Regulation of amiloride-sensitive sodium absorption in murine
airway epithelium by C-type natriuretic peptide
Thomas J.
Kelley1,
Calvin U.
Cotton1, and
Mitchell L.
Drumm1,2
1 Department of Pediatrics and
2 Department of Genetics, Center
for Human Genetics, Case Western Reserve University, Cleveland,
Ohio 44106-4948
 |
ABSTRACT |
We have previously
shown that C-type natriuretic peptide (CNP), a guanylate
cyclase agonist, can stimulate cystic fibrosis transmembrane
conductance regulator (CFTR)-mediated chloride secretion in murine
airway epithelial cells via protein kinase (PK) A activation through
the inhibition of cGMP-inhibited phosphodiesterases. In this paper, we
show that CNP is also capable of reducing amiloride-sensitive sodium
absorption in murine airway epithelium through a cGMP-dependent mechanism that is separate from the CFTR regulatory signaling pathway.
Both murine tracheal and nasal tissues exhibit sensitivity to
amiloride-sensitive sodium regulation by exogenously added CNP. CNP
depolarized the nasal transepithelial potential difference by 6.3 ± 0.5 mV, whereas the cGMP-inhibited phosphodiesterase inhibitor
milrinone actually hyperpolarized the nasal transepithelial potential
difference by 2.0 ± 1.2 mV in mice homozygous for a CFTR stop
mutation [CFTR(
/
)]. Inhibition of guanylate
cyclase activity and PKG activity in normal mice resulted in an
increase in amiloride-sensitive sodium absorption, suggesting that
tonic regulation of amiloride-sensitive sodium absorption is in part due to basal cGMP levels and PKG activity.
guanylate cyclase; guanosine 3',5'-cyclic
monophosphate
 |
INTRODUCTION |
AT THE CELLULAR LEVEL, loss of cystic fibrosis (CF)
transmembrane conductance regulator (CFTR) function results in two
primary electrophysiological abnormalities in the airway epithelia of CF patients. First is the loss of chloride permeability in response to
cAMP, consistent with the role of the CFTR as a cAMP-regulated chloride
channel (21, 24). Second, increased absorption of sodium
across the airway epithelium through amiloride-sensitive sodium
channels has been demonstrated in CF airways (19). The role that
increased absorption of sodium plays in disease progression, if any, is
not known at this time. It has been postulated that increased sodium
absorption coupled with decreased chloride secretion serves to
desiccate mucus secretions, which prevent proper clearance and enhance
bacterial growth and infection. The activity of the epithelial sodium
channel (ENaC) has been shown to be altered by the presence or absence
of CFTR in transfected MDCK cells (29). ENaC-dependent sodium
absorption is increased in the absence of the CFTR and is further
stimulated by cAMP-increasing agents compared with cells cotransfected
with both CFTR and ENaC. How the loss of CFTR function or expression
results in sodium hyperabsorption is not yet clearly
understood. The expression of the ENaC is reportedly unchanged between
non-CF and CF airways, indicating that regulation of ENaC activity is
in some way modulated by the CFTR, perhaps via a disruption of some
physical interaction between CFTR and ENaC (2, 29).
Although CF mice do not develop detectable airway disease, the nasal
epithelia of CFTR(
F508/
F508) and CFTR(
/
) mice serve as good models for the ion transport defect found in the human CF
airway; they not only exhibit the chloride transport abnormality associated with reduced or lost CFTR function but also exhibit the
hyperabsorption of sodium characteristic of human CF airways (8).
