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

Modulation of osteoclastogenesis through adrenomedullin receptors on osteoclast precursors: initiation of differentiation by asymmetric cell division

Adrenomedullin (ADM), a member of the calcitonin family of peptides, is a potent vasodilator and was shown to have the ability to modulate bone metabolism. We have previously found a unique cell surface antigen (Kat1 antigen) expressed in rat osteoclasts, which is involved in the functional regulation of the calcitonin receptor (CTR). Cross-linking of cell surface Kat1 antigen with anti-Kat1 antigen monoclonal antibody (mAbKat1) stimulated osteoclast formation only under conditions suppressed by calcitonin. Here, we found that ADM provoked a significant stimulation in osteoclastogenesis only in the presence of calcitonin; a similar biological effect was seen with mAbKat1 in the bone marrow culture system. This stimulatory effect on osteoclastogenesis mediated by ADM was abolished by the addition of mAbKat1. 125I-labeled rat ADM (125I-ADM)-binding experiments involving micro-autoradiographic studies demonstrated that mononuclear precursors of osteoclasts abundantly expressed ADM receptors, and the specific binding of 125I-ADM was markedly inhibited by the addition of mAbKat1, suggesting a close relationship between the Kat1 antigen and the functional ADM receptors expressed on cells in the osteoclast lineage. ADM receptors were also detected in the osteoclast progenitor cells in the late mitotic phase, in which only one daughter cell of the dividing cell express ADM receptors, suggesting the semiconservative cell division of the osteoclast progenitors in the initiation of osteoclastogenesis. Messenger RNAs for the receptor activity-modifying-protein 1 (RAMP1) and calcitonin receptor-like receptor (CRLR) were expressed in cells in the osteoclast lineage; however, the expression of RAMP2 or RAMP3 was not detected in these cells. It is suggested that the Kat1 antigen is involved in the functional ADM receptor distinct from the general ADM receptor, consisting of CRLR and RAMP2 or RAMP3. Modulation of osteoclastogenesis through functional ADM receptors abundantly expressed on mononuclear osteoclast precursors is supposed to be important in the fine regulation of osteoclast differentiation in a specific osteotrophic hormonal condition with a high level of calcitonin in blood.

Calcitonin induces expression of the inducible cAMP early repressor in osteoclasts

The cAMP response element modulator gene (Crem) encodes a variety of transcriptional regulators including the inducible cAMP early repressor, ICER. We previously showed that Crem knockout mice, which are deficient in CREM and ICER factors, display slightly increased long bone mass and decreased osteoclast number. These data are consistent with the notion that Crem regulates bone mass in part through an effect on osteoclast formation and/or function. Since ICER is strongly induced by cAMP, we asked whether the calcium-regulating hormone calcitonin, which stimulates cAMP production and inhibits osteoclastic bone resorption, could induce ICER in osteoclasts. The monocytic cell line RAW264.7 was treated with receptor activator of NF-kappaB ligand (RANKL) to induce osteoclast formation. Calcitonin caused a time- and dose-dependent induction of ICER mRNA and an increase in ICER protein abundance in RANKL-treated RAW264.7 cells. Calcitonin also induced ICER mRNA and protein in osteoclasts derived from primary mouse bone marrow cell cultures. Calcitonin-treated osteoclasts showed immunoreactivity with an anti-CREM antibody. Calcitonin decreased the activity of wild-type and Crem knockout osteoclasts in vitro, and this inhibitory effect was greater in Crem knockout osteoclasts. Furthermore, calcitonin decreased calcitonin receptor mRNA expression in wildtype osteoclasts, but not in Crem knockout osteoclasts. These data suggest that calcitonin induction of ICER in osteoclasts might regulate osteoclast activity.

Effects of nasal administration of calcitonin in oophorectomized women: 2-year controlled double-blind study

OBJECTIVE

The aim of this study was to assess the effects of nasal salmon calcitonin (SCT) administration on bone turnover in ovariectomized women.

METHODS

Patients who had undergone bilateral ovariectomy 7 days previously, received either calcium supplementation (1000 mg/day, together with nasal SCT (100 IU/day) (n = 19) or the same calcium supplementation together with a placebo intranasal spray daily (n = 19), for 2 years.

