Osteoclast Ion Channels: Potential Targets for Antiresorptive Drugs

This review summarizes the types of ion channels that have been identified in osteoclasts and considers their potential as targets for therapeutic agents aimed at the treatment of osteoporosis and other bone disorders. We focus on channels that have been identified using molecular and electrophysiological approaches. Numerous ion channels have been characterized, including K(+), H(+), Na(+), nonselective cation and Cl(-) channels. K(+) channels include an inward rectifier K(+) channel (Kir2.1) that is regulated by G proteins, and a transient outward rectifier K(+) channel (Kv1.3) that is regulated by cell-matrix interactions and