Hypertension and Vascular Research Division, Henry Ford Hospital, Detroit, Michigan 48202
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ABSTRACT |
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First published September 21, 2001; 10.1152/ajprenal.00357.2001.The
macula densa expresses a luminal
Na+-K+-2Cl
cotransporter and a
basolateral Cl
conductance. Although it is known that
cotransport of Na+, K+, and Cl
is
the first step in tubuloglomerular feedback (TGF), subsequent steps are
unclear. We hypothesized that
Na+-K+-2Cl
entry via the luminal
Na+-K+-2Cl
cotransporter elevates
intracellular Cl
, increases electrogenic Cl
efflux across the basolateral membrane, and depolarizes the macula densa, initiating TGF. We perfused afferent arterioles with macula densa attached. The macula densa was perfused with solutions containing either 5 mM Na+ and 3 mM Cl
(low NaCl) or 80 mM Na+ and 77 mM Cl
(high NaCl). When the
macula densa perfusate was changed from low to high NaCl, afferent
arteriole diameter decreased from 15.8 ± 0.8 to 13.1 ± 0.7 mm (P < 0.05). Adding 10 µM furosemide to the macula
densa lumen blocked TGF. When nystatin, a group I cation ionophore, was
added to the macula densa lumen together with furosemide in the
presence of low NaCl, it induced TGF (from 18.0 ± 1.5 to 15.6 ± 1.6 mm; P = 0.003). When valinomycin, a
K+-selective ionophore, was added to the macula densa lumen
together with furosemide in the presence of low NaCl containing 5 mM
K+, it did not induce TGF. Subsequent addition of 50 mM KCl
to the macula densa perfusate induced TGF (from 21.7 ± 0.8 to
17.5 ± 1.3 mm; P = 0.0047; n = 6). Adding 50 mM KCl without valinomycin did not induce TGF. When
5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB; 1 µM), a
Cl
channel blocker, was added to the bath, it blocked TGF
induced by high NaCl, but did not block TGF induced by valinomycin plus 50 mM KCl. NPPB did not alter afferent arteriole constriction induced
by norepinephrine. We concluded that increased NaCl in the lumen of the
macula densa leads to influx of Cl
via the
Na+-K+-2Cl
cotransporter. The
accelerated transport increases intracellular Cl
. The
subsequent exit of Cl
across the basolateral membrane via
Cl
channels in turn leads to depolarization of the
macula densa and thereby induces TGF.
afferent arteriole; transport; chloride channel
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INTRODUCTION |
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TUBULOGLOMERULAR
FEEDBACK (TGF) is generally thought to be mediated by the macula
densa, which detects changes in NaCl concentration ([NaCl]) in the
distal tubule and transmits a feedback signal to the glomerular vessels
(5, 24). TGF is a major component of renal autoregulation,
maintaining homeostasis by regulating renal blood flow and glomerular
filtration of a single nephron (23, 33). Many
investigators have attempted to define the various steps in TGF signal
transmission and characterize the factors that modulate this process.
Numerous in vivo single-nephron micropuncture studies have established
that increased [NaCl] and/or osmolality of the tubular fluid at the
macula densa decreases the single-nephron glomerular filtration rate.
In vivo micropuncture (15, 33) and in vitro studies
(2) showed that furosemide and other loop diuretics that
inhibit NaCl cotransport effectively block TGF responses.
Electrophysiological studies have shown that macula densa basolateral
membrane potential (Vbl) is depolarized in
response to increased luminal fluid [NaCl] (1). Addition of furosemide induced hyperpolarization or blocked depolarization of
Vbl in response to an increase in luminal
[NaCl] (11), thereby indicating that NaCl transport by
the cotransporter is necessary for the change in voltage
(21). Both the TGF response and the Vbl depolarization occurred over the same range
of luminal [NaCl]. Although an active
Na+-K+-2Cl cotransporter is
clearly necessary for TGF, it is not known 1) how macula
densa cells are able to sense changes in tubular Na+,
K+ and/or Cl
concentration
([Na+], [K+], and [Cl
],
respectively) to initiate a TGF response; or 2) whether
changes in macula densa Vbl are indeed necessary
for a TGF response. We hypothesized that Na+,
K+, and Cl
entry via the luminal
Na+-K+-2Cl
cotransporter of the
macula densa results in elevation of intracellular Cl
activity, electrogenic Cl
efflux across the basolateral
membrane, and depolarization of the membrane potential, thereby
initiating TGF.