Kelley et al. (18) have previously demonstrated that the
combination of forskolin and milrinone is effective in restoring some
CFTR function in the nasal epithelium of CFTR(
F508/
F508) mice but
not in that of CFTR(
/
) mice. Kelley and colleagues (14,
15) have shown that activation results from blocking the activity of
cGMP-inhibited phosphodiesterase (cGI-PDE), thereby raising the local
concentration of cAMP near the apical plasma membrane of epithelial
cells to a level sufficient to activate
F508 CFTR. This signaling
pathway would indicate that cGMP could play a significant role in the
regulation of CFTR activity. We have been able to substantiate this
mechanism by demonstrating nearly identical activation of chloride
permeability in CFTR(
F508/
F508) mice with the combination of
forskolin and C-type natriuretic peptide (CNP), an agonist for the
membrane-associated guanylate cyclase (GC) B receptor (17). Natriuretic
peptides have been shown to regulate chloride transport in shark rectal
gland cells (28) and in porcine colon (1). Heat-stable enterotoxin
(Sta) and guanylin, peptides that stimulate membrane-bound GCs, have been shown to activate the CFTR through protein kinase (PK) A-dependent pathways. These peptides are structurally and functionally related to
the natriuretic peptides and have been shown to stimulate CFTR-mediated chloride transport in the human colonic cell lines T84 and Caco-2 (3,
5) through apparent cross-activation of PKA by cGMP.
In addition to their role as
Cl
secretagogues,
natriuretic peptides within the renal and vascular systems have been
shown to effectively inhibit sodium transport through cGMP-dependent mechanisms (reviewed in Ref. 25). For example, atrial natriuretic peptide stimulates cGMP formation via activation of GC-A and causes a
decrease in amiloride-sensitive sodium absorption in renal inner medullary collecting duct cells (23). Similarly, decreasing cGMP
production through the inhibition of GC activity by the compound LY-83583 increases amiloride-sensitive sodium absorption (23). Clearly,
the regulation of electrolyte and fluid levels in the airways is an
important process. An intriguing aspect of CNP as a natural modulator
of ion transport in airway epithelium is the possibility that this
peptide hormone may regulate sodium, as well as chloride, transport.
Coupled with the ability of CNP to increase ciliary beat frequency (6),
modulating both chloride secretion and sodium absorption through the
production of cGMP would make CNP an excellent candidate for a primary
regulator of airway clearance. It has been demonstrated that apical
application of CNP can effectively increase cGMP levels and regulate
ciliary beat frequency in airway epithelial cells (6). Production of CNP in the lumen has been shown to likely take place in stimulated macrophages and to be secreted in response to lipopolysaccharide or
other challenges (30). In this paper, we explore whether CNP is capable
of influencing sodium absorption in murine airways and examine the
signaling pathways involved in sodium regulation compared with those
involved in the cGMP-mediated stimulation of chloride secretion.
 |
MATERIALS AND METHODS |
Measurement of mouse nasal transepithelial potential
difference values. Mouse nasal transepithelial
potential difference (TEPD) was measured with the protocols of Grubb et
al. (8) and Kelley et al. (16). Briefly, mice were
anesthetized with 10 µl/g body weight of 0.4 mg/ml of acepromazine,
11 mg/ml of ketamine, and 2 mg/ml of xylazine in PBS. PE-10 tubing
drawn out to approximately one-half of its original diameter was
inserted 2-3 mm into the nostril of the mouse. The solutions were
perfused at room temperature by use of a Razel A-99 (Razel Scientific
Instruments, Stamford, CT) syringe pump at a rate of ~7 µl/min. A
series of valves was used to change solutions, with a delay time of
~45 s between solution change and solution contact with the nasal
epithelium. Ringer solutions consisted of chloride-replete
HEPES-buffered Ringer (HBR; 10 mM HEPES, pH 7.4, 138 mM NaCl, 5 mM KCl,
2.5 mM
Na2HPO4, 1.8 mM CaCl2, and 1.0 mM
MgSO4) and chloride-free HBR (10 mM HEPES, pH 7.4, 138 mM sodium gluconate, 5 mM potassium gluconate,
2.5 mM
Na2HPO4,
3.6 mM hemicalcium gluconate, and 1.0 mM
MgSO4; all chemicals from Sigma,
St. Louis, MO).
