HCO
3 reabsorption in renal
collecting duct of NHE-3-deficient mouse: a compensatory response
Suguru
Nakamura1,
Hassane
Amlal1,
Patrick J.
Schultheis2,
John H.
Galla1,
Gary E.
Shull2, and
Manoocher
Soleimani1
Departments of 1 Internal
Medicine and 2 Molecular Genetics,
Biochemistry and Microbiology, University of Cincinnati School of
Medicine, Cincinnati, Ohio 45267
 |
ABSTRACT |
Mice with a targeted disruption of
Na+/H+
exchanger NHE-3 gene show significant reduction in
HCO
3 reabsorption in proximal tubule,
consistent with the absence of NHE-3. Serum HCO
3, however, is only mildly
decreased (P. Schulties, L. L. Clarke, P. Meneton, M. L. Miller, M. Soleimani, L. R. Gawenis, T. M. Riddle, J. J. Duffy, T. Doetschman, T. Wang, G. Giebisch, P. S. Aronson, J. N. Lorenz, and G. E. Shull.
Nature Genet. 19: 282-285, 1998),
indicating possible adaptive upregulation of
HCO
3-absorbing transporters in
collecting duct of NHE-3-deficient (NHE-3
/
) mice.
Cortical collecting duct (CCD) and outer medullary collecting duct
(OMCD) were perfused, and total
CO2 (net
HCO
3 flux,
JtCO2) was
measured in the presence of 10 µM Schering 28080 (SCH, inhibitor of
gastric H+-K+-ATPase)
or 50 µM diethylestilbestrol (DES, inhibitor of
H+-ATPase) in both mutant and
wild-type (WT) animals. In CCD,
JtCO2 increased in NHE-3 mutant mice (3.42 ± 0.28 in WT to 5.71 ± 0.39 pmol · min
1 · mm
tubule
1 in mutants,
P < 0.001). The SCH-sensitive net
HCO
3 flux remained unchanged, whereas
the DES-sensitive HCO
3 flux increased
in the CCD of NHE-3 mutant animals. In OMCD,
JtCO2 increased in NHE-3 mutant mice (8.8 ± 0.7 in WT to
14.2 ± 0.6 pmol · min
1 · mm
tubule
1 in mutants,
P < 0.001). Both the SCH-sensitive
and the DES-sensitive HCO
3 fluxes
increased in the OMCD of NHE-3 mutant animals. Northern hybridizations
demonstrated enhanced expression of the basolateral
Cl
/HCO
3
exchanger (AE-1) mRNA in the cortex. The gastric
H+-K+-ATPase
mRNA showed upregulation in the medulla but not the cortex of NHE-3
mutant mice. Our results indicate that
HCO
3 reabsorption is enhanced in CCD
and OMCD of NHE-3-deficient mice. In CCD,
H+-ATPase, and in the OMCD, both
H+-ATPase and gastric
H+-K+-ATPase
contribute to the enhanced compensatory
HCO
3 reabsorption in NHE-3-deficient animals.
acid-base; proton-potassium-adenosinetriphosphatase; AE-1; proton-adenosinetriphosphatase; bicarbonate reabsorption; cortical
collecting duct; outer medullary collecting duct; NHE-3 knockout
 |
INTRODUCTION |
MORE THAN 85% OF FILTERED
HCO
3 is reabsorbed in the proximal
tubule by a process of active acid secretion, with the remaining
filtered HCO
3 being reabsorbed in the
thick ascending limb and collecting duct (3, 13, 14, 15, 16). The
acid-secreting transporters that are responsible for the reabsorption
of HCO
3 in the proximal tubule are the
Na+/H+
exchanger NHE-3 and the H+-ATPase,
with NHE-3 mediating ~60% of total
HCO
3 reabsorption in this nephron
segment (3, 6, 14, 17, 29). The collecting duct reabsorbs 5-10%
of total filtered HCO
3 and plays a
major role in the final control of urine pH (15, 16, 18, 23).
HCO
3 reabsorption in the cortical
collecting duct (CCD) is predominantly mediated via the H+-ATPase (23, 25, 32), whereas in
outer medullary collecting duct (OMCD), the gastric
H+-K+-ATPase
(HKAg) is the major transporter involved in
HCO
3 reabsorption (21, 28, 33). A
nongastric HKA (colonic HKA, or HKAc) is also expressed in the
collecting duct but does not play a significant role in
HCO
3 reabsorption under normal
conditions (1, 10, 19, 21).
