Identification and characterization of a sodium/calcium exchanger, NCX-1, in osteoclasts and its role in bone resorption

We provide the first demonstration for a Na+/Ca2+ exchanger, NCX-1, in the osteoclast. We speculate that by using Na+ exchange, NCX-1 couples H+ extru sion with Ca2+ fluxes during bone resorption. Microspectrofluorimetry of fu ra-a-loaded osteoclasts revealed a rapid and sustained, but reversible, cyt osolic Ca2+ elevation upon Na+ withdrawal. This elevation was abolished by the cytosolic introduction (by gentle permeabilization) of a highly specifi c Na+/Ca2+ exchange inhibitor peptide, XIP, but not its inactive analogue, sXIP. Confocal microscopy revealed intense plasma membrane immunofluorescen ce with an isoform-specific monoclonal anti-NCX-1 antibody applied to gentl y permeabilized osteoclasts. Electrophysiological studies using excised out side-in membrane patches showed a low-conductance, Na+-selective, dichlorob enzamil-sensitive, amiloride-insensitive channel that we tentatively assign ed as being an NCX. Finally, to examine for physiological relevance, an ost eoclast resorption (pit) assay was performed. There was a dramatic reductio n of bone resorption following NCX-1 inhibition by dichlorobenzamil and XIP (but not with S-XIP). Together, the results suggest that a functional NCX, likely NCX-1, is involved in the regulation of osteoclast cytosolic Ca2+ a nd bone resorption. (C) 2001 Academic Press.