1,25(OH)2D and 24,25(OH)2D production in the developing kidney.

To clarify the state of vitamin D production by the developing kidney, firstly, we measured serum levels of 1,25(OH)2D and 24,25(OH)2D in humans of different ages (pregnant and nonpregnant women, adult males, children and newborn infants) and secondly, we measured 1 alpha- and 24-hydroxylase activity in the kidney mitochondria of rats at different ages. The mean serum levels of 1,25(OH)2D in pregnant women, cord blood and newborns were significantly higher than those in children and nonpregnant women and adult males. In newborns, the level increased with gestational age. Synthesis of 1,25(OH)2D was, at least in part, under the control of the fetus and newborn, rather then being solely a reflection of the conditions prevailing in the mother. The 1 alpha-hydroxylase activity in mitochondria was highest in the 1- to 2-month-old rats, and it decreased gradually thereafter. The change in 1 alpha-hydroxylase activity with age was due to a change in the Vmax of the system.