Role of luminal anion and pH in distal tubule potassium
secretion
J. B. O.
Amorim1,
M. A.
Bailey2,
R.
Musa-Aziz3,
G.
Giebisch2, and
G.
Malnic3
1 Basic Science Department, Faculdade de Odontologia
de São José dos Campos, and 3 Department
of Physiology and Biophysics, Instituto de Ciências
Biomédicas, Universidade de São Paulo, 05508-900 São Paulo, Brazil; and 2 Department of Cellular
and Molecular Physiology, Yale University School of Medicine, New
Haven, Connecticut 06510
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ABSTRACT |
Potassium secretory flux
(JK) by the distal nephron is regulated by
systemic and luminal factors. In the present investigation, JK was measured with a double-barreled
K+ electrode during paired microperfusion of superficial
segments of the rat distal nephron. We used control solutions
(100 mM NaCl, pH 7.0) and experimental solutions in which
Cl
had been replaced with a less permeant anion and/or
pH had been increased to 8.0. JK increased
when Cl
was replaced by either acetate (~37%), sulfate
(~32%), or bicarbonate (~62%), and also when the pH of the
control perfusate was increased (~26%). The majority (80%) of
acetate-stimulated JK was Ba2+
sensitive, but furosemide (1 mM) further reduced secretion (~10% of
total), suggesting that K+-Cl
cotransport was
operative. Progressive reduction in luminal Cl
concentration from 100 to 20 to 2 mM caused increments in
JK that were abolished by inhibitors of
K+-Cl
cortransport, i.e., furosemide and
[(dihydroindenyl)oxy]alkanoic acid. Increasing the pH of the luminal
perfusion fluid also increased JK even in the
presence of Ba2+, suggesting that this effect cannot be
accounted for only by K+ channel modulation of
K+ secretion in the distal nephron of the rat.
Collectively, these data suggest a role for
K+-Cl
cotransport in distal nephron
K+ secretion.
distal tubule; collecting duct; potassium secretion; postassium-chloride cotransport
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INTRODUCTION |
A LARGE NUMBER OF
FACTORS modulate potassium (K+) secretion by the
initial and cortical collecting tubules (10). Luminal modulators include changes in the delivery of Na+
(12, 26), the transepithelial potential difference
(9), the concentration of Ca2+
(21), the presence of impermeant anions, and the
concentration of Cl
relative to that of other anions
(6).
K+ secretion rises sharply when impermeant anions are
present in the lumen of the distal tubule (27). Earlier
studies attributed this effect to the increase in the lumen-negative
transepithelial potential difference generated by persistent
Na+ reabsorption in the face of the decrease in anion
conductance resulting from the replacement of permeable
Cl
by a much less permeant species, such as sulfate
(5, 11). In vivo voltage-clamp experiments confirm the
transepithelial potential difference as an important modulator of
K+ secretion in the distal tubule (9).
Nevertheless, the stimulatory effect of sulfate on K+
secretion persists even when the lumen-negative potential difference is
collapsed by amiloride (27) and is not affected by the
inclusion of Ba2+ in the perfusate (6).
Therefore, it is not possible to attribute the effect of anions on
K+ secretion solely to changes in transepithelial potential
difference, and it has been proposed that substitution of
Cl
by sulfate may enhance electroneutral K+
secretion by creating a favorable transmembrane gradient for K+-Cl
cotransport from cell to lumen
(6).
Similar conclusions can be reached from studies in humans. Infusions of
Na sulfate have a greater kaliuretic effect than do those of NaCl
(4), although this is only apparent when the urinary
Cl
concentration is <15 mM. That the ability of the
impermeant sulfate to modulate K+ excretion is critically
dependent on urinary Cl
concentration provides strong, if
circumstantial, evidence for K+-Cl
cotransport as a pathway for K+ secretion in the distal
nephron. These studies further reveal that infusions of Na bicarbonate
stimulate K+ excretion to a greater extent than do those of
Na sulfate (4). In contrast to the actions of sulfate, the
kaliuretic effect of bicarbonate was not dependent on a concomitantly
low urinary Cl
concentration. These observations would
suggest that bicarbonate ions wield an additional influence on
K+ secretion and thus excretion.
