Skeletal actions of intermittent parathyroid hormone: effects on bone remodelling and structure

Intermittent administration of parathyroid hormone peptides has anabolic skeletal effects and reduces fracture risk in postmenopausal women with osteoporosis but the cellular and structural mechanisms by which these effects are mediated have not been fully established. In cancellous and endocortical bone, there is evidence that both modelling and remodelling-based formation contribute to the increase in bone mass although the contribution of these at different time points in the response to PTH has not been established. Despite the large increase in spine bone mineral density, however, significant increases in iliac crest cancellous bone volume and trabecular thickness have not been consistently demonstrated, possibly reflecting site-specific differences in PTH-induced skeletal effects and/or the large sampling and measurement variance associated with assessment of iliac crest cancellous bone volume and structure. In iliac crest cortical bone, increased cortical thickness has been demonstrated, due at least in part to increased endosteal bone formation; there is also some evidence for increased formation on periosteal surfaces. At some sites an increase in cortical porosity may also occur and the overall effects on cortical bone strength, particularly at the hip, remain to be established. Studies in iliac crest bone indicate a trend towards a lower mineralisation density of bone matrix and increased heterogeneity of mineralisation, consistent with new bone formation. In addition, there is a reduction in mineral crystallinity and a shift towards more divalent collagen cross-links, indicating a change towards a younger bone profile.

The potential clinical implications of these effects on bone are currently unknown. The stimulatory effect of PTH peptides on bone formation may favour their use in low turnover bone disease and in states of advanced bone loss. Furthermore, if beneficial effects on cortical bone strength are confirmed, efficacy at non-vertebral sites might be superior to those observed with antiresorptive drugs. Better definition of the effects of intermittent PTH administration on cancellous and cortical bone remodelling and structure at different skeletal sites may inform these speculations and is an important area for future research.     

Experimental determination of “the law of bone remodeling” and effect of rat parathyroid hormone (1–34) infusion on derived parameters

Summary The dependence of bone remodeling probability on bone age was investigated by fitting a previously described mathematical model [1] to data describing the decline of radioactivity in the femurs of growing rats prelabeled with [³H]tetracycline. The remodeling probability declined with increasing bone age, but because the data were obtained at the whole bone level, several explanations for this finding are possible, and the results cannot be extrapolated beyond the duration of the experiment. A simpler exponential model that assumes remodeling probability is constant can be used to calculate bone resorption rate within a limited time period. A significantly increased bone resorption rate in PTH-infused rats could readily be demonstrated using the exponential model. A more sophisticated statistical analysis was required to discriminate between control and PTH-infused rats using the more complex model in which resorption rate was not constant.     

Bone marrow fat content in relation to bone remodeling and serum chemistry in intact and ovariectomized dogs

Summary It was previously shown that 11 months after ovariectomy the volume fraction of trabecular bone in the spine and 11th rib medullary canal of Beagle dogs (6 control, 9 ovariectomized) was significantly reduced. In this paper it is shown that these changes are accompanied by increased marrow fat volume in the 11th rib (59.0±9.5% vs. 44.3 ±10.0%). Conversely, the volume fraction of functional (hematopoietic) cells in the marrow was reduced by ovariectomy. Additionally, variations in marrow fat volume were tested for correlation with 22 other variables pertinent to bone physiology. Marrow fat volume was significantly positively correlated with serum osteocalcin, rib trabecular bone porosity, rib cross-sectional area, and gains in body weight. It was negatively correlated with serum estrogen concentrations and the extent of rib trabecular surfaces labeled with tetracycline.     

