Recent clinical studies indicated that angiotensin receptor blockers (ARBs) would decrease the risk of bone fractures in the elderly populations. There is little known about the role of the ARBs in the process of fracture healing. The purpose of the present study was to verify the hypothesis that systemic treatment with telmisartan has the ability to promote fracture healing. In this study, femur fractures were produced in 96 mature male BALB/c mice. Animals were treated with the ARBs telmisartan or vehicle. Fracture healing was analysed after 2, 5 and 10 weeks postoperatively using X-ray, biomechanical testing, histomorphometry, immunohistochemistry and micro-computed tomography (micro-CT). Radiological analysis showed the diameter of the callus in the telmisartan treated animals was significantly increased when compared with that of vehicle treated controls after two weeks of fracture healing. The radiologically observed promotion of callus formation was confirmed by histomorphometric analyses, which revealed a significantly increased amount of bone formation when compared with vehicle-treated controls. Biomechanical testing further showed a significantly greater peak torque at failure, and a higher torsional stiffness in telmisartan-treated animals compared with controls. There was an increased fraction of PCNA-positive cells and VEGF-positive cells in telmisartan-treated group compared with vehicle-treated controls. From the three-dimensional reconstruction of the bony callus, telmisartan-treated group significantly increased the values of BV/TV by 21.7% and CsAr by 26.0% compared to the vehicle-treated controls at 5 weeks post-fracture. In summary, we demonstrate in the current study that telmisartan could promote fracture healing in a mice model via increasing mechanical strength and improving microstructure. The most mechanism is probably by an increase of cell proliferation and neovascularization associated with a decreased VEGF expression in hypertrophic chondrocytes.
Citation: Zhao X, Wang J-x, Feng Y-f, Wu Z-x, Zhang Y, Shi L, et al. (2014) Systemic Treatment with Telmisartan Improves Femur Fracture Healing in Mice. PLoS ONE 9(3): e92085. https://doi.org/10.1371/journal.pone.0092085
Editor: Michael Glogauer, University of Toronto, Canada
Received: November 25, 2013; Accepted: February 17, 2014; Published: March 18, 2014
Copyright: © 2014 Zhao et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This research was supported by the National Natural Science Foundation of China (Grant No. 81300710). The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
The renin–angiotensin system (RAS) is an endocrine system that controls body fluid and electrolyte balance and blood pressure . The main effector peptide in this system is angiotensin II (Ang II), which is formed from angiotensin I (Ang I) by angiotensin-converting enzyme (ACE), a key molecule in this system. Angiotensin II (Ang II) acts via angiotensin II type 1 (AT1) and type 2 (AT2) receptors, which are members of the 7-transmembrane- spanning G-protein-coupled receptors. AT1 and AT2 receptors exhibit limited sequence homology (34% amino acid sequence identity) . Many actions of Ang II appear to be through AT1 receptors. The RAS has been an important target of antihypertensive drugs, particularlly angiotensin-converting enzyme (ACE), inhibitors and angiotensin receptor blockers (ARBs) , .
Recent clinical studies indicated that beta blockers and anti-hypertension drugs would reduce the risk of bone fractures in the elderly populations –. This suggests a possible link between vascular and skeletal systems. Renin angiotensin system (RAS) is operating not only systemically but also locally in several tissues, and bone microenvironments have been studied in this regard –. In bone tissue, Ang II was reported to promote bone resorption via the AT1 receptor in cell culture system and in ovariectomized mice and rats . Expression of AT1 receptor was observed in cultured osteoblasts, and Ang II inhibit differentiation and bone formation via the AT1 receptor in rat calvarial osteoblastic cells , .
A recent study showed that inhibition of angiotensin-converting enzyme has improved fracture healing and periosteal callus formation in a murine femur fracture model . However, there is still little known about the role of the ARBs in the process of fracture healing. Therefore, the purpose of the present study was to verify the hypothesis that systemic treatment with telmisartan has the ability to promote fracture healing.
