SYNTHESIS OF DIHYDROXYCHOLECALCIFEROLS IN MATERNAL AND FETAL ORGANS OF THE RAT IN THE LAST DAYS OF GESTATION

The in vitro synthesis of (3)H1,25(OH)(2)D-3 and (3)H24,25-(OH)(2)D-3 in different maternal (kidney and spleen), placental (maternal and fet al sides) and fetal (kidney, intestine, liver and skeleton) tissues an d the relative contribution of each of these organs to total productio n was studied in the last six days of gestation in rats. On day 16, sy nthesis of both metabolites was higher in fetal tissues than in matern al kidney, and decreased as gestation advanced. On day 16, placental c ontribution represented more than 50% of the total production of (3)H1 ,25(OH)(2)D-3, while the maternal kidneys and the fetal tissues contri buted only 16% and 26%, respectively. On day 18, the synthesis of (3)H 1,25(OH)(2)D-3 by maternal placenta and fetal tissues was significantl y reduced in comparison with that observed on day 16. Between days 16 and 19, the plasma concentrations of 1,25(OH)(2)D in mothers and fetus es were associated with the magnitude of its in vitro production. Star ting on day 19, however, the in vitro production remained at the same level while the plasma concentration increased, suggesting lower utili zation or lower catabolism of this metabolite. Similarly, the total sy nthesis of (3)H24,25(OH)(2)D-3 decreased on day 19. Between days 16 an d 18, a higher synthesis of (3)H24,25(OH)(2)D-3 corresponded with lowe r plasma concentration of this metabolite suggesting greater utilizati on. In contrast, between days 19 and 21, the in vitro synthesis and pl asma concentration of 24,25(OH)(2)D increased in paralel fashion. in s ummary we report the following findings: a) inhibition of the in vitro synthesis of (3)H1,25(OH)(2)D-3 and (3)H24,25(OH)(2)D-3 on day 19 of gestation in the rat; b) the contribution of each of the different mat ernal, placental and fetal tissues to the total synthesis of these met abolites in the last six days of gestation; and c) a paralelism betwee n in vitro production and plasma concentration of both metabolites.