OPPOSING EFFECTS ON MITOCHONDRIAL-MEMBRANE POTENTIAL BY MALONATE AND LEVAMISOLE, WHOSE EFFECT ON CELL-MEDIATED MINERALIZATION IS ANTAGONISTIC

The act of chondrocyte preparation for primary, enchondral, mineraliza tion is associated with a decline in mitochondrial respiration toward the end of the proliferative zone and the hypertrophic zone in the gro wth plate. Dexamethasone (Dex)-stimulated cultures of rat marrow strom a constitute a differentiation model simulating, in its energy metabol ism, chondrocyte mineralization. In this model, early inhibition of su ccinate dehydrogenase (SDH) enriches the culture with mineralizing cel ls, whereas levamisole inhibits mineralization. Dex also increases mit ochondrial membrane potential in stromal cells, especially on days 7-8 of stimulation. In the present study, suicide inhibition of SDH, by n itropropionic acid (NPA), in Dex-stimulated cells showed a dose-depend ent increase in day 21 mineralization; the maximal effect was induced on days 2-4 of stimulation. Mineralization under 2-day-long exposure t o NPA showed a similar trend to the previously studied effect of conti nuous exposure to malonate applied between days 3-11. Unlike malonate, the effect of NPA required its presence in the cultures for only 2 da ys and resulted in higher mineralization than that seen under 8 days o f malonate. NPA delineated a period, days 2/4 to 7/9, in which inhibit ion of succinate oxidation is necessary to augment mineralization. Dur ing this period, NPA also exhibited OPC selection capacity. Early appl ication of levamisole, under conditions previously shown to decrease d ay 21 mineralization, maintained mitochondrial membrane potential at t he beginning of Dex stimulation but decreased or had little effect on it during days 5-10. By contrast, malonate previously found to increas e day 21 mineralization decreased the membrane potential at the beginn ing of Dex stimulation but increased it later on day 7, or during days 5-10. These results indicate that during osteoprogenitor differentiat ion, before the mineralization stage, a surge in mitochondrial inner m embrane potential during late matrix maturation may be a marker that h eralds the extracellular matrix mineralization. (C) 1996 Wiley-Liss, I nc.