Parathyroid hormone and calcitonin interactions in bone: Irradiation-induced inhibition of escape in vitro

Summary Calcitonin (CT) inhibits hormonally stimulated bone resorption only transiently in vitro. This phenomenon has been termed “escape,” but the mechanism for the effect is not understood. One possible explanation is that bone cell differentiation and recruitment of specific precursor cells, in response to stimulators of resorption, lead to the appearance of osteoclasts that are unresponsive to CT. To test this hypothesis, cell proliferation in neonatal mouse calvaria in organ culture was inhibited by irradiation from a cobalt-60 source. At a dose of 6000 R, [³H]thymidine incorporation into intact calvaria was inhibited approximately 90%. Irradiation had no effect on the resorptive response to 0.1 U/ml parathyroid hormone (PTH). However, irradiation induced a dose-dependent inhibition of the escape response which was maximal at 6000 R. A dose of 6000 R did not affect the binding of¹²⁵I-salmon CT to calvaria and decreased PTH stimulation of cyclic AMP release from bone without affecting the cyclic AMP response to CT. Although irradiation caused a dose-dependent inhibition of DNA synthesis, the dose-response curves for that effect and inhibition of escape were not superimposable. A morphologic study of hormonally treated calvaria demonstrated that irradiation prevented the early increase in number of osteoclasts in PTH-treated calvaria that had been observed previously in unirradiated bones. Autoradiography showed that irradiation also prevented the PTH-stimulated recruitment of newly divided mononuclear cell precursors into osteoclasts. This may be correlated with the effect of irradiation to prevent the loss of responsiveness to CT in the presence of PTH.     

Parathyroid hormone as a stimulus to mast cell accumulation in bone

Low doses of parathyroid hormone were administered over a 30-day period to Sprague-Dawley weanlings rats. In 7 of 12 animals, mast cell hyperplasia occurred in the proximal tibial metaphyseal marrow, without an associated fall in serum phosphorus or rise in serum calcium. Further, no alteration occurred in body weight or skeletal ash weight relative to the control population. A third group, on a calcium-deficient diet containing adequate vitamin D, showed marked bone marrow mast cell hyperplasia in all animals.

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Zusammenfassung Entwöhnte Sprague-Dawley-Ratten erhiclten während einer Periode von 30 Tagen niedrige Dosen von Parathyroidhormon. 7 von 12 Tieren zeigten eine Mastzell-Hyperplasie im proximalen Teil des Tibiametaphysenmarks, ohne gleichzeitige Erniedrigung des Serumphosphates oder Anstieg des Serumcalciums. Femer traten im Vergleich zur Kontrollgruppe keine Veränderungen im Körpergewicht oder im Gewicht des Aschenrückstandes des Skelets auf. Eine dritte Gruppe, die eine Calcium-arme, entsprechend mit Vitamin D angereicherte Nahrung erhielt, zeigte eine ausgeprägte Mastzell-Hyperplasie im Knochenmark aller Tiere.

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Résumé Des doses faibles d'hormone parathyroidienne sont administrées pendant 30 jours à des rats sevrés de l'espèce Sprague-Dawley. Chez 7 des 12 animaux utilisés, une hyperplasie des mastocytes est observée dans la moëlle tibiale métaphysaire proximale, sans chute associée du phosphore sérique ou élévation du calcium sérique. En outre, aucun changement de poids corporel ou du poids des cendres squelettiques n'est observé par rapport aux témoins. Un troisième groupe, soumis à une alimentation pauvre en calcium, mais contenant une quantité suffisante de vitamine D, présente une hyperplasie mastocytaire marquée au niveau de la moëlle osseuse de tous les animaux.

     

Treatment of Osteoporosis with Parathyroid Hormone and Teriparatide

Nowadays osteoporosis treatment is based primarily on therapy with antiresorptive agents, like the bisphosphonates. Parathyroid hormone (Preotact) and human recombinant parathyroid hormone peptide 1–34 (Teriparatide) are relatively new for the treatment of osteoporosis and belong to the group of anabolic agents. Both agents demonstrated an increase in bone mineral density and a significant reduction in vertebral fractures in postmenopausal women with osteoporosis when given for 18–24 months. Data on nonvertebral fractures are, however, not clear-cut, and so far only bisphosphonates and strontium ranelate have been demonstrated to reduce all types of fractures and therefore remain the front-line option for treatment of osteoporosis. As the safety, tolerability, and cost of the therapy also influence the choice of therapy, Preotact and Teriparatide might be useful additions to the armamentarium for (second-line) treatment of osteoporosis. An erratum to this article can be found at     