Recently, an NHE-3-deficient mouse was developed that shows a
significant reduction in HCO
3
reabsorption in the proximal tubule (22). Serum
HCO
3 concentration, however, was
only mildly decreased (22), indicating possible adaptive upregulation
of HCO
3absorbing transporters in
collecting duct of NHE-3-deficient (NHE-3
/
) mice. The
purpose of the current experiments was to examine
HCO
3-absorbing transporters in the CCD
and OMCD of NHE-3-deficient mice.
 |
METHODS |
HCO
3
transport measurement in CCD and OMCD.
After removal, both kidneys of wild-type (WT) or NHE-3-deficient mice
were decapsulated, sectioned into three to four cross sections per
kidney, and immediately placed in a Petri dish containing dissecting
solution. Each section was stripped from the papillary tip to the
cortex into smaller wedges and transferred into a second Petri dish
containing dissecting solution maintained at 14°C under a
dissecting microscope. Segments of CCD and OMCD were dissected as
before (11, 12). Tubules were then transferred to a Lucite chamber
containing bathing solution (Table 1) initially at room temperature. One end of the tubule was pulled into an outer holding pipette. Once secure, the inner perfusion pipette was advanced, and the
tubule was opened with a slight positive pressure. The opposite end of
the tubule was then pulled into an outer collecting pipette. The
tubules were warmed to 37°C in a temperature-controlled chamber and
bathed with solution replaced every 30 min. Perfusion rates were
maintained at 1-2 nl/min. Collections were made at 20- to 30-min
intervals in a precalibrated constant-bore collection pipette. Three
collections were made per each tubule. The collected samples were
placed in a Petri dish under mineral oil. The solutions used are shown
in Table 1. HCO
3-containing solutions were bubbled with 5%
CO2-95%
O2 gas. Bath pH was nominally 7.4. The osmolarity of the solutions was adjusted to 290 mosM by addition of
sucrose.
Inhibitors. To assess the contribution
of different pathways of proton secretion for net
HCO
3 reabsorption (JtCO2), three
inhibitors were added to the perfusate. To block H+-ATPase, 50 µM
diethylestilbestrol (DES) or 10 nM bafilomycin was used. The inhibitory
effects of bafilomycin and DES were not additive (please see Fig. 5
later), indicating that DES inhibits JtCO2
via the bafilomycin-sensitive
H+-ATPase. To inhibit HKAg, 10 µM Schering 28080 (SCH) was used. The inhibitory effects of SCH and
DES on JtCO2 in
the mouse collecting duct are additive (20), indicating inhibition of
HKAg and H+-ATPase,
respectively. Two inhibitors were tested in each collection. Three
collections were made in each tubule, one without and two with an
inhibitor. In all perfusions, the sequence of control and inhibitor was varied.
Measurement of
tCO2 flux.
tCO2 in nanoliter samples from
collectate and perfusate was measured by microfluorometry
(Nanoflow; World Precision Instruments). The
JtCO2
(in pmol · min
1 · mm
tubule length
1) across
the tubule epithelium was calculated as
where
C0 is the concentration of
tCO2 in the perfusion fluid (in
pmol/nl), C1 is the concentration
of tCO2 in the collected fluid (in
pmol/nl), V0 is the perfusion rate
(nl/min), V1 is the collection
rate (in nl/min) (note that in the absence of vasopressin, V0 = V1), and
L is the length of the tubule (in mm).
RNA isolation. Total cellular RNA was
extracted from kidney (whole kidney, cortex, or medulla) by the method
of Chomczynski and Sacchi (8). In brief, 0.2 g of tissue was
homogenized at room temperature in 10 ml Tri Reagent (Molecular
Research Center, Cincinnati, OH). RNA was extracted by
phenol/chloroform, precipitated by isopropanol, and quantitated by
spectrophotometry. RNA was stored at
80°C until used.
Northern hybridization. Total RNA
samples (30 µg/lane) were fractionated on a 1.2%
agarose-formaldehyde gel and transferred to Magna NT nylon membranes
(MSI). Membranes were then cross-linked by ultraviolet light and baked
as described. Hybridization was performed according to Church and
Gilbert (9). The cDNA probe was labeled with
32P-labeled deoxynucleotides using
the RadPrime DNA labeling kit (GIBCO-BRL). The membranes were washed,
blotted dry, and screened by a Phosphor Imager screen (Molecular
Dynamics). For HKAg, HKAc, and
H+-ATPase Northern blots, rat
cDNA-specific probes were used as before. For AE-1, a 650-bp cDNA
(Sac
I-Bgl II fragment) from rat AE-1 cDNA
was used as specific probe. The hybridizations were performed under
high-stringency conditions to prevent any cross-hybridization with
other isoforms.