Materials and Methods
Ninety-six mature male BALB/c mice (purchased from the Experimental Animal Center of The Fourth Military Medical University, Xi'An, China) with an average weight of 24 g at the beginning of the study were used. The mice were kept five per cage with free access to mouse chow and water ad libitum. All experimental procedures of animal were approved by the Ethics in Animal Research Committee of The Fourth Military Medical University.
The mice were anesthetized by i.p. injection of xylazine (25 mg/kg BW) and ketamine (75 mg/kg BW). Under sterile conditions a 4 mm medial parapatellar incision was performed at the right knee and the patella was dislocated laterally. After drilling a hole (Ø = 0.5 mm) into the intracondylar notch a distally flattened 24 G needle was implanted intramedullary. The patella was then repositioned over the knee joint and the muscles and skin were sutured separately. Then the middle of the femur was exhibited through a lateral approach and an osteotomy was created in the middle of the femur, using a gigly-wire-saw , . Wound closure completed the operative procedure.
Forty-eight mice were treated daily with the ARBs Telmisartan (Boehringer-Ingelheim, Germany) (30 mg/kg) in their drinking water. An additional 48 mice served as controls, given only drinking water. Animals were killed after 2, 5 and 10 weeks by cervical dislocation. Fracture healing was evaluated by biomechanical (n = 8 each group) and by by histomorphometric and immunohistochemical anlysis (n = 8 each group). Additional animals were killed after 5 and 10 weeks for micro-CT analysis (n = 8 each group).
At the end of the 2, 5 and 10 weeks post-fracture observation periods, the mice were anesthetized (xylazine and ketamine, same dosage as above), and ventro-dorsal radiographs of the healing femora were taken. X-rays were taken by a DMR+ Mo target mammography machine (18 kV, 160 mAs, GE, USA). The callus diameter was measured in relation to the femora diameter (%).
The fractured and nonfractured femurs were subjected to torsional testing to assess biomechanical properties. Proximal and distal ends were placed into device pots and embedded using polymethylmethacrylate (PMMA). The bones were then mounted on a commercial material testing system (Instron, Norwood, MA, USA). Applying a gradually increasing angle (0.15 s-1), parameters of the fractured femora are given in percent of the corresponding unfractured contralateral femora: Peak torque at failure (%), Peak rotation angle at failure (%), Torsional Stiffness (%).
For histomorphometric analysis, the healed femora bones were fixed in 4% phosphate-buffered formalin for 24 h, decalcified in 13% EDTA solution for 2 weeks and then embedded in paraffin. Longitudinal sections of 5 um thickness were stained with hematoxylin and eosin. The central slide of each specimen was analyzed at a magnification of 5× (Leica Instruments GmbH, Germany). In adherence to the recommendations of the American Society of Bone and Mineral Research (ASBMR)  the following histomorphometric parameters were calculated: total callus area in relation to femoral bone diameter at the fracture gap [Cl.Ar B.Dm−1(%)], bone callus area (mineralized bone, osteoid and bone marrow) in relation to total callus area [B.Cl.Ar Cl.Ar−1 (%)], cartilaginous callus area in relation to total callus area [Cg.Cl.Ar Cl.Ar−1 (%)] and fibrous callus area in relation to total callus area [Fb.Cl.Ar Cl.Ar−1 (%)]
Immunohistochemical staining for proliferating cell nuclear antigen (PCNA) and VEGF was used. Sections were deparaffinized using xylene and rehydrated using graded alcohols. The endogenous peroxidase was quenched using 3% hydrogen peroxide for 20 min. Antigen retrieval was achieved by microwaving (10 min; 700 W) specimens in citrate buffer (pH 6.0). After blocking unspecific binding sites with PBS and goat normal serum (30 min; room temperature), sections were incubated overnight with mouse monoclonal anti-PCNA (1∶50 PBS; Sigma, St. Louis, MO, USA)) or with mouse monoclonal anti-VEGF (1∶50 PBS; Sigma, St. Louis, MO, USA) antibodies at room temperature. Peroxidase-conjugated goat anti-mouse antibodies (1∶100; Millipore, Temecula, CA, USA) were used as secondary antibodies (incubation for 1 h at room temperature). DAB (Dako Cytomation) served as the chromogen and Mayer's hemalum as the counterstain. The immunohistochemical data were qualitatively assessed.