Measurement of mouse tracheal potential
difference. Excised tracheae were mounted on holding
pipettes, and the luminal and bath compartments were perfused
independently. All experiments were performed at 37°C by placing
all solutions and the mounted trachea in an Isolette infant incubator
(Narco, Hatboro, PA). HBR was perfused to both the luminal and
basolateral sides by gravity. Luminal perfusion rates were between 3 and 5 ml/min. TEPD was measured via 4% agar bridges in HBR placed on
both the luminal and basal sides and connected through calomel
electrodes to a DVC 1000 voltage-current clamp (WPI). Data were
collected on a MacLab/4e from Advanced Instruments.
Mice. Mice were genotyped from tail
clip DNA.
F508 mice (Cftrtm1Kth) were a
generous gift from Kirk Thomas (University of Utah School of Medicine)
and were genotyped by the procedures previously described (31).
Cftrm1Unc mice (27) were obtained from
Jackson Laboratories and were genotyped as described by Koller et al.
(22). To increase survival of CF animals, the mice were fed a liquid
diet as described by Eckman et al. (4). The mice were cared for in
accordance with Case Western Reserve University Institutional Animal
Care and Use Committee guidelines.
 |
RESULTS |
Effect of CNP on amiloride-sensitive sodium absorption
in mouse trachea. The effects of CNP and 8-bromo-cGMP
(8-BrcGMP) on amiloride-sensitive sodium absorption were studied by
measuring the TEPD in excised tracheae of normal mice (Fig.
1). The GC agonist CNP was tested for its
ability to change sodium absorption by measuring depolarization of TEPD
in both the presence and absence of amiloride. The results were
compared with the effects of the direct addition of 8-BrcGMP. The
baseline TEPD of these tissues was
3.7 ± 1.1 mV
(n = 7 mice). CNP (1 µM) and
8-BrcGMP (100 µM) added to the luminal perfusate reversibly
depolarized TEPD by 21.6 ± 8.1 and 23.3 ± 8.4%, respectively.
Amiloride (100 µM) induced a 65.4 ± 2.9% depolarization of
baseline TEPD. When tracheae were exposed to amiloride before and
during CNP addition, CNP no longer had any effect on TEPD. These
results indicate that CNP is able to reduce amiloride-sensitive sodium
absorption across the epithelium of mouse tracheae, likely through a
cGMP-dependent mechanism.

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Fig. 1.
Effect of C-type natriuretic peptide (CNP) and 8-bromo-cGMP
(8-BrcGMP) on tracheal transepithelial potential difference
(TEPD). A: raw trace of TEPD assay
with excised murine trachea in presence of HEPES-buffered Ringer (HBR)
solution, CNP (1 µM), amiloride (Amil; 100 µM), and 8-BrcGMP (100 µM). B: average depolarization
mediated by each compound given as percentage of baseline TEPD.
Amil-pretreated CNP was measured relative to depolarization caused by
Amil alone. Values are means ± SE and represent an average of 4 experiments.
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|
Differential effects of the phosphodiesterase
inhibitor milrinone and CNP on nasal TEPD of CFTR(
/
)
mice. Other groups (13, 29) have demonstrated that
agonists of PKA activity increased amiloride-sensitive sodium
absorption in cells lacking CFTR but expressing the amiloride-sensitive
ENaC. In the nasal epithelium of CFTR(
/
) mice, milrinone
(100 µM) hyperpolarized the lumen negative TEPD by 2.0 ± 1.2 mV
(n = 4 experiments) when measured in
chloride-replete HBR in the absence of amiloride (Fig.
2). This result is consistent
with our hypothesis that milrinone is stimulating PKA activity through
the inhibition of cGI-PDE. Kelley et al. (16) have previously shown
that CNP is capable of stimulating chloride secretion through a
mechanism indistinguishable from that of milrinone in wild-type and
F508 homozygous mice. In CFTR(
/
) mice,
CNP caused a depolarization in the lumen negative potential of 6.3 ± 0.5 mV (31.5 ± 7.7% of baseline TEPD;
n = 4 experiments; Fig. 2).