Intermittent treatment with parathyroid hormone (PTH) as well as a non-peptide small molecule agonist of the PTH1 receptor inhibits adipocyte differentiation in human bone marrow stromal cells

Whereas continuous PTH infusion increases bone resorption and bone loss, intermittent PTH treatment stimulates bone formation, in part, via reactivation of quiescent bone surfaces and reducing osteoblast apoptosis. We investigated the possibility that intermittent and continuous PTH treatment also differentially regulates osteogenic and adipocytic lineage commitment of bone marrow stromal progenitor/mesenchymal stem cells (MSC). The MSC were cultured under mildly adipogenic conditions in medium supplemented with dexamethasone, insulin, isobutyl-methylxanthine and troglitazone (DIIT), and treated with 50 nM human PTH(1–34) for either 1 h/day or continuously (PTH replenished every 48 h). After 6 days, cells treated with PTH for 1 h/day retained their normal fibroblastic appearance whereas those treated continuously adopted a polygonal, irregular morphology. After 12–18 days numerous lipid vacuole and oil red O-positive adipocytes had developed in cultures treated with DIIT alone, or with DIIT and continuous PTH. In contrast, adipocyte number was reduced and alkaline phosphatase staining increased in the cultures treated with DIIT and 1 h/day PTH, indicating suppression of adipogenesis and possible promotion of early osteoblastic differentiation. Furthermore, intermittent but not continuous PTH treatment suppressed markers of differentiated adipocytes such as mRNA expression of lipoprotein lipase and PPARγ as well as glycerol 3-phosphate dehydrogenase activity. All of these effects of intermittent PTH were also produced by a 1 h/day treatment with AH3960 (30 μM), a small molecule, non-peptide agonist of the PTH1 receptor. AH3960, like PTH, activates both the cAMP and calcium signaling pathways. Treatment with the adenylyl cyclase activator forskolin for 1 h/day, mimicked the anti-adipogenic effect of intermittent PTH, whereas pretreatment with the protein kinase-A inhibitor H89 prior to intermittent PTH resulted in almost complete conversion to adipocytes. In contrast, the MAP kinase inhibitor PD 98059 failed to prevent the anti-adipocytic effect of intermittent PTH, suggesting that the inhibitory effect of PTH on adipocyte differentiation is predominantly cAMP-dependent. These results demonstrate a differential effect of PTH1 receptor agonists on the adipocytic commitment and differentiation of adult human bone marrow mesenchymal stem cells. This response may represent an additional mechanism that contributes to the overall bone anabolic action of intermittent PTH.     

Time-course of mast cell accumulation in rat bone marrow after ovariectomy

We previously reported that mast cells accumulate in the tibia bone marrow of ovariectomized (OVX) rats. In this study, the timing of mast cell accumulation and osteoclast generation were compared to determine whether or not mast cell accumulation preceded osteoclast recruitment after ovariectomy. This may be significant because of the number of cytokines released by mast cells that are potentially active on resorption. Sprague-Dawley rats (120) aged 12 weeks were OVX or sham-operated, and killed on days 4, 7, 14, 28, and 56 postsurgery. Ten additional intact rats were used as baseline controls. Ovariectomy was confirmed by a sharp and sustained fall in serum estradiol. The loss in trabecular bone volume (BV/TV) began on day 7, reaching 80% on day 56 (P<0.001 vs baseline controls). The number of osteoclasts (N.OC/TBPm) increased in the OVX rats between days 4 and 7 (+130%;P<0.001), and continued rising to day 28. During the next month, it decreased greatly (−63%,P<0.001 on day 56 vs day 28). In the sham-treated rats, few mast cells were scattered in the bone marrow (1.9 cells/mm² in the baseline controls). Their number fluctuated during the experimental period, but at each time-point it was lower than in the OVX rats. They were predominantly (90%) of the mucosal subtype. In the OVX rats, their number doubled between days 4 and 14 (P<0.001), reached 8.6 cells/mm² on day 28 (a 5.4-fold increase compared with day 4 OVX rats), and plateaued for the next 4 weeks. OVX had no effects on mast cell subtypes. In conclusion, mast cell accumulation and osteoclast differentiation are precocious and concomitant; this does not support a direct role for mast cells in osteoclast recruitment. Rather, the two cell populations may derive from a common precursor or be targeted simultaneously by estrogen depletion through common stimulator(s). Mast cell hyperplasia appears to be a significant, and usually unknown, manifestation of ovariectomy in the bone marrow environment.     

