Role of 20-HETE in mediating the effect of dietary K intake on
the apical K channels in the mTAL
Ruimin
Gu,
Yuan
Wei,
Houli
Jiang,
Michael
Balazy, and
Wenhui
Wang
Department of Pharmacology, New York Medical College, Valhalla, New
York 10595
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ABSTRACT |
We have used the patch-clamp
technique to study the effect of dietary K intake on the apical K
channels in the medullary thick ascending limb (mTAL) of rat kidneys.
The channel activity, defined by the number of channels in a patch and
the open probability (NPo), of the 30- and 70-pS
K channels, was 0.18 and 0.11, respectively, in the mTAL from rats on a
K-deficient diet. In contrast, NPo of the 30- and 70-pS K channels increased to 0.60 and 0.80, respectively, in the
tubules from animals on a high-K diet. The concentration of
20-hydroxyeicosatetraenoic acid (20-HETE) measured with gas chromatography-mass spectrometry was 0.8 pg/µg protein in the mTAL
from rats on a high-K diet and increased significantly to 4.6 pg/µg
protein in the tubules from rats on a K-deficient diet. Addition of
N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) or
17-octadecynoic acid (17-ODYA), agents that inhibit the formation of
20-HETE, had no significant effect on the activity of the 30-pS K
channels. However, DDMS/17-ODYA significantly increased the activity of
the apical 70-pS K channel from 0.11 to 0.91 in the mTAL from rats on a
K-deficient diet. In contrast, inhibition of the cytochrome
P-450 metabolism of arachidonic acid increased NPo from 0.64 to 0.81 in the tubules from
animals on a high-K diet. Furthermore, the sensitivity of the 70-pS K
channel to 20-HETE was the same between rats on a high-K diet and on a
K-deficient diet. Finally, the pretreatment of the tubules with DDMS
increased NPo of the 70-pS K channels in the
mTAL from rats on a K-deficient diet to 0.76. We conclude that an
increase in 20-HETE production is involved in reducing the activity of
the apical 70-pS K channels in the mTAL from rats on a K-deficient diet.
cytochrome P-450; arachidonic acid; hypokalemia; hyperkalemia; medullary thick ascending limb; 20-hydroxyeicosatetraenoic acid
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INTRODUCTION |
HYPOKALEMIA HAS BEEN
REPORTED to impair urinary concentrating ability in humans and
animals (6, 22, 23) and to result in Cl wasting
(27). Because the thick ascending limb (TAL) has a key
role in the urinary concentrating ability, it has been proposed that a
diminished Na transport in the TAL of the loop of Henle was involved in
the hypokalemia-induced decrease in urinary concentrating ability
(6). This hypothesis is supported by several independent studies. Gutsche et al. (12) have shown that Na transport
in the TAL was inhibited in kidneys from hypokalemic rats. Similar results were also reported by Unwin et al. (29). Moreover,
micropuncture studies have reported that K depletion reduced Cl
reabsorption in the loop of Henle of rat kidneys (17, 18).
Thus the data strongly indicate that an impaired NaCl transport in the
medullary TAL (mTAL) is responsible for the diminished urinary
concentrating ability induced by hypokalemia.
Recently, it has been demonstrated that the protein expression levels
of Na-K-Cl cotransporters and ROMK, a renal K channel that is
responsible for K recycling (2), were significantly lower
in kidneys from rats on a K-deficient diet than those from animals on
normal chow (20). Because K recycling is essential for the
function of the cotransporter, a decrease in the apical K conductance
is expected to inhibit the Na-K-Cl cotransporter. Thus studying the
regulation of the apical K channels in the mTAL may elucidate the
mechanism by which K depletion impairs the epithelial transport in the mTAL.
