CHRONIC EXTRACELLULAR ACIDOSIS INDUCES PLASMALEMMAL VACUOLAR-TYPE H-ATPASE ACTIVITY IN OSTEOCLASTS()

Proton extrusion into an extracellular resorption compartment is an es sential component of bone degradation by osteoclasts. Chronic metaboli c acidosis is known to induce negative calcium balance and bone loss b y stimulating osteoclastic bone resorption, but the underlying mechani sm is not known. The present studies were undertaken to evaluate wheth er chronic acidosis affects proton extrusion mechanisms in osteoclasts cultured on glass coverslips. Acidosis, mimicked experimentally by ma intaining the cells at extracellular pH 6.5, rapidly lowered intracell ular pH to 6.8. However, after 2 hours, a proportion of cells demonstr ated the capacity to restore intracellular pH to near normal levels, T o define the mechanism responsible for this recovery, the activity of individual H+ transport pathways was analyzed. We found that chronic a cid treatment for up to 6 h did not significantly affect the cellular buffering power or Na+/H+ antiport activity, In contrast, chronic acid osis activated vacuolar H+ pumps in the osteoclasts. Although only sim ilar to 5% of the control cells displayed proton pump activity, about 40% of cells kept at extracellular pH 6.5 for 4-6 h were able to recov er from the acute acid load by means of bafilomycin A(1)-sensitive pro ton extrusion, Conversely, the H+-selective conductance recently descr ibed in the plasma membrane of osteoclasts was clearly inhibited in th e cells exposed to chronic acidosis, Following acid treatment, the act ivation threshold of the H+ conductance was shifted to more positive p otentials, and the current density was significantly reduced. Consider ed together, these results suggest that induction of plasmalemmal vacu olar type ATPase activity by chronic acidosis, generated either system ically due to metabolic disease or locally at sites of inflammation, i s likely to stimulate osteoclastic bone resorption and thus to promote bone loss.