Clinical Science (2000) 98, 313–319 (Printed in Great Britain)
Does a high concentration of calcium in the
urine cause an important renal concentrating
defect in human subjects?
George S. S. LAM*, John R. ASPLIN† and Mitchell L. HALPERIN*
*Renal Division, St. Michael’s Hospital, University of Toronto, Toronto, Ontario M5B 1A6, Canada, and
†Renal Division, University of Chicago, Chicago, IL 60637, U.S.A.
A
B
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The objective of this study was to evaluate the hypothesis that a high concentration of ionized
calcium in the lumen of the medullary collecting duct causes an osmole-free water diuresis. The
urine flow rate and osmolality were measured in normal human subjects, as well as in patients
with a history of nephrolithiasis who excreted more than 5 mmol of calcium per 24 h. There was
an inverse relationship between the concentration of calcium in the urine and the 24 h urine
volume both in normal subjects and in patients with a history of nephrolithiasis. When the
concentration of calcium in the urine was greater than 5 mmol/l, the urine volume was less than
1 litre per day in the majority of subjects. After 16 h of water deprivation, when the
concentration of calcium in the urine was as high as 17 mmol/l (ionized calcium 7.4 mmol/l), urine
osmolality was 1258 mOsm/kg of water and the urine flow rate was 0.30 ml/min. We conclude
that, although a calcium receptor may be present in the lumen of the medullary collecting duct
in human subjects, an extremely high concentration of urinary total and ionized calcium does not
cause a clinically important defect in the renal concentrating process.
INTRODUCTION
Calcium (Ca) is a critical ion in human physiology. At
one extreme, Ca is a principal constituent of bone ; at the
other, the concentration of ionized Ca is very low in the
cytosol, where it is intimately involved in signal transduction, enzyme activity, ion transport and the
contraction–relaxation process [1]. New insights into the
physiology of Ca have emerged since a receptor for
ionized Ca was discovered on the surface of cells [2,3] ;
this receptor is very sensitive to minor variations in the
physiological, ionized concentration of Ca in the extracellular fluid. Its fundamental role is documented for
control of the release of parathyroid hormone, and
changes in this receptor’s binding capacity and\or affinity
for ionized Ca or surrogate cationic ligands, such as
magnesium (Mg), may explain many of the findings in
certain disease states [4–6].
Riccardi et al. [7] proposed a novel hypothesis to
explain why polyuria might develop when the urinary
Key words : antidiuretic hormone, arginine vasopressin, hypercalcaemia, hypercalciuria, loop of Henle, nephrogenic diabetes
insipidus, polyuria, urine osmolality, water.
Abbreviations : VP, vasopressin ; DDAVP, [1-deamino,8-D-arginine]vasopressin (desmopressin) ; DI, diabetes insipidus ; MCD,
medullary collecting duct ; mTAL, medullary thick ascending limb.
Correspondence : Professor M. L. Halperin, Professor of Medicine, St. Michael’s Hospital Annex, Lab F1, Research Wing, 38 Shuter
Street, Toronto, Ontario M5B 1A6, Canada (e-mail mitchell.halperin!utoronto.ca).
# 2000 The Biochemical Society and the Medical Research Society
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G. S. S. Lam, J. R. Asplin and M. L. Halperin
Scheme 1
Sites of action of ionized calcium in the kidney
The effects of hypercalciuria are shown to the right of the vertical broken line. If the luminal concentration of ionized Ca2+ rises sufficiently in the MCD, ionized Ca2+
will bind to its receptor in the lumen and impair water reabsorption in the MCD. The effects of hypercalcaemia are shown to the left of the vertical broken line. When
the concentration of ionized Ca2+ in plasma rises, Ca2+ binds to its receptor on the basolateral side of the mTAL cells, which leads to inhibition of their ROM-K channels
(the major K+ conductance channel in the luminal membrane of the mTAL of the loop of Henle). With less entry of K+ into the lumen of the mTAL, there will be less
reabsorption of Na+ and Cl− via the Na+/K+/2Cl− co-transporter. In addition, the lumen of the mTAL becomes less positively charged, and this retards the
passive reabsorption of Na+, Ca2+ and Mg2+.
calcium concentration rises (Scheme 1). In this formulation, when the urinary Ca concentration rises, Ca
ions bind to the luminal Ca receptor in the medullary
collecting duct (MCD), which leads ultimately to a major
defect in renal concentrating ability. As a result of the
larger volume of urine, the risk of formation of Cacontaining kidney stones would be diminished.
