1 Geriatric Research, Education, and Clinical Center, St. Louis Veterans Administration Medical Center, 63125, and 2 Division of Geriatric Medicine and 3 Department of Biochemistry and Molecular Biology, St. Louis University Health Sciences Center, St. Louis, Missouri 63104
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ABSTRACT |
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The
purpose of this study was to determine whether there are differences in
intestinal Ca and phosphate transport in mice having different peak
bone densities. Intestinal transport was measured in C57BL/6 (C57, low
bone density) and C3H/He (C3H, high bone density) female mice.
Unidirectional (mucosal to serosal) transport of Ca was 58% higher in
C3H compared with C57 mice, as measured by everted duodenal sacs. The
capacity of the duodenal mucosa to take up Ca was also higher in the
C3H mice. This uptake highly correlated with Ca transport
across the intestine. 1,25-Dihydroxyvitamin D3
[1,25(OH)2D3], which stimulates
intestinal Ca absorption, markedly stimulated unidirectional Ca
transport and uptake to similar levels in both strains of mice. On the
other hand, unidirectional phosphate transport in C3H mice was only
36% that of C57 mice. mRNA levels of the plasma membrane Ca pump were
90% higher in the duodenum of C3H mice. There was no difference
between strains in duodenal calbindin or 24-hydroxylase mRNA levels.
Regarding vitamin D metabolism, there was no difference in serum
1,25(OH)2D3 levels or in renal 1-hydroxylase
mRNA levels. The combination of high intestinal Ca transport and low
phosphate transport may contribute to the high peak bone density seen
in the C3H relative to the C57 mouse.
1,25-dihydroxyvitamin D3; calbindin; intestinal 24-hydroxylase; duodenum; plasma membrane calcium pump
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INTRODUCTION |
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MICE HAVE BEEN SHOWN TO HAVE a wide range of bone densities (10). In particular, C57BL/6 (C57) mice have a low peak bone density, and C3H/He (C3H) mice have a high peak bone density. Peak bone density is an important determinant of bone mass later in life (1, 31). Therefore, C57 and C3H strains have been used as model systems for determining important parameters regarding peak bone mass. Differences in a number of systemic and bone-related factors have been reported between these strains (13, 18, 23).
Absorption of dietary calcium (Ca) by the small intestine is an important component of Ca homeostasis (30). A major factor in attaining peak bone density in humans is the intestinal absorption of dietary Ca (1, 31). Ca absorption early in life may play a role in the attainment of peak bone mass and in prevention of low bone mass in older individuals. Intestinal Ca absorption may also be related to bone mass in mice. Ca balance studies have shown that Ca balance is decreased in C57 compared with C3H mice (11). In addition, Ca supplementation has been reported to improve peak bone mass in mice (25).
The purpose of this study was to determine whether there were differences in the intestinal absorption of Ca between the low bone density C57 and the high bone density C3H strains. Ca absorption was measured in vitro using everted intestinal sacs. A major regulator of intestinal Ca absorption is 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], which is the major biologically active metabolite of vitamin D3 (30). Therefore, the responsiveness of the two strains to 1,25(OH)2D3 in terms of Ca absorption was determined. Finally, intestinal absorption of phosphorus was measured in vitro, because phosphorus is the other important mineral component of bone.
Expression of several key regulators of intestinal Ca absorption in C57 and C3H mice was compared. In the intestine, the mRNA levels for calbindin, the plasma membrane Ca pump, and the vitamin D 24-hydroxylase were determined. Calbindin, which is a soluble Ca-binding protein, and the Ca pump, which pumps Ca across the basolateral membrane of the absorptive cell, are two of the major components of the Ca absorptive pathway (29, 30). The vitamin D 24-hydroxylase is involved in the catabolism of 1,25(OH)2D3 in the intestine.
