Potassium channels modulate canine pulmonary vasoreactivity to
protein kinase C activation
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 protein kinase C (PKC) activation
was determined in the isolated blood-perfused dog lung. Pulmonary
vascular resistances and compliances were measured with vascular
occlusion techniques. The PKC activators phorbol 12-myristate
13-acetate (PMA; 10
7 M) and
thymeleatoxin (THX; 10
7 M)
significantly increased pulmonary arterial and pulmonary venous resistances and pulmonary capillary pressure and decreased total vascular compliance by decreasing both microvascular and large-vessel compliances. The Ca2+-activated
K+-channel blocker
tetraethylammonium ions (1 mM), the ATP-sensitive K+-channel inhibitor glibenclamide
(10
5 M), and the delayed
rectifier K+-channel blocker
4-aminopyridine (10
4 M)
potentiated the pressor response to both PMA and THX on the arterial
and venous segments and also further decreased pulmonary vascular
compliance. In contrast, the ATP-sensitive
K+-channel opener cromakalim
(10
5 M) attenuated the
vasoconstrictor effect of PMA and THX on both the arterial and venous
vessels. In addition, membrane depolarization by 30 mM KCl elicited an
increase in the pressor response to PMA. These results indicate that
pharmacological activation of PKC elicits pulmonary vasoconstriction.
Closure of the Ca2+-activated
K+ channels, ATP-sensitive
K+ channels, and delayed rectifier
K+ channels as well as direct
membrane depolarization by KCl potentiated the response to PMA and THX,
indicating that K+ channels
modulate the canine pulmonary vasoconstrictor response to PKC activation.
pulmonary vascular resistance; pulmonary vascular compliance; thymeleatoxin
 |
INTRODUCTION |
PROTEIN KINASE C (PKC) represents an important
component of a signal transduction pathway that regulates vascular
smooth muscle contraction. The role of PKC in vascular smooth muscle
contraction has been investigated with phorbol esters. Phorbol esters
appear to exert their effect through the activation of the enzyme PKC by substituting for diacylglycerol (DAG) (10, 31). DAG is thought to be
one of the endogenous lipids that activates PKC by increasing the
affinity of the enzyme for Ca2+
and phosphatidylserine at normal
Ca2+ levels (32). By activating
PKC, phorbols can potentiate vascular contraction by increasing the
influx of Ca2+ through
Ca2+ channels into vascular smooth
muscle cells (11, 44). Activation of PKC by phorbols induces a
slow-developing vascular smooth muscle contraction (20). Phorbol
12-myristate 13-acetate (PMA), an ester derivative of croton oil, has
been used to study PKC-induced pulmonary vasoconstriction (3, 24, 25,
29). PMA enhances the vasoconstrictor response produced by vasoactive
agents such as acetylcholine, serotonin, and
K+ (5, 11). Also, Orton et al.
(35) observed that PMA potentiation of hypoxic and
K+ vasoconstriction in isolated
rat lungs was mediated through the activation of PKC.
Ion channels, including K+
channels, have been identified in vascular smooth muscle cells (12, 34,
35, 40) and are reported to be involved in the regulation of vascular
tone (14, 22). Several different
K+ channels, including
ATP-sensitive, Ca2+-activated, and
nonspecific voltage-gated K+
channels, are present on vascular smooth muscle. Activation of these
channels causes an increase in K+
efflux, membrane hyperpolarization, inhibition of
Ca2+ influx, and subsequent
vascular smooth muscle relaxation. In the pulmonary circulation,
vascular smooth muscle tone is an important determinant of pulmonary
vascular resistance, pulmonary vascular compliance, and lung blood
flow, and 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 identifying the specific vascular segments in the
pulmonary circulation that constrict reflects the net effect on
pulmonary capillary pressure
(Ppc) that is determined by the
distribution of vascular resistance in the pulmonary arteries and veins.