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METHODS |
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Rabbit afferent arterioles with the macula densa attached were isolated and perfused according to the method routinely used in our laboratory (8-10). Briefly, the left kidney was removed and sliced along the corticomedullary axis, and the slices were placed in ice-cold minimum essential medium (MEM) containing 5% BSA. Superficial afferent arterioles with glomeruli intact were microdissected together with adherent tubular segments consisting of portions of the thick ascending limb of the loop of Henle, macula densa, and early distal tubule. The microdissected complex was transferred (with a micropipette) to a temperature-regulated chamber mounted on an inverted microscope with Hoffmann modulation. Both the afferent arteriole and the end of either the distal tubule or the thick ascending limb were cannulated with an array of glass pipettes. Intraluminal pressure was measured by the Landis technique via a fine pipette introduced into the arteriole through the perfusion pipette. The afferent arteriole was perfused with oxygenated MEM supplemented with 5% BSA and (in mM) 5 NaHCO3, 10 NaCl, 10 HEPES, and 10 NaOH. Intraluminal pressure was maintained at 60 mmHg throughout the experiment. The macula densa was perfused with physiological saline consisting of (in mM) 10 HEPES, 3 KCl, 1.2 MgSO4, 2.0 KPO4, 5 NaHCO3, 5.5 glucose, 1.0 calcium lactate2, and either 74 NaCl (high [NaCl]) or 0 NaCl (low [NaCl]). MEM and physiological saline were oxygenated with 100% O2, and the pH of each solution was 7.4. The bath was similar to the arteriolar perfusate except that it contained 0.1% BSA. Images of the afferent arteriole were displayed at magnifications up to ×1,980 and recorded with a video system consisting of a camera, monitor, and video recorder. Diameter was measured using an image-analysis system (Universal Imaging, West Chester, PA) within 10 min after switching luminal or basolateral solutions.
Nystatin, a group I cation ionophore, and valinomycin, a
K+-selective ionophore, were purchased from Sigma Chemical
(St. Louis, MO), and 5-nitro-2-(3-phenylpropylamino)benzoic acid
(NPPB), a Cl channel blocker, was a generous gift from
Hoechst Marion Roussel Chemical Research (Frankfurt, Germany).
Statistics. Each separate experiment was approached in an analogous manner. The designs were repeated measures with the same afferent arteriole being measured under different conditions. In all cases, a subset of all possible contrasts was specified in advance as being of primary importance. To analyze a specific difference, a paired t-test was used. All tests were two sided, and results are presented as means ± SE. For each experiment, comparisons of interest were considered to be a set, and Holm's multiple-comparisons procedure was used to adjust for multiple testing (26). A value of P < 0.05 was considered significant.
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RESULTS |
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We first demonstrated that furosemide blocks the TGF response
induced by high [NaCl] in our preparation as it does in in vivo micropuncture experiments. When the macula densa perfusate was changed
from low to high [NaCl], afferent arteriole diameter decreased from
17.7 ± 0.9 to 15.0 ± 1.5 µm (P < 0.01)
within 10 min (Fig. 1A). When
the luminal solution was changed back to low [NaCl], diameter
measurements returned to baseline values. After 10 µM furosemide was
added to the lumen of the macula densa, afferent arteriole diameter
increased slightly from 17.6 ± 1.0 to 18.2 ± 0.6 µm. When
the luminal perfusate was changed from low to high [NaCl] in the
presence of furosemide, diameter did not change significantly (from
18.2 ± 0.6 to 18.0 ± 1.0 µm; n = 5). In
time controls, afferent arteriole diameter decreased from 15.8 ± 0.8 to 13.1 ± 0.7 µm (P < 0.05; Fig.
1B) when the solution perfusing the macula densa was first
changed from low to high [NaCl]. When we repeated the process,
diameter decreased from 15.6 ± 0.6 to 13.1 ± 1.1 µm
(P < 0.02; n = 5). Taken together,
these data show that furosemide blocks TGF in our preparation.
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Because it is not clear whether entry of Na+,
Cl, K+, or some combination of these ions is
required for TGF, we first examined the effect of nystatin, a group I
cation ionophore, on TGF. Owing to the differing electrochemical
gradients of Na+ and K+, nystatin primarily
allows Na+ entry. When the solution perfusing the macula
densa was changed from low to high [NaCl], afferent arteriole
diameter decreased from 17.6 ± 1.6 to 15.2 ± 1.8 µm
(P < 0.001). Diameter then increased to 18.0 ± 1.5 µm when the lumen was perfused with low [NaCl] and furosemide.