Micro-CT measurement was performed using a desktop micro-CT system (GE Healthcare, USA). Bones were thawed at room temperature, and excess tissue was removed. The scanning protocol was set at X-ray energy settings of 80 kV and 80 μA, and the sample was scanned over one entire 360°rotation, with an exposure time of 3000 ms/frame. A constrained 3-D Gaussian filter was used to partly suppress the noise in the volumes. The high and low radio-opacity mineralized tissues were differentially segmented by a two-level global thresholding procedure , .The callus volume of interest (VOI) was defined as the newly-formed bone tissues; the medullary canal volume and the original bone tissue were excluded from evaluation according to previous reports , . After segmentation, bone volume/tissue volume (BV/TV) and average cross-sectional area (CsAr) were quantified within the VOI.
All data were presented as the mean with their standard deviation (means±SD) in the text and the tables. Statistical analyses were conducted using the statistics package SPSS 16.0 software (SPSS Inc., Chicago, IL, USA). Differences among treatment groups were tested by one-way analysis of variance (ANOVA). If significant differences were indicated, differences between the means of two groups were tested by Fisher's protected least significant difference (PLSD). Observations were considered to be statistically significant at P<0.05.
Radiological analyses at 2 weeks after fracture demonstrated that the diameter of the callus in the telmisartan treated animals was significantly increased when compared with that of vehicle treated controls. After 5 weeks the callus size showed no difference between the groups, whereas after 10 weeks the callus in the control animals was greater than in telmisartan treated animals. In telmisartan-treated animals, the relative callus size decreased continuously from weeks 2 to 10, while in control animals, it had increased to maximum values by week 5 and it then significantly decreased until the end of the 10 week observation period (Fig. 1).
At 2 weeks post-fracture, biomechanical analysis showed a significantly greater peak torque at failure and a higher torsional stiffness in telmisartan-treated animals compared with controls (P<0.05). The peak rotation angle did not differ between the two groups. At 5 weeks post-fracture, no significant differences in maximal torque, rotation angle and torsional stiffness could be detected between telmisartan-treated animals and vehicle-treated controls. After 10 weeks post-fracture, both groups showed complete bone healing, as indicated by a torsional stiffness of almost 100% of the contralateral femur in both groups (Table 1).
At 2 weeks after fracture, telmisartan-treated animals showed some more cartilage and markedly less fibrous tissue. Of interest, we observed a significant (P<0.05) reduction of the gap width from weeks 2 to 5 in the telmisartan-treated animals when compared with vehicle treated controls. This resulted in a significantly smaller gap at week 5 after fracture in telmisartan-treated animals. At 10 weeks after fracture, cartilaginous and fibrous tissue had disappeared in almost all samples of both groups (Fig. 2, Table 2).
A, C and E show the callus of vehicle treated controls, B, D and F that of telmisartan-treated animals.
The results of the histomorphometric analysis showed that periosteal callus formation was significantly greater in telmisartan-treated animals at 2 and 5 weeks when compared with vehicle treated controls. This was indicated by a significantly greater relative callus diameter. Accordingly, we observed also a significantly (P<0.05) greater periosteal callus area after 2 and 5 weeks. These differences were completely lost after 10 weeks. At this time point, all animals showed complete bone bridging of the osteotomy (Table 2).