Eicosanoids have an important role in regulating the membrane transport
in the mTAL. PGE2 has been demonstrated to attenuate the
effect of vasopressin on Cl reabsorption (5) and to
inhibit Na reabsorption in the loop of Henle (26). It was
recently reported that PGE2 inhibits the reabsorption of
bicarbonate in the mTAL (9). In addition, cytochrome
P-450-dependent metabolites of arachidonic acid (AA) are
involved in the regulation of transport function of the mTAL.
Cytochrome P-450 metabolites of AA such as
20-hydroxyeicosatetraenoic acid (20-HETE) and 20-COOH-AA have been
shown to inhibit the Na-K-Cl transporter (7) and
bicarbonate reabsorption in the mTAL (10). We have
reported previously that 20-HETE inhibited the apical 70-pS K channel
(34). However, the role of eicosanoids in mediating the
effect of K intake on transport function in the TAL is not clear.
Because the 70-pS K channels make a significant contribution to the
apical K conductance, we explored the role of 20-HETE in mediating the
effect of dietary K intake on the apical K channels in the mTAL.
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METHODS |
Preparation of mTAL.
Pathogen-free Sprague-Dawley rats (Taconic Farms, Germantown, NY) were
used in the experiments. Animals were kept either on a high-K diet
(10%, wt/wt) or on a K-deficient diet for 7-10 days before
use. The method for preparation of the mTAL has been described previously (34). To immobilize the mTALs, we placed the
tubules on a 5 × 5-mm cover glass coated with Cell-Tak
(Collaborative Research, Bedford, MA). The cover glass was transferred
to a chamber mounted on an inverted microscope (Nikon, Melville, NY),
and the tubules were superfused with bath solution containing (in mM) 140 NaCl, 5 KCl, 1.8 MgCl2, 1.8 CaCl2, 5 glucose, and 10 HEPES (pH 7.4). We used a sharpened pipette to open the
mTAL to gain access to the apical membranes.
Measurement of 20-HETE.
Freshly isolated mTAL tubules (100-200 µg protein) were
resuspended in 100-µl bath solution and incubated for 10 min at
37°C. The incubation was terminated by decreasing temperature to
0°C with ice. As an internal standard, 1 ng of deuterated
[14,15-2H2]20-HETE was mixed with the
tubules. The tissue was spun down, and the supernatant was collected
and evaporated. Distilled water (1 ml) was added to the tube containing
the tubules, and lipids were extracted with ethyl acetate acidified
with formic acid (pH 3.5). The extract was dissolved in 100 µl
methanol and separated by reverse-phase HPLC using a gradient of
acetonitrile in water (50-100% in 20 min) at a flow rate of 1 ml/min. The 20-HETE fraction was dried, resuspended in 100 µl
acetonitrile, and converted to pentaflurobenzyl (PFB) ester by adding
10 µl PFB bromide and 10 µl of
N,N-diisopropylethylamine. The mixture was
incubated at room temperature for 30 min. The sample was evaporated
under nitrogen and further incubated with 80 µl
N,O-bis(trimethylsilyl)trifluoroacetamide for 30 min to
collect the PFB ester, trimethylsilyl (TMS) ether derivative of
20-HETE. The sample was dried and dissolved in 50 µl of isooctane for
gas chromatography-mass spectrometry (GC-MS) analysis (HP 5989A mass
spectrometer interfaced with a HP 5890 gas chromatograph). The samples
were injected into a 10 × 0.25-mm DB-1 capillary column with
0.25-µm film thickness (J & W Scientific, Folsom, CA). Helium was
used as the carrier gas to raise the temperature to 180-300°C
with 25°C/min of step increase. A selected ion monitoring negative
chemical ionization (NCl) was used to record ion abundances at
mass-to-charge ratio (m/z) 391 and m/z 393, which
corresponded to the endogenous and deuterated derivatized 20-HETE (PFB
ester TMS ether), respectively. The concentration of total 20-HETE in the purified biological samples was calculated by comparison of the ion
abundance ratio (m/z 391/393) vs. a standard curve of 20-HETE-PFB-TMS/2H2-20-HETE-PFB-TMS
molar ratio constructed by NCl GC-MS analysis.
