Calcitonin secretion in postmenopausal osteoporosis

A moderately high intake compared to a low intake of zinc depresses magnesium balance and alters indices of bone turnover in postmenopausal women

OBJECTIVES

To determine whether moderately high or low intakes of zinc adversely affect the copper status of postmenopausal women to result in unfavorable changes in calcium and magnesium metabolism and other indicators of bone turnover.

DESIGN

After a 10-day equilibration period in which the diet provided 31.5 micromol (2 mg) Cu and 137.7 micromol (9 mg) Zn/8.4 MJ (2000 kcal), the subjects were randomly divided into two groups, with one group fed the basal diet supplemented to provide 15.7 micromol (1 mg) Cu/8.4 MJ, and the other group fed the same diet supplemented to provide 47.2 micromol (3 mg) Cu/8.4 MJ. After equilibration, both groups were fed the basal diet with no zinc supplemented (provided 45.9 micromol [3 mg] Zn/8.4 MJ) for 90 days; this was followed by another 10-day equilibration period before the basal diet was supplemented with zinc to provide 811 micromol (53 mg)/8.4 MJ for 90 days.

SETTING

The metabolic unit of the Grand Forks Human Nutrition Research Center, Grand Forks, ND, USA.

SUBJECTS

A total of 28 postmenopausal women recruited by advertisement throughout the United States of America. Among them, 25 women (64.9+6.7 y) completed the study; 21 as designed.

RESULTS

The moderately high intake compared to the low intake of zinc increased the excretion of magnesium in the feces and urine, which resulted in a decreased magnesium balance. In the women fed low dietary copper, plasma osteocalcin was higher during the low-zinc than high-zinc dietary period. The urinary excretion of N-telopeptides was increased and the serum calcitonin concentration was decreased by high dietary zinc regardless of dietary copper.

CONCLUSIONS

A moderately high intake of zinc (811 micromol/day; 53 mg/day) did not induce changes in copper metabolism that resulted in unfavorable changes in bone or mineral metabolism. However, low dietary zinc (45.9 micromol/day; 3 mg/day) apparently resulted in undesirable changes in circulating calcitonin and osteocalcin. As a moderately high intake of zinc decreased magnesium balance, further study of the possibility that a high intake of zinc is a health concern for individuals consuming less than the recommended amounts of magnesium is warranted.

Estrogen and bone--a reproductive and locomotive perspective

The primary function of the skeleton is locomotion, and the primary function of estrogen is reproduction. When the skeleton is considered within this locomotive context, the onset of estrogen secretion at puberty leads to packing of mechanically excess mineral into female bones for reproductive needs. Accordingly, the unpacking of this reproductive safety deposit at menopause denotes the origin of type I osteoporosis.

INTRODUCTION

According to the prevailing unitary model of involutional osteoporosis, female postmenopausal bone loss can be described as having an initial accelerated, transient phase (type I), followed by a gradual continuous phase (type II). Estrogen withdrawal is generally accepted as the primary cause of the type I osteoporosis. Thus, the quest to uncover the origin of type I osteoporosis has focused on the estrogen withdrawal-related skeletal changes at and around the menopause. However, considering that the cyclical secretion of estrogen normally begins in early adolescence and continues over the entire fertile period, one could argue that focusing on perimenopause alone may be too narrow.

MATERIALS AND METHODS

This is not a systematic review of the literature on the skeletal function of estrogen(s), but rather, an introduction of a novel structure- and locomotion-oriented perspective to this particular issue through pertinent experimental and clinical studies.

RESULTS AND CONCLUSIONS

When considering locomotion as the primary function of the skeleton and integrating the classic findings of the pubertal effects of estrogen on female bones and the more recent hypothesis-driven experimental and clinical studies on estrogen and mechanical loading on bone within this context, a novel evolution-based explanation for the role of estrogen in controlling female bone mass can be outlined: the onset of estrogen secretion at puberty induces packing of mechanically excess bone into female skeleton for needs of reproduction (pregnancy and lactation). Accordingly, the unpacking of this reproductive safety deposit of calcium at menopause denotes the accelerated phase of bone loss and thus the origin of type I osteoporosis.

Effect of daily hormone therapy and alendronate use on bone mineral density in postmenopausal women

OBJECTIVE

To evaluate the effect of daily oral and transdermal hormone therapy alone or in combination with alendronate on bone mineral density in postmenopausal women.

DESIGN

Comparative prospective clinical study.

SETTING

Outpatient clinic of a training and research hospital.

PATIENT(S)

One hundred seventy-three consecutive postmenopausal women with no previous hormone therapy and a bone mineral density T score <-1 SD were randomly enrolled.

INTERVENTION(S)

Oral conjugated estrogen, alone or with alendronate, or transdermal estrogen, alone or with alendronate, given for 1 year. All patients also received medroxyprogesterone acetate and calcium.

MAIN OUTCOME MEASURE(S)

Bone density measurement at L2 to 4 region by dual-energy X-ray absorptiometry.

RESULTS

At the end of 1 year, significant increase in bone density measurements were seen in all groups. Oral conjugated estrogen and transdermal estrogen have the same effect on bone mineral density loss. Hormone therapy alone stabilized the bone mineral density loss. Hormone therapy together with alendronate resulted in better values in all groups.

