOX group.
Significantly different (P < 0.05)
from: * zero; ** all other treatment groups.
|
|
Figures 3 and 4 show the
effects of the treatment groups on pulmonary arterial and venous
resistances, respectively. Figure 3 shows that hypoxia significantly
increased pulmonary arterial resistance, an effect potentiated by TEA,
Glib, 4-AP, and TEA+Glib. In contrast, Crom inhibited the hypoxic
increase in pulmonary arterial resistance during hypoxia. The effect of
hypoxia on pulmonary venous resistance is shown in Fig. 4. Hypoxia
significantly increased postcapillary resistance, which, in contrast to
precapillary resistance, was not potentiated by either TEA, Glib, 4-AP,
or TEA+Glib. However, pretreatment with Crom did block the initial
pulmonary venoconstrictor response to hypoxia.

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Fig. 3.
Effect of TEA, Glib, and Crom on pulmonary arterial resistance response
to hypoxia. Control groups represent no treatment under normoxic
conditions. Values are means ± SE;
n = 5 lungs for all groups except
n = 6 lungs for HYPOX group.
Significantly different (P < 0.05)
from: * respective control group;
+ HYPOX group; ** all
other treatment groups.
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Fig. 4.
Effect of TEA, Glib, 4-AP, and Crom on pulmonary venous resistance
response to hypoxia. Control groups represent no treatment under
normoxic conditions. Values are means ± SE;
n = 5 lungs for all groups except
n = 6 lungs for HYPOX group.
* Significantly different from respective control group,
P < 0.05.
|
|
The effect of hypoxia on Ppc,
which is determined by the distribution of precapillary and
postcapillary resistance, is presented in Fig.
5. Hypoxia significantly increased
Ppc, a phenomenon that was blocked
by Crom. Closing of the
Ca2+-activated
K+ channels with TEA, blocking the
ATP-sensitive K+ channels with
Glib, using TEA+Glib, or inhibiting the delayed rectifier
K+ current with 4-AP did not
significantly increase Ppc
relative to the initial increase in
Ppc observed during hypoxia. With
respect to voltage-dependent
Ca2+-channel function, Fig.
6 shows that verapamil inhibited the
hypoxic pressor response on both the arterial and venous segments. In addition, a small number of experiments showed that verapamil also
inhibited the potentiated hypoxic response to
K+-channel inhibition by all three
K+-channel blockers used in this
study (data not shown). These results indicate that blockade of L-type
voltage-dependent Ca2+ channels
modulates hypoxic vasoconstriction.

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Fig. 5.
Effect of TEA, Glib, 4-AP, and Crom on pulmonary capillary pressure
response to hypoxia. Control groups represent no treatment under
normoxic conditions. Values are means ± SE;
n = 5 lungs for all groups except
n = 6 lungs for HYPOX group.
* Significantly different from respective control group,
P < 0.05.
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Fig. 6.
Effect of verapamil on pulmonary arterial and venous resistances during
hypoxia. Values are means ± SE; n = 5 lungs.
|
|
Table 1 summarizes the effect of hypoxia on
pulmonary segmental vascular compliance. Hypoxia significantly
decreased total vascular compliance by lowering both middle-compartment
and large (arterial and venous)-vessel compliances, effects potentiated by TEA, Glib, or 4-AP but subsequently reversed by Crom. Specifically, TEA, Glib, or 4-AP significantly potentiated the decrease in
middle-compartment compliance, and TEA and Glib together potentiated
the effect on the middle-compartment, arterial, and venous compliances.
In contrast, Crom or verapamil inhibited the effect of hypoxia on
arterial, venous, and middle-compartment compliances.
 |
DISCUSSION |
In this study, hypoxia increased the precapillary and postcapillary
resistances and Ppc and decreased
the microvascular and large-vessel compliances. Audi et al. (6)
observed that hypoxic dog lungs elicited vasoconstriction in both
precapillary and postcapillary pulmonary vessels, with preferential
vasoconstriction in the arterial segments. In addition, studies using
direct micropuncture measurements in isolated cat lungs suggested that
hypoxia constricted small-artery vessels (27), and it has been proposed
that the small distensible vessels in the capillary bed region
constrict to hypoxia (20). Linehan and Dawson (25) observed that
hypoxia decreased total vascular compliance, and Hofman and Ehrhart
(23) reported that pulmonary vascular compliance under hypoxic
conditions was further decreased in the presence of cyclooxygenase
inhibition. In the present study, the decrease in vascular
compliance that occurred when vascular pressure was increased reflected
the relative indistensibility of the pulmonary vasculature on hypoxic
stimulation. In addition, there was a tendency for large-vessel
compliance (C1 + C3) to comprise a smaller
percentage of total vascular compliance when vascular pressures were
increased by hypoxia compared with control conditions (17 vs. 21%).
