Vol. 274, Issue 6, F1037-F1044, June 1998
Physiological disposal of the potential alkali load in diet of
the rat: steps to achieve acid-base balance
Shih-Hua
Lin1,
Surinder
Cheema-Dhadli2,
Sorasak
Chayaraks2,
Ching-Bun
Chen2,
Manjula
Gowrishankar3, and
Mitchell L.
Halperin2
1 Renal Division, Tri-Service
General Hospital, National Defense Medical Center, Taipei 100, Republic
of China; 2 Renal Division, St.
Michael's Hospital, University of Toronto, Toronto, Ontario M5B
1A6; and 3 Renal Division,
Department of Pediatrics, University of Alberta, Edmonton, Alberta,
Canada T6G 2B7
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ABSTRACT |
The purpose of
this study was to provide a better understanding of the physiological
role of endogenous net organic acid production in rats consuming their
usual diet. Balance studies were performed over 24 h, and
urine was collected in the day and night portions of the diurnal cycle.
A supplemented low-electrolyte diet (LED) was fed to determine whether
urinary organic anions were identical to those in the diet. A titration
procedure was developed to determine the
pK of titratable groups in the urine
of rats studied with and without an acid load. Although normal rats
excreted net acid (NAE), the latter was inversely related to the amount
of food consumed. The rates of excretion of bicarbonate
(HCO
3), citrate, unmeasured organic
anions, and NH+4 were higher in the night
portion of the diurnal cycle. NAE rose dramatically when alkali intake
was decreased by consuming the LED. Dietary and urinary organic anions
were not identical because rats fed the LED supplemented with potassium
citrate excreted <10% of this alkali load as citrate and <25% as
HCO3. In the 24 h after 3,000 µmol
NH4Cl was given intraperitoneally,
H+ did not appear to be retained,
yet NAE rose by only close to 2,000 µeq. The rate of excretion of
titratable groups with a pK in the 3 to 5 pH range fell by close to 1,000 µeq; most of these changes
occurred in the first 7 h after
NH4Cl was given. We conclude that
rat chow provides a large net alkali load. There appear to be two types
of endogenous acid production, a form associated with a rise in NAE
(e.g., sulfuric acid) and dietary alkali-driven endogenous net acid
production, which titrates this alkali. Renal excretion of organic
anions makes these acids end products of metabolism.
acid-base balance; ammonium; bicarbonate; citrate; diurnal cycle; endogenous acid production; net acid excretion; organic anions
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INTRODUCTION |
HYDROGEN IONS
(H+) are produced if the valence
of the end products of metabolism have a greater net anionic charge
than their substrates. The contribution of adenine nucleotides and
cofactors to this net valence can be ignored because they are present
in catalytic amounts and are both formed and removed in the overall metabolic sequences (10, 14). To decide whether metabolism of dietary
constituents plus reactions in the body and the lumen of the
gastrointestinal (GI) tract and bone cause a net daily H+ or bicarbonate
(HCO3) load, the composition of the
urine is examined in steady state. If the excretion of ammonium
(NH+4) plus titratable acid exceeds that of
HCO3 (called net acid excretion), the
diet is said to yield an acid load; the converse is also true (27, 28,
31, 32, 35).
In acid-base balance studies in rats in steady state performed in our
laboratory, more microequivalents of sulfate
(SO24) than net acid were excreted
(4). Moreover, there were two other sources of
H+ production, because these rats
excreted some monovalent phosphate (H2PO4) and a large quantity
of unmeasured
anions.1
With respect to the latter, it is likely that many of them represented endogenous acid production (for review, see Refs. 1, 3, 5, 8, 28, 29,
38, 41). Hence, if net acid production exceeds net acid excretion and
acid-base balance is present, there must be a source of alkali to
titrate this extra H+ load.
Accordingly, the issues to be addressed were to establish whether the
usual diet of the laboratory rat produces a net alkali load, to
evaluate its diurnal variation, to identify whether the ingested and
excreted unmeasured anions are identical, and to study what factors
might control the disposition of ingested alkali (i.e., excretion or
whether the alkali load can be used to titrate an exogenous acid load).
