Department of Physiology and Biophysics, Weill Medical
College of Cornell University, New York, New York 10021
The present clinical taxonomy of distal
renal tubular acidoses includes "gradient," "secretory," and
"voltage" defects. These categories refer to presumed collecting
duct defects in which the epithelium may be abnormally permeable and
unable to sustain an ion gradient, in which luminal proton ATPases are
defective, or in which electrogenic Na+ reabsorption is
impaired and luminal electronegativity is reduced. Classification of
affected individuals is based on urinary pH and ion concentrations
during spontaneous acidosis and during SO
infusion,
as well as urinary PCO2 during
HCO
loading. A model of rat CD has been developed
that has been used to examine determinants of urinary acidification
(Weinstein AM. Am J Physiol Renal Physiol 283:
F1252-F1266, 2002) and the interplay of HCO
and
PO
loads to generate a disequlibrium pH and
equilibrium PCO2. In this paper, pure forms of
gradient, voltage, and secretory defects are simulated, with attention
to variability in the locus of the defect in the cortical (CCD), outer
medullary (OMCD), or inner medullary collecting duct (IMCD). The
objective of these calculations is to discover whether the intuitive
description of these defects is sustained quantitatively. The most
important positive finding is that the locus of the transport defect
along the CD plays a critical role in the apparent severity of the
lesion, with more proximal defects being less severe and more easily
correctable. In particular, model calculations suggest that for
gradient or secretory defects to be clinically detectable they need to
involve the OMCD and/or IMCD. Additionally, the calculations reveal a
possible mechanism for CD K+ wasting, which does not
involve failure of H+-K+-ATPase but derives
from a paracellular anion leak and thereby a more electronegative
lumen. The most important negative finding is the lack of support for
the category of renal tubular acidosis associated with a voltage
defect. Although CD lesions that present with both K+ and
H+ secretory defects suggest mediation by transepithelial
electrical potential difference (PD), both PD changes and proton pump
PD sensitivity appear too small to account for the observed abnormalities.
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INTRODUCTION |
DISTAL RENAL TUBULAR
ACIDOSIS refers to a group of disorders that manifest failure to
achieve an acidic urinary pH (<5.5) despite systemic acidemia. It has
long been an objective of workers in the field to develop a taxonomy of
these disorders based on the underlying transport defects within the
collecting duct (CD). Ideally, these defects were to be revealed
through administration of a battery of provocative tests (3, 7,
14, 27). The most widely utilized tests have been
SO
administration, with measurement of urinary pH,
net acid, and K+ excretion and HCO
or
PO
loading, with measurement of urinary
PCO2. From the responses to these tests, some
distinct patterns have emerged, namely "gradient," "voltage,"
or "secretory" defects (14). With a gradient defect, the urine acidifies with infusion of SO
, and
PCO2 of alkaline urine increases normally. The
CD fails only in circumstances where a tubule-to-blood pH gradient
needs to be established, and the presence of an abnormal
transepithelial leak pathway is envisioned. With a voltage defect, both
K+ and H+ secretion are diminished in acidosis,
and there is blunting of the PCO2 response in
HCO
loading. The putative defect is composed of
electrogenic Na+ reabsorption, so to the extent that proton
secretion depends on a luminal negative PD, it too will be affected. In
the secretory defect, urinary acidification cannot be restored by
SO
, and proton transport into a
high-HCO
urine is markedly decreased. Presumably,
the proton pumps themselves are dysfunctional, so that no manipulation
of transepithelial driving forces can restore a normal proton secretory
rate. Beyond this taxonomy, some have tried to use the pattern of test
responses to infer the site, cortical or medullary, of the CD defect
(7).
Thus far, all of the discussion of acidification defects has
advanced in the absence of model simulations of the proposed pathophysiology. In the foregoing papers, a model of rat CD has been
developed that could acidify the urine, and specific attention had been
given to the effects of luminal flow on urinary pH and K+
excretion and to simulation of SO
infusion
(36). The model CD also produced a high urinary
PCO2 in response to HCO
loading, and specific attention was given to the interplay of
HCO
and PO
loads to determine
the final urinary PCO2 (37). The
program for this paper is to consider three defects, intended to
represent pure forms of gradient, voltage, and secretory defects. The
gradient defect will be a high-conductance, anion-selective
paracellular shunt in one or more CD segments. The voltage defect will
be a sharp reduction in luminal membrane Na+ conductance in
either cortical (CCD) or inner medullary CD (IMCD), or both. The
secretory defect will be a nearly complete reduction in luminal proton
pumps in one or more CD segments. For each defect, urinary solute
excretion under acidotic conditions, without and with
SO
infusion, will be examined. Similarly, urinary
PCO2 with HCO
loading will
also be simulated. The objective of these calculations is to discover
whether the intuitive description of these defects actually holds up
under scrutiny in a model. One important theme that will emerge is that
the distal defects (IMCD or full CD) are more severe and less
susceptible to correction by solute infusion.