Time-controlled traces with HBR alone varied only 0.3 ± 0.4 mV
(n = 3 experiments) in the direction
of depolarization. Treatment of the nasal epithelium of
CFTR(
/
) mice with amiloride before and during CNP
addition reduced any further depolarization of TEPD by CNP to ~1.5 ± 0.6 mV (Fig. 3). Although milrinone
and CNP both act through cGI-PDE inhibition to stimulate chloride secretion, they differ in their ability to regulate sodium absorption. The results obtained with milrinone are consistent with previous reports that show that PKA activation increases
amiloride-sensitive sodium absorption in CF cells (13, 29). However,
the effects of CNP suggest that cGMP may have dual roles in sodium and
chloride transport regulation by acting through different signaling
pathways.

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Fig. 2.
Effects of milrinone and CNP on nasal TEPD in CFTR( / )
mice. A: averaged nasal TEPD traces
showing points every 15 s. Data are change in TEPD ( TEPD), with zero
TEPD and zero time being defined as time either CNP (1 µM) or
milrinone (100 µM) was added. Control, HBR time control. Positive
values, hyperpolarization of lumen negative TEPD; negative values,
depolarization. Values are means ± SE; nos. in parentheses, no. of
mice. B: effects of conditions shown
in A 150 s after drug addition. Values
are means ± SE; nos. in parentheses, number of experiments.
Significant difference compared with control:
* P = 0.05;
** P < 0.001 by Duncan's
multiple range test.
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Fig. 3.
Effect of CNP on nasal TEPD in CFTR(+/+, )
(n = 5) and CFTR( / )
(n = 4) mice in presence of Amil.
Averaged traces show points every 15 s after addition of Amil (100 µM). Amil and CNP were perfused in presence of HBR. Zero time, time
of CNP addition. Positive values, hyperpolarization of lumen negative
TEPD; negative values, depolarization. Values are means ± SE.
* P < 0.05 by
t-test compared with equivalent time
points in traces from CFTR( / ) mice.
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|
Effects of GC inhibition on mouse nasal
TEPD. It has previously been shown that the GC
inhibitor LY-83583 increases amiloride-sensitive sodium transport in
rabbit medullary collecting duct cells (23). With the use of the mouse
nasal TEPD assay, the GC inhibitors methylene blue (100 µM) and
LY-83583 (50 µM) were perfused onto the nasal epithelium of normal
mice. Lumen negative TEPD was hyperpolarized 3.3 ± 0.7 (n = 5 experiments) and 2.5 ± 0.6 mV (n = 8 experiments) with LY-83583
and methylene blue, respectively (Fig. 4).
The addition of amiloride to the perfusing solutions depolarized the
TEPD in the presence of either GC inhibitor. GC inhibitors added after the addition of amiloride had no effect on the TEPD. The
hyperpolarization induced by LY-83583 also showed sensitivity to the
addition of 8-BrcGMP, suggesting that cGMP is involved in the basal
regulation of sodium absorption. The addition of milrinone, an agent to
increase cAMP levels, had no effect other than a slight
hyperpolarization of nasal TEPD. This result is consistent with the
hyperpolarizing effect milrinone exhibited on the nasal TEPD of
CFTR(
/
) mice as shown in Differential
effects of the phosphodiesterase inhibitor milrinone and CNP on nasal
TEPD of CFTR(
/
) mice.

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Fig. 4.
Effect of guanylate cyclase (GC) inhibition on nasal TEPD in CFTR(+/+)
mice before (A) and after
(B) Amil addition.
C: effect of addition (open bar) of
milrinone (100 µM) and 8-BrcGMP (100 µM) on LY-83583-induced
hyperpolarization of lumen negative TEPD. In
A and
C, TEPD was allowed to reach steady
state before addition of drugs. In B,
steady state with Amil was reached before addition of GC inhibitors.