RESULTS

In the calcium-only-treated subjects, lumbar bone mineral density (BMD) was found to have decreased significantly (P < 0.001), 6 months after surgery and remained at this level until the end of the study. In the SCT-treated group, BMD remained stable during the 1st year and then decreased gradually, reaching a statistically significant level in the 2nd year. Mean serum osteocalcin concentration was unchanged during the 1st year of SCT treatment but was significantly elevated during the 2nd year (P < 0.01). The observed rise in serum osteocalcin concentration and urinary hydroxyproline excretion during the 2nd year of treatment with SCT was accompanied by a significant rise in serum calcitonin levels (P < 0.001 after 18 months and P < 0.01 after 24 months).

CONCLUSION

This study shows that continuous treatment with intranasal SCT is able to prevent the bone loss that follows ovariectomy.

Effects of continuous calcitonin treatment on osteoclast-like cell development and calcitonin receptor expression in mouse bone marrow cultures

Continuous treatment with calcitonin (CT) to inhibit osteoclastic bone resorption results in acquired resistance. The mechanisms of this "escape" phenomenon are not yet established. The aim of this study was to examine the effects of continuous treatment with CT on the generation of osteoclasts and calcitonin receptor (CTR) expression in mouse bone marrow cultures. This was done by daily CT treatment of mouse bone marrow cultures from day 0, when only undifferentiated mononuclear precursors of osteoclast-like cells were present, or commencing from day 6, when differentiated osteoclast-like cells were abundant. The response to CT treatment was determined by quantitation of cells positive for tartrate-resistant acid phosphatase (TRAP) and binding of 125I-salmon CT. Calcitonin receptor and TRAP mRNA levels were determined using semi-quantitative reverse transcription/polymerase chain reaction. When cultures were treated with CT from day 0, TRAP-positive multinucleated cells appeared. These cells expressed only very low levels of CTR or CTR mRNA and were morphologically indistinguishable from osteoclast-like cells formed in control cultures. They also displayed the ability to resorb bone. Continuous CT treatment of cultures from day 6 rapidly reduced the CTR mRNA levels, with a t1/2 of 6 to 12 h, and these levels remained low thereafter. 125I-salmon CT binding capacity, as determined by autoradiography, was lost in parallel. These effects were specific for the CTR since there was no consistent effect on TRAP mRNA levels. Based on these data, we suggest that the "escape" phenomenon may result from a prolonged CT-induced loss of CT responsiveness due, at least in part, both to reduced synthesis of CTR, and to the appearance in bone of CTR-deficient osteoclasts.

Downregulation of calcitonin receptor mRNA expression by calcitonin during human osteoclast-like cell differentiation

Calcitonin inhibits both osteoclast formation and bone resorption, and is a primary treatment for patients with hypercalcemia and increased bone turnover. However, the clinical utility of calcitonin is limited because patients become refractory to calcitonin after several days (the calcitonin "escape phenomenon"). The molecular basis for calcitonin "escape" is unclear. To determine the regulatory mechanisms controlling calcitonin receptor (CTR) expression in osteoclasts and their precursors, we treated immature mononuclear precursors for human osteoclast-like multinucleated cells (MNC) formed in vitro with 1,25-(OH)2D3, to induce their differentiation to committed mononuclear precursors, and mature multinucleated osteoclasts, and used reverse transcriptase (RT)-PCR to assess expression of CTR mRNA in both committed mononuclear precursors and MNC. The PCR fragment produced was cloned and sequenced to confirm that it was derived from CTR mRNA. CTR mRNA expression was detected in mononuclear MNC precursors after 7 d of 1,25-(OH)2D3 treatment. It was also present in osteoclast-like MNC and highly purified giant cells from osteoclastomas, but not in monocytes or macrophage polykaryons formed in vitro. Calcitonin markedly decreased CTR but not actin mRNA expression in giant cells and MNC after 12 h, and removal of calcitonin restored CTR mRNA expression. Similarly, calcitonin decreased calcitonin-induced adenylate cyclase activity. These data suggest: (a) downregulation of CTR gene expression by calcitonin may in part explain the calcitonin "escape phenomenon"; and (b) expression of CTR mRNA occurs in mononuclear osteoclast precursors within 7 d after exposure to 1,25-(OH)2D3.

