Lack of biological activity of vitamin D3-3 beta sulfate during lactation in vitamin D-deficient rats.

Sulfoconjugated vitamin D has been claimed to have an important antirachitic activity and to be present at higher amounts than free vitamin D in maternal milk. We have previously shown that vitamin D3-3 beta sulfate (SD3) administration has little effect on calcium and bone metabolism during pregnancy in rats. In the present work, we have compared the biological activity of free vitamin D3 (D3) and SD3 during the lactation period. After delivery, D-depleted (-D) female rats were orally treated with D3 or SD3 (1,300 pmoles/every two days) during 20 days of lactation. Vitamin D status was determined before, during and at the end of the treatment for mother rats and at days 1 and 20 of life in suckling pups. Both mothers and pups were sacrificed at day 20 of lactation and subjected to hormonal and mineral determinations and to histomorphometric analysis of bone metabolism. After 12 days of SD3 treatment, mother rats showed a slight but significant elevation in plasma concentrations of calcium phosphorus and vitamin D metabolites. This effect was reversed at the end of lactation; at this time most maternal plasma parameters did not differ from those observed in -D non-treated mothers. By contrast, 20 days of D3 administration in mothers normalized plasma biochemical parameters. These results were confirmed by analysis of both static and dynamic parameters of bone formation. Maternal SD3 treatment did not improve either plasma biochemical or histological parameters of bone formation and resorption in suckling pups which remained comparable to that of D-deficient pups; by contrast, pups from D3-treated mothers normalized most biochemical plasma parameters although bone metabolism remained abnormal. In conclusion, the biological activity of SD3 on bone and mineral metabolism during lactation in rats is as low as in the nonreproductive stages.