Radiocalcium metabolism in disuse osteoporosis in man

The Pathophysiology, Identification and Management of Fracture Risk, Sublesional Osteoporosis and Fracture among Adults with Spinal Cord Injury

BACKGROUND

The prevention of lower extremity fractures and fracture-related morbidity and mortality is a critical component of health services for adults living with chronic spinal cord injury (SCI).

METHODS

Established best practices and guideline recommendations are articulated in recent international consensus documents from the International Society of Clinical Densitometry, the Paralyzed Veterans of America Consortium for Spinal Cord Medicine and the Orthopedic Trauma Association.

RESULTS

This review is a synthesis of the aforementioned consensus documents, which highlight the pathophysiology of lower extremity bone mineral density (BMD) decline after acute SCI. The role and actions treating clinicians should take to screen, diagnose and initiate the appropriate treatment of established low bone mass/osteoporosis of the hip, distal femur or proximal tibia regions associated with moderate or high fracture risk or diagnose and manage a lower extremity fracture among adults with chronic SCI are articulated. Guidance regarding the prescription of dietary calcium, vitamin D supplements, rehabilitation interventions (passive standing, functional electrical stimulation (FES) or neuromuscular electrical stimulation (NMES)) to modify bone mass and/or anti-resorptive drug therapy (Alendronate, Denosumab, or Zoledronic Acid) is provided. In the event of lower extremity fracture, the need for timely orthopedic consultation for fracture diagnosis and interprofessional care following definitive fracture management to prevent health complications (venous thromboembolism, pressure injury, and autonomic dysreflexia) and rehabilitation interventions to return the individual to his/her pre-fracture functional abilities is emphasized.

CONCLUSIONS

Interprofessional care teams should use recent consensus publications to drive sustained practice change to mitigate fracture incidence and fracture-related morbidity and mortality among adults with chronic SCI.

Selected discoveries from human research in space that are relevant to human health on Earth

A substantial amount of life-sciences research has been performed in space since the beginning of human spaceflight. Investigations into bone loss, for example, are well known; other areas, such as neurovestibular function, were expected to be problematic even before humans ventured into space. Much of this research has been applied research, with a primary goal of maintaining the health and performance of astronauts in space, as opposed to research to obtain fundamental understanding or to translate to medical care on Earth. Some people—scientists and concerned citizens—have questioned the broader scientific value of this research, with the claim that the only reason to perform human research in space is to keep humans healthy in space. Here, we present examples that demonstrate that, although this research was focused on applied goals for spaceflight participants, the results of these studies are of fundamental scientific and biomedical importance. We will focus on results from bone physiology, cardiovascular and pulmonary systems, and neurovestibular studies. In these cases, findings from spaceflight research have provided a foundation for enhancing healthcare terrestrially and have increased our knowledge of basic physiological processes.

Metabolic and calcium kinetic studies in idiopathic hypercalciuria

Calcium balances and calcium kinetic studies using (47)Ca were performed in nine male patients with idiopathic hypercalciuria and in three normal male subjects. A sharp reduction in calcium intake in eight patients with idiopathic hypercalciuria caused a decrease in urinary calcium excretion, the latter remaining elevated above that reported for normal subjects on a low calcium diet. The hypercalciuric patients had an enlarged miscible calcium pool size, an increased calcium turnover rate, increased bone formation and bone resorption rates, and an elevated true intestinal calcium absorption rate, the increase of the latter three parameters being proportional to the increase of the turnover rate. The fraction of the calcium turnover rate excreted in the urine was elevated whereas that constituted by the endogenous fecal calcium excretion was decreased. Arguments are presented for the concept that the primary abnormality in idiopathic hypercalciuria is neither renal calcium hyperexcretion nor intestinal calcium hyperreabsorption, but a more fundamental disturbance in calcium metabolism of as yet unknown cause, leading to a high calcium turnover.

Bony pathology in the cancer patient

Bony pathology in the cancer patient represents a significant source of morbidity and mortality. Complications include insufficiency and pathological fractures resulting from either medical treatments or bony metastases that can cause significant functional limitations. Additional complications include spinal cord compression, hypercalcemia, and bone marrow failure. Rehabilitation management of such conditions is reviewed, with an emphasis on diagnostic and therapeutic management. Bracing and focused rehabilitation programs facilitate maximal participation and functional outcomes, which can result in an enhanced quality of life. Specific rehabilitation goals and strategies are discussed, with an emphasis on tailoring these according to the functional staging of the patient.

Clinical evaluation of the effects of immobilization followed by remobilization and exercise on the metacarpophalangeal joint in horses

OBJECTIVES

To evaluate clinical effects of immobilization followed by remobilization and exercise on the metacarpophalangeal joint (MPJ) in horses.

ANIMALS

5 healthy horses.

PROCEDURE

After lameness, radiographic, and force plate examinations to determine musculoskeletal health, 1 forelimb of each horse was immobilized in a fiberglass cast for 7 weeks, followed by cast removal and increasing amounts of exercise, beginning with hand-walking and ending with treadmill exercise. Lameness examination, arthrocentesis of both MPJ, single-emulsion radiographic examination, nuclear scintigraphic examination, ground-reaction force-plate analysis, and computed tomographic examination were done at various times during the study.

RESULTS

All horses were lame in the immobilized MPJ after cast removal; lameness improved slightly with exercise. Force plate analysis revealed a significant difference in peak forces between immobilized and contralateral limbs 2 weeks after cast removal. Range of motion of the immobilized MPJ was significantly decreased, and joint circumference was significantly increased, compared with baseline values, during the exercise period. Osteopenia was subjectively detected in the immobilized limbs. Significant increase in the uptake of radionucleotide within bones of the immobilized MPJ after cast removal and at the end of the study were detected. Loss of mineral opacity, increased vascular channels in the subchondral bone, and thickening within the soft tissues of the immobilized MPJ were detected.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicate that 8 weeks of enforced exercise after 7 weeks of joint immobilization did not restore joint function or values for various joint measurements determined prior to immobilization.

Increased bone resorption during the first year after stroke

BACKGROUND AND PURPOSE

Significant bone mineral density (BMD) reduction occurs in stroke patients on the hemiplegic side compared with the intact side. To elucidate the pathogenesis of hip fractures in this population, we measured serum markers of bone metabolism and BMD in the stroke patients within 1 year (early group) and between 1 and 2 years after onset of hemiplegia (long-term group).

METHODS

Sera were collected from 51 patients from the early group and 93 patients from the long-term group. All patients had hemiplegia. Sera were assayed for pyridinoline cross-linked carboxy-terminal telopeptide of type I collagen (ICTP; a bone resorption marker) and bone Gla protein (a bone formation marker). The z score of BMD was determined in both second metacarpals.