Results suggest that normal rat chow provides a net alkali load. When
the alkali source in the synthetic diet was solely potassium (K+) citrate, citrate accounted
for <10% of the unmeasured anions in the urine. Moreover, when rats
were given an exogenous acid load that closely approximated the rate of
excretion of unmeasured anions, only two-thirds of this
H+ load was eliminated by a rise
in the rate of net acid excretion. The remainder appeared to be
titrated by a mechanism involving a decline in the rate of excretion of
unmeasured titratable groups, leaving a residual input of
HCO3. The urinary titratable groups
that changed in amount in response to an acid-base load had a
pK primarily in the 3 to 5 pH range.
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METHODS |
Rats
Rats were cared for in accordance with the principles and guidelines of
the Canadian Council on Animal Care. This study protocol was approved
by the Animal Care Committee at St. Michael's Hospital. Adult male
Wistar rats (weight 300-400 g) were housed in individual metabolic
cages, so that complete collections of urine could be obtained and that
balance data could be calculated. The amount of chow given and
remaining was weighed daily. Most rats typically consumed 18-22 g
of chow per day (Purina Rodent Laboratory Chow: 174 mmol/kg
Na+, 282 mmol/kg
K+, 183 mmol/kg
Cl). Some rats
spontaneously consumed as much as 32 g of chow/day. Urine was collected
(with thymol as the preservative) in two portions (0-7 h and
7-24 h) beginning at 0900. Blood was drawn under light anesthesia
at times 0 and 24 h.
Experimental Protocols
Studies in rats fed a variable amount of
chow. This experiment was designed to evaluate whether
the diet produced a net acid or alkali load. Six groups of rats were
given as little as 3-7 g of their usual chow per day or as much as
28 to 32 g of chow/day for 24-36 h. The rate of production and
excretion of H+ was assessed by
measuring the rates of excretion of
SO24, H2PO4, and organic anions
collected over 24 h; blood samples were obtained at 0- and 24-h time
points.
Studies in rats fed a synthetic, electrolyte-poor
diet. To evaluate the renal response to removal of the
potential alkali load in the diet, rats
(n = 10) were fed 18-22 g of a
low-electrolyte diet for 3 days prior to study (ICN Pharmaceuticals,
Cleveland, OH; 0 mmol/kg Na+, 1 mmol/kg K+, 16 mmol/kg
Cl).
To compare the nature of the organic anions consumed and excreted, a
second group of rats (n = 6) consumed
the low-electrolyte diet with all the
Na+ and
Cl of the usual diet
replaced as NaCl and all the K+
replaced as its citrate salt (3,857 ± 63 µeq NaCl, 7,761 ± 119 µeq K+ citrate). Urine was
collected for the 24-h period, with blood samples at times 0 and 24 h,
as described above. A quantitative comparison was made of the rate of
consumption and excretion of citrate and unmeasured anions in the 24 h
urine.
Studies in rats given an acute acid
load. These experiments were designed to evaluate how
many of the unmeasured anions in the urine represent "potential
HCO3," i.e., if not excreted, could
the dietary alkali be used to titrate
H+ from an exogenous acid load?
The control group (n = 12) ate their usual 18-22 g of rat chow but had no other treatment; the
experimental group (n = 14) consumed a
similar amount of chow and received an amount of
NH4Cl approximating the usual rate
of unmeasured anion excretion (3,000 µmol). The
NH4Cl was given by the
intraperitoneal route in two equally divided doses at 0900 and 1600 (see APPENDIX for an explanation of
the rationale for using the intraperitoneal route). Urine and blood
were collected as described above: urine samples were taken in the day
and night periods to evaluate the time course for the responses. The
urine was titrated to identify the amount and the
pK of the titratable groups with and
without the load of NH4Cl.
Analytical Techniques
Na+ and
K+ in plasma and urine were
determined by flame photometry,
Cl was determined by
electromimetic titration, and blood gas analysis was performed at
37°C with a digital pH/blood gas analyzer (Corning 178 blood pH
analyzer). The HCO3 concentration was
calculated from the pH and PCO2 using
a solubility factor of 0.0301 (plasma) or 0.0309 (urine), and a
pK' was adjusted for ionic
strength (16, 43). NH+4, citrate, creatinine,
Ca2+,
Mg2+, phosphate, and
SO24 were measured as previously described (4, 15).