We have used stationary microperfusion of superficial segments of the
rat distal tubule to investigate the role of luminal anions and pH,
either separately or in combination, in modulating K+
secretion. We observed that K+ secretion was greater under
conditions in which Cl
was replaced by either sulfate or
acetate. This increment in K+ secretion could be abolished
by known inhibitors of K+-Cl
cotransport,
suggesting that the effect of both sulfate and acetate was indirect,
attributable to low luminal Cl
concentration. Replacement
of Cl
with bicarbonate induced a further increment in
K+ excretion. Our experiments suggest that modulation of
K+-Cl
cotransport by luminal fluid pH plays a
key role in this action of bicarbonate on K+ secretion.
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METHODS |
Male Wistar rats (wt range 180-320 g), kept on a standard
rat chow (plasma K+ concentration = 4.0 ± 0.7 mM), were anesthetized with Inactin (100 mg/kg ip) and prepared
surgically for micropuncture experiments. During the experiments, body
temperature was maintained at 37°C, and isotonic saline,
containing 3% mannitol, was infused intravenously at a rate of 4 ml/h.
Stationary microperfusion.
The technique employed in these experiments (1) is
illustrated in Fig. 1. A proximal tubule
was impaled by a double-barreled micropipette, and FD&C green control
solution was injected from barrel P to localize distal
tubule segments with one or more surface loops (Fig. 1A).
Once the K+-selective microelectrode (see below) had been
inserted into the last surface loop of the distal segment (Fig.
1B), a large column of Sudan-black-colored castor oil was
injected into the proximal tubule through barrel C to
prevent downstream flow of native tubular fluid. The oil column was
subsequently split by injection of the control solution at a rate
sufficient to lower distal tubular K+ concentration, as
recorded by the electrode, to the level of the perfusion fluid. After
control measurements, the experimental solution was injected through a
single-barreled micropipette (P) inserted into either a late proximal
loop or the early distal tubule (Fig. 1C), and recordings
were made again. In a typical experiment, each nephron was perfused two
to three times with both an experimental and control solution, allowing
paired measurement of K+ secretion. In each animal, one to
three nephrons were perfused in the manner described.

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Fig. 1.
Schematic drawing of the stationary microperfusion
method. A: a superficial segment of a proximal convoluted
tubule is punctured with a double-barreled micropipette containing
Sudan-black colored castor oil (C) and control perfusate (P).
B: a late superficial segment of the distal tubule is
punctured with a double-barreled microelectrode, consisting of a
potassium-selective barrel (IE) and a reference barrel (Ref) connected
to a high-impedance electrometer (E). C: a single-barreled
micropipette containing the experimental solution (S) is inserted into
a late segment of the proximal tubule or, as shown, the early distal
tubule.
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Solutions and chemicals.
The composition of the solutions used in the present study are
presented in Table 1. It should be noted
that the control solution (100 mM NaCl) was bicarbonate free and used
at either pH 7.0 or 8.0. In all of the other solutions, NaCl, reduced
to 20 or 2 mM, was replaced by sodium salts of acetate, sulfate, or
bicarbonate. Acetate was used because it exerted a similar effect to
that of sulfate and has a soluble barium salt. Raffinose pentahydrate
(Riedel-de Haën, Hannover, Germany) was added to each solution to
establish isosmolality with plasma. Under these equilibrium conditions,
some Na+ reabsorption may persist but fluid reabsorption is
minimized so that increases in luminal K+ concentration
relate directly to K+ secretion. Furosemide and
[(dihydroindenyl)oxy]alkanoic acid (DIOA), both established
inhibitors of K+-Cl
cotransport (7,
29), were dissolved in dimethylsulfoxide before dilution;
vehicle alone (0.5%) was included in the control solutions.
Hexamethyldisilazane and K ionophore cocktail A were both obtained from
Fluka (Buchs, Switzerland). All other reagents were from Sigma.