Staining for PCNA was found in osteoblasts, periosteal cells and endothelial cells at 2 weeks post-fracture. There was an increased fraction of PCNA-positive cells in telmisartan-treated group compared with vehicle-treated controls. At 2 weeks post-fracture, VEGF-positive staining was found in hypertrophic chondrocytes. The quantitative analysis also indicated a higher fraction of positive cells in telmisartan treatment animals (21.7±3.5%) compared with vehicle-treated controls (3.7±3.7%) (Fig. 3 and Fig. 4).
Micro-CT analysis showed greater amount of bony callus was found in the telmisartan-treated animals than in the vehicle-treated controls at 5 weeks post-fracture, which demonstrates the anabolic effect of telmisartan during the early period of fracture healing. After 10 weeks the callus size showed no difference between the groups (Fig. 5 and Table 3).
A, C for the vehicle-treated group and B, D for the telmisartan-treated group.
The results of the fractured femur (Table 3) from the Micro-CT evaluation were expressed as BV/TV and CsAr. At 5 weeks post-fracture, telmisartan-treated group significantly increased the values of BV/TV by 21.7% and CsAr by 26.0% (P<0.05) compared to the vehicle-treated controls. After 10 weeks, the values of BV/TV and CsAr showed no difference between the groups (P>0.05).
In the present study, we analyzed the effect of telmisartan on the process of fracture healing in mice. The results point to a new possibility in the treatment of fractures. We demonstrate for the first time that telmisartan can improve fracture repair, as indicated by a signicantly increased bone formation compared to vehicle-treated controls.
Osteoporosis and atherosclerosis, two multifactorial and degenerative entities, are major public health problems. These diseases accompany the aging process and share common risk factors . A correlation exists between osteoporosis and atherosclerosis regardless of age, body mass index (BMI) and cardiovascular risks. Cases of low BMD of the hip have a higher risk of cardiovascular disease (CVD) mortality in both genders . Postmenopausal women with atherosclerotic changes of the abdominal aorta have a significantly decreased value of BMD at the lumbar spine and hip, as well as a higher incidence of fatal and non-fatal coronary events . Pathophysiological mechanisms connecting atherosclerosis and osteoporosis are complex and can be dependent or independent of the production of vitamin D . Hyperproduction of inflammatory markers such as C-reactive protein, interleukin-1 (IL-1), interleukin-6 (IL-6) and tumour necrosis factor-α (TNF-α) can be considered risk factors and some are directly related to the severity of atherosclerosis .
The renin-angiotensin system (RAS) is an endocrine system that governs body fluid and electrolyte balance and blood pressure . In the classic endocrine RAS, angio-tensinogen produced in the liver is sequentially cleaved by peptidases to form the biologically active octapeptide angiotensin II (AngII). Renin produced by the juxtaglo-merular apparatus of the kidney and secreted into the circulation cleaves angiotensinogen to the inactive deca-peptide angiotensin I, which is cleaved by angiotensin-converting enzyme (ACE) to generate AngII. The initial reaction between the enzyme renin and the substrate an-giotensinogen is the rate-limiting step of the RAS, for which strict species specificity exists. The RAS has been an important target of antihypertensive drugs, especially ACE inhibitors and angiotensin receptor blockers (ARBs) , .
The ACE is a key component of the RAS, generating Ang II as the main effector molecule. Different in vitro studies have shown that Ang II can be synthesized by osteoblastic cells through ACE and that Ang II influences osteoblastic cell functions through specific receptor binding –. Ang II exerts its biological effects mainly by two receptors, the AT1 and AT2 receptors. Ang II has a multitude of biological effects in different tissues and influences inflammation, angiogenesis, cell proliferation, cell differentiation and apoptosis. Accordingly, the RAS has been shown to significantly influence tissue remodelling in various tissues .