It appears that Ca2+-activated
K+ channels, ATP-sensitive
K+ channels, and delayed rectifier
K+ channels play a role in
maintaining normal pulmonary vascular tone. TEA, Glib, and 4-AP caused
a small but significant increase in pulmonary vascular pressure under
normoxic conditions, which was potentiated during hypoxia. Specifially,
TEA, Glib, and 4-AP potentiated the response to hypoxia on the arterial
but not on the venous segments and also further decreased pulmonary
vascular compliance. In contrast, Crom inhibited the vasoconstrictor
effect of hypoxia on both the arterial and venous vessels during
hypoxia. The increase in precapillary resistance but not in
postcapillary resistance on
K+-channel blockade had the net
effect of no further increase in Ppc, which is directly correlated
to the distribution of pulmonary vascular resistance (1, 14, 16).
Recently, K+-channel inhibition
has been implicated as a critical event in the initiation of HPV (33,
34, 41). Post et al. (33) reported that hypoxia inhibited the delayed
rectifier K+ current, and Yuan et
al. (41) found that the ATP-sensitive K+-channel opener Crom inhibited
hypoxic vasoconstriction in pulmonary arterial rings, an effect
reversed by the ATP-sensitive
K+-channel blocker Glib. Wiener et
al. (40) reported that ATP-sensitive K+ channels modulated pulmonary
vasoconstriction to hypoxia in isolated ferret lungs. Post et al. (34)
observed that hypoxia inhibited a
Ca2+-dependent
K+ channel in canine pulmonary
vascular smooth muscle and suggested that
K+-channel inhibition is a key
event that associates hypoxia with pulmonary vasoconstriction by
causing membrane depolarization and subsequent
Ca2+ entry. In addition, Albarwani
and Nye (2) suggested that hypoxia inhibited both
Ca2+-activated and ATP-sensitive
K+ channels.
Although not tested in this study,
K+-channel activation by
vasodilators that elevate cGMP or cAMP may also attenuate the pressor response to hypoxia. Cabell et al. (11) suggested that
large-conductance Ca2+-activated
K+ channels were important in the
cAMP-mediated but not in the cGMP-mediated relaxation of coronary
resistance arteries. In other species, it has been shown that the
activity of high-conductance
Ca2+-activated
K+ channels is potentiated by the
cGMP-dependent activation of protein kinase G (35), and
Archer et al. (3) provided compelling evidence for the opening of
Ca2+-activated
K+ channels by increased cGMP
levels in rat pulmonary vascular smooth muscle cells. In their
experimental model, Archer et al. suggested that pulmonary vascular
signal transduction is modulated by both cGMP and cAMP, which regulate
K+-channel phosphorylation.
Specifically, increased levels of these cyclic nucleotide second
messengers promote the opening of
Ca2+-activated
K+ channels, which leads to
membrane hyperpolarization, inhibition of voltage-gated
Ca2+ channels, and subsequent
vasorelaxation or an attenuation of vasoconstriction. Torphy (38), in a
recent review, documented that the increases in cAMP and cGMP can
simultaneously activate the protein kinase A and protein kinase G
pathways to promote the opening of
Ca2+-activated
K+ channels and elicit membrane
hyperpolarization. Therefore, vasodilatory mechanisms involving the
stimulation of cAMP or cGMP may act to attenuate HPV.
Evidence indicates that hypoxia blocks voltage-gated
K+ channels in pulmonary vascular
smooth muscle cells (41), and hypoxia-induced membrane depolarization
has been associated with the inhibition of whole cell
K+ current, leading to an increase
in pulmonary arterial tension (32). In contrast, ATP-sensitive
K+-channel openers such as
lemakalim, pinacidil, and minoxidil appear to attenuate hypoxia-induced
effects through membrane hyperpolarization (12, 28). Yuan et al. (41)
demonstrated that Crom inhibited hypoxia-induced contractions in
isolated rat pulmonary arteries, which was antagonized by Glib. In
addition, Post et al. (34) provided strong evidence for a direct role
of Ca2+-activated
K+-channel inhibition in hypoxic
pulmonary vasoactivity.
Studies (33, 34) suggested that L-type voltage-dependent
Ca2+ channels also modulate the
potentiated hypoxic pressor response by
K+-channel blockade. Post et al.