Ion-selective microelectrodes.
Double-barreled microelectrodes were pulled from assymetric
borosilicate glass to a tip diameter of ~1 µm. The smaller,
reference barrel was backfilled with a solution containing 0.24 M NaCl
and 0.76 M Na acetate, colored by FD&C green. The total cation/anion mobility of this solution was similar because the mobility in a
solution of Cl
is larger than that of Na+,
whereas that of acetate is smaller. The animal was grounded (tail), and
measurements of transepithelial potential difference were made through
the reference barrel.
The larger barrel was exposed for 30 min at room temperature to
hexamethyldisilazane before the tip was filled with K+
ionophore. Silanized electrodes were kept in a desiccator until the day
of the experiment, when they were filled with the electrolyte solutions
(1). The electrode was calibrated before and after each
impalement by superfusion onto the kidney surface of standards (kept at
37°C). Standards of 3, 10, and 30 mM KCl were used, and 100 mM NaCl
was added to each standard to compensate for the fact that the
K+ ionophore has an inherent sensitivity to sodium. The
mean voltage difference per decade change in K+
concentration was similar to values previously reported by us (1). The voltage difference between the reference and
ion-selective barrels, representing luminal K+ activity,
was recorded using a differential, high-impedance electrometer (model
223, WPI, New Haven, CT) and sampled every second by an analog-digital
converter (Lynx, São Paulo, Brazil).
Analysis and statistics.
For each perfusion, luminal K+ activity increased from the
initial value of 0.5 mM, given as [K+]o, to a
stationary K+ concentration
([K+]s) as a result of K+
secretion into the nephron. These data were analyzed by a
Visual Basic program that fit an exponential to the approach of luminal K+ activities to the stationary level, i.e., by plotting
the increase in luminal K+ activity against time. The
half-time (t1/2) of the approach of K+ activity to the stationary level was calculated from
this exponential, and secretory K+ fluxes
(JK) were obtained by the following
relationship, where r is tubule radius and other terms are
as defined above
A value for each variable was obtained from individual
perfusions: the mean of repeated perfusions under each condition was calculated to give a pair of values for each tubule, and n
corresponds to the number of tubules perfused. Statistical
comparisons were made using Student's paired t-test or,
when nonpaired groups were compared, analysis of variance and the
Bonferroni contrast test. The probability of 0.05 was taken as the
limit of statistical significance.
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RESULTS |
A representative graph plotting the approach of the luminal
K+ concentration to the stationary value (i.e.,
Ks
luminal K+ concentration)
against time is shown in Fig. 2.
Figure 2A details the secretory response under control
conditions (100 mM NaCl). Similar data obtained with a solution in
which Cl
was largely replaced by sulfate is depicted in
Fig. 2B. Data were obtained from the same superficial distal
tubule, and paired data sets were acquired for each set
of experimental conditions. These curves were used to calculate
t1/2, an index of the rate of K+
secretion. According to the equation cited in METHODS,
JK depends on [K+]s
and on the rate of the approach to [K+]s from
the initial perfusion value of 0.5 mM (i.e.,
[K+]o), given as
ln2/t1/2. From an inspection of Table
2, it is apparent that paired values for
[K+]s were similar for Cl
replacement with acetate, sulfate, or bicarbonate. Modulation of
K+ secretion by acute changes in luminal components thus
reflects increases or decreases in t1/2. It is
apparent that the rate at which the steady-state luminal K+
concentration is established is greater, and
t1/2 is smaller, when Cl
is
replaced by a less permeant anionic species.

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Fig. 2.
Approach of luminal K+ concentration
([K+]) to steady-state level
([K+]s) from the initial perfusate
concentration of 0.5 mM ([K+]o). The change
in [K+], which equals
[K+]s [K+] at time
t, is plotted as a function of time (s). ,
Experimental points; solid lines, exponentials fitted to data.
A: control perfusion (100 mM NaCl). B: perfusion
of the same nephron segment using a solution in which 80 mM
Cl has been replaced by sulfate.
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Table 2.
