Role of 1,25-(OH)2D3 during pregnancy; studies with pigs suffering from pseudo-vitamin D-deficiency rickets, type I.

Biochemical parameters of Ca homeostasis in fetal and maternal plasma and placental transfer of Ca, Pi and vitamin D metabolites were measured during pregnancy and at parturition. Control pigs and pigs with inherited pseudo-vitamin D-deficiency rickets, type I (PVDR), which are devoid of renal calcitriol production, were used. Although sows with PVDR normally require treatment with massive doses of vitamin D at intervals of 4 weeks to stabilize Ca and Pi concentrations in plasma, these plasma parameters tended to normalize during the first 3 months of pregnancy when the vitamin D treatment was discontinued about 4 weeks before conception. In homozygote sows plasma calcitriol concentrations remained unphysiologically low and the 25OHD3 concentrations increased steadily during pregnancy reaching higher levels than those found in control sows. Despite hypocalcaemia and hypophosphataemia of the homozygote sows at term, fetal Ca and Pi concentrations were normal. This demonstrated a higher feto-maternal concentration gradient in PVDR pigs than in control pigs. It is concluded that maintenance of Ca and Pi homeostasis of the feto-maternal system, including active transplacental transport in pigs, is at least partly independant from calcitriol.