Clinical Chemistry 46: 248-251, 2000;
(Clinical Chemistry. 2000;46:248-251.)
© 2000 American Association for Clinical Chemistry, Inc.
Strontium as a Marker for Intestinal Calcium Absorption: The Stimulatory Effect of Calcitriol
Marieke Dijkgraaf-ten Bolscher1,a,
J. Coen Netelenbos1,
Rob Barto2 and
Wim J.F. van der Vijgh2
1
Department of Endocrinology and
2
Clinical Research Laboratory of Internal Medicine, University Hospital Vrije Universiteit, De Boelelaan 1117, 1081 HV Amsterdam, The Netherlands.
a Address correspondence to this author at: University Hospital Vrije Universiteit, Room Br 232, De Boelelaan 1117, 1081 HV Amsterdam, The Netherlands. Fax 31-20-4443844; e-mail M.tenBolscher{at}azvu.nl
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Abstract
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Background: Intestinal strontium absorption is becoming
accepted as a clinical and diagnostic tool for assessing intestinal
calcium absorption in humans. However, little is known about whether
intestinal strontium absorption, like that of calcium, is stimulated by
calcitriol in healthy humans.
Methods: The effect of calcitriol on intestinal strontium
absorption was measured in eight healthy men, ages 20–60 years. Before
administration of calcitriol, two tests were performed with an interval
of 10 days for calculating the within-subject variation
(SER). Before the third test, 0.5 µg of calcitriol was
given twice daily for 3 days. In each test, the fractional strontium
absorption (Fc240) and the area under the
concentration-time curve (AUC0–240) 4 h after an oral
strontium load of 2.5 mmol were calculated.
Results: The within-subject SER of Fc240
and AUC0–240 was 1.7 ± 0.7 and 0.83 ± 0.1,
respectively. The stimulatory effect of calcitriol on
Fc240 and AUC0–240 was 35% (21.8 ± 2.0
to 28.8 ± 2.4; P = 0.003) and 61% (8.97
± 0.97 to 14.4 ± 1.3 mmol · L-1 · min;
P = 0.001), respectively.
Conclusions: Although the reproducibility of
AUC0–240 and its sensitivity to calcitriol were better
than those of Fc240, the Fc240 of strontium is
preferred for a clinical test because of its simplicity, requiring only
two instead of five blood samples.
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Introduction
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Abnormal or inadequate intestinal calcium absorption is a
contributing factor in certain disease states, e.g., osteoporosis. The
study of intestinal calcium absorption and calcium metabolism in
animals and humans is essential for further elucidating basic
mechanisms, for understanding disease processes, and for assessing
therapeutic strategies.
Strontium has become a clinical and diagnostic tool for measuring
calcium absorption in humans (1)(2)(3)(4)(5)(6). Various studies have
demonstrated a close correlation between the intestinal absorption of
(radioactive) calcium and nonradioactive strontium (7)(8).
Previously, we determined the
bioavailability
(AUCpo
/AUCiv x
Doseiv/Dosepo) of
SrCl2 in healthy male volunteers to obtain a
simple parameter that is most representative for intestinal absorption.
We found that the bioavailability of SrCl2
correlated best with the fractional absorption of strontium at 240 min
(Fc240)1
after the oral intake of the test solution (1).
Using this test, we undertook a double-blind placebo-controlled study
to investigate whether 17ß-estradiol stimulates intestinal strontium
absorption in healthy postmenopausal women (9). In that
study, 17ß-estradiol did not affect the Fc240
of strontium after the test solution was ingested. A possible
explanation could be that the calcium/strontium load (9.0 mmol of
calcium and 5.0 mmol of strontium) given to the women was too high,
which could render the test insensitive with respect to the
possible stimulation of active transport by 17ß-estradiol. Therefore,
we reduced the load of the test to 2.5 mmol of strontium only.
The aim of this study was to assess the reproducibility of the renewed
strontium absorption test. Thereafter, the stimulatory effect of
calcitriol [1,25(OH)2D3] on
intestinal strontium absorption, as reflected by
Fc240, was investigated. To check for possible
shifts in the absorption profile, plasma strontium concentrations at 1,
2, and 3 h were also determined.
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Materials and Methods
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study population
Eight healthy male volunteers, ages 20–60 years, participated in
this study. All subjects gave informed consent to participate in the
study. The ethics committee of our hospital approved the study. All
procedures followed were in accordance with the Declaration of
Helsinki. The mean body weight of the volunteers was 74.9 kg (range,
55–90 kg); their mean body mass index was 23.8
kg/m2 (range, 20.2–25.9
kg/m2).
study design
All subjects arrived in the outpatient clinic after overnight
fasting. Body weight was measured, and blood samples were withdrawn for
the determination of plasma strontium, calcium, albumin, phosphate and
1,25(OH)2D. Subjects received 200 mL of a test
solution containing 2.5 mmol of SrCl2 without
additional calcium. The test solution was consumed within 1 min. One,
2, 3, and 4 h thereafter, a blood sample was withdrawn for the
determination of strontium. Blood samples were centrifuged at
1500g for 10 min. Subsequently, plasma was separated and
stored at -20 °C until analysis. All plasma samples were analyzed
for strontium by graphite furnace atomic absorption spectrophotometry (10). The SrCl2 solution was prepared
by the Pharmacy Department of our hospital.
