Effects of vitamin D deprivation and 1,25-dihydroxyvitamin D3 treatment on ion release from rat tibiae in vitro.

Skeletal effects of dietary vitamin D and of 1,25-dihydroxyvitamin D3[1,25(OH)2D3] in vivo and in vitro were investigated using bones from vitamin D-deficient suckling rats. Tibiae from 19- to 21-day-old pups were incubated for up to 8 h in a defined medium and the net releases of Ca, inorganic phosphate (Pi) and Mg into and from the bones were determined. The pups were delivered and nursed by mothers fed either a diet containing vitamin D3 (+D) or a D-free diet (-D) starting on the sixth day of pregnancy. Effects of metabolic inhibition with iodoacetic acid (IAA, 1 mM); of elevated medium Pi concentration (2 mM); and of 1,25(OH)2D3 in vitro (2 and 10 ng/ml of medium) or in vivo (500 ng/mother ip, 2x) were evaluated. Although no effects of vitamin D on glucose metabolism or net Pi release were seen, tibiae from -D pups showed a marked decrease in net Ca release (up to 70%), while net Mg release was increased by 27%. IAA completely inhibited net Ca release in this system, had no effect on net Pi release, and reduced lactate production and net Mg release by 86 and 33%, respectively. Elevating the medium Pi concentration from 1 to 2 mM did not affect the differences between +D and -D bones. Treatment with 1,25(OH)2D3 in vitro and in vivo returned ion releases by the -D tibiae partly or completely to the +D levels. The results support the following conclusions: (1) vitamin D deficiency during suckling reduces net Ca release and increases net Mg release from bone which may contribute to the changes in circulating concentrations of these ions seen in D deficiency; (2) these effects on bone appear to be specific for Ca and Mg, since the vitamin has little or no effect on net Pi release; (3) net Ca and Mg release from rat bone in vitro presumably are energy-dependent processes, while net Pi release is not; and (4) 1,25(OH)2D3 can affect net Ca and Mg release from -D bones by a direct action.