In the kidney, the mRNA levels of the 25(OH)D-1-hydroxylase, the
1,25(OH)2D3-24-hydroxylase, and calbindin were
determined. The 1
-hydroxylase synthesizes
1,25(OH)2D3 from 25(OH)D, which increases
plasma 1,25(OH)2D3 levels. The 24-hydroxylase
hydroxylates both 1,25(OH)2D3 and 25(OH)D,
which decreases plasma 1,25(OH)2D3 levels.
Renal calbindin may play a role in the tubular reabsorption of Ca.
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METHODS |
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Animals. Female C57 and C3H mice were purchased from Harlan Industries (Indianapolis, IN) and were used between 8 and 12 wk of age. This age range was chosen because Ca balance studies suggested strain differences in Ca absorption at this age (11). Growth curves of the strains are very similar, and both strains increased in weight by ~5% over this age range. Mice were fed a semisynthetic diet containing 1.2% Ca, 0.8% phosphorus, and 3.3 IU/g vitamin D3 (Purina Rodent Chow, Ralston-Purina, St. Louis, MO). To study the effect of 1,25(OH)2D3 on intestinal Ca absorption, mice were given intraperitoneal injections of 1,25(OH)2D3 (100 ng/100 g body wt) at 48, 24, and 6 h before death. This multidose protocol was used to study both the short-term and long-term effects of 1,25(OH)2D3 in rats (5). All studies were approved by the Animal Studies Committee of the St. Louis Veterans Administration Medical Center.
Measurement of intestinal Ca transport. Intestinal Ca transport was measured in vitro using everted intestinal sacs as previously described for rats (5, 8, 9). On the day of the experiment, the mice were killed, and blood was collected into heparinized tubes. Plasma was frozen for later determination of plasma Ca, phosphorus, and 1,25(OH)2D3.
To measure intestinal transport, the abdominal cavity was exposed by midline incision, and a 5-cm segment of the intestine was removed. Intestinal segments were everted by first flushing them out with a fine, blunt-end needle. The segment end was then tied to the end of the needle, and the segment was carefully everted over the end of the needle. A small-gauge needle was then used to fill the segment with incubation buffer, and the ends were tied off. The incubation buffer consisted of (in mM) 125 NaCl, 10 fructose, 30 Tris, and 0.25 CaCl2 (pH 7.4 at 37°C). To study the duodenum, the region immediately distal to the pylorus was used, and to study the jejunum, the region at the midpoint of the small intestine was used. Leakage of the sacs was detected by a loss of internal fluid during the 1-h incubation. This occurred <5% of the time. Everted, fluid-filled sacs were incubated in flasks containing 10 ml of the incubation buffer. To measure unidirectional Ca transport, 45CaCl2 (ICN Radiochemicals, Costa Mesa, CA) was added to the buffer. Flasks were then gassed with 95% O2, stoppered, and incubated for 1 h at 37°C. At the end of the incubation period, sacs were removed from the flask and drained of internal fluid, and the amount of fluid in the sacs was determined. Samples of internal fluid from each sac were assayed in triplicate for 45Ca using a scintillation counter. The total amount of Ca transported was calculated by multiplying radioactive counts per microliter of fluid by volume of fluid in the sac and by specific activity. Ca transport measured in this way is sensitive to age, segment, 1,25(OH)2D3 treatment, and dietary Ca in the rat (8, 5). In addition to the amount of Ca that moved across the sac, the amount taken up by the intestinal segment was also determined in these experiments. After the internal fluid was drained, the intestinal segment was washed, solubilized, and counted for 45Ca. Tissue uptake was then expressed as Ca accumulation per milligram wet weight of tissue. Ca uptake by the small intestinal mucosa is sensitive to age, segment, 1,25(OH)2D3 treatment, and dietary Ca in the rat (2).Measurement of intestinal phosphate transport. Phosphate transport was measured in separate experiments using the same procedures described for Ca transport. During measurement of unidirectional phosphate transport, 1 mM NaH2PO4 and [32P]H2PO4 were present in the incubation buffer. The amount of phosphate transported was calculated by multiplying radioactive counts per microliter of fluid by volume of fluid in the sac and by specific activity. Tissue uptake of phosphate was expressed as phosphate accumulation per milligram wet weight of tissue. Phosphate transport and phosphate uptake is sensitive to age, segment, 1,25(OH)2D3 treatment, and dietary Ca in the rat (2).
