Faculty Bibliography

Previous cross-sectional studies using delayed gamma neutron activation analysis and whole body counting suggested that the relationship of total body calcium (TBCa) to total body potassium (TBK) (muscle mass, body cell mass) remained constant with age. This led to the hypothesis that the muscle mass and skeletal mass compartments are integrated in their response to aging. It had also been hypothesized that loss of skeletal and muscle mass was similar between races. In the current study, delayed gamma neutron activation analysis and whole body counting were performed on 90 black and 143 white women 20-69 yr of age. Black women had higher TBCa and TBK values than white women, even when the data were adjusted for age, height, and weight. TBCa was correlated with height and TBK with weight. The estimated decline of skeletal mass (TBCa) from 20 to 70 yr was 18% in black women and 19% in white women. However, the lifetime decline of TBK was only 8% for black women, compared with 22% for white women. Black women may lose TBK more slowly than TBCa with aging, compared with white women. In particular, correlation of TBCa and age was similar for blacks and whites (r = -0.44 and r = -0.54, respectively). However, for TBK these correlations were r = -0.14 and r = -0.42. These data confirm a higher musculoskeletal mass in black women and suggest that the loss of muscle mass with age may be lower in black than in white women. These ethnic differences do not support the hypothesis of an integrated musculoskeletal system, so that these two components should be considered separately. A prospective study is needed to confirm these findings.

In a previous work, we demonstrated that the osteoprogenitors derived from the marrow stroma of the hypophysectomized (HX) rat demonstrate enhanced proliferative and differentiation capacities when placed in an optimal microenvironment. In this study, we sought to investigate the potential of the trabecular osteoblast-like cells of the HX rat. These cells represent a more mature pool of osteoblasts than the progenitors derived from the marrow stroma. We examined all three stages of osteoblast development using trabecular osteoblast-like cells derived from age-matched intact rats as a control. Using thymidine incorporation and cell number as indicators of proliferation, we found that these cells, like the osteoprogenitors derived from the HX rat, demonstrate augmented proliferation when placed in culture. Additionally, type I collagen expression remained at significant levels past the end stages of proliferation, at which point it is expected to be downregulated. Matrix maturation markers, such as alkaline phosphatase activity and bone sialoprotein expression, however, were significantly lower than in the controls. Mineralization potential, as measured by mineralized nodule formation, Ca(2+) content, and OPN and OCN expression, was also significantly reduced. Our results have uncovered an aberrant model of osteoblast development in which proliferation is deregulated, resulting in a minimal capacity of these cells to develop into fully differentiated mineralizing osteoblasts.

Black women have lower levels of serum 25-hydroxyvitamin D (25OHD) with higher serum PTH levels than white women. Correction of these alterations in the vitamin D-endocrine system could lead to less bone loss in postmenopausal women and, consequently, preservation of bone mass. Ten healthy postmenopausal black women were given 20 microg vitamin D3 daily for 3 months. At the end of the study, mean serum 25OHD levels had increased from 24 to 63 nmol/L. Serum intact PTH and nephrogenous cAMP declined significantly, and there was a 21% drop in the fasting urinary N-telopeptide of type I collagen. Vitamin D3 supplementation raises serum 25OHD levels in postmenopausal black women, decreases secondary hyperparathyroidism, and reduces bone turnover. These findings should spur further investigation of the use of vitamin D supplementation in the prevention of osteoporosis in this population.

The bone mineral density (BMD) of the spine was measured in the posteroanterior (PA) and lateral projections as well as the total body BMD in 447 black and white women. The lateral projection is comprised predominantly of cancellous bone whereas the total body BMD is predominantly cortical bone, and the PA spine is intermediate in composition. Black women had a higher BMD than white women for each measurement, but the difference was greatest in the lateral spine. Similarly, black women showed less decline in cancellous bone density with aging. The development of a high peak cancellous bone mass with reduced involutional loss may provide a major contribution towards protection against osteoporotic fractures in black women. Metabolic and pharmacologic studies in black and white women should consider the possibility of the influence of a larger cancellous bone mass.

Osteoporosis in men is a disease that is increasing in incidence, and with an increasing elderly population it poses a serious health problem. Since both testosterone (T) and growth hormone (GH) have an anabolic effect on bone and both decrease with aging, we were prompted to test whether the administration of these hormones in combination would increase bone mass in orchiectomized (orx) senile rats more than administration of either agent alone. Twenty-month-old male Wistar rats were divided into five groups with seven animals each: (a) age-matched intact control, (b) orx, (c) orx+GH (2.5 mg/kg/day), (d) orx+T [10 mg/kg, subcutaneous (s.c.), injection given twice a week], and (e) orx+GH+T. Testosterone and GH were given subcutaneously for 4 weeks. Bone histomorphometry of the tibial shaft showed that the orx group had lower cortical bone area than the intact control group. The decrease in cortical bone area was due to increased intracortical porosis as well as decreased periosteal bone formation rate (BFR). Administration of T to the orx animals prevented the development of the porosis and the decrease in periosteal BFR. The bone mineral content (BMC) and bone mineral density (BMD) of the femur as tested by dual-energy X-ray absorptiometry were significantly higher in the orx+T than in the orx group and were not significantly different from that of the intact control group. Administration of GH to the orx rats increased periosteal BFR significantly; however, the BMC and BMD measured were not increased significantly in comparison to the orx group. When GH and T were combined in treatment, the cortical bone area, periosteal BFR, and femoral BMD were all significantly higher than that of the orx and even higher than the intact control rats. Two-way analysis of variance shows that the individual effect of GH and T treatment on the periosteal BFR and cortical bone area was significant. The effect of T, but not GH, on femoral BMC and BMD was also significant; however, there is no synergistic interaction between the two treatments. Four weeks of orx with or without GH or T administration had no significant effect on tibial metaphyseal cancellous bone volume. In conclusion, this short-term study suggests that the combined intervention of GH and T in androgen-deficient aged male rats may have an independent effect in preventing osteopenia. The significant effect of GH+T may be attributed to the prevention of intracortical porosis, and an increase in periosteal bone formation and cortical bone mass.