Relationship between serum intact parathyroid hormone concentrations and bone remodeling in type I osteoporosis: Evidence that skeletal sensitivity is increased

To define the role of parathyroid gland function in the pathophysiology of bone loss in type I (postmenopausal) osteoporosis, we measured serum intact parathyroid hormone (PTH) concentration by immunoradiometric assay (IRMA) and by multisite immunochemiluminometric assay (ICMA) in 63 postmenopausal osteoporotic women (PMOp) with vertebral compression fractures and in 75 age-comparable postmenopausal normal women (PMNl). Also, tetracycline-based histomorphometric indices in cancellous bone were assessed in iliac biopsy samples from 61 PMOp and 28 PMNl women. Serum PTH concentrations by IRMA were similar in PMOp and PMNl (medians, 3.92 and 3.77 pmol/l; NS) but were significantly lower in PMOp by the more sensitive ICMA (medians, 2.82 and 3.14 pmol/l;P<0.01). By multiple linear regression analysis, serum PTH was directly related (P<0.001) to activation frequency, bone resorption rate, bone formation rate, and the calculated rate of bone loss. For each unit (pmol/l) increase in serum PTH by ICMA, activation frequency increased by 1.3%/year more (P=0.01), bone resorption rate increased by 3.9%/year more (P=0.003), and the rate of cancellous bone loss was 2.8% greater (P= 0.0003) in the PMOp women compared with the PMNl women. Concentrations of serum estradiol, but not serum estrone, had a weak opposing effect to PTH, especially for bone formation rate. These data suggest that in PMOp the bone has increased sensitivity to the biologic effects of PTH. This may represent one of the fundamental pathophysiologic defects in PMOp and, in the setting of estrogen deficiency, may explain, in part, their greater rate of bone loss.     

新伐他汀体外对破骨细胞性骨吸收及大鼠骨代谢的影响

目的探讨新伐他汀体外对破骨细胞骨吸收功能的作用及其大鼠骨代谢的影响.方法采用体外成熟破骨细胞和大鼠颅盖骨培养体系,检测新伐他汀作用7 d后破骨细胞骨吸收陷窝和培养上清钙的变化;检测大鼠颅盖骨培养上清碱磷酶和钙含量,组织学观察颅盖骨形态学变化.结果新伐他汀体外可明显抑制破骨细胞骨吸收陷窝的形成及培养上清钙的释放,新伐他汀体外可增强大鼠颅盖骨培养上清碱磷酶的活性,组织学观察到新伐他汀使大鼠颅盖骨矿化增强.结论新伐他汀体外不仅可促进大鼠颅盖骨的成骨活性,并且可明显抑制破骨细胞骨吸收功能,对骨质疏松有重要的防治作用.     

