In vivo metabolism of calcitriol in the pregnant rabbit doe

The production rate, the disappearance rate and the half life of calcitriol in gravid rabbit does at 24 days of gestation were compared, under unstressed steady state conditions, to those of nonpregnant animals. The contribution of the fetoplacental unit to the circulating levels of fetal calcitriol was also assessed. The calcitriol levels (139.6 +/- 19.9 vs 55.3 +/- 8.8 pmol/l and production rates (113.9 +/- 8.8 vs 59.2 +/- 9.2 pmol/min/Kg) were higher in pregnant than in nonpregnant animals (P less than 0.01). However, clearance rates (1.07 +/- 0.18 vs 1.12 +/- 0.20 ml/min/Kg and circulating half life (442 +/- 49 vs 368 +/- 35 min; NS) were similar in both groups of animals. Fetal levels (62.3 +/- 1.6 pmol/l) and specific activity (11166 +/- 864 dpm/pmol) of calcitriol were lower than those of the respective mothers (P less than 0.005). Taken together these data suggest that, calcitriol is transported through the placenta; and that the fetoplacental unit contributes to the fetal and perhaps to the maternal calcitriol levels.