CONCLUSION

Hormone therapy is adequate in osteopenic women. However, hormone therapy plus alendronate is advantageous in women with considerable bone mineral density loss.

Protective role of n-3 lipids and soy protein in osteoporosis

It is well established that bone loss due to estrogen deficiency after menopause is greater in women consuming higher quantities of animal protein than in women consuming vegetable protein, particularly soy protein. Besides the dietary protein source altering bone loss, it has also been postulated recently that the source of a higher n-6/n-3 ratio in dietary oils is implicated in causing osteoporosis. Both animal and human studies have indicated that an increased intake of n-6 fatty acids from vegetable oils elevates prostaglandin E(2) levels as well as pro-inflammatory cytokines such as IL-1, IL-6 and TNF-alpha. Interestingly, it has been found that lack of estrogen also increases the production of these cytokines by immune cells and thereby activates osteoclasts during the peri-menopausal period. We speculated that the use of n-3 fatty acids and soy protein, which are known to act as anti-inflammatory and down regulate pro-inflammatory cytokines, may also protect against bone loss by decreasing osteoclast activation and bone resorption. Similar to the results of others, our ongoing studies indeed show that the bone loss in ovariectomized mice is significantly attenuated by feeding diets enriched with either fish oil or soy protein when compared to corn oil and casein-fed mice. One of the mechanisms appears to be decreasing the activation of receptor activator of NF-kappaB ligand (RANKL) on T cells, which has been found to increase osteoclast activation along with increasing pro-inflammatory cytokines in OVX mice. Since hormone replacement therapy has been found to cause adverse effects, further both animal and human studies are required with moderate soy protein and fish oil supplements in understanding the mechanisms involved in altering immune function and bone loss during menopause in women and aging in men.

Calcitonin receptor mRNA in mononuclear leucocytes from postmenopausal women: decrease during osteoporosis and link to bone markers with specific isoform involvement

Calcitonin inhibits bone resorption via its receptor (CTR) on osteoclasts. Two hCTR isoforms, hCTR1 and hCTR2, give proteins that differ in their structure and signaling pathways. We investigated whether specific isoforms or quantitative changes in total hCTR mRNA were associated with high bone resorption and turnover in menopause or osteoporosis. The hCTR mRNA in mononuclear blood cells of premenopausal (PreM), healthy (PostM), and osteoporotic (OsteoP) postmenopausal women was assessed using reverse-transcriptase polymerase chain reaction. hCTR1 and hCTR2 were investigated for 59 total RNA samples, and semiquantitative analysis of total hCTR mRNA was performed for 71. Serum calcitonin, free urinary deoxypyridinoline (D-Pyr), serum bone alkaline phosphatase (SBAP), and osteocalcin (SOC) were also evaluated. Serum calcitonin levels did not differ in PostM and OsteoP. The prevalence of each isoform was similar in the three groups. Healthy postmenopausal women and OsteoP with hCTR2 had lower bone turnover (D-Pyr: 6.79 +/- 0.54, n = 25; SBAP: 11.63 +/- 1.47, n = 26; SOC: 8.31 +/- 0.58, n = 26) than those without hCTR2 (D-Pyr: 9.90 +/- 1.95, n = 5; SBAP: 21 +/- 5.19, n = 5; SOC: 11.9 +/- 2.10, n = 5; p < 0.05). Total hCTR mRNA levels were not different in PreM and PostM. By contrast, values were strikingly lower in OsteoP (0.57 +/- 0.17, n = 28) than in PostM (2. 25 +/- 0.61, n = 19, p < 0.05) and negatively correlated with bone markers values in both. We suggest that a specific isoform and amounts of total hCTR mRNA are linked to increased bone resorption in postmenopausal osteoporosis.

The justification for providing dietary guidance for the nutritional intake of boron

Because a biochemical function has not been defined for boron (B), its nutritional essentiality has not been firmly established. Nonetheless, dietary guidance should be formulated for B, because it has demonstrated beneficial, if not essential, effects in both animals and humans. Intakes of B commonly found with diets abundant in fruits, vegetables, legumes, pulses, and nuts have effects construed to be beneficial in macromineral, energy, nitrogen, and reactive oxygen metabolism, in addition to enhancing the response to estrogen therapy and improving psychomotor skills and cognitive processes of attention and memory. Perhaps the best-documented beneficial effect of B is on calcium (Ca) metabolism or utilization, and thus, bone calcification and maintenance. The paradigm emerging for the provision of dietary guidance that includes consideration of the total health effects of a nutrient, not just the prevention of a deficiency disease, has resulted in dietary guidance for chromium (Cr) and fluoride; both of these elements have beneficial effects in humans, but neither has a defined biochemical function. Knowledge of B nutritional effects in humans equals or is superior to that of Cr and fluoride; thus, establishing a dietary reference intake for B is justified. An analysis of both human and animal data suggests that an acceptable safe range of population mean intakes of B for adults could well be 1-13 mg/d. Recent findings indicate that a significant number of people do not consistently consume more than 1 mg B/d; this suggests that B could be a practical nutritional or clinical concern.