Titration of Urine
Titrations were carried out using a pH meter model AT 1 perpHect meter
370, using a Ross Sureflow electrode (AT 1 Orion; Laboratory Products
Group, Boston, MA). Urines were titrated with 0.1 N NaOH from its pH to
the pH of plasma to quantitate titratable acid. Another portion of
urine was acidified with 6 N HCl to pH 2.7 and then titrated at room
temperature with 0.1 N NaOH to the pH of plasma, noting the volumes
needed to change the pH by 0.5 pH unit increments. The NaOH reagent was
prepared fresh each week. In each case, a water blank was treated in an
identical fashion to the urine and titrated as above.
Calculation of the Difference in Excretion of Potential
HCO3
This is defined as the difference in the number of titratable groups
excreted in normal rats, compared with rats consuming an identical diet
but given an acute NH4Cl load. The
difference in rate of excretion of potential
HCO3 was calculated as follows: first,
native urine was titrated from pH 2.7 to the pH of plasma; second, once
the concentrations of the components of net acid (phosphate and
NH+4), creatinine, and citrate were known,
their titration curves were measured individually at the ionic strength
of the urine from each rat, and the values were subtracted from the
titration curve of the corresponding untreated urine; third, the
difference for these curves in microequivalents per liter in control
and NH4Cl-treated rats was
multiplied by the respective urine volumes. This difference should
reflect the decline in excretion of potential
HCO3 in response to an
NH4Cl load over the 24-h period.
Statistical Analysis
Results are reported as means ± SE. Statistical analysis was
performed on the group mean values, using an unpaired Student's t-test.
P < 0.05 was considered to be
statistically significant.
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RESULTS |
When rats consumed 18-22 g of chow/day, net acid (458 ± 95 µeq) was excreted because the urine contained more
NH+4 (594 ± 59 µeq/day) plus titratable
acid (176 ± 35 µeq) than HCO3 (312 ± 48 µeq/day) (Table 1). A
minimum estimate of their rate of production of
H+ is the rate of excretion of
SO24 (845 ± 53 µeq/day). This
estimate ignores endogenously produced monovalent phosphate and organic
acids; the latter were potentially very large, as judged from the rate
of excretion of unmeasured anions (Table 1). Despite this apparent
imbalance in H+ production and
excretion, the plasma pH, PCO2, and
[HCO3] were normal (Table
2). Hence, experiments were
designed to identify the source of alkali needed to achieve acid-base
balance.
To evaluate the impact of the amount of rat chow consumed on net
H+ production and excretion, rats
were given 5-32 g of chow per day. The greater the amount of food
consumed, the lower the excretion of NH+4 and
net acid; the converse was true for the excretion of
HCO3, unmeasured anions, and citrate
(Table 1, Fig. 1). Because there was no
major difference in the daily excretion of
SO24 and phosphate with the changes in
food consumption, the diet appeared to provide a net alkali load.
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Fig. 1.
Effect of the amount of food ingested on the excretion of net acid.
Amount of food ingested in g/day is shown on the
x-axis. Excretions of
NH+4 ( ),
HCO3 ( ), and citrate ( ) are
depicted on the y-axis,
left. Unmeasured anions (circles with
crosses) are depicted in the y-axis,
right.
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To provide a diet without alkali, rats were fed a low-electrolyte diet.
On this chow, rats had a much higher daily excretion of
NH+4 (1,816 vs. 594 µeq/day) and net acid
(2,046 vs. 458 µeq/day), whereas the daily excretion of unmeasured
anions (913 vs. 2,655 µeq/day), citrate (35 vs. 606 µeq/day), and
HCO3 (6 vs. 312 µeq/day) were much
lower than on regular chow (Tables 1 and
3). On the low-electrolyte diet, the rate
of net acid excretion closely approximated the sum of
SO24, phosphate, and organic anion
excretion rates.
When the amount of K+ and alkali
in the low-electrolyte diet was returned to control values but the
alkali was given solely as a citrate salt, the daily excretion of all
the above acid-base parameters returned to close to their control
values (compare Tables 1 and 3 in rats consuming 18-32 g of
food/day). Excretion of citrate accounted for <10% of the unmeasured
anions in the urine (351 µeq vs. 4,343 µeq). There was no
significant difference in plasma pH,
PCO2,
HCO3 concentration ([HCO3]), or the anion gap
in both groups of rats on the low-electrolyte diet (results not shown).
Rats were given NH4Cl to determine
whether alkali in their diet could be used to titrate an exogenous acid
load. Because the pH, PCO2,
[HCO3], and anion gap in plasma were in the normal range 24 h after receiving 3,000 µmol of
NH4Cl (Table 2), this suggests
that the H+ load was eliminated.