Amil (100 µM) and GC inhibitors [methylene blue (MethBlue; 100 µM) and LY-83583 (50 µM)] were perfused in presence of HBR.
Zero time is time of GC inhibitor addition. Positive values,
hyperpolarization of lumen negative TEPD; negative values,
depolarization. Values are means ± SE;
n, no. of experiments.
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|
The effects of the GC inhibitor LY-83583 on mouse nasal TEPD were
compared between normal mice and CFTR(
F508/
F508) mice. The nasal
epithelium of the CFTR(
F508/
F508) mice exhibits the CF airway
characteristic of hyperabsorption of sodium compared with that in
non-CF mice. Baseline nasal TEPD values of CFTR(
F508/
F508) and
non-CF mice were
19.3 ± 2.0 (n = 10) and
8.7 ± 1.3 mV
(n = 10), respectively. Unlike the
normal mice, the CFTR(
F508/
F508) mice exhibited no further
hyperpolarization of lumen negative TEPD with the addition of LY-83583
(Fig. 5). After 2 min of exposure to
LY-83583 (50 µM), the nasal TEPD of CFTR(
F508/
F508) mice depolarized an average of 0.3 ± 1.1 mV
(n = 4). These data suggest that
abnormal regulation of the cGMP signaling pathway may be at least
partially responsible for the increased absorption of sodium across the
airway epithelium, which is characteristic of CF.

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Fig. 5.
Effect of GC inhibition on nasal TEPD in CFTR(+/+) and
CFTR( F508/ F508) mice. LY-83583 (50 µM) in HBR was perfused onto
nasal epithelium of CFTR(+/+) and CFTR( F508/ F508) mice followed
by perfusion with Amil (100 µM). Zero time is time of GC inhibitor
addition. Positive values, hyperpolarization of lumen negative TEPD;
negative values, depolarization. Values are means ± SE;
n, no. of experiments.
*P < 0.05 by
t-test compared with equivalent time
points in trace from CFTR( F508/ F508) mice.
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Differential regulation of ion transport by PKA and
PKG. GC control of sodium absorption implies a role for
cGMP in this regulatory pathway. To assess the extent to which
cGMP-dependent protein kinase is involved in the regulation of sodium
absorption, the PKA- and PKG-specific inhibitors
Rp diastereomers of adenosine 3',5'-cyclic monophosphothioate
(Rp-cAMPS) and
8-(p-chlorophenylthio)guanosine 3',5'-cyclic monophosphothioate
(Rp-8-pCPT-cGMPS), respectively, were
tested for their effect on the nasal TEPD in wild-type mice (Fig.
6). Measured 3.5 min after
Rp-8-pCPT-cGMPS application, PKG
inhibition resulted in a 2.4 ± 0.9-mV hyperpolarization
(n = 4 experiments) of lumen negative
TEPD that was amiloride sensitive. PKA inhibition resulted in a further
depolarization of TEPD. Nasal TEPD continued to depolarize an average
of 1.8 ± 1.2 mV (n = 3 experiments) after 3.5 min and responded much less to amiloride in the
presence of Rp-cAMPS.

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Fig. 6.
Effect of protein kinase (PK) A and PKG inhibition on nasal TEPD in
CFTR(+/+) mice. Rp diastereomers of
8-(p-chlorophenylthio)guanosine
3',5'-cyclic monophosphothioate
(Rp-cGMPS; 100 µM;
n = 4;
A) and adenosine
3'.5'-cyclic monophosphothioate
(Rp-cAMPS; 100 µM;
n = 3;
B) in HBR were perfused onto
nasal epithelium of CFTR(+/+) mice followed by perfusion with Amil (100 µM). Averaged traces show points every 15 s. Positive values,
hyperpolarization of lumen negative TEPD; negative values,
depolarization. Values are means ± SE.