The effect of intranasal salmon calcitonin on biochemical parameters of bone turnover in postmenopausal women

This study aims to evaluate the effect of intranasal salmon calcitonin on variables of bone metabolism in 12 postmenopausal women during 24 months of treatment. A treatment regime of 100 U of intranasal salmon calcitonin on alternate days and 1500 mg daily of oral elementary calcium was applied. The control group consisted of 35 postmenopausal women distributed according to time since menopause. Biochemical and hormonal evaluations of calcium metabolism were performed at the start of treatment and after 6, 12, 18 and 24 months of treatment. Mean serum osteocalcin concentration was unchanged during the 1st year of treatment but was significantly elevated during the 2nd year (p = 0.03 and p = 0.005 after 18 and 24 months, respectively) when compared to levels at 12 months. Similar elevation of osteocalcin levels was observed in untreated women during the first 12 postmenopausal months. Mean 24-h hydroxyproline excretion decreased during the first 12 months of therapy but increased in the subsequent 6 months. The observed rise in serum osteocalcin concentration and urinary hydroxyproline excretion during the 2nd year of treatment with calcitonin was accompanied by a significant rise in serum calcitonin level. No significant differences in serum calcium, phosphorus, alkaline phosphatase or parathormone concentration, or urinary calcium excretion, were observed between treated and untreated women during the 24-month period. This study shows that 12 months' treatment with intranasal salmon calcitonin decreases bone resorption in early postmenopausal women, while bone formation remains unchanged. Longer treatment with intranasal salmon calcitonin, however, seems to be ineffective, most probably due to secondary resistance to calcitonin.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term changes in the haversian systems following high-dose irradiation. An ultrastructural and quantitative histomorphological study

The effects of high-dose irradiation on the morphology of haversian bone were studied, over a fifty-two-week period, in seventy-seven adult rabbits, after the administration of a single dose of radiation (therapeutic x-ray; twenty-five, fifty, or 100 gray) to one knee joint. The specimens of bone were examined with microangiography, light and transmission electron microscopy, and histomorphometry. Analysis was performed on the haversian bone in the subchondral bone plate of weight-bearing portions of the femoral condyles. Microangiography demonstrated dilatation of the microvasculature four weeks after irradiation. Beginning at twelve weeks, there was a marked decrease in vascularity; no obvious recovery of the subchondral bone had occurred by fifty-two weeks. At four weeks, morphological analysis revealed two changes in the haversian canals: simple occlusion of the haversian vessels with loss of cells in the canal, and dilatation of the capillaries with abnormal resorption of the perivascular bone matrix by osteoclasts. The abnormal bone resorption was not coupled with subsequent new-bone formation, resulting in increased porosity. Beginning at four weeks, a progressive decrease in the number of haversian vessels and in cellularity became prominent. The decrease in cellularity involved all types of cells, including endothelial cells, pericytes, perivascular mesenchymal cells, osteoblasts, osteocytes, and osteoclasts. The loss of perivascular cells was often but not always associated with occlusion of the haversian vessels. Histomorphometry revealed both time-dependent and dose-dependent decreases in capillary density (the number of intraosseous capillaries per unit area) and in the number of osteocytes in the subchondral bone plate. The porosity of the same areas showed a significant increase by four weeks (p < 0.001 after administration of twenty-five gray and p < 0.01 after administration of both fifty and 100 gray), but between twelve and fifty-two weeks, there was only a slight additional increase. Statistical analysis revealed significant correlations between capillary density and osteocyte survival (p < 0.001) and between capillary density and porosity (p < 0.001). The portion of the subchondral bone plate that was located farthest from the non-irradiated normal bone showed progressive damage and no sign of recovery at fifty-two weeks.

Histochemical and autoradiographic studies on elcatonin internalization and intracellular movement in osteoclasts

The binding sites and chronologic localization of elcatonin (eCT) in osteoclasts were examined by autoradiography using [125I]elcatonin (125I-eCT). In addition to the structural changes induced by calcitonin (CT) reported so far, changes were also observed in the structure of Golgi apparatus. These changes continued until 48-72 h after incubation with eCT. Developed silver grains of 125I-eCT were localized into multinucleated osteoclasts and mononuclear cells that were ultrastructurally defined as "preosteoclasts." The silver grains located on plasma membranes of those cells and were then internalized; they accumulated, especially in the Golgi apparatus, and remained for 48-72 h. A few silver grains were also detected in lysosomes and small vesicles. The decrease in the number of silver grains in the Golgi apparatus accompanied the recovery of osteoclast structures--Golgi apparatus and then ruffled borders. These findings suggest that (1) CT especially inhibits the sorting function of Golgi apparatus in osteoclasts, resulting in prolonged retention of CT in this organelle. (2) The CT in Golgi apparatus may keep its activity and cause the prolonged effect of CT on osteoclast activity.