Measurement of mRNA levels.
Total RNA was isolated from intestinal mucosal scrapings using
guanidinium isothiocyanate followed by cesium chloride centrifugation (12). mRNA levels of mouse intestinal calbindin,
intestinal 24-hydroxylase, renal 1-hydroxylase, and renal
24-hydroxylase were determined by ribonuclease protection assay (RPA).
The RPA was performed as previously described, using specific probes
(7). The RPA probe for the mouse intestinal calbindin
D9k consisted of nucleotides 9-308 (27),
the probe for the 24-hydroxylase consisted of nucleotides 945-1191
of the mouse renal cytochrome P-450 subunit CYP24
(17), and the probe for the 1
-hydroxylase consisted of
nucleotides 127-610 of the mouse renal cytochrome P-450
subunit CYP1
(26). The RPA probe for the plasma
membrane Ca (PMCA) pump was specific for the PMCA1b form of Ca
pump. This is the major form found in the intestine and is the
form thought to be involved in Ca absorption (16). The
probe was generated by RT-PCR using total mouse intestinal RNA and
primers based on rat PMCA1 sequence (forward primer: nucleotides
3557-3577; reverse primer: nucleotides 3869-3889)
(24), as described by Varadi et al. (28). The
178-nucleotide PCR product was confirmed to be the PMCA1b form of the
Ca pump by sequencing and comparison to the rat sequence (>98%
homology). All probe sequences excluded substrate or cofactor binding
sites and did not show homology to related proteins. The actin probe
was rat
-actin (Ambion). RPA was performed using the RPAII kit from
Ambion (Austin, TX). Probes were labeled with [32P]UTP
(ICN Radiochemicals) using the Maxiscript T7 polymerase kit from
Ambion. Bands were quantitated by scanning densitometry and normalized
to actin mRNA.
Plasma measurements. Plasma Ca and phosphorus were measured colorimetrically using commercial kits (Sigma, St. Louis, MO). Plasma 1,25(OH)2D3 was measured using a commercial kit (ImmunoNuclear, Stillwater, MN).
Statistics. Data from these experiments are reported as means ± SE of the number of animals indicated. Statistical analyses were performed using Student's two-tailed t-test. A confidence level of 95% or greater was considered significant.
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RESULTS |
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Intestinal Ca transport was compared between C57 and C3H mice
(Fig. 1A). In the duodenum, Ca
transport by C3H mice was 58% higher than the transport seen in C57
mice. In the jejunum, there was no difference between the two strains.
In the same experiments, Ca uptake into the intestinal tissue was
determined (Fig. 1B). Ca uptake by the intestine paralleled
Ca transport across the intestine. Ca uptake was 45% higher in the
duodenum of the C3H mouse, but there was no difference in Ca uptake in
the jejunum. In general, Ca transport and Ca uptake in both strains
were less in the jejunum than in the duodenum. Ca transport was
7.07 ± 0.88 and 5.94 ± 0.92 pmol in the C57 mouse duodenum
and jejunum, respectively. Ca uptake was 108 ± 6 and 65 ± 6 pmol/mg in the duodenum and jejunum, respectively. In the C3H mouse, Ca
transport was 11.2 ± 1.6 and 5.52 ± 0.57 pmol in the
duodenum and jejunum, respectively. Ca uptake was 157 ± 17 and
72 ± 9 pmol/mg in the duodenum and jejunum, respectively.
