Bone formation is not impaired by hibernation (disuse) in black bears Ursus americanus
1 Department of Biomedical Engineering, Michigan Technological University,
Houghton, MI 49931, USA
2 US Geological Survey, Virginia Cooperative Fish and Wildlife Research
Unit, Virginia Polytechnic Institute and State University, Blacksburg, VA
24061-0321, USA
3 Departments of Pathology and Medicine
4 Department of Orthopaedics and Rehabilitation, The Pennsylvania State
University, Hershey, PA 17033, USA
* Author for correspondence (e-mail: swdonahu{at}mtu.edu)
Accepted 15 August 2003
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Summary |
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Key words: black bear, Ursus americanus, bone formation, hibernation, metabolism, adaptation, collagen, disuse
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Introduction |
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Approximately 90% of the organic component of the extracellular matrix of
bone is type I collagen. The serum concentrations of type I collagen peptide
fragments are useful for assessing bone turnover in patients with osteoporosis
(Watts, 1999). The
bone-forming osteoblasts secrete type I procollagen molecules, which have a
central triple helical domain and telopeptide and propeptide domains on both
the amino- and carboxy-terminal ends of the molecule. During bone formation,
the carboxy-terminal propeptide of type I procollagen (PICP) is cleaved off
and released into the circulation. The concentration of PICP in serum has been
positively correlated with histomorphometric measurements of bone formation
(Eriksen et al., 1993
). During
resorption of bone, the cross-linked telopeptides are released into the serum.
The degradation of type I bone collagen by the proteinase cathepsin K produces
cross-linked amino-terminal telopeptide (NTX) fragments
(Atley et al., 2000
). The
degradation of type I bone collagen by matrix metalloproteinases produces
cross-linked carboxy-terminal telopeptide (ICTP) fragments
(Garnero et al., 2003
).
However, cathepsin K destroys the ICTP epitope. Thus, different modes of
enzymatic digestion of type I bone collagen produce different telopeptide
fragments. The serum ICTP concentration has been positively correlated with
bone resorption and negatively correlated with bone mineral density
(Eriksen et al., 1993
;
Yasumizu et al., 1998
). In
patients immobilized for 30 to 180 days following a stroke, PICP levels were
significantly lower and ICTP levels significantly higher than in healthy
age-matched male and female controls (Fiore
et al., 1999
). Serum NTX levels are significantly (approximately
twofold) higher in post-menopausal women than in pre-menopausal women
(Garnero et al., 1996
). These
finding suggest that there are different mechanisms of collagen degradation
for disuse and post-menopausal osteoporosis.
Hibernating black bears have been called `metabolic marvels' for their
unique physiological characteristics
(Nelson, 1987). For example,
hibernating bears remain dormant for 57 months, during which time they
do not urinate or defecate, they efficiently recycle urea and amino acids, and
the females give birth and nurse (Nelson,
1987
; Wright et al.,
1999
). There is some evidence to suggest that bears do not lose
bone or muscle mass during hibernation
(Floyd et al., 1990
;
Tinker et al., 1998
).
Histomorphometric analyses of bone biopsies from bears showed that bone
resorption and formation surfaces increased several-fold during winter
hibernation relative to active summer values, and bone volume was unchanged
(Floyd et al., 1990
). Floyd et
al. proposed that bears produce an osteoregulatory substance, which promotes
increased bone formation during disuse to compensate for increased resorption,
thus making bears uniquely resistant to disuse osteoporosis
(Floyd et al., 1990
).
Hibernating black bears provide a unique and naturally occurring model for
studying the physiology of bone disuse with inactivity. Recently, we assayed
serum PICP and ICTP levels from 17 wild black bears, collected during active
and hibernating periods, using radioimmunoassay
(Donahue et al., 2003). As in
human bed-rest studies (Fiore et al.,
1999
), we found that serum ICTP concentration significantly
increased in bears during hibernation (i.e. disuse). However, unlike human
disuse studies, PICP levels were unchanged during hibernation in black bears
when compared to the active period prior to hibernation. These findings
suggested that bone resorption and formation was unbalanced during
hibernation, resulting in net bone loss. However, in the months immediately
following their arousal from hibernation there was a 34-fold increase
in PICP levels in both a young and an old (17 year) bear, suggesting
accelerated bone formation during remobilization, which was not compromised
with aging. These findings on bear bone metabolism were provocative; however,
in that study only one sample was collected from each bear during each period
(immobilization and remobilization). Therefore, the time course of seasonal
variations in serum markers of bone resorption and formation was not defined
in our previous study. The goals of the current study were (1) to assess the
time-course of seasonal variations in bone resorption and formation markers to
find out if the changes in bone resorption and formation are sustained during
the entire disuse period, which would give an indication of the amount of bone
lost, and (2) to gain some insight on how the lost bone may be recovered
during remobilization.