* P < 0.05 by
t-test compared with equivalent time
points in trace from Rp-cAMPS-treated
mice.
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|
Conversely, chloride secretion measured in response to forskolin and
CNP was sensitive to PKA inhibition (Fig.
7). The addition of
Rp-cAMPS reduced forskolin- and
CNP-induced hyperpolarization from ~3 to 0.4 ± 0.6 mV
(n = 4 experiments).
Rp-8-pCPT-cGMPS did not significantly
reduce hyperpolarization mediated by the combination of forskolin and
CNP (2.3 ± 0.7 mV; n = 3 experiments). These data are consistent with previous findings (16)
that CNP-induced chloride secretion in Calu-3 cells and in mouse nasal
epithelia is dependent on PKA activity, whereas PKG activity has no
significant effect on this pathway. Both sodium and chloride secretion
in murine airway epithelium can be regulated through cGMP, although the
specific signaling pathways appear to diverge (Fig.
8).

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Fig. 7.
Effect of PKA and PKG inhibition on chloride secretion in
CFTR( F508/ F508) mice. Either Rp-cGMPS (100 µM) or
Rp-cAMPS (100 µM) (Rp-cN) was perfused in presence of Amil
(100 µM), forskolin (Forsk; 10 µM), and CNP (1 µM) in
chloride-free HBR onto nasal epithelium of CFTR( F508/ F508) mice.
No Agonist, time-controlled trace with chloride-free HBR and Amil (100 µM). A: averaged traces showing points every 15 s. Values
are means ± SE; n, no. of experiments. Zero time and
zero TEPD are point at which Forsk and CNP were added. B:
TEPD 2 min after drug treatment. Positive values, hyperpolarization
of lumen negative TEPD; negative values, depolarization. Significant
difference compared with Forsk+CNP in absence of inhibitors:
* P = 0.01; ** P = 0.002 (both by
Duncan's multiple range test).
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Fig. 8.
Schematic diagram showing dual regulation of chloride and sodium
transport by cGMP at apical membrane of an airway epithelial cell. (+),
Stimulation of activity; ( ), inhibition. PDE, phosphodiesterase;
PDE III, cGMP-inhibited, cAMP-specific PDE; PDE V, cGMP-specific PDE.
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|
 |
DISCUSSION |
Kelley and colleagues (14, 15, 18) have previously shown that CNP is
capable of regulating CFTR-mediated chloride secretion through the
inhibition of cGI-PDE by increasing PKA activity, presumably through
localized elevations of cAMP concentration. CNP can substitute
effectively for the cGI-PDE inhibitor milrinone in stimulating nasal
epithelial chloride secretion in CFTR(
F508/
F508) mice when used
in combination with forskolin. This observation raises the possibility
that CNP is a natural regulator for airway epithelial chloride
secretion (16).
As their name implies, the natriuretic peptides are most well known for
their ability to regulate sodium transport. Along with a loss of
cAMP-stimulated chloride secretion in CF airway epithelia, an increase
in the rate of sodium absorption is also associated with the CF
phenotype. A role for CNP in the regulation of sodium absorption in the
airways would be consistent with the previously established role for
natriuretic peptides. We have been able to show that CNP, via
a cGMP-dependent pathway, decreases amiloride-sensitive ion
transport. The larger CNP-induced decrease of TEPD in
CFTR(
/
) mice compared with that in
CFTR(+/+,
) mice suggests that CF-related sodium
hyperabsorption can be modulated by cGMP. Consistent with this
hypothesis, we have shown that inhibition of either GC or PKG activity
leads to an increase in amiloride-sensitive sodium transport in the
nasal epithelia of non-CF mice. However, GC inhibition has no effect on
sodium transport when tested in CFTR(
F508/
F508) mice, thus
providing evidence that this pathway may already be involved in
CF-associated sodium hyperabsorption (Fig. 5). Our data suggest that
abnormal regulation of cGMP-dependent pathways may be at least
partially involved in elevated sodium absorption found in CF airway
epithelia. Although these data are consistent with other reports (23,
25) showing the regulation of sodium absorption by natriuretic
peptides, there are data that indicate that cGMP has other effects on
airway epithelial ion transport. Geary et al. (6) reported that CNP has
no effect on either chloride or sodium transport in primary nasal
epithelial cells obtained from scrapings placed in culture. It is
possible that culture conditions altered the electrical response to
CNP, although cGMP-mediated effects on ciliary beat frequency were preserved. Also, a recent paper by Schwiebert et al. (26) demonstrated that cGMP could stimulate an increase in sodium and chloride currents in rat tracheal epithelial cells. Our differing results may reflect the
different techniques employed in each of the studies or differences between cGMP-mediated regulation of sodium transport in rat tracheal and mouse nasal and tracheal epithelial cells. Rat tracheal epithelial sodium transport appears to be regulated primarily through cyclic nucleotide-gated nonselective cation channels, whereas this study suggests that amiloride-sensitive channels play a larger role in mouse
airways.