In vitro studies on bone resorption in neonatal mouse calvariae using a modified dissection technique giving four samples of bone from each calvaria

Bone resorption in a modified bone culture system, based on incubation of small fragments from neonatal mouse calvarial bones, has been studied. Four bone fragments were dissected out from each mouse calvaria and were thereafter cultured in CMRL 1066 medium in plastic multiwell dishes. Bone resorption was assessed by 45Ca release from prelabeled bones. The rate of bone resorption in response to parathyroid hormone (PTH) was less in the anterior part of the calvaria compared to the posterior part. After removing the anterior region, four parietal bone fragments that showed identical basal and PTH-stimulated release of 45Ca could be dissected out from each mouse. Excretion of lactate dehydrogenase and beta-glucuronidase was the same in bones cultured submerged or on grids. Uptake of [3H]thymidine in bones cultured submerged was 54% of [3H]thymidine uptake in bones cultured on grids. Dose-response curves, established by using parietal bone fragments, showed that the sensitivity and the magnitude of the increase in 45Ca release seen after stimulation with PTH, prostaglandin E2, and 1 alpha-hydroxyvitamin D3 were the same for bones cultured submerged or on grids. The 45Ca release in response to stimulation with PTH, prostaglandin E2, and 1 alpha-OHD3 was the same in calvarial fragments cultured submerged and those previously obtained with calvarial halves cultured on grids. Thus, even though the rate of DNA synthesis was slower in bones cultured submerged, the rate and the magnitude of resorption were the same in bones cultured on grids or submerged. These data show that it is possible to perform studies on bone resorption with small fragments of neonatal mouse parietal bones.

Nasal administration of salmon calcitonin for prevention of glucocorticoid-induced osteoporosis in children with nephrosis

To determine the effect of intranasal administration of salmon calcitonin on glucocorticoid-induced osteoporosis in children with nephrosis, we gave 100 U of calcitonin intranasally on alternate days with 1 alpha-hydroxyvitamin D3 to five children, 8 to 12 years of age, with frequently relapsing nephrosis. Four patients with osteoporosis, 10 to 14 years of age, were treated only with 1 alpha-hydroxyvitamin D3 and served as control subjects. Both groups were treated with an almost equal amount of glucocorticoids previously and during this study period. Bone mineral content of the spine was measured by a quantitative computed tomographic technique. The bone mineral content was preserved in both cortical and spongeous areas of the vertebrae during the 16-month period in the calcitonin-treated group but was decreased significantly in the control group. Urinary hydroxyproline and calcium excretion decreased significantly in the calcitonin-treated group. The serum calcium and phosphorus concentrations and the parathyroid function did not change significantly in either group. We conclude that calcitonin suppressed bone resorption and might be useful for the long-term treatment of osteoporosis, in combination with 1 alpha-hydroxyvitamin D3, in children with nephrosis requiring long-term glucocorticoid therapy.

Osteotropic factor responsiveness of highly purified populations of early and late precursors for human multinucleated cells expressing the osteoclast phenotype

Recently we have adapted human long-term bone marrow cultures to form multinucleated cells (MNC) that express the osteoclast phenotype and used semisolid culture techniques to identify early (bipotent) and late (unipotent) mononuclear precursors for these MNC. The early precursor can form both osteoclast-like MNC and macrophage polykaryons; the late precursor forms only osteoclast-like MNC. In this study we examined the effects of osteotropic hormones and cytokines of MNC formation from highly purified populations of these early or late mononuclear precursor cells. MNC expressing the osteoclast phenotype were identified by their cross-reactivity with the 23c6 monoclonal antibody, which preferentially identifies osteoclasts. 1,25-(OH)2D3 (10(-8) M), IL-1 beta (10 u/ml), and IL-6 (100 pg/ml) stimulated formation of 23c6-positive MNC from highly purified populations of early or late precursor cells. In contrast, PTH (50 ng/ml) did not act directly on late precursor cells but only stimulated 23c6-positive MNC formation from early precursors. These results show that (1) 1,25-(OH)2D3, IL-1 beta, and IL-6 can stimulate 23c6-positive MNC formation from a highly enriched population of early and late precursors, and (2) PTH does not act on late precursors but may act indirectly on the late precursors.