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The effect of 1,25(OH)2D3 given in vivo on Ca
transport and Ca uptake by the duodenum was then determined.
1,25(OH)2D3 significantly increased Ca
transport to similar levels in both strains of mice (Fig.
2A). The component of Ca
transport stimulated by 1,25(OH)2D3 [1,25(OH)2D3 minus control] was calculated,
and this also was not different between the two strains. The effect of
1,25(OH)2D3 on Ca uptake was similar (Fig.
2B). 1,25(OH)2D3 significantly increased Ca uptake in both strains of mice. Although the maximal level
of stimulation was 29% less in the C57, this difference was not
statistically significant. In addition, there was no difference in the
1,25(OH)2D3-stimulated component of Ca uptake.
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Changes in Ca transport paralleled changes in Ca uptake in these
experiments (Figs. 1 and 2). This suggested that these two physiological processes were correlated. To determine whether there was
a correlation, Ca transport was plotted as a function of Ca uptake.
Data from Fig. 2, A and B were used, because this experiment produced a wide range of Ca transport and uptake. There was
a high degree of correlation between Ca uptake and Ca transport (Fig.
3). This suggests that Ca transport
across the intestine is a reflection of Ca uptake into the intestinal
mucosa.
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Intestinal phosphate transport was also measured in C57 and C3H mice
(Fig. 4). Phosphate transport was
measured in the duodenum, because this region showed significant
differences regarding Ca transport between the two strains (Fig. 1).
Duodenal phosphate transport in the C3H mouse was only 36% that of the
C57 mouse (Fig. 4). Likewise, phosphate uptake by the C3H mouse was
55% that of the C57 mouse (Fig. 4). In absolute terms, phosphate
transport was 88.6 ± 17.0 and 31.9 ± 3.5 nM in the C57 and
C3H mice, respectively. Phosphate uptake was 1,534 ± 100 and
843 ± 46 pmol/mg, respectively.
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Regarding plasma levels, there was no significant difference in plasma
Ca, phosphorus, or 1,25(OH)2D3 between the two
strains (Table 1). Plasma
1,25(OH)2D3 levels were increased 12- to
14-fold in both strains by 1,25(OH)2D3
treatment, and there was no significant difference in the final levels.
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To characterize the mechanisms responsible for these strain differences
in intestinal Ca absorption, basal expression was measured as well as
expression in response to chronic 1,25(OH)2D3 treatment. There was no difference in the basal (control) level of mRNA
for calbindin in the duodenum of the two strains (Fig. 5). 1,25(OH)2D3
treatment had no significant effect on calbindin mRNA levels in these
studies. Previous studies have shown that this chronic treatment with
1,25(OH)2D3 markedly increases calbindin mRNA
levels in 1,25(OH)2D3-depleted rats
(4). However, because these mice were not vitamin D
deficient, they had high levels of calbindin mRNA in the duodenum at
the beginning of the study. Thus it may be difficult for
1,25(OH)2D3 to further increase calbindin mRNA
levels under these conditions.
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In contrast to calbindin, the basal Ca pump mRNA levels were 90%
higher in the C3H mice (Fig. 6). This
strain difference was also seen in the
1,25(OH)2D3-treated groups. As with calbindin, 1,25(OH)2D3 treatment had no significant effect
on Ca pump mRNA levels. This again may be due to the fact that these
mice were not vitamin D-deficient.
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In the same experiments, the mRNA levels of CYP24 were measured in the
duodenum (Fig. 7). mRNA levels were
virtually undetectable in the control animals, but they were markedly
stimulated by 1,25(OH)2D3. This has been seen
previously in the rat (3). There was no difference in the
maximal level of stimulation by 1,25(OH)2D3 between the two strains.