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Materials and methods |
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Radioimmunoassays were performed to determine the serum concentrations of PICP (bone formation marker), ICTP (bone resorption marker) and cortisol. Reagents for the serum ICTP and PICP assays were obtained from Diasorin (Stillwater, MN, USA). 100 µl serum samples were run in duplicate. The intraassay coefficient of variation were 4.8% for ICTP and 2.8% for PICP. The serum cortisol levels were determined using reagents from Diagnostic Products Corporation (Los Angeles, CA). 50 µl serum samples were run in duplicate; the intraassay coefficient of variation was 5%. Enzyme-linked immunosorbant assay (ELISA) was performed to determine the serum concentrations of NTX (bone resorption marker) using reagents from Ostex International (Seattle, WA, USA). 50 µl serum samples were run in duplicate; the intra-assay coefficient of variation was 4.6%. For all four assays, measurements were repeated on samples with an intra-sample coefficient of variation greater than 10%.
For each of the five bears, a mean value for each period (prehibernation, hibernation and post-hibernation) was determined for PICP, ICTP, NTX and cortisol. For each bear there were ten pre-hibernation samples, nine hibernation samples, and one post-hibernation sample. One-way analyses of variance (ANOVA) were used to compare the mean (N=5) serum levels of PICP, ICTP, NTX and cortisol between the three study periods. Significant ANOVA values were followed up with Fisher's PLSD tests for multiple-means comparisons. Linear regressions were performed to assess correlations between cortisol and ICTP and cortisol and PICP to assess the potential role of cortisol in mediating bone remodeling. A significance level of 0.05 was used for all statistical analyses.
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Results |
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The bone resorption marker ICTP showed a sustained increase during the hibernation period (Fig. 2). In the 3-month prehibernation period the mean ICTP concentration showed very little fluctuation, remaining between 9 and 13 µg l1. Within the first 2 weeks of the hibernation period, mean ICTP levels increased to 20 µg l1 and remained elevated for the duration of hibernation. 2 weeks after arousal, ICTP levels dropped rapidly towards prehibernation values. Unlike ICTP, the serum concentration of the bone resorption marker NTX did not show seasonal variations (Fig. 3). The serum concentration of cortisol was greatest during the hibernation period and dropped following arousal from hibernation, similar to ICTP concentration (Fig. 4). However, cortisol showed greater variability than ICTP.
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There was no significant (P=0.207) difference in the mean concentration of PICP between the pre-hibernation period (203±28 µg l1) and the hibernation period (158±21 µg l1) (Fig. 5). However, the mean concentration of ICTP significantly (P=0.0028) increased by more than twofold during the hibernation period (26±3.9 µg l1) compared to the prehibernation period (10±1.1 µg l1) (Fig. 6). This finding suggests that bone formation was uncoupled from bone resorption, resulting in a net bone loss during disuse. However, the mean values of serum NTX concentration did not significantly (P=0.7215) change during hibernation compared to the pre-hibernation period (Fig. 7). The discrepancy in seasonal variations in the levels of bone resorption markers ICTP and NTX is probably due to the way the bone collagen was broken down and is addressed in detail in the discussion.
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During the remobilization period, 2 weeks after arousal from hibernation, the concentration of the bone formation marker PICP significantly (P=0.0001) increased over pre-hibernation and hibernation values (Fig. 5). This finding suggests that bone formation rapidly increased during remobilization. 2 weeks into the post-hibernation period the ICTP concentration had returned to pre-hibernation levels (Fig. 6). Taken together, these findings suggest that immediately upon remobilization, bears begin to replace any bone that was lost during hibernation.
Serum concentrations of cortisol showed similar behavior to ICTP, significantly (P=0.0073) increasing during hibernation and returning to pre-hibernation values during remobilization (Fig. 8). Regression analysis showed a significant (P=0.0236), but weak (r2=0.254) negative correlation between serum cortisol and PICP levels. Serum ICTP concentration showed a significant (P=0.0062), but weak (r2=0.348) positive correlation with serum cortisol.