When nystatin (100 µM) was added to the macula densa lumen in the
presence of low [NaCl] and furosemide, the diameter decreased from
18.0 ± 1.5 to 15.6 ± 1.6 µm (P < 0.003;
Fig. 2) within 10 min. The TGF response
induced by nystatin was not significantly different from that induced
by high [NaCl]. To verify that constriction of the afferent arteriole
was not due to a direct effect of nystatin on the afferent arteriole,
we investigated the ability of the ionophore to constrict isolated
perfused vessels. Nystatin at 10
6 M and 10
5
M had no effect on diameter. However, adding 10
4 M
nystatin to the bath caused a transient decrease in diameter (lasting
<30 s) from 16.8 ± 1.1 to 7.9 ± 2.2 µm, which then
returned to baseline. To test arteriolar contractility, at the end of
the experiment 10
6 M norepinephrine was added to the
bath. All of the arterioles reacted normally. These data show that
nystatin can cause TGF after the
Na+-K+-2Cl
cotransporter is
blocked by furosemide.
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Even with low [Na+] perfusing the macula densa lumen,
nystatin initially increased Na+ entry into the macula
densa due to the negative membrane potential of the cell. To test
whether Na+ entry per se is the initiating step of TGF, we
next examined the effect of valinomycin, a K+-selective
ionophore, on TGF. When the solution perfusing the macula densa was
changed from low to high [NaCl], afferent arteriole diameter
decreased from 21.1 ± 0.8 to 17.8 ± 1.0 µm
(P = 0.003). Diameter then increased to 21.6 ± 0.8 µm when we changed to low [NaCl] plus furosemide. Adding 1 µM
valinomycin to the macula densa lumen in the presence of low [NaCl]
and furosemide did not change diameter significantly. However, when the
KCl concentration ([KCl]) of this solution was increased by 50 mM,
afferent arteriole diameter decreased from 21.7 ± 0.8 to
17.5 ± 1.3 µm (P = 0.0047; n = 6; Fig. 3A).
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To show that the TGF response is not due to the change in
[Cl] and cannot be elicited by high [K+]
alone, we reversed the order of the addition of KCl and valinomycin (Fig. 3B). With low [NaCl] and furosemide perfusing the
macula densa, increasing [KCl] by 50 mM did not affect afferent
arteriole diameter in the absence of valinomycin. However, when
valinomycin was subsequently added, the afferent arteriole diameter
decreased from 19.2 ± 0.34 to 15.7 ± 0.61 µm
(P = 0.0003; n = 6). These data show
that neither 50 mM KCl nor valinomycin alone can induce TGF, but if
both are added to the solution perfusing the macula densa, TGF is initiated.
Blockade of the basolateral Cl channel with NPPB has been
shown to prevent the depolarization induced by increasing [NaCl] at
the macula densa (20, 21). To investigate whether blocking the basolateral Cl
channel blunts TGF, we examined the
effect of NPPB on TGF. We first investigated whether NPPB is able to
block TGF induced by high [NaCl]. When the solution perfusing the
macula densa was changed from low to high [NaCl], afferent arteriole
diameter decreased from 18.2 ± 1.2 to 15.0 ± 1.5 µm
(P = 0.0024; Fig. 4).
Diameter then increased to 18.3 ± 1.1 µm when we changed to low
[NaCl]. Adding 1 µM NPPB to the bath did not alter afferent
arteriole diameter when the macula densa was perfused with low
[NaCl], but did inhibit constriction when the macula densa perfusate
was changed to high [NaCl] (18.8 ± 1.2 vs. 18.7 ± 1.2 µm; n = 6).
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Because NPPB blocked TGF in response to changing [NaCl] at the macula
densa, we next investigated whether NPPB could block arteriolar
constriction induced by depolarization. In the presence of low
[NaCl], afferent arteriole diameter was 20.4 ± 1.4 µm. When
the macula densa perfusate was changed to high [NaCl], diameter decreased to 17.1 ± 1.31 µm. When 50 mM KCl was added to the
macula densa perfusate in the presence of NPPB and valinomycin,
afferent arteriole diameter decreased from 20.7 ± 1.2 to
16.8 ± 1.5 µm (P = 0.002; n = 6;
Fig. 5). These data suggest that NPPB
cannot block TGF induced by valinomycin plus 50 mM KCl.