As shown in Table 4, there was an increase in the excretion of NH+4 of close to 1,000 µeq and a decline in the excretion of
HCO3 of close to 500 µeq over 24 h.
Together with a rise in titratable acid excretion, net acid excretion
could explain the elimination of only two-thirds of this
H+ load. Of interest, the daily
excretion of citrate fell by close to 250 µeq and that of unmeasured
anions fell by almost 1,300 µeq in the 24 h after
NH4Cl was given. When the urine
was titrated between pH 2.7 and the urine pH, there were fewer
titratable groups excreted in the
NH4Cl group. Moreover, when the
titration curves for phosphate, NH+4,
citrate, and creatinine measured at a similar temperature and ionic
strength were subtracted from that of the untreated urine, the majority
of the titratable groups that disappeared when
NH4Cl was given had
pK values in the pH range of 3-5
(Fig. 2). When expressed as 24 h
excretions, there was a decline of 887 ± 27 µeq of
these titratable groups in the NH4Cl group (called residual
titratable groups in Table 4).
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Fig. 2.
Titration of 24 h urine. For details, see text. Curve depicted by the
open squares represents the titration of urine from fed rats minus the
titration curves for NH+4, phosphate,
citrate, and creatinine (n = 7). Curve
depicted by the open circles represents the titration curve of urine
from the NH4Cl-treated rats minus
the titration curves for their urinary NH+4,
phosphate, citrate, and creatinine (n = 6).
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Because unmeasured anions appeared to play an important role in
acid-base balance, more detailed studies were carried out. Urine was
collected in two time periods, 7 h during the day and 17 h overnight in
the normal and NH4Cl-treated rats.
To avoid collection errors, data were expressed per 100 µmol
creatinine (close to the usual 24 h creatinine excretion). Nocturnal
urine in normal rats had a higher content of
HCO3, unmeasured anions, and citrate
but a lower content of net acid (Table 5
and Fig. 3). This is consistent with the
hypothesis that the rat, which is a nocturnal eater, consumes a net
load of alkali. It is of interest that the rate of excretion of
NH+4 was also higher in the overnight period,
consistent with the fact that the diet produces an acid load as well,
but that its H+ and
HCO3 loads are handled in a
semi-independent fashion. Rats given
NH4Cl received their acid load
during the day and had a higher excretion of
NH+4 and a lower excretion of
HCO3, unmeasured anions, and citrate at this time (Table 5 and Fig. 3).
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Fig. 3.
Excretion of acid-base parameters in the day and night portions of the
diurnal cycle. For details, see text. "Anions" refers to
unmeasured anions (see footnote in Introduction). Open bars at
left are the daytime 7 h, and solid
bars at right represents the nocturnal
17 h in the diurnal cycle. A:
excretions in the control-fed rats. B:
excretions in the fed rats that received
NH4Cl in 2 divided doses. TA,
titratable acid; NAE, net acid excretion.
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DISCUSSION |
The purpose of these experiments was to evaluate whether rat chow
provided a net acid or alkali load. There are two lines of evidence to
suggest that their diet produces a net acid load. First, the urine
contained an appreciable amount of
SO24 and
H2PO4 (anions produced along
with H+; Refs. 10, 26, 28, 33, 35,
41). Second, net acid was excreted (Table 1). In the following, we
shall attempt to show that the premises for this daily net acid load
are only partial truths and that much more emphasis needs be given to
the role of dietary alkali with the excretion of unmeasured organic
anions in the urine, as previously suggested (1, 3, 5, 8, 28, 29, 38,
41).
Although organic anions are excreted in appreciable amounts in the rat,
one cannot deduce how many of them are potential
HCO3 for a number of reasons. First,
they consist of a large number of individual compounds with different
possible metabolic fates (25). Second, attempting to determine their
amount by net valence does not discriminate among anions that were
merely ingested and excreted as their
K+ salt, for example, compared
with those produced endogenously along with
H+. Third, although titration
reveals pK values, it does not
indicate whether the excreted anion entered the body along with
H+ or
K+. Moreover, titration does not
distinguish between anions and cations (i.e., creatinine and
-hydroxybutyric acid have a pK close to 5.0, and both are titrated between pH 4 and 6). Nevertheless, the following points were established. First, dietary and urinary unmeasured anions were not identical, because, when
K+ was added back to the
low-electrolyte diet as its citrate salt, the rate of excretion of
unmeasured anions rose markedly, but citrate accounted for <10% of
the unmeasured anions excreted (Table 3). Hence, the body deals with an
alkali load by producing a variety of organic acids and uses the
H+ so produced to eliminate
dietary alkali; this requires that organic anions become end products
of metabolism (being excreted in the urine; see Ref. 14). Nevertheless,
one cannot be certain whether these new organic acids were produced by
bacteria in the GI tract or by a metabolic process in the body.