Recent clinical studies indicated that beta blockers and anti-hypertension drugs would reduce the risk of bone fractures in the elderly populations –. This suggests a possible link between vascular and skeletal systems. Renin angiotensin system (RAS) is operating not only systemically but also locally in several tissues, and bone microenvironments have been studied in this regard –. Osteoblasts and osteoclasts express angiotensin II type 1 receptor in cell cultures, suggesting the existence of local RAS in bone , , . One study showed that activation of RAS induces not only hypertension but also osteopenia with microstructural deterioration, reminiscent of osteoporosis, suggesting that aberrant activation of RAS may contribute to the co-occurrence of the hypertensive disorders and osteoporosis, which is often seen with advancing age .
It has been proposed that inhibition of RAS with ACE inhibitors or ARBs has beneficial effects beyond those resulting from lowering blood pressure alone, such as renoprotective effects and cardiovascular outcomes . Epidemiological studies indicate that patients who have undergone stroke have an increased risk of hip fracture –. However, one study using rats reported that exogenous Ang II administration suppressed bone mass levels through AT1 receptor in animals subjected to ovariectomy and five-sixths nephrectomy . Also, a study of mice with complete NO synthase deficiencies revealed that increased bone mass by genetic disruption was normalized by AT1 blockade .
ARBs are widely used in cardiovascular medicine, it is of particular interest to elucidate how these drugs may interfere with bone homeostasis. A clinical study reported that angiotensin-receptor blocker use was associated with a large and significant reduction. They believed that the fracture reduction associated with angiotensin-receptor blockers suggests a possible mechanism for affecting fracture risk . In addition, a previous study using spontaneous hypertensive rats revealed that AT1 blocker inhibited the reduction of bone loss, but no effect of AT2 was observed in fracture risk .
In the present study, systemic treatment with telmisartan during fracture healing resulted in an increased callus formation after 2 and 5 weeks. This was most probably due to an increase of cell proliferation in the periosteal callus, as indicated by an increased fraction of PCNA-positive cells and VEGF-positive cells in telmisartan-treated group compared with vehicle-treated controls. The improved periosteal callus formation resulted in an accelerated healing process with an earlier histological bridging of the fracture gap and an advanced remodelling after 10 weeks. The improved histological healing of the fractures after telmisartan treatment is further supported by the results of our biomechanical analysis, which demonstrated a greater torque to failure and a higher torsional stiffness after 2 and 5 weeks in telmisartan-treated animals. Of interest, after 2 weeks of fracture healing, the callus of telmisartan-treated animals showed less fibrous tissue compared with controls.
It is well known that VEGF is one of the key growth factors promoting endochondral ossification also during secondary fracture healing. This stage of fracture healing consists of terminal chondrocyte differentiation and cell death, extracellular matrix degradation and calcification as well as angiogenesis and osteogenesis . VEGF has been reported to be expressed in terminally differentiating chondrocytes, coincidently with the onset of vascularization and subsequent ossification of the callus . Hence, the positive action of telmisartan on fracture repair at 2 weeks post-fracture might be due to an inhibition of VEGF expression in hypertrophic chondrocytes as shown by immunohistochemical analysis. On the other hand, the increased amount of VEGF expression might also be a consequence of altered cell proliferation and differentiation as indicated by the increase PCNA content in the callus of telmisartan-treated animals when compared with vehicle-treated controls.
This study has one major limitation. Our fracture model was established based on a surgical osteotomy in contrast with a closed fracture model, the latter of which better describes common clinical fractures. However, this model enabled us to make a consistent fracture line, which is essential for making reliable and accurate evaluations both mechanically and histologically.
In summary, we demonstrate in the current study that telmisartan could promote fracture healing in a mice model via increasing mechanical strength and improving microstructure. The most mechanism is probably by an increase of cell proliferation and neovascularization associated with a decreased VEGF expression in hypertrophic chondrocytes. Considering the increasing administration of telmisartan in hypertension patients, the results of this study suggest that physicians could prescribe telmisartan to fracture patients who have hypertension, for the purpose of not only for hypertension, but also promoting fracture healing.