RESULTS

Serum ICTP concentrations (ng/mL) were higher in the early group (15.4+/-4.1) than in the long-term group (6.7+/-4.4). Bone Gla protein was normal or low in both groups. Multiple regression analysis identified Barthel Index, degree of hemiplegia, and illness duration as independent determinants of ICTP in the early group, whereas Barthel Index, degree of hemiplegia, and serum calcium were determinants of ICTP in the long-term group. There were statistically significant correlations between the z score of the hemiplegic side and age, Barthel Index, degree of hemiplegia, illness duration, 25-hydroxyvitamin D (25-OHD), and ICTP in the early group and between the z score and degree of hemiplegia and 25-OHD level in the long-term group.

CONCLUSIONS

The pathogenesis of reduced BMD differed between the early and long-term stroke groups. These results suggest that in the early group, increased bone resorption caused by immobilization was responsible for osteopenia on the hemiplegic side, whereas the degree of hemiplegia and 25-OHD level were the determinants of osteopenia in the long-term group.

Zinc stimulates protein synthesis in the femoral-metaphyseal tissues of normal and skeletally unloaded rats

The effect of zinc on protein synthesis in the femoral-metaphyseal tissues of normal and skeletally unloaded rats was investigated. Skeletal unloading was designed using the model of hindlimb suspension in rats. Animals were fed for 2 or 4 days during the unloading. [3H]Leucine was added to the reaction mixture containing the 5500 g supernatant fraction of the homogenate prepared from the femoral-metaphyseal tissues. In vitro protein synthesis was significantly decreased in the bone tissues from the rats which had undergone unloading for 2 or 4 days. When the metaphyseal tissues were cultured for 24 h in the presence of zinc sulfate (10(-5) M) or beta-alanyl-L-histidinato zinc (AHZ, 10(-5) M), zinc compounds clearly stimulated protein synthesis in the metaphyseal tissues from the 4-day unloaded rats. The zinc effect was also seen in the metaphyseal tissues from normal rats. The addition of zinc sulfate (10(-5) M) or AHZ (10(-7) to 10(-5) M) into the reaction mixture containing the 5500 g supernatant fraction of metaphyseal homogenate from normal or unloaded rats produced a significant increase in protein synthesis. This increase was clearly inhibited in the presence of cycloheximide (10(-7) M). The present result demonstrates that protein synthesis is impaired in the femoral-metaphyseal tissues of rats with skeletal unloading, and that this impairment is clearly restored by zinc supplementation.

Histomorphological confirmation of bone loss in the femoral-metaphyseal tissues of rats with skeletal unloading

The alteration of mineral content in the femoral metaphysis of rats with skeletal unloading was investigated using a model of hindlimb suspension. Animals were fed for 4 days during the unloading. The femoral length, femoral dry weight and femoral mineral density were significantly decreased by the unloading. The calcium, phosphorus and zinc contents in the femoral metaphysis were appreciably reduced by the unloading, although, except for zinc, similar decreases were not seen in the femoral diaphysis. Moreover, the trabecular bone at the femoral metaphysis was clearly reduced by the unloading. Skeletal unloading caused a decrease in osteoid tissue in the primary and secondary spongiosa, indicating that osteoblastic bone formation may be inhibited. The present results clearly demonstrate that skeletal unloading can induce bone loss in the femoral metaphysis.

CALCIUM ABSORPTION IN MAN: BASED ON LARGE VOLUME LIQUID SCINTILLATION COUNTER STUDIES

A technique has been developed for the in vivo measurement of absorption of calcium in man after oral administration of 1 to 5 microcuries of calcium-47 and continuous counting of the radiation in the subject's arm with a large volume liquid scintillation counter. The maximum value for the arm counting technique is proportional to the absorption of tracer as measured by direct stool analysis. The rate of uptake by the arm is lower in subjects with either the malabsorption syndrome or hypoparathyroidism. The administration of vitamin D increases both the absorption rate and the maximum amount of calcium absorbed.

Effect of essential trace metal on bone metabolism in the femoral-metaphyseal tissues of rats with skeletal unloading: comparison with zinc-chelating dipeptide

The effect of essential trace metals on bone metabolism was investigated in the femoral-metaphyseal tissues obtained from skeletal-unloaded rats. Skeletal unloading was designed by using the model of hindlimb suspension in rats; the animals were fed for 4 days with the unloading. Femoral-metaphyseal tissues were cultured for 24 hours in a medium containing either vehicle (control), nickel, manganese, cobalt, copper, zinc, or zinc-chelating dipeptide (beta-alanyl-L-histidinato zinc; AHZ) in the concentration range of 10(-6) to 10(-4) M. Bone biochemical components (alkaline phosphatase activity, glucose consumption, and DNA content) were significantly decreased by skeletal unloading. The presence of zinc sulfate or AHZ (10(-5) and 10(-4) M) caused a significant increase of alkaline phosphatase activity in the bone tissues from unloaded rats. This effect was not seen by nickel, manganese, cobalt and copper (10(-6) to 10(-4) M). The culture medium glucose was clearly consumed by the bone tissues. This consumption was inhibited by nickel, manganese, or copper (10(-5) and 10(-4) M), while cobalt, zinc, and AHZ had no effect. DNA content in the bone tissues from unloaded rats was significantly increased by all metal compounds (10(-5) M). The effect of AHZ on bone components was greater than zinc sulfate. The AHZ (10(-5) M)-increased alkaline phosphatase activity in the bone tissues from unloaded rats was clearly blocked by the presence of cycloheximide (10(-6) M), staurosporine (10(-7) M), dibucaine (10(-4) M), or okadaic acid (10(-7) M). The present study demonstrates that, of various essential trace metals, zinc compounds have an unique anabolic effect on bone metabolism in the femoral-metaphyseal tissues of rats with skeletal unloading. Zinc-chelating dipeptide may stimulate bone protein synthesis through the mechanism that is involved in protein kinases.

Skeletal response to immobilization in Paget's disease of bone: a case report

The authors discuss a case of Paget's disease of the forearm in which the Pagetic radius and non-Pagetic ulna responded differently to immobilization. The high turnover state of Pagetic bone is more sensitive to physical unloading than normal bone, and thus more rapidly develops osteopenia of immobilization. The different responses to immobilization between Pagetic bone and site controlled normal bone illustrate how physical and metabolic factors interact at a skeletal site to regulate bone remodeling and bone mass.