The aim of the present study was to evaluate whether
a high urinary concentration of total and\or ionized Ca
is associated with an important defect in the ability to
excrete a very concentrated urine when vasopressin
(VP) acts. Data were obtained in normal human subjects,
who excreted less than 200 mg (5 mmol) of Ca per
day, and in patients with a history of nephrolithiasis,
with a rate of excretion of Ca exceeding 200 mg (5 mmol)
per day. The results indicate that very high urine
osmolalities and low urine flow rates occurred when the
concentration of total Ca in urine was greater than
5 mmol\l. Hence a clinically important defect in the renal
concentrating ability could not be attributed to a high
urinary concentration of Ca.
# 2000 The Biochemical Society and the Medical Research Society
METHODS
The study protocols were approved by the Ethics
Committee for experiments in humans at the University
of Chicago and at St. Michael’s Hospital. Informed
consent was obtained for all protocols.
Studies with 24 h urine collections
The first study group consisted of 41 normal subjects
who excreted more than 200 mg (5 mmol) of Ca per
24 h. They had no history of kidney stones nor had firstdegree relatives ; moreover, the subjects had no medical
problems and were not taking any drugs known to affect
mineral metabolism. Each subject remained on their
ambient diet with an unrestricted fluid intake, and
provided a 24 h urine sample.
The second study group consisted of all stone-forming
patients who entered the University of Chicago Kidney
Stone Program from 1988 [8]. Three consecutive 24 h
urines were collected from 864 patients ; thymol was used
as the preservative and the urines were not refrigerated.
Calcium and nephrogenic diabetes insipidus
Figure 1 Plot of measured and calculated concentrations of
ionized Ca in urine
Concentrations were calculated and measured in 21 urine samples (r l 0.97). The
line is the line of identity. For details, see the text.
Subjects remained on their ambient diet and fluid intake.
Stone-formers with a plasma creatinine concentration of
greater than 1.3 mg\dl were excluded, as were subjects
with a urine pH greater than 6.3 (the latter was to avoid
potential errors in the calculation of the concentration of
ionized Ca in the urine). Hence a total of 1630 urine
samples were included from patients with a positive
history of kidney stones and a rate of excretion of Ca that
was greater than 200 mg (5 mmol)\24 h. This is the group
of subjects most likely to have a concentration of Ca in
the urine that, in theory, should bind to the luminal Ca
receptor in the MCD. The completeness of each collection was assessed by examining the rate of excretion of
creatinine [9].
The concentrations of sodium (Na+), potassium (K+),
chloride (Cl−), Ca, phosphorus, Mg, uric acid and
creatinine in urine were measured by standard laboratory
methods using a Synchron CX5CE analyser (Beckman
Instruments, Brea, CA, U.S.A.). Oxalate and citrate were
measured enzymically using oxalate oxidase and citrate
lyase respectively. Sulphate was measured by barium
precipitation [10], and pH was measured using a glass
electrode. The concentration of ionized Ca in the urine
was measured using an ion-selective electrode in samples
from 21 patients ; concentrations in other urine
samples were estimated from these measurements using
the iterative computer program EQUIL2, which solves
simultaneously for all ion-pair interactions [11]. The
mean values for the directly measured and calculated
values for ionized Ca in the urine were very strongly
correlated, as shown in Figure 1 (r# l 0.97 ; P 0.001) ;
hence we report the calculated values for ionized Ca in
the survey portion of the data.
Studies with 16 h of water deprivation
Volunteers (n l 14 ; age 17–59 years) consumed their
usual diets and took no medication. Only subjects who
had a concentration of Ca in the urine that exceeded
200 mg\l (5 mmol\l) were included, because they were
most relevant to test the hypothesis outlined in Scheme 1.
Each subject avoided water ingestion for 16 h, beginning
at 20.00 hours on the evening prior to urine measurements
(zero time). Subjects voided voluntarily at 08.00 hours
the next morning (12 h time point), after which they were
given desmopressin acetate o[1-deamino,8-D-arginine]vasopressin (DDAVP) ; 0.3 µg\kg body weightq by nasal
insufflation ; two additional urine samples were provided
voluntarily at 2 h intervals after taking DDAVP.