Although relatively scant, evidence points to a relationship between
PKC and K+ channels in vascular
smooth muscle cells. Minami et al. (30) observed that PKC inhibited the
Ca2+-activated
K+ channel in coronary arterial
smooth muscle, and Aiello et al. (2) showed that PKC activation by
phorbol esters blocks the delayed rectifier channel. Most recently, a
study by Shimoda et al. (39) reported that endothelin-1 depolarized
intrapulmonary arterial smooth muscle cells by inhibiting the delayed
rectifier K+ current, an effect
dependent on activation of the phospholipase C/PKC signal transduction pathway.
In light of these previous investigations that appear to establish a
relationship between K+ channels
and PKC activation, it was hypothesized that closing K+ channels would potentiate
PKC-mediated canine pulmonary vasoconstriction. Therefore, the present
study was done to determine the role of K+-channel modulation on the
effect of PKC 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 theory 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. (4) was used to determine the effect of PKC on segmental
vascular resistance and compliance. These vascular occlusion techniques
have previously been used to measure the pulmonary vascular
resistance-compliance profile in normal lungs and in lungs challenged
with vasoactive agents (6-9).
Specific to this study, the role of ATP-sensitive
K+ channels was investigated with
cromakalim (Crom; an activator of ATP-sensitive K+ channels) and glibenclamide
(Glib; a blocker of ATP-sensitive K+ channels) to determine whether
ATP-sensitive K+ channels modulate
the vasoconstrictor response to PKC. In addition, the
Ca2+-activated
K+-channel blocker
tetraethylammonium ions (TEA) and the delayed rectifier
K+-channel inhibitor
4-aminopyridine (4-AP) were used to determine whether these specific
K+ channels also potentiate the
pulmonary vascular response to PKC.
 |
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 previously been described in detail (6,
7, 9, 18, 36, 42). Briefly, the lung was perfused at a constant flow
with 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
(arterial PO2 100-110 Torr and
arterial PCO2 30-40 Torr) and pH
in normal ranges. 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 with an electromagnetic flow probe (Carolina Medical SF 300A) positioned in the venous outflow
line 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 or
losing weight in zone III conditions
(Ppa > Ppv > Paw).
Ppc.
Ppc was determined with the
double-occlusion technique (42). When both the arterial and venous
cannulas are simultaneously occluded,
Ppa and
Ppv quickly equilibrate to the
same pressure (Ppc). If
Ppa and
Ppv did not exactly equilibrate 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 the double-occlusion pressure is an excellent estimate
of Ppc (42).
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 a precapillary resistance
(R1) and
Ppc is separated from
Ppv by a postcapillary resistance
(R2).
R1 and
R2 were
calculated with the following equations
|
(2)
|
|
(3)
|
All
pulmonary vascular resistances are reported in 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)
|
Middle-compartment
compliance (C2) was calculated
with the equation derived by Audi et al. (4)
|
(5)
|
Arterial
compliance (C1) was determined
by the following equation (2)
|
(6)
|
where
A2 is the area
bounded by the arterial pressure curve after arterial
occlusion and Pdo is
the double-occlusion pressure calculated by numerical integration.
Venous compliance (C3) was then
calculated by the following relationship with
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. Q 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. 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 13 treatment groups.
Group 1 (PMA control lungs) consisted
of isolated lung lobes (n = 5) treated
with 10
7 M PMA (Sigma) for
60 min to achieve a peak pressor response. To study the effect that
Ca2+-activated
K+-channel modulation has on the
vasoactive response to PMA, lobes (group 2;
n = 5) were pretreated with 1 mM TEA
(Sigma) for 30 min before the addition of PMA. For
group 3 (n = 5 lobes),
10
5 M Glib (Sigma) was used
as a pretreatment for 15 min before the addition of PMA to evaluate the
importance of ATP-sensitive K+
channels on the vasoactive response to PKC activation. In
group 4, the lobes
(n = 5) were pretreated with
10
5 M Crom (an
ATP-sensitive K+-channel opener)
for 15 min before the addition of PMA. In group 5, the lobes (n = 5)
were pretreated with 10
4 M
4-AP (an inhibitor of delayed rectifier
K+ channels; Sigma) for 15 min
before the addition of PMA. For groups 6-10
(n = 5 lobes/group), the PKC activator
thymeleatoxin (THX; 10
7 M;
Calbiochem) was substituted for PMA, with the experimental protocols
being identical to groups
1-5. In
group 11, the
Ca2+-dependent PKC isoform (
and
) inhibitor Gö 6976 (10
7 M) was used as a
pretreatment for 15 min before the addition of PMA.