Patch-clamp technique.
Electrodes were pulled with a Narishige model PP83 vertical pipette
puller and had resistances of 4-6 M
when filled with 140 mM
NaCl. The channel current recorded by an Axon 200A patch-clamp amplifier was low-pass filtered at 1 kHz using an eight-pole Bessel filter (902LPF; Frequency Devices, Haverhill, MA). The current was
digitized at a sampling rate of 44 kHz using a VR-10B digital data
recorder (Instrutech) and stored on videotape (Hitachi FX600). For
analysis, data stored on the tape were transferred to an IBM-compatible Pentium computer (Gateway 2000) at a rate of 4 kHz and analyzed using
the pClamp software system v. 6.04 (Axon Instruments, Burlingame, CA).
Channel activity was defined as NPo, a product
of channel open probability (Po) and channel
number (N). The NPo was calculated from data samples of 30- to 60-s duration in the steady state as
follows
where ti is the fractional open time
spent at each of the observed current levels. Because three types of K
channels have been identified in the mTAL (1, 28, 31), we
measured the channel current at three different membrane potentials in
each patch to estimate the conductance of the K channel in the patch.
Solution and statistics.
The pipette solution was composed of (in mM) 140 KCl, 1.8 Mg2Cl, 1.8 Ca2Cl and 5 HEPES (pH 7.4).
Staurosporine, calphostin C, and 17-octadecynoic acid (17-ODYA) were
purchased from Biomol and dissolved in pure ethanol. Indomethacin and
nordihydroguaiaretic acid were obtained from Sigma and dissolved in
methanol. The final concentrations of ethanol or methanol were less
than 0.1% and had no effect on channel activity. Data are presented as
means ± SE. We used paired and unpaired Student's
t-tests to determine the statistical significance.
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RESULTS |
We examined the activity of apical K channels, as defined by
NPo, in the mTAL obtained from animals on a
K-deficient diet and on a high-K diet for 7-10 days, and results
are summarized in Fig. 1. In
the mTAL from rats on a K-deficient diet, the mean NPo/patch of the 70- and 30-pS K channels
were 0.11 ± 0.01 and 0.18 ± 0.01 (n = 41),
respectively. In tubules from rats on a high-K diet, the mean
NPo/patch of the 70- and 30-pS K channel increased to 0.80 ± 0.07 and 0.60 ± 0.06 (n = 63), respectively. Thus it is apparent that the activity of the
apical K channels was significantly lower in the mTAL from rats on a
K-deficient diet than those on a high-K diet. Because previous studies
have indicated that 20-HETE blocked the 70-pS K channel in the mTAL (34), we examined the possibility that an increase in
20-HETE production was responsible for decreasing the activity of the apical 70-pS K channel in the mTAL from rats on a K-deficient diet. We
used GC-MS to measure the intracellular 20-HETE concentrations. Figure
2 is a representative recording from four
such experiments showing that the 20-HETE concentration in the mTAL
from rats on a K-deficient diet was 4.6 pg/µg protein. This was
significantly higher than that observed in tubules from rats on a
high-K diet (0.8 pg/µg protein).

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Fig. 1.
Channel activity, defined by the number of channels in a
patch and the open probability of single channels
(NPo)/patch, of the apical 30- and 70-pS K
channels in the tubules from rats on a high-K diet or on a K-deficient
diet (low K), respectively. *Difference is significant
(P < 0.05).
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Fig. 2.
20-Hydroxyeicosatetraenoic acid (20-HETE)
concentrations measured by gas chromatography-mass spectrometry in the
tubules from animals on a K-deficient diet (left) and on a
high-K (middle) diet, respectively. Bottom trace
shows the peak of 20-HETE standard. Right: results of 4 experiments. *Significantly different from control, P < 0.05.