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To rule out the possibility that the effect of NPPB on TGF is due to
direct alteration of afferent arteriole contractility by NPPB, we
examined the effect of norepinephrine on the afferent arteriole with
and without NPPB. Norepinephrine constricted afferent arterioles in a
dose-dependent manner. In the presence of NPPB, norepinephrine caused
afferent arteriole constriction that was identical to control: diameter
decreased to 73% and 29% of baseline at 107 M and
5 × 10
7 M norepinephrine, respectively. Thus we
eliminated the possibility that NPPB inhibits TGF due to reduced
afferent arteriole contractility. Taken together, our results
demonstrate that Cl
efflux is important for initiation of
the TGF response.
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DISCUSSION |
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The nature of the luminal activator of TGF remains unclear. Early
studies provided evidence that osmolality, Na+,
Cl, and NaCl are critical for initiation of TGF
(14). Accumulated data indicate that an active
Na+-K+-2Cl
cotransporter in the
luminal membrane of the macula densa is necessary for TGF (21,
23) for reasons that remain unclear. Transport of
Na+, Cl
, and/or K+ has numerous
effects, including changes in membrane potential and intracellular
[Na+], [K+], and [Cl
], any
or all of which may be necessary for TGF. In the present study, we
found that 1) nystatin in the macula densa lumen initiates TGF, 2) valinomycin plus 50 mM KCl in the macula densa lumen
initiates TGF, 3) inhibition of basolateral Cl
channels blocks TGF induced by changing [NaCl] at the macula densa,
and 4) inhibition of either the luminal cotransporter or the
basolateral Cl
channels will not block TGF induced by depolarization.
To investigate whether changes in intracellular ion concentrations in
the macula densa initiate TGF, we measured the afferent arteriole
diameter after addition of cation ionophores that increase [Na+] or [K+]. First, we investigated
whether nystatin could initiate TGF. The dramatic increase in
Na+ permeability caused by nystatin increases intracellular
[Na+] (27, 32). The fact that nystatin
causes Na+ entry into the macula densa even in the presence
of 5 mM Na+ (as in our experiments) is based on the
reported resting membrane potential of the macula densa. In the absence
of nystatin, the macula densa membrane potential is approximately 61
mV. When nystatin was added to the macula densa perfusate, the
Na+ and K+ permeabilities of the luminal
membrane dramatically increased. Although in theory nystatin is
nonselective, Na+ permeates nystatin-induced channels to a
greater extent than K+, because K+ is near its
equilibrium potential and Na+ is not.
Our data show that nystatin induces TGF in the presence of low [NaCl]
and furosemide concentration. The vasoconstriction induced by perfusing
the macula densa with nystatin cannot be due to nystatin exiting the
lumen of the tubule and affecting either the basolateral membrane of
the macula densa or the afferent arteriole directly. Once nystatin is
incorporated into the luminal membrane, it does not diffuse past the
tight junctions to the basolateral membrane, and owing to its
lipophilicity it does not easily leave the membrane once it has
entered. Additionally, the flow rate of the macula densa perfusate was
~100 nl/min, whereas the bath flow rate was 106 times
greater (1 ml/min). Finally, when we added nystatin to the bath at
106 M and 10
5 M, it had no effect on
afferent arteriole diameter. However, adding 10
4 M
nystatin did cause a transient decrease in diameter (lasting <30 s) in
isolated afferent arterioles, which then returned to baseline. Given
that 1) the time course was different from the nystatin-induced TGF response, 2) only 10
4 M
nystatin induced constriction, and 3) we used
10
4 M nystatin in the macula densa to induce TGF and thus
the concentration reaching the afferent arterioles must have been
lower, it appears improbable that nystatin-induced TGF was due to a
direct effect on the afferent arterioles.
The discrepancy between nystatin-induced TGF and the direct effect of nystatin on afferent arterioles is likely due to the concentration of nystatin that we used. Different cells require different concentrations of nystatin to change the intracellular [Na+]. Additionally, nystatin would be expected to bind albumin, which would decrease the effective concentration. Because we had albumin in the bath, the effective concentration of nystatin was likely lower than the calculated concentration.
To investigate whether Na+ entry into the macula densa per
se initiates TGF, valinomycin (a K+-selective ionophore)
and high [K+] in the macula densa lumen were used to
enhance K+ entry. This technique is commonly employed to
depolarize vascular smooth muscle cells as well as other cell types
(25). After the
Na+-K+-2Cl cotransporter was
blocked by furosemide, addition of 50 mM KCl to the low-[NaCl] macula
densa perfusate in the presence of valinomycin induced TGF. These data
indicate that Na+ entry per se was not likely the initiator
of TGF.