Vitamin D deficiency in veterans with chronic spinal cord injury

Chronic spinal cord injury (SCI) is associated with osteopenia, increasing the prevalence of long-bone fractures. Although disuse may be the primary cause of osteopenia, identification of any additional mechanisms of bone loss may lead to potential therapeutic interventions. We investigated the relationships of serum calcium (Ca), phosphorus (PO4), albumin, alkaline phosphatase (Alk P), and parathyroid hormone (PTH) with serum 25-hydroxyvitamin D [25(OH)D] in 100 subjects with chronic SCI and 50 control subjects. in a subgroup of 50 subjects with SCI and 50 control subjects, we correlated these parameters with serum 1,25-dihydroxyvitamin D [1,25(OH)2D]. Mean ages for the group with SCI and the controls were the same. In subjects with SCI, the duration of injury was 20 +/- 1 years (mean +/- SD). Thirty-two of 100 subjects with SCI, as compared with eight of 50 controls, had serum 25(OH)D levels less than the normal range (chi2 = 4.36, P < .05). In subjects with SCI, a negative correlation was demonstrated between serum 25(OH)D and PTH (r = .29, P < .005). Mean serum 1.25(OH)2D levels were significantly elevated in subjects with SCI as compared with controls (61 +/- 21 v 46 +/- 18 pg/mL, P < .0005). Twenty of 50 subjects with SCI had serum 1.25(OH)2D levels greater than 62 pg/mL, as compared with 10 of 50 controls (chi2 = 4.76 P < .05). A positive correlation was found between serum PTH and 1,25(OH)2D in subjects with SCI and controls (r = .41, P < .005 and r = .30, P < .05, respectively). Twelve subjects with SCI had serum PTH levels greater than the normal range. In this high-serum PTH subgroup, serum 15(OH)D concentration was significantly lower (P < .05) and serum 1,25(OH)2D and Alk P concentrations were significantly higher (P < .005 and P < .05, respectively) as compared with the subgroup with serum PTH values within the normal range. In subjects with SCI, 17 had a serum Ca concentration less than 8.5 mg/dL. In persons with SCI, depressed levels of serum 25(OH)D, as well as other factors, may result in forces inclined to reduce the serum calcium concentration. A state of mild secondary hyperparathyroidism may result, thus increasing the conversion of serum 25(OH)D to 1.25(OH)2D. These data suggest that in chronic SCI subjects, as in the general population, secretion of PTH and the increase of circulating 1.25(OH)2D are subject to control by negative-feedback mechanisms. Higher levels of serum PTH would be expected to accelerate bone resorption of a skeleton already regionally osteoporotic as a consequence of the bone mineral loss due to acute immobilization.

Zinc decrease and bone metabolism in the femoral-metaphyseal tissues of rats with skeletal unloading

Whether the decrease of zinc content in the femoral-metaphyseal tissues of rats with skeletal unloading is involved in the alteration of bone metabolism was investigated. Skeletal unloading was designed using the model of hindlimb suspension in rats. Animals were fed for 4 days with the unloading. The metaphyseal zinc content were significantly decreased by the unloading. Zinc accumulation in the metaphyseal tissues by a single oral administration of zinc sulfate (20 mg Zn/100 g body weight) was partially depressed by the unloading, although serum zinc concentration was higher than that in normal rats, suggesting an impaired movement of zinc from serum into bone tissues by the unloading. Skeletal unloading caused a significant decrease of alkaline phosphatase activity and deoxyribonucleic acid (DNA) content in the metaphyseal tissues. These decreases were completely restored by addition of zinc sulfate (10(-4) M) or beta-alanyl-L-histidinato zinc (AHZ; 10(-5) M) in a culture medium with the metaphyseal tissues in vitro. The effects of zinc compounds were abolished by the presence of cycloheximide (10(-8) M), suggesting that the zinc effect is based on a newly synthesized protein. Dipicolinate (10(-4) and 10(-5) M), a potent zinc-chelating agent, caused an appreciable decrease of zinc content and alkaline phosphatase activity in the metaphyseal tissues. This decrease was restored by zinc supplement. The present results suggest that the skeletal unloading-induced decrease of zinc content in the femoral-metaphyseal tissues plays a role in the deterioration of bone metabolism in the unloaded rats.

Skeletal unloading induces selective resistance to the anabolic actions of growth hormone on bone

Loss of skeletal weight bearing or physical unloading of bone in the growing animal inhibits bone formation and induces a bone mineral deficit. To determine whether the inhibition of bone formation induced by skeletal unloading in the growing animal is a consequence of diminished sensitivity to growth hormone (GH) we studied the effects of skeletal unloading in young hypophysectomized rats treated with GH (0, 50, 500 micrograms/100 g body weight/day). Skeletal unloading reduced serum osteocalcin, impaired uptake of 3H-proline into bone, decreased proximal tibial mass, and diminished periosteal bone formation at the tibiofibular junction. When compared with animals receiving excipient alone, GH administration increased bone mass in all animals. The responses in serum osteocalcin, uptake of 3H-proline and 45Ca into the proximal tibia, and proximal tibial mass in non-weight bearing animals were equal to those in weight bearing animals. The responses in trabecular bone volume in the proximal tibia and bone formation at the tibiofibular junction to GH, however, were reduced significantly by skeletal unloading. Bone unloading prevented completely the increase in metaphyseal trabecular bone normally induced by GH and severely dampened the stimulatory effect (158% vs. 313%, p < 0.002) of GH on periosteal bone formation. These results suggest that while GH can stimulate the overall accumulation of bone mineral in both weight bearing and non-weight bearing animals, skeletal unloading selectively impairs the response of trabecular bone and periosteal bone formation to the anabolic actions of GH.

Differential effects of insulin and insulin-like growth factor-I in the femoral tissues of rats with skeletal unloading

The alteration of bone metabolism in the femur of rats with skeletal unloading for 4 days was investigated. Skeletal unloading was designed using the model of hindlimb hang in rats. Skeletal unloading caused a significant decrease in femoral weight, calcium, and phosphorus contents in the metaphysis but not diaphysis. Also, the unloading induced a significant decrease of zinc content, alkaline phosphatase activity, and deoxyribonucleic acid (DNA) content in the femoral diaphysis and metaphysis. When the femoral-diaphyseal and metaphyseal tissues from normal and skeletal-unloading rats were cultured in the presence of insulin (10(-9) and 10(-8) M) for 24 hours in vitro, the hormonal effect to increase alkaline phosphatase activity and DNA content in the diaphysis, but not metaphysis, was lost in the bone tissues from unloading rats. However, the culture with insulin-like growth factor-I (IGF-I; 10(-8) and 10(-7) M) produced a significant increase of alkaline phosphatase activity and DNA content in both the diaphyseal and metaphyseal tissues from normal and unloading rats. These results demonstrate that skeletal unloading causes an impairment of insulin effect, but not IGF-I effect, on bone metabolism in femoral tissues.