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If an increase in 20-HETE concentrations was responsible for decreasing
the channel activity, inhibition of cytochrome P-450
-hydroxylation of AA with agents such as
N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS)
(21, 30) should increase channel activity in the mTAL from
rats on a K-deficient diet. We examined the effect of DDMS on the 70-pS
K channel (30) in the tubules obtained from rats on a
K-deficient or a high-K diet. Figure 3
shows the effect of DDMS on the apical 70-pS K channel in the mTAL from
rats on a K-deficient diet. In the absence of DDMS, channel activity
was low (NPo = 0.05) in this particular
case. Addition of 5 µM DDMS stimulated the activity of the 70-pS K
channel and increased NPo to 1.90. Figure
4 summarizes the results of experiments
in which the effect of DDMS on channel activity was tested. Inhibition of cytochrome P-450 oxygenase increased the mean
NPo of the 70-pS K channel from 0.11 ± 0.01 to 0.91 ± 0.1 (n = 17) in the tubules from
rats on a K-deficient diet. In contrast, DDMS had no significant effect
on the activity of the 30-pS K channel (Fig. 4). This suggested that
high concentrations of cytochrome P-450 metabolites of AA were involved in suppressing the activity of the 70-pS K channel in the
tubules from rats on a K-deficient diet. In contrast, inhibiting cytochrome P-450 metabolism of AA caused a modest increase
in NPo of the 70-pS K channel in the mTAL from
rats on a high-K diet (Fig. 5). In 19 experiments, we observed that inhibiting cytochrome P-450
metabolism of AA increased NPo by 25 ± 3%, from 0.64 ± 0.07 to 0.81 ± 0.1, in the tubules
harvested from rats on a high-K diet.

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Fig. 3.
Channel recording showing the effect of
N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) on the
activity of the 70-pS K channel in the medullary thick ascending limb
(mTAL) from animals on a K-deficient diet. The experiments were
performed in a cell-attached patch, and the pipette holding potential
was 0 mV. Top trace is time course of the experiments. Two
parts of the data, indicated by numbers, are extended at a fast-time
resolution. The channel closed level is indicated by C.
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Fig. 4.
Channel activity (NPo/patch) of
the apical 30- and 70-pS K channels in the mTAL from animals on a
K-deficient diet. The channel activity was measured before and after
treatment with DDMS/17-octadecynoic acid (17-ODYA) for 20 min.
*Significantly different from control, P < 0.05.
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Fig. 5.
Channel recording showing the effect of DDMS on the activity of the
70-pS K channel in the mTAL from animals on a high-K diet. The
experiments were performed in a cell-attached patch, and the pipette
holding potential was 0 mV. Top trace is time course of the
experiments. Two parts of the data, indicated by numbers, are extended
at a fast-time resolution. The channel-closed level is indicated by
C.
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It has been reported that renal PGE2 production increased
during hypokalemia (17, 18). To determine the role of
cyclooxygenase metabolites of AA in mediating the effect of dietary K
intake on channel activity, we explored the effect of indomethacin on channel activity. Figure 6 summarizes
results demonstrating that indomethacin (5 µM) increased
NPo slightly from 0.12 ± 0.02 to 0.25 ± 0.06 (n = 6) in the mTAL from rats on a K-deficient
diet. This suggests that cyclooxygenase metabolites of AA are not
mainly responsible for suppressing channel activity in the mTAL from animals on a K-deficient diet. Also, Fig. 6 shows that inhibiting lipooxygenase with 5 µM nordihydroguaiaretic acid had no effect on
channel activity (NPo = 0.11 ± 0.02;
n = 3).

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Fig. 6.
Effects of indomethacin (5 µM) and nordihydroguaiaretic
acid (NDGA, 5 µM) on 70-pS K channel activity in the mTAL from rats
on a K-deficient diet. Experiments were performed in cell-attached
patches. *Significantly different from control, P < 0.05.