Stationary potassium concentration and approach to half-time during
paired perfusion of the late distal tubule
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Calculation of JK from
[K+]s
[K+]o
and t1/2 yields the data presented in Figs.
3-6.
A significant increase in JK was observed when
either sulfate or acetate replaced luminal Cl
. When the
principal anion was bicarbonate, a further increment was recorded (Fig.
3). As shown in Table 3, perfusion with
sulfate-containing solutions resulted in only a minor increase in the
lumen-negative transepithelial potential difference, whereas neither
bicarbonate nor acetate altered this difference. These data indicate
that the role of changes in transepithelial potential difference in mediating K+ secretion is minor.

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Fig. 3.
Net K+ secretory flux
(JK) calculated from
[K+]s and t1/2,
during paired perfusion of the distal tubule with control solutions
(Cl ) and solutions in which Cl has been
replaced by acetate, sulfate, or bicarbonate, as detailed in Table 1.
Values are means ± SE; n = 9, 14, and 18 tubules,
respectively. *P < 0.05, **P < 0.01, paired t-test. The effect of bicarbonate was significantly
greater than that of either acetate or sulfate (P < 0.05, unpaired t-test).
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Fig. 4.
Net JK during paired perfusion of
Cl and acetate and of acetate plus barium (Ba) and
acetate plus barium plus furosemide (furo). Ba (3 mM) reduced
acetate-stimulated JK by ~80%
(P < 0.01). On addition of furosemide (1 mM) to this
perfusate, the flux decreased further to ~11% of control values
(P < 0.01). Values are means ± SE;
n = 14 tubules for each paired condition. *P < 0.05, **P < 0.01, compared with Cl .
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Fig. 5.
Top: JK increases as
luminal Cl concentration decreases from 100 (P < 0.05) to 2 mM (P < 0.01), ANOVA.
Bottom: stimulatory effect of reducing luminal
Cl concentration is abolished by either furosemide (1 mM;
P < 0.01) or [(dihydroindenyl)oxy]alkanoic acid
(DIOA; 100 µM; P < 0.01), inhibitors of K-Cl
cotransport. DIOA (100 µM) has no significant effect during
perfusions with 100 mM Cl .
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Fig. 6.
K+ secretion in the distal tubule during
luminal perfusion with the control perfusate (100 mM NaCl) at pH 7.0 and 8.0. Values are means ± SE; n = 11 (pH) and
18 (bicarbonate) tubules. Increasing the pH of the luminal fluid
significantly increased JK (*P < 0.05 vs. pH 7.0), although this effect was smaller than that of
bicarbonate at the same pH (**P < 0.05 vs.
bicarbonate).
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Table 3.
Transepithelial potential difference in the late distal tubule during
perfusion with solutions of differing principal anions
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To assess the role of channels in mediating K+ secretion,
Ba2+ was added to an acetate-containing perfusate.
Ba2+ markedly reduced JK (Fig. 4),
leaving a residual K+ secretion of 0.18 nmol · cm
2 · s
1. The
remaining JK was of a similar magnitude to the
increment in K+ secretion resulting from replacement of
Cl
with acetate (0.23 nmol · cm
2 · s
1),
suggesting that the effect of acetate was not mediated through ion
channels. Addition of furosemide (1 mM) to the solution further reduced
K+ secretion (Fig. 4). The remainder was still
significantly greater than zero (P < 0.05) and
probably represents passive paracellular flux of K+ driven
along the favorable electrochemical gradient (
42.0 ± 1.7 mV).
Taken together, these observations strongly suggest that electroneutral
K+-Cl
cotransport is operative after the
establishment of a favorable cell-to-lumen Cl
concentration gradient. This hypothesis is supported by the observation that increasing the driving force for K+-Cl
cotransport by reducing luminal Cl
concentration from 20 to 2 mM evoked a further increase in K+ secretion (Fig. 5,
top). With conditions employed herein, a reduction in
luminal Cl
is associated with an increase in luminal
sulfate, and we cannot preclude a direct effect of the replacement
anion. Nevertheless, it is noteworthy that either furosemide or DIOA (a
more selective inhibitor of K+-Cl
cotransport) was able to reduce anion/low-Cl
-stimulated
K+ secretion to levels observed with 100 mM
Cl
(Fig. 5, bottom). In addition, in the
presence of 100 mM Cl
in the lumen, 100 µM DIOA did not
affect K+ secretion significantly. These experiments thus
imply that the magnitude of the cell-to-luminal Cl
concentration gradient rather than the nature of the luminal anion
accounts for the augmentation of K+ secretion.
