Cyclosporine A inhibits calcemic hormone-induced bone resorption in vitro

We have investigated the in vitro effects of cyclosporine (CsA), a potent immunosuppressive agent, on bone resorption induced by calcemic hormones. CsA inhibited parathyroid hormone (PTH), prostaglandin E2, 1,25-dihydroxy vitamin D3 (1,25(OH)2D3), and osteoclast-activating factor induced resorption of fetal rat limb bones in a dose-dependent manner. Established ongoing resorptive activity in bone was also inhibited by CsA. The CsA inhibition of bone resorption could be partially surmounted by higher concentrations of PTH and 1,25(OH)2D3. The inhibitory effects of CsA on limb bone resorption were reversible. Neither protein nor DNA synthesis were inhibited by treatment of limb bones with CsA. Thus, the inhibitory effect of this agent on bone resorption is not a cytotoxic one. These data could suggest that the induction of bone resorption by the calcemic hormones involves an immune cell derived mediator such as a lymphokine.     

Effects of parathyroid hormone on alkaline phosphatase activity and mineralization of cultured chick embryo tibiae

Summary The objectives of this study were (a) to determine if decreased bone alkaline phosphatase (AlPase) activity, resulting either from exposure to parathyroid hormone (PTH) or from direct inhibition of AlPase with levamisole, was concomitant with net changes in bone Ca and P content; and (b) to determine the duration of the effect of PTH on bone AlPase activity after the hormone was removed. Tibiae from 7-day chick embryos were incubated in chemically defined medium for 3–4 days. The Ca and P content of bones incubated for 72 h in this system increased from 2-to 9-fold. Addition of PTH (1 U/ml) to the medium resulted in a 60% decrease in bone AlPase activity, and this decrease was accompanied by a marked inhibition of Ca and P accumulation in bone. When levamisole, a specific inhibitor of AlPase activity, was added to the medium, a similar inhibition of bone mineral accumulation resulted. A 24-h incubation in medium containing PTH (1 U/ml) resulted in a 30% decrease in bone AlPase activity. Following the 24-h exposure to PTH, incubation was continued in medium containing no hormone. After 72 h in hormone-free medium, bones that had been exposed to PTH earlier contained more AlPase activity than paired control bones never exposed to PTH. The PTH-treated bones also recovered their ability to accumulate Ca and P. The results of this study show that: (a) decreased bone AlPase activity resulting from either exposure to PTH or direct inhibition by levamisole is associated with a marked decrease in mineral accumulation in bone; (b) exposure to PTH followed by removal of the hormone leads to recovery and even an increase in bone AlPase activity; and (c) following removal of PTH from the culture system, tibiae recover their ability to accumulate Ca and P. These results suggest that one of the ways PTH may alter mineral accretion is through its effects on bone AlPase activity.     

驱铜联合活性维生素D3治疗对Wilson''''s病患者骨骼及代谢的影响

目的观察Wilson's病患者骨骼X线摄片表现,及驱铜治疗联合活性维生素D3补充治疗对骨代谢的影响。方法对35例入选Wilson's病患者入院时进行骨骼X线摄片,并分别于治疗前及二巯基丙磺酸钠驱铜辅以活性维生素D3治疗8疗程后,放射免疫法测定骨代谢相关激素:PTH、CT、及血清BGP水平;用生化法测定血钙、血磷,尿钙、尿磷等。并用单光子吸收法(SPA)测定尺桡骨中远端1/3处平均骨密度值。结果Wilson's病患者手腕部X线检测异常率达60%。治疗后,血PTH、血钙均降低,骨密度(BMD)值升高,较治疗前差异有统计学意义(P<0·05);血磷、尿钙、磷及血清CT、BGP水平较治疗前差异无显著性。结论Wilson's病患者常并发骨质疏松或骨质软化等代谢性骨病,驱铜治疗联合活性维生素D3补充治疗可更好改善骨骼代谢,更快改善其骨密度。     

Human cystatin C, a cysteine proteinase inhibitor, inhibits bone resorption in vitro stimulated by parathyroid hormone and parathyroid hormone-related peptide of malignancy.

