The Journal of Clinical Endocrinology & Metabolism Vol. 83, No. 6 1916-1920
Copyright © 1998 by The Endocrine Society
From the Clinical Research Centers |
A Defect in Renal Calcium Conservation May Contribute to the Pathogenesis of Postmenopausal Osteoporosis1
Hassan M. Heshmati,
Sundeep Khosla,
Mary F. Burritt,
W. Michael O’Fallon and
B. Lawrence Riggs
The Endocrine Research Unit, Division of Endocrinology and
Metabolism (H.M.H., S.K., B.L.R.), Department of Laboratory Medicine
and Pathology (M.F.B.), and the Section of Biostatistics, Department of
Health Sciences Research (W.M.O.), Mayo Clinic and Mayo Foundation,
Rochester, Minnesota 55905
Address all correspondence and requests for reprints to: B. Lawrence Riggs, M.D., Mayo Clinic, 200 First Street SW, Plummer North 6, Rochester, Minnesota 55905.
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Abstract
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Although all postmenopausal women are estrogen deficient, women who
have postmenopausal osteoporosis may have a defect, in addition to
estrogen deficiency, that accounts for their higher rates of bone
resorption and greater bone loss, relative to those who do not. To test
the hypothesis that one defect is an impairment in renal calcium
conservation, we measured renal calcium transport in 19 osteoporotic
and 19 normal postmenopausal women, whose ages (median and 25th–75th
percentile range) were 70 yr (range, 67–72) and 72 yr (range, 69–74),
respectively. There was no difference between groups in values for
serum ionized calcium and PTH concentrations or in renal filtered load
of calcium. However, before PTH infusion, the osteoporotic women had
lower (P = 0.0046) values for tubular reabsorption
of calcium (TRCa) of 96.8% (range, 96.0–97.1) vs.
98.0% (range, 97.2–98.3) and higher (P = 0.0154)
urinary calcium excretion of 0.194 mg/dL of glomerular filtrate (GF)
(0.154–0.239) vs. 0.125 mg/dL of GF (0.103–0.173) than
the normal women. After infusion of 200 U of synthetic PTH (synthetic
1–34 analog of human PTH), TRCa increased and calcium excretion
decreased comparably in both groups, so that the differences between
groups after intervention remained: for TRCa, 98.3% (97.7–98.6)
vs. 98.9% (98.4–99.3; P = 0.0042);
and for calcium excretion, 0.099 mg/dL of GF (0.080–0.138)
vs. 0.066 mg/dL of GF, (0.045–0.097,
P = 0.0180). In conclusion, postmenopausal women
with osteoporosis have a PTH-independent defect in renal calcium
conservation. This defect is of sufficient magnitude to contribute to
the greater negative calcium balance in postmenopausal women with
osteoporosis vs. their postmenopausal peers.
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Introduction
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ALL postmenopausal women are estrogen
deficient, but in only a minority of them does osteoporosis develop
within the first 20 yr after menopause. Moreover, there are large
differences between normal and osteoporotic postmenopausal women in
their levels of bone resorption (1), degrees of negative calcium
balance (2), and rates of bone loss (3).
These differences between osteoporotic and normal postmenopausal women
led Riggs and Melton (4) to hypothesize that women in whom
postmenopausal osteoporosis develops have some condition, in addition
to estrogen deficiency, that predisposes them to a higher rate of and,
possibly, a more prolonged postmenopausal bone loss. Potential factors
or combinations of factors that have been suggested to account for the
greater bone loss in osteoporotic women include a greater degree of
postmenopausal estrogen deficiency (5), increased responsiveness of
bone to PTH (6), intestinal calcium malabsorption (7), and possibly an
enhanced paracrine secretion in bone of cytokines that may mediate the
effect of estrogen deficiency (8, 9). However, not one of these has
been clearly established as the major causal factor, and other
investigators have failed to confirm that postmenopausal osteoporotic
women had differences in residual postmenopausal levels of sex steroids
(10), bone responsiveness to PTH (11), intestinal calcium absorption
(12), or enhanced skeletal production of bone-resorbing cytokines from
bone marrow (13).