The aim of the present study was to examine cancellous bone changes induced by exercise on three different skeletal sites, the lumbar vertebra, the proximal, and the distal tibia, in the young growing rat. Forty 4-week-old female Sprague-Dawley rats were randomized into 4 groups of 10 animals each; 8 weeks exercise (8EX), 8 weeks sedentary control (8CON), 12 weeks exercise (12EX), and 12 weeks sedentary control (12CON). The exercise regimen consisted of treadmill running at 24 m/min 1 hr per day 5 days a week. After each period of exercise, the proximal and distal tibial metaphyses (PTM and DTM, respectively) and the fifth lumbar (L5) vertebral body were processed for histomorphometry of the cancellous bone (secondary spongiosa) and cortical periosteum. Eight and twelve weeks of exercise significantly increased the mineral apposition rate and bone formation rate in the PTM and DTM, and 12 weeks of exercise significantly increased the labeled perimeter in the DTM, compared with the age-matched controls. Eight and twelve weeks of exercise significantly increased cancellous bone volume in the PTM (mean +/- standard deviation, 8EX; 19.1 +/- 2.9% vs 8CON; 14.3 +/- 3.1%, P < 0.05 and 12EX; 18.8 +/- 3.5% vs 12CON; 15.2 +/- 3.3%, P < 0.05), and 12 weeks exercise significantly increased cancellous bone volume in the DTM, compared with age-matched control (12EX; 32.5 +/- 7.7%, 12CON; 22.2 +/- 4.8%, P < 0.05). The increase in cancellous bone volume by 12 weeks exercise was higher in the DTM than that in the PTM (43.4% and 24.0%, respectively). On the other hand, the exercise did not significantly affect cancellous bone volume and bone formation in the L5 vertebral body, although the cortical periosteal bone formation rate and the L5 vertebral bone mass were increased. These findings suggest that cancellous bone adaptation to treadmill exercise is site specific, and the effect may be influenced by factors such as mechanical loading and metaphyseal bone architecture in the young growing rat.

Conditions such as estrogen deficiency, skeletal unloading, and aging have all been demonstrated to have various effects on the proliferation and differentiation of bone marrow stroma-derived osteoprogenitor cells. Here we have sought to examine the effects of pituitary hormone deficiency on the proliferation and the differentiation of these osteoprogenitor cells using the hypophysectomized (HX) rat as a model. In the present study, we use an in vitro culture system to examine the effects of HX on the osteogenic potential of rat bone marrow stroma. With the intact animal as a control, we used [3H]thymidine incorporation and cell number as indexes of proliferation. We also measured alkaline phosphatase enzyme activity, relative levels of osteocalcin expression with RT-PCR, and osteopontin and bone sialoprotein steady-state levels by Northern blot to delineate the effect on differentiation. Our results indicate that osteoprogenitor cells exposed to a pituitary hormone-deficient environment in vivo demonstrate an enhanced proliferative capacity and also exhibit an augmented expression of differentiation markers when exposed to an optimal environment in vitro.

Total body calcium (TBCa) in 270 black and white women age 21-79 years was measured concurrently by delayed gamma neutron activation analysis (DGNA) and dual-energy X-ray absorptiometry (DXA). The mean value for TBCa calculated from DXA was 933 g compared with 730 g for DGNA. By regression, TBCa(DXA(g)) = 1.35 x TBCa(DGNA(g)) -54 (r = 0. 90, r(2) = 81.4%, SEE = 66.9 g). This remarkable difference of 203 g suggests that one or both these methods is not accurate. Adjustment of the regression of DXA versus DGNA for body mass index or trunk thickness explained 8.5-10% of the variability between methods. The unadjusted slope for the DXA values regressed against the DGNA values was 1.35, indicating significant discordance between the methods. There is greater agreement between the two DGNA facilities (Brookhaven National Laboratory and Baylor College of Medicine) and between the various DXA instruments. Either DGNA underestimates TBCa or DXA overestimates total-body bone mineral content. Resolution of these disparate results may possibly be achieved by concurrent measurement of whole human cadavers of different sizes with chemical determination of the calcium content of the ash. In the interim, cross-calibration equations between DGNA and standardized values for DXA for total-body bone mineral content may be used, which will permit reporting of consistent values for TBCa from the two technologies.