(34) observed that the dihydropyridine Ca2+-channel blocker nisoldipine
prevented hypoxic inhibition of K+
currents in pulmonary arterial smooth muscle cells. In addition, it has
been shown that a decrease in oxygen from a normoxic to a hypoxic level
causes depolarization of the resting membrane potential in pulmonary
vascular smooth muscle and subsequent
Ca2+ entry through voltage-gated
Ca2+ channels (4, 26). In the
present study, the L-type voltage-dependent Ca2+-channel blocker verapamil
inhibited the hypoxic pressor response. In addition, a small number of
experiments showed that verapamil also inhibited the potentiated
hypoxic response to K+-channel
inhibition by all three K+-channel
blockers used in this study (data not shown).
In the pulmonary circulation, vascular smooth muscle tone is an
important determinant of pulmonary vascular resistance, pulmonary vascular compliance, and lung blood flow. The membrane potential of
vascular smooth muscle is regulated by
K+ channels, which, in turn,
modulate vascular smooth muscle tone and vasoconstriction. The
importance in the identification of the specific vascular segments
responding to hypoxia and the K+
channels that modulate the pressor response relates to the effect on
Ppc, which is determined by the
distribution of vascular resistance in the pulmonary arteries and veins
and the maintenance of ventilation-perfusion matching by constriction
of pulmonary arteries that direct blood flow away from hypoxic
regions (22).
The results of this study show that hypoxia increased pulmonary
arterial resistance, pulmonary venous resistance, and
Ppc and decreased pulmonary
vascular compliance. TEA, Glib, and 4-AP potentiated the response to
hypoxia on the arterial but not on the venous segments and also further
decreased pulmonary vascular compliance. In contrast, the ATP-sensitive
K+-channel opener Crom and the
L-type voltage-dependent
Ca2+-channel blocker verapamil
inhibited the vasoconstrictor effect of hypoxia on both the arterial
and venous vessels. These results indicate that closure of the
Ca2+-activated
K+ channels, ATP-sensitive
K+ channels, and delayed rectifier
K+ channels potentiate the canine
pulmonary arterial response under hypoxic conditions and that
K+-channel inhibition may be a key
event that links hypoxia to pulmonary vasoconstriction by eliciting
membrane depolarization and subsequent Ca2+ influx through
voltage-dependent Ca2+ channels.
 |
ACKNOWLEDGEMENTS |
I thank Louise Meadows for excellent technical assistance.
 |
FOOTNOTES |
This work was supported by the American Heart Association
Georgia
Affiliate.
Address reprint requests to S. A. Barman.
Received 8 September 1997; accepted in final form 1 April 1998.
 |
REFERENCES |
1.
Agostoni, E.,
and
J. Piiper.
Capillary pressure and distribution of vascular resistance in isolated lung.
Am. J. Physiol.
202:
1033-1036,
1962.
2.
Albarwani, S.,
and
P. Nye.
An ATP-activated potassium channel in smooth muscle cells from the pulmonary artery.
In: Ion Flux Pulmonary Vascular Control. New York: Plenum, 1993, p. 149-157.
3.
Archer, S. L.,
J. M. C. Huang,
V. Hampl,
D. P. Nelson,
P. J. Schultz,
and
E. K. Weir.
Nitric oxide and cGMP cause vasorelaxation by activation of a charybdotoxin-sensitive K channel by cGMP-dependent protein kinase.
Proc. Natl. Acad. Sci. USA
91:
7583-7587,
1994[Abstract].
4.
Archer, S. L.,
R. D. Yankovich,
E. Chesler,
and
E. K. Weir.
Comparative effects of nisoldipine, nifedipine, and bepridil on experimental pulmonary hypertension.
J. Pharmacol. Exp. Ther.
233:
12-17,
1985[Abstract].
5.
Audi, S. H.,
C. A. Dawson,
and
J. H. Linehan.
A method for analysis of pulmonary arterial and venous occlusion data.
J. Appl. Physiol.
73:
1190-1195,
1992[Abstract/Free Full Text].
6.
Audi, S. H.,
C. A. Dawson,
D. A. Rickaby,
and
J. H. Linehan.
Localization of the sites of pulmonary vasomotion by use of arterial and venous occlusion.
J. Appl. Physiol.
70:
2126-2136,
1991[Abstract/Free Full Text].
7.
Barman, S. A.
Pulmonary vasoreactivity to endothelin-1 at elevated vascular tone is modulated by potassium channels.
J. Appl. Physiol.
80:
91-98,
1996[Abstract/Free Full Text].
8.
Barman, S. A.
Pulmonary vasoreactivity to serotonin during hypoxia is modulated by ATP-sensitive potassium channels.