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Finally, the mRNA levels of the vitamin D hydroxylases and calbindin
D9k in the kidney were measured in these experiments (Table
2). There was no significant difference
in the basal (control) levels of CYP1. This is consistent with the
fact that there was no significant difference in plasma
1,25(OH)2D3 levels between strains (Table
1). Treatment with 1,25(OH)2D3 significantly
decreased the CYP1
mRNA in both strains. There was also no
significant difference in the basal (control) levels CYP24.
Treatment with 1,25(OH)2D3
significantly increased CYP24 mRNA levels in both strains. Finally, there was no difference in renal
calbindin D9k mRNA levels with either strain or
1,25(OH)2D3 treatment. This is similar to what
was seen in the duodenum (Fig. 5).
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DISCUSSION |
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These studies demonstrate that the Ca-transporting capacity of the duodenum of the C3H mouse is 58% greater than that of the C57 mouse (Fig. 1). There is no difference seen in the jejunum, suggesting that the duodenum is the major site of increased Ca transport by the C3H mouse. In the previously reported balance study, the apparent Ca absorption by C3H mice was ~55% greater than that seen in C57 mice (11). However, differences in apparent Ca absorption may reflect differences in Ca secretion as well as Ca transport (6). The present in vitro studies strongly suggest the increase seen in the balance study is due to greater Ca transport rather than decreased Ca secretion by the C3H mouse.
The question arises as to why there is increased Ca transport by the duodenum of the C3H mouse. Intestinal Ca transport involves uptake of Ca by the absorptive cells, transport of Ca across the cells with the aid of calbindin, and extrusion of Ca out of the cells by an ATP-dependent Ca pump (29). Intestinal calbindin is a soluble, vitamin D-regulated, Ca-binding protein. Steady-state levels of calbindin correlate well with intestinal Ca transport under a variety of conditions in the rat (30). However, in these studies there were no differences in the calbindin mRNA levels between the two strains (Fig. 5).
In addition to calbindin, the ATP-dependent Ca pump plays a major role in Ca transport. A number of studies have shown a correlation between Ca pump mRNA levels and Ca transport regarding 1,25(OH)2D3 administration, dietary Ca, and dietary phosphorus (29). In the rat, Ca pump protein levels correlate with Ca transport in terms of age and 1,25(OH)2D3 administration (5). Interestingly, in the present study, Ca pump mRNA levels were 90% higher in C3H mice compared with C57 mice (Fig. 6). This was greater in magnitude than the increase in Ca transport seen in the C3H mice (58%). Thus the increase in duodenal Ca pump mRNA in C3H mice could account for the observed increase in Ca transport, assuming the increased mRNA levels resulted in increased Ca pump activity.
There has been some debate as to whether calbindin or the Ca pump is most important as the rate-limiting step of Ca transport. In many studies, there are parallel changes in the expression of these two proteins in response to vitamin D, dietary Ca, and age (5). However, in the present study, only Ca pump expression was significantly higher in the C3H mice, suggesting that changes in the Ca pump alone may alter overall Ca transport. However, there may also be strain differences in other components of the Ca transport system, such as the newly discovered Ca channel (15). This channel may play a role in the uptake of Ca into absorptive cells.
In these studies, the strain differences in Ca absorption were not due
to differences in the production and action of
1,25(OH)2D3. First, plasma
1,25(OH)2D3 levels were not significantly
different in the two strains (Table 1). Second, the basal mRNA levels
for CYP1 and CYP24 in the kidney were not different (Table 2).
Finally, the capacity of 1,25(OH)2D3 to
increase duodenal Ca transport was the same in both strains (Fig. 2).
Also, 1,25(OH)2D3 increased duodenal CYP24 mRNA
to the same level (Fig. 6). This is consistent with the fact that
vitamin D receptor levels in the duodenum have been reported to be the
same in both strains (11).