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MODEL CALCULATIONS |
In the calculations that follow, model parameters and peritubular
conditions are those previously used (36). CD entering solute concentrations for simulation of acidosis ("acid control") have been modified from baseline conditions by HCl titration of luminal
HCO
from 7.0 to 3.5 mM, keeping total phosphate and
ammonia constant (Table 1). To secure
maximal acidification of final urine, the input flow rate has also been decreased from 54 to 48 µl/min for all collecting ducts (7.5-6.7 nl/min per CCD). Figures 1 and
2 display the model solution for this
acid control condition. Figure 1 displays the tableau of luminal
potential difference (PD), volume flow, and concentrations and flows of
the nonreacting species, in which the solid curves are those of
acidosis, whereas the dotted curves are the baseline curves of model
development (36). The obvious changes with acidosis are
the decrease in entering volume flow, and the parallel decreases in all
of the entering solute flows. In a comparison of the two conditions,
there is no important difference in luminal PD along the entire CD. In
the CCD and OMCD, there are also no substantive differences in
Na+, K+, or Cl
concentration;
however, in the IMCD, cation concentrations are lower, whereas
Cl
remains little changed from baseline. With reference
to Fig. 2, one may see that the depression in IMCD Na+ and
K+ concentrations derives from the increase in titratable
acid (TA) and NH
in this segment. Despite the decrease in flow, there is negligible change in both absolute Na+ reabsorption (2.54 and 2.56 µmol/min under baseline
and acidosis, respectively) and K+ reabsorption (1.05 and
1.00 µmol/min under baseline and acidosis, respectively).
Nevertheless, luminal K+ concentration remains adequate to
sustain H+-K+-ATPase activity. Figure 2
contains the concentrations and flows relevant to the CD acid-base
economy, under acid control (solid curves) and baseline conditions
(dotted curves). It is apparent that the model CD can acidify the
urine, and it achieves nearly complete HCO
reabsorption within the first 2 mm of the IMCD, to a minimal urinary pH
of 4.3. Nevertheless, with the smaller delivered loads under acid
control, the absolute increase in TA excretion is small, and the
absolute increase in NH
excretion is negligible. The
noteworthy feature of the acid control simulation is that the low pH of
the final urine is achieved early and sustained over distance, so that
minor parameter changes will not yield an apparent acidification defect.

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Fig. 1.
Electrolyte transport along the model collecing duct (CD) with 3.5 mM HCO in entering fluid and perfusion at 48 µl/min (solid curves), and under control conditions, 7.0 mM
HCO and perfusion at 54 µl/min (dotted curves).
Left: luminal potential difference (PD; mV), and
concentrations of Na+, K+, and Cl
(mM; brackets). Right: volume flow within the aggregate of
all tubule segments (µl/min) as well as the axial solute flows
(µmol/min) within the entire CD. The abcissa is distance along the
CD, with x = 0 the initial cortical point, and cortical
(CCD), outer medullary (OMCD), and inner medullary CD (IMCD) accounting
for 2, 2, and 5 mm of CD length, respectively.
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Fig. 2.
Acid-base transport along the model CD with
low-HCO perfusate as in Fig. 1 (solid curves) and
under control conditions (dotted curves). Left: luminal
pH and concentrations of HCO , titratable acid
(TA), and NH (mM). Right: flows
within the aggregate of all CD tubule segments of
HCO , TA, and NH (µmol/min),
along with their sum-to-net acid flow (TA + NH HCO ).
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There are a number of possible parameter selections that might
represent a gradient acidification defect in this CD model. If one
considers amphotericin toxicity as the paradigm for such defects, then
studies in lipid bilayers suggest that the drug could be an
anion-selective channel within cell membranes (9, 17).
However, on the basis of studies of turtle urinary bladder, Steinmetz
and co-workers (18, 30) concluded that amphotericin provided a large conductance pathway for H+ and
K+ through luminal membranes. With respect to the kidney,
amphotericin can induce nonselective anion-conducting channels in
isolated proximal tubule brush-border membranes (28).
However, amphotericin toxicity does not appear to have been studied in
any perfused tubule preparation, so there are no data identifying
affected segments or the transport abnormalities. From the perspective of the model, the simple inclusion of a large H+- and
K+-conductive pore within the luminal membrane of OMCD
cells does not produce an acidification defect. Thus absent the ability
to simulate amphotericin with any degree of confidence, a pure gradient defect will be considered in which only tight junctional
Cl
and HCO
permeabilities are both
increased from control by approximately a factor of 30. The increase in HCO
permeability creates a tubule segment that
cannot sustain a transepithelial HCO
gradient, and
the parallel increase in Cl
permeability was included
simply because it seems most likely that a paracellular pathway with a
high HCO
permeability would conduct both species.
