ACID EXTRUSION IS INDUCED BY OSTEOCLAST ATTACHMENT TO BONE INHIBITIONBY ALENDRONATE AND CALCITONIN

Acid extrusion is essential for osteoclast (OC) activity, We examined Na+ and HCO3- -independent H+ extrusion in rat- and mouse OCs by measu ring intracellular pH (pH(i)) changes, with the pH(i) indicator BCECF (biscarboxyethyl-5-(6) carboxyfluorescein) after H+ loading with an am monium pulse, 90% of OCs attached to glass do not possess HCO3- and Na +-independent H+-extrusion (rate of pH(i) recovery = 0.043+/-0.007 (SE M) pH U/min, n = 26), In contrast, in OCs attached to bone, the pH(i) recovery rate is 0.228+/-0.011 pH(i) U/min, n = 25. OCs on bone also p ossess a NH4+-permeable pathway not seen on glass, The bone-induced H extrusion was inhibited by salmon calcitonin (10(-8) M, for 2 h), and was not present after pretreating the bone slices with the aminobisph osphonate alendronate (ALN). At ALN levels of 0.22 nmol/mm(2) bone, H extrusion was virtually absent 12 h after cell seeding (0.004+/-0.002 pH U/min) and similar to 50% inhibition was observed at 0.022 pmol AL N/mm(2) bone, The Na+-independent H+ extrusion was not inhibited by ba filomycin A(1) (up to 10(-7) M), although a bafilomycin A(1) (10(-8) M )-sensitive H+ pump was present in membrane vesicles isolated from the se osteoclasts, These findings indicate that Na+-independent acid extr usion is stimulated by osteoclast attachment to bone and is virtually absent when bone is preincubated with ALN, or when osteoclasts are tre ated with salmon calcitonin.