SrCl2 · 6 H2O (pro
analysis) was obtained from Merck.
The reproducibility of the strontium absorption test was assessed by
repeating the test with an interval of 10 days. Seven days after the
second baseline test, participants received 0.5 µg of
1,25(OH)2D3 (Rocaltrol; Roche
Nederland) twice daily for 3 days before the third strontium absorption
test.
methods
Plasma 1,25(OH)2D was measured by
radioimmunoassay after immunoextraction (IDS). The intra- and
interassay coefficients of variation (CVs) were 6.3% and 9.7%,
respectively.
data analysis
For each subject, three plasma strontium concentration-time curves
were obtained. All concentrations measured were corrected for
endogenous strontium concentrations, determined at t =
0 of each test. The time at which the highest plasma
concentration (cmax) was observed was
designated tmax. The area under the
concentration-time curve up to time t
(AUC0–t), expressed as
mmol · L-1 · min, was calculated
by the trapezoidal rule, using the pharmacokinetic computer program
Topfit 2.0 (11). The Fc of strontium 4 h after an oral
strontium load (Fc240) was calculated
according to the following formula (1):
in which c240 is the plasma
concentration 4 h after ingestion (mmol/L);
c0 is the plasma concentration at 0
min (mmol/L); V is the central volume of distribution,
represented by 15% of the body weight (L); and
D is the dose of strontium (2.5 mmol).
The within-subject variation of Fc240 was
determined from its values in test 1 (baseline,
xi1) and test 2 (replicated test,
xi2) by the equation:
in which n represents the number of volunteers (n = 8).
statistical analysis
Data are expressed as the mean (± SE). Between- and
within-subject variations were assessed for Fc240
and AUC0–240. The significance of the
stimulatory effect of 1,25(OH)2D3 on
intestinal strontium absorption (Fc240 and
AUC0–240) between baseline and treatment test
was determined by a paired-sample t-test. The correlation
between the plasma 1,25(OH)2D concentration and
the values of Fc240 and
AUC0–240 at baseline and after treatment with
1,25(OH)2D3 were tested for
significance by means of a Spearman correlation test. Differences were
significant at P <0.05 (two-tailed). All analyses were
performed using the Statistical Package for the Social Sciences (SPSS
7.5 for Windows).
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Results
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The plasma concentration-time curves of strontium at baseline and
after 1,25(OH)2D3 treatment of each
subject are shown in Fig. 1
. After oral administration of the test solution, the mean
maximum plasma strontium concentration of the baseline test was
51.9 ± 5.0 µmol/L (range, 25.4–69.3 µmol/L; n = 8).
This maximum concentration (cmax) was
reached at a mean time (tmax) of 173
min (range, 120–240 min) after administration.
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Figure 1. Concentration-time curves of strontium in plasma after an
oral load of 2.5 mmol of SrCl2 in eight healthy male
volunteers at baseline and after 1,25(OH)2D3
treatment.
Symbols indicate individual volunteers.
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The within- and between-subject variation of
Fc240 and AUC0–240 are
shown in Table 1
. The within-subject variation
(SER) of the AUC0–240 was
approximately twofold lower than that of the
Fc240 (0.83 ± 0.1 and 1.7 ±
0.7, respectively).
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Table 1. Between- and within-subject variation of the fractional
absorption (Fc240) and area under the concentration-time
curve (AUC0–240) of strontium 4 h after an oral load of
2.5 mmol of SrCl2 in eight healthy male
volunteers.1
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The biochemical variables (Table 2
) did not show a significant change after treatment with 0.5
µg of 1,25(OH)2D3 twice daily for 3
days in these eight volunteers. Despite a nonsignificant increase of
the plasma 1,25(OH)2D concentration,
1,25(OH)2D3 significantly increased
the Fc240 and AUC0–240 of
strontium (Table 3
).
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Table 3. Effect of 1,25(OH)2D3 on
Fc240 and AUC0–240 of strontium 4 h after an
oral load of 2.5 mmol of SrCl2 in eight healthy male
volunteers.1
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The stimulatory effect of 1,25(OH)2D3
on Fc240 and AUC0–240 was
35% (21.8% ± 2.0% to 28.8% ± 2.4%; P =
0.003) and 61% (8.97 ± 0.97 to 14.4 ± 1.3
mmol · L-1 · min; P =
0.001), respectively.