Group 12 lobes were pretreated with
norepinephrine (NE; 10
7 M;
Sigma) for 15 min before the addition of PMA to determine whether a
vasoactive agent that increases pulmonary vascular tone independent of
K+ blockade would have a similar
effect on PMA. In group 13, KCl (30 mM; Sigma) was used to determine the effect of membrane depolarization on the vasoactive response to PMA. All drugs were dissolved in DMSO
except TEA, KCl, and NE that were dissolved in saline. The volume of
DMSO or saline used to dissolve the drugs had no significant effect
alone (vehicle control) on lung hemodynamics relative to baseline
control measurements (data not shown). All drugs were given as a bolus
into the venous reservoir, and all drug concentrations were calculated
based on 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. 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 control
conditions, TEA, Glib, and 4-AP elicited a small but significant
increase in Ppa. As shown in Fig.
2, PMA caused a significant increase in
pulmonary arterial resistance, an effect potentiated when the Ca2+-activated
K+ channels were blocked with TEA,
the ATP-sensitive K+ channels were
blocked with Glib, and the delayed rectifier
K+ channels were inhibited with
4-AP. In contrast, opening the ATP-sensitive K+ channels with Crom attenuated
the pressor response to PMA. Figure 3 shows
the effects of PMA on pulmonary venous resistance. PMA significantly
increased pulmonary venous resistance, an effect potentiated by TEA,
Glib, and 4-AP. In contrast, Crom attenuated the increase in pulmonary
venous resistance by PMA. Figures 4 and
5 show the effect of the PKC activator THX
on pulmonary vascular resistance. THX increased both pulmonary arterial
resistance (Fig. 4) and pulmonary venous resistance (Fig. 5) similarly
to PMA. In addition, TEA, Glib, and 4-AP potentiated the response to
THX on both the precapillary and postcapillary vessels, and Crom
inhibited the response in the precapillary vessels and partially
blocked the pressor response to THX in the postcapillary vessels, with the remaining partial pressor response being insignificant.

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Fig. 1.
Effect of tetraethylammonium ions (TEA), glibenclamide (Glib), and
4-aminopyridine (4-AP) on pulmonary arterial perfusion pressure. Values
are means ± SE; n = 5 lungs/group.
* Significantly different from zero,
P < 0.05.
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Fig. 2.
Effect of Glib, TEA, 4-AP, and cromakalim (Crom) on pulmonary arterial
resistance response to phorbol 12-myristate 13-acetate (PMA). Control
groups represent no treatment. Values are means ± SE;
n = 5 lobes/group. Significantly
different (P < 0.05) from:
* respective control group;
+ PMA alone.
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Fig. 3.
Effect of Glib, TEA, 4-AP, and Crom on pulmonary venous resistance
response to PMA. Control groups represent no treatment. Values are
means ± SE; n = 5 lobes/group.
Significantly different (P < 0.05) from: * respective control group;
+ PMA alone.
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Fig. 4.
Effect of Glib, TEA, 4-AP, and Crom on pulmonary arterial resistance
response to thymeleatoxin (THX). Control groups represent no treatment.
Values are means ± SE; n = 5 lobes/group. Significantly different
(P < 0.05) from: * respective
control group; + THX
alone.
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Fig. 5.