The concentrations of Na+ and K+ in these urine
samples were determined by flame photometry (Radiometer FLM 3 ; London Scientific Ltd, London, Ontario,
Canada) ; Cl− was determined by electromimetic titration
(Chloridemeter CMT 10 ; London Scientific Ltd) ; and
ionized Ca was measured with an ion-selective electrode
(Model 92-20 ionpair ; Orion Research Co., Beverly,
MA, U.S.A.). Osmolality was measured by freezingpoint depression (advanced micro-osmometer model
3MO ; Advanced Instruments Inc., Needham Heights,
MA, U.S.A.). The pH and partial pressure of CO in
#
urine were measured at 37 mC with a digital pH\blood
gas analyser (Corning 178 blood pH analyser) ; the
concentration of bicarbonate (HCO−) was calculated
$
from the pH and partial pressure of CO using a
#
solubility factor of 0.0309 and a pK adjusted for ionic
strength [12,13]. Ammonium (NH+), citrate, creatinine,
%
urea, Ca, Mg, sulphate, uric acid and phosphorus were
measured as previously described [14,15].
RESULTS
There was an inverse correlation between the concentration of total or ionized Ca in the urine and the 24 h
urine volume for normal subjects (Figure 2). For those
normal subjects who had concentrations of total and
ionized urinary Ca that exceeded 8 mmol\l and 4 mmol\l
respectively, the urine volume was less than 1 litre per
day. A similar survey was conducted in patients with a
history of nephrolithiasis who excreted more than
200 mg (5 mmol) of Ca per day ; urines with the highest
concentration of Ca corresponded to the lowest urine
volumes (Figure 3). Although these patients were not
given DDAVP and their urine flow rate may not have
been at its nadir, the majority of urine samples with a
concentration of total Ca that was greater than 10 mmol\l
had a volume that was less than 1 litre per day.
Since individual urine samples throughout the 24 h
period might have had a high concentration of Ca and a
high flow rate, a second protocol with much shorter
collection periods was carried out with 14 normal
subjects who excreted less than 200 mg (5 mmol) of Ca
per day. In this acute study, DDAVP was administered
so that the ability of hypercalciuria to cause a water
diuresis of the nephrogenic diabetes insipidus (DI) type
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G. S. S. Lam, J. R. Asplin and M. L. Halperin
Figure 2
Relationship between the concentration of calcium in the urine and the 24 h urine volume for normal subjects
For details, see the Methods section. Each point represents the data from one subject. Log–log plots of the concentration of total Ca in the urine against volume (left
panel) and of the concentration of ionized Ca in urine against urine volume (right panel) are shown for non-stone-forming subjects. Each point represents a single 24 h
urine collection. The solid lines are the linear regressions (left panel, r 2 l 0.68, P 0.001 ; right panel, r 2 l 0.61, P 0.001). The urine samples with the highest
concentrations of Ca are those with the lowest volumes.
Figure 3 Relationship betweenthe concentration of calcium in the urine and the 24 h urine volume for patients with a history
of nephrolithiasis
For details, see the Methods section. Each point represents the data from one patient. Log–log plots of the concentration of total Ca in the urine against urine volume
(left panel) and the concentration of ionized Ca in the urine against urine volume (right panel) are shown for patients with nephrolithiasis. Only urine samples containing
200 mg (5 mmol) of Ca per 24 h were included. Each point represents a single 24 h urine collection. The solid lines are the linear regressions (left panel, r 2 l
0.59, P 0.001 ; right panel, r 2 l 0.52, P 0.001).
could be evaluated. The range of concentrations of total
Ca in the urine from these 14 subjects was from 5 to
17 mmol\l (Table 1, Figure 4). There was no evidence for
a low urine osmolality or a high urine flow rate when the
concentrations of total and ionized Ca in the urine were
at these high values.
We considered the possibility that stone formers may
have an abnormal response to the concentration of Ca in
the urine in the lumen of the MCD which leads to higher
concentrations of Ca in the urine than for normal
subjects. To examine this hypothesis, we compared the
slopes of the plots of Ca concentration against urine
volume using the least-squares statistical model (Figures
2 and 3). There was no significant difference between the
# 2000 The Biochemical Society and the Medical Research Society
stone formers and normal subjects with regard to the
concentration of either total or ionized Ca in the urine.