Fractures after thyroidectomy in men: a population-based cohort study

Bone mass is purportedly reduced by an endogenous or exogenous excess of thyroid hormone or, perhaps, by calcitonin deficiency. Patients who have undergone thyroidectomy could be subject to all of these effects, yet their practical implications in terms of fracture risk are poorly defined. Interpretation is further hampered by the focus on women, where results may be influenced by involutional osteoporosis. Consequently, we assessed the potential for fractures among the 136 Rochester, Minnesota men who underwent thyroidectomy between 1935 and 1979, relative to a group of age-matched control men from the community. With 2194 person-years of follow-up in each group, survival free of any fracture of vertebra, proximal humerus, distal forearm, pelvis, or proximal femur was similar in the two groups (p = 0.23), and the relative risk of any of these fractures for thyroidectomized patients versus their controls was increased only 1.5-fold (95% CI, 0.7-3.2). The difference was entirely accounted for by a statistically significant excess of proximal femur fractures in the men with thyroidectomy. Risk factors for fractures among men with thyroidectomy included greater age at surgery, greater extent of surgery, and the presence of risk factors for secondary osteoporosis. Thus, thyroidectomy, performed mainly for adenoma or goiter, seems to have little overall influence on the risk of age-related fractures in men. However, the association with hip fractures requires further evaluation.

Effect of estrogen status on bone regulating hormones

The effect of surgically induced menopause and a subsequent estrogen monotherapy on the secretion of calciotropic hormones and serum minerals was evaluated in 11 perimenopausal menstruating women. In seven of them, plasma insulin growth factor-I was also assessed. In the 12th-16th week after bilateral oophorectomy, a decline of serum PTH (p < 0.01) and an increase of calcemia (p < 0.05), phosphatemia (p < 0.05), and IGF-I (p < 0.01) were documented. A subsequent transdermal estrogen substitution (100 micrograms/day for 6 weeks) returned these values to the preoperative range. However, neither the operation nor the estrogen treatment altered calcitonin secretion (basal and calcium stimulated), serum 1,25(OH)2, vitamin D3, or magnesium. The effectiveness of oophorectomy as well as compliance of estrogen substitution was documented by serum estradiol, FSH, and LH response. The results demonstrate a stimulating effect of estrogen on PTH secretion, secondary to an estrogen-induced reduction in plasma calcium. They further demonstrate an inhibitory effect on phosphatemia and IGF-I production, but no effect on calcitonin secretion and vitamin D metabolism.

Exercise and reproductive factors as predictors of bone density among osteoporotic women in Mexico City

We examined the association between physical activity and reproductive factors with bone density among 313 Mexican women, aged 26-83 years. Bone mineral density (BMD) was measured with a HOLOGIC QRD 1000 w, DXA densitometer at the lumbar spine and femoral region. We explored the relation between BMD and parity, age, body mass index (BMI), type of menopause, and level of exercise. Multiple regression models examining determinants of bone density at the lumbar and femoral regions showed that increasing age and lack of exercise were statistically significant predictors of bone demineralization. The number of pregnancies also had a deletereous effect on bone density, especially for lumbar spine, as well as BMI <20 kg/m2. Our results suggest that physical activity, parity, and BMI are important determinants of bone density in this population.

Circulating monomer-like calcitonin in osteoporotic patients

Physiological concentrations of monomeric calcitonin can inhibit osteoclastic bone resorption in vitro. We therefore investigated the circulating molecular forms, including monomer-like calcitonin, and their concentrations in 9 men and 9 women with established osteoporosis. Calcitonin was immunoextracted from serum by the use of rabbit calcitonin antibodies coupled to Sepharose 4B. The lyophilized extracts were incubated with 6 M urea overnight and gel chromatographed in a fast protein liquid chromatography (FPLC) system; calcitonin was measured by radioimmunoassay in the fractions. FPLC disclosed immunoreactive calcitonin of three different molecular sizes in the patients. The two largest forms were approximately 30 and 10 kDa and one eluted at the same position as monomeric calcitonin (3.4 kDa). After extraction and FPLC we found slightly higher calcitonin concentrations in osteoporotic women than previously reported levels in age-matched healthy women. Male patients had higher levels than female patients. None of the osteoporotic patients lacked monomer-like calcitonin. There was no significant correlation between the extracted total or monomer-like calcitonin and bone mineral density of the femoral neck. It is concluded that the circulating calcitonin in both male and female patients comprises three different molecular forms and that there is no deficiency of the monomer-like form. The calcitonin levels in the female patients were slightly higher than in a previous control group.

Evolution of the pathogenesis of postmenopausal bone loss

At a time when the investigation of bone diseases appears to be on the threshold of a new era, it is appropriate to pause and contemplate past and present thinking on the etiology of postmenopausal osteoporosis. This brief review traces the evolution of ideas on the nature of the pathogenesis of this disease from Albright's hypoosteoblastic hypothesis through other hypotheses that include disturbance in osteoclasia, negative calcium balance, disturbance of calcium homeostatic control mechanisms, increased skeletal sensitivity to parathyroid hormone, deficiency of calcitriol and calcitonin, altered activities of growth factors and cytokines, alterations in the local regulation of osteoclastogenesis, and changes in mechanical usage set points.