In a series of further experiments, we investigated the effect of
changes in luminal pH on K+ secretion because the effects
of bicarbonate could be attributed to alkalinization of the tubular
fluid. Increasing pH of the control (100 mM NaCl) solution from 7.0 to
8.0 resulted in a significant stimulation of K+ secretion
(Fig. 6). However, this increment was significantly less than that
resulting from perfusion of the lumen with a bicarbonate solution, also
at pH 8.0. It is notable that the increment in JK resulting from alkalinzation of the tubular
fluid was similar in magnitude to the difference between the respective
stimulations resulting from bicarbonate and from sulfate. To further
investigate the mechanism by which alkalinization of the luminal
perfusate enhanced K+ secretion, we performed the
experiments depicted in Fig. 7.
Increasing the pH of the acetate-containing perfusate from 7.0 to 8.2 resulted in a marked increase in K+ secretion, despite the
presence of Ba2+ in both solutions.

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Fig. 7.
Increasing the pH of the acetate perfusate from 7.0 to
8.2 stimulates JK despite the continued presence
of Ba (3 mM). Values are means ± SE; n = 16 tubules; **=P < 0.01.
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DISCUSSION |
K+ secretion in the distal nephron proceeds by two
distinct processes: active uptake of K+ in exchange for
Na+ by the Na+-K+-ATPase across the
basolateral membrane provides the driving force for translocation
across the apical membrane along a favorable electrochemical gradient.
The present investigation concerns the mechanisms by which changes in
the composition of the tubular fluid, independently of maneuvers that
alter the peritubular fluid, affect K+ secretion. We have
specifically focused on the mechanisms by which changes in the
concentration of distal tubular bicarbonate and other anions modulate
K+ transport.
Previous studies have firmly established several luminal factors known
to alter K+ secretion in the distal convoluted tubule and
initial and final collecting ducts. These factors include the delivery
of Na+ (12), fluid (13), and
Ca2+ (21), the pH of fluid (3),
and the nature of the principal anionic species (2, 26,
27). Studies in humans have defined the important role of
bicarbonate as a powerful stimulus of K+ secretion
(4, 15, 16) and have shown that its kaliuretic potency
exceeds that of other poorly reabsorbable anions such as sulfate.
In the present study, K+ secretion in the initial
collecting duct was measured under equilibrium conditions in which
Na+ reabsorption was not associated with significant fluid
movement. The main findings were 1) replacement of
Cl
with either sulfate or acetate increased the rate of
K+ secretion; 2) replacement of Cl
with bicarbonate elicited a further increment in K+
secretion; and 3) increasing the pH of the control perfusate stimulated K+ secretion.
The effect of sulfate and acetate.
It has been suggested that impermeant anions increase the luminal
negativity of the transepithelial potential difference, thereby
imposing a more favorable electrochemical gradient for K+
secretion (5, 11). That the transepithelial potential
difference regulates K+ secretion is beyond doubt
(9). The importance of this variable in the present study,
however, must be questioned because sulfate induced only a small
increase in the lumen-negative transepithelial potential difference and
both acetate and bicarbonate were without effect. Our data imply that a
change in potential difference would contribute in only a minor way to
the anion-stimulated K+ secretion and are in accord with
earlier studies which found that sulfate can stimulate K+
secretion even when the negative transepithelial potential difference is abolished (27).
K+ channels and
K+-Cl
cotransport.