J. Appl. Physiol.
83:
569-574,
1997[Abstract/Free Full Text].
9.
Barman, S. A.,
J. R. Pauly,
and
C. M. Isales.
Canine pulmonary vasoreactivity to serotonin: role of protein kinase C and tyrosine kinase.
Am. J. Physiol.
272 (Heart Circ. Physiol. 41):
H740-H747,
1997[Abstract/Free Full Text].
10.
Blatz, A. L.,
and
K. L. Magleby.
Calcium-activated potassium channels.
Trends Neurosci.
10:
463-467,
1987.
11.
Cabell, F.,
D. S. Weiss,
and
J. M. Price.
Inhibition of adenosine-induced coronary vasodilation by block of large-conductance Ca2+-activated K+ channels.
Am. J. Physiol.
267 (Heart Circ. Physiol. 36):
H1455-H1460,
1994[Abstract/Free Full Text].
12.
Clapp, L. H.,
R. Davey,
and
A. M. Gurney.
ATP-sensitive K+ channels mediate vasodilation produced by lemakalim in rabbit pulmonary artery.
Am. J. Physiol.
264 (Heart Circ. Physiol. 33):
H1907-H1915,
1993[Abstract/Free Full Text].
13.
Cook, N. S.
The pharmacology of potassium channels and their therapeutic potential.
Trends Pharmacol. Sci.
9:
21-28,
1988[Medline].
14.
Cope, D. K.,
R. C. Allison,
J. L. Parmentier,
J. N. Miller,
and
A. E. Taylor.
Measurement of effective pulmonary capillary pressure using the pressure profile after pulmonary artery occlusion.
Crit. Care Med.
14:
16-21,
1986[Medline].
15.
Cornfield, D. N.,
J. A. McQueston,
I. F. McMurtry,
D. M. Rodman,
and
S. H. Abman.
Role of ATP-sensitive potassium channels in ovine pulmonary vascular tone.
Am. J. Physiol.
263 (Heart Circ. Physiol. 32):
H1363-H1368,
1992[Abstract/Free Full Text].
16.
Dawson, C. A.,
T. A. Bronikowski,
J. H. Linehan,
and
D. A. Rickaby.
Distributions of vascular pressure and resistance in the lung.
J. Appl. Physiol.
64:
274-284,
1988[Abstract/Free Full Text].
17.
Drake, R. E.,
K. A. Gaar,
and
A. E. Taylor.
Estimation of the filtration coefficient of pulmonary exchange vessels.
Am. J. Physiol.
234 (Heart Circ. Physiol. 3):
H266-H274,
1978[Abstract/Free Full Text].
18.
Euler, U. S. von,
and
G. Liljestrand.
Observations on the pulmonary arterial blood pressure in the cat.
Acta Physiol. Scand.
12:
301-320,
1946.
19.
Hakim, T. S.
Identification of constriction in large versus small vessels using the arterial-venous and double occlusion techniques in isolated canine lungs.
Respiration
54:
61-69,
1988[Medline].
20.
Hakim, T. S.,
C. A. Dawson,
and
J. H. Linehan.
Hemodynamic responses of dog lung lobe to lobar venous occlusion.
J. Appl. Physiol.
47:
145-152,
1979[Abstract/Free Full Text].
21.
Hasunuma, K.,
D. M. Rodman,
and
I. F. McMurtry.
Effects of K+ channel blockers on vascular tone in the perfused rat lung.
Am. Rev. Respir. Dis.
144:
884-887,
1991[Medline].
22.
Hillier, S. C.,
J. A. Graham,
C. H. Hanger,
P. S. Godbey,
R. W. Glenny,
and
W. W. Wagner, Jr.
Hypoxic vasoconstriction in pulmonary arterioles and venules.
J. Appl. Physiol.
82:
1084-1090,
1997[Abstract/Free Full Text].
23.
Hofman, W. F.,
and
I. C. Ehrhart.
Pulmonary vascular reactivity and permeability to alveolar hypoxia in the dog.
J. Appl. Physiol.
69:
1828-1835,
1990[Abstract/Free Full Text].
24.
Hoshino, Y.,
H. Obara,
M. Kusunoki,
Y. Fujii,
and
S. Iwai.
Hypoxic contractile response in isolated human pulmonary artery: role of calcium ion.
J. Appl. Physiol.
65:
2468-2474,
1988[Abstract/Free Full Text].
25.
Linehan, J. H,
and
C. A. Dawson.
A three-compartment model of the pulmonary vasculature: effects of vasoconstriction.
J. Appl. Physiol.
55:
923-928,
1983[Abstract/Free Full Text].