Treatment of osteoporosis

Osteoporosis is a common disorder affecting the health of many adults. Strategies for fracture prevention include optimization of peak bone mass and prevention of bone loss at menopause and with aging. Genetic, nutritional, and life-style factors influence peak bone mass and may be used to focus preventive efforts. Once peak bone mass is reached, increased bone resorption may be the major pathogenetic factor. Calcium plus vitamin D, estrogen replacement therapy, calcitonin, and etidronate are agents currently available for treatment of osteoporosis; they act by inhibiting bone resorption. The failure of bone formation to keep pace with bone resorption also contributes to bone loss. Fluoride and intermittent parathyroid hormone therapy increase bone formation; however, more data are needed to determine efficacy. Insulin-like growth factors, transforming growth factor-beta (TGF-beta), and bone morphogenetic proteins may stimulate bone formation, but they have not yet been tested clinically. New approaches to treatment of osteoporosis will emerge as our understanding of the pathogenesis increases.

Serum calcitonin forms and concentrations in young and elderly healthy females

To further investigate the role of calcitonin (CT) in normal physiology we studied circulating forms and the secretion after "calcium clamp" in young and elderly healthy females. Heterogeneity of CT in serum was disclosed after immunoextraction, fast protein liquid chromatography, and radioimmunoassay in young (27 +/- 3 years; mean +/- SD, n = 6) and elderly females (69 +/- 6 years, n = 11). Three distinct molecular forms appeared with approximate mol wt of 30, 10, and 3-4 kDa. All young women studied had considerable amounts of circulating monomer-like CT whereas several elderly had undetectable or low levels. The influence of age on basal and calcium stimulated, immunoextracted CT in serum was also studied in young (26 +/- 4 years; mean +/- SD, n = 13) and elderly (63 +/- 6 years; n = 12) healthy females. The calcium stimulation was carried out by means of the standardized calcium clamp method, where calcium was kept on a presettled level at 1.45 mmol/liter (+/- 2%) for 60 minutes. CT was immunoextracted from serum in all series of experiments with a polyclonal antiserum directed against the mid- and carboxyterminal region of the CT molecule, and the amount of extracted CT was determined by radioimmunoassay using another polyclonal antiserum against the carboxyterminal portion. After calcium infusion, the increase in CT was significantly higher in young women than in elderly (P < 0.05). At basal conditions, the CT levels were not significantly different but slightly higher in young than in elderly females. In conclusion, several elderly women lack monomer-like calcitonin in serum in contrast to young women.(ABSTRACT TRUNCATED AT 250 WORDS)

The long-term effect of a calcium diet on the bone tissue, C-cells and parathyroid glands of the rat

In this experiment of 11 month-duration, 18 female Wistar rats received controlled amounts of calcium with food and water, to produce a state of either hypocalcemia or hypercalcemia. After a long-term low calcium diet hypocalcemia disappeared. This group of animals showed insignificant increases in the nuclear-cytoplasmic ratio of the parathyroid cells as well as in the total volume of the parathyroid glands, and a significant increase in the volume density of the osteoid. In the group receiving a high calcium diet, hypercalcemia was still present after 11 month. The insignificantly smaller and more numerous C-cells produced more calcitonin than normally. The parathyroid cells were significantly smaller and the numerical areal density of the osteoclasts was significantly lower than in the control group. It can be concluded that the parathyroid glands and C-cells are involved in the maintenance of blood calcium homeostasis during a long-term experiment on rats receiving low or high calcium diet.

Effect of 131I treatment on the calcitonin response to calcium infusion in hyperthyroid patients

OBJECTIVE

The objective was to evaluate the effect of 131I treatment for hyperthyroidism on calcitonin secretion by thyroid C-cells.

DESIGN

Determination of basal calcitonin levels and calcitonin secretory reserve before and after 131I administration.

PATIENTS

Seventeen hyperthyroid patients (15 female, two male) were studied before, and 2 months after 131I treatment, and 12 of these patients were restudied 8 months after 131I treatment.

MEASUREMENTS

Calcitonin response was assessed by measuring basal and post calcium infusion calcitonin levels. Basal TSH, T3, and T4 levels were also determined at each study.

RESULTS

The rise of plasma calcium resulted in statistically significant increase of plasma calcitonin levels before 131I treatment (10.9 +/- 2.4 pmol/l), while this response was significantly diminished 2 and 8 months after treatment (2.6 +/- 0.7 and 1.6 +/- 0.3 pmol/l, respectively). No correlation was found between the calcitonin response and age or plasma TSH.

CONCLUSION

Our results demonstrate that 131I treatment for hyperthyroidism may seriously damage thyroid C-cells and cause calcitonin deficiency.