Three such defects will be considered, in which junctional anion
conductance is increased solely in the CCD, OMCD, or IMCD.
In Figs.
3-5,
the three leaky junctions are examined under acid control conditions or
when 60 mM luminal Cl
has been replaced by
SO
(Table 1). Figure 3 is a tableau of bar graphs
in which the abcissa references one of seven simulations: acid control
perfusion with control parameters, acid control perfusion with the
three gradient defects, or the gradient defects with an attempt at
SO
correction; the panels display electrical PD,
and solute concentrations and flows at the CD terminus. Figure 4
displays the axial profiles of luminal PD, K+
concentration, and flow for each of the leaky tight junctions. In Fig.
3, it may be seen that when the abnormality is in the CCD, there is a
55% decrease in Na+ excretion and nearly absent
K+ excretion (87% decrease). This derives from the
paracellular Cl
shunt of the principal cell
Na+ current, producing a 20-mV depolarization within the
CCD (Fig. 4). The impact of the IMCD defect is smaller but parallels
that of the Na+-reabsorbing CCD, with a decrease in
Na+ and K+ excretion of 18 and 27%,
respectively. With reference to Fig. 4, the IMCD effect is more complex
than in the CCD: early in the IMCD, the leaky junctions short circuit
the transport potential, thus depolarizing the epithelium and enhancing
K+ reabsorption; late in the IMCD, the PD is
hyperpolarized, consistent with an accentuated Cl
diffusion potential that enhances K+ excretion. Because the
IMCD coalesces and the bulk of luminal area is early, the net
effect is the small decrease in K+ excretion. The OMCD
defect imposes increases in both Na+ and K+
excretion (28 and 27%, respectively), due to a luminal negative PD
generated by blood-to-lumen anion gradients (Fig. 4). With this
observation, the model CD offers a possible mechanism for an
acidification defect accompanied by renal K+ wasting that
does not involve an associated defect in K+ reabsorption
via the H+-K+-ATPase. With CCD and IMCD
defects, there are increases in Cl
reabsorption, whereas
with the leaky OMCD, urinary Cl
increases. Regardless of
the defect, however, it appears that the hyperpolarization of
SO
administration is a strong perturbation that
provides nearly identical cation excretion and urinary flow rate (Fig.
3).

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Fig. 3.
Urinary composition and flows under acid perfusion, with tight
junctions leaky to anions. Gradient defects are simulated by increasing
tight junctional anion permeability to both Cl and
HCO by a factor of 30, in the CCD, OMCD, or IMCD. In
each bar graph, the abcissa references 1 of 7 simulations: acid control
perfusion with control parameters (Con), acid control perfusion with
the 3 gradient defects, or the gradient defects when 60 mM luminal
Cl has been replaced by SO (Table 1).
The panels display electrical PD and solute concentrations and flows at
the CD terminus.
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Fig. 4.
K+ handling by the CD with tight junctional (TJ) anion
leaks. The 3 tiers correspond to the simulations of Fig. 3, in which
the paracellular Cl and HCO
permeabilities of CCD, OMCD, or IMCD have been increased by a factor of
30 over control. For each parameter set, the panels display luminal PD,
K+ concentration, and axial K+ flow as a
function of distance along the CD. In each panel, the dotted curve
corresponds to the control parameters (Fig. 1).
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Figure 5 displays the data relevant to acid excretion by the model CD:
end-luminal pH and concentrations of HCO
, TA, and
NH
(left) and urinary excretion of
HCO
, TA, NH
, and net acid
(right). These data, along with K+ excretion,
are also summarized in Table 2. All
three leaky junctions lead to an acidification defect, in the sense
that all of the urinary pH values in acidosis are well above that in
control, and all of the values for net acid excretion are diminished.
Nevertheless, the CCD defect appears to be mild and might not even be
discerned if the diagnostic threshold were a urinary pH >5.5. With the
CCD defect, urinary TA and NH
concentrations are
higher than in control, but urinary excretion is diminished as a result
of decreased urinary flow. Both OMCD and IMCD defects are severe, with
urinary pH values near 6.4 and net acid excretion ~60% of the
control CD. Nevertheless, these two defects may behave differently with
respect to their response to SO
infusion. With the
leaky OMCD, SO
brought the urinary pH to 5.3, below
the diagnostic threshold, and thus "corrected" the acidification
defect. With the leaky IMCD, urinary pH with SO
fell to only 5.7, suggesting that with the most distal defect there was just not the opportunity to correct the HCO
leak.
