CELL-MEDIATED MINERALIZATION IN CULTURE AT LOW-TEMPERATURE ASSOCIATEDWITH SUBTLE THERMOGENIC RESPONSE

In both the growth plate and in marrow stromal cell cultures cell-medi ated mineralization is preceded by characteristics of anaerobic and lo w efficiency energy metabolism. Reagents that increase mineralization like malonate and dexamethasone (DEX) also increase the mitochondrial membrane potential (MtMP) especially 1 week after DM stimulation. Cont rarily, levamisole, which decreases mineralization, also decreases MtM P. Modulation of MtMP and energy metabolism could be linked to regulat ion of mineralization by the uncoupling of oxidative phosphorylation. This uncoupling should be associated with thermogenesis in cells that induce mineralization. We examined whether cold temperature affects mi neralization, and whether cellular thermogenesis takes place at cold t emperature in parallel to changes in MtMP. Osteoprogenitor cells (OPC) induced, in DM stimulated rat marrow stroma, higher mineralization at 33 degrees C than at 37 degrees C. Increased mineralization by cold t emperature required long incubation since incubation in the cold durin g short intervals, 3-4 days, did not increase mineralization relative to (37 degrees C) controls. Marrow stromal cells in the presence of va linomycin responded to incubation at 33 degrees C by retaining all the vital dye after 4 h, unlike the cells at 37 degrees C; however, after 24 h the level of dye retention at 33 degrees C was the same as at 37 degrees C. The delayed response of the temperature-dependent (> 37 de grees C) K+ ionophor to incubation in the cold indicated that certain cells may respond to low temperature by local intracellular heating, a nd by heat conduction to the plasma membrane. DEX-stimulated stromal c ells, unlike unstimulated cells, showed increased mitochondrial rhodam ine 123 retention in the presence of valinomycin after 24 h in the col d, which corresponds to day 4 of OPC induction. This is consistent wit h the concept that valinomycin-induced cell damage is mediated by (col d-induced) local heating. The mechanism of this cell damage should sel ectively prefer nonthermogenic (rhodamine retaining) over thermogenic (rhodamine leaking) cells such as OPC. At cold temperature DEX-stimula ted stromal cells showed the best anti-OPC selection under exposure to valinomycine between days 3-7, concurrent with the period of rhodamin e leakage from the mitochondria These results indicate that thermogene sis is enhanced during the period of low MtMP in mineralizing cells, a nd prolonged exposure to cold increases mineralization also due to ind uction of subtle thermogenesis. (C) 1996 Wiley-Liss, Inc.