The effect of human recombinant cystatin C, a cysteine proteinase inhibitor, on bone resorption in vitro was evaluated. Bone resorption was assessed by analyzing the release of 45Ca and 3H from mouse calvarial bones prelabeled in vivo by injections with 45Ca or [3H]proline, respectively. In 24 h cultures, cystatin C (50 micrograms/ml) significantly inhibited the release of 45Ca and 3H stimulated by parathyroid hormone (PTH, 15 nmol/liter) or parathyroid hormone-related peptide of malignancy (PTHrP, 15 nmol/liter). The degree of inhibition caused by cystatin C in these 24 h cultures was similar to that caused by calcitonin (30 ng/ml). The inhibitory effect of cystatin C on 45Ca release induced by PTH was sustained in 96 h cultures, whereas the initial inhibition caused by calcitonin was transient. Cystatin C, 10-100 micrograms/ml, caused a dose-dependent inhibition of PTH (15 nmol/liter), and PTHrP (15 nmol/liter) stimulated 45Ca release. Addition of 50 micrograms/ml of cystatin C to mouse bone cultures inhibited the release of 45Ca induced by PTH and PTHrP at a wide range of submaximal and maximal concentrations of hormones (0.01-10 nmol/liter). No effect of cystatin C on 45Ca release in dead bones could be observed, nor did the inhibitor decrease the release of calcium in control bones. The inhibition by cystatin C on PTH-induced mineral mobilization was reversible. Cystatin C (1-100 micrograms/ml) did not affect protein synthesis or mitotic activities in mouse calvarial bones as assessed by the incorporation of [3H]proline and [3H]thymidine, respectively. These data show that cystatin C is a potent inhibitor of mineral mobilization and matrix degradation in cultured bones stimulated to resorb by PTH and PTHrP and that this effect is not due to general cytotoxicity.     

甲状腺机能亢进症患者骨密度及骨代谢改变

目的：探讨甲状腺机能亢进症（甲亢）患骨密度与骨代谢指标的改变。方法：本用二维骨密度仪测定65例甲亢患及38例健康对照骨密度,同时测定血清钙、碱性磷酸酶（AKP）、甲状旁腺激素（PTH）、降钙素（CT）、并计算PTH/CT比值及24小时尿钙值。分为3组：甲亢合并OP组;甲亢骨矿含量正常组及健康对照组。结果：甲亢合并OP组24小时尿钙、血AKP高于另两组（P＜0.05)甲亢组CT明显低于对照组,PTH/CT比值明显高于对照组,且甲亢合并OP组更明显（P＜0.05、P＜0.01)。结论：甲亢为高转换型OP,PTH、CT、PTH/CT比值对研究甲亢合并OP有重要参考价值。     

绝经后妇女血清调钙激素水平与骨代谢关系探讨

目的 探讨绝经后妇女血清调钙激素水平对骨形成、骨吸收及骨密度的影响。方法 142名健康绝经后妇女测定血雌二醇（E2）、睾酮（T）、总三碘甲状腺原氨酸（TT3）、总甲状腺素（TT4)、甲状旁腺激素全段（PTH－SP）、降钙素（CT）、骨钙素（BGP）、尿脱氧吡啶啉（DPD）、尿肌酐（Cr）。双能X线骨密度仪测定腰椎、髋部、前臂骨密度（BMD）,对检测结果进行分析。结果 本组骨质疏松患病率为60．48％,低骨量32．39％,正常骨量7．13％。血清6种调钙激素中T、E2、TT3、TT4、CT与BMD、BGP呈正相关,与年龄负相关;PTH－SP与BMD呈负相关,与年龄、DPD／Cr呈正相关。结论 绝经后妇女血调钙激素水平影响骨形成、骨吸收和骨密度,使骨代谢趋向于负平衡,是绝经后妇女易发生骨质疏松症的重要原因。     

Evaluation of the role of second messenger systems in tumor necrosis factor-stimulated resorption of fetal rat limb bones