淫羊藿苷对小鼠骨髓源性破骨细胞诱导生成及骨吸收功能的影响

目的探讨淫羊藿苷对破骨细胞诱导产生及骨吸收功能的影响。方法用终浓度分别为25ng·mL^-1、30ng·mL^-1、10^-8mol·L^-1的M—CSF、RANKL、1，25（OH）2VitD3体外诱导培养小鼠骨髓源性破骨细胞，在此过程中加入终浓度分别为0、10^-7mol·L^-1、10^-6mol·L^-1、10^-5mol·L^-1的淫羊藿苷。倒置相差显微镜下观察活体细胞、HE染色、TRAP染色及降钙素受体染色鉴定破骨细胞，计数骨片上骨吸收陷窝数及面积，玻片上TRAP阳性多核细胞数。结果加药组随淫羊藿苷浓度的增加，骨片上形成的骨吸收陷窝数及面积，玻片上的TRAP阳性多核细胞数呈量的依赖性的减少，与非加药组比较，10^-5mol·L^-1、10^-5mol·L^-1浓度的淫羊藿苷组，差异有显著性（P〈0．05）。结论淫羊藿苷具有抑制破骨细胞诱导产生及骨吸收功能的作用，并随浓度增加抑制作用增强。     

The effect of monocyclic and bicyclic analogs of human parathyroid hormone (hPTH)-(1-31)NH2 on bone formation and mechanical strength in ovariectomized rats

The [Leu²⁷]cyclo(Glu²²-Lys²⁶)-hPTH-(1-31)NH₂ lactam is a stronger stimulator of adenylyl cyclase activity and a better stimulator of trabecular bone in the ovariectomized (OVX) rat model of osteopenia than hPTH-(1-31)NH₂. This enhanced activity is due in large part to the stabilization of the amphiphilic receptor-binding α-helix in the Ser¹⁷-Gln²⁹ region. The goal of the present study was to determine whether further cyclization could produce a more active hPTH analog. To this end, we compared the relative bioactivities of the bicyclic hPTH analog [Glu¹⁷,Leu²⁷]cyclo(Lys¹³-Glu¹⁷,Glu²²-Lys²⁶)-hPTH-(1-31)NH₂, made by replacing Ser¹⁷ with Glu¹⁷ and introducing a second lactam linkage between Lys¹³ and Glu¹⁷. The relative EC₅₀ for adenylyl cyclase stimulation by the bicyclic hPTH analog was similar to the EC₅₀ of the monocyclic [Leu²⁷]cyclo(Glu²²-Lys²⁶)-hPTH-(1-31)NH₂, but the bicyclic analog was still more active than hPTH-(1-31)NH₂. As expected from adenylyl cyclase stimulation being responsible for PTH’s anabolic action, the bicyclic hPTH analog [Glu¹⁷, Leu²⁷]cyclo(Lys¹³-Glu¹⁷, Glu²²-Lys²⁶)-hPTH-(1-31)NH₂ was able to increase femoral trabecular volume and thickness and mechanical strength in OVX rats, but it was no more effective than [Leu²⁷]cyclo(Glu²²-Lys²⁶)-hPTH-(1-31)NH₂ when injected once daily in a dose of 0.8 nmol/100 g body weight. Thus, further constraint of the conformation of hPTH-(1-31)NH₂ by introducing two lactam linkages between Lys¹³-Glu¹⁷ and Glu²²-Lys²⁶ did not raise the osteogenicity above that of the monocyclic analog.     

Non-responders to hormone replacement therapy for the prevention of postmenopausal bone loss: Do they exist?

Hormone replacement therapy (HRT) prevents postmenopausal bone loss, but the prevalence of non-responders in healthy early postmenopausal women is not known. In order to study this, we reviewed data from three published studies, each carried out in a randomized, placebo-controlled, longitudinal design over 2 year, that used seven hormone replacement therapies. Bone mineral content (BMC) was measured in the distal forearm by single photon absorptiometry. A mathematical model for elimination of measurement errors was applied to published BMC data. After this correction, we found that only 1.2% of early healthy postmenopausal women who are receiving HRT in conventional doses will lose more than 1% of forearm BMC per year. In conclusion, most, if not all, healthy early postmenopausal women who might need HRT against loss of bone will respond positively in forearm BMC to such therapy.     