Statistical analysis. The data are
expressed as mean ± SE where appropriate. For statistical analysis
of the differences in the levels of mRNA expression, Phosphor Imager
readings from three separate experiments were obtained and analyzed.
Statistical analysis was determined using ANOVA.
P < 0.05 was considered
statistically significant.
Materials.
32P was purchased from New England
Nuclear (Boston, MA). Nitrocellulose filters and other chemicals were
purchased from Sigma Chemical (St. Louis, MO). The GIBCO-BRL RadPrime
DNA labeling kit was purchased from Life Technologies.
 |
RESULTS |
JtCO2 in CCD of WT and
NHE-3 knockout mice.
In the first series of experiments,
JtCO2 in CCD of
WT and NHE-3 knockout mice was examined. As shown in Fig.
1,
JtCO2 was enhanced in NHE-3 mutant animals, with
JtCO2 increasing
from 3.42 ± 0.28 to 5.71 ± 0.39 pmol · min
1 · mm
tubule
1
(P < 0.001, n = 6 and 7 for WT and deficient mice,
respectively).

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Fig. 1.
Net HCO 3 reabsorption
(JtCO2)
in cortical collecting duct (CCD) of wild-type (WT, +/+) and NHE-3
knockout ( / ) mice.
JtCO2 was
measured in CCD of WT and NHE-3-deficient mice, according to
METHODS. As indicated,
JtCO2 increased
in NHE-3-deficient mice (* P < 0.001).
|
|
Effect of inhibitors on
JtCO2 in CCD of WT and NHE-3 knockout
mice.
To examine the contribution of HKA and
H+-ATPase, respective
specific inhibitors of these transporters were used. For
HKAg and H+-ATPase inhibition, 10 µM SCH and 50 µM DES was added to the perfusate, respectively.1
Figure 2,
A and
B, show the inhibitory effects of DES
and SCH on JtCO2
in CCD of WT or NHE-3 knockout mice. These data permit the calculation
of SCH-sensitive and DES-sensitive
JtCO2, which are
summarized in Fig. 2, C and
D. Figure
2C shows that the SCH-sensitive JtCO2 remained
unchanged in CCD of NHE-3-deficient mice. Figure 2D shows that the DES-sensitive
JtCO2 increased
in CCD of NHE-3-deficient mice.

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Fig. 2.
Effect of inhibitors on
JtCO2 (in
pmol · min 1 · mm 1)
in CCD of WT and NHE-3 knockout mice.
A and
B: effects of diethylestilbestrol
(DES) and Sch-28080 (SCH) on
JtCO2 in CCD of
WT (A) or NHE-3 knockout mice
(B). In WT mice
(A),
JtCO2
(n = 6) was decreased by both DES
(1.42 ± 0.48; P < 0.007) and SCH
(1.82 ± 0.24; P < 0.002)
compared with controls (CON, 3.42 ± 0.28). In mutant mice
(B),
JtCO2
(n = 7) was also decreased by both DES
(2.27 ± 0.32; P < 0.001) and SCH (4.12 ± 0.34; P < 0.01). These data permit calculation of the SCH-sensitive components
(WT 1.60 ± 0.21; mutant 1.58 ± 0.22), which did not
differ (P = not significant)
(C), and the DES-sensitive
components, which were higher (P < 0.02) in mutant (3.44 ± 0.23) than in WT mice (2.0 ± 0.25) (D).
* P < 0.05.
|
|
JtCO2 in OMCD of WT or
NHE-3 knockout mice.
The purpose of the next series of experiments was to examine
JtCO2 in OMCD of
WT or NHE-3 knockout mice. As shown in Fig. 3,
JtCO2 was
enhanced in OMCD of NHE-3 mutant animals, with
JtCO2 increasing
from 8.84 ± 0.68 to 14.20 ± 0.60 pmol · min
1 · mm
tubule
1
(P < 0.001, n = 5 and 6 for WT and deficient mice,
respectively).

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Fig. 3.
JtCO2 in outer
medullary collecting duct (OMCD) of WT or NHE-3 knockout mice.