Tumor necrosis factor (TNF) actions in target tissues are mediated by various signalling pathways. The effect of TNF to stimulate resorption in fetal rat limb bones is not inhibited by indomethacin. The current studies were designed to assess the role of cyclic AMP (cAMP) and calcium as second messengers in this prostaglandin-independent action of TNF on bone resorption. TNF alone failed to increase cyclic AMP in fetal rat limb bones after either brief (15 min) or long-term (72 h) treatment. TNF-stimulated resorption in fetal rat limb bones was enhanced by the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX). In short term incubations, the combination of TNF + IBMX did not elicit increases in cAMP in the limb bones. In 72 h cultures, addition of IBMX revealed a dose-dependent effect of TNF to increase cAMP. TNF produced a significant increase in inositol phosphate turnover in limb bones, with a greater response at 5 min than at 1 or 20 min. The calcium channel blocker nitrendipine inhibited TNF-stimulated resorption in the fetal rat limb bones. TNF-stimulated resorption was attenuated by pretreatment with pertussis toxin (PTx). PTx did not inhibit the effect of TNF to increase inositol phosphate turnover. TNF did not increase cAMP, intracellular calcium, or inositol phosphates in the UMR-106 cells. The data suggest the following: (a) cyclic mucleotides may play a role in TNF-stimulated resorption, although an increase in cAMP is not a direct rapid effect of TNF per se; (b) inositol phosphates could also play a signalling role in the action of TNF; (c) a pertussis toxin-sensitive step is required for TNF action on bone resorption.     

Comparison of fetal rat limb bones and neonatal mouse calvaria: effects of parathyroid hormone and 1,25-dihydroxyvitamin D3

The relative responses of fetal rat limb bones and neonatal mouse calvaria to parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) were examined in organ culture. Limb bones were cultured in dishes in BGJ + 1 mg/ml bovine serum albumin (bSA) or DMEM + 15% heat-inactivated horse serum (hS). Calvaria were either cultured in dishes with one of the above two media or in roller tubes in DMEM + hS. The order of sensitivity to PTH was: calvaria in roller tubes greater than limb bones in dishes in DMEM + hS greater than limb bones in dishes in BGJ + bSA greater than or equal to calvaria in dishes in DMEM + hS greater than calvaria in dishes in BGJ + bSA. The most sensitive system (i.e., calvaria in roller tubes) showed significant resorption in response to 10(-10)M PTH at 48 h. The least sensitive system did not respond to 3 X 10(-8)M PTH after the same length of time. The greater response in DMEM + hS compared with BGJ + bSA appeared to be due to the protein component of the medium. The order of sensitivity to 1,25(OH)2D3 was: calvaria in roller tubes = long bones in culture dishes in BGJ + bSA greater than long bones in culture dishes in DMEM + hS greater than calvaria in dishes in DMEM + hS greater than calvaria in dishes in BGJ + bSA. The most sensitive systems showed resorption in response to 10(-11)M 1,25(OH)2D3 by 72 h. The least sensitive system failed to respond to 10(-9)M 1,25(OH)2D3 after the same length of time. The nature of the protein constituent did not seem to influence the response of the limb bones to 1,25(OH)2D3. The results indicated that although the responses of the two bone systems to the calcemic hormones were qualitatively similar, media and culture conditions could markedly affect the sensitivity.     

Repetition of continuous PTH treatments followed by periodic withdrawals exerts anabolic effects on rat bone

Various animal experiments and human studies have shown that intermittent injections of parathyroid hormone (PTH) exert anabolic effects on bone, whereas continuous PTH treatment decreases the bone mass and causes hypercalcemia in animals. However, limited data are available with regard to the effects of a repetitive regimen of continuous treatments of PTH followed by periodic withdrawals on the bone metabolism. We investigated the effects of this regimen by comparing the findings of intermittent and continuous PTH treatments in rats. Infusions of PTH for 24 h followed by 6-day withdrawal periods from PTH transiently increased the serum calcium levels on day 1, but these levels were within the normocalcemic range. The repetition of 4 cycles of continuous PTH infusions followed by PTH withdrawals as well as intermittent PTH treatment increased the trabecular bone thickness, osteoblast surface, and bone formation rate. Continuous PTH infusions followed by PTH withdrawals also increased the cortical thickness of the femoral diaphysis and the osteoid volume in trabecular bones, whereas the continuous treatment failed to induce these changes. These findings suggest that continuous PTH treatment followed by PTH withdrawal is a potential regimen that can induce the anabolic effects of PTH in bone metabolism without inducing hypercalcemia.     