Calcitonin metabolism in senile (type II) osteoporosis

The exact role of calcitonin (CT) in the pathogenesis of senile (Type II) osteoporosis remains unknown. Whole plasma calcitonin (iCT) and extracted monomeric calcitonin (eCT) basal levels, metabolic clearance rate (MCR) and production rate (PR) of iCT and eCT were measured in 41 postmenopausal women, including 14 hip fractures (OP II) and 27 healthy controls. No significant difference appeared for basal iCT levels between OP II (mean +/- SEM: 41.9 +/- 3.4 pg/ml) and controls (mean +/- SEM: 46.2 +/- 5 pg/ml). eCT basal levels were similar in OP II (mean +/- SEM: 5.42 +/- 0.5 pg/ml) and in controls (mean +/- SEM: 7.3 +/- 0.7 pg/ml). MCR were similar in the two groups. iCT PR were similar in OP II (mean +/- SEM: 17.2 +/- 1.5 micrograms/24 h) and controls (mean +/- SEM: 18.6 +/- 1.1 micrograms/24 h). No difference appeared between eCT PR in OP II (mean +/- SEM: 2.3 +/- 0.2 micrograms/24 h) and controls (mean +/- SEM: 3.2 +/- 0.3 pg/ml). From these data, no evidence appears that calcitonin might be one of the determinant factors in the pathogenesis of senile osteoporosis.

Influence of estrogen replacement therapy on endogenous calcitonin production rates

Calcitonin is now a well-accepted therapy for inhibition of bone loss, both in the first years of menopause and in established osteoporosis. However, its exact role in the pathogenesis of that disease as well as the interactions between calcitonin production and estrogen metabolism remain unsolved. In order to clarify the influence of estrogen replacement therapy (ERT) on calcitonin secretory capacity, we measured whole plasma immunoreactive calcitonin basal levels, metabolic clearance rates and production rates in a group of postmenopausal women, before and after a daily intake for 28 days of 0.625 mg/day of conjugated equine estrogens, and again 4 weeks after the withdrawal of that estrogen replacement therapy. No significant changes appeared in immunoreactive calcitonin or immunoreactive calcitonin metabolic clearance rate but the production rate significantly increased over the 28 days (mean +/- SEM, from 21.3 +/- 5.1 pg/ml to 25.2 +/- 5.9 pg/ml, p less than 0.05), and then decreased 4 weeks after therapy was withdrawn to the initial level (17.9 +/- 3.6 pg/ml). We concluded that estrogen replacement therapy significantly increases calcitonin secretory capacity. This confirms the interactions between calcitonin production and estrogen metabolism, and may provide an explanation concerning the mode of action of estrogen replacement therapy in prevention of postmenopausal bone loss.

The ovariectomized rat model of postmenopausal bone loss

An animal model of postmenopausal bone loss can be defined as a living animal in which spontaneous or induced bone loss due to ovarian hormone deficiency can be studied, and in which the characteristics of the bone loss and its sequalae resemble those found in postmenopausal women in one or more respects. Although in comparison to humans, the skeletal mass of rats remains stable for a protracted period during their lifespan, rats can be ovariectomized to make them sex-hormone deficient, and to stimulate the accelerated loss of bone that occurs in women following menopause. Ovariectomy induced bone loss in the rat and postmenopausal bone loss share many similar characteristics. These include: increased rate of bone turnover with resorption exceeding formation; and initial rapid phase of bone loss followed by a much slower phase; greater loss of cancellous than cortical bone; decreased intestinal absorption of calcium; some protection against bone loss by obesity; and similar skeletal response to therapy with estrogen, tamoxifen, bisphosphonates, parathyroid hormone, calcitonin and exercise. These wide-ranging similarities are strong evidence that the ovariectomized rat bone loss model is suitable for studying problems that are relevant to postmenopausal bone loss.

Effect of calcitonin deficiency on bone density and bone turnover in totally thyroidectomized patients

To investigate the influence of calcitonin deficiency on bone turnover and density we studied 25 premenopausal female and 12 male patients (age 23 to 49 years) who had undergone total thyroidectomy for differentiated thyroid cancer 1 to 15 years previously. Basal and calcium stimulated extractable calcitonin, representing the monomeric, biologically active form of the hormone, was lacking or markedly decreased in all patients. There was a relative increase of urine hydroxyproline excretion (an index of osteoclastic bone degradation) in relation to serum osteocalcin (an index of osteoblastic bone formation) indicating an imbalance of bone turnover with a tendency to increased degradation in all patients. Total and trabecular bone density, measured with quantitative computed tomography at the distal forearm were significantly decreased in the male and normal in the female patients, without a relation to the duration of the calcitonin deficiency. The study indicates that patients with calcitonin deficiency, suppressive thyroid hormone treatment, or both may have a higher risk of increased bone degradation and osteopenia. Whether the effect is more due to calcitonin deficiency or thyroid hormone therapy, cannot be concluded from this study design. The fact that only the male patients had a decreased bone density may be due to a lower parathyroid activity in our female patients and the greater thyroidectomy-induced decrement of monomeric calcitonin in our male patients compared with male controls.