Effect of Glib, TEA, 4-AP, and Crom on pulmonary venous resistance
response to THX. Control groups represent no treatment. Values are
means ± SE; n = 5 lobes/group.
Significantly different (P < 0.05) from: * respective control group;
+ THX alone.
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The effect of PMA and THX on Ppc,
which was determined by distribution of the precapillary and
postcapillary resistances, is presented in Figs.
6 and 7,
respectively. PMA and THX significantly increased
Ppc, a phenomenon that was
decreased by Crom. Closing the
Ca2+-activated
K+ channels with TEA, blocking the
ATP-sensitive K+ channels with
Glib, or inhibiting the delayed rectifier
K+ current with 4-AP significantly
increased Ppc relative to the initial increase in Ppc observed
with PMA or THX, which was reflective of the initial increase in
pulmonary venous resistance with PMA (Fig. 3) and THX (Fig. 5). Figure
8 shows that Gö 6976 inhibited the
pressor response to PMA, indicating that PMA activates specific Ca2+-dependent PKC isoforms to
induce pulmonary vasoconstriction. The possibility also existed that
other vasoactive agents as well as membrane depolarization could
potentiate the pressor response to PMA, so experiments were done with
the pulmonary vasoconstrictors NE (6) and KCl to compare the effect of
the K+-channel blockers on the
response to PMA. Figure 9 shows that 10
7 M NE, which is a
concentration that mimics the increase in
Ppa with the
K+-channel inhibitors (Fig. 1) in
this experimental model, had no significant effect on the pressor
response to PMA. In contrast, 30 mM KCl, which also elicits an increase
in Ppa similar to that observed
with the K+-channel blockers (Fig.
1), did increase the pressor response to PMA (Fig.
10). These data indicate that membrane
depolarization as well as direct pharmacological
K+-channel blockade by specific
inhibitors may be mechanisms of K+-channel modulation that
potentiate the vasoconstrictor effect of PMA.

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Fig. 6.
Effect of Glib, TEA, 4-AP, and Crom on pulmonary capillary pressure
response to PMA. Control groups represent no treatment. Values are
means ± SE; n = 5 lobes/group.
Significantly different (P < 0.05) from: * respective control group;
+ PMA alone.
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Fig. 7.
Effect of Glib, TEA, 4-AP, and Crom on pulmonary capillary pressure
response to THX. Control groups represent no treatment. Values are
means ± SE; n = 5 lobes/group.
Significantly different (P < 0.05) from: * respective control group;
+ THX alone.
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Fig. 8.
Effect of Gö 6976 on total pulmonary vascular resistance response
to PMA. Values are means ± SE; n = 5 lobes. There was no effect of vehicle alone or inhibitor alone on
pulmonary vascular resistance. * Significantly different from
respective control group, P < 0.05.
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Fig. 9.
Effect of norepinephrine (NE) on total pulmonary vascular resistance
response to PMA. Control groups represent no treatment. Values are
means ± SE; n = 5 lobes.
* Significantly different from respective control group,
P < 0.05.
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Fig. 10.
Effect of KCl on total pulmonary vascular resistance response to PMA.
Values are means ± SE; n = 5 lobes. Significantly different (P < 0.05) from: * respective control group; ** KCl alone.
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Tables 1 and 2
summarize the effects of PMA and THX, respectively, on pulmonary
segmental vascular compliance. PMA and THX significantly decreased
total vascular compliance by lowering both middle-compartment and
large-vessel (arterial and venous) compliances, effects potentiated by
TEA, Glib, or 4-AP but subsequently reversed by Crom. Specifically,
TEA, Glib, and 4-AP significantly potentiated the decrease in
middle-compartment compliance, arterial compliance, and venous
compliance by PMA and THX. In contrast, Crom inhibited the effect of
PMA and THX on the arterial, venous, and middle-compartment
compliances.