Characteristics of calcium uptake by calcitonin-treated mouse calvariain vitro

The effect of calcitonin (CT) on calcium metabolism by bone was investigated in a stationary organ culture system, using half-calvaria from 5-day-old mice. CT induced the uptake of calcium by bones from the culture medium. This uptake was not accompanied by phosphate, so that the final Ca:P ratio of CT-treated bones (1.80±0.06) was significantly (p<0.02) higher than the initial values before culture (1.59±0.03). The calcium uptake reached a plateau after approximately 48 hours of culture, and was reversed by PTH within 48 hours. Calcium uptake rppears to be an active effect of CT, and cannot be explained fully by an inhibition of bone aesorption, stimulation of new bone formation, or maturation of the mineral phase.     

Second messengers in thrombin-stimulated bone resorption

Characterized human thrombins and two commercial bovine thrombin preparations were examined for their effects on bone resorption and on the cyclic AMP and phosphoinositide second messenger systems in bone. Human alpha- and gamma-thrombins, as well as both bovine thrombin preparations, stimulated bone resorption in vitro, whereas catalytically inactivated human diisopropylfluorophosphate (DIP)-alpha-thrombin did not significantly stimulate resorption. Human alpha-thrombin and a commercial bovine thrombin preparation increased cyclic AMP production in fetal rat limb bones, but another bovine commercial thrombin preparation and gamma-thrombin did not. Except for DIP-alpha-thrombin, all thrombins increased production of inositol phosphates in fetal rat limb bones at concentrations that stimulated resorption. In time course studies, bovine thrombin increased label in inositol trisphosphate at 30 s, with decreasing effects at later times. Inositol monophosphate increased progressively over 30 min. Our results are consistent with thrombin-stimulated bone resorption being mediated at least partially through the inositol phosphate pathway.     

Parathyroid hormone secretory pattern,circulating activity,and effect on bone turnover in adult growth hormone deficiency

Adult growth hormone deficiency (AGHD) is associated with osteoporosis. Reports have associated parathyroid hormone (PTH) circadian rhythm abnormalities with osteoporosis. Furthermore, there is evidence of relative PTH insensitivity in AGHD patients. Factors regulating PTH circadian rhythm are not fully understood. There is evidence that serum phosphate is a likely determinant of PTH rhythm. The aim of this study was to investigate PTH circadian rhythm and its circulating activity and association with bone turnover in untreated AGHD patients compared to healthy individuals. We sampled peripheral venous blood at 30-min and urine at 3-h intervals during the day over a 24-h period from 1400 h in 14 untreated AGHD patients (7 M, 7 W; mean age, 49.5 +/- 10.7 years) and 14 age (48.6 +/- 11.4 years; P = NS) and gender-matched controls. Cosinor analysis was performed to analyze rhythm parameters. Cross-correlational analysis was used to determine the relationship between variables. Serum PTH (1-84), phosphate, total calcium, urea, creatinine, albumin, type I collagen C-telopeptides (CT(x)), a bone resorption marker, and procollagen type I amino-terminal propeptide (PINP), a bone formation marker, were measured on all samples. Nephrogenous cyclic adenosine monophosphate (NcAMP), which reflects the renal activity of PTH, was calculated from plasma and urinary cAMP. Urinary calcium and phosphate were measured on all urine samples. Significant circadian rhythms were observed for serum PTH, phosphate, CT(x), and PINP in AGHD and healthy subjects (P < 0.001). No significant rhythm was observed for serum-adjusted calcium. PTH MESOR (rhythm-adjusted mean) was significantly higher (P < 0.05), whereas the MESOR values for phosphate, CT(x) (P < 0.05), and PINP (P < 0.001) were lower in AGHD patients than in controls. AGHD patients had significantly lower 24-h NcAMP (P < 0.001) and higher urinary calcium excretion (P < 0.05). Maximum cross-correlation between PTH and phosphate (r = 0.75) was observed when PTH was lagged by 1.5 h in healthy individuals, suggesting that changes in phosphate precede changes in PTH concentration. PTH/CT(x) and PTH/PINP showed maximum correlation when CT(x) (r = 0.68) and PINP (r = 0.71) were lagged by 3 h. In AGHD patients, compared to controls the maximum correlation between PTH/phosphate (r = 0.88, P = 0.007), PTH/CTx (r = 0.61, P = 0.027), and PTH/PINP (r = 0.65, P = 0.028) was observed when the lag time was reduced by 1.5 h in all variables, with changes in PTH and phosphate occurring at concurrent time points. Our data suggest decreased end-organ sensitivity to the effects of PTH in AGHD patients, resulting in a significantly lower NcAMP, low bone turnover, and higher calcium excretion in the presence of significantly higher PTH concentrations. We have also demonstrated that changes in serum phosphate precede those of PTH, which in turn precede changes in bone resorption and formation in healthy individuals. This relationship was altered in AGHD patients. These results suggest a possible role for GH in regulating PTH secretion and the bone remodeling process.     

