Dogmas and controversies in the handling of nitrogenous wastes: Excretion of nitrogenous wastes in human subjects
Renal Division, St Michael's Hospital, University of Toronto, Toronto, Ontario, M5B 1A6 Canada
* Author for correspondence (e-mail: mitchell.halperin{at}utoronto.ca)
Accepted 6 April 2004
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Summary |
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Key words: acidbase, ammonium, urea, urine osmolality, water, human
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Introduction |
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I. Excretion of urea and the urine volume |
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What is a safe urine flow rate?
To illustrate this point, the rise in the ion product for calcium x
oxalate, the constituents of the commonest form of kidney stones in humans
(Coe and Parks, 2000), are
calculated at a 24 h average urine flow rate (1.2 ml min1),
at the usual mean overnight urine flow rate of 0.6 ml min1
(Halperin et al., 2002
), and at
a lower urine flow rate of 0.3 ml min1. For the purpose of
this calculation, we shall assume that the excretion rates for calcium and
oxalate remain constant (Table
1). The calcium x oxalate ion product is 4- and 16-fold
higher at the 0.6 and 0.3 ml min1 flow rates than at the 1.2
ml min1 rate. The major danger of precipitation occurs when
the urine flow rate decreases from 0.6 to 0.3 ml min1.
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Control of the urine flow rate
Vasopressin is released when the sodium (Na+) concentration in
plasma (PNa) exceeds its low-normal value of 138 mmol
l1 (Robertson,
2000). This hormone causes water channels (aquaporin 2; AQP-2) to
be inserted into the luminal membrane of the late distal convoluted tubule
(DCT) and the collecting ducts (Nielsen et
al., 2002
). In this setting, the urine volume should be directly
proportional to the number of effective urine osmoles and inversely
proportional to their concentration in the urine (Equation 1):
![]() | (1) |
Of great interest, vasopressin also causes the insertion of urea
transporters into the luminal membrane of the inner medullary collecting duct
(MCD) (Sands, 1999). In rats
consuming their usual diet, the MCD is sufficiently permeable to urea that its
concentrations are similar in the urine and in the renal papilla
(Gowrishankar et al., 1998
)
(Table 2). Hence urea is not an
effective osmole in the urine and it does not control the urine flow rate in
this setting.
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Conundrum
When the urine is electrolyte-poor, its flow rate could decline
sufficiently to cause a precipitate to form because of the low rate of
excretion of effective osmoles (Equation 1,
Table 1). An example of this
conundrum is a human in a hot environment who sweats and ingests little salt
or water. A deficit of Na+, Cl and water will
develop and the urine will contain very few of the usual effective osmoles
(Na+ and Cl).
Possible ways to resolve this conundrum
(i) Increase the excretion of new effective osmoles
Ketonuria of prolonged fasting seems to waste useful energy. Moreover, to
achieve acidbase balance, these ketoacid anions must be excreted with
NH4+, adding to the apparent disadvantage in their
excretion because the source of the nitrogen would be from lean body mass
(Halperin et al., 1989). Kamel
et al. (1998
) postulated that
there was an advantage in excreting NH4+ plus ketoacid
anions instead of urea during prolonged fasting, when viewed as a
physiological adaptation to allow for the excretion of urine with a flow rate
in a safer range (
0.5 ml min1). In more detail, when 2
mmoles of NH4+ are excreted along with 2 mmoles of
ketoacid anions, this results in acidbase balance; however, the urine
will contain four effective milliosmoles containing the same two nitrogens
that would be present in one millimole of the ineffective urine osmole, urea.
While useful in the prolonged fasted state, this mechanism does not provide a
means to ensure a safe minimum urine flow rate in fed mammals.
(ii) Make urea become an effective osmole in electrolyte-poor urine when vasopressin acts
The only non-electrolyte osmole that is excreted in a sufficient quantity
in a fed mammal is urea, but urea does not seem to be an effective urine
osmole when vasopressin acts in rats consuming the usual amount of
electrolytes (Table 2).