Skeletal unloading induces resistance to insulin-like growth factor I

In previous studies with a hindlimb elevation model, we demonstrated that skeletal unloading transiently inhibits bone formation. This effect is limited to the unloaded bones (the normally loaded humerus does not cease growing), suggesting that local factors are of prime importance. IGF-I is one such factor; it is produced in bone and stimulates bone formation. To determine the impact of skeletal unloading on IGF-I production and function, we assessed the mRNA levels of IGF-I and its receptor (IGF-IR) in the proximal tibia and distal femur of growing rats during 2 weeks of hindlimb elevation. The mRNA levels for IGF-I and IGF-IR rose during hindlimb elevation, returning toward control values during recovery. This was accompanied by a 77% increase in IGF-I levels in the bone, peaking at day 10 of unloading. Changes in IGF binding protein levels were not observed. Infusion of IGF-I (200 micrograms/day) during 1 week of hindlimb elevation doubled the increase in bone mass of the control animals but failed to reverse the cessation of bone growth in the hindlimb-elevated animals. We conclude that skeletal unloading induces resistance to IGF-I, which may result secondarily in increased local production of IGF-I.

Impact of skeletal unloading on bone formation: role of systemic and local factors

We have developed a model of skeletal unloading using growing rats whose hindlimbs are unweighted by tail suspension. The bones in the hindlimbs undergo a transient cessation of bone growth; when reloaded bone formation is accelerated until bone mass is restored. These changes do not occur in the normally loaded bones of the forelimbs. Associated with the fall in bone formation is a fall in 1,25(OH)2D3 production and osteocalcin levels. In contrast, no changes in parathyroid hormone, calcium, or corticosterone levels are seen. To examine the role of locally produced growth factors, we have measured the mRNA and protein levels of insulin like growth factor-1 (IGF-1) in bone during tail suspension. Surprisingly, both the mRNA and protein levels of IGF-1 increase during tail suspension as bone formation is reduced. Furthermore, the bones in the hindlimbs of the suspended animals develop a resistance to the growth promoting effects of both growth hormone and IGF-1 when given parenterally. Thus, the cessation of bone growth with skeletal unloading is apparently associated with a resistance to rather than failure to produce local growth factors. The cause of this resistance remains under active investigation.

Changes in markers of bone formation and resorption in a bed rest model of weightlessness

To study the mechanism of bone loss in physical unloading, we examined indices of bone formation and bone resorption in the serum and urine of eight healthy men during a 7 day -6 degrees head-down tilt bed rest. Prompt increases in markers of resorption--pyridinoline (PD), deoxypyridinoline (DPD), and hydroxyproline (Hyp)/g creatinine--during the first few days of inactivity were paralleled by tartrate-resistant acid phosphatase (TRAP) with significant increases in all these markers by day 4 of bed rest. An index of formation, skeletal alkaline phosphatase (SALP), did not change during bed rest and showed a moderate 15% increase 1 week after reambulation. In contrast to SALP, serum osteocalcin (OC) began increasing the day preceding the increase in Hyp, remained elevated for the duration of the bed rest, and returned to pre-bed rest values within 5 days of reambulation. Similarly, DPD increased significantly at the onset of bed rest, remained elevated for the duration of bed rest, and returned to pre-bed rest levels upon reambulation. On the other hand, the other three indices of resorption, Hyp, PD, and TRAP, remained elevated for 2 weeks after reambulation. The most sensitive indices of the levels of physical activity proved to be the noncollagenous protein, OC, and the collagen crosslinker, DPD. The bed rest values of both these markers were significantly elevated compared to both the pre-bed rest values and the post-bed rest values. The sequence of changes in the circulating markers of bone metabolism indicated that increases in serum OC are the earliest responses of bone to head-down tilt bed rest.

Skeletal unloading decreases production of 1,25-dihydroxyvitamin D

The plasma concentration of 1,25-dihydroxyvitamin D [1,25(OH)2D] decreases during skeletal unloading and increases when normal weight bearing is restored. To determine whether these changes in plasma 1,25(OH)2D reflect changes in production, metabolic clearance, or both we measured the kinetics of 1,25(OH)2D metabolism in rats whose skeletons were normally loaded, unloaded, or reloaded after a period of nonweight bearing. Skeletal unloading produced a transient but striking fall in the production (-73%) and plasma concentration (-72%) of 1,25(OH)2D without having a significant effect (< 20%) on metabolic clearance. Skeletal reloading returned production to normal. Bone formation predictably decreased during unloading and returned to normal after return to weight bearing. No consistent changes in blood ionized calcium, plasma immunoreactive parathyroid hormone (irPTH), or plasma phosphorus occurred. These data suggest that the changes in plasma 1,25-(OH)2D associated with changes in skeletal weight bearing primarily reflect changes in 1,25(OH)2D production. The data provide no evidence that the changes in 1,25(OH)2D production are a consequence of changes in blood ionized calcium, plasma irPTH, or phosphorus.

Simulated weightlessness and bone metabolism: gravitational stimulation enhances insulin sensitivity

The effect of simulated weightlessness on bone metabolism was investigated in skeletal unloading for 4 days. Skeletal unloading was designed using the model of hindlimb hang in rats. Skeletal unloading with hindlimb hang cased a significant decrease of alkaline phosphatase activity, deoxyribonucleic acid (DNA) content, and glucose consumption in the femoral diaphysis, but not in the calvaria. When femoral-diaphyseal tissues were cultured in the presence of insulin (10(-8) M), the hormone produced a significant increase of alkaline phosphatase activity and decrease of glucose consumption in the femoral-diaphyseal tissues obtained from normal rats. This hormonal effect was not seen in the femoral diaphysis, but in the calvaria, of rats with skeletal unloading. However, insulin effect was seen in the femoral diaphysis obtained at 3 days after the removal of skeletal unloading. Meanwhile, the presence of other bone-regulating factors (10(-8) M parathyroid hormone [1-34] and 10(-4) M zinc sulfate) revealed an appreciable effect on alkaline phosphatase activity in the femoral diaphysis from rats with skeletal unloading. These results suggest that gravitational stimulation can directly enhance a specific insulin sensitivity in the regulation of bone metabolism.