Ionophore A23187 promotes osteoclast formation in bone organ culture

Summary Morphologic effects of the ionophore A23187 were compared with those of PTH and 1,25(OH)₂ vitamin D₃ on fetal rat limb bone cultures. All three treatments resulted in loss of bone and the appearance of multinucleate osteoclasts. Only PTH caused proliferation of fibroblast-like cells within the marrow cavity. Cleanly dissected (“stripped”) bones were unresponsive to A23187 and exhibited many dead and dying cells after 48 h in culture with this agent; in contrast, stripped bones were responsive to PTH. All three treatments showed a similar time course of appearance of osteoclasts. The results indicate that all three agents promote osteoclast formation. There are several possible explanations for the differential effects of A23187 and PTH in stripped bones, including the possibility that the cells of the periosteum are required for the action of A23187 but not of PTH.     

Cellular localization of neprilysin in mouse bone tissue and putative role in hydrolysis of osteogenic peptides.

The regulation of osteoblast and osteoclast metabolism is mediated by both hormones and local bone peptide factors. Peptides and hormones are under control of membrane peptidases such as Neprilysin (NEP). NEP is a widely distributed cell-surface zinc-metallopeptidase that is involved in the regulation of several important physiological processes by controlling the half-life of bioactive peptides. Although NEP is known to be present in skeletal tissues, neither its cellular localization nor its function have been established. To address this question, we examined NEP distribution in bones of postnatal mouse. In situ hybridization (ISH) and immunohistochemistry showed that NEP messenger RNA (mRNA) and protein are associated with bone-forming cells including presumptive osteoblast precursors, preosteoblasts, osteoblasts, and osteocytes. NEP levels in newborn and adult mice bones also were compared by immunoblotting. Higher amounts of NEP immunoreactivity were observed in newborn as compared with adult bones, suggesting a relationship between NEP expression and bone growth. To further explore this hypothesis, we monitored in vitro NEP proteolytic activity using a series of synthetic osteogenic peptides such as parathyroid hormone-related peptide 1-43 (PTHrP1-34), osteostatin (PTHrP107-139), osteogenic growth peptide (OGP), calcitonin, alpha-calcitonin gene-related peptide (alpha-CGRP), and PTH1-34. Except for PTH1-34, all peptides were found to be NEP substrates.     

Interferon gamma and calcitonin induce differential changes in cellular kinetics and morphology of osteoclasts in cultured neonatal mouse calvaria

The present study compares the effects of calcitonin (CT) and interferon gamma (IFN-gamma) on the size, distribution, and ultrastructure of osteoclasts in cultured neonatal mouse calvaria. The number and cross-sectional area of osteoclasts in cultured bones was increased by the addition of parathyroid hormone (PTH) to the culture medium for 24-48 h. Prolonged treatment (up to 72 h) with PTH led to extensive rarefication and formation of holes in the mineralized matrix. PTH-activated osteoclasts exhibited an elaborate ruffled border and showed a typical zonal arrangement of intracellular organelles with the outer cytoplasmic region containing numerous membrane-bound vesicles. CT (20 mU/ml) within 90 minutes caused a complete loss of the ruffled border in PTH-activated osteoclasts. The typical zonal architecture disappeared with intracellular vesicles spread through the entire cytoplasm. Prolongation of CT treatment to 24-48 h led to the appearance of vesicles with dark granular content in inactivated osteoclasts. This morphologically distinct vesicle population allowed us to identify the latter cells as "postosteoclasts" and thereby to distinguish them from osteoclasts precursors. Small active osteoclasts that had originated very likely from these precursors appeared in calvarial bones coincidentally with escape from CT inhibition of bone resorption and resulted in increased total number of osteoclasts. In contrast, IFN-gamma (500 U/ml) acting as a proliferation inhibitor, reduced the total number of osteoclasts. In the presence of PTH it caused no immediate (90 minutes) change in the ultrastructure of PTH-induced osteoclasts apart from an increase in the number of autophagic vacuoles. After prolonged exposure (24-48 h) in the presence of PTH, osteoclasts with low resorbing activity exhibited intermediate borders at their contact zone with the mineralized matrix. In the absence of PTH, the short-term effect of the immune interferon on osteoclast morphology was almost comparable to that of CT. After prolonged treatment in the absence of PTH, postosteoclasts comparable in vesicle population and size to those after CT treatment were found.     