In addition to differences in Ca absorption, there are also differences in duodenal phosphate transport between the two species (Fig. 4). Interestingly, phosphate transport is higher in the C57 mouse. This argues against some kind of generalized defect in transepithelial ion transport in the C57 mouse. Because phosphate, along with Ca, is a major component of bone, this increased phosphate transport may have physiological significance. There is evidence that the ratio of dietary Ca to phosphate is important for optimal bone mineralization. In adult and aged mice, high dietary phosphate in relation to dietary Ca may result in reduced bone mineralization (22). In vitamin D receptor knockout mice, dietary phosphorus must be low relative to dietary Ca to prevent the low bone density associated with these animals (19). Thus it is possible that reduced Ca transport combined with increased phosphate transport may contribute to the reduced peak bone mass seen in the C57 mouse.
This is not to suggest that differences in mineral absorption alone account for all of the increased peak bone mass seen in the C3H mice. It is likely that several factors are involved, some of which may be related to Ca absorption. For example, C3H mice have higher levels of insulin-like growth factors (IGF-I) in serum and bone than C57 mice (21). Higher levels of serum IGF-I may increase bone formation directly. In addition, they could contribute to the increased Ca absorption seen in the C3H strain. It has been shown that growth hormone and IGF-I stimulate intestinal Ca absorption (14). Regarding bone itself, histomorphometric studies have shown that bone formation rate is greater in the C3H mice compared with the C57 mice (23). This is consistent with the fact that C3H mice have higher levels of alkaline phosphatase and higher osteoprogenitor cell numbers (13). There is also evidence that bone resorption may be slowed in the C3H mice, because they produce fewer osteoclasts (18).
Finally, these studies demonstrate that the everted intestinal sac is a useful in vitro technique for characterizing Ca absorption in the mouse. Previous studies of Ca absorption in the mouse have used in vivo techniques such as balance studies (11) and the ligated intestinal loop (20). In the present study, everted intestinal sacs detected the expected differences between intestinal segments (Fig. 1) and the marked stimulation by 1,25(OH)2D3 (Fig. 2). They have also been used to study the effect of age and dietary Ca in mice (unpublished studies). With the large number of transgenic mice now available, it may be possible to use this technique to dissect out the important components and regulators of the intestinal Ca transport system itself.
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ACKNOWLEDGEMENTS |
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This work was supported by the St. Louis Geriatric Research,
Education, and Clinical Center, the Medical Research Service of the
Department of Veterans Affairs, and National Institute on Aging Grant
AG-12587. The probe for the mouse cytochrome P-450 subunit
of the 1-hydroxylase was kindly provided by Dr. Anthony Portale,
University of California, San Francisco, CA.
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FOOTNOTES |
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10.1152/ajpgi.00175.2001
Address for reprint requests and other correspondence: H. J. Armbrecht, Geriatric Center (11G-JB), St. Louis Veterans Administration Medical Center, St. Louis, MO 63125 (E-mail: hjarmbrec{at}aol.com).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 25 April 2001; accepted in final form 20 September 2001.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Anderson, JJ,
and
Rondano PA.
Peak bone mass development of females: can young adult women improve their peak bone mass?
J Am Coll Nutr
15:
570-574,
1996[Abstract].
2.
Armbrecht, HJ.
Age-related changes in calcium and phosphorus uptake by rat small intestine.
Biochim Biophys Acta
882:
281-286,
1986[ISI][Medline].
3.
Armbrecht, HJ,
and
Boltz MA.
Expression of 25-hydroxyvitamin D 24-hydroxylase cytochrome P450 in kidney and intestine. Effect of 1,25-dihydroxyvitamin D and age.
FEBS Lett
292:
17-20,
1991[ISI][Medline].
4.
Armbrecht, HJ,
Boltz MA,
Christakos S,
and
Bruns ME.
Capacity of 1,25-dihydroxyvitamin D to stimulate expression of calbindin D changes with age in the rat.
Arch Biochem Biophys
352:
159-164,
1998[ISI][Medline].
5.
Armbrecht, HJ,
Boltz MA,
and
Kumar VB.