Moreover, it is not clear whether regulation was at a GI, a metabolic,
and/or a renal level.
The first support for the hypothesis that the diet of the rat produces
a net alkali load comes from experiments where the quantity of food
consumed was varied. If rat chow was really a net source of
H+, one would expect to find a
direct relationship between net acid excretion and chow intake.
Although there was little change in the rate of excretion of endogenous
acids
(SO24 + H2PO4)
with different amounts of chow consumed over 24 h, the rate of
excretion of NH+4 and net acid
fell when more food was consumed (Table 1, Fig. 1). More evidence suggesting that rat chow provides an alkali load can be
derived from experiments where rats were given a synthetic diet lacking
alkaline salts. Rats consuming this diet excreted much more
NH+4 and net acid compared with rats on their
usual diet (Tables 1 and 3); the number of unmeasured anions in the
urine was much lower on the low-electrolyte diet.
To examine whether organic anions in the diet and urine were identical,
K+ citrate was added to the
low-electrolyte diet so that it was virtually the sole input of alkali
(Table 3); this alkali load was much larger than the net rate of
H+ production, as judged by the
rate of excretion of SO24 plus
H2PO4. Nevertheless, it led
to little excretion of HCO3, so
endogenous net acid production appeared to be stimulated (Table 3). In
support of this concept, the rate of excretion of unmeasured anions
rose by a quantitatively appropriate amount (Table 3). A similar
conclusion appears to be valid in rats that consumed their usual chow,
because the daily excretion of HCO3,
citrate, and unmeasured anions was directly proportional to the
quantity of chow consumed (Fig. 1).
When 3,000 µmol of NH4Cl was
administered intraperitoneally (see
APPENDIX), virtually all of this
salt was excreted because there was an appropriate rise in the
excretion of nitrogen and Cl (data not shown);
moreover, the acid-base parameters in plasma returned to normal values
within 24 h (Table 2). Two renal components could explain the removal
of this H+ load. First, close to
2,000 µmol of the 3,000 µmol given was eliminated by the rise in
net acid excretion. Second, there was close to a 1,000 µeq fall in
the daily excretion of unmeasured organic anions. If this fall in
excretion is equivalent to the retention of a
HCO3 load, which could be used to
titrate endogenously supplied H+,
it could have eliminated the remaining
H+ produced when
NH+4 was converted to urea (19, 34).
The diurnal patterns for the excretion of net acid and unmeasured
organic anions were explored. The rates of excretion of HCO3, citrate, and unmeasured anions
were somewhat higher in the night collection; this probably reflects
the alkali load in the diet because rats are nocturnal feeders. Of
interest, the rate of excretion of NH+4 was
also higher in nocturnal collections, suggesting that the diet provided
an acid plus an alkali load but that these were handled in a
semi-independent fashion (Table 5, Fig. 3). It is not clear from the
present study why NH+4 excretion was higher
overnight. Because there was no decline in the urine pH, perhaps the
higher osmole excretion rate, which should lead to a higher urine flow
rate for any given urine osmolality (40), caused a lower
NH3 concentration in the lumen and
thereby was the factor responsible for the higher nocturnal rate of
excretion of NH+4. Consistent with this
impression is the fact that the concentration of creatinine was lower
in overnight urines (6.7 ± 0.7 daytime and 5.1 ± 0.6 overnight). Because bladder emptying is unlikely to be complete over 7 h, these indirect estimates of urine flow rate were utilized. In
contrast, when NH4Cl was ingested
during the day, it stimulated a large increase in the rate of excretion
of NH+4 and a parallel decline in the rate of
excretion of unmeasured anions (Table 5, Fig. 3). It appears that the
acid load was eliminated in less than 24 h, because the rate of
excretion of NH+4 fell, and there was a rise
in the excretion of unmeasured anions overnight. Most of the titratable
groups that disappeared from the urine in the rats given
NH4Cl had a
pK in the 3 to 5 pH range (Fig. 2).