Thus with similar transport defects, namely, high tight junctional
anion permeability, the CCD lesion might be inapparent, the OMCD lesion
severe but correctable, and the IMCD lesion not correctable.

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Fig. 5.
Urinary composition and flows under acid perfusion, with tight
junctions leaky to anions. Using the parameters and perfusion
conditions of Fig. 3, the panels display end-luminal pH and
concentrations of HCO , TA, and NH
(left) and urinary excretion of HCO , TA,
NH , and net acid (right).
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HCO
loading with these gradient defects has been
simulated using the high-HCO
perfusion solution of
the previous paper (37), and its composition and flow rate
are specified in Table 1. Figure 6 shows
the results of these calculations, specifically, end-luminal pH and its
disequilibrium value, end-luminal HCO
concentration and flow, total CD CO2 generation, and the end-luminal
equilibrium PCO2. Some of these quantities are
also indicated in Table 3. The most
prominent feature of these simulations is that with each anion leak,
there is a substantial and nearly identical decrease in urinary
HCO
excretion, due to paracellular flux of
HCO
from the lumen. That this reflects
HCO
leak is seen by noting that despite the leaky
junctions, the changes in luminal titration of
HCO
("CO2 generation") are quite a
bit smaller than the steep decreases in luminal HCO
flow. With each of the defects, there is a substantial decrease in the
disequilibrium pH, reflecting the decrease in luminal
HCO
concentration. In a comparison of the CCD defect
with control, there is a decrease in urinary flow rate (a 48% decrease
in HCO
concentration compared with a 67% reduction
in HCO
flow) and thus a commensurate increase in
urinary phosphate concentration. This increase in phosphate buffer
concentration contributes to both the decrease in disequilibrium pH and
the maintenance of the near-identical value of the equilibrium
PCO2 (37). With the OMCD and IMCD
lesions, urinary volume flows are little different from control, so
that phosphate concentrations are comparable, and the decline in
disequilibrium pH is reflected in the decline in the equilibrium
PCO2 predicted for these lesions. Nevertheless, the equilibrium PCO2 for both OMCD and IMCD
defects are still substantially (104 and 83 mmHg, respectively) greater
than the ambient PCO2.

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Fig. 6.
Urinary composition and flows under HCO loading,
with tight junctions leaky to anions. Using the parameters of Fig. 3,
HCO loading with these gradient defects has been
simulated using a perfusion solution in which HCO
concentration is 32 mM, obtained by Cl replacement, and
entering at 54 µl/min. The panels display end-luminal pH and its
disequilibrium value, end-luminal HCO concentration
and flow, total CD CO2 generation, and the end-luminal
equilibrium PCO2. Calculation of the
disequilibrium pH (and equilibrium PCO2)
assumes a system with no CO2 escape.
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With respect to choices for simulating secretory acidification defects,
a number of possibilities derive from the fact that both
H+-ATPase and H+-K+-ATPase have
been identified along the entire rat CD and that the integrity of the
peritubular HCO
exit pathways may also impact on
luminal proton secretion. In the prior OMCD model (34), it
had been observed that not only abundance of luminal proton pumps but
also the activity of the peritubular
Cl
/HCO
exchanger and peritubular
Cl
(and HCO
) conductance could
modulate proton secretion. In the present work, three segmental
secretory defects are examined, namely, 99% reductions in the
abundance of both luminal proton ATPases in the CCD, OMCD, and IMCD.
Input conditions for acidosis and SO
correction are
the same as those used to investigate the gradient defects (Table 1).
Figures 7 and
8 contain the results of those
simulations, with tableaux of end-luminal concentrations and flows. The
panels in Fig. 7 indicate that with each of the three secretory
defects, there is little change in urinary flow rate or Na+
excretion. K+ excretion, however, varies substantially with
the site of the lesion: with CCD pump defects, K+ excretion
is identical to control; with OMCD defects, K+ excretion is
60% greater than control; and without IMCD pumps, K+
excretion is more than double control (Table 2). Infusion of SO
hyperpolarizes the CD and increases volume,
Na+, and K+ excretion nearly equally in all of
the defects. With respect to K+ excretion during
SO
infusion, the absolute differences among the
three lesions are maintained, but they become small relative to total
urinary K+ delivery.

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Fig. 7.
Urinary composition and flows under acid perfusion, with defective
proton secretion. Three segmental secretory defects are examined: 99%
reductions in the abundance of both luminal H+-ATPase and
H+-K+-ATPase in the CCD, OMCD, and IMCD. Input
conditions for the acidosis and for SO correction
are the same as those used for the gradient defects (Fig. 3). The
panels display electrical PD and solute concentrations and flows at the
CD terminus.
