Citations

Abstract We studied the mechanism of impaired calcium absorption with aging in 51 healthy women whose ages ranged from 26 to 88 years. Serum concentrations of 1,25-dihydroxyvitamin D [1,25-(OH)2D, mean of four measurements per subject] increased with age by 22% (P < 0.05) but, by split-point analysis, plateaued or decreased slightly after age 65. In a subset of 20 subjects, [3H]1,25-(OH)2D3 kinetic analysis showed that this increase with age resulted from both increased production and decreased metabolic clearance of 1,25-(OH)2D. Despite the increase in serum 1,25-(OH)2D concentration, true calcium absorption did not change with age. The expected inverse correlation between true fractional calcium absorption and dietary calcium intake, however, was easily demonstrated (r = 0.66, P < 0.001). Serum intact parathyroid hormone (PTH) increased with age by 35% (P < 0.02) and serum bone gla protein (BGP, osteocalcin) increased by 47% (P < 0.001); the increases in serum PTH and serum BGP were directly correlated (r = 0.32, P < 0.05). The data are consistent with the following hypothetical model: (1) intestinal resistance to 1,25-(OH)2D action accounts for the increase in serum 1,25-(OH)2D concentrations with aging with no change in true calcium absorption; (2) this results in a compensatory increase in PTH secretion and in 1,25-(OH)2D production that prevents true calcium absorption from decreasing; (3) the previously described defect in 25-hydroxyvitamin D (25-OHD) 1α-hydroxylase activity in aging animals and humans acccounts for the leveling off in serum 1,25-(OH)2D concentration after age 65 years; and (4) the secondary hyperparathyroidism leads to increased bone turnover and thus contributes to age-related bone loss.

Abstract The roles of parathyroid hormone (PTH) and calcitonin (CT) in the pathogenesis of familial benign hypercalcemia (FBH, or hypocalciuric hypercalcemia) are uncertain. Thus we performed studies in 26 patients with FBH, 12 patients with primary hyperparathyroidism (HPT), and 20 normal volunteers, to answer these questions: are plasma levels of intact or biologically active PTH frequently elevated in FBH? Is plasma intact PTH nonsuppressible during calcium infusion? Is there blunting of the C cell CT response to calcium infusion as occurs in primary HPT? We used three methods for measurement of PTH: a mid region-specific radioimmunoassay (iPTH, antiserum GP-1M), an extraction-concentration bioassay (bioPTH, stimulation of cAMP generation in osteoblastlike cells), and a two-site immunoradiometric assay (IRMA) for intact PTH. PTH levels were significantly elevated in primary HPT by all three methods, but mean PTH was normal in FBH and 85–92% of values overlapped the normal range. During 5 minute calcium infusions (2 mg Ca2+ per kg) iPTH values fell little, but bioPTH and intact PTH fell sharply in all three groups. Mean calcium-induced decreases of intact and bioPTH were indistinguishable from normal in FBH, but PTH levels generally remained elevated at 5 minutes in primary HPT. In FBH basal and postinfusion CT levels were normal. The data show that, in the majority of patients with FBH, PTH concentrations and bioactivity in blood are within the normal range and are suppressed rapidly to very low levels with further increases of calcium. The data suggest that the abnormality of parathyroid function in FBH differs from that in primary HPT. There was no deficiency of CT or C cell responsiveness in FBH, another difference from primary HPT. Nonetheless, in 8–15% of cases, measurement of PTH could not discriminate FBH from primary HPT.