It is of interest that both NH+4 and
HCO3 appeared in the urine in
appreciable quantities thoughout the 24-h period (Figs. 1 and 3). On
the one hand, this could be a technical problem due to the hydrolysis
of urea by a bacterial urease. Although thymol was added to the
collection vessel, this does not rule out the possibility that
hydrolysis of urea occurred in the bladder or in the metabolic cage.
Two points make this hydrolysis unlikely: first, the excretion of NH+4 and
HCO3 moved in opposite directions when
the quantity of food ingested was varied (Fig. 1); and second,
collections made over shorter time periods did not change the
proportion of or the excretion rates of NH+4 and HCO3 in the urine (Table 5).
Hence, it is likely that there were stimuli for the excretion of both
NH+4, potential
HCO3, and actual
HCO3 thoughout the 24-h period. In
keeping with this view, when rat chow was consumed by rabbits, there
was a much larger excretion of HCO3,
with almost no NH+4 in the urine (36). Hence,
specific metabolic features in a given species affect the amount and
proportion of HCO3 and
NH+4 excreted, and excreting both ions
throughout the day seems to be a normal process in the rat (Table 5) as it is in other species (6).
On the surface, it seems surprising that such a large dietary alkali
load did not simply titrate the H+
produced as
H2SO4
for example, thereby totally removing the daily need to excrete
NH+4. Nevertheless, continued production of
NH+4 in cells of the proximal convoluted tubule will lead ultimately to the accumulation of the
NH3 in the medullary interstitium
(13, 24). Moreover, when H+ ions
are secreted in the collecting duct, its luminal pH will fall, lowering
the [NH3] in the
lumen, which, in turn, enhances NH3 entry by diffusion, producing
a given NH+4 concentration in this luminal
fluid (and also in the urine). In fact, distal
H+ secretion is not totally
inhibited by a large alkali load because the
PCO2 in alkaline urine is high (9).
Hence, it appears that the kidney is poised to excrete
NH+4 in case there is an additional acid
load. This helps to preserve acid balance and avoids extra loss of
Na+ and
K+ without requiring a time lag
for the induction of enzymes [i.e., during prolonged fasting (2,
22) or with diarrhea]. Likewise, at the normal intracellular pH
of proximal tubular cells, the kidney is also poised to excrete organic
anions rather than HCO3 when an alkali
load is ingested, because excretion of citrate could minimize the risk
of formation of brushite stones in an alkaline urine (30). The
reabsorption of citrate in the proximal convoluted tubule appears to be
controlled by its intracellular pH (12); its reabsorption is increased
during extrarenal acidosis (7, 39). Indeed, at this time, the
transporter for citrate in the luminal membrane of the proximal
convoluted tubule has a higher activity (18). That reabsorption of
citrate may decrease in patients with proximal renal tubular acidosis
is another interesting observation that is consistent with a more
alkaline intracellular fluid pH in this nephon segment.
Concluding Remarks
Athough the rat normally excretes net acid, its diet supplies an
overall alkali load. This alkali load is eliminated for the most part
by having a larger net endogenous organic acid production, together
with the excretion of some of these organic anions as their
Na+ and/or
K+ salts. This response is prompt
and somewhat larger in the night portion of the diurnal cycle. The
unmeasured anions in the urine differ in nature from those ingested on
the low-electrolyte diet.
Perspectives
The usual daily net production of
H+ can be conveniently thought of
as existing in two major groups. One subtype (e.g.,
SO24) requires an increase in net acid
excretion for its elimination (10, 26, 28, 33, 35, 41). The second type
does not require net acid excretion for its elimination; rather, it
occurs in response to an alkali load. Most of this alkali stimulates endogenous net organic acid production by obligating the excretion of
some of their conjugate bases (organic anions), thereby making them end
products of metabolism and leaving the
H+ so produced to titrate the
alkali load (10, 14, 26, 35).
From the preceding, it follows that the traditional analysis of the
renal response to acid-base perturbations relying on net acid excretion
should be modified, because this analysis led to the inappropriate
conclusion that the diet of the rat had a net acid load (17, 23, 33,
35). In fact, its alkali load was 10-fold higher than its acid load.