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Fig. 8.
Urinary composition and flows under acid perfusion, with defective
proton secretion. Using the parameters and perfusion conditions of Fig.
7, the panels display end-luminal pH and concentrations of
HCO , TA, and NH (left)
and urinary excretion of HCO , TA,
NH , and net acid (right).
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Of the three secretory defects, only the OMCD and IMCD lesions yield an
acidification defect discernible from the final urinary composition.
While the CCD defect yields a urinary pH greater than control, it
is well below any standard diagnostic threshold, and there is a
negligible difference in TA and NH
excretion (Fig. 8
and Table 2). Both the OMCD and IMCD defects are severe, with high
urinary pH and net acid excretion rates 40 and 50% below control,
respectively. Of interest, the OMCD pump lesion produces a greater
reduction in urinary NH
excretion than the IMCD
defect. This reflects the fact that in the model CD, there is
NH
secretion that occurs mainly in the OMCD (Fig. 2).
Attempts to correct either OMCD or IMCD secretory defects with
SO
infusion fail to lower urinary pH even close to
the diagnostic threshold of 5.5, with the more distal lesion the more
refractory to correction.
The response of the CD with a secretory defect to
HCO
loading is examined in Fig.
9 and Table 3. In addition to the three
segmental defects, the calculations include the case in which all three
segmental defects are in place simultaneously and also the case in
which this totally defective CD is undergoing water diuresis. (Luminal
membrane water permeability for CCD, OMCD, and IMCD principal cells is
decreased by a factor of 30 from control.) The first observation from
these calculations is that with absent proton secretion from the CCD,
OMCD, and IMCD, luminal CO2 titration decreases by 0.13, 0.35, and 0.21 µmol/min, or by 14, 38, and 23%, respectively. This
yields a measure of the overall importance of each segment to proton
secretion along this CD. When all of the segments are simultaneously
defective, CO2 titration is reduced by 0.74 µmol/min, or
80%. This is slightly larger than sum of the three individual defects,
0.69 µmol/min (74%), and due to the fact that, with an isolated
proximal defect, more distal segments may increase proton secretion.
The prediction that with nearly complete proton pump inhibition there
may still be titration of 20% of the delivered HCO
derives from the assumption of antidiuresis in these simulations. With
water abstraction from luminal fluid, there is a concentration of
HCO
and phosphate at a constant PCO2. As examined in detail in the companion
paper (37), this urinary alkalinization yields an acid
disequilibrium pH and titration of HCO
by
H2PO4. Indeed, with water diuresis and global
CD secretory defects, CO2 titration falls further to 7.5%
of control. Figure 9 (and Table 3) display the disequilibrium pH in
HCO
loading for each of the secretory defects. With
either CCD or OMCD proton pumps missing, there is no decrease (and
actually a small increase) in the end-luminal disequilibrium pH, and
this derives from intact acidification within the IMCD. However, when
IMCD proton secretion is defective, the disequilibrium pH does collapse
and is sustained only by the osmotic effect. These observations provide
the rationalization for the predicted changes in urinary equilibrium
PCO2. In the case of either CCD or OMCD
defects, equilibrium PCO2 is normal; with IMCD
or full CD secretory failure, the equilibrium
PCO2 falls by ~100 mmHg. In the absence of
antidiuresis, the equilibrium PCO2 with
defective CD proton secretion is only 4 mmHg greater than the ambient
value.

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Fig. 9.
Urinary composition and flows under HCO loading,
with defective proton secretion. Labels on the abcissa refer to the
segmental defects of Fig. 7, plus one in which all proton secretory
defects are present simultaneously (CD), and one in which all defects
are present and principal cell luminal membrane water permeability is
1/30 of control in all segments [CD-antidiuretic hormone (ADH)]. The
perfusion solution is that of Fig. 6, in which HCO
concentration is 32 mM, entering at 54 µl/min. The panels display
end-luminal pH and its disequilibrium value, end-luminal
HCO concentration and flow, total CD CO2
generation, and end-luminal equilibrium PCO2.
Calculation of the disequilibrium pH (and equilibrium
PCO2) assumes a system with no CO2
escape.