Abstract We characterized gene expression in the reparative callus that formed after fracture of the rat femur. The callus was divided into regions of bone formation (hard callus) and cartilage formation (soft callus), and gene expression was examined separately in each region. Expression of extracellular matrix protein genes varied with the progression of repair and differed between hard and soft calluses. Messenger ribonucleic acids (mRNAs) for osteonectin, alkaline phosphatase, and type I procollagen were detected in the hard callus at maximal levels during endochondral ossification and bone remodeling (day 15) and at 50% maximal levels during intramembranous bone formation (day 7). Messenger RNAs for these proteins in the soft callus were detected at low levels during chondrogenesis (day 9) but increased to 80% of maximal levels with chondrocyte hypertrophy and mineralization of the cartilage matrix (day 13). Messenger RNAs for type II procollagen and proteoglycan core protein were detected at maximal levels in the soft callus during chondrogenesis (day 9). Osteocalcin gene expression was detected in the hard callus during endochondral ossification and remodeling but not during intramembranous bone formation or at any time in the soft callus. Osteonectin mRNA was detected in both the hard and soft callus throughout the entire course of fracture repair. Expression of cartilage and bone-related genes correlated with the temporal sequence of histologic changes, suggesting transcriptional regulation of gene expression during repair. Differences in gene expression between hard and soft callus and in each of these regions as repair progressed suggest local regulation of gene expression during cell differentiation and matrix synthesis.

Abstract Osteoporosis is widely viewed as a major public health concern, but the exact magnitude of the problem is uncertain and likely to depend on how the condition is defined. Noninvasive bone mineral measurements can be used to define a state of heightened fracture risk (osteopenia), or the ultimate clinical manifestation of fracture can be assessed (established osteoporosis). If bone mineral measurements more than 2 standard deviations below the mean of young normal women represent osteopenia, then 45% of white women aged 50 years and over have the condition at one or more sites in the hip, spine, or forearm on the basis of population-based data from Rochester, Minnesota. A smaller proportion is affected at each specific skeletal site: 32% have bone mineral values this low in the lumbar spine, 29% in either of two regions in the proximal femur, and 26% in the midradius. Although this overall estimate is substantial, some other serious chronic diseases are almost as common. More importantly, low bone mass is associated with adverse health outcomes, especially fractures. The lifetime risk of any fracture of the hip, spine, or distal forearm is almost 40% in white women and 13% in white men from age 50 years onward. If the enormous costs associated with these fractures are to be reduced, increased attention must be given to the design and implementation of control programs directed at this major health problem.

AbstractPursuing the hypothesis that quantitation of skeletal alkaline phosphatase (ALP) activity in canine serum would provide an index of the rate of bone formation, we compared three methods for isoenzyme-specific identification of skeletal ALP activity in canine serum: heat inactivation, wheat germ agglutinin (WGA) precipitation, and concanavalin A (ConA) precipitation. ALP isoenzyme activities were extracted from canine bone, intestine, and liver, diluted into heat-inactivated canine serum (i.e., serum without ALP activity), and used as calibrators of ALP isoenzyme activities. Differential sensitivity to inhibition by 10 mM L-homoarginine was used to distinguish intestinal ALP activity from hepatic and skeletal ALP activities (i.e., 9, 80, and 72% inhibition, respectively). To allow resolution of skeletal ALP activity from hepatic ALP activity, we tested two established methods (heat inactivation and WGA precipitation) and a novel method, ConA precipitation. The organ-derived skeletal and hepatic ALP isoenzyme activities were used to compare these three methods with respect to linearity, isoenzyme separation, and precision. All three methods were linear, but the WGA and ConA methods afforded greater isoenzyme separation and precision. The relative extent of isoenzyme separation (i.e., the difference in percentage remaining skeletal and hepatic ALP isoenzyme activities) averaged 23, 40, and 47% remaining ALP activity for the heat, WGA, and ConA methods, respectively. However, when these methods were applied to the quantitation of skeletal ALP activity in sera from 10 young and 10 adult beagles, the WGA method was found to be unacceptable because most of the results fell outside the range of the WGA assay calibrators (i.e., >100% skeletal ALP activity). The heat and ConA methods showed that the amount of skeletal ALP activity in the beagle sera decreased with age, both as ALP activity per liter and as percentage of total serum ALP activity (p < 0.001 for each). Skeletal ALP activity levels determined by ConA were correlated with values determined by heat inactivation (r = 0.87, p < 0.001) but not with WGA-determined levels (r = 0.26). Intestinal ALP activity was detected in only 1 of these 20 sera. We conclude that ConA precipitation can be used for quantitation of skeletal ALP activity in beagle serum.