Fc240 did not correlate with the plasma
1,25(OH)2D concentrations. However, a positive
significant correlation was observed between the plasma
1,25(OH)2D concentrations and
AUC0–240. Fig. 1
illustrates that under the
influence of 1,25(OH)2D3, the mean
value of cmax increased to 74 ±
7.6 µmol/L (range, 51.2–114.1 µmol/L; n = 8) and the mean
value of tmax decreased to 128 min
(range, 60–240 min).
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Discussion
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As expected (12), reducing the load of the strontium
absorption test to 2.5 mmol of strontium only, instead of 9.0 mmol of
calcium and 5.0 mmol of strontium (1), produced an increased
value of Fc240, i.e., from 7.0% to 21.8%. The
within-subject variation for Fc240 of strontium
was considerably better than that described earlier (1)(7). Furthermore, the CV for
AUC0–240 was 10%, which is quite
similar to the CV of 11.8% reported by Vezzoli et al. (3),
who used a comparable strontium load of 30.2 µmol/kg. The mean
AUC0–240 values of strontium of 9.36 (baseline)
and 8.58 (repeated test)
mmol · L-1 · min in our present study
were comparable with the 8.0 and 9.43
mmol · L-1 · min found by Vezzoli and
co-workers (3)(4) in two separate studies.
The stimulating effect of
1,25(OH)2D3 on intestinal
calcium absorption has already been well established (13)(14). With regard to strontium, different
opinions exist about the mechanism of strontium transport through the
intestinal wall. Dumont et al. (15) have concluded from
their experiments that transport of strontium is passive only. Others
have found evidence for active intestinal strontium absorption and
indications for a common transport mechanism of calcium and strontium (16)(17). Our previous studies in rats gave
evidence for active transport of strontium across the intestinal wall,
which was 1,25(OH)2D3 dependent (18). To apply strontium as a marker for intestinal calcium
absorption in humans, it is necessary that the effect of intervention
on intestinal calcium absorption is reflected by the strontium
absorption test.
1,25(OH)2D3 stimulated intestinal
strontium absorption by increasing the
cmax, as illustrated in Fig. 1
;
tmax also decreased. This means that
strontium, like calcium, is at least partially absorbed by active
transport. To date, only one study in humans has discussed the
stimulatory effect of 1,25(OH)2D3 on
intestinal strontium absorption (19). However, the authors
determined fractional strontium absorption by means of deconvolution,
for which purpose 45Ca was given orally and
85Sr was administered intravenously. This
procedure is debatable because the two elements have comparable, but
not identical, pharmacokinetic behaviors.
Our present study measured intestinal strontium absorption and shows
that strontium, like calcium, can be stimulated by
1,25(OH)2D3. Because of its good
reproducibility, it is a sensitive test. It can be concluded,
therefore, that the strontium absorption test provides an appropriate
measure for intestinal calcium absorption and that this test is a good
alternative for measuring modulating effects of interventions on
intestinal calcium absorption as discussed recently by Heaney (20).
The Fc240 can be used for the assessment of
intestinal strontium absorption after limited sampling.
Fc240 did not correlate with plasma
1,25(OH)2D concentrations. A possible explanation
for this finding is that the tmax is
not always near 240 min. Nevertheless, the Fc240
of strontium was still highly significantly increased after
1,25(OH)2D3 treatment. Therefore, the
Fc240 of strontium can still be used as a measure
for intestinal calcium absorption. However, when more information is
desired about possible changes in the absorption profile, the
concentration-time curve of strontium must be considered. Under these
circumstances, quantitative information can then be obtained by the
measurement of an absorption parameter that is based on more than two
samples, e.g., AUC0–240.
The present study supports the view that the strontium absorption test
is a good measure for intestinal calcium absorption. Although the
reproducibility of the AUC0–240 and its
sensitivity to 1,25(OH)2D3 were
better than those of Fc240, the
Fc240 of strontium is preferred as a clinical
test because of its simplicity, requiring only two instead of five
blood samples.
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Acknowledgments
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We thank Simone Neele for collecting the numerous blood samples.
The volunteers are acknowledged for their enthusiastic participation in
this study. We thank the Laboratory of Clinical Chemistry and the
Laboratory of Endocrinology, University Hospital Vrije Universiteit,
Amsterdam, for the measurement of the biochemical variables and the
Department of Pharmacy for preparing the test solution. We also thank
J. Kuik for advice concerning the statistical analysis.
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Footnotes
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1 Nonstandard abbreviations: Fc, fractional absorption; 1,25(OH)2D3, calcitriol; and AUC, area under the curve.
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Abstract
Introduction