After consideration of the valinomycin data, we asked what effects
nystatin has on cells in addition to increasing Na+ entry.
The dramatic increase in Na+ permeability caused by
nystatin has been shown to depolarize the membrane potential (27,
32). The fact that nystatin depolarizes the macula densa even in
the presence of 5 mM Na+ (as in our experiments) can be
shown using the Goldman-Hodgkin-Katz equation and the reported resting
membrane potential of the macula densa. In the absence of nystatin, the
macula densa membrane potential is approximately 61 mV. When nystatin
is added to the macula densa perfusate, the Na+ and
K+ permeabilities of the luminal membrane dramatically
increase. Because nystatin increases the luminal Na+ and
K+ permeabilities to values much larger than those of
Cl
or basolateral K+, these can be ignored
when calculating the membrane potential (E). Although in
theory nystatin is nonselective, as noted earlier, Na+
permeated nystatin-induced channels to a greater extent than K+, because K+ was near its equilibrium
potential and Na+ was not. Thus the equation becomes
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One would expect valinomycin and high [K+] to depolarize the membrane potential due to the large increase in K+ permeability and the dramatic reduction in the gradient across the luminal membrane. Although valinomycin and high luminal [K+] induced TGF, from these data alone one cannot conclude whether TGF was initiated by the increase in intracellular [K+] or depolarization. TGF was only initiated when both valinomycin and high [K+] were present simultaneously; neither alone induced TGF. These data indicate that neither valinomycin nor high [K+] had a direct effect on the afferent arterioles.
When the nystatin and valinomycin data are taken together, it appears
that TGF is induced by depolarization of the macula densa rather than
changes in intracellular [Na+] or [K+].
However, it must be noted that these maneuvers have other effects in
addition to causing depolarization. Given that we have not actually
measured membrane potential, some degree of caution is warranted. For
instance, we cannot exclude the role of K+ in TGF signal
transmission. Given that the macula densa has a luminal K+
conductance (7), Vallon et al. (28) and
Schnermann (22) used a micropuncture technique and found
that luminal addition of a K+ channel blocker attenuated
TGF. The mechanism(s) by which a change in stop-flow pressure caused
parallel changes in macula densa [K+] remains unclear.
However, it is not surprising that changes in [K+] would
alter TGF, because K+ is required for
Na+/K+/2Cl cotransport activity
both directly and through K+ recycling across the luminal membrane.
One may ask why high [K+] alone did not induce TGF when
increased extracellular [K+] depolarized the membrane
potential by >20 mV in whole-cell patch-clamp experiments on the
macula densa and also in isolated perfused thick ascending
limb-glomerulus preparations (12, 20). There is a
potential explanation for the apparently disparate results. In our
experiments, only the luminal membrane was exposed to elevated [K+]. In the experiments of Schlatter et al. (20,
21), either both the luminal and basolateral membrane or the
basolateral membrane alone was exposed to high [K+].
Given that the macula densa has a luminal K+ conductance, a
basolateral K+ conductance, and a basolateral
Cl conductance, depolarization of the luminal membrane
would be buffered by the basolateral conductances; thus the
depolarization induced by luminal high [K+] alone may not
have been sufficient to initiate TGF.
Changes in macula densa membrane potential as a mediator of TGF were first suggested by Schlatter et al. (21). Since that report, several authors have confirmed that a change in [NaCl] in the macula densa perfusate alters membrane potential (1, 2, 11, 15). However, in all of these reports, only macula densa membrane potential was measured. None of the authors measured changes in afferent arteriole diameter in response to depolarization of the macula densa. Thus we believe our data are the first to show that depolarization of the macula densa is sufficient and necessary to cause TGF.