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The last category of abnormal acidification is that which follows from
defective electrogenic Na+ reabsorption. It has been
assumed that the intermediate abnormality in this defect is the failure
to hyperpolarize the CD lumen and thus drive electrogenic proton
secretion or HCO
reabsorption. Thus this has been
termed a voltage defect in acidification, and the prototype is the
application of amiloride. To simulate this defect in the CCD, the
principal cell luminal membrane Na+ channel has had its
permeability reduced to 5% of control; to simulate this defect in the
IMCD, the luminal membrane cation channel (equally permeable to both
Na+ and K+) has had its permeability reduced to
5% of control. Figures 10 and
11 and Table 2 display CD function
in acidosis, with either segmental defect or with a global lesion
comprising both CCD and IMCD defects. Figure 10 contains the bar graphs
of end-luminal PD, urinary flow, and end-luminal concentration and
flows of the nonreacting solutes. As expected, the defective cation
channel in the IMCD produces a depolarization of end-luminal PD. For
either segment, the channel defect is mildly natriuretic with the CCD and IMCD lesions enhancing Na+ excretion by 42 and 29%,
respectively. Both segments together produce a 75% increase, with
total Na+ excretion corresponding to 42% of delivery to
the CD. The most profound impact of these lesions is on K+
excretion. With the CCD lesion alone, urinary K+ is
one-third of control, and with both CCD and IMCD defects, the urine is
essentially K+ free (Table 2). With reference to Fig. 10,
infusion of SO
is clearly effective in
hyperpolarizing luminal PD and in restoring K+ excretion.

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Fig. 10.
Urinary composition and flows under acid perfusion, with defective
cation channels. Two segmental and one global defect are examined: 95%
reduction in CCD principal cell luminal membrane Na+
permeability, 95% reduction in IMCD luminal membrane cation
(Na+ and K+) permeability (IM), and the
combination of these 2 (CD). Input conditions for the acidosis and for
SO correction are the same as those used for the
gradient defects (Fig. 3). The panels display electrical PD and solute
concentrations and flows at the CD terminus.
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Fig. 11.
Urinary composition and flows under acid perfusion, with defective
cation channels. Using the parameters and perfusion conditions of Fig.
10, the panels display end-luminal pH and concentrations of
HCO , TA, and NH (left)
and urinary excretion of HCO , TA,
NH , and net acid (right).
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Figure 11 and Table 2 indicate that of the three tubules, only the
combined CCD and IMCD cation channel defects produce a mild renal
tubular acidosis, with a urinary pH of 5.9. The nature of this defect,
however, has little to do with the altered luminal PD, but rather the
absence of urinary K+ acts to shut off proton secretion by
the H+-K+-ATPase. With this combined defect,
net acid excretion is barely abnormal (decreased by 8%), because the
bulk of ammonia addition occurs in the unaffected OMCD (Fig. 2) and the
H+-ATPase is still functional. With both CCD and IMCD
defects, urinary acidification is restored (pH = 4.6) with
SO
infusion, in parallel with the delivery of
K+ to the IMCD. Despite the voltage defects, urinary
acidification with HCO
loading is entirely normal, with no significant differences from control in HCO
titration, disequilibrium pH, or equilibrium
PCO2. The end-luminal PD under these
circumstances, for the combined CCD and IMCD defects is
7 mV, which
may be compared with the end-luminal PD of the control tubule of
17
mV. This 10-mV difference is of minor impact on H+
secretion via the H+-ATPase, in view of the fact that this
proton pump can transport protons against a full 180-mV electrochemical
potential (1). With respect to the CD response to
HCO
loading, Fig. 12
and Table 3 indicate that the absence of functional Na+
channels in either the CCD, IMCD, or both has no significant impact on
luminal pH or CO2 concentrations, or their changes after equilibration. In the taxonomy of renal tubular acidosis by Arruda and
Kurtzman (3), the high urinary
PCO2 with HCO
loading is
compromised with the voltage-dependent defect but may be corrected
(e.g., lithium) or may not be corrected (e.g., amiloride) during
phosphate loading. In this model CD, because the voltage-dependent defect shows no compromise of urinary PCO2,
further testing with phosphate is not undertaken.

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Fig. 12.
Urinary composition and flows under HCO
loading, with defective cation channels. Model calculations use the
transport defects in Fig. 10 under perfusion conditions in Fig. 6, in
which HCO concentration is 32 mM, entering at 54 µl/min. The panels display end-luminal pH and its disequilibrium
value, end-luminal HCO concentration and flow, total
CD CO2 generation, and the end-luminal equilibrium
PCO2. Calculation of the the disequilibrium pH
(and equilibrium PCO2) assumes a system with no
CO2 escape.
|
|
 |
DISCUSSION |
The objective of these calculations was to visit the taxonomy of
distal renal tubular acidoses from the perspective of a model CD. The
classification scheme recognizes "gradient defects," in which
proton secretion is intact but the epithelium is leaky; "secretory
defects," in which the proton pumps are defective; and "voltage
defects," in which failure of electrogenic Na+
reabsorption induces an acidification defect. This taxonomy had been
developed intuitively from a substantial body of experimental data with
intact kidneys, but it has remained inferential, with limited support
from micropuncture or microperfusion studies. The premise of this work
is that if the postulated defects are operative in vivo, then they
should produce acidification defects in the model CD, with the expected
responses to provocative testing. It should be acknowledged that this
task is much simpler than the reverse problem of starting with a set of
test results and then trying to infer the transport defect. The major
assumptions of this work are that the model CD captures the key
features of the segmental physiology and that the simulations with
specified peritubular conditions are applicable to the tubule in vivo.