Severe hypercalcaemia four months after acute oliguric renal failure--successful treatment with intravenous clodronate

A 33 year old man developed acute oliguric failure lasting 66 days, eight days after admission with multiple gun shot wounds. On day 99 after admission, serum calcium was elevated mildly at 2.54 mmol/l (normal range 2.1-2.5 mmol/l). Serum parathormone was undetectable. He was discharged soon afterwards. He presented again on day 164 with nausea, vomiting and blurred vision. Fundoscopy revealed an ischaemic retinopathy and extensive keratopathy. Serum calcium was 3.48 mmol/l and serum creatinine 262 umol/l (normal range 40-110 umol/l). Repeat parathormone was undetectable and there was no evidence of myeloma, sarcoidosis or malignancy. Following treatment with intravenous saline and frusemide, serum calcium fell to a nadir of 3.05 mmol/l. On day 168 an infusion of sodium clodronate 300 mg was given. Twenty-four hours later serum calcium was 2.65 mmol/l and 48 hours later calcium was 2.26 mmol/l. Normocalcaemia was maintained for 17 days and severe hypercalcaemia never recurred. This is the first report in which biphosphonates have been successfully used to treat hypercalcaemia following acute renal failure thus obviating the need for further dialysis.

Osteoporosis after spinal cord injury

Dual-photon absorptiometry characterized bone loss in males aged less than 40 years after complete traumatic paraplegic and quadriplegic spinal cord injury. Total bone mass of various regions and bone mineral density (BMD) of the knee were measured in 55 subjects. Three different populations were partitioned into four groups: 10 controls (healthy, age matched); 25 acutely injured (114 days after injury), with 12 reexamined 16 months after injury; and 20 chronic (greater than 5 years after injury). Significant differences (p less than 0.0001) in bone mass mineral between groups at the arms, pelvis, legs, distal femur, and proximal tibia were found, with no differences for the head or trunk. Post hoc analyses indicated no differences between the acutely injured at 16 months and the chronically injured. Paraplegic and quadriplegic subjects were significantly different only at the arms and trunk, but were highly similar at the pelvis and below. In the acutely injured, a slight but statistically insignificant rebound was noted above the pelvis. Regression techniques demonstrated early, rapid, linear (p less than 0.0001) decline of bone below the pelvis. Bone mineral loss occurs throughout the entire skeleton, except the skull. Most bone loss occurs rapidly and below the pelvis. Homeostasis is reached by 16 months at two thirds of original bone mass, near fracture threshold.

Estimated intracortical bone turnover in the femur of growing macaques: implications for their use as models in skeletal pathology

Although macaques have been used widely to study the dynamics of bone remodeling, there have been few comparisons to skeletal tissue turnover in humans. This study analyzes variation in bone microstructure with respect to gender, age, and body weight in growing macaques to determine whether the pattern of osteonal remodelling in macaques is like that in analogous bones of humans. Histomorphometric measurements were made on femoral midshaft cross-sections of 54 macaques from the Cayo Santiago skeletal collection. These data show that variation in bone microstructure occurs independent of gender, age, and body weight in macaques. Moreover, intracortical bone turnover in the macaque femur is much slower than in humans, making them poor models to study the effects of weightlessness, but good models to study low turnover skeletal dysplasias.

Simulated weightlessness and bone metabolism: impairment of insulin effect on alkaline phosphatase activity in bone tissue

The effect of simulated weightlessness on bone alkaline phosphatase was investigated after skeletal unloading for up to 4 days. The skeletal unloading was designed by using the model of hindlimb hang in rats. The femoral-diaphyseal fragments obtained from rats bred with skeletal unloading were cultured for 24 h at 37 degrees C in 5% CO2/95% air in Dulbecco's Modified Eagle Medium (high glucose). The bone alkaline and acid phosphatase activity were significantly decreased by skeletal unloading. When the bone tissue was cultured with synthetic [Asu1,7] eel calcitonin (3 and 30 nM), the hormone caused a significant increase of alkaline phosphatase activity in the bone tissues from rats with normal and skeletal-unloading. In culture with insulin (1.0 and 10 nM), skeletal unloading impaired the effect on insulin to increase bone alkaline phosphatase activity. Meanwhile, the culture with zinc sulfate (10 and 100 microM), which can increase bone protein synthesis, caused a remarkable elevation of alkaline phosphatase activity in the bone tissues form rats with normal and skeletal-unloading. Insulin (10 nM) did not alter the zinc effect. These findings suggest that the skeletal unloading with hindlimb hang causes the impairment of insulin's effect to increase alkaline phosphatase activity in the femoral diaphysis of rats, although the effects of calcitonin and zinc were not altered.

Simulated weightlessness and bone metabolism: evidence for direct gravitational effect and its related insulin action

The effect of simulated weightlessness on bone metabolism was investigated in skeletal unloading for 4 days. Skeletal unloading was designed using the model of hind-limb hang in rats. In hypokinetic state, rats were fed while the right hind limb was weighed down and the left hind limb was unloaded (a state of weightlessness). Bone metabolism in normal rats did not vary significantly in the femoral diaphysis in right and left hind limb. Alkaline phosphatase activity and DNA content in the femoral diaphysis were not significantly altered by hypokinetic state without skeletal unloading, while the unloading caused an appreciable decrease in the enzyme activity and DNA content. However, femoral-diaphyseal zinc content and glucose consumption was significantly decreased by hypokinetic state with and without skeletal unloading. When femoral-diaphyseal tissues were cultured in the presence of insulin (10 nM), the hormone produced a significant increase of alkaline phosphatase activity and decrease of glucose consumption in the femoral-diaphyseal tissues obtained from normal and hypokinetic rats. This hormonal effect was not seen in bone tissues from hypokinetic rats with skeletal unloading. These results suggest that skeletal unloading-induced disorder of bone metabolism is directly related to gravitational effect and that gravitational stimulation may be involved in insulin action.

Simulated weightlessness and bone metabolism: impairment of glucose consumption in bone tissue

The effect of simulated weightlessness on bone glucose consumption was investigated in skeletal unloading for up to 4 days. Skeletal unloading was designed using the model of hind-limb hang in rats. Femoral-diaphyseal fragments obtained from rats bred with skeletal unloading were cultured for 24 h at 37 degrees C in 5% CO2/95% air in Dulbecco's Modified Eagle Medium (high glucose, 4.5 g/dl). The consumption of medium glucose by bone tissues was significantly decreased by skeletal unloading for 2 and 4 days, while the production of lactic acid from bone tissues was not significantly altered. In addition, adenosine triphosphate (ATP) content in the cultured bone was significantly decreased by skeletal unloading. The presence of calcitonin (synthetic [Asu1,7] eel, 3 and 30 nM) fairly restored bone glucose consumption impaired by skeletal unloading. However, the effect of insulin (1 and 10 nM) on bone glucose consumption and ATP content was not seen in the bone tissues with skeletal unloading. These results suggest that the sensitivity of bone response for insulin action is reteriorated by skeletal unloading.