 |
DISCUSSION |
In this study, pharmacological activators of PKC (PMA and THX)
increased precapillary and postcapillary resistances and
Ppc and decreased microvascular
and large-vessel compliances. The effect of PMA and THX was greater in
the veins (postcapillary resistance) than in the arteries (precapillary
resistance), which reflects the net increase in
Ppc due to the greater
constriction in the veins. The greater vasoconstrictor response to PMA
and THX observed in the veins in this study may relate to the degree of
upregulation and/or translocation of PKC from the cytosol to the
membrane in pulmonary venous smooth muscle compared with that in
pulmonary arterial smooth muscle. By activating PKC, phorbols can
potentiate vascular contraction by increasing the influx of Ca2+ through
Ca2+ channels into vascular smooth
muscle cells (11, 44). Previous studies (3, 20) have shown that
activation of PKC by phorbols induces a slow-developing vascular smooth
muscle contraction, which was a phenomenon also observed in the present
study. In isolated perfused canine lungs, Allison et al. (3) reported that PMA increased both precapillary and postcapillary resistances, but
they did not measure the effect of PMA on pulmonary vascular compliance. In the present study, the decrease in pulmonary vascular compliance that occurred when vascular pressure was increased by either
PMA or THX reflected the relative indistensibility (lack of compliance
in the vessels) of the pulmonary vasculature in response to PKC
activation. Although both PMA and THX decreased all compartmental
compliances (arterial, microvascular, and venous), the percentage of
large-vessel compliance (C1 + C3) to microvascular compliance
(C2) was similar to that
measured under control conditions (C1 + C3 = 24% for control lobes, 22%
for PMA-treated lobes, and 23% for THX-treated lobes).
PKC represents a family of at least 11 isoforms that are presently
classified into four separate groups: group 1 comprises the classic PKC
isoforms (
,
I,
II, and
) that are calcium dependent, group
2 consists of the novel PKC isoforms (
,
, µ,
, and
) that
are calcium independent, group 3 are the atypical isoforms (
/
and
) that are calcium independent and DAG insensitive, and group 4 (PKC-µ) is an isoform that is similar to the isoforms in group 3 but
contains a unique signal peptide and transmembrane domain (43).
Numerous PKC isoforms are expressed in vascular smooth muscle (
,
,
,
, and
) that may be dependent on species, type of
vessel, and age of the vessel (21, 26, 27, 33). A recent study by
Damron et al. (15) reported the expression of at least six PKC isoforms
including PKC-
, PKC-
, and PKC-
in cultured canine pulmonary
vascular smooth cells. PMA is widely accepted as an activator of PKC,
and THX has recently been shown to cause translocation and
downregulation of multiple PKC isoforms (38). Thus, based on the above
identification of specific PKC isoforms present in canine pulmonary
vascular smooth muscle, the data obtained with PMA and THX suggest that
the PKC isoforms
,
, and
may be among those activated by
these agents in canine pulmonary vascular smooth muscle to elicit
vasoreactivity. In addition, inhibition of the PMA response by Gö
6976 (calcium-dependent PKC-
and PKC-
isoform inhibitor)
strengthens the possibility that PKC-
may be activated to induce
pulmonary vasoreactivity.
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
control conditions. Hasanuma et al. (22) observed that 4-AP and TEA
caused vasoconstriction in perfused rat lungs, and Pinheiro and Malik
(37) reported that activation of ATP-sensitive
K+ channels had a small but
minimal role in maintainng pulmonary vascular tone in piglet lungs. In
contrast, ATP-sensitive K+-channel
blockade appears to have no effect on basal tone in the pulmonary
vascular bed of the cat (17) or lamb (41). Thus K+-channel modulation of basal
pulmonary vascular tone appears to be
K+-channel selective and species
dependent. Under conditions of PKC activation, TEA, Glib, and 4-AP
potentiated the response to PMA and THX on the arterial and venous
segments and also further decreased pulmonary vascular compliance. Crom
significantly attenuated the vasoconstrictor effect of PMA and THX on
both the arterial and venous vessels. It is worth noting that the
potentiation in arterial and venous resistances by
K+-channel inhibition during PKC
activation was greater in magnitude than by either
K+-channel blockade or PKC
activation separately, indicating a synergistic response rather than an
additive effect on precapillary and postcapillary resistances. The
increase in postcapillary resistance on
K+-channel blockade had the net
effect of further increasing Ppc, which is directly correlated to the distribution of pulmonary vascular
resistance (1, 13, 16).