Conceived and designed the experiments: WL YBY. Performed the experiments: XZ JXW YFF. Analyzed the data: XZ JXW YFF ZXW YZ. Contributed reagents/materials/analysis tools: YZ LS QCT. Wrote the paper: XZ JXW YFF WL YBY.
- 1. Peach MJ (1977) Renin-angiotensin system: biochemistry and mechanisms of action. Physiol Rev 57: 313–370.
- 2. Senbonmatsu T, Saito T, Landon EJ, Watanabe O, Price EJ, et al. (2003) A novel angiotensin II type 2 receptor signaling pathway: possible role in cardiac hypertrophy. Embo J 22: 6471–6482.
- 3. Ondetti MA, Rubin B, Cushman DW (1977) Design of specific inhibitors of angiotensin-converting enzyme: new class of orally active antihypertensive agents. Science 196: 441–444.
- 4. Sanada M, Taguchi A, Higashi Y, Tsuda M, Kodama I, et al. (2004) Forearm endothelial function and bone mineral loss in postmenopausal women. Atherosclerosis 176: 387–392.
- 5. Solomon DH, Mogun H, Garneau K, Fischer MA (2011) Risk of fractures in older adults using antihypertensive medications. J Bone Miner Res 26: 1561–1567.
- 6. Song HJ, Lee J, Kim YJ, Jung SY, Kim HJ, et al. (2012) beta1 selectivity of beta-blockers and reduced risk of fractures in elderly hypertension patients. Bone 51: 1008–1015.
- 7. Lavoie JL, Sigmund CD (2003) Minireview: overview of the renin-angiotensin system—an endocrine and paracrine system. Endocrinology 144: 2179–2183.
- 8. Sakai K, Agassandian K, Morimoto S, Sinnayah P, Cassell MD, et al. (2007) Local production of angiotensin II in the subfornical organ causes elevated drinking. J Clin Invest 117: 1088–1095.
- 9. Shimizu H, Nakagami H, Osako MK, Hanayama R, Kunugiza Y, et al. (2008) Angiotensin II accelerates osteoporosis by activating osteoclasts. Faseb J 22: 2465–2475.
- 10. Hagiwara H, Hiruma Y, Inoue A, Yamaguchi A, Hirose S (1998) Deceleration by angiotensin II of the differentiation and bone formation of rat calvarial osteoblastic cells. J Endocrinol 156: 543–550.
- 11. Bandow K, Nishikawa Y, Ohnishi T, Kakimoto K, Soejima K, et al. (2007) Low-intensity pulsed ultrasound (LIPUS) induces RANKL, MCP-1, and MIP-1beta expression in osteoblasts through the angiotensin II type 1 receptor. J Cell Physiol 211: 392–398.
- 12. Garcia P, Schwenzer S, Slotta JE, Scheuer C, Tami AE, et al. (2010) Inhibition of angiotensin-converting enzyme stimulates fracture healing and periosteal callus formation - role of a local renin-angiotensin system. Br J Pharmacol 159: 1672–1680.
- 13. Holstein JH, Menger MD, Culemann U, Meier C, Pohlemann T (2007) Development of a locking femur nail for mice. J Biomech 40: 215–219.
- 14. Garcia P, Holstein JH, Histing T, Burkhardt M, Culemann U, et al. (2008) A new technique for internal fixation of femoral fractures in mice: impact of stability on fracture healing. J Biomech 41: 1689–1696.
- 15. Parfitt AM, Drezner MK, Glorieux FH, Kanis JA, Malluche H, et al. (1987) Bone histomorphometry: standardization of nomenclature, symbols, and units. Report of the ASBMR Histomorphometry Nomenclature Committee. J Bone Miner Res 2: 595–610.
- 16. Muller R, Ruegsegger P (1997) Micro-tomographic imaging for the nondestructive evaluation of trabecular bone architecture. Stud Health Technol Inform 40: 61–79.