It is reasonable to conclude that a large fraction of K+
secretion in our experiments was mediated by diffusion across the apical membrane through K+ channels because addition to the
tubular fluid of Ba2+, an effective blocker of both the
small-conductance and the maxi-K channel (10), reduced
JK by ~80% (Figs. 4 and 7). However, the persistence of a residual JK suggests additional
secretory pathways. It is possible that K+-Cl
cotransport participated in distal nephron K+ secretion as
first proposed on the basis of experiments demonstrating that
K+ secretion was stimulated by low concentrations of
luminal Cl
(27), even in the presence of
Ba2+ (6). Ba2+-insensitive
K+ secretion has also been reported in the isolated
cortical collecting duct (20, 24) but only in animals
treated with deoxycorticosterone acetate.
The present study confirmed the presence of a
Ba2+-insensitive component of K+ secretion in
the distal nephron. This component was almost completely inhibited by
both furosemide and DIOA (Figs. 4 and 5). Similarly, maneuvers designed
to increase the cell-to-lumen Cl
concentration gradient
evoked a gradient-dependent increase in JK, an
increase that was eradicated by K+-Cl
cotransport inhibition (Fig. 5). Although the specificity of K+-Cl
cotransport inhibitors is limited, the
most likely interpretation of these findings is that
K+-Cl
cotransport contributes to the
stimulation of JK by acetate, sulfate, and
bicarbonate. Ba2+-insensitive K+ secretion in
the late distal tubule of control rats had previously been demonstrated
(1) and supports the possibility that
K+-Cl
cotransport is operative under normal
conditions. In support of this hypothesis, addition of either
furosemide or DIOA to low- Cl
perfusates reduced
K+ secretion to a level that was slightly below that found
with 100 mM Cl
. However, we observed that addition of
DIOA to 100 mM Cl
-containing perfusates did not cause
significant alteration of K+ transport, suggesting that
K+-Cl
cotransport is only operative when the
cell-to-lumen gradient is favorable (Fig. 5). The observation of a
component of Ba2+-insensitive K+ secretion at
higher luminal or urinary Cl
concentrations (4, 15,
20, 24) suggests the possibility of K+ secretion via
a paracellular transport route.
Effect of bicarbonate and pH.
K+-Cl
cotransport would be expected to
contribute to the stimulation of JK resulting
from the replacement of Cl
with bicarbonate (Fig. 3) for
the reasons cited above. Nevertheless, bicarbonate has additional
effects associated with the alkaline pH of such solutions. There is a
strong correlation between the pH of the tubular fluid and
K+ secretion in the distal nephron (17, 18),
and reducing the perfusate pH from 7.4 to 6.8 impairs K+
secretion in the rabbit cortical collecting duct (3). In
the present study, we found that increasing the pH of the control solution (100 mM NaCl) stimulated distal K+ secretion.
Although the effect of cytoplasmic pH on K+ channel
activity has been repeatedly demonstrated (8, 18), several
observations make it very unlikely that increasing luminal pH may
enhance secretion through the small-conductance K+ channel
of the principal cell (ROMK). First, ROMK channels are insensitive to
changes in extracellular pH (8, 10, 33). Second,
intracellular pH of the principal cell was shown to be unaffected by
changes in luminal pH due to the apparent lack of an acid-base
transporter in the apical membrane (30). Sensitivity of
ROMK to pH seems thus to be limited to the cytosole (8, 18). Finally, addition of Ba2+ to the acetate
perfusate did not ablate the stimulatory effect on
JK of increasing pH from 7.0 to 8.2. This
suggests that luminal pH augments K+ efflux through a
Ba2+-insensitive pathway. Two possibilities should be
considered. The first is that changes in luminal pH activate
K+-Cl
cotransport. Because the cell-to-lumen
Cl
concentration gradient remained constant during
experiments, extracellular pH would have to exert a direct modulating
action on K+-Cl
cotransport. An alternative
hypothesis invokes a role for the TASK2 K+ channel, which
is activated by extracellular alkalinization over the physiological
range yet is relatively insensitive to Ba2+ block
(22). In situ hybridization studies localize TASK2
expression to the cortical distal tubules and collecting ducts, but a
clearly defined role for these channels is lacking.
