1 , Z & Dihydroxyvitamin D 3 Increases Citrate Secretion from Osteosarcoma Cells

Rat osteosarcoma cells respond to 1,25-dihydroxyvitamin D3 with a 6to lo-fold increase in the secretion of citric acid. The time required to attain a half-maximal response is 12 h, a time course which is consistent with the postulated steroidal hormone action of this vitamin D metabolite. The citrate response is achieved by physiological concentrations of 1,25-dihydroxyvitamin Ds, with half of the maximal response at a vitamin concentration of 0.03 rig/ml. Both the time course and the dose dependence of the citrate response closely parallel the previously reported stimulation of bone Gla protein synthesis by 1,25-dihydroxyvitamin DS in these cells. Citrate and bone Gla protein bind avidly to bone mineral and are numerically the most abundant organic acid and protein in bone. The parallel secretion of both in 1,25-dihydroxyvitamin Da-treated osteoblastic cells suggests that they may act in tandem to mediate an action of this vitamin D metabolite on the mineral phase of bone.