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Discussion |
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Notwithstanding these provocative data, some important limitations to our
observations need to be considered. First, because of the limited number of
post-hibernation samples, it is uncertain if, and for how long, bone formation
is elevated following hibernation. In our previous study we measured serum
PICP in 17 wild black bears over the course of the summer, although only one
sample was taken from each bear (Donahue et
al., 2003). In that previous study, we found that serum PICP
levels were 45-fold higher in early to mid-June than during
hibernation; in July and August PICP concentrations were similar to
hibernation values. Given the results of these two studies, it is reasonable
to hypothesize that bone formation is higher in black bears in the first
23 months following hibernation than during the remaining portion of
their physically active life and during hibernation. This hypothesis is
supported by the histological data of Floyd et al.
(1990
), which showed that the
bone formation rate in black bears was severalfold higher in the spring than
during hibernation. A more thorough analysis of the post-hibernation bone
formation indicators is clearly needed to further support this hypothesis.
Second, serum markers of bone metabolism reflect bone remodeling in the entire
skeleton. Therefore, it is unclear if all bones behave similarly. Third, there
is a discrepancy between the seasonal variations in serum ICTP and NTX in
black bears. The rise in ICTP indicates that bone resorption is increased
during hibernation, but the unchanged level of NTX suggests that bone
resorption is unchanged. However, it has been shown that different enzymes
that degrade type I bone collagen produce different epitopes of the type I
collagen telopeptide fragments (ICTP and NTX)
(Atley et al., 2000
;
Garnero et al., 2003
). Bone
resorption by the proteinase cathepsin K produces NTX fragments
(Atley et al., 2000
);
resorption by matrix metalloproteinases produces ICTP fragments
(Garnero et al., 2003
). Thus,
it is likely that bone resorption is increased in hibernating bears by
increased matrix metalloproteinase activity.
Bone remodeling has been studied in other hibernating animals, most notably
in ground squirrels, golden hamsters and little brown bats
(Haller and Zimny, 1977;
Kwiecinski et al., 1987
;
Steinberg et al., 1979
,
1981
,
1986
). These studies suggest
that bone is lost during hibernation by reduced osteoblastic formation and
increased bone resorption. Bone resorption is believed to occur in these
animals by osteocytic osteolysis, although intracortical osteoclastic
resorption cavities have been observed post-hibernation in little brown bats
Myotis lucifugus (Kwiecinski et
al., 1987
). In little brown bats bone loss is manifest by reduced
cortical thickness and mineral density at the end of hibernation compared to
the beginning of hibernation (Kwiecinski
et al., 1987
). Little brown bats also have significantly higher
plasma calcium levels in some hibernation months compared to the active summer
months, unlike black bears, which show no significant difference in plasma
calcium levels between active and hibernation periods
(Floyd et al., 1990
). This
discrepancy in seasonal plasma calcium levels may be due to the observation
that osteoblastic bone formation is arrested during hibernation in other
animals (Steinberg et al.,
1986
), whereas black bears possibly prevent hypercalcemia during
hibernation by maintaining osteoblastic bone formation, as our data and those
of Floyd et al. (1990
)
suggest.
Both little brown bats and black bears mate prior to hibernation and have
delayed implantation (Hellgren,
1998; Kwiecinski et al.,
1991
). However, black bears give birth and nurse their cubs during
hibernation, while gestation and lactation in little brown bats occurs in the
summer after hibernation. During pregnancy and lactation, calcium is liberated
from bones (Cross et al.,
1995
). The additional burden of pregnancy and lactation on bones
during hibernation may explain, at least in part, the disproportionately small
birth weight of black bears compared to little brown bats. At birth, black
bears only weigh about 0.3% of their mother's mass, while little brown bats
are about 30% of their mother's mass
(Kwiecinski et al., 1987
;
Oftedal et al., 1993
).
However, despite this additional burden of pregnancy and lactation on the
bone's calcium supply, female black bears may be able to recover bone during
the active summer months as well as males can. We previously found that bone
strength and mineral content were significantly higher in female bears than in
male bears near the end of their active period (i.e. October)
(Harvey and Donahue 2003
).
Female black bears possibly attain a higher bone mineral content than males
during the active summer months, in preparation for the additional burden of
pregnancy and lactation during hibernation. Furthermore, we previously found
that bone strength, mineral content and porosity did not change with age in
either gender. These findings suggest that the bone lost during annual
hibernation periods, due to increased resorption, can be recovered annually by
bears during the active summer months.