As additional evidence that depolarization of the macula densa is
required for a TGF response, and to begin to study what mediates
depolarization when the [NaCl] in the macula densa lumen increases,
we investigated the role of Cl channels. A central
mechanism in macula densa-induced TGF is the apical
Na+-K+-2Cl
cotransporter. This
transporter is electroneutral, and therefore changes in its activity
will not alter membrane potential. However, activation of this
transporter would increase the intracellular concentrations of all
three ions. The mechanisms responsible for efflux of Na+
and K+ either are electroneutral or favor hyperpolarization
of the membrane potential. In contrast, Cl
efflux occurs
via basolateral channels (19) and therefore depolarizes the macula densa (16, 21). Thus we questioned whether
Cl
efflux via the basolateral Cl
channels
is necessary to induce TGF by increasing [NaCl] in the lumen of the
macula densa. We examined the effect of NPPB on TGF. Early studies
(17) showed that NPPB inhibited half-maximal
Cl
conductance in the thick ascending limb of Henle's
loop at a concentration of 8 × 10
8 M. However, the
blocking effect of NPPB varied considerably from tissue to tissue. In
isolated thick ascending limb-glomerulus preparations, adding 10 µM
NPPB to the bath hyperpolarized macula densa basolateral membrane
potential from
66 ± 7 mV to
75 ± 5 mV, and this effect
was fully reversible (21). Therefore we chose 1 µM NPPB
in our preparation. Our data show that when NPPB was added to the bath,
it completely inhibited TGF but did not block the TGF induced by
valinomycin plus 50 mM KCl. Given that others have shown that NPPB
blocked the depolarization of the macula densa caused by increasing
[NaCl] in the lumen of the macula densa (16, 21), our
data indicate that Cl
efflux via basolateral channels is
necessary for TGF and that a TGF response can be elicited even when
these channels are blocked if the macula densa is depolarized. However,
one must always keep in mind that inhibitors have effects other than
those desired; thus the effects of NPPB may not have been due to
inhibition of Cl
channels.
Changes in luminal [NaCl] at the macula densa alter membrane
voltages. This appears to be one of the first steps in the signal transduction chain of the TGF loop. Although the next step is unclear,
both Ca2+ and ATP appear to be involved. Peti-Peterdi and
Bell (16) showed that increasing lumen [NaCl]
elevated cytosolic Ca2+ concentration through a
signaling pathway that included
Na+/K+/2Cl cotransport,
basolateral membrane depolarization via Cl
channels, and
Ca2+ entry through voltage-gated Ca2+ channels.
It was therefore reasoned that this large depolarization of membrane
voltages in the presence of high [NaCl] at the macula densa could
promote Ca2+ entry across the basolateral membrane.
Ca2+ plays an important role in intracellular signal
transduction by activating transport processes or stimulating the
release of a chemical mediator. Previous micropuncture studies
(4, 6) showed that a cytosolic Ca2+ system was
involved in macula densa cell signaling. In these studies, luminal
perfusion of the Ca2+ ionophore A-23187 with
Ca2+ in the perfusate enhanced TGF, whereas
8-(N,N-diethylamino)octyl-3,4,5-trimetoxybenzoate (TMB-8),
which inhibits intracellular release of Ca2+, reduced
TGF. However, the exact role of Ca2+ in TGF remains undefined.
There is also substantial evidence that ATP and adenosine play an
important role in TGF responses. Mitchell and Navar (13) demonstrated that infusion of ATP into the peritubular capillaries decreased proximal tubule stop-flow pressure, which indicated that ATP
caused preglomerular vasoconstriction. Previously we reported that
adenosine added to the interstitium (bath and afferent arteriole
perfusate) significantly augmented the afferent arteriole constriction
induced by high [NaCl] at the macula densa (18). Recently, Bell et al. (3) found that macula densa cells
released ATP via Cl channels in response to increases in
bath [NaCl]. Interestingly, the macula densa expresses
5'-nucleotidase, which converts AMP to adenosine (29);
moreover, the terminal segment of the afferent arteriole expresses high
quantities of the A1 adenosine receptor (30),
and adenosine is thought to mediate TGF (31).
From these data, a unified and testable model of TGF can be generated.
First, the [NaCl] in the tubular fluid increases. This causes an
increase in Na+/K+/2Cl
cotransport activity due to enhanced gradients for Na+ and
Cl
entry. Activation of the cotransporter increases
intracellular [Na+], [K+], and
[Cl
]. Cl
exits the cell via the
basolateral Cl
channels, which in turn leads to
depolarization of the macula densa and thereby induces TGF.
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ACKNOWLEDGEMENTS |
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This work was supported in part by National Heart, Lung, and Blood Institute Grant HL-28982. J. L. Garvin was supported in part during this work by Research Career Development Award HL-02891.
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FOOTNOTES |
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Address for reprint requests and other correspondence: J. L. Garvin, Hypertension and Vascular Research Division, Henry Ford Hospital, 2799 West Grand Blvd., Detroit, MI 48202 (E-mail: jgarvin1{at}hfhs.org).
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 September 21, 2001;10.1152/ajprenal.00357.2001
Received 1 December 2000; accepted in final form 16 July 2001.
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