JtCO2 was
measured in OMCD of WT and NHE-3-deficient mice, according to
METHODS.
JtCO2 increased
in NHE-3-deficient mice (* P < 0.001).
|
|
Effect of inhibitors on
JtCO2 in OMCD of WT or NHE-3 knockout
mice.
To examine the contribution of HKA and
H+-ATPase, 10 µM SCH or 50 µM
DES was added to the perfusate, respectively. Figure
4, A and
B, shows the inhibitory effects of DES
and SCH on JtCO2 in OMCD of WT or NHE-3-deficient animals. Calculated SCH-sensitive and
DES-sensitive
JtCO2 values are
depicted Fig. 4, C and
D, which shows that both the
SCH-sensitive and the DES-sensitive
JtCO2 increased
in OMCD of NHE-3-deficient mice.

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Fig. 4.
Effect of inhibitors on
JtCO2 (in
pmol · min 1 · mm 1)
in OMCD of WT or NHE-3 knockout mice.
A and
B: effects of DES and SCH on
JtCO2 in OMCD of
WT (A) or NHE-3-deficient mice
(B). In WT mice
(A),
JtCO2
(n = 5) was decreased by both DES
(5.89 ± 0.58; P < 0.02) and SCH
(4.8 ± 0.78; P < 0.01) compared
with control (CON) perfusions (8.84 ± 0.68). In NHE-3-deficient
mice (B),
JtCO2
(n = 6) was also decreased by both DES
(9.45 ± 0.49; P < 0.001) and SCH
(5.34 ± 0.52; P < 0.0001). Similar to the CCD, the calculated DES-sensitive component was
greater (P < 0.04) in mutant (4.71 ± 0.59) than in WT (2.95 ± 0.28, D). However, unlike the CCD, the
SCH-sensitive component was greater (P < 0.0001) in mutant (8.81 ± 0.52) than in WT mice (4.04 ± 0.30, C).
* P < 0.05.
|
|
DES inhibits JtCO2
via the bafilomycin-sensitive
H+-ATPase.
To determine the specificity of DES inhibition on
H+-ATPase, the effect of DES and
bafilomycin, a known specific inhibitor of the
H+-ATPase, was compared. As shown
in Fig. 5, the inhibitory effects of
bafilomycin and DES were not additive, indicating that DES decreases
JtCO2 via
inhibition of H+-ATPase.

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Fig. 5.
DES inhibits
JtCO2 (in
pmol · min 1 · mm 1)
via the bafilomycin (BAF)-sensitive
H+-ATPase.
JtCO2
(n = 4) in tubules perfused with BAF
only (2.0 ± 0.08) did not differ
(P = not significant) from that in
those perfused with both BAF and DES (1.75 ± 0.10).* P < 0.05.
|
|
AE-1 mRNA expression in kidneys of WT and
NHE-3-deficient mice.
HCO
3 transport across the basolateral
membrane of CCD cells is predominantly mediated via the
Cl
/HCO
3
exchanger AE-1, which is exclusively expressed on the basolateral
membranes of acid-secreting intercalated cells (2, 24, 30). As shown in
Fig. 6, AE-1 mRNA expression is increased
in the whole kidney of NHE-3 knockout animals. Zonal examinations
showed enhancement of AE-1 mRNA levels in the cortex of NHE-3 knockout
mice (Fig. 6), consistent with enhanced
HCO
3 reabsorption in CCD. The
expression of AE-1 in the medulla remained unchanged (Fig. 6). The AE-1
to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA expression
ratio is shown in Fig. 6 (bottom).

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Fig. 6.
Northern hybridization of kidney AE-1 in WT or NHE-3 knockout mice.
Representative Northern blots of AE-1 mRNA expression in whole kidney,
cortex, and medulla: ratio of AE-1 mRNA to GAPDH mRNA. Results indicate
that AE-1 mRNA expression is increased in whole kidney and cortex of
NHE-3 knockout mice but remained unchanged in medulla
(n = 3 for each group). NS, not
significant.
|
|
HKAg mRNA expression in kidneys of WT and
NHE-3-deficient mice. To correlate the activity of the
SCH-sensitive HKA with its mRNA expression, HKAg mRNA levels were
measured in whole kidney, cortex, and medulla. As shown in Fig.