In humans, high serum concentrations of cortisol, as well as glucocorticoid
therapy, are associated with increased bone resorption and decreased bone
formation, which decrease bone mass and increase the incidence of spontaneous
fracture (Libanati and Baylink,
1992). Additionally, a significant negative correlation between
serum cortisol levels and bone mineral density has been shown in men
(Dennison et al., 1999
). These
findings suggest a role for cortisol in the regulation of bone turnover in
hibernating bears. However, it is unclear why the increased serum cortisol
during hibernation did not significantly change bone formation. One possible
explanation is that other factors influence the metabolic activity of
bone-forming osteoblasts. The hormone leptin is possibly involved in the
regulation of bone metabolism during hibernation, since leptin is an appetite
and bone-formation suppressor, although its effects on bone metabolism may be
secondary to its effect on appetite suppression. Steep and continuously rising
leptin levels, beginning about 1 month prior to hibernation, have been
reported for European brown bears, coincident with the period when bears
become anorectic (Hissa et al.,
1998
). We also noted in our study that the bears stopped eating in
December shortly before hibernating, despite the availability of food.
Interestingly, our PICP data show an apparent negative correlation with the
data of Hissa et al. (1998
):
pre-hibernation, bone formation is low and leptin levels are high; at arousal,
bone formation is high and leptin levels low. Thus, when hibernating bears
emerge from their winter dens, decreased serum leptin levels would permit
increases in bone formation, allowing the bears to recover the bone lost
during hibernation (due to increased resorption).
The question we set out to answer was, how can black bears maintain bone
mass and strength when annual hibernation (i.e. disuse) and active periods are
approximately equal (i.e. 6 months)? Our results suggest the answer to be that
bears maintain their bone mass by maintaining normal bone formation during
disuse and by rapidly increasing bone formation during remobilization to
recover the bone lost by increased bone resorption during hibernation.
However, further investigation is need to substantiate this hypothesis. A new,
and perhaps equally intriguing question is, what are the biological mechanisms
that regulate bone remodeling in hibernating black bears? Are parathyroid
hormone, calcitonin, and leptin involved? It is known that hormones, including
PTH, sensitize bone cells to mechanical stimulation in vitro
(Ryder and Duncan, 2000;
Sekiya et al., 1999
). Thus,
one possible way of elevating bone formation following hibernation is the
sensitization of bone-forming osteoblasts, by circulating hormone levels,
during remobilization after spring arousal.
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Acknowledgments |
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References |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Atley, L. M., Mort, J. S., Lalumiere, M. and Eyre, D. R. (2000). Proteolysis of human bone collagen by cathepsin K: characterization of the cleavage sites generating by cross-linked N-telopeptide neoepitope. Bone 26,241 -247.[CrossRef][Medline]
Collet, P., Uebelhart, D., Vico, L., Moro, L., Hartmann, D., Roth, M. and Alexandre, C. (1997). Effects of 1- and 6-month spaceflight on bone mass and biochemistry in two humans. Bone 20,547 -551.[CrossRef][Medline]
Cross, N. A., Hillman, L. S., Allen, S. H. and Krause, G. F. (1995). Changes in bone mineral density and markers of bone remodeling during lactation and postweaning in women consuming high amounts of calcium. J. Bone Min. Res. 10,1312 -1320.[Medline]
Dauty, M., Perrouin Verbe, B., Maugars, Y., Dubois, C. and Mathe, J. F. (2000). Supralesional and sublesional bone mineral density in spinal cord-injured patients. Bone 27,305 -309.[CrossRef][Medline]
Dennison, E., Hindmarsh, P., Fall, C., Kellingray, S., Barker,
D., Phillips, D. and Cooper, C. (1999). Profiles of
endogenous circulating cortisol and bone mineral density in healthy elderly
men. J. Clin. Endocrinol. Metab.
84,3058
-3063.
Donahue, S. W., Vaughan, M. R., Demers, L. M. and Donahue, H. J. (2003). Serum markers of bone metabolism show bone loss in hibernating bears. Clin. Orth. Relat. Res. 408,295 -301.