Physical bone changes in carragheenin-induced arthritis evaluated by quantitative computed tomography

Repeated non-invasive measurements were performed in dogs of trabecular bone density (TBD), low density bone area (LDBA), and high density bone area (HDBA) in chronic arthritis using quantitative computed tomography (QCT). Unilateral chronic arthritis of the knee had been induced by weekly instillation of 2 ml carragheenin into the right knee joint for 12 weeks with the left knee serving as a control. CT scanning of the distal femoral condyles was performed in 12 mature dogs with chronic arthritis. Another 6 dogs underwent a longitudinal CT study starting immediately prior to induction of arthritis. During induction of arthritis TBD decreased (P less than 0.01), LDBA increased (P less than 0.05) and HDBA decreased (P less than 0.01) in the arthritic bone. Opposite changes were found on the control side, i.e. TBD increased (P less than 0.01), LDBA decreased (P less than 0.01) and HDBA increased (P less than 0.01). The chronic arthropathic bone showed 20% lower TBD (P less than 0.0001), greater LDBA (P less than 0.0001) and lower HDBA (P less than 0.0001) as compared with the control bone. Reproducibility tests of TBD showed a coefficient of variation of 0.8%. Indentation tests and histomorphometric analyses confirmed the bone density changes as measured by CT.

Homeostatic control of bone structure: an application of feedback theory

The regulation of bone mass and structure in the weight-bearing skeleton is governed to a great extent by the mechanical demands placed upon the bone tissue. The apparent biological goal is the maintenance of a minimum adequate structure, in which the margin of safety between normal mechanical demands and fracture is balanced by the cost of excessive bone mass on mobility. Frost has developed two powerful postulates concerning bone adaptation: (a) there exist threshold levels of mechanical strain, above or below which bone adaptation is turned on, and (b) the set point for normal bone structure can be modulated by hormones. A model was developed, using Frost's postulates and simple feedback theory, that describes the interaction between biochemical influences and mechanical influences on bone structure. The model predicts that biochemical agents that influence bone structure independently of the mechanical feedback system (e.g., calcitonin) are capable of only limited anabolic effects on bone mass because their influences conflict with mechanical influences. However, biochemical agents that influence bone structure by changing the set point of the mechanical feedback system (e.g., estrogen) will provide lasting changes in bone structure. Age-related changes occur within the effector and transduction components of the mechanical feedback system that tend to make it sluggish. These changes may lead to increased bone fragility because the system is no longer capable of maintaining adequate bone structure.

Beta-alanyl-L-histidinato zinc prevents skeletal unloading-induced disorder of bone metabolism in rats

The effect of a new zinc compound beta-alanyl-L-histidinato zinc (AHZ) on the disorder of bone metabolism caused by skeletal unloading was investigated. Skeletal unloading was designed using the model of hind-limb hang in rats. Skeletal unloading for up to 4 days caused a remarkable decrease of zinc content, alkaline phosphatase activity, and DNA content in the femoral diaphysis of rats. Oral administration of AHZ (2.5, 5.0, and 10.0 mg/100 g) caused a significant increase in zinc, DNA, and calcium contents in the femoral diaphysis of rats with the skeletal unloading. Bone alkaline phosphatase activity was significantly increased by doses of 5.0 and 10.0 mg/100 g. These results clearly indicate that skeletal unloading-induced disorder of bone metabolism is prevented by the oral administration of AHZ. AHZ may be useful as a therapeutic tool in bone disorder.

Immobilization-related hypercalcaemia--a possible novel mechanism and response to pamidronate

Immobilization-related hypercalcaemia is an uncommon but important condition being associated not infrequently with both urolithiasis and osteoporosis. In this study 5 patients who had been immobilized for a mean of 3 months and had a mean adjusted serum calcium of 3.15 mmol/l were treated with doses of intravenous pamidronate ranging between 10 mg and 45 mg. All patients became normocalcaemic by day 3. Patients 1-3 mobilized shortly after treatment and remained normocalcaemic. In those patients who continued to be immobile hypercalcaemia recurred after an interval of several weeks. Retreatment with pamidronate again resulted in normocalcaemia. No side effects were noted with treatment. All of the patients studied had increased rates of bone resorption as shown by elevated urinary hydroxyproline/creatinine ratios (median:range) of 0.101:0.045-0.180 (normal less than 0.033) and elevated calcium/creatinine ratios of 2.50:0.69-3.63 (normal less than 0.50). None of the patients in this study had any of the usual risk factors for developing immobilization-related hypercalcaemia though all 5 patients had problems with significant sepsis which we postulate may have lead to cytokine release which in turn contributed to the development of hypercalcaemia. We conclude that pamidronate (at doses as low as 10 mg) is safe and effective in immobilization-related hypercalcaemia and suggest that sepsis should be added to the list of risk factors for development of this syndrome.

Effect of physical activity on calciotropic hormones and calcium balance in rats

The response of calciotropic hormones and bone turnover to exercise and immobilization was examined by the measurement of calcium balance, bone turnover indexes, levels of parathyroid hormone, nephrogenous adenosine 3',5'-cyclic monophosphate (cAMP), and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] weekly for 6 wk in three groups of rats: control, exercise trained, and immobilized. Early in the experiment, increases were observed in excretion of urinary calcium, hydroxyproline, and in serum alkaline phosphatase after both exercise and immobilization. It was not until the latter part of the experimental period that changes were observed in nephrogenous cAMP and intestinal absorption efficiency of calcium. In the fasting state, the exercise group had a drop in serum calcium and phosphate and a rise in nephrogenous cAMP and serum 1,25(OH)2D3 compared with the control group. The exercised animals experienced an increase in bone mass, whereas the immobilized animals had a decline in bone mass. Thus exercise stimulates bone growth, resulting in an increased demand for minerals that is satisfied by an increase in serum 1,25(OH)2D3 levels and increased intestinal absorption of calcium. The increase in calcium absorption suppresses parathyroid hormone production (nephrogenous cAMP) in the exercised animal. Immobilization resulted in increased bone resorption that suppressed parathyroid hormone, nephrogenous cAMP, and the intestinal absorption of calcium.

Inhibition of osteoclastic bone resorption by mechanical stimulation in vitro

The influence of mechanical stimulation by intermittent compressive force (ICF) of physiologic magnitude on osteoclastic bone resorption was investigated in cultures of fetal mouse cartilaginous long bones. Exposure to ICF resulted in a significant decrease in mineral resorption, as indicated by the decreased release of 45Ca and a decreased number of osteoclasts in the diaphysis. Conditioned medium (CM) from ICF-exposed periosteum-free cultures (ICF-CM), but not from control cultures (Co-CM), inhibited mineral resorption in fresh bones cultured under control conditions. Co-CM increased, but ICF-CM decreased, the number of tartrate-resistant acid phosphatase-positive cells in 7-day bone marrow cultures. Direct exposure of bone marrow cultures to ICF yielded the same results. Thus, osteoclastic bone resorption in cartilaginous long bones is inhibited by ICF in vitro. A soluble factor(s) acting on tartrate-resistant acid phosphatase-positive, osteoclast precursor-like cells seems to play a role in this effect.

