No abstract

Local administrations of prostaglandins E1 or E2 combined with orthodontic tooth movement can approximately double the rate of tooth movement in monkeys compared to that of the control. Macroscopically, no side-effect was observed in the gingiva and associated structures.

Bone metabolism is often affected by a variety of mechanical forces, but the cytological basis of their action is not known. In this study, we examined the effect of a continuously applied compressive pressure (CCP) on the growth and differentiation of clonal mouse osteoblast-like cells (MC3T3-E1) cultured in a specifically devised culture chamber. The gas phase of the chamber was maintained at a pressure of 2 atmospheres (atm) above ambient (3 atm total, 3.1 kg/cm2; 3.0 x 10(5) Pa) by continuously infusing a compressed mixed gas (O2: N2:CO2 = 7.0%:91.3%:1.7%). The pO2, pCO2, and pH in the culture medium at 37 degrees C under 3 atm were maintained at the same levels as those under 1 atm. MC3T3-E1 cells were cultured in alpha-minimal essential medium containing 10% fetal bovine serum under either 3 atm in the CCP culture chamber or 1 atm in an ordinary CO2 incubator. Alkaline phosphatase activity, a marker of osteoblasts, was greatly suppressed by the CCP treatment. The inhibition of alkaline phosphatase activity was rapidly restored when the cells were transferred to an ordinary CO2 incubator under 1 atm, indicating that the inhibition of alkaline phosphatase activity by CCP is reversible. Cell growth was not altered under CCP. The CCP treatment greatly increased the production and secretion of prostaglandin E2 (PGE2). Adding either conditioned medium from the CCP culture or exogenous PGE2 to the control culture under 1 atm suppressed alkaline phosphatase activity dose-dependently. The CCP treatment also suppressed collagen synthesis and calcification. These results suggest that CCP causes the cells to produce and secrete PGE2, which, in turn, inhibits differentiation of osteoblasts and the concomitant calcification.

A cell culture system was designed to study the effect of an electric field on bone cells. This system subjected cultured bone cells to a series of high-voltage pulses at varying amplitudes of electric field without changing the ion concentrations of the culture medium. When mouse osteoblast-like cells (MC3T3-E1) were stimulated for 5 min to 20 hr by an electric field (3.19 kV/m, 3 msec, 10 Hz pulses), DNA synthesis was greatly increased. The stimulation occurred only during the growth phase. The electric field also increased incorporation of 45Ca into the cells, but it did not stimulate cAMP production. Adding calcium ionophore A23187 stimulated both 45Ca uptake and DNA synthesis, but dibutyryl cAMP did not stimulate either of them. These results suggest that the electric field stimulates the DNA synthesis of growing osteoblasts by a mechanism involving calcium ions.

In previous research, we devised a specific culture chamber to examine the effect of continuously applied compressive pressure (CCP) on bone formation and resorption. The chamber was infused with compressed mixed gases with different O2 and CO2 composition to maintain the pO2, pCO2, and pH in the culture medium under pressures of +0.5 atm (1.5 atm total) to +2.0 atm (3.0 atm total) at the same levels as those at the ordinary pressure (1 atm). Using the specific culture chamber, we demonstrated that CCP greatly suppressed the differentiation of mouse osteoblast-like MC3T3-E1 cells. The inhibition by CCP appeared to be mediated by prostaglandin E2 (PGE2). In the present study, we examined the effect of CCP on osteoclastic bone resorption. CCP treatment of mouse bone marrow culture markedly increased both the PGE2 production and the number of tartrate-resistant acid phosphatase (TRACP)-positive mononuclear cells (possibly precursors of multinucleated osteoclasts). An autoradiographic study using [125I]-salmon calcitonin showed clearly that those TRACP-positive cells had calcitonin receptors. The CCP effect was the greatest at +1.0 atm (2.0 atm total). Isobutylmethylxanthine potentiated the production of TRACP-positive cells induced by CCP. Adding indomethacin completely inhibited both the TRACP-positive cell formation and the PGE2 production induced by CCP. CCP also increased the release of 45Ca from prelabeled mouse calvaria during later stages (2-6 days) of the 6-day culture period. CCP markedly increased PGE2 but not interleukin 1 in the culture media of mouse calvaria. These results indicate that, besides inhibiting osteoblast differentiation, CCP stimulates bone resorption by generating new osteoclasts through a mechanism involving PGE2 production.