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D. N. KALU, CARMEL HILLYARD, and G. V. FOSTER

SUMMARY

The effect of glucagon on bone was studied in rats. Urinary hydroxyproline excretion and incorporation of [3H]proline into bone hydroxyproline were used as indices of bone collagen breakdown and formation respectively. Parathyroid extract (15 USP units/rat/h, i.v.), infused into thyroparathyroidectomized animals, increased urinary hydroxyproline excretion. This increase was nullified by simultaneous administration of glucagon (50 μg/rat/h, i.v.). Rats treated with glucagon for 12 days (30 μg/100 g/day, s.c.) excreted slightly less hydroxyproline in their urine than controls. In both intact and thyroparathyroidectomized rats, glucagon (10 μg/100 g/h, s.c.) decreased incorporation of [3H]proline into bone. Similar results were obtained in nephrectomized rats, evidence that changes produced by glucagon were not solely due to alterations in proline pool size caused by increased renal excretion. From these data it is concluded that: (1) glucagon can inhibit bone resorption (the effect being slight in normal rats, but easily demonstrable in parathyroid hormone-treated thyroparathyroidectomized rats), (2) release of endogenous calcitonin is not required to produce this effect, (3) parathyroid hormone and glucagon may act on the same target cell in bone, (4) inhibition of skeletal resorption may contribute to glucagon-induced hypocalcaemia, and (5) the hormone possibly decreases bone formation. Since pharmacological doses of glucagon were used in our studies, the relationship of the observations made to the physiological role of glucagon is unknown.

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K. W. Kan and R. L. Cruess

ABSTRACT

In order to assess glucocorticoid actions on fetal cartilage development, [3H]dexamethasone binding site levels in fetal bovine cartilaginous tissues from long bones were measured, using a whole cell assay at 37 °C. Displaceable [3H]dexamethasone binding in epiphysial growth cartilage was maximal (16·2 fmol/106 cells) in fetuses of 10–15 cm crown–rump length (CR), and declined to 22% of the maximum in fetuses of 20–30 cm CR. Subsequently, [3H]dexamethasone binding rose to a plateau (13·0 fmol/106 cells) in fetuses of 30–80 cm CR and declined in those of 80–100 cm CR. When measured in growth plate cartilage, [3H]dexamethasone binding was significantly higher in fetuses of 40–80 cm CR (39 fmol/106 cells) than in those of 80–100 cm CR. There was no significant change of [3H]dexamethasone binding affinities in epiphysial chondrocytes of 5–100 cm CR fetuses or in growth plate chondrocytes of 40–100 cm CR fetuses.

These results demonstrate that fetal cartilaginous tissues during development possess varying cellular levels of glucocorticoid binding and may thus have temporal changes in sensitivity to glucocorticoid hormones.

J. Endocr. (1986) 110, 257–262

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Kristin M Aasarød, Masoud Ramezanzadehkoldeh, Maziar Shabestari, Mats P Mosti, Astrid K Stunes, Janne E Reseland, Vidar Beisvag, Erik Fink Eriksen, Arne K Sandvik, Reinhold G Erben, Christiane Schüler, Malcolm Boyce, Bjørn H Skallerud, Unni Syversen, and Reidar Fossmark

Epidemiological studies suggest an increased fracture risk in patients taking proton pump inhibitors (PPIs) for long term. The underlying mechanism, however, has been disputed. By binding to the gastric proton pump, PPIs inhibit gastric acid secretion. We have previously shown that proton pump (H+/K+ATPase beta subunit) KO mice exhibit reduced bone mineral density (BMD) and inferior bone strength compared with WT mice. Patients using PPIs as well as these KO mice exhibit gastric hypoacidity, and subsequently increased serum concentrations of the hormone gastrin. In this study, we wanted to examine whether inhibition of the gastrin/CCK2 receptor influences bone quality in these mice. KO and WT mice were given either the gastrin/CCK2 receptor antagonist netazepide dissolved in polyethylene glycol (PEG) or only PEG for 1year. We found significantly lower bone mineral content and BMD, as well as inferior bone microarchitecture in KO mice compared with WT. Biomechanical properties by three-point bending test also proved inferior in KO mice. KO mice receiving netazepide exhibited significantly higher cortical thickness, cortical area fraction, trabecular thickness and trabecular BMD by micro-CT compared with the control group. Three-point bending test also showed higher Young’s modulus of elasticity in the netazepide KO group compared with control mice. In conclusion, we observed that the gastrin receptor antagonist netazepide slightly improved bone quality in this mouse model, suggesting that hypergastrinemia may contribute to deteriorated bone quality during acid inhibition.