Eriksen, E. F., Charles, P., Melsen, F., Mosekilde, L., Risteli, L. and Risteli, J. (1993). Serum markers of type I collagen formation and degradation in metabolic bone disease: Correlation with bone histomorphometry. J. Bone Min. Res. 8, 127-132.[Medline]
Fiore, C. E., Pennisi, P., Ciffo, F., Scebba, C., Amico, A. and Di Fazzio, S. (1999). Immobilization-dependent bone collagen breakdown appears to increase with time: Evidence for a lack of new bone equilibrium in response to reduced load during prolonged bed rest. Horm. Metab. Res. 31,31 -36.[Medline]
Floyd, T., Nelson, R. A. and Wynne, G. F. (1990). Calcium and bone metabolic homeostasis in active and denning black bears (Ursus americanus). Clin. Orth. Rel. Res. 255,301 -309.[Medline]
Garland, D. E., Stewart, C. A., Adkins, R. H., Hu, S. S., Rosen, C., Liotta, F. J. and Weinstein, D. A. (1992). Osteoporosis after spinal cord injury. J. Orth. Res. 10,371 -378.
Garnero, P., Ferreras, M., Karsdal, M. A., Nicamhlaoibh, R., Risteli, J., Borel, O., Qvist, P., Delmas, P. D., Foged, N. T. and Delaisse, J. M. (2003). The type I collagen fragments ICTP and CTX reveal distinct enzymatic pathways of bone collagen degradation. J. Bone Min. Res. 18,859 -867.[Medline]
Garnero, P., Hausherr, E., Chapuy, M. C., Marcelli, C., Grandjean, H., Muller, C., Cormier, C., Braeart, G., Meunier, P. J. and Delmas, P. D. (1996). Markers of bone resorption predict hip fracture in elderly women: the EPIDOS Prospective Study. J. Bone Min. Res. 11,1531 -1538.[Medline]
Haller, A. C. and Zimny, M. L. (1977). Effects of hibernation on interradicular alveolar bone. J. Dent. Res. 56,1552 -1557.[Abstract]
Harvey, K. B. and Donahue, S. W. (2003). Analysis of bone histology, composition, and mechanical properties of black bear tibias in relation to disuse osteoporosis. In 2003 ASME Summer Bioengineering Conference, pp. B53. Key Biscayne, FL.
Hellgren, E. C. (1998). Physiology of hibernation in bears. Ursus 10,467 -477.
Hissa, R., Hohtola, E., Tuomala-Saramaki, T., Laine, T. and Kallio, H. (1998). Seasonal changes in fatty acids and leptin contents in the plasma of the European brown bear (Ursus arctos arctor). Ann. Zool. Fennici 35,215 -224.
Houde, J. P., Schulz, L. A., Morgan, W. J., Breen, T., Warhold, L., Crane, G. K. and Baran, D. T. (1995). Bone mineral density changes in the forearm after immobilization. Clin. Orth. Rel. Res. 317,199 -205.[Medline]
Jaworski, Z. F. and Uhthoff, H. K. (1986). Reversibility of nontraumatic disuse osteoporosis during its active phase. Bone 7,431 -439.[Medline]
Kaneps, A. J., Stover, S. M. and Lane, N. E. (1997). Changes in canine cortical and cancellous bone mechanical properties following immobilization and remobilization with exercise. Bone 21,419 -423.[CrossRef][Medline]
Kwiecinski, G. G., Damassa, D. A. and Gustafson, A. W. (1991). Patterns of plasma sex hormone-binding globulin, thyroxine and thyroxine-binding globulin in relation to reproductive state and hibernation in female little brown bats. J. Endocrinol. 128,63 -70.[Abstract]
Kwiecinski, G. G., Krook, L. and Wimsatt, W. A. (1987). Annual skeletal changes in the little brown bat, Myotis lucifugus lucifugus, with particular reference to pregnancy and lactation. Am. J. Anat. 178,410 -420.[Medline]
Leblanc, A. D., Schneider, V. S., Evans, H. J., Engelbretson, D. A. and Krebs, J. M. (1990). Bone mineral loss and recovery after 17 weeks of bed rest. J. Bone Min. Res. 5, 843-850.[Medline]
Lewis, P. L., Brewster, N. T. and Graves, S. E. (1998). The pathogenesis of bone loss following total knee arthroplasty. Orth. Clin. N. Amer. 29,187 -197.
Libanati, C. R. and Baylink, D. J. (1992). Prevention and treatment of glucocorticoid-induced osteoporosis: A pathogenetic perspective. Chest 102,1426 -1435.[Abstract]
Lindgren, U. and Mattsson, S. (1977). The reversibility of disuse osteoporosis: studies of bone density, bone formation, and cell proliferation in bone tissue. Calc. Tissue Res. 23,179 -184.