The possibility existed that other vasoactive agents as well as
membrane depolarization could potentiate the pressor response to PMA
independent of K+-channel
blockade, so experiments were done with the pulmonary vasoconstrictors
NE (6) and KCl to compare the effect of the K+-channel blockers on the
response to PMA. NE, at a concentration that mimicked the increase in
Ppa by the
K+-channel inhibitors under
baseline conditions in this experimental model, had no significant
effect on the pressor response to PMA. In contrast, KCl, at a
concentration that also elicited an increase in
Ppa similar to that observed with
the K+-channel blockers did
increase the pressor response to PMA. These results suggest that
membrane depolarization has a similar effect as direct pharmacological
K+-channel blockade on the
vasoconstrictor response to PMA.
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
physiological significance of
K+-channel modulation on the
pulmonary vasoactive response to PKC activation may be related to
vascular hyperreactivity. It has been shown that PKC inhibits the
delayed rectifier K+ current in
rabbit vascular smooth muscle cells (2), and PKC activators such as PMA
also block the Ca2+-dependent
K+ channel in coronary arterial
smooth muscle cells (30). Shimoda et al. (39) recently reported that
inhibition of the voltage-gated K+
current in intrapulmonary arterial myocytes by endothelin-1 was markedly attenuated by staurosporine and GF-109203X, inhibitors of PKC.
In addition, it has been hypothesized that
K+ channels are secondarily
activated in response to increased vasoreactivity to regulate the
pulmonary vascular bed under pathophysiological conditions involving
pulmonary hypertension (28). Collectively, these results suggest that
K+ channels play an important role
as a safety mechanism to regulate pulmonary vasoreactivity and prevent
hyperreactivity. K+ channels may
also modulate endothelium-derived hyperpolarizing factor in vascular
smooth muscle through the activation of
Ca2+-dependent
K+ channels (23), and it has
recently been suggested that endothelium-derived hyperpolarizing factor
may be K+ that effluxes through
charybdotoxin- and apamin-sensitive
K+ channels (19). Activation of
these hyperpolarizing K+ channels
would attenuate pulmonary vasoconstriction and lower capillary
pressure, a major determinant of lung fluid balance.
In summary, the results of this study showed that agonist activation of
PKC by PMA and THX increased pulmonary arterial resistance, pulmonary
venous resistance, and Ppc and
decreased pulmonary vascular compliance. TEA, Glib, and 4-AP
potentiated the response to PMA and THX on the arterial and venous
segments and also further decreased pulmonary vascular compliance. In
contrast, the ATP-sensitive K+-channel opener Crom attenuated
the vasoconstrictor effect of PKC activation on both the arterial and
venous vessels. Although pharmacological agents were used to activate
PKC and inhibit K+ channels, these
results indicate that physiological closure of the
Ca2+-activated
K+ channels, ATP-sensitive
K+ channels, and delayed rectifier
K+ channels potentiate the canine
pulmonary arterial response during PKC activation and that
K+-channel modulation may be a key
physiological response toward regulating pulmonary vasoreactivity.
 |
ACKNOWLEDGEMENTS |
I thank Louise Meadows for excellent technical assistance.
 |
FOOTNOTES |
This work was supported by the American Lung Association of Georgia and
the Medical College of Georgia Research Institute.
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: S. A. Barman,
Dept. of Pharmacology and Toxicology, Medical College of Georgia,
Augusta, GA 30912.
Received 14 September 1998; accepted in final form 17 May 1999.
 |
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