IN-VIVO PARATHYROID-HORMONE STIMULATES IN-VITRO BONE-RESORPTION BY BOVINE MONOCYTES

Cells of the monocyte-macrophage lineage have been thought to play a r ole in bone resorption. We examined the effects of in vivo administrat ion of parathyroid hormone and 1,25-dihydroxyvitamin D-3 on the abilit y of monocytes to degrade bone in vitro. Administration of parathyroid hormone for 4 d resulted in sustained hypercalcemia and a transient 1 -d increase in plasma 1,25-dihydroxyvitamin D-3. Parathyroid hormone s ignificantly stimulated bone degradation by monocytes 2.6 times more t han that of pretreatment controls. Parathyroid hormone treatment signi ficantly enhanced (threefold) release of superoxide anion by monocytes stimulated with phorbol 12-myristate 13-acetate and increased migrati on of monocytes to bone particles in vitro. Continuous 7-d infusion of 1,25-dihydroxyvitamin D-3 (50 mu g/d) elevated plasma 1,25-dihydroxyv itamin D-3 until infusions were discontinued. Increased 1,25-dihydroxy vitamin D-3 was associated with hypercalcemia, which continued for sev eral days postinfusion. In vivo administration of 1,25-dihydroxyvitami n D-3 did not affect in vitro ability of monocytes to degrade bone. We concluded that in vivo administration of parathyroid hormone enhanced in vitro responsiveness of isolated monocytes in a manner consistent with a role for monocytes in bone remodeling. Furthermore, these data suggested that circulating monocytes could be a useful experimental mo del for further studies on parathyroid hormone responsiveness and bone resorption for the cow with milk fever.