The question becomes, What should be used to replace the net acid
excretion term? At a theoretical level, net acid excretion should be
thought of as NH+4 + titratable
acid 
HCO3 potential
HCO3 in the urine. For practical
considerations, if one wishes to know whether there is a net acid or
alkali load in steady state or whether the kidney is providing a net
acid or alakli load to the body, the initial step should be to examine
the difference between the excretions of the major urinary cations
(Na+ + K+)
and the major urinary anion,
Cl. Once the overall
acid-base effect is known, one can assess the individual contributions
of NH+4, titratable acid, HCO3, and unmeasured anions. The
potential error of using the simple initial calculation is that certain
organic anions will be excreted and not contribute to acid-base balance (Na+ or
K+ salts that are both ingested
and excreted unchanged); a second error could occur with major changes
in the excretion of unmeasured cations, such as
Ca2+ or
Mg2+. Because the rates of
excretion of these latter cations are virtually always small, this
second potential error should not create a problem with human
patients. A quantitative analysis of the disposal of the
NH4Cl load in this study using the
clinical approach outlined above is provided in Table
6.
Several clinical examples could illustrate the potential advantage of
this form of analysis. First, when subjects fasted for a prolonged
period were given NH4Cl, they
eliminated the H+ load without
increasing net acid excretion. Rather, the
H+ load was consumed by retaining
potential HCO3 because the rate of
excretion of organic anions (largely -hydroxybutyrate anions)
declined, so that the same amount of NH+4 could now be excreted as its
Cl salt (21). Second,
patients having a reduced rate of excretion of
NH+4 (type IV renal tubular acidosis)
achieved acid-base balance because they have a parallel decline in the rate of excretion of unmeasured anions (20). Third, subjects with
end-stage renal disease have a lower rate of elimination of organic
anions and therefore a lower overall endogenous net acid production
(42). One should also appreciate that changes in their diet not only
lower the production of sulfuric acid (lower protein intake) but that
they also lower the intake of potential HCO3, if part of the restricted intake
of K+ was due to a lower intake of
alkaline K+ salts, such as those
contained in fruit.
| |
APPENDIX |
Rationale for Intraperitoneal Use of NH4Cl
Precipitation reactions in the lumen of the GI tract can remove a
potential acid load when NH+4, a potential H+ precursor, is absorbed and
metabolized to urea (19). In more detail, if
Mg2+, inorganic phosphate, and
NH+4 are present together in the lumen of the
GI tract,
Mg(NH4)PO4
precipitates. If the Mg salt ingested was
MgCO3, the net result would be the
conversion of the potential acid load in
NH4Cl to an alkali load by
converting nonabsorbable alkali into an absorbable form
(NaHCO3, Eq.
1). Alternatively, these same precipitation reactions
in the GI tract could result in a larger
H+ load if the Mg salt in
Eq. 1 was
MgSO4 (instead of
MgCO3) and the inorganic
phosphate was ingested as
NaH2PO4,
i.e., one NH4Cl is ingested, but 2 H+ are added to the body
(Eq. 2). Moreover, a more alkaline
environment in the GI lumen could permit bacteria in this location to
generate a larger amount of organic acids (reviewed in Ref. 11). To
avoid these potentially confounding problems,
NH4Cl was given by the intraperitoneal route in the experiments described in this
study
|
(A1)
|
|
(A2)
|
| |
ACKNOWLEDGEMENTS |
We are extremely grateful to Drs. Kamel S. Kamel and Bobby J. Stinebaugh for very helpful discussions and suggestions during the
preparation of this manuscript. We are also indebted to Stella Tang and
Eleanor Singer for expert technical assistance and to Jolly Mangat and
Rebecca Esrock for expert secretarial assistance.
| |
FOOTNOTES |
This work was supported by Grant 5623 from the Medical Research Council
of Canada and by a grant from the Kidney Foundation of Canada.
1
Concentration of unmeasured anions in
the urine was calculated as the difference between the number of
microequivalents of the measured cations
(Na+ + K+ + Ca2+ + Mg2+ + NH+4)
and the number of microequivalents of the measured anions
(Cl +HCO3 + SO24 + phosphate). Valence on phosphate was calculated from the urine pH and a
pK corrected for temperature and ionic
strength (37).
Address for reprint requests: M. L. Halperin, St. Michael's Hospital
Annex, Lab 1, Research Wing, 38 Shuter St., Toronto, ON, Canada M5B
1A6.
Received 22 October 1997; accepted in final form 12 February 1998.
| |
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Abstract
Introduction
