Effect of calcitonin in pregnant rats on bone resorption in fetuses

Fetal bone resorption was measured by an organ culture technique using fetuses from intact or thyroparathyroidectomized pregnant rats. These experiments were performed to investigate the effects of 1,25-dihydroxycholecalciferol (1,25-DHCC) and salmon calcitonin (SCT) in pregnant rats, on both fetal growth and fetal bone resorption. Pregnant rats were given 0.1-0.5 microgram 1,25-DHCC per day from day 17 of gestation: in intact rats bone resorption was increased and fetal growth decreased; 1,25-DHCC probably modified fetal bone resorption in the absence of fetal parathyroid secretion. Infusion of SCT in minipumps (30 mu./h) did not modify plasma calcium levels in either the mother or fetuses, neither was bone resorption altered. In 1,25-DHCC-treated rats, SCT infusion resulted in an increase in fetal weight and a decrease in fetal bone resorption. On the other hand, SCT infusion was found to facilitate phosphate accumulation in fetuses. At the end of the SCT infusion the SCT concentration was 450 ng/l in maternal plasma and 553 +/- 60 ng/l in fetal plasma. Salmon calcitonin was shown to cross the placental barrier in the rats; it may interact with the effects of 1,25-DHCC in the fetus.