Thus there was no support for the hypothesis that stone
formers and normal subjects have different responses in
renal concentrating ability that are related to the concentration of Ca in the urine.
Additional studies were carried out to evaluate whether
the concentration of total Ca in the urine might differ
markedly from that of its ionized form, which is the form
likely to bind to the Ca receptor in the lumen of the
MCD. First, those 24 h urine samples with the highest
concentrations of ionized Ca also had the lowest volume
(Figure 3). It is notable that the range of concentrations
of ionized Ca in the urine was very wide. Secondly, the
Calcium and nephrogenic diabetes insipidus
Table 1 Composition of urine samples from normal subjects
in which the concentration of Ca was greater than 5 mmol/l
DISCUSSION
The data are meanspS.E.M. for urine samples that had a concentration of Ca that
exceeded 5 mmol/l in a specimen collected after 12–16 h of water deprivation
and following administration of DDAVP. Only one urine sample was included for
each patient ; individual values are shown in Figure 4. The concentration of ionized
Ca was measured in samples from eight subjects.
The objective of the present study was to test the
hypothesis that a high concentration of Ca in the lumen
of the MCD, and thereby in the urine, would cause an
important defect in the ability to excrete a very concentrated urine, i.e. induce an VP-resistant type of nephrogenic DI.
There are three types of experiment described in the
literature that permit an evaluation of the relationship
between the concentration of Ca in the urine and
nephrogenic DI. At one extreme are data obtained from
in vitro studies in animals. For example, Sands and coworkers [16] perfused isolated inner MCD segments
from rats, adding VP to the bath and either 1 or 5 mmol
of Ca per litre to the luminal fluid. With the higher
luminal concentration of Ca, the permeability for water
was significantly reduced and there was no change in the
permeability for urea. Although these data support the
view that a high concentration of Ca in the urine causes
nephrogenic DI in qualitative terms, the decline in water
permeability was quantitatively small (33 % decrease).
This raises the possibility that the residual permeability
for water could still be high enough to prevent a major
increase in excretion of electrolyte-free water in vivo.
The second category of experimental data was obtained
in human subjects given an acute infusion of Ca to raise
the concentration of Ca in plasma to 15 mg\dl
(3.75 mmol\l) [17]. These subjects had a larger urine
volume when hypercalciuria was present. Notwithstanding, although the urine flow rate was high, the urine
osmolality was never less than the plasma osmolality.
This high urine flow rate was largely the result of an
increase in the rate of excretion of Na+ and Cl− (rise in
Na+ excretion from 100 to 536 µmol\min). Therefore an
important reason for the high urine flow rate accom-
Parameter
Value
Flow rate (ml/min)
Osmolality (mOsm/kg of water)
pH
Concn. (mmol/l)
Na+
K+
Cl−
Total Ca
Ionized Ca2+
Mg2+
Phosphate
NH4+
Urea
Creatinine
Citrate
SO42−
0.6p0.1
959p41
5.9p0.2
167p12
53p4
173p13
9.5p0.7
6.2p0.8
7.7p0.7
28p3
45p6
431p28
16p1.4
2.7p0.6
35p7.7
concentration of ionized Ca in the urine was measured
for eight of the 14 normal subjects reported in Figure 4,
and ranged from 2.5 to 8.8 mmol\l. The concentration of
citrate in the urine was relatively constant, and much
lower in magnitude than the concentration of total or
ionized Ca (Table 1). Hence it is unlikely that urines with
a very high concentration of total Ca had a low
concentration of ionized Ca due to the formation of
Ca–citrate ion complexes.
Figure 4
Effect of concentration of calcium in the urine on renal concentrating ability in human subjects given DDAVP
Urine flow rate (right panel) and osmolality (left panel) are plotted against urine Ca concentration. For details, see the Methods section. Each point represents the data
from one of the 14 normal human subjects.
# 2000 The Biochemical Society and the Medical Research Society
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G. S. S. Lam, J. R. Asplin and M. L. Halperin
panying acute hypercalcaemia and hypercalciuria was the
very high rate of excretion of Na+ and Cl−, which may
reflect the effects of hypercalcaemia in the medullary
thick ascending limb (mTAL) in the loop of Henle
(Scheme 1).