Taken together, our data suggest an important role of
K+-Cl
cotransport in mediating anion effects
on K+ secretion. Because distal K+ secretion
has been attributed mostly to the principal cell of cortical collecting
duct, we have incorporated K+-Cl
cotransport
into this cell. This suggestion is shown in our model of K+
secretion in the distal nephron (Fig.
8A). The molecular nature of
this transport is unclear, although three of the four known isoforms of
the K+-Cl
cotransporter are present in the
kidney: KCC3 and KCC4 have been found in renal proximal tubules,
particularly in the basolateral membrane (19). KCC1 is
expressed in the apical membrane of the thick ascending limb; more
recent evidence localizes this transporter also to distal tubule and
collecting duct (Mount DB, personal communication). Nevertheless,
alternative explanations for our findings should be considered (Fig.
8B). Stimulation of JK by both the
high luminal pH and bicarbonate could also involve known transport
mechanisms in B-intercalated cells. In these cells, coupling of the
apically located Cl
/bicarbonate exchanger (14, 23,
25, 28, 32) with H+-K+-ATPase has been
suggested to mediate Cl
reabsorption (32).
Under special circumstances, such coordinated activity could also be
involved in KCl secretion. We consider the inversion of active KCl
reabsorption into secretion highly unlikely. However, alkalinization of
B-intercalated cells by reversal of Cl
/ bicarbonate
exchange could reduce H+-K+-ATPase activity and
thus diminish K+ reabsorption. In parallel with continued
K+ secretion via principal cells, enhanced net secretion of
K+ would result. However, this hypothesis must be
considered uncertain because the activity of
H+-K+-ATPase appears to be significant only
when these cells are acidified (31). This issue requires
further investigation, especially in view of the fact that intercalated
cells have been found to display significant heterogeneity with respect
to expression of acid-base transporters (28, 31).

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Fig. 8.
Possible models of collecting duct K+
transport. Top: K+ secretion in the principal
cell involves active uptake of K+ in exchange for
Na+ by Na+-K+-ATPase across the
basolateral membrane, which provides the driving force for
translocation across the apical membrane along a favorable
electrochemical gradient. Luminal K+ transport involves
K+ channels and K-Cl cotransport. Luminal modulators of
K+ secretion include the transepithelial potential
difference, the concentration of luminal Cl and
bicarbonate, and pH. pH affects K+ channels from the
cytoplasmic side, and K-Cl cotransport from the lumen.
Bottom: alternative model involving B-type intercalated cell
with apical H+-K+-ATPase and
Cl /HCO exchange. Modified according to
Zhou et al. (32). CCD, cortical collecting duct.
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In conclusion, our studies show that elevation of bicarbonate in distal
tubules is uniquely potent in stimulating K+ secretion. The
data suggest an important role of K+-Cl
cotransport or coordinated changes in
H+-K+-ATPase activity coupled to
HCO
/Cl
exchange. The present findings
also suggest that from a physiological point of view the proposed
transport mechanism (K+-Cl
cotransport) would
be operative in low-Cl
urines, found in patients with
low-salt diets, and particularly in conditions of increased bicarbonate
excretion such as metabolic alkalosis.
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ACKNOWLEDGEMENTS |
M. A. Bailey was funded by the Wellcome Trust. G. Giebisch was
supported by National Institute of Diabetes and Digestive and Kidney
Diseases Grant DK-17433. R. Musa-Aziz and G. Malnic were supported by
the Fundação de Amparo à Pesquisa do Estado de São Paulo and by Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (Pronex).
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FOOTNOTES |
Address for reprint requests and other correspondence: G. Malnic, Dept. Fisiologia e Biofisica, Inst. Ciencias Biomedicas, USP,
Av. Prof. Lineu Prestes, 1524, 05508-900 São Paulo, Brazil (E-mail: gemalnic{at}usp.br).
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. Section 1734 solely to indicate this fact.
First published October 22, 2002;10.1152/ajprenal.00236.2002
Received 25 June 2002; accepted in final form 15 October 2002.
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