Abstract Appendicular bone mass is inversely related to the risk of hip fracture in short-term prospective studies, but hip fractures typically occur about 30 years after menopause. We developed a model that estimates a woman's lifetime risk of hip fracture based on measurement of radial bone mass at age 50 using short-term prospective data relating bone mass to hip fracture, the correlation between bone mass at age 50 and later years, the age-specific incidence of hip fracture and mortality, and prospective data about bone mass and mortality. We estimate that a 50-year-old white woman has a 19% lifetime risk of hip fracture if her radial bone mass is at the 10th percentile for her age and an 11% lifetime risk if her bone mass is at the 90th percentile. Improved measurement techniques that have a higher predictive value for hip fracture in short-term studies could substantially increase this gradient of lifetime risk and therefore be more clinically useful.

AbstractIn a series of postmenopausal women previously studied by our group, pernicious anemia was associated with reduced bone mineral density of the spine and with vertebral fractures. To determine whether osteoporotic fractures represent an important clinical problem for such patients, we conducted a population-based retrospective cohort study of all Rochester, Minnesota residents newly diagnosed with pernicious anemia from 1950 through 1979. These 131 individuals were followed through the inpatient and outpatient records of all medical care providers in the community for 1550 person-years. In comparison with fracture rates from the general community, patients with pernicious anemia had a 1.9-fold increase in proximal femur fractures, a 1.8-fold increase in vertebral fractures, and a 2.9-fold increase in distal forearm fractures (all p < 0.05). Increases in the risk of proximal humerus and pelvis fractures were not statistically significant. Clinicians should be aware that pernicious anemia is associated with an increased risk of osteoporotic fractures, but the mechanism is unknown.

Abstract High caffeine consumption has been proposed as a risk factor for osteoporotic fracture, but the evidence associating high caffeine intake with low bone density is inconsistent. We therefore examined the influence of caffeine consumption on bone mineral at six skeletal sites in an age-stratified random sample of white women residing in Rochester, Minnesota. After age adjustment, there was no association between overall caffeine consumption and bone mineral at five of the six sites. In the femoral shaft, however, there was a statistically significant interaction between age and caffeine consumption so that high caffeine intake was associated with slight reductions in bone mineral among elderly subjects but with modestly increased bone mineral at younger ages. When caffeine intake was categorized by source, no consistent influence of coffee, tea, or other caffeinated beverage consumption could be detected on bone mineral. Caffeine intake was, however, positively associated with cigarette smoking and alcohol consumption. After adjusting for age, caffeine consumption was not correlated with biochemical indices of bone turnover, circulating concentrations of estradiol and estrone, or other dietary and musculoskeletal variables. These data suggest that caffeine intake in the range consumed by a representative sample of white women is not an important risk factor for osteoporosis. Among elderly women, however, in whom calcium balance performance is impaired, high caffeine intake may predispose to cortical bone loss from the proximal femur.

Abstract A new bone cell line was established by transfecting normal adult human osteoblast-like (hOB) cells, derived from a 68-year-old woman, with the plasmid pSV3 neo. The plasmid included coding sequences and promotors for the large and small T antigens of the SV40 virus as well as resistance to the antibiotics neomycin and G418. A single antibiotic-resistant colony was located and cloned. Large tumor antigen production in the clonal cell line was confirmed by indirect immunofluorescence study. Treatment with 1,25-dihydroxy-vitamin D3 increased steady-state concentrations of protein and mRNA for osteocalcin and for alkaline phosphatase. Northern blot analyses also demonstrated the presence of mRNAs for α(I)-procollagen, osteopontin 1a, transforming growth factor β, and interleukin-1β. The plasma membrane calcium pump and osteonectin were identified by immunocytochemical analysis. These cells produced a matrix that mineralized when β-glycerophosphate was added to their cultures. As assessed by functional receptor assays, both estrogen and androgen receptors were present and functional, although at low concentrations. Treatment with parathyroid hormone did not stimulate adenylate cyclase activity. Thus, these cells are a well-differentiated, steroid-responsive clonal cell line that closely approximates the phenotype of the mature osteoblast. They should serve as an excellent model for the study of osteoblast biology.