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Viji Vijayan and Sarika Gupta

Hyperhomocysteinemia (HHCY) is a risk factor for osteoporosis but whether HHCY affects bone mineralization or not is still ambiguous. Herein we evaluated whether homocysteine affects tissue mineral density (TMD) of cortical bone and if so the role of osteocytes. CD1 mice administered with homocysteine (5 mg/100 g body weight, i.p.) for 7, 15 and 30 days showed temporal changes in TMD and osteocyte lacunar density in femoral cortices. Short-term administration of homocysteine (day 7) increased osteocyte lacunar density and reduced TMD evidenced by microCT50 while prolonged administration of homocysteine (day 30) reinstated TMD and lacunar density to baseline values. Major differences were decreased number of nucleated osteocyte lacunae, increased number of empty lacunae and cleaved caspase 3-positive osteocyte lacunae in day 30 HHCY bone evidenced by H&E staining and immunohistochemistry. Other differences were induction in mineralization genes like Dmp1, Phex and Sost in cortical bone by real-time PCR and increased number of Dmp1- and Sost-positive osteocyte lacunae in day 30 HHCY bone evidenced by immunohistochemistry. Both HHCY day 7 and day 30 samples showed reduced Young’s modulus demonstrating that biomechanical property of bone was lost during early HHCY itself, which did not improve with recovery of TMD. Our results thus demonstrate occurrence of two phases in cortical bone upon HHCY: the early phase that involved loss of TMD and increase in osteocyte numbers and a late phase that involved osteocyte reprogramming, apoptosis and mineralization, which reinstated TMD but compromised biomechanical property. To conclude, osteocytes have a potential role in arbitrating bone pathogenesis during HHCY.

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MK Lindberg, SL Alatalo, JM Halleen, S Mohan, JA Gustafsson, and C Ohlsson

There are two known estrogen receptors, estrogen receptor-alpha (ER alpha) and estrogen receptor-beta (ER beta), which may mediate the actions of estrogen. The aim of the present study was to compare fat content, skeletal growth and adult bone metabolism in female mice lacking ER alpha (ERKO), ER beta (BERKO) or both ERs (DERKO). We demonstrate that endogenous estrogens decrease the fat content in female mice via ER alpha and not ER beta. Interestingly, the longitudinal bone growth was decreased in ERKO, increased in BERKO, but was intermediate in DERKO females, demonstrating that ER alpha and ER beta exert opposing effects in the regulation of longitudinal bone growth. The effects on longitudinal bone growth were correlated with similar effects on serum levels of IGF-I. A complex regulation of the trabecular bone mineral density (BMD), probably caused by a disturbed feedback regulation of estrogen and testosterone, was observed in female ER-inactivated mice. Nevertheless, a partial functional redundancy for ER alpha and ER beta in the maintenance of the trabecular BMD was observed in the female mice at 60 days of age. Thus, ER alpha and ER beta may have separate effects (regulation of fat), opposing effects (longitudinal bone growth) or partial redundant effects (trabecular BMD at 60 days of age), depending on which parameter is studied.

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M. Zaidi, T. J. Chambers, R. E. Gaines Das, H. R. Morris, and I. MacIntyre

ABSTRACT

The calcitonin gene encodes a small family of peptides: calcitonin, calcitonin gene-related peptide (CGRP) and katacalcin. Whereas calcitonin is concerned with skeletal maintenance, the function, if any, of katacalcin is still unknown. In the present study we have assessed resorption of human cortical bone substrate by isolated rat osteoclasts and have shown that CGRP acts directly on the osteoclast to inhibit bone resorption. The three CGRP peptides (rat, human(a) and human(β)) caused an almost equivalent decrease in osteoclastic bone resorption and were approximately 1000-fold less potent than human calcitonin in this respect. The responses of human calcitonin and human CGRP(α) were additive. Furthermore, prior treatment with trypsin to destroy receptors abolished the responsiveness of osteoclasts to CGRP and calcitonin. The carboxyl- and amino-terminal fragments of CGRP were found not to inhibit bone resorption, suggesting that the whole molecule of CGRP is necessary for biological activity. We have therefore suggested that the calcitonin-like effects of CGRP, seen both in vivo in the rat bioassay and in vitro in organ cultures, are due to the direct action of CGRP on the osteoclast, probably mediated through the calcitonin receptor. Though it is unlikely that CGRP is involved in the regulation of plasma calcium, the peptide may be an important local regulator of bone cell function.

