1,25-Dihydroxyvitamin D3 decreases alkaline phosphatase activity in cultures of embryonic chick tibiae

The influence of 1,25-dihydroxyvitamin D3 1,25(OH)2D3] on the alkaline phosphatase (AlPase) activity in cultures of chick embryo tibiae was determined. A dose-related, decreased release (30-47%) of AlPase from the bones was seen with the metabolite at 0.05-0.5 ng/ml of medium with a similar effect on the bone content of enzyme. The highest dose (1 ng/ml) decreased the bone content by 38% without further effect on AlPase release. Combining a low level of 1,25(OH)2D3 (0.05 ng/ml) with parathyroid hormone (PTH, 1 U/ml) reduced release of enzyme additively, but caused no greater decrease in bone content of activity than PTH alone. No effects of 24,25-dihydroxyvitamin D3 24,25(OH)2D3, 0.5 ng/ml] on release or bone content of AlPase were found when this metabolite was added alone or in combination with PTH; however, 24,25(OH)2D3 did prevent the inhibition of release of AlPase when added with 1,25(OH)2D3. After a 1-day exposure to 1,25(OH)2D3, continued incubation in metabolite-free medium resulted in an 89% increase in bone content of AlPase. The results suggest that 1,25(OH)2D3, as well as PTH, may have regulatory roles in bone growth through their effects on AlPase.