Intestinal plasma membrane calcium pump protein and its induction by 1,25(OH)2D3 decrease with age.
Am J Physiol Gastrointest Liver Physiol
277:
G41-G47,
1999
6.
Armbrecht, HJ,
Gross CJ,
and
Zenser TV.
Effect of dietary calcium and phosphorus restriction on calcium and phosphorus balance in young and old rats.
Arch Biochem Biophys
210:
179-185,
1981[ISI][Medline].
7.
Armbrecht, HJ,
Hodam TL,
Boltz MA,
and
Kumar VB.
Capacity of a low calcium diet to induce the renal vitamin D 1-hydroxylase is decreased in adult rats.
Biochem Biophys Res Commun
255:
731-734,
1999[ISI][Medline].
8.
Armbrecht, HJ,
Wasserman RH,
and
Bruns ME.
The effect of 1,25-dihydroxyvitamin D3 on intestinal calcium absorption in strontium-fed rats.
Arch Biochem Biophys
192:
466-473,
1979[ISI][Medline].
9.
Armbrecht, HJ,
Zenser TV,
Bruns ME,
and
Davis BB.
Effect of age on intestinal calcium absorption and adaptation to dietary calcium.
Am J Physiol Endocrinol Metab Gastrointest Physiol
236:
E769-E774,
1979
10.
Beamer, WG,
Donahue LR,
Rosen CJ,
and
Baylink DJ.
Genetic variability in adult bone density among inbred strains of mice.
Bone
18:
397-403,
1996[ISI][Medline].
11.
Chen, C,
and
Kalu DN.
Strain differences in bone density and calcium metabolism between C3H/HeJ and C57BL/6J mice.
Bone
25:
413-420,
1999[ISI][Medline].
12.
Chirgwin, J,
Przybyla AE,
MacDonald RJ,
and
Rutter WJ.
Isolation of biologically active ribonucleic acid from sources enriched in ribonuclease.
Biochemistry
18:
5294-5299,
1979[ISI][Medline].
13.
Dimai, HP,
Linkhart TA,
Linkhart SG,
Donahue LR,
Beamer WG,
Rosen CJ,
Farley JR,
and
Baylink DJ.
Alkaline phosphatase levels and osteoprogenitor cell numbers suggest bone formation may contribute to peak bone density differences between two inbred strains of mice.
Bone
22:
211-216,
1998[ISI][Medline].
14.
Fleet, JC,
Bruns ME,
Hock JM,
and
Wood RJ.
Growth hormone and parathyroid hormone stimulate intestinal calcium absorption in aged female rats.
Endocrinology
134:
1755-1760,
1994[Abstract].
15.
Hoenderop, JGJ,
van der Kemp AWCM,
Hartog A,
van de Graaf SFJ,
van Os CH,
Willems PHGM,
and
Bindels RJM
Molecular identification of the apical Ca2+ channel in 1,25-dihydroxyvitamin D3-responsive epithelia.
J Biol Chem
274:
8375-8378,
1999
16.
Howard, A,
Legon S,
and
Walters JRF
Human and rat intestinal plasma membrane calcium pump isoforms.
Am J Physiol Gastrointest Liver Physiol
265:
G917-G925,
1993
17.
Itoh, S,
Yoshimura T,
Iemura O,
Yamada E,
Tsujikawa K,
Kohama Y,
and
Mimura T.
Molecular cloning of 25-hydroxyvitamin D-3 24-hydroxylase (Cyp-24) from mouse kidney: its inducibility by vitamin D-3.
Biochim Biophys Acta
1264:
26-28,
1995[ISI][Medline].
18.
Linkhart, TA,
Linkhart SG,
Kodama Y,
Farley JR,
Dimai HP,
Wright KR,
Wergedal JE,
Sheng M,
Beamer WG,
Donahue LR,
Rosen CJ,
and
Baylink DJ.
Osteoclast formation in bone marrow cultures from two inbred strains of mice with different bone densities.