J. Endocr. (1987) 115,511–518

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Russell T Turner, Michael Dube, Adam J Branscum, Carmen P Wong, Dawn A Olson, Xiaoying Zhong, Mercedes F Kweh, Iske V Larkin, Thomas J Wronski, Clifford J Rosen, Satya P Kalra, and Urszula T Iwaniec

Excessive weight gain in adults is associated with a variety of negative health outcomes. Unfortunately, dieting, exercise, and pharmacological interventions have had limited long-term success in weight control and can result in detrimental side effects, including accelerating age-related cancellous bone loss. We investigated the efficacy of using hypothalamic leptin gene therapy as an alternative method for reducing weight in skeletally-mature (9 months old) female rats and determined the impact of leptin-induced weight loss on bone mass, density, and microarchitecture, and serum biomarkers of bone turnover (CTx and osteocalcin). Rats were implanted with cannulae in the 3rd ventricle of the hypothalamus and injected with either recombinant adeno-associated virus encoding the gene for rat leptin (rAAV-Leptin, n=7) or a control vector encoding green fluorescent protein (rAAV-GFP, n=10) and sacrificed 18 weeks later. A baseline control group (n=7) was sacrificed at vector administration. rAAV-Leptin-treated rats lost weight (−4±2%) while rAAV-GFP-treated rats gained weight (14±2%) during the study. At study termination, rAAV-Leptin-treated rats weighed 17% less than rAAV-GFP-treated rats and had lower abdominal white adipose tissue weight (−80%), serum leptin (−77%), and serum IGF1 (−34%). Cancellous bone volume fraction in distal femur metaphysis and epiphysis, and in lumbar vertebra tended to be lower (P<0.1) in rAAV-GFP-treated rats (13.5 months old) compared to baseline control rats (9 months old). Significant differences in cancellous bone or biomarkers of bone turnover were not detected between rAAV-Leptin and rAAV-GFP rats. In summary, rAAV-Leptin-treated rats maintained a lower body weight compared to baseline and rAAV-GFP-treated rats with minimal effects on bone mass, density, microarchitecture, or biochemical markers of bone turnover.

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MP Warren and NE Perlroth

Women have become increasingly physically active in recent decades. While exercise provides substantial health benefits, intensive exercise is also associated with a unique set of risks for the female athlete. Hypothalamic dysfunction associated with strenuous exercise, and the resulting disturbance of GnRH pulsatility, can result in delayed menarche and disruption of menstrual cyclicity. Specific mechanisms triggering reproductive dysfunction may vary across athletic disciplines. An energy drain incurred by women whose energy expenditure exceeds dietary energy intake appears to be the primary factor effecting GnRH suppression in athletes engaged in sports emphasizing leanness; nutritional restriction may be an important causal factor in the hypoestrogenism observed in these athletes. A distinct hormonal profile characterized by hyperandrogenism rather than hypoestrogenism is associated with athletes engaged in sports emphasizing strength over leanness. Complications associated with suppression of GnRH include infertility and compromised bone density. Failure to attain peak bone mass and bone loss predispose hypoestrogenic athletes to osteopenia and osteoporosis. Metabolic aberrations associated with nutritional insult may be the primary factors effecting low bone density in hypoestrogenic athletes, thus diagnosis should include careful screening for abnormal eating behavior. Increasing caloric intake to offset high energy demand may be sufficient to reverse menstrual dysfunction and stimulate bone accretion. Treatment with exogenous estrogen may help to curb further bone loss in the hypoestrogenic amenorrheic athlete, but may not be sufficient to stimulate bone growth. Treatment aimed at correcting metabolic abnormalities may in fact prove more effective than that aimed at correcting estrogen deficiencies.

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Lucie E Bourne, Caroline PD Wheeler-Jones, and Isabel R Orriss

Biomineralisation, the deposition of mineral onto a matrix, can be both a physiological and pathological process. Bone formation involves the secretion of an extracellular matrix (ECM) by osteoblasts and subsequent mineralisation of that matrix. It is regulated by a number of local and systemic factors and is necessary for maintenance of normal bone health. Conversely, mineralisation (or calcification) of soft tissues, including the vasculature, is detrimental to that tissue, leading to diseases such as arterial medial calcification (AMC). The mechanisms underlying AMC development are not fully defined, though it is thought that vascular smooth muscle cells (VSMCs) drive this complex, cell-mediated process. Similarly, AMC is regulated by a variety of enzymes and molecules, many of which have already been implicated in the regulation of bone mineralisation. This review will provide an overview of the similar, and sometimes opposing effects of these signalling molecules on the regulation of bone mineralisation and AMC.

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Colin Farquharson

In this issue of Journal of Endocrinology, Lanham et al. investigated the effects of hypothyroidism on the developing skeleton of the ovine foetus in utero. Their analyses indicated that, following thyroidectomy, bone growth, structure and mechanical properties were all altered at late gestation or at term. Adrenalectomy, whilst preventing the prepartum rise in triiodothyronine, did not modify skeletal development. The hypothyroid-mediated skeletal defects of the developing foetus described in this study may have clinical implications for bone health in later life.