Aluminum deposition at the osteoid-bone interface. An epiphenomenon of the osteomalacic state in vitamin D-deficient dogs

Abstract

Although aluminum excess is an apparent pathogenetic factor underlying osteomalacia in dialysis-treated patients with chronic renal failure, the mechanism by which aluminum impairs bone mineralization is unclear. However, the observation that aluminum is present at osteoid-bone interfaces in bone biopsies of affected patients suggests that its presence at calcification fronts disturbs the cellular and/or physiochemical processes underlying normal mineralization. Alternatively, aluminum at osteoid-bone interfaces may reflect deposition in preexistent osteomalacic bone without direct effects on the mineralization process. We investigated whether aluminum accumulates preferentially in osteomalacic bone and, if so, whether deposition of aluminum occurs at calcification fronts and specifically inhibits mineralization. Aluminum chloride (1 mg/kg) was administered intravenously three times per week for 3 wk to five normal and five vitamin D-deficient osteomalacic dogs. Before administration of aluminum the vitamin D-deficient dogs had biochemical and bone biopsy evidence of osteomalacia. Bone aluminum content in the osteomalacic dogs (15.1 +/- 2.2 micrograms/g) and the plasma aluminum concentration (10.4 +/- 2.1 micrograms/liter) were no different than those of normal dogs (10.5 +/- 3.5 micrograms/g and 11.9 +/- 1.2 microgram/liter, respectively). After the 3 wk of aluminum administration the plasma phosphorus, parathyroid hormone, and 25-hydroxyvitamin D concentrations were unchanged in normal and vitamin D-deficient dogs. Similarly, no alteration in bone histology occurred in either group. In contrast, bone aluminum content increased to a greater extent in the vitamin D-deficient dogs (390.3 +/- 24.3 micrograms/g) than in the normal dogs (73.6 +/- 10.6 micrograms/g). Moreover, aluminum localized at the osteoid-bone interfaces of the osteomalacic bone in the vitamin D-deficient dogs, covering 42.9 +/- 9.2% of the osteoid-bone surface. Further, in spite of continued aluminum chloride administration (1 mg/kg two times per week), vitamin D repletion of the vitamin D-deficient dogs for 11 wk resulted in normalization of their biochemistries. In addition, while normal dogs maintained normal bone histology during the period of continued aluminum administration, vitamin D repletion of the vitamin D-deficient dogs induced healing of their bones. Indeed, the appearance of aluminum in the cement lines of the healed bones indicated that mineralization had occurred at sites of prior aluminum deposition. These observations illustrate that aluminum deposition in osteomalacic bone may be a secondary event that does not influence bone mineralization. Thus, although aluminum may cause osteomalacia in chronic renal failure, its presence at mineralization fronts may not be the mechanism underlying this derangement.