No abstract

Disorders in which magnesium (Mg) depletion is common have an associated high incidence of osteoporosis. Mg depletion in humans results in hypocalcemia, low serum parathyroid hormone (PTH) and 1,25(OH)₂-vitamin D levels, as well as PTH and vitamin D resistance which may serve as mechanisms for the development of osteoporosis. In order to determine if isolated Mg depletion will result in bone loss, we have induced dietary Mg deficiency in the rat. Adult (290 g) female rats were given either a low-Mg diet (2 mg/100 g chow; n = 6) or a normal control Mg diet (63 mg/100 g chow; n = 6). Dietary calcium (Ca) was normal in both groups (592 mg/100 g chow). At 12 weeks, blood was obtained for serum Mg, Ca, PTH, 1,25(OH)₂-vitamin D, and osteocalcin determinations. The rats were then euthanized and the femurs obtained for mineral analysis and histomorphometry. Serum Mg in the low-Mg group was less than control (0.4 ± 0.2 vs. 1.9 ± 0.2 mg/dl, p < 0.001; mean ± SD) while serum Ca was higher (11.7 ± 0.5 vs. 9.3 ± 0.4 mg/dl, p < 0.001). PTH was suppressed in the Mg-deficient group (36 ± 16 vs. 109 ± 30 pg/ml in controls, p < 0.002). Serum 1,25(OH)₂-vitamin D was also suppressed in the Mg-deficient animals (7.1 ± 4.8 vs. 28.5 ± 8.2 pg/ml in controls, p < 0.002). Serum osteocalcin levels were not different (19.8 ± 2.5 ng/ml in Mg-deficient rats vs. 15.3 ± 3.4 ng/ml in controls). While the ash weight of Ca and phosphorus in the femur did not change, the ash weight of Mg fell (low-Mg group 0.55 ± 0.01%, controls 0.65 ± 0.02%, p < 0.001). Histomorphometry demonstrated reduction in bone mass; the trabecular bone volume in the femur of the low-Mg group was reduced from control (7.7 ± 0.2 vs. 13.7 ± 1.9%, p < 0.002). A surprising new observation was an increase in osteoclast (OC) bone resorption with Mg depletion. The number of OC per millimeter bone surface was 16.9 ± 1.3 in the low-Mg group versus 7.8 ± 1.5 in controls (p < 0.001). The percentage of bone surface occupied by OC was 38.3 ± 3.7 in the low-Mg group versus 17.7 ± 2.4 in controls (p < 0.001). This increased resorption occurred with an inappropriate non-altered bone-forming surface relative to control (% osteoid surface: low-Mg group 2.4 ± 0.7 vs. controls 2.6 ± 0.4; % osteoid volume: low-Mg group 0.25 ± 0.09 vs. controls 0.38 ± 0.06; number of osteoblasts per millimeter bone surface: low-Mg group 0.9 ± 0.3 vs. controls 1.3 ± 0.3). No increase in bone-forming surface or osteoblast number despite an increase in OC-resorbing surface and OC number strongly suggests impaired activation of osteoblasts and an uncoupling of bone formation and bone resorption. Our data demonstrate that Mg depletion in the rat alters bone and mineral metabolism which results in bone loss.

No abstract

Undemineralized methacrylate embedded bone biopsies and other bone specimens can be processed much more rapidly by application of acidified 2,2-dimethoxypropane (DMP) dehydration, which requires two hours, than by traditional graded ethanol dehydration, which requires at least four days. This shortened processing time is valuable when biopsy results are urgently needed to detect osteomalacia or to determine bone features prior to possible parathyroidectomy. We have processed over 200 bone specimens with DMP and have compared DMP dehydration to graded ethanol dehydration in 11 biopsies in which two plugs were available from the same patient. DMP dehydration does not compromise the following: tetracycline retention, Goldner's stain, acid phosphatase localization or histochemical identification of aluminum. Cement lines, which provide a record of past remodelling, are useful in clinical interpretation of bone biopsies. We have adapted two stains, toluidine blue and methylene blue/basic fuchsin, for improved cement line identification. Five-micrometer sections individually demineralized in acetate buffer prior to cement line staining show best results with toluidine blue at pH 5.5 and with methylene blue/basic fuchsin at pH 2.5-3.5.