Intranasal salmon calcitonin. A review of its pharmacological properties and potential utility in metabolic bone disorders associated with aging

Salmon calcitonin, a polypeptide hormone secreted by the parafollicular C cells of the thyroid gland, lowers serum calcium levels by decreasing bone resorption and renal tubular calcium reabsorption. An analgesic action, possibly mediated via beta-endorphins, is also evident. In the past, parenteral formulations of salmon calcitonin have been used in the management of metabolic bone disorders, but their routine use has been limited by the inconvenience of this route of administration and by poor tolerability. The development of an intranasal preparation of salmon calcitonin will provide a more convenient means of administering the drug. In clinical trials published to date intranasal salmon calcitonin has been effective and well tolerated in small numbers of recently postmenopausal women at risk of developing osteoporosis, and in patients with established osteoporosis, Paget's disease, or osteoporosis secondary to corticosteroid usage, multiple myeloma or ovariectomy. For periods of up to 2 years the drug reduces bone resorption and improves bone architecture, relieves pain and increases functional status. Further research is needed to confirm longer term efficacy (in particular, effects on fracture rate), optimal dosage schedules and the role of intermittent and combination treatment regimens.

Pathogenesis of osteoporosis

The final clinical outcome of the osteoporotic process is a fracture, which can occur as a result of minimal trauma or even spontaneously. At present low bone mass is regarded as the main contributor to bone fragility, but possible qualitative changes in the bone matrix must also be considered. Two factors which determine the level of bone mass at any age are the obtained peak bone mass and duration and rate of bone loss. Peak bone mass is achieved during the first three decades of life. Genetic and nutritional factors as well as mechanical stress on the skeleton obviously play crucial roles in determining peak bone mass. Two phases of bone loss--age-related and menopause-related--dictate the final bone mass at old age. Postmenopausal osteoporosis is a particular example of unbalanced bone resorption leading to net bone loss. An increasing number of systemic and local factors have been found to participate in the regulation of bone remodeling.

Role of oestrogen in the development of osteoporosis

The evidence now strongly supports a central role for oestrogen in the maintenance of the normal female skeleton. Loss of endogenous oestrogen leads to a period of accelerated bone loss which can be prevented with appropriate oestrogen replacement. The route of administration is not important provided adequate dosages are prescribed. Oestrogens may exert their actions by a direct steroid receptor mechanism or via local and/or systemic factors such as cytokines, prostaglandins or calcitonin. As the requirement for osteoporosis prevention grows, it is essential that the extent of the involvement oestrogen has in skeletal homeostasis is fully recognized.

Postprandial response of calcium metabolism in postmenopausal women to meals varying in protein level/source

The postprandial response of calcium metabolism to single meals varying in protein source and level was determined in eight postmenopausal women, age 51 to 65 years. Following an overnight fast, subjects consumed a liquid meal containing either 15 g or 45 g of protein from beef (B), cottage cheese (C), or soy isolate (S; 45 g only); one meal provided 0 g protein (basal). Blood was collected before the meal and at 30, 60, 120, and 180 minutes post-meal. Urine was collected every 30 minutes post-meal. Urinary Ca excretion significantly increased following 45-g protein meals when compared with basal meals, but not when compared with 15 g protein meals. Although glomerular filtration rate (GFR) was unaffected by diet, percent renal tubular Ca reabsorption was significantly reduced following C45 or S45 meals. No consistent changes in serum levels of parathyroid hormone (PTH) and calcitonin (Ct) were observed in response to diet. Serum phosphorus levels were significantly reduced following high-protein meals when compared with no-protein meals. Insulin response varied with protein level and source (C45 greater than S45 greater than C15 greater than B45 greater than B15 greater than basal). Significant associations found between insulin and calcium metabolism indicate a possible role of insulin in the mechanism of protein-induced calciuria.

Calcitonin, estrogens and the bone

Estrogen deficiency following natural or surgical menopause, is thought to be the main factor leading to postmenopausal bone loss. Furthermore, after estrogen failure a significant reduction of intestinal calcium absorption and a negativization of calcium balance has been observed. The mechanism of estrogen effect on skeletal tissue is not yet fully elucidated. Recently, specific receptors for estrogens in osteoblastic cells have been described; however their low density does not give a full explanation about their functional role. Therefore estrogens act, at least in part, indirectly through calciotropic hormones. In order to further elucidate this issue, we performed some studies in postmenopausal osteoporotic patients and in fertile oophorectomized women. In the first double blind placebo controlled study, after a 1-year estrogen treatment period we observed an increase in bone mineral content in the hormone-treated patients. Furthermore, in all treated patients an improvement of intestinal calcium absorption was detected, while 1,25-dihydroxy-vitamin D serum levels did not show significant changes. To further analyse the relationship between estrogens (E) and calcitonin (CT) in postmenopausal osteoporosis, we performed a double blind placebo controlled study to evaluate the effects of 1-yr estro-progestative treatment on CT secretory reserve, evaluated by calcium infusion test. Blood levels of CT showed a progressive increase during the study period in the hormone-treated group, with a significant increase in the CT response to calcium stimulation test, suggesting a modulation of CT secretion by E. Recently, we performed two studies in fertile oophorectomized women. In the first, we followed longitudinally 24 fertile women for 1 yr. In these patients we measured, before and after oophorectomy, biochemical indexes of bone metabolism and bone mass. During the observation period a significant increase in bone resorption and a significant drop in intestinal calcium absorption was observed. In the second study, performed on 14 women before and 6 months after oophorectomy, a treatment with conjugated estrogens allowed the correction of the primary intestinal defect responsible for the reduced calcium absorption.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcitonin and estrogens

