Elevated levels of vitamin D-dependent calcium-binding protein (calbindin-D9k) in the osteosclerotic (oc) mouse

The osteosclerotic (oc) mouse is an osteopetrotic mutation that has recently been identified as having rickets associated with its osteopetrosis. The presence of this rachitic lesion, unexplainable from a nutritional standpoint, prompted an investigation into the vitamin D endocrine system in these animals. The developmental appearance of vitamin D-dependent calcium-binding protein (calbindin-D9k) and alkaline phosphatase was studied in oc mutant and normal mice from birth to weaning, as were serum concentrations of 25-hydroxyvitamin D3 (25OHD3), 1,25-dihydroxyvitamin D3 1,25-(OH)2D3], calcium, and phosphorus. Intestinal and renal calbindin-D9k levels were markedly and precociously elevated (4- to 9-fold) in young suckling, but not newborn, mutant mice compared to values in normal controls. Serum 25OHD3 levels were very low to undetectable in 2-week-old mutant mice compared to normal values, while 1,25-(OH)2D3 levels were 6 times higher in mutants. The exact cause of this premature induction in mutants is unknown, but may be due to elevated circulating levels of 1,25-(OH)2D. Alkaline phosphatase activity was similar between phenotypes at all ages. These studies indicate that the rachitic lesion present in oc mutants may be the result of some inherited disorder in vitamin D metabolism in these animals. Alternatively, these data are also consistent with a normal appropriate response to hypocalcemia and hypophosphatemia resulting from decreased osteoclastic bone resorption.