2型糖尿病患者绝经期骨密度与甲状旁腺素、雌激素相关性研究

目的 观察2型糖尿病妇女绝经期骨密度与甲状旁腺素、雌激素相关性研究.方法 测定绝经期2型糖尿病妇女伴骨质疏松（A）组及绝经期2型糖尿病妇女无骨质疏松（B）组的左侧髋部股骨颈、大转子、华氏三角区、及腰椎L2～L4正侧位的骨密度和血清中骨代谢指标,如：骨钙素、碱性磷酸酶、钙、磷、甲状旁腺素、雌二醇、Ⅰ型胶原羧基末端终肽（β-CTx）的浓度,对骨密度与多个变量之间的关系进行相关分析,并对（A）组血清中的甲状旁腺素、雌二醇、骨钙素、β-CTx与不同部位的骨密度之间的关系进行多元逐步回归分析.结果绝经期2型糖尿病妇女（B）组的腰椎、大转子、华氏三角区、股骨颈等骨密度指标明显低于对照组（A）（P＜0.05）;2型糖尿病绝经期妇女血清中雌二醇水平与腰椎L2～L4骨密度呈正相关（P＜0.032）;甲状旁腺素水平与股骨颈骨密度呈负相关（P＜0.034）.结论 绝经期2型糖尿病患者甲状旁腺素和雌激素水平与骨密度密切相关,分别可以用于预测骨质疏松发生的不同部位.     

Home-based resistance training improves femoral bone mineral density in women on hormone therapy

This study tested whether moderate resistance training would improve femoral bone mineral density (BMD) in long-term users of hormone therapy with low BMD. The study was a 2-year randomized, controlled, trial (RCT) of moderate resistance training of either the lower extremity or the upper extremity. Eighty-five women participated in a 6-month observation period. The setting was center-based and home-based training. The participants were 189 women aged 59–78 years, with total femur T-scores from –0.8 to –2.8 and on hormone therapy (HT) for a minimum of 2 years (mean 11.8 years); 153 completed the trial. Lower extremity training used weight belts (mean 7.8 kg) in step-ups and chair rises; upper extremity training used elastic bands and dumbbells. Measurements were BMD and body composition [dual-energy X-ray absorptiometry (DXA)], bone turnover markers. Total femoral BMD showed a downward trend during the observation period: 0.35%±0.18% (P=0.14). The response to training was similar in the upper and lower groups in the primary outcomes. At 2 years, total femoral BMD increased 1.5% (95% CI 0.8%–2.2%) in the lower group and 1.8% (95% CI 1.1%–2.5%) in the upper group. Trochanter BMD increased 2.4% (95% CI 1.3%–3.5%) in the lower group and 2.5% (95% CI 1.4%–3.6%) in the upper group (for both analyses time effect P<0.001). At 1 year, a bone resorption marker (C-telopeptide) decreased 9% (P=0.04). Bone formation markers, bone-specific alkaline phosphatase, decreased 5% (P<0.001), and N-terminal type I procollagen peptide decreased 7% (P=0.01). Body composition (percent lean and percent body fat) was maintained in both groups. We concluded that long-term moderate resistance training reversed bone loss, decreased bone turnover, increased femur BMD, and maintained body composition. The similarity of response in upper and lower groups supports a systemic response rather than a site-specific response to moderate resistance training.     

The time-dependent effect of ibandronate on bone graft remodeling in an ovariectomized rat spinal arthrodesis model

Background contextIn osteoporotic patients undergoing spinal arthrodesis, the use of bisphosphonates (BPs) remains controversial with regard to bone fusion. There is no consensus about the appropriate time to give BPs to patients with osteoporosis undergoing spinal arthrodesis.PurposeWe aimed to study the effect of BPs, given at different times, on the bone response to osteoporotic spinal arthrodesis.Study design/settingRadiological, histologic, and molecular assessments of bone formation after the different administration time of ibandronate in an ovariectomized (OVX) rat spinal fusion model.MethodsFemale Sprague-Dawley rats (n=100) were OVX (n=80) or non-OVX operated (n=20) and randomized into five groups: non-OVX, osteoporosis, and osteoporosis with early, simultaneous, and late BP groups. Eight weeks after ovariectomy, lumbar spinal arthrodesis was performed using autologous tailbones. Animals were killed 4 and 8 weeks after arthrodesis, and bone formation was assessed by measuring bone mineral density (BMD), messenger RNA expression, manual palpation, radiological evaluation, and histomorphometry.ResultsCompared with late administration, early administration of ibandronate increased femur BMD in OVX rats and did not hinder bone fusion. Radiological analysis showed that groups given early ibandronate had increased bone volume in the grafted site 8 weeks after surgery. Histomorphometric analysis showed that ibandronate positively affected endochondral and intramembranous ossification. In the OVX groups, ibandronate increased bone turnover to a level similar to that in the non-OVX group. These findings suggested that early administration of ibandronate did not inhibit osteogenesis, including endochondral and intramembranous ossification and fusion rate.ConclusionsOur results suggest that the early administration of BPs may not hinder the bone fusion of osteoporotic patients undergoing spinal arthrodesis.     

