Studies on the effects of placental lactogen on calcium metabolism during pregnancy

In order to clarify the dynamics in maternal calcium metabolism during pregnancy, serum concentrations of ionized calcium, calcium regulating hormones and intestinal calcium absorption were measured in pregnant and hypophysectomized(HX) rats. Serum concentrations of ionized calcium decreased significantly late in pregnancy. Serum levels of 1 alpha,25-(OH)2 vitamin D3 (D3) increased late in pregnancy, however, those of parathyroid hormone (PTH) increased not significantly throughout pregnancy. Serum levels of calcitonin(CT) and intestinal calcium absorption increased as pregnancy progressed. Administration of human placental lactogen(hPL), bovine growth hormone(bGH) and ovine prolactin(oPRL) to the HX-rats remarkably enhanced intestinal calcium absorption. Serum concentrations of 1 alpha,25-(OH)2D3 significantly increased by administration of bGH and hPL to the HX-rats, but they did not increase significantly by oPRL administration. These data suggest that 1) maternal intestinal calcium absorption might be increased by the action of increased serum 1 alpha,25-(OH)2D3 and the maternal bone might be kept at the same density throughout pregnancy because serum CT protects the maternal skeleton by resisting the bone-resorption activities of 1 alpha,25-(OH)2D3; and 2) placental lactogen may play an important role on the increase of intestinal calcium absorption by stimulating the production of 1 alpha,25-(OH)2D3 during pregnancy. From these results, it is considered that these alterations of calcium metabolism in the maternal side are the rational responses to supply Ca to the fetus and newborn for keeping their calcium homeostasis.