Marchetti, M. E., Houde, J. P., Steinberg, G. G., Crane, G. K., Goss, T. P. and Baran, D. T. (1996). Humeral bone density losses after shoulder surgery and immobilization. J. Shoulder Elbow Surg. 5,471 -476.[Medline]
Nelson, R. A. (1987). Black bears and polar bears: Still metabolic marvels. Mayo Clin. Proc. 62,850 -853.[Medline]
Oftedal, O. T., Alt, G. L., Widdowson, E. M. and Jakubasz, M. R. (1993). Nutrition and growth of suckling black bears (Ursus americanus) during their mothers winter fast. Br. J. Nutr. 70,59 -79.[Medline]
Rantakokko, J., Uusitalo, H., Jamsa, T., Tuukkanen, J., Aro, H. T. and Vuorio, E. (1999). Expression profiles of mRNAs for osteoblast and osteoclast proteins as indicators of bone loss in mouse immobilization osteopenia model. J. Bone Min. Res. 14,1934 -1942.[Medline]
Ryder, K. D. and Duncan, R. L. (2000). Parathyroid hormone modulates the response of osteoblast-like cells to mechanical stimulation. Calc. Tissue Int. 67,241 -246.[CrossRef][Medline]
Sekiya, H., Mikuni-Takagaki, Y., Kondoh, T. and Seto, K. (1999). Synergistic effect of PTH on the mechanical responses of human alveolar osteocytes. Biochem. Biophys. Res. Commun. 264,719 -723.[CrossRef][Medline]
Steinberg, B., Singh, I. J. and Mitchell, O. G. (1979). Changes observed in bone during hibernation using Procion red dye as a matrical marker. J. Exp. Zool. 210,537 -541.[Medline]
Steinberg, B., Singh, I. J. and Mitchell, O. G. (1981). The effects of cold-stress. Hibernation, and prolonged inactivity on bone dynamics in the golden hamster, Mesocricetus auratus.J. Morphol. 167,43 -51.[Medline]
Steinberg, B., Singh, I. J. and Mitchell, O. G. (1986). An autoradiographic study of the uptake of tritiated proline by osteoblasts during hibernation. Histol. Histopathol. 1,155 -160.[Medline]
Tinker, D. B., Harlow, H. J. and Beck, T. D. (1998). Protein use and muscle-fiber changes in free-ranging, hibernating black bears. Physiol. Zool. 71,414 -424.[Medline]
Trebacz, H. (2001). Disuse-induced deterioration of bone strength is not stopped after free remobilization in young adult rats. J. Biomech. 34,1631 -1636.[CrossRef][Medline]
Vico, L., Collet, P., Guignandon, A., Lafage-Proust, M. H., Thomas, T., Rehaillia, M. and Alexandre, C. (2000). Effects of long-term microgravity exposure on cancellous and cortical weight-bearing bones of cosmonauts. Lancet 355,1607 -1611.[CrossRef][Medline]
Watts, N. B. (1999). Clinical utility of
biochemical markers of bone remodeling. Clin. Chem.
45,1359
-1368.
Weinreb, M., Patael, H., Preisler, O. and Ben-Shemen, S. (1997). Short-term healing kinetics of cortical and cancellous bone osteopenia induced by unloading during the reloading period in young rats. Virchows Arch. 431,449 -452.[CrossRef][Medline]
Weinreb, M., Rodan, G. A. and Thompson, D. D. (1989). Osteopenia in the immobilized rat hind limb is associated with increased bone resorption and decreased bone formation. Bone 10,187 -194.[Medline]
Wright, P. A., Obbard, M. E., Battersby, B. J., Felskie, A. K., LeBlanc, P. J. and Ballantyne, J. S. (1999). Lactation during hibernation in wild black bears: Effects on plasma amino acids and nitrogen metabolites. Physiol. Biochem. Zool. 72,597 -604.[CrossRef][Medline]
Yasumizu, T., Hoshi, K., Iijima, S. and Asaka, A. (1998). Serum concentration of the pyridinoline cross-linked carboxyterminal telopeptide of type I collagen (ICTP) is a useful indicator of decline and recovery of bone mineral density in lumbar spine: Analysis in Japanese postmenopausal women with or without hormone replacement. Endocr. J. 45,45 -51.[Medline]