These findings may have implications for the use of pharmacological
therapies for CF. The effects on sodium absorption should be considered
when various pharmacological approaches are used to stimulate mutant
CFTR activity. Our data are consistent with other reports that show
that stimulation of PKA activity by agents that raise cAMP levels
increases sodium absorption in CF airway epithelia (Fig. 2). Each of
the compounds commonly tested, milrinone (18), genistein (11, 12),
NS004 (7), and CPX (9, 10), requires adenylate cyclase activation to
optimize the stimulatory effects of these compounds on CFTR activity.
Although each of these compounds may be effective at stimulating
chloride permeability through some mutant form of CFTR, a concomitant
increase in already elevated sodium absorption will likely occur. It is
not currently known what effect sodium absorption plays in disease
progression, and further increasing the rate of absorption may be
detrimental to the health of the patient. Most experimental protocols
used for in vivo measurements of chloride permeability, such as nasal TEPD, utilize high concentrations of amiloride- and chloride-free Ringer solutions to optimize both electrical and chemical gradients for
chloride secretion and generation of TEPD. Such conditions that promote
net secretion would not be present in a therapeutic setting. As
discussed by Knowles et al. (20), an increase in chloride permeability
in a CF airway might be expected to result in a net absorption of fluid
rather than secretion. This concept has led to the notion that chloride
secretion, coupled with the inhibition of sodium absorption by
amiloride, is likely to provide the most effective mode for driving the
net balance toward fluid secretion. Finding agents such as CNP that may
help restore a balance to chloride and sodium transport, as opposed to
merely increasing CFTR-mediated chloride permeability, may have a more therapeutic benefit.
In summary, our data identify a crucial role for cGMP in the
coregulation of ion transport in the airway epithelia. A previous report (6) demonstrated that CNP increases ciliary beat frequency in
primary human airway epithelia. These observations suggest that CNP may
play an important role in acute fluid balance and mucociliary clearance
in mammalian cells. CNP is reportedly produced by murine peritoneal and
bone marrow macrophages in response to lipopolysaccharide (30). Thus,
if CNP is also produced by airway macrophages, CNP may represent an
excellent candidate for a natural regulator of airway clearance.
Perhaps modified forms of CNP and other possible airway clearance
regulators that inversely regulate both chloride secretion and sodium
absorption should be examined for therapeutic potential.
 |
ACKNOWLEDGEMENTS |
This work was supported by National Heart, Lung, and Blood
Institute Grant HL-50160; National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-51878; and grants from the Cystic Fibrosis
Foundation.
 |
FOOTNOTES |
Address for reprint requests: M. L. Drumm, Dept. of Pediatrics, Case
Western Reserve Univ., 8th floor BRB, 10900 Euclid Ave., Cleveland, OH
44106-4948.
Received 5 December 1997; accepted in final form 2 March 1998.
 |
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