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To exclude the possibility that the sensitivity of the 70-pS K channel
to 20-HETE diminished in the mTALs from rats on a high-K diet, we
examined the effect of 20-HETE on the 70-pS K channel in an inside-out
patch in the mTAL from animals on a high-K diet or on a K-deficient
diet. Figure 7 shows that the
dissociation constant value required for inhibition of the channel
activity by 50% is ~5 nM in the tubules obtained from animals on a
high-K diet as well as on a K-deficient diet. This finding excluded the possibility that the diminished response of the 70-pS K channel to
inhibition of P-450 metabolism of AA is due to an alteration in the sensitivity of the 70-pS K channel to 20-HETE. This notion was
also supported by observations that application of 20-HETE (100 nM)
inhibited channel activity in a cell-attached patch by 90% in the
presence of DDMS and that removal of 20-HETE restored the channel
activity (Fig. 8).

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Fig. 7.
Dose-response curves of the 70-pS K channel to 20-HETE in
the mTAL from rats on a high-K diet ( ) and on a
K-deficient diet ( ). The experiments were carried out in inside-out
patches.
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Fig. 8.
Effect of 20-HETE (100 nM) on the 70-pS K channel in the mTAL from
a rat on a high-K diet. DDMS was present throughout the experiments
that were carried out in a cell-attached patch. Top trace is
time course of the experiments. Three parts of the data are extended to
show the fast-time resolution. The channel closed level is indicated by
C.
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That high concentrations of 20-HETE were responsible for decreasing
channel activity in the mTAL from rats on a K-deficient diet was also
indicated by observations that pretreatment of the mTAL with DDMS
significantly increased the activity of the 70-pS K channels (Fig.
9). The pretreatment of the tubules with
DDMS/17-ODYA increased NPo from 0.10 ± 0.01 to 0.76 ± 0.07 (n = 34). In contrast, pretreatment of the mTALs failed to increase the channel activity significantly in the mTALs from animals on a high-K diet (control 0.80 ± 0.07, experiment 0.87 ± 0.07; n = 37).

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Fig. 9.
Channel activity (NPo/patch) of
the apical 70-pS K channels in the mTAL from animals on a K-deficient
diet (low K) and on a high-K diet. The channel activity was measured
before and after treatment with DDMS/17-ODYA for 20 min. *Significantly
different from control, P < 0.05.
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DISCUSSION |
The TAL is responsible for the reabsorption of 20-25% of the
filtrated Na load and has a key role in urinary concentrating mechanisms (11, 13). Na enters the cell through the
luminal Na-K-Cl cotransporter, energized by a favorable electrochemical gradient of Na, and then is pumped out of the cell through the basolateral Na-K-ATPase. K recycling is essential for maintaining the
normal function of the Na-K-Cl cotransporter (11, 14). First, K recycling hyperpolarizes the cell membrane and accordingly provides the driving force for Cl leaving the cell. Second, K recycling
participates in generating the lumen positive potential, which is the
driving force for transepithelial Na, Ca2+, and
Mg2+ transport. Third, K recycling is important for
providing an adequate supply of K to the Na-K-Cl cotransporter in the
cortical TAL where K concentration is at least one order of magnitude
lower than that of Na and Cl.
Three types of K channels (1, 31),
Ca2+-activated large-conductance (>100 pS),
intermediate-conductance (70-74 pS), and small conductance
(30-40 pS), have been identified in the apical membrane of the rat
TAL (28). The large-conductance K channel has a low open
probability under physiological conditions. Thus it is unlikely that
the large-conductance K channel could play a significant role in K
recycling across the apical membrane (32). It is generally
believed that the 70- and 30-pS K channels are mainly responsible for K
recycling (32) because both channels have a high channel
open probability and are frequently found under physiological
conditions (1, 31). Furthermore, patch-clamp experiments
revealed that the 70- and the 30-pS K channel contribute to the apical
K conductance by ~80 and 20%, respectively (34). Thus
the 70-pS K channel should have a key role in K recycling.