We report here that women with postmenopausal osteoporosis have a
PTH-independent abnormality of renal calcium conservation that is of
sufficient magnitude that it could explain why they have a greater
negative calcium balance than their postmenopausal peers.
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Subjects and Methods
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Subjects
After approval of the protocol by the Mayo Institutional Review
Board, 19 women with established postmenopausal osteoporosis and 19
postmenopausal normal women were studied. All subjects gave informed
consent. Subjects with significant medical diseases had been excluded
from the study. No subject was taking any medication known to affect
calcium and bone metabolism. Thoracic and lumbar spine radiographs and
spine and hip bone mineral density measurements were obtained for all
subjects. Osteoporosis was defined as a history of nontraumatic
fracture of vertebrae, hip, or distal forearm (Colles’ fracture) and
bone density of the lumbar spine more than 2.5 SD below the
young normal mean. Urinary calcium levels were not considered in the
selection process. The main clinical characteristics of the subjects
are shown in Table 1
.
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Table 1. Main clinical characteristics of 19 postmenopausal
osteoporotic women and 19 postmenopausal normal women
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Design
The nutritional status of the subjects was assessed by a trained
dietitian. Throughout the study period, the subjects maintained their
habitual calcium intake. Twenty-four hours before the study morning,
the subjects commenced a 24-h urine collection for the determination of
urinary creatinine, calcium, and cross-linked N-telopeptides of type I
collagen (NTx), a marker for bone resorption. They were admitted to the
Mayo General Clinical Research Center at 1700 h on the evening
before the study morning. After an overnight fast and bed rest, at
0630 h, each emptied her bladder to complete the 24-h urine
collection. The subjects remained fasting and at bed rest except to
urinate. Commencing at 0700 h, a 41/2-h inulin clearance
study was performed as previously reported (14). At 0930 h (t
= 0), a pharmacologic dose (200 U) of the synthetic 1–34 analog of
human PTH (Rorer Pharmaceuticals, Fort Washington, PA) dissolved in 50
mL of 5% dextrose/5% human albumin (Plasbumin, Miles, Elkhart, IN)
was infused at a constant rate over 15 min. Blood and urine were
collected before (t = -90, -30, and 0 min) and after (t =
30, 60, and 120 min) PTH infusion (Fig. 1). The following variables were measured
before and after PTH infusion: serum total calcium, ionized calcium,
ultrafiltrable calcium (UFCa), phosphorus (P), sodium (Na), and
creatinine; plasma cAMP and inulin; and urinary calcium, P, Na,
creatinine, cAMP, and inulin. Serum PTH was determined at -30 and 0
min.
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Figure 1. Timing of blood sampling, urine collection,
and PTH infusion. Urine collections A and B are for baseline
measurements, and urine collections C, D, and E are for post-PTH
infusion measurements.
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Assay methods
Serum PTH was measured by an immunochemiluminometric assay
(Sanofi, Chaska, MN), plasma and urinary cAMP by RIA (Incstar
Corporation, Stillwater, MN), serum estrone and estradiol by RIA
(Diagnostic Product Corporation, Los Angeles, CA), urinary NTx by
enzyme-linked immunosorbent assay (Ostex International, Seattle, WA),
serum and urinary calcium by atomic absorption spectroscopy
(Instrumentation Laboratories, Boston, MA), serum ionized calcium with
a Radiometer ICA 2 Analyzer (Radiometer America, Westlake, OH), UFCa
after separation in a micropartition cartridge (Amicon Centrifree
Micropartition System, Beverly, MA), serum and/or urinary P, Na, and
creatinine by routine automated methods (Hitachi 911 Analyzer,
Indianapolis, IN), and plasma and urinary inulin by standard
colorimetric methods (15).
We used standard formulas for the calculation of glomerular filtration
rate (GFR), calcium filtration rate, tubular reabsorption of calcium
(TRCa), calcium excretion, tubular reabsorption of P (TRP), tubular
reabsorption of Na (TRNa), and nephrogenous cAMP (NcAMP) (14).
The plasma or serum values used in the calculations were the mean of
blood samples drawn at the beginning and end of urine collection.
Statistical analysis
Statistical evaluation was performed with the Wilcoxon and the
Spearman rank correlation tests. All results were expressed as median
and range.
