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Nephrology Unit, Department of Medicine, University of Rochester School of Medicine, Rochester, New York 14642
Chronic metabolic
acidosis induces net calcium efflux from bone mineral through an
increase in osteoclastic resorption and a decrease in osteoblastic
matrix deposition and mineralization. To determine the effects of
chronic metabolic acidosis on the expression of genes necessary for
mineralization, we grew primary bone cells, which are principally
osteoblasts, to confluence in neutral pH (7.5) medium and then switched
the cells either to a neutral pH or to an acidic pH (7.1)
differentiation medium. Cells were harvested for RNA at 4- to 7-day
intervals for up to 44 days. By 36 days, there was extensive bone
nodule formation and mineralization in cells cultured in neutral
medium; however, there was a substantial decrease in nodule formation
and mineralization in cells cultured in acidic medium. There was a
marked increase in matrix Gla protein RNA and an increase in
osteopontin RNA in neutral cultures; however, acidic medium almost
completely prevented any increase. In contrast, RNA levels for
osteonectin and transforming growth factor-
1 were not altered by
chronic acidosis. Additional cells were incubated in acid
differentiation medium for 1, 2, or 3 wk and then transferred to
neutral medium; in each case, there was recovery of matrix Gla protein
RNA and osteopontin RNA expression. Still other cells were incubated in
neutral differentiation medium for 1, 2, or 3 wk and then transferred
to acid medium; in each case there was inhibition of matrix Gla protein
RNA and osteopontin RNA expression. Thus metabolic acidosis appears to specifically inhibit RNA accumulation of certain genes whose products may be essential for formation of mature bone matrix.
calcium; bone; hydrogen ion; mineralization
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