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.

Comparison between the effects of forskolin and calcitonin on bone resorption and osteoclast morphology in vitro

The adenylate cyclase activator forskolin (1-10 mumol/L) inhibited 45Ca release from parathyroid hormone (PTH; 10 nmol/L) stimulated prelabeled neonatal mouse calvaria in short term culture (24 h). This effect of forskolin was potentiated by rolipram, Ro 20-1724, and isobutyl-methylxanthine, three structurally different inhibitors of cyclic AMP phosphodiesterase. Forskolin (10 mumol/L) and calcitonin (30 mU/mL) inhibited the mobilization of stable calcium and inorganic phosphate as well as the release of the lysomal enzymes beta-glucuronidase and beta-N-acetylglucosaminidase from PTH-stimulated unlabeled bones. Osteoclasts in PTH-stimulated calvaria showed active ruffled borders with numerous membrane infoldings. Treatment of PTH-stimulated bones with forskolin and calcitonin resulted in a rapid (2 h) loss of the active ruffled border. In addition, forskolin and calcitonin induced similar changes with respect to the number and size distribution of cytoplasmic vesicles in PTH-activated osteoclasts. After 24 h, all signs of osteoclast inactivation were still prominent, whereas after 48 h of treatment with forskolin or calcitonin, the reappearance of a ruffled border on a number of osteoclasts signaled an escape from the inhibitory action of both calcitonin or forskolin. These data indicate that forskolin inhibits bone resorption by a cyclic AMP dependent mechanism and that the effect of forskolin and calcitonin on bone resorption and osteoclast morphology are comparable. These observations lend further support to the view that cyclic AMP may be an intracellular mediator of the inhibitory action of calcitonin on multinucleated osteoclasts.

Bone cell biology: the regulation of development, structure, and function in the skeleton

Bone cells compose a population of cells of heterogeneous origin but restricted function with respect to matrix formation, mineralization, and resorption. The local, mesenchymal origin of the cells which form the skeleton contrasts with their extraskeletal, hemopoietic relatives under which bone resorption takes place. However, the functions of these two diverse populations are remarkably related and interdependent. Bone cell regulation, presently in its infancy, is a complicated cascade involving a plethora of local and systemic factors, including some components of the skeletal matrices and other organ systems. Thus, any understanding of bone cell regulation is a key ingredient in understanding not only the development, maintenance, and repair of the skeleton but also the prevention and treatment of skeletal disorders.

Colony-stimulating factors regulate the development of multinucleated osteoclasts from recently replicated cells in vitro

Osteoclasts mediate the process of bone resorption. However, little is known about the mechanisms that regulate the formation of either osteoclasts or osteoclast precursors. In contrast, colony-stimulating factors (CSFs) are well-known to regulate the formation of myeloid cells and their precursors. Because osteoclasts and myeloid cells may originate from a common stem cell, we examined the effects of two CSFs, granulocyte-macrophage CSF (GM-CSF) and interleukin 3 (IL-3), on bone resorption, osteoclast formation, and the incorporation of recently replicated nuclei into the osteoclasts of mouse bone cultures. CSFs had little effect on the formation rate of osteoclasts or their resorptive activity but significantly decreased the percentage of recently replicated osteoclast progenitor cell nuclei present in the osteoclasts of bones treated with parathyroid hormone. GM-CSF also increased the number of myeloid cells in the marrow space of the cultures and the percentage of these cells derived from recently replicated progenitors. These results demonstrate that GM-CSF and IL-3 can regulate the development of osteoclasts from recently replicated precursor cells in cultured fetal mouse long bones. However, the mechanisms by which CSFs influence osteoclast formation are difficult to determine from these studies because markers for the osteoclast progenitor and precursor do not exist. These data also provide evidence that the differentiation of osteoclast progenitors is regulated by different factors at different points in their ontogeny.