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Results
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Except for number of vertebral fractures and level of bone
density at the lumbar spine, the two groups were comparable in general
characteristics (Table 1
). The difference in bone density, 27%,
indicates a moderately severe degree of osteoporosis. Secondary
analytical variables at baseline (Table 2) were similar, except the osteoporotic
women had significant decreases in serum levels of UFCa and significant
increases in 24-h urinary excretion of calcium.
The variables related to renal calcium conservation, measured at
baseline and after PTH infusion (Table 3), were the primary data used to test
the hypothesis that women with postmenopausal osteoporosis have
impaired renal calcium conservation. At baseline, the osteoporotics had
a higher urinary calcium excretion (adjusted for glomerular
filtration). There was no difference in the renal filtered load of
calcium between the two groups. However, the osteoporotic women had a
significantly lower TRCa, a difference that was of sufficient magnitude
to account for their excess calcium excretion. We then evaluated the
possibility that this was the result of decreased sensitivity to the
action of PTH on the renal tubule, by repeating the measurements after
infusion of PTH. After this infusion, TRCa increased and calcium
excretion decreased significantly in both groups. However, the
proportional mean increase in TRCa and mean decrease in urinary calcium
excretion were the same in both the osteoporotic and control groups, so
the significant differences noted in these two measurements at baseline
were maintained despite near-maximal PTH stimulation.
Table 4 gives the secondary variables for
aspects of renal function other than calcium at baseline and after PTH
infusion. There were no differences in P transport or NcAMP production
at baseline. However, there was a trend for a larger increase in NcAMP
in the osteoporotic women after the PTH infusion. TRNa was slightly,
but significantly, higher in the osteoporotic women at baseline;
however, the Na filtration rate was similar in both groups. No
significant correlation was observed between serum PTH and TRCa (both
basal and peak values) and between peaks of NcAMP and TRCa (data not
shown).
Values for individual subjects for percent TRCa and percent TRP in the
two groups, before and after PTH infusion, are given in Figs. 2 and 3,
respectively. Because nonparametric statistics were used in the
analysis, exclusion of the outlier in the osteoporotic group in Fig. 2
did not alter the statistical significance of the differences in the
percent TRCa between the osteoporotic and control women either at
baseline or after PTH infusion.
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Figure 2. TRCa at baseline and after PTH infusion in
postmenopausal control and osteoporotic women (lines represent the mean
values).
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Figure 3. TRP at baseline and after PTH infusion in
postmenopausal control and osteoporotic women (lines represent the mean
values).
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Discussion
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The data from this study clearly show that renal calcium
conservation is impaired in osteoporotic postmenopausal women, compared
with control women of similar age. This defect is in addition to the
increase in bone resorption occurring as a consequence of estrogen
deficiency. Although it has been suggested previously that renal
calcium homeostasis is abnormal in postmenopausal osteoporotic women
(16, 17), those studies measured renal calcium transport indirectly. In
the present study, we measured all contingent variables directly and
thus established unequivocally that the osteoporotic women had abnormal
renal calcium transport: a decrease in TRCa associated with an increase
in the renal calcium excretion. Because our patients had severe
osteoporosis, we do not know whether those with only mild disease will
also have this defect.
We have investigated several possible mechanisms that might account for
this abnormality. First, it could be caused solely by the increase in
bone resorption caused by estrogen deficiency, in the absence of an
additional defect in renal calcium transport. This would result in an
increase in skeletal calcium outflow, leading to a decrease in PTH
secretion and, thus, to a decrease in the stimulatory effect of PTH on
renal TRCa. Our data are not compatible with this mechanism. Serum PTH
and NcAMP values were not decreased significantly, and the renal
calcium filtration rate was not increased significantly, in contrast to
what would be predicted to occur if this mechanism were operable. Most
important, the abnormality in renal tubular calcium transport continued
after PTH infusion. Because the dosage of PTH that we used has
previously been shown to induce a maximal increase in TRCa (18), the
defect seems to be independent of PTH.