To gain insights into how to resolve this conundrum, the concentration of urea was measured in the urine and in the interstitial compartment of the papilla in rats consuming a low-electrolyte diet while they received vasopressin. The concentration of urea was significantly higher in the urine than in the papillary interstitial compartment (Table 2). This implies that urea is not freely permeable in the inner MCD when the urine is electrolyte-poor. Accordingly, we speculated that under this circumstance, urea could become an effective osmole in the inner MCD despite the continuing presence of vasopressin (Fig. 1). In fact, the change in urea permeability need only be modest to ensure the needed small rise in the urine flow rate.
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In summary, to allow for water conservation, urea is not an effective
osmole when vasopressin acts in fed mammals
(Gamble et al., 1934).
Nevertheless, when the urine is electrolyte-poor, there is a risk of
precipitate formation in the urinary tract if urea does not become an
effective osmole to ensure a safe minimum urine flow rate. The mechanism that
could permit this physiological alteration in urea reabsorption is a subject
of ongoing studies.
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II. Excretion of NH4+ and control of the urine pH |
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Uric acid stone formation, importance of the urine pH
Uric acid is a sparingly soluble constituent of the urine; its
concentration rises when the urine pH is too low (Equation 2):
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Traditional view of acid balance; define the dogma
H+ are produced when new anions are formed during the metabolism
of neutral compounds (Equation 3):
![]() | (3) |
![]() | (4) |
|
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Second, for high rates of secretion of NH4+, ammonia
(NH3) must diffuse across the renal medullary interstitial
compartment to the MCD. In this process, NH4+ are
reabsorbed in the thick ascending limb of the loop of Henle, replacing
K+ on the Na+, K+, 2-Cl
cotransporter (NKCC). This generates the `single effect' for recycling of
NH4+ in the loop of Henle and the build-up of the high
concentration of NH3 in the medullary interstitial compartment to
facilitate its diffusion into the lumen of the MCD
(Fig. 3). To
complete the transfer of NH3 to the final urine as
NH4+, the pH of the luminal fluid in this nephron
segment should be less than 6.0 (Knepper
et al., 1989); this low urine pH is caused by H+
secretion in the MCD, primarily by the H+-ATPase.
Conundrum
A low urine pH is needed to achieve high rates of excretion of
NH4+. Hence the price to pay to defend acidbase
balance would be to accept a risk of precipitation of uric acid in the
terminal nephron. This led us to reconsider the traditional view of the
physiology of the excretion of NH4+ and the role of its
medullary shunt pathway.
Resolution; the physiology of NH4+ excretion revisited
(i) Is a low urine pH required for high rates of NH4+ excretion?
As judged from the data in subjects fed NH4Cl on a chronic basis
(Madison and Seldin, 1958;
Simpson, 1971
) and during the
ketoacidosis of prolonged fasting (Kamel
et al., 1998
), a low urine pH is not needed to augment the
excretion of NH4+ because maximum excretion rates occur
when the urine pH is
6.0.
(ii) Is a medullary shunt required for high rates of NH4+ excretion?
We address this question under three headings: theoretical considerations,
an experiment using loop diuretics in rats to inhibit the reabsorption of
NH4+ in the loop of Henle, and data from rats with
chronic metabolic acidosis that permit a quantitative analysis of the
contribution of the medullary interstitial shunt pathway to
NH4+ excretion.
Theoretical concerns
Diffusion is a slow process, which requires a high concentration of the
substance that will diffuse and the absence of a barrier for its diffusion.
Although NH3 is the species of
NH4+/NH3 transported across the basolateral
membrane out of cells of the medullary thick ascending limb of the loop of
Henle (Kikeri et al., 1989),
owing to the interstitial fluid pH and the pK for NH4+,
the interstitial concentration of NH3 is 1/100 that of
NH4+. Of greater importance, there are barriers for the
diffusion of NH3 in the renal medullary interstitial compartment,
i.e. the lipid component of cell membranes of the MCD.