Estrogen deficiency is thought to be the main factor leading to postmenopausal osteoporosis (PMO). A role for calcitonin (CT) has been proposed as mediator of estrogen action on bone, and therefore, as pathogenetic factor of PMO. However, this hypothesis is still controversial. To further analyze the relationships between estrogens and CT in PMO, we studied the effects of one-year estro/progesterone therapy on CT secretory reserve, evaluated by a calcium infusion test in 12 postmenopausal women, as compared to 12 placebo treated subjects. In the hormone treated group, blood levels of CT showed a progressive increase during the study and a plateau was reached at 9 months, indicating that CT production achieved a new steady state. Hormonal therapy also significantly improved the CT response to calcium stimulation test. A concomitant increase of vertebral bone mass was observed in the hormone treated women, who also maintained initial bone density of femoral dyaphyses. On the contrary, the placebo treated group continued to lose bone mineral at both sites. Our study demonstrates that estrogens regulate CT secretion in postmenopausal women; thus, CT may be considered a mediator of estrogen action on bone.

Clinically useful calcitonin assays

Calcitonin assays are essential tools for research into calcitonin (CT) and C-cell physiology and pathophysiology. Several existing radio immunoassays for CT, based on polyclonal antisera, are generally reliable for detection and follow-up of patients having medullary thyroid carcinoma. Occasionally, however, these assays suffer from problems of specificity and sensitivity. Two-site immunometric techniques for CT offer potential for great improvement in sensitivity, specificity, and turnaround time over those based on classical competitive-binding radioimmunoassays.

Calcitonin secretion rate in elderly normal subjects

The aim of the study was to investigate the metabolic clearance rate (MCR) and the secretion rate (SR) of calcitonin (CT) in elderly men and women (mean age +/- SE: 75 +/- 3 and 78 +/- 3 yr, respectively). The basal levels of CT were higher in men than in women (9.6 +/- 3.2 vs 6.0 +/- 1.5 pg/ml; p less than 0.05). The MCR was not significantly different between sexes (905 +/- 54.2 vs 810 +/- 165.1 l/day). In contrast, CT SR was significantly higher in male subjects compared to women (8.7 +/- 3.1 vs 4.8 +/- 1.5 micrograms/day; p less than 0.05). These findings suggest that only a few units of CT are secreted daily by elderly subjects and that the lower basal values of the hormone in women are probably the result of a lower production rate.

The pathogenesis of osteoporosis

Changes in the calciotropic hormones with age contribute significantly to the pathogenesis of osteoporosis. In both postmenopausal (Type I) and senile osteoporosis (Type II) it is common to find reduced levels of serum 1,25-dihydroxyvitamin D and malabsorption of calcium. In Type I patients a reduced level of serum parathyroid hormone causes a real decrease in serum 1,25-dihydroxyvitamin D production and malabsorption of calcium, whereas in Type II patients the decline in 1 alpha-hydroxylase activity in the kidney causes a decline in serum 1,25-dihydroxyvitamin D which leads to malabsorption of calcium and secondary hyperparathyroidism. In the final analysis both pathways lead to bone loss. In some Type II patients there may be a decline also in the function or number of the vitamin D-binding receptors in the gut. Treatment of patients with vitamin D analogues, however, normalizes calcium absorption and improves calcium balance. The improvement in calcium balance reduces bone resorption and prevents further bone loss; in addition recent studies have shown that therapy with vitamin D analogues leads to a reduction in fracture incidence.

Estrogen in prevention and treatment of osteoporosis

Estrogen has clearly been shown to decrease bone loss and frequency of osteoporotic fractures. Calcitonin has been shown in several studies to reduce the rate of bone loss, although no data are yet available demonstrating a reduction in fracture frequency. Studies of osteoporosis intrinsically assume that prevention of further loss, or increments in bone mass, will be associated with declines in fracture recurrence. That may not always be the case. Recent controlled studies in which fluoride was used to increase bone mass in vertebral bodies resulted in no significant decline in fracture recurrence, and there was even a suggestion of increased fracture risk at some sites. Thus further data on calcitonin will be important, even though calcitonin is not expected to alter bone quality as does fluoride. Alternative therapies to calcitonin and estrogen are being investigated in clinical studies since both are currently limited in their use. Because calcitonin currently requires nearly daily injections, estrogen remains the principal agent available for both prevention and treatment in spite of its wide effect on multiple body systems. Bisphosphonates, given continuously or intermittently, appear to be relatively safe oral alternatives to calcitonin. The long-term effects of these agents need to be evaluated in greater detail before they can be recommended for prevention, but a role in therapy of the established disorder seems likely. The skeletal effect of bisphosphonates is also inhibition of bone remodeling and therefore prevention of further bone loss. Thus, they add nothing to the other therapeutic regimens from this perspective and as with calcitonin therapy, documentation of decreased frequency of fracture is lacking. Agents to increase bone mass are purely investigational at this time and many years may elapse before efficacy can be shown for such interventions.