In the present study we have found that the apical 30- and 70-pS K
channels are regulated by a dietary intake of K; a high intake of K
increases, whereas a low intake of K decreases the activity of the
apical K channels. Because K recycling is essential for maintaining
normal function of the Na-K-Cl cotransporter, it is conceivable that
diminished K conductance in the apical membrane could reduce the
turnover rate of the cotransporter. Thus it is possible that the K
depletion-induced decrease in channel activity in the mTAL is involved
in decreasing NaCl transport and urinary concentrating ability in the
kidney. The mechanism by which the dietary K intake changes the apical
K channel activity in the mTAL is not completely understood. It was
reported that K-depletion decreases the expression of ROMK channels as
well as the Na-K-Cl cotransporter (20). This indicates
that a downregulation of the K channels and the cotransporters in the
mTAL should be partially responsible for the hypokalemia-induced
impairment of NaCl transport. On the other hand, the expression of both
ROMK channels and the Na-K-Cl cotransporter is the same in the tubules from animals on a high-K diet as those on a normal diet
(20). This indicates that a posttranslation is also
involved in mediating the effect of the dietary K intake on the apical
K channels and the cotransporters. This notion is strongly suggested by
the present finding that inhibiting cytochrome
P-450-dependent metabolism of AA increased the activity of
the apical 70-pS K channel in the mTAL from rats on a K-deficient diet.
Because we could observe the effect of DDMS in 15 min, it is unlikely
that the effect of DDMS was the result of increasing protein synthesis
of the 70-pS K channel. Therefore, it is most likely that inhibition of
cytochrome P-450 metabolism of AA activates the previously
silent K channels. This view is also supported by our unpublished
observations that the NPo of the 70-pS K channel
increased after excision to form inside-out patches in the mTAL from
rats on a K-deficient diet. This suggests that the channel activity is
suppressed by an endogenous inhibitor. Because renal PGE2
concentrations increased significantly during hypokalemia,
PGE2 was suggested to be responsible for impairing urinary
concentrating ability (17, 18). However, addition of
indomethacin to block cyclooxygenase could only partially improve the
transport function of the TAL in the loop of Henle (17), suggesting that PGE2 is not mainly responsible for the
hypokalemia-induced impairment of NaCl transport in the TAL. This
notion also has been supported by the finding that indomethacin only
modestly increased channel activity in the mTAL from rats on a
K-deficient diet.
Several lines of evidence indicate that 20-HETE, a major cytochrome
P-450-dependent metabolite of AA in the mTAL
(4), has a key role in mediating the effect of dietary
intake of K on the apical 70-pS K channels. First, the production of
20-HETE was fourfold higher in the mTALs from animals on a K-deficient
diet than from those on a high-K diet. Second, inhibition of cytochrome P-450 monooxidase resulted in a significantly larger
increase in the activity of the 70-pS K channel in the mTAL from rats
on a K-deficient diet than from those on a high-K diet. Third,
pretreatment of the mTAL with DDMS/17-ODYA raised the
NPo of the 70-pS K channel in the mTAL from rats
on a K-deficient diet to an extent similar to that observed in the
tubules from animals on a high-K diet. We have demonstrated that the
effect of inhibiting cytochrome P-450 monooxidase was
significantly attenuated in the mTAL from rats on a high-K diet
compared with those on a K-deficient diet. This was not the result of
diminished response of the 70-pS K channel to 20-HETE because the
dissociation constant value required for inhibition of the channel
activity by 50% was almost identical in tubules from rats on a high-K
diet or a K-deficient diet. Therefore, it is possible that an increase
in 20-HETE production is involved in decreasing the activity of the
apical 70-pS K channels.