J Bone Miner Res
14:
39-46,
1999[ISI][Medline].
19.
Masuyama, R,
Nakaya Y,
Tanaka S,
Tsurukami H,
Nakamura T,
Watanabe S,
Yoshizawa T,
Kato S,
and
Suzuki K.
Dietary phosphorus restriction reverses the impaired bone mineralization in vitamin D receptor knockout mice.
Endocrinology
142:
494-497,
2001
20.
Meyer, MH,
and
Meyer RA.
Increased intestinal absorption of calcium in young and adult X-linked hypophosphatemic mice after administration of 1,25-dihydroxyvitamin D3.
J Bone Miner Res
3:
151-157,
1988[ISI][Medline].
21.
Rosen, CJ,
Dimai HP,
Vereault D,
Donahue LR,
Beamer WG,
Farley JR,
Linkhart S,
Linkhart T,
Mohan S,
and
Baylink DJ.
Circulating and skeletal insulin-like growth factor-I (IGF-I) concentrations in two inbred strains of mice with different bone mineral densities.
Bone
21:
217-223,
1997[ISI][Medline].
22.
Shah, BG,
Krishnarao VG,
and
Draper HH.
The relationship of Ca and P nutrition during adult life and osteoporosis in aged mice.
J Nutr
92:
30-42,
1967[ISI][Medline].
23.
Sheng, MH,
Baylink DJ,
Beamer WG,
Donahue LR,
Rosen CJ,
Lau KH,
and
Wergedal JE.
Histomorphometric studies show that bone formation and bone mineral apposition rates are greater in C3H/HeJ (high-density) than C57BL/6J (low-density) mice during growth.
Bone
25:
421-429,
1999[ISI][Medline].
24.
Shull, GE,
and
Greeb J.
Molecular cloning of two isoforms of the plasma membrane Ca2+-transporting ATPase from rat brain.
J Biol Chem
263:
8646-8657,
1988
25.
Takahashi, K,
Tsuboyama T,
Matsushita M,
Kasai R,
Okumura H,
Yamamuro T,
Okamoto Y,
Kitagawa K,
and
Takeda T.
Effective intervention of low peak bone mass and bone modeling in the spontaneous murine model of senile osteoporosis, SAM-P/6, by Ca supplement and hormone treatment.
Bone
15:
209-215,
1994[ISI][Medline].
26.
Takeyama, K,
Kitanaka S,
Sato T,
Kobori M,
Yanagisawa J,
and
Kato S.
25-Hydroxyvitamin D3 1-hydroxylase and vitamin D synthesis.
Science
277:
1827-1830,
1997
27.
Tatsumi, K,
Higuchi T,
Fujiwara H,
Nakayama T,
Itoh K,
Mori T,
Fujii S,
and
Fujita J.
Expression of calcium binding protein D-9k messenger RNA in the mouse uterine endometrium during implantation.
Mol Hum Reprod
5:
153-161,
1999
28.
Varadi, A,
Molnar E,
and
Ashcroft SJH
A unique combination of plasma membrane Ca2+-ATPase isoforms is expressed in islets of Langerhans and pancreatic -cell lines.
Biochem J
314:
663-669,
1996[ISI][Medline].
29.
Wasserman, RH,
Chandler JS,
Meyer SA,
Smith CA,
Brindak ME,
Fullmer CS,
Penniston JT,
and
Kumar R.
Intestinal calcium transport and calcium extrusion processes at the basolateral membrane.
J Nutr
122:
662-671,
1992[ISI][Medline].
30.
Wasserman, RH,
and
Fullmer CS.
Vitamin D and intestinal calcium transport: facts, speculations, and hypotheses.
J Nutr
125:
1971S-1979S,
1995[Medline].
31.
Weaver, CM.
Age related calcium requirements due to changes in absorption and utilization.
J Nutr
124:
1418S-1425S,
1994[Medline].