- 17. Fu L, Tang T, Miao Y, Hao Y, Dai K (2009) Effect of 1,25-dihydroxy vitamin D3 on fracture healing and bone remodeling in ovariectomized rat femora. Bone 44: 893–898.
- 18. Gerstenfeld LC, Sacks DJ, Pelis M, Mason ZD, Graves DT, et al. (2009) Comparison of effects of the bisphosphonate alendronate versus the RANKL inhibitor denosumab on murine fracture healing. J Bone Miner Res 24: 196–208.
- 19. Kakar S, Einhorn TA, Vora S, Miara LJ, Hon G, et al. (2007) Enhanced chondrogenesis and Wnt signaling in PTH-treated fractures. J Bone Miner Res 22: 1903–1912.
- 20. Braithwaite RS, Col NF, Wong JB (2003) Estimating hip fracture morbidity, mortality and costs. JAGS 51: 364–370.
- 21. Schulz E, Arfai K, Liu X, Sayre J, Gilsanz V (2004) Aortic calcification and the risk of osteoporosis and fractures. J Clin Endocrinol Metab 89: 4246–4253.
- 22. Bagger YZ, Tanko LB, Alexandersen P, Hansen HB, Qin G, et al. (2006) Radiographic measure of aorta calcification is a site-specific predictor of bone loss and fracture risk at the hip. J Intern Med 259: 598–605.
- 23. Stojanovic OI, Lazovic M, Lazovic M, Vuceljic M (2011) Association between atherosclerosis and osteoporosis, the role of vitamin D. Arch Med Sci 7: 179–188.
- 24. Danilevicius CF, Lopes JB, Pereira RM (2007) Bone metabolism and vascular Calcification. Braz J Med Biol Res 40: 435–442.
- 25. Hatton R, Stimpel M, Chambers TJ (1997) Angiotensin II is generated from angiotensin I by bone cells and stimulates osteoclastic bone resorption in vitro. J Endocrinol 152: 5–10.
- 26. Hayashi Y (1998) The expressive psychopathology of the Japanese poet, Sakutaro Hagiwara. Psychiatry Clin Neurosci 52: 621–627.
- 27. Schurman SJ, Bergstrom WH, Shoemaker LR, Welch TR (2004) Angiotensin II reduces calcium uptake into bone. Pediatr Nephrol 19: 33–35.
- 28. Hiruma Y, Inoue A, Hirose S, Hagiwara H (1997) Angiotensin II stimulates the proliferation of osteoblast-rich populations of cells from rat calvariae. Biochem Biophys Res Commun 230: 176–178.
- 29. Asaba Y, Ito M, Fumoto T, Watanabe K, Fukuhara R, et al. (2009) Activation of renin-angiotensin system induces osteoporosis independently of hypertension. J Bone Miner Res 24: 241–250.
- 30. Casas JP, Chua W, Loukogeorgakis S, Vallance P, Smeeth L, et al. (2005) Effect of inhibitors of the renin-angiotensin system and other antihypertensive drugs on renal outcomes: systematic review and meta-analysis. Lancet 366: 2026–2033.
- 31. Kanis J, Oden A, Johnell O (2001) Acute and long-term increase in fracture risk after hospitalization for stroke. Stroke 32: 702–706.
- 32. Dennis MS, Lo KM, McDowall M, West T (2002) Fractures after stroke: frequency, types, and associations. Stroke 33: 728–734.
- 33. Izu Y, Mizoguchi F, Kawamata A, Hayata T, Nakamoto T, et al. (2009) Angiotensin II type 2 receptor blockade increases bone mass. J Biol Chem 284: 4857–4864.
- 34. Tsiridis E, Upadhyay N, Giannoudis P (2007) Molecular aspects of fracture healing: which are the important molecules? Injury 38 Suppl 1S11–25.
- 35. Ferguson C, Alpern E, Miclau T, Helms JA (1999) Does adult fracture repair recapitulate embryonic skeletal formation? Mech Dev 87: 57–66.