Experiment using loop diuretics
Another way to gain insights into the importance of a pathway is to inhibit
it. Because NH4+ is reabsorbed in the loop of Henle, a
loop diuretic should markedly reduce medullary NH4+
shunting. Following action of the loop diuretic, ethacrynic acid, rather than
reducing the rate of excretion of NH4+ as one would
expect if this NH3 shunt pathway were critical for
NH4+ excretion, the rate of NH4+
excretion rose significantly (Vasuvattakul
et al., 1993). Because the urine pH fell in the experiments
employing ethacrynic acid, we used the loop diuretic, furosemide, which has a
minor carbonic anhydrase effect that prevents the fall in the urine pH.
Furosemide led to a marked rise in the rate of excretion of
NH4+, but in these experiments, this rise could not be
attributed to a fall in the urine pH (Table
4). Therefore this medullary shunt pathway lowers rather than
raises the rate of NH4+ excretion. Hence another
hypothesis is needed to identify a possible function of this shunt
pathway.
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Quantitative analysis of the medullary shunt process
Sajo et al. (1981) measured
NH4+ excretion in rats with chronic metabolic acidosis
using a microcatheterization technique to sample fluid from the junction of
the cortical and medullary collecting ducts. They found that approximately 75%
of the NH4+ excreted was added prior to entry into the
MCD (Fig. 3). Therefore a high
rate of shunting of NH4+ from the loop of Henle to the
MCD is not required to achieve a high rate of excretion of
NH4+ in the rat.
Reinterpretation of the physiology of NH4+ excretion; an attempt at resolution
The major function of the medullary NH3 shunt pathway is not to
achieve high rates of excretion of NH4+, but possibly to
prevent a large fall in the urine pH. This can be accomplished by having a
robust luminal NH4+/H+ ion exchanger to
remove H+ secreted by the MCD during chronic metabolic acidosis
(Verlander et al., 2003;
Weiner and Verlander, 2003
).
In this hypothesis, distal H+ secretion will provide the driving
force for the transport of NH4+ into the lumen of the
MCD and hence this process functions as an adjuster of the urine pH.
The overall process is described in more detail in
Fig. 4. It begins with the
reabsorption of NH4+ from the loop of Henle, which adds
NH3 to the medullary interstitial compartment (the H+ to
convert it to NH4+ are added at site 4 in
Fig. 4). Recycling of
NH4+ in the loop of Henle raises the concentration of
NH4+ in the medullary interstitium
(Knepper et al., 1989) (site
1, Fig. 4).
NH4+ can diffuse rapidly enough through the renal
medullary interstitial compartment because its concentration is high.
NH4+ crosses both lipid-containing cell membranes of the
MCD via two different Rh-glycoproteins that function as
NH4+/H+ exchangers, one on the basolateral
and another on the luminal membrane of these cells (sites 3 and 4 in
Fig. 4)
(Verlander et al., 2003
;
Weiner and Verlander, 2003
).
The combination of H+ exit from, and NH4+
entry into, the lumen of the MCD could adjust the urine pH upward (towards
6.0) by removing luminal H+ despite continuing H+
secretion by the H+-ATPase. The net result is a final urine pH that
is
6.0 and a somewhat higher rate of NH4+
excretion.
In summary, we suggest that NH4+ plays a direct role
in both the traditional physiology (generates new
HCO3) and in the integrative physiology (adjusts
the urine pH to 6.0). For the latter, NH4+ diffuses
across the medullary interstitial compartment and that there is a
H+-linked counter-transport system with NH4+
across both polar membranes of MCD cells. Moreover, the driving force for this
process is the secretion of H+ by the MCD, because the
concentration of NH4+ in the urine is higher than in the
papillary interstitial compartment (Table
2). Thus, the activity of the
NH4+/H+ exchanger in the luminal membrane
might act as an `adjuster' for the final urine pH to minimize the risk of uric
acid stone formation (Fig.
4).
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Concluding remarks |
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Acknowledgments |
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References |
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