The third category of data was obtained from studies in
patients with a chronic elevation in the concentration of
Ca in the urine. The report of Zeffren et al. [18] is very
important because 12 patients were studied, the concentration of Na+ in plasma was reported, the urine
osmolalities before and after VP administration were
available, and they provided a longer-term follow-up.
These data were again not consistent with a nephrogenic
DI-type of lesion, because all but one of these patients
had hyponatraemia rather than a tendency to hypernatraemia, a urine\plasma osmolality ratio of greater
than 1, and a small further rise in urine osmolality when
VP was administered. In the single patient who had
data consistent with a diagnosis of nephrogenic DI
(hypernatraemia and a urine\plasma osmolality ratio of
less than 1 when AVP was administered), these findings
persisted after the hypercalcaemia and hypercalciuria
were reversed.
There have been many studies in animals in which
hypercalcaemia
and\or hypercalcuria
was\were
associated with a larger volume of urine (e.g. see [19–24]).
In all of these studies, the increased urine volume was not
associated with a urine osmolality that was excessively
low ; rather, an electrolyte-induced osmotic diuresis was
present, consistent with the effects of hypercalcaemia
acting on the mTAL (Scheme 1). Overall, the net effects
of hypercalcaemia on the mTAL are similar to those of a
loop diuretic. Therefore one can anticipate the excretion
of a large volume of urine containing appreciable
quantities of Na+ and Cl−.
Perspectives
Although a Ca receptor faces the lumen of the MCD,
extremely high urinary concentrations of total and
ionized Ca did not compromise the renal concentrating
process to a measurable extent (Figures 2–4). Rather,
polyuria can be explained by the actions of hypercalcaemia on the mTAL of the loop of Henle, which lead
to both a lower medullary interstitial osmolality and a
higher distal delivery of Na+ and Cl−. Nevertheless, can
the hypothesis outlined in Scheme 1 be rescued ? Clearly
the portion in the MCD is the component of the
hypothesis that is in jeopardy. A high degree of complexformation of Ca with citrate is a possible explanation as
to why nephrogenic DI was not present. However, this
explanation is not likely, because the concentration of
ionized Ca was high and the concentration of total Ca
was much greater than that of citrate (Table 1). Moreover,
the excretion of citrate declines markedly in patients with
metabolic acidosis [25,26], a condition that is not
# 2000 The Biochemical Society and the Medical Research Society
associated with nephrogenic DI. Further, although it
might be logical for a high pH to attenuate the effects of
urinary Ca on water permeability in the MCD (higher
proportion of HPO#−), this was not supported by our
%
data (Table 1 ; Figures 2–4).
Perhaps there are other ways by which an elevated
luminal concentration of Ca could increase the urine
volume by a small, but significant, amount and thereby
lessen the risk of Ca stone formation. Two points are
relevant in this context. First, one need only achieve a
small increase in urine volume to reduce the risk of this
precipitation reaction. In more detail, if the urine flow
rate doubled within the oliguric range, for example from
0.3 to 0.6 ml\min, the Caioxalate ion product would
decrease by 4-fold [(0.5iconcentration of Ca)i(0.5i
concentration of oxalate)]. Secondly, the volume of
urine will increase if the rate of excretion of osmoles
rises at a given urine osmolality. The two major
categories of osmoles to consider are electrolytes (Na+
and Cl− for the most part) and urea. Since the
reabsorption of urea in the inner MCD is enhanced
during oliguria owing to the actions of VP [27], it
would be theoretically possible to have a small increment
in urine flow rate when the concentration of Ca in the
urine is high if the net response of the binding of ionized
Ca to its luminal receptor diminished the permeability of
the inner MCD to urea. Consistent with this speculation
is the observation of Gowrishankar et al. [28], who
demonstrated that urea did not approach a diffusion
equilibrium between the lumen of the inner MCD and
the papillary interstitial compartment when the rate of
excretion of electrolytes was low. Having urea as an
‘ effective ’ osmole in this setting would prevent an even
lower urine volume [28,29] and thereby a higher concentration of Ca in the urine (Scheme 1).
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Received 6 August 1999/1 November 1999; accepted 6 December 1999
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