Increased bone formation by intermittent parathyroid hormone administration is due to the stimulation of proliferation and differentiation of osteoprogenitor cells in bone marrow

In order to examine the mechanism of the anabolic effect of parathyroid hormone (PTH) on bone formation, human PTH(1-34) [hPTH(1-34)] (30 μg/kg) was injected subcutaneously to 9-week-old rats 5 times a week for 1 or 3 weeks. Trabecular bone volume (BV/TV) in the tibial metaphysis was not significantly different between the PTH- and vehicletreated groups, but the parameters related to bone formation, including osteoid surface (OS/BS), mineralizing surface (MS/BS), mineral apposition rate (MAR), and bone formation rate (BFR/BS), were significantly increased as early as 1 week after PTH treatment. And the parameters related to bone resorption including eroded surface (ES/BS) and osteoclast number (N.Oc/BS) were also significantly increased as early as 1 week after PTH treatment. Treatment with PTH for 1 week induced no significant increase in bone mineral density at the femoral metaphysis, whereas the same treatment for 3 weeks induced a significant increase. When bone marrow cells isolated from femora and tibiae of either PTH- or vehicle-treated rats were cultured at a high density (2 × 10⁷ cells/one well of 24-multiwell plate), cellular alkaline phosphatase (ALP) activity was significantly increased in the cells isolated from PTH-treated rats compared with vehicletreated rats. When bone marrow cells were cultured at a low density (4 × 10⁶ cells/a one well of 6-multiwell plate) to generate colonies (colony forming unit-fibroblastic, CFU-F), PTH induced apparent increases in both the total number of CFU-F and the number of ALP-positive CFU-F. The ratio of the latter to the former was significantly higher in the PTH-treated group than in the vehicle-treated group. These findings suggest that the anabolic effect of PTH is, at least in part, due to the stimulation of proliferation and differentiation of osteoprogenitor cells in bone marrow.     

Effects of exercise on bone mineral density in calcium-replete postmenopausal women with and without hormone replacement therapy

Osteoporosis is a major public health concern. The combination of exercise, hormone replacement therapy, and calcium supplementation may have added benefits for improving bone mineral density compared to a single intervention. To test this notion, 320 healthy, non-smoking postmenopausal women, who did or did not use hormone replacement therapy (HRT), were randomized within groups to exercise or no exercise and followed for 12 months. All women received 800 mg calcium citrate supplements daily. Women who exercised performed supervised aerobic, weight-bearing and weight-lifting exercise, three times per week in community-based exercise facilities. Regional bone mineral density (BMD) was assessed by dual energy X-ray absorptiometry. Women who used HRT, calcium, and exercised increased femoral neck, trochanteric and lumbar spine bone mineral density by approximately 1–2%. Trochanteric BMD was also significantly increased by ~1.0% in women who exercised and used calcium without HRT compared to a negligible change in women who used HRT and did not exercise. The results demonstrate that regional BMD can be improved with aerobic, weight-bearing activity combined with weight lifting at clinically relevant sites in postmenopausal women. The response was significant at more sites in women who used HRT, suggesting a greater benefit with hormone replacement and exercise compared to HRT alone.     

