In humans, loss-of-function mutations in
parathyroid hormone (PTH) and 25-hydroxyvitamin D3-1alpha-hydroxylase [1alpha(
OH)ase] genes lead to isolated
hypoparathyroidism and
vitamin D-dependent
rickets type I, respectively. To better understand the relative contributions of PTH and
1,25-dihydroxyvitamin D3 [
1,25(OH)2D3] to skeletal and
calcium homeostasis, we compared mice with targeted disruption of the PTH or 1alpha(
OH)ase genes to the double null mutants. Although PTH-/- and 1alpha(
OH)ase-/- mice displayed only moderate
hypocalcemia, PTH-/-1alpha(
OH)ase-/- mice died of
tetany with severe
hypocalcemia by 3 weeks of age. At 2 weeks, PTH-/- mice exhibited only minimal dysmorphic changes, whereas 1alpha(
OH)ase-/- mice displayed epiphyseal dysgenesis which was most severe in the double mutants. Although reduced osteoblastic bone formation was seen in both mutants, PTH deficiency caused only a slight reduction in long bone length but a marked reduction in trabecular bone volume, whereas 1alpha(
OH)ase ablation caused a smaller reduction in trabecular bone volume but a significant decrease in bone length. The results therefore show that PTH plays a predominant role in appositional bone growth, whereas
1,25(OH)2D3 acts predominantly on endochondral bone formation. Although PTH and
1,25(OH)2D3 independently, but not additively, regulate osteoclastic
bone resorption, they do affect the renal
calcium transport pathway cooperatively. Consequently, PTH and
1,25(OH)2D3 exhibit discrete and collaborative roles in modulating skeletal and
calcium homeostasis and loss of the renal component of
calcium conservation might be the major factor contributing to the lethal
hypocalcemia in double mutants.