26.
McMurtry, I. F.,
A. B. Davidson,
J. T. Reeves,
and
R. F. Grover.
Inhibition of hypoxic pulmonary vasoconstriction by calcium antagonists in isolated rat lungs.
Circ. Res.
38:
99-104,
1976[Abstract].
27.
Nagasaka, Y.,
J. Bhattacharya,
S. Nanjo,
and
M. A. Gropper.
Micropuncture measurement of lung microvascular pressure profile during hypoxia in cats.
Circ. Res.
54:
90-95,
1984[Abstract].
28.
Nelson, M. T.,
J. B. Patlak,
J. F. Worley,
and
N. B. Standen.
Calcium channels, potassium channels, and voltage dependence of arterial smooth muscle tone.
Am. J. Physiol.
259 (Cell Physiol. 28):
C3-C18,
1990[Abstract/Free Full Text].
29.
Nelson, M. T.,
and
J. M. Quayle.
Physiological roles and properties of potassium channels in arterial smooth muscle.
Am. J. Physiol.
268 (Cell Physiol. 37):
C799-C822,
1995[Abstract/Free Full Text].
30.
Okabe, K.,
K. Kitamura,
and
H. Kuriyama.
Features of 4-aminopyridine sensitive outward current observed in single smooth muscle cells from the rabbit pulmonary artery.
Pflügers Arch.
409:
561-568,
1987[Medline].
31.
Parker, J. C.,
P. R. Kvietys,
K. P. Ryan,
and
A. E. Taylor.
Comparison of isogravimetric and venous occlusion capillary pressures in isolated dog lungs.
J. Appl. Physiol.
55:
964-968,
1983[Abstract/Free Full Text].
32.
Peng, W.,
S. V. Karwande,
J. R. Hoidal,
and
I. S. Farrukh.
Potassium currents in cultured human pulmonary arterial smooth muscle cells.
J. Appl. Physiol.
80:
1187-1196,
1996[Abstract/Free Full Text].
33.
Post, J. M.,
C. H. Gelband,
and
J. R. Hume.
[Ca2+]i inhibition of K+ channels in canine pulmonary artery.
Circ. Res.
77:
131-139,
1995[Abstract/Free Full Text].
34.
Post, J. M.,
J. R. Hume,
S. L. Archer,
and
E. K. Weir.
Direct role for potassium channel inhibition in hypoxic pulmonary vasoconstriction.
Am. J. Physiol.
262 (Cell Physiol. 31):
C882-C890,
1992[Abstract/Free Full Text].
35.
Robertson, B. E.,
R Schubert,
J. Hescheler,
and
M. T. Nelson.
cGMP-dependent protein kinase activates Ca-activated K channels in cerebral artery smooth muscle cells.
Am. J. Physiol.
265 (Cell Physiol. 34):
C299-C303,
1993[Abstract/Free Full Text].
36.
Shirai, M.,
K. Sada,
and
I. Ninomiya.
Effects of regional alveolar hypoxia and hypercapnia on small pulmonary vessels in cats.
J. Appl. Physiol.
61:
440-448,
1986[Abstract/Free Full Text].
37.
Standen, N. B.,
J. M. Quayle,
N. W. Davies,
J. E. Brayden,
and
M. T. Nelson.
Hyperpolarizing vasodilators activate ATP-sensitive K+ channels in arterial smooth muscle.
Science
245:
177-180,
1989[Medline].
38.
Torphy, T. J.
-Adrenoceptors, cAMP, and airway smooth muscle relaxation: challenges to the dogma.
Trends Pharmacol. Sci.
15:
370-374,
1994[Medline].
39.
Townsley, M. I.,
R. J. Korthuis,
B. Rippe,
J. C. Parker,
and
A. E. Taylor.
Validation of double vascular occlusion method for Pc,i in lung and skeletal muscle.
J. Appl. Physiol.
61:
127-132,
1986[Abstract/Free Full Text].
40.
Wiener, C. M.,
A. Dunn,
and
J. T. Sylvester.
ATP-dependent K+ channels modulate vasoconstrictor responses to severe hypoxia in isolated ferret lungs.
J. Clin. Invest.
88:
500-504,
1991[Medline].
41.
Yuan, X.,
W. F. Goldman,
M. L. Tod,
L. J. Rubin,
and
M. P. Blaustein.
Hypoxia reduces potassium currents in cultured rat pulmonary but not mesenteric arterial myocytes.
Am. J. Physiol.
264 (Lung Cell. Mol. Physiol. 8):
L116-L123,
1993[Abstract/Free Full Text].
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