Evidence for two pathways for stimulation of collagenolysis in bone

The effect of parathormone (PTH), lipopolysaccharide (LPS), or interleukin-1 (IL-1) on calcium release and collagen degradation in bone was examined in vitro using labeled neonatal calvaria of normal mice and also of osteopetrotic microphthalmic (mi/mi) mice that have defective osteoclasts. All three agents stimulated calcium release from normal bone but not from mi/mi bone. PTH stimulated the degradation of both noncalcified and calcified collagen in normal bone as well as the degradation of noncalcified collagen in mi/mi bone. However, LPS and IL-1 only stimulated the degradation of calcified collagen in normal bone. One-half maximal stimulation of noncalcified collagen degradation in normal or mi/mi bone was achieved by about 3 nM PTH compared with about 1 nM PTH for that of calcium release from normal bone. While calcitonin (CT) and leupeptin inhibited calcium release and thereby the degradation of calcified collagen, neither agent inhibited PTH-stimulated noncalcified collagen degradation in normal or mi/mi bone. The data indicate the existence of two pathways that lead to collagen degradation in bone. One is intimately connected with the resorptive process stimulated by a variety of agents, and is probably mediated by osteoclasts. A second mechanism is sensitive only to PTH and appears to be associated with nonosteoclastic cells since it can operate under conditions in which osteoclasts are thought to be inactive or are inhibited.

Whole-body irradiation inhibits the escape phenomenon of osteoclasts in bones of calcitonin-treated rats

The escape phenomenon is characteristic of osteoclastic bone resorption in organ cultures, and calcitonin only transiently inhibits parathyroid hormone (PTH)-stimulated resorption. The present study demonstrated that the transient inhibition of osteoclastic bone resorption, a phenomenon reminiscent of escape, occurs in the bones of calcitonin (ECT)-treated rats and that whole-body irradiation inhibits this escape. Rats were treated with daily subcutaneous injections of ECT for 72 h. At 24 h ECT decreased the incidence of osteoclast profiles with ruffled borders both in the growth plate-metaphysis junction (GPMJ) and the metaphyseal trabecular bone region (MT). However, by 72 h the incidence in the MT had been restored to the level of the control. The trabecular bone volume in the ECT-treated bone did not differ significantly from the control value. Whole-body irradiation (600 rad) before the first injections of ECT prevented the re-activation of the ruffled border formation and increased the trabecular bone volume at 72 h. Irradiation diminished the number of osteoclasts in the ECT-treated bones to the level of the control. ECT-treated bones contained a greatly increased number of macrophage-like cells (MO). Irradiation prevented this ECT-induced increase in the number of MO. These results strongly suggest that the escape phenomenon in vivo involves the calcitonin-induced proliferation of cells in the mononuclear phagocyte system, with resultant increases in the number of osteoclasts and in the bone resorption activity of osteoclasts.

DNA synthesis is not necessary for osteoclastic responses to parathyroid hormone in cultured fetal rat long bones

Osteoclasts, the principal cells mediating bone resorption, are believed to increase their size, number, and resorbing activity in response to parathyroid hormone (PTH) through mechanisms dependent upon the fusion of specific mononuclear precursor cells into either new or existing multinucleated osteoclasts. To address the question of whether these actions of PTH are dependent on the replication of osteoclast precursor cells, we examined the ability of an inhibitor of DNA synthesis, hydroxyurea (HU), to alter bone resorption, osteoclast formation, and DNA synthesis in cultured fetal rat bones treated with PTH. We found that HU significantly reduced [3H]thymidine incorporation into the bones and labeling of osteoclast nuclei by greater than 90%, but did not prevent PTH from stimulating bone resorption, measured as the release of 45Ca, or from increasing the number of osteoclasts in the bones. In bones cultured without PTH, HU decreased the rate of bone resorption, but not the number of osteoclasts per bone. We conclude that in fetal rat bone cultures, PTH can increase osteoclast number and stimulate bone resorption by affecting existing osteoclasts and osteoclast precursors, and that replication of osteoclast precursor cells is not necessary for PTH to stimulate a resorptive response. In unstimulated cultures it appears that HU inhibits bone resorption by affecting mechanisms that are independent of changes in osteoclast number and that may be influenced by cell replication or other unknown factors.