Obviously, every effort has been made to develop reliable segmental
models (32, 33, 34, 35). The first of the two companion
papers described the synthesis of these segmental models into a full CD
and its ability to excrete an acidic urine (36). It had
been known from the IMCD model that maximal acidification required fully functional H+-K+-ATPase, and thus
required adequate K+ delivery to the IMCD
(33). In the full CD model, obtaining a minimal urinary pH
also depended on limiting buffer delivery to the CD. Thus in the
simulations here, urinary flow rates are lower than the typical
infusion rates in whole animal studies, and CCD entering concentrations
included low HCO
and high K+. The second
of the two companion papers examined the response of the model CD
during HCO
loading and specifically its ability to
generate a high equilibrium PCO2 (37). The magnitude of the PCO2
was found to depend on the acid disequilibrium pH, along with the
end-luminal concentrations of HCO
and
H2PO4.
There has been general agreement that amphotericin toxicity could be
considered the prototype of a gradient defect. Data from several
studies have been summarized in Table 4
and indicate that amphotericin can induce a relatively high urinary pH
despite systemic acidosis, with correction of the pH by
SO
infusion. With HCO
loading,
the animals are fully capable of generating a high equilibrium
PCO2. Isolated tubule segments from
amphotericin-treated animals have not been studied in vitro, so the
precise defect in rats remains unknown. On the basis of experiments in
turtle urinary bladder, it had been suspected that the defect of
amphotericin was a luminal membrane channel permeable to H+
and K+ (18, 30). Such a defect had the
intuitive appeal of providing an explanation for the renal
K+ wasting and hypokalemia associated with amphotericin
(21). In the model CD, an isolated OMCD luminal membrane
cation channel, permeable to K+ and H+, did not
yield a simulated acidification defect, and this was not explored
extensively. More recently, the membrane pathway induced by
amphotericin has been recognized to shift from a selective cation
channel to a nonselective anion-conducting pore, as drug level and time
of incubation increase, due to interaction with membrane lipid
(10). In the absence of experimental guidance regarding
luminal and peritubular membrane effects of amphotericin along the CD,
the simplest example of a true gradient defect was created by
introducing high tight junctional HCO
(and
Cl
) permeabilities in the CCD, OMCD, or IMCD. The notable
finding of these simulations was that the OMCD anion leak produced
acidification failure, which was correctable with
SO
, and a relatively normal increase in urinary
PCO2 with HCO
loading. With
leaky CCD junctions, urinary acidification was negligibly impaired;
with a leaky IMCD, the defect was too late along the tubule to be
corrected by SO
. One unexpected finding of these
simulations was that with the OMCD anion leak, diffusion potentials
rendered the lumen more electronegative than control and thus enhanced
urinary K+ excretion. Thus one might achieve compatibility
with the observed hypokalemia of amphotericin treatment despite the
absence of any new leak pathway for K+.
While it is likely that many patients with distal renal tubular
acidosis have a secretory defect (8), a satisfactory
animal model is not available for comparison with model calculations. For such a secretory model, consideration has been given to vanadate intoxication, which produces acidification failure with hypokalemia, in
association with profound inhibition of CD
H+-K+-ATPase (12). However, the
response of these animals to SO
infusion or
HCO
loading was not determined. At one time, the
acidification defect that follows transient ureteral obstruction had
been considered exemplary of a secretory defect (3).
Representative findings appear in Table 4, where one sees acidification
failure that does not correct with SO
and a
distinctly low urinary PCO2 during
HCO
loading. Nevertheless, subsequent reviews have
been more likely to categorize the postobstructed kidney as a voltage
defect, perhaps in view of the prominent K+ retention
(14, 27). The finding in the model CD is that with isolated CCD proton pump absence, more distal segments compensate fully, and no acidification defect is perceived. When proton pumps are
absent from the OMCD, there is acidification failure that is not
correctable with SO
, but because of the intact
IMCD, there is a completely normal PCO2
response to HCO
loading. When IMCD pumps are missing, the acidification defect is compatible with the pattern observed in the obstructed kidney. Indeed, when ATPase activities have
been examined in isolated tubule segments from obstructed kidneys, the
observations included near obliteration of both H+-ATPase
and H+-K+-ATPase activities from medullary
collecting ducts (16, 26). An additional observation in
the model CD was the impact of water diuresis to further blunt the
increase in urinary PCO2 with
HCO
loading. This is also coherent with findings in
the obstructed kidney, in which tubular resistance to ADH is expected.