7, HKAg mRNA is decreased mildly in the
whole kidney of NHE-3 knockout mice; however, the difference was not
statistically significant. Zonal analysis showed that HKAg mRNA
expression is increased in the medulla but mildly decreased in the
cortex (Fig. 7). The expression ratio of HKAg mRNA to GAPDH mRNA is
shown in Fig. 7
(bottom).

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Fig. 7.
Northern hybridization of kidney gastric
H+-K+-ATPase
(HKAg) in WT or NHE-3 knockout mice. Representative Northern blots of
HKAg mRNA expression in the whole kidney, cortex, and medulla: ratio of
HKAg mRNA to GAPDH mRNA. HKAg mRNA expression is increased in the renal
medulla but is mildly decreased in the cortex
(n = 3 for each group).
|
|
H+-ATPase mRNA expression in kidneys of WT and
NHE-3-deficient mice. The expression of mRNA encoding the 31-kDa
subunit of the H+-ATPase was
examined in the whole kidney, cortex, and medulla. As shown in Fig.
8,
H+-ATPase mRNA abundance remained
unchanged in whole kidney, cortex, and medulla of NHE-3 knockout mice.
The H+-ATPase mRNA to GAPDH mRNA
expression ratio is shown in Fig. 8 (bottom).

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Fig. 8.
Northern hybridization of kidney
H+-ATPase in WT or NHE-3 knockout
mice. Representative Northern blots of
H+-ATPase mRNA in the whole
kidney, cortex, and medulla: ratio of
H+-ATPase mRNA to GAPDH mRNA.
H+-ATPase mRNA expression remains
unchanged in both cortex and medulla of NHE-3 knockout mice
(n = 3 for each group,
P > 0.05).
|
|
 |
DISCUSSION |
The current experiments demonstrate that
HCO
3 reabsorption is enhanced in CCD
and OMCD of NHE-3 mutant mice (Figs. 1 and 3). The increase in
HCO
3 reabsorption in CCD is
predominantly via H+-ATPase. In
OMCD, increased HCO
3 reabsorption is
mediated via both HKAg and
H+-ATPase (Figs. 2 and 4).
Northern hybridizations indicated enhanced expression of
AE-1 in the cortex (Fig. 6) and HKAg in the medulla (Fig. 7).
The distal nephron, and particularly CCD and medullary collecting duct,
plays an essential role in reabsorbing the
HCO
3 and final acid-base composition
of the urine (15, 16, 23). In CCD,
HCO
3 reabsorption occurs predominantly by type A intercalated cells (A-type IC) (23, 25, 32). Two acid-secreting transporters, HKAg and
H+-ATPase, are
expressed at the apical membrane of the A-type IC (4, 7) and are
responsible for the majority of HCO
3 reabsorption in this nephron segment. B-type IC are involved in HCO
3 secretion, which is predominantly
mediated via the apical
Cl
/HCO
3
exchanger (16, 23, 26, 27). In B-type IC, both HKAg and
H+-ATPase are expressed
on the basolateral membrane domain (4, 7) and are
predominantly involved with intracellular
HCO
3 generation, which would then be
secreted into the lumen via the apical
Cl
/HCO
3
exchanger (16, 23). Our results indicate enhanced AE-1 mRNA expression
in the cortex of NHE-3-deficient mice (Fig. 5), which is consistent
with the finding of increased HCO
3
reabsorption in the CCD. Lack of upregulation of AE-1 mRNA expression
in medulla, despite enhanced HCO
3 reabsorption in OMCD, suggests that either the amount of exchanger is
not rate limiting for overall bicarbonate reabsorption in OMCD or the
majority of AE-1 is originating from segments other than the medullary
collecting duct (such as descending and ascending limb of Henle). This
is in contrast to the cortex, where AE-1 expression is limited only to CCD.
Our results demonstrated enhancement of
H+-ATPase activity in CCD and OMCD
of NHE-3-deficient mice. Lack of alteration in
H+-ATPase mRNA expression
indicates that enhancement of this transporter in NHE-3 mutant mice is
likely a posttranscriptional event. This conclusion is consistent with
previous studies demonstrating that adaptation of
H+-ATPase in acid-base disorders
is via alteration in the rate of insertion (exocytosis) or retrieval
(endocytosis) of the transporter to or from the membrane rather than
via alterations in the rate of transcription (23). Studies examining
the localization of H+-ATPase
protein (membrane bound vs. cytosolic) by immunocytochemistry should
provide a more definite answer to this question.