Bone response to normal weight bearing after a period of skeletal unloading

Skeletal unloading in the growing rat induces a temporary inhibition of bone formation and thereby a deficit in bone calcium compared with age-matched, normally loaded animals. To determine whether this deficit can be restored by skeletal reloading we measured bone formation rate at the tibiofibular junction and total bone calcium in the tibia and lumbar vertebra in rats whose hindlimbs were unloaded for 2 wk and then reloaded by return to normal weight bearing. Continuously loaded or unloaded animals were also studied. Skeletal unloading reduced bone formation by 34% and tibial and vertebral calcium by 12 and 22%, respectively. Reloading significantly increased the rates of bone formation and calcium accretion 30-34% above normally loaded animals, and by 2 wk had decreased the deficit in tibial and vertebral calcium by 36 and 23%, respectively. These data indicate that the deficit in bone calcium induced by skeletal unloading in the growing rat can be restored in part by return to normal weight bearing. However, the time required to restore bone calcium exceeds the time required to produce the original calcium deficit.

Simulated weightlessness and bone metabolism: decreases of protein and DNA syntheses in the femoral diaphysis of rats

The effect of simulated weightlessness on bone protein and DNA syntheses was investigated in the skeletal unloading for up to 4 days. The skeletal unloading was designed by using the model of hindlimb hang in rats. The femoral-diaphyseal fragments obtained from rats which bred with skeletal unloading were cultured for 3 h at 37 degrees C in 5% CO2/95% air in Dulbecco's Modified Eagle Medium (high glucose). When the bone tissues were pulsed with [3H]proline, the incorporation of [3H]proline into the bone protein was significantly decreased by skeletal unloading. In the pulse with [14C]uridine, the incorporation of [14C]uridine into the bone RNA was significantly reduced by skeletal unloading. Moreover, the incorporation of [3H]thymidine into the acid-insoluble residues of bone tissue was significantly decreased by skeletal unloading. Also, DNA content in the femoral diaphysis was significantly reduced by skeletal unloading. These findings suggest that skeletal unloading causes the decreases of protein and DNA syntheses in the femoral diaphysis of rats.

Hypercalcaemia causing declining cognitive function in a head injured patient

Although hypercalcaemia in young children or adolescents immobilized by fractures or spinal cord injury is well recognized, hypercalcaemia in adult immobilized patients is rare without a pre-existing bone disease. To our knowledge, hypercalcaemia in a head-injured, immobilized patient has not been previously reported. We report here a case where a previously normocalcaemic, immobilized, head-injured adult patient developed cognitive decline secondary to hypercalcaemia five months after injury, when transient interruption of enteral feedings led to mild dehydration. Indices of bone turnover were elevated and parathyroid hormone was appropriately suppressed. Possible predisposing factors in our patient included a severe degree of immobilization and a very high level of athletic activity prior to injury. Careful fluid management and specific monitoring of calcium levels, even several months post-injury, should be performed to avoid the added complications of hypercalcaemia in head-injured patients.

Immobilization osteoporosis: a review

Bone mass is not only subject to systemic hormonal homeostatic mechanisms, but also to local mechanical influences. The importance of the mechanical balance of bone has been more recently stressed by the research on the effect of weightlessness on bone, and by the introduction of the concept of "mechanostat" in the pathogenesis of osteoporotic conditions. Immobilization osteoporosis has clinical (fractures, sometimes hypercalcemia, urinary lithiasis) and radiological features. Immobilization has an effect on bone modeling and remodeling, through an increased activation of remodeling loci, and a decrease of the osteoblastic stimulus. This leads directly to a local reduction in bone mass, the increased activation multiplying the effect of the deficit in bone formation. The prevention is based on exercise if the load is applied intermittently for a daily period. It seems also that muscle weight is an important determinant of bone mass. There is a potential for recovery during the active early phase of immobilization osteoporosis that may disappear in the subsequent late (about six months) inactive phase. Permanent losses could be prevented by appropriate measures, pharmacology or exercises, applied during the first months of immobilization. No recovery has been demonstrated after the inactive phase has been reached, whatever the treatment. The cumulative effect of repeated periods of immobilization remains hypothetical.

Hypercalcemia of immobilization in an adult patient with peripheral neuropathy

A 48-year-old man developed a marked and persistent hypercalcemia 3 months after admission for paraplegia resulting from severe peripheral neuropathy most likely of alcoholic etiology. Serum ionized calcium was elevated, and parathyroid hormone levels were low normal by the two separate radioimmunoassays. Urinary calcium excretion was markedly elevated, and serum 1,25-dihydroxyvitamin D level was decreased. An extensive clinical evaluation for possible occult malignancy, myeloma, and sarcoidosis as a cause of hypercalcemia produced no positive findings. Treatment with calcitonin caused prompt normalization of serum calcium, and its discontinuation resulted in recurrence of hypercalcemia. With improvement of neuropathy, the patient started active physical therapy. We gradually discontinued calcitonin, and the patient's serum calcium remained normal during the following 11 months. We discuss difficulties in both clinical and laboratory diagnosis of hypercalcemia of immobilization in the adult patient because no specific laboratory test is available.

Simulated weightlessness and bone metabolism: decrease of alkaline phosphatase activity in the femoral diaphysis of rats

The effect of simulated weightlessness on bone metabolism was investigated in the skeletal unloading for up to 9 days. The skeletal unloading was designed by using the model of hindlimb hang in rats. The food ingestion of rats with the skeletal unloading was not altered in comparison to that of normal rats. Calcium concentration in the serum was not significantly altered by the skeletal unloading for 9 days, while the serum inorganic phosphorus concentration was significantly decreased at 6 and 9 days. Calcium content in the femoral diaphysis was not altered by the skeletal unloading for 9 days. However, the activities of alkaline and acid phosphatases in the femoral diaphysis were markedly decreased by the skeletal unloading. The decrease in bone alkaline phosphatase activity was seen at 2 days with the skeletal unloading. The present results demonstrate that the skeletal unloading with hindlimb hang can induce the disorder of bone metabolism. This model is useful for studying the effects of simulated weightlessness on bone metabolism.