Effects of salmon calcitonin on synthesis and mineralization of collagen in rats

The effects of salmon calcitonin (CT) on collagen metabolism and mineral deposition in fractures and intact femora, and on collagen metabolism in healing skin wounds and intact skin have been studied in young male rats. Serum calcium and serum phosphorus were reduced 3 h after the daily subcutaneous CT injection (3 MRC-U/kg body weight), whereas a rebound increase in the serum levels of both minerals was observed at 24 hours after the injection. CT had an early transient inhibitory influence on the collagen synthesis, and this resulted in a reduced total content of collagen in bones and skin specimens from treated rats compared to controls. The concentration of collagen in bone and skin was, however, increased in treated animals compared to controls after prolonged CT administration. Following an early transient increase, the incorporation of strontium-85 into the fractured bones was impaired after 30 days of CT treatment. This resulted in a reduced mineral concentration in the fractures of treated rats compared to controls in the last part of the experiment. The recorded effects of CT treatment, which were most pronounced in healing fractures and intact skin specimens, may be interpreted as an inhibitory influence of CT both on synthesis, mineralization and degradation of collagen.     

Interleukin-6 is produced by bone and modulated by parathyroid hormone

Interleukin-6 (IL-6) is a cellular regulatory molecule, the diverse functions of which relate to cells both within and outside the immune system. In this report we demonstrated that bone tissue, specifically osteoblasts, produce interleukin-6 and that this function can be modulated by the osteotrophic hormone parathyroid hormone (PTH). Given that the complex process of bone remodeling is now thought to be regulated not only by systemic hormones but also by locally produced factors, the existence of a parathyroid hormone-stimulated production of interleukin-6 by osteoblasts may have important physiological significance.     

Salmon stanniocalcin and bovine parathyroid hormone have dissimilar actions on mammalian bone.

Stanniocalcin (STC), a calcium-regulating glycoprotein hormone isolated from the corpuscles of Stannius of salmon, was tested for effects on bone and calcium metabolism in mammalian species (rats and mice). STC generally failed to alter serum calcium of parathyroidectomized rats at concentrations equimolar with effective concentrations of parathyroid hormone (PTH). STC did not increase cAMP in ROS 17/2.8 or UMR-108 osteosarcoma cells, OK kidney cells, fetal rat limb bones, or neonatal mouse calvariae, and similarly failed to increase urinary cAMP in rats. STC did not consistently stimulate resorption in any of the rodent bone culture systems, although variable resorptive responses were elicited in fetal mouse calvariae. The results indicate that this fish hormone has limited, if any, PTH-like activity on calcium metabolism in mammalian systems.     

Parathyroid hormone, calcitonin, and vitamin D metabolites during normal fracture healing in humans. A preliminary report

The metabolism of calcium is regulated by hormones: parathyroid hormone, (PTH) calcitonin (CT), and vitamin D metabolites. To study the physiologic role of these hormones during the process of fracture healing in humans, the blood levels of PTH, CT, 25-(OH)-D3, 24,25-(OH)2-D3, and calcium were determined in 13 young patients with fractures of long bones. The parameters were measured first on admission and again after six to eight weeks. A group of healthy volunteers of similar age and sex served as control subjects. Plasma calcium level on the day of fracture was significantly reduced, 8.50 +/- 0.23 mg% (p less than 0.001). Serum CT level on the day of fracture was significantly increased, 0.18 +/- 0.02 ng/ml (p less than 0.05), and it continued to increase during the healing period, up to 0.23 +/- 0.02 ng/ml (p less than 0.001) after six to eight weeks. A significant rise was noted in plasma level of 24,25-(OH)2-D3, from 2.02 +/- 0.42 ng/ml on the day of fracture to 2.84 +/- 0.41 ng/ml six weeks later (p less than 0.05). No significant changes were found in serum PTH and plasma 25-(OH)-D3 levels on the day of fracture or during the healing process. The results suggest a possible physiologic active role for CT and 24,25-(OH)2-D3 in fracture healing in humans.     

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.