One aspect of the model CD that is not compatible with obstruction is
the prediction of prominent urinary K+ wasting, due to the
absence of IMCD H+-K+-ATPase. In this regard,
Sabatini and co-workers (16, 26) have shown that in the
postobstructed kidney, there is a sharp decrease in CCD
Na+-K+-ATPase and an increase in CCD
H+-K+-ATPase, additional enzyme defects that
sharply decrease CD K+ secretion. Furthermore, a study in
humans with hyperkalemic distal renal tubular acidosis (most of whom
were postobstructive), demonstrated that amiloride and bumetanide could
still modulate K+ excretion comparably to control subjects
(29). Thus the postobstructed kidney exhibits secretory
defects of both protons and K+, although it is not clear
that transepithelial voltage per se is critical to this pathophysiology.
Both amiloride administration and lithium toxicity have been used as
illustrative of a voltage-dependent acidification defect. In the
open-circuited turtle bladder, luminal application of lithium reduced
Na+ reabsorption, transepithelial PD, and proton secretion
(2). In the short-circuited preparation, lithium had no
effect on proton secretion, and in particular, when epithelial PD was
held at the control value, proton secretion proceeded at control rates.
In vivo, both lithium and amiloride result in a high urinary pH during acidosis, as well as abnormally low urinary
PCO2 during HCO
loading;
with lithium, but not with amiloride, urinary acidification can be
restored by SO
infusion (Table 4). The
lithium-induced acidification defect was localized to the CD via
micropuncture (6). In the model CD, the voltage defect was
simulated with 95% reductions in luminal Na+ permeability
either in the CCD or IMCD or both. With the defects in both segments,
there was natriuresis and nearly complete elimination of K+
from the urine. This combined defect did produce acidification failure,
but this was due to lack of K+ availability to the
H+-K+-ATPase and not to the minor
depolarization of luminal PD. In contrast to experimental observation
with amiloride, the model CD with the voltage defect showed no
depression of urinary PCO2 with
HCO
loading, and the acidification defect in
acidosis was completely correctable by SO
. In this
regard, examination of CD ATPases during lithium and amiloride administration have shown pronounced depression of both cortical and
medullary H+-ATPase activity (11), and with
lithium, additional depression of CCD
H+-K+-ATPase (16). Thus model
calculations do not support acidification failure mediated by a
voltage, and the prototypes for this defect have additional associated
enzyme defects. One must consider the possibility that the
pathophysiology defined in the turtle bladder for the lithium effect
may not be applicable to the kidney tubule.
In summary, the most important positive finding from this modeling
effort has been that the locus of the transport defect along the CD
plays a critical role in the apparent severity of the lesion, with more
proximal defects being less severe and more easily correctable. In
particular, model calculations suggest that for gradient or secretory
defects to be clinically detectable they need to involve the OMCD
and/or IMCD. Additionally, the calculations have revealed a possible
pathophysiology for CD K+ wasting, which does not involve
failure of H+-K+-ATPase but that derives from a
paracellular anion leak and thereby a more electronegative lumen. The
most important negative finding in this study is the inability to
support the category of renal tubular acidosis associated with a
voltage defect. Certainly, CD lesions that present with both
K+ secretory defects and H+ secretory defects
suggest transepithelial electrical potential as a common intermediate.
However, no prior model of CD function has supported this construct,
nor can this one. Finally, from an overall perspective, the CD
complexity, which is apparent in the calculations of this paper, does
not encourage undertaking the inverse problem: to define a set of
clinical tests that can pinpoint a CD transport defect. Alternatively,
modern investigations of renal tubular acidosis have focused attention
on genetic defects in specific transporters (8). These
studies have already provoked questions as to whether a specific
transport defect can, in isolation, account for the full phenotype of
the acidosis or whether one bad transporter is bringing down others
within the cell (13). Such questions fall easily within
the scope of a mathematical model of the CD.
This investigation was supported by Public Health Service Grant
1-R01-DK-29857 from the National Institute of Arthritis, Diabetes, and
Digestive and Kidney Diseases.
Address for reprint requests and other correspondence:
A. M. Weinstein, Dept. of Physiology and Biophysics, Weill
Medical College of Cornell University, 1300 York Ave., New York, NY
10021 (E-mail:
alan{at}nephron.med.cornell.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
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in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.