The major isoform of cytochrome P-450 monooxidase in the
mTAL is the CYP4A family (21, 30), which converts AA to
20-HETE and epoxides (epoxyeicosatrienoic acids, EETs) such as
11,12-EET and 5,6-EET (3, 19, 21). Several studies
have demonstrated that EETs have an important role in the regulation of
ion channels. Whereas application of 20-HETE inhibited the
Ca2+-activated maxi-K channels, EETs have been reported to
activate the maxi-K channel (15, 19). Our preceding
observation that addition of 11,12-EET had no significant effect on the
70-pS K channel did not support the role of EETs in regulating the
channel activity directly (16). Moreover, in the present
study we have found that 5,6-EET had no effect on the activity of the
70-pS K channel (unpublished observation). However, we need further experiments to explore the possibility that EETs could reverse the
inhibitory effect of 20-HETE. Accordingly, if concentrations of EETs
fall in the mTAL from rats on a K-deficient diet, the inhibitory effect
of 20-HETE should be enhanced.
Several studies have shown that 20-HETE is an important second
messenger for regulation of renal function, including epithelial transport (7, 8, 24, 34). Escalante et al. (7,
8) have shown that 20-HETE inhibits the activity of Na-K-Cl
cotransporter. We have demonstrated previously that 20-HETE mediated
the effect of stimulation of Ca2+-sensing receptors and the
effect of angiotensin II (16) on the apical 70-pS K
channel (35). In addition, salt intake is an important
regulator for 20-HETE production. An increase in Na intake has been
reported to increase the formation of 20-HETE (19, 25). A
decrease in 20-HETE generation in response to high intake of Na was
involved in inducing the salt-sensitive hypertension in animal models
(19). In the present study, we have observed that low
intake of K could also increase the 20-HETE generation. However, the
mechanism by which low intake of K enhanced the formation of 20-HETE is
not clear. There are at least three possibilities: 1) the AA
release may be stimulated in the mTALs from rats on a K-deficient diet,
2) the activity of cytochrome P-450 monooxidase
may be upregulated, and 3) the ratio between 20-HETE and
EETs may change in response to the dietary intake of K. Further
experiments are required to explore the mechanism of the effect of
dietary K intake on 20-HETE formation.
Another finding of the present study is that inhibition of cytochrome
P-450 monooxidase had no significant effect on the 30-pS K
channel. This observation is also consistent with our previous finding
that 20-HETE failed to inhibit ROMK channels expressed in oocytes. On
the other hand, it is clearly shown that the dietary intake of K
regulates the number of the 30-pS K channel. Thus it is possible that
the mechanisms by which the low-K intake decreases the activity of the
apical 30 pS are different from those of the apical 70-pS K channel. We
have shown previously that protein tyrosine kinase has a key role in
mediating the effect of dietary K on the apical low-conductance K
channels in the cortical collecting duct; inhibition of protein
tyrosine kinase increased the number of the apical K channels
(33). We have postulated that an increase in the activity
of protein tyrosine kinase is responsible for suppressing the activity
of the apical low-conductance K channels in the collecting duct from
rats on a K-deficient diet. Because the apical 30-pS K channel in the
mTAL and the apical low conductance (30- to 40-pS) K channel in the
collecting duct are generally believed to be closely related to ROMK
channels, it is possible that the same mechanism may be responsible for
mediating the effect of dietary intake of K on both K channels.
We conclude that the activity of the apical K channels in the mTAL is
regulated by dietary intake of K and that an increase in 20-HETE
generation is involved in inhibiting the activity of the apical 70-pS K
channels in the mTAL from rats on a K-deficient diet.
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ACKNOWLEDGEMENTS |
The work was supported by National Institutes of Health Grants
HL-34300 and DK-47402.
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FOOTNOTES |
Address for reprint requests and other correspondence: W.-H.
Wang, Dept. of Pharmacology, New York Medical College, Valhalla, NY
10595 (E-mail: wenhui_wang{at}nymc.edu).
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"
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Received 12 June 2000; accepted in final form 3 October 2000.
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