Calcitonin, estradiol, and hydroxyproline as parameters in the early diagnosis of involutional osteoporosis. The importance of the "second calcitonin phenomenon"

Bone metabolism was studied in a group of 92 subjects. A greater age-related decrease in calcitonin and estradiol concentrations exists in women than in men, though this difference was not significant; it was significant, however, when the values of the three different groups of women were compared. We present what we have called the second calcitonin phenomenon, that is, a highly significant difference in women between a second basal calcitonin level and the primary admission value. In the calcium infusion test, men considered to be osteoporotic showed a deceleration in the rapid loss of reserve calcitonin deposits.

Calcitonin and lumbar bone mineral content during oestrogen-progestogen administration in post-menopausal women

It now appears to be accepted that oestrogens and progestogens can help to prevent post-menopausal bone loss. This study accordingly evaluated vertebral bone mineral content (BMC) patterns and changes in calcitonin (CT) secretion in 12 women who had been ovariectomized in the previous 6 mth and in 12 others who had had a natural menopause, all of whom received oestrogen-progestogen replacement therapy for 12 mth. We also studied 12 oophorectomized and 21 normal-menopause women who did not receive any treatment and hence constituted the corresponding control groups. A significant difference was found between the lumbar BMC in the treated women and the controls. Moreover, the CT levels rose significantly after replacement therapy in both the oophorectomized and the natural-menopause subjects. It was concluded that combined oestrogen-progestogen treatment can prevent post-menopausal bone loss and increase CT secretion.

Calcitonin reserve in healthy women and patients with postmenopausal osteoporosis

A deficit of immunoreactive calcitonin (iCT) has been found in women with postmenopausal osteoporosis (PM-OP), however, recent studies assessing the monomeric fraction of calcitonin (exCT) do not seem to confirm these findings. We have measured serum levels of iCT by radioimmunoassay (RIA) and exCT (chromatography and RIA) at 0, 5, 10, and 20 minutes after the i.v. infusion of 2 mg calcium/kg body weight in four different groups of women: (1) 12 healthy premenopausal women (HPM), (2) 16 early postmenopausal women (EPM), (3) 16 postmenopausal women within more than 2 years of menopause (LPM), and (4) 24 women with PM-OP. In the HPM group, iCT levels increased significantly 5 and 10 minutes after finishing the calcium infusion (P less than 0.05); this did not occur in the other three groups. The exCT levels in the HPM and LPM groups showed a significantly greater increase than in the EPM and PM-OP groups at 5 minutes (P less than 0.05) and at 10 and 20 minutes (P less than 0.01) after infusion. The behavior of the PM-OP and EPM groups was similar throughout the study. We conclude that there is a calcitonin reserve deficiency in the first years after menopause, which recovers later. This hormone deficiency could explain the accelerated bone loss that takes place at this time of life. The patients with PM-OP also show this deficit, and this may play an ethiopathogenic role in the production of the disease.

Osteoporosis. Overcoming a costly and debilitating disease

As the aging population continues to increase, so too will the problem of osteoporosis. Because the first manifestation of the disease may appear at a stage when prevention is no longer possible, early education of both men and women regarding causes and treatment is desirable. Estrogen replacement is the most valuable tool in treating women and should be started within the first 3 years of menopause. Calcium alone or in combination with vitamin D is a safe preventive measure for women over age 40 and men over age 50.

Ovarian steroid treatment blocks a postmenopausal increase in blood monocyte interleukin 1 release

In previous studies, we showed that blood monocyte elaboration of interleukin 1 (IL-1), a known stimulator of bone resorption, was higher in osteoporotic patients with rapid bone turnover than in those with slow turnover and in nonosteoporotic subjects. Since an acceleration of bone loss following menopause contributes to the risk of osteoporosis in women, we have studied the effects of menopause and ovarian steroid treatment on IL-1 release by monocytes obtained from nonosteoporotic and osteoporotic women. IL-1 activity in the monocyte culture medium derived from untreated postmenopausal women (nonosteoporotic and osteoporotic) was higher than in the medium derived from either untreated premenopausal or estrogen/progesterone-treated postmenopausal women. A significant negative correlation was found between IL-1 and years since menopause in both the healthy (r = -0.75; P less than 0.005) and the osteoporotic (r = -0.61; P less than 0.01) untreated postmenopausal women. The difference between the two slopes was significant at P less than 0.05. Premenopausal IL-1 levels were achieved within 8 years of menopause in the nonosteoporotic, but not in the osteoporotic, subjects in whom increases were evident as long as 15 years after menopause. IL-1 also correlated inversely with vertebral mineral density (r = -0.37; P less than 0.05), as measured by quantitative computed tomography. In prospective studies, treatment with estrogen/progesterone for 1 month caused a substantial highly significant decrease in IL-1 activity in each of three nonosteoporotic and five osteoporotic women, confirming the apparent effect of hormone therapy observed in the cross-sectional analysis. Although a cause-effect relationship has not been established, it is our hypothesis, based on these data, that alterations in IL-1 production may underlie the postmenopausal acceleration in bone loss and its inhibition by ovarian steroids. Persistent elevation of IL-1 secretion appears to be a feature of postmenopausal osteoporosis.