The signal responsible for upregulation of
H+-ATPase in the collecting duct
of NHE-3-deficient mice remains speculative at this time. The primary
renal defect in NHE-3 mutant mice is decreased reabsorption of
HCO
3 in the proximal tubule, with
subsequent increased delivery to the distal tubule. In addition to the
kidney, NHE-3 mutant mice show decreased reabsorption of HCO
3 in the small intestine (22). As a
result of the HCO
3 absorption defect
in the kidney proximal tubule and small intestine, NHE-3 mutant mice
show a mild metabolic acidosis (22, 31). One likely signal involved in
the upregulation of H+-ATPase is
increased delivery of HCO
3 to the distal nephron. According to this scheme,
H+-ATPase is upregulated and
enhances HCO
3 reabsorption in the
collecting duct of NHE-3 knockout mice which in turn attenuates HCO
3 loss in the urine. Alternatively,
it is possible that H+-ATPase
upregulation in CCD (and OMCD) is due to the mild acidosis that is
present in NHE-3 knockout mice (22, 31). In support of this latter
hypothesis, several studies have shown that metabolic acidosis
upregulates HCO
3 absorption in A-type IC of CCD via increased apical
H+-ATPase and basolateral
Cl
/HCO
3
exchange (AE-1) activities (23, 26, 27). Increased apical
H+-ATPase activity under these
circumstances was shown to be via enhanced insertion of this pump into
the luminal membrane of A-type IC (5, 26, 27).
HKAg shows differential regulation at mRNA level in cortex and medulla
in NHE-3 mutant mice. HKAg mRNA levels are increased in the medulla but
mildly decreased in the cortex of NHE-3 knockout mice (Fig. 7).
Upregulation of HKAg mRNA in the medulla correlates with its increased
activity in OMCD as measured by SCH-sensitive JtCO2,
and indicates transcriptional upregulation (or increased mRNA
stability) of this transporter in NHE-3 mutant mice. Lack of
enhancement of HKAg mRNA in the cortex of NHE-3 mutant animals remains
suspect. Immunocytochemical studies in CCD have indicated that HKAg and
H+-ATPase are colocalized on the
apical membranes of A-type IC and on the basolateral membranes of
B-type IC (4, 7). Studies in rabbits have shown that the CCD adapts to
metabolic acidosis by downregulating
HCO
3 secretion in B-type IC cells via
decreased apical
Cl
/HCO
3
exchange activity (26). Downregulation of apical
Cl
/HCO
3
exchange activity results in alkaline intracellular pH in B-type IC
cells, which in turn can suppress the basolateral HKAg. It is possible
that the lack of upregulation of HKAg mRNA in the cortex (Fig. 6) may
reflect the net effect of suppression of basolateral HKAg and
upregulation of apical HKAg. However, comparable SCH-sensitive HKA
activity in CCD of WT and mutant mice (Fig. 2) does not support this conclusion.
In summary, JtCO2
is increased in both CCD and OMCD of NHE-3 knockout mice. In CCD,
JtCO2 is
increased predominantly via upregulation of
H+-ATPase activity, whereas in
OMCD, JtCO2 is
increased via upregulation of both HKAg and
H+-ATPase
activities. Compensatory
HCO
3 reabsorption in the collecting
duct reduces HCO
3 wasting in NHE-3
knockout mice and helps to maintain acid-base homeostasis.
 |
ACKNOWLEDGEMENTS |
These studies were supported by National Institute of Diabetes and
Digestive and Kidney Diseases Grants DK-46789, DK-52821, and DK-54430
(to M. Soleimani) and DK-50594 (to G. E. Shull) and by grants from
Dialysis Clinic Incorporated (to M. Soleimani and J. H. Galla).
 |
FOOTNOTES |
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.
1
Colonic
H+-K+-ATPase
mRNA remained undetectable in NHE-3 knockout animals. Furthermore, 1 mM
ouabain in the perfusate had no effect on net
HCO
3 transport in wild-type or knockout animals. Taken together, these results indicate that colonic
H+-K+-ATPase
does not have an increased role in the NHE-3 knockout mice.
Address for reprint requests and other correspondence: M. Soleimani,
Division of Nephrology and Hypertension, Dept. of Internal Medicine,
Univ. of Cincinnati, 231 Bethesda Ave., MSB 5502, Cincinnati, OH
45267-0585 (E-mail: Manoocher.Soleimani{at}uc.edu).
Received 16 December 1998; accepted in final form 12 March 1999.
 |
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