Phosphorus deficiency,parathyroid hormone and bone resorption in the growing rat

Sham-operated and parathyroidectomized (PTX) rats were divided into two pair-fed groups, one on a normal mineral intake (0.5% Ca, 0.3% P), the other on a regimen low in phosphorus (0.5% Ca, 0.03% P). P depletion led to a drop in plasma P and urine P, a rise in plasma Ca and a marked rise in urine Ca, a drop in serum magnesium and a rise in urine Mg. The changes were more pronounced in the PTX animals, but final values were the same in both groups. Parallel bone-seeking isotope (⁸⁵Sr,¹⁷⁷Lu,²³⁷Np) studies in nonablated animals revealed an increased in the urinary nuclide output and in the urine/tibia ratio in P-deficient animals. Normal and primary bone osteocytes decreased and enlarged osteocytes increased as a result of P deficiency; osteoclasts and osteoblasts also increased. Bone composition showed a drop in ash content and a rise in water, with a light decrease in both Ca and P, and a corresponding rise in hydroxyproline and nitrogen in the P-deficient animals. The results are interpreted to mean that P-deficiency in the young growing rat leads to an increase in bone resorption which occurs also in the absence of parathyroid hormone (PTH). The fact that final values were similar in the control and PTX P-deficient animals suggests that steady-state regulation can also occur without PTH. Because P-deficiency leads to rapid hypercalcemia and rapid marked hypercalciuria, there may exist a mechanism for phosphate regulation which would then supersede Ca homeostasis. The change in serum and urine Mg levels may reflect a decrease in tubular Ca and Mg reabsorption associated with P-deficiency.     

Parathyroid hormone (1–34) increases vertebral bone mass, compressive strength, and quality in old rats

Human parathyroid hormone 1–34 (PITH) exerts an anabolic effect on bone in younger rats. The aim of the present study was to examine the effect of PTH on vertebral bone in 2-year-old male rats. The rats were treated with daily injections of 15 nmol/kg PTH or vehicle (V) for 56 days. Tetracycline and calcein were injected on day 15 and day 40 of the treatment period, respectively. The PTH treatment did not influence the body weights of the rats, the volumes of whole vertebra, or the vertebral body heights. However, the PTH treatment induced profound changes in the bone structure. Histomorphometric analyses of the vertebral bodies (L-6) revealed an approximate doubling of the cancellous bone volume after PTH treatment from 24.6 ± 1.3% to 54.9 ± 2.0% (p < 0.001) as well as a doubling of the trabecular thickness while the bone surface/bone volume decreased by 60%. PTH treatment also increased bone formation as indicated by an increase in mineral apposition rate (from 0.42 ± 0.01 to 0.89 ± 0.01 μm/day, p < 0.01), increased mineralizing surface (from 7.8 ± 1.4 to 43.8 ± 1.9%, p < 0.01) and an increase in both volume-related and surface-related bone formation rates (5 and 11 times, respectively). The biomechanical properties were analyzed using standardized bone specimens from the vertebral bodies of L-4 by applying cranial-caudal compression in a materials testing machine. The PTH treatment induced a substantial increase in the strength of the vertebral body: ultimate load increased by 66%, ultimate stiffness by 47%, and energy absorption by 98%. The increase in vertebral body strength was also evident after normalizing the parameters to the cross sectional area and the ash content of the vertebral body specimens. PTH treatment increased ultimate stress from 26 ± 3 to 44 ± 3 N per mm² (p < 0.01) and increased ultimate load normalized to ash content per mm specimen height from 59 ± 4 to 72 ± 4 N (mm/mg) (p < 0.05). The PTH treatment induced an increase in dry defatted bone density and ash density of both the vertebral body specimen (L-4) and the whole vertebra (L-5). In conclusion, PTH showed a remarkable ability to stimulate bone formation in the vertebral body of old rats. Furthermore, the biomechanical analysis revealed an enhanced compressive bone strength, even after correction for the increased bone mass, indicating an improved bone quality after the PTH treatment.