A second possibility is that the decrease in renal tubular calcium
transport results from decreased renal sensitivity to the action of
normal serum levels of PTH. Contradicting this possibility is the
finding that there was a somewhat greater increase in NcAMP in the
women with osteoporosis. Moreover, the increases in TRCa in the two
groups, after PTH infusion, were similar: the osteoporotic women had
lower values at baseline and continued to have lower values after PTH
infusion.
A third possibility is that there is a lifelong, primary impairment of
renal tubular function but that this defect is too subtle to be
recognized clinically before the osteoporosis becomes evident. If so,
it is limited to renal calcium transport, because we found no
abnormalities in GFR or in renal tubular transport of P. The
significance, if any, of the minimal increase in TRNa at baseline is
unclear. Also, we did not test for the possibility of a defect in renal
acid-base handling.
A fourth possibility is that the defect could be related to a greater
degree of estrogen deficiency in the osteoporotic women. There is ample
evidence that estrogen has direct effects on the kidney (14, 19, 20).
However, in keeping with our previous results (10), we failed to find a
difference between the two groups in the postmenopausal low levels of
serum estrone and estradiol, the principal physiologic estrogens.
The final possibility, and the one that we currently favor, is that
women with postmenopausal osteoporosis have an inherent, possibly
genetically-determined, abnormality in the paracrine mediation of the
estrogen effect, so that the tissue response to estrogen deficiency is
amplified both in bone and in the renal tubule. If so, postmenopausal
osteoporotic women would have greater bone loss than their peers,
because the same degree of estrogen deficiency would induce a greater
increase in net bone resorption and a greater impairment in TRCa. The
proportional increase in skeletal calcium outflow and increase in renal
calcium losses would tend to cancel the respective effects of the two
processes on decreasing or increasing serum PTH levels. Previous
studies have shown that the effects of estrogen deficiency on the
skeleton may be mediated by paracrine production of cytokines, such as
interleukin-1, tumor necrosis factor 
, and interleukin-6 (8, 9, 21, 22). We have recently shown that estrogen seems to have direct effects
on renal calcium handling (14). It remains to be determined, however,
whether the same factors that mediate the effects of estrogen
deficiency on the skeleton also do so in the renal tubule.
Whatever the mechanism, continued renal calcium wastage, if
uncompensated for by an increase in intestinal calcium absorption,
would lead to continued bone loss, because bone contains 99% of the
body calcium stores (23). Because intestinal calcium absorption has
been shown to be either decreased or normal in women with
postmenopausal osteoporosis (12, 24), it is unlikely that a significant
degree of intestinal compensation for the renal calcium loss
occurs.
Although the degree of calcium loss caused by the impairment of renal
calcium conservation in the osteoporotic women may seem small, it is
substantial, in terms of body calcium homeostasis. If the amount of
excess calcium excretion in the osteoporotic women, over that in the
control women, that we have found is projected to occur over 24 h,
it would amount to an estimated wastage of 65 mg/day. We recognize,
however, that this extrapolation may represent an overestimate, because
the urine collections were made in the early morning, a time at which
circadian studies have shown that bone resorption is maximal, as
estimated by urinary pyridinium cross-link levels (16). Nonetheless, a
loss of this degree is comparable with the excess negative calcium
balance that has been demonstrated in women with postmenopausal
osteoporosis (2). Thus, the calcium wastage caused by this defect could
contribute substantially to the excessive bone loss that leads to
postmenopausal osteoporosis.
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Acknowledgments
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We wish to thank the women who volunteered to participate in
this study; J. M. Muhs for recruiting the subjects; B. J.
Norby and L. A. Wahlstrom for performing the renal clearance
study; L. Oenning, H. M. O’Connor, and S. Nayar for nutritional
assessment; R. A. Soderberg, C. A. McAlister, S. K.
Bonde, M. A. Anderson, D. M. Hanson, S. H. Showalter,
K. C. Hicok, and D. W. Heser for technical assistance; N.
Geller for the illustration; and P. C. Wollan and K. S. Egan
for statistical help.
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Footnotes
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1 This work was supported, in part, by Research Grants AG-04875 and
MO1-RR-00585 from the National Institutes of Health, USPHS. 
Received May 19, 1997.
Accepted February 18, 1998.
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Abstract
Introduction
