Effects of calcium and (inorganic) phosphate on bone resorption produced by vitamin D3 metabolites in vitro

The effects of changing the concentrations of calcium (Ca2+) and inorganic phosphate (Pi) on 45Ca release from prelabeled fetal rat bones caused by parathyroid hormone (PTH), 25-hydroxyvitamin D3 (25-OH-D3) and 1, 25-dihydroxyvitamin D3 (1,25-(OH)2-D3) were tested, in vitro. Increasing total (Ca2+) from 1.25 mM to 2.5 mM caused a significant increase in bone resorption caused by submaximal concentrations of these materials. Further increase in total (Ca2+) to 3.5 mM abolished this potentiating effect of bone resorption. Increasing the (Pi) from 1 mM to 2-3 mM caused significant inhibition of bone resorption caused by these resorbing materials. In addition, a (Pi) of 3 mM caused significant inhibition of higher concentrations of PTH and 25-OH-D3. (Pi) of 2 and 3 mM also caused significant inhibition of 45Ca release from control bones. It is concluded that a moderate increase in medium (Ca2+) mimics the effect of these bone resorbing materials on bone. Phosphate appears to be a direct inhibitor of bone resorption. Therefore, changing the (Ca2+) and (Pi) in the medium can affect the bone resorbing effect of vitamin D3 metabolites, similar to their effect on PTH.