Effect of physical activity on calcium and phosphorus metabolism in the rat

Changes in skeletal mass, nutritional calcium and phosphorus balance, and intestinal calcium absorption were studied in four groups of rats: control, exercise allowing free access to food, exercise with pair-feeding to control levels, and immobilization. The exercise regimen consisted of treadmill running 25 m/min, 60 min/day, 5 days/wk for 13 wk; rats were immobilized by bilateral sciatic denervation. The total body Ca (TBCa) was measured by neutron activation analysis as an index of skeletal mass. Standard metabolic balance techniques were used to determine calcium and phosphorus balance, and an in situ duodenal loop ligation preparation was used to study the active and passive intestinal Ca transport processes. Exercise promoted a positive Ca and P balance and increased the skeletal mass, largely as a result of an increase in 1,25-dihydroxyvitamin D and an enhancement of the intestinal Ca absorption efficiency. Urinary excretion of Ca and P did not differ from control levels and food intake was not a factor because pair-fed rats responded to exercise almost identically to those fed ad libitum. Conversely, immobilization caused a decrease in TBCa and a lower Ca and P balance. These effects are the result of an increased urinary mineral excretion, greater endogenous fecal excretion, and decreased mineral absorption efficiency in the intestine.

Immobilization hypercalcaemia with severe bone mineral loss and hypogonadism

Moderate hypercalcaemia occurred in a 17-year-old male who was immobilized with abdominal and right hip sepsis for 9 months after a motor vehicle accident. The hypercalcaemia was due to bone resorption, with a urine hydroxyproline:creatinine ratio of 0.203 (normal less than 0.017) and a urine calcium loss of 22.9 mmol/24 hr, associated with impaired renal function. There was radiological evidence of severe bone demineralization in the pelvis over 42 weeks. Radiocalcium absorption, using 47Ca, was decreased (0.17, normal range 0.35-1.30), renal tubular maximum for calcium reabsorption was decreased (1.61 mmol/1 glomerular filtrate, normal range 1.8-2.2), the serum parathyroid hormone concentration was in the low normal range (3.2, 3.6 u/l, normal range 2-6) and the plasma 1,25-dihydroxy-vitamin D concentration was decreased despite a normal 25-hydroxy-vitamin D concentration, indicating suppression of the parathyroid, 1,25-dihydroxy-vitamin D axis. The patient was found to be hypogonadal at 41 weeks after admission and testosterone therapy was begun, with associated improvement in mobilization and a reduction of the hypercalcaemia.

Calcium and vitamin D metabolism in Guamanian Chamorros with amyotrophic lateral sclerosis and parkinsonism-dementia

We evaluated 16 Guamanian Chamorros with amyotrophic lateral sclerosis and 33 patients with parkinsonism-dementia for disturbances of calcium and vitamin D metabolism. The serum immunoreactive parathyroid hormone level was mildly elevated in 6 patients with amyotrophic lateral sclerosis and in 5 patients with parkinsonism-dementia. There were significant positive correlations between serum immunoreactive parathyroid levels and duration of illness in male patients with motor neuron disease, but not in female patients or in patients with parkinsonism-dementia. Intestinal absorption of calcium, as assessed by serum and urinary activity of calcium 47 following oral administration, was decreased in 2 patients with amyotrophic lateral sclerosis and in 4 patients with parkinsonism-dementia, all of whom had low levels of serum 1,25-dihydroxyvitamin D. Reductions in cortical bone mass were striking in patients with motor neuron disease. A significant negative correlation was found between the percentage of cortical area of the second metacarpal bone and muscle atrophy and weakness, and significant positive correlations were found between degree of immobility and ratio of urinary hydroxyproline to creatinine in patients with amyotrophic lateral sclerosis and parkinsonism-dementia. In general, abnormalities in calcium metabolism were subtle. Thus, if the demonstrated deposition of metals, particularly calcium and aluminum, in central nervous system tissues of Guamanians with these two conditions is a cause of the diseases and of the early appearance of neurofibrillary tangles in neurons, the accumulation has apparently occurred long before onset of symptoms, and detectable abnormalities of calcium and vitamin D metabolism may already have been corrected.

Hypercalcaemia in association with renal failure: the role of immobilisation

Hypercalcaemia occurring after ten weeks of immobilisation was observed in four adult patients all of whom had had prior renal failure sufficient to require renal dialysis. In all patients parathyroid hormone levels were normal or low and in three plasma 1,25(OH)2D3 levels were low. These findings are consistent with immobilisation induced increases in bone calcium resorption. Renal excretion of calcium may have been impaired by renal dysfunction resulting in hypercalcaemia and suppression of plasma PTH and 1,25(OH)2D3 levels. Resolution of the hypercalcaemia was associated with remobilisation. Parathyroidectomy is inappropriate treatment.

Calcium homeostasis in immobilization: an example of resorptive hypercalciuria

Prolonged immobilization may result in hypercalcemia, hypercalciuria, and osteoporosis. Although bone resorption is central to this syndrome, the mechanism of resorption is uncertain. In particular, the role of systemic calcium-regulating hormones remains unclear. In 14 immobilized subjects we measured fasting calcium excretion, 24-hour urinary calcium excretion during restricted calcium intake, the renal phosphorus threshold, plasma 1,25-dihydroxyvitamin D, nephrogenous cyclic AMP, and immunoreactive parathyroid hormone. Mean serum calcium levels were normal, but fasting and 24-hour calcium excretion were markedly elevated (0.28 mg per deciliter of glomerular filtrate and 314 mg per 24 hours, respectively). The mean levels of serum phosphorus (4.8 mg per deciliter) and the renal phosphorus threshold (4.3 mg per deciliter) were elevated. Mean plasma 1,25-dihydroxyvitamin D was strikingly reduced (9.9 pg per milliliter), as were nephrogenous cyclic (0.64 nmol per deciliter of glomerular filtrate) and immunoreactive parathyroid hormone in both assays. These findings indicate that the parathyroid--1,25-dihydroxyvitamin D axis is suppressed in patients with immobilization-induced hypercalciuria, as would be predicted by a model of resorptive hypercalciuria.

Correlation of kinetics and absorption of 47Ca with estrogen and hormonal ovarian function in postmenopause

The relationship between the metabolism of 47Ca bone kinetics and intestinal absorption with estrogen and hormonal ovarian function in physiologic and artificial postmenopause has been studied. The augmented exchangeable pool of calcium in bone is related to the duration of postmenopause and estradiol values. The time from menopause/oophorectomy includes all changes occurring in postmenopause, so other undefined factors, besides estrogens, are seemingly indispensable for postmenopausal changes of calcium kinetics. The diminished calcium absorption and augmented exchangeable pool of calcium correlate only with the absence of ovaries and not with estrogen levels in physiologic and artificial postmenopause. It seems that the total hormonal and especially ovarian function in postmenopause cannot be neglected, and although estrogens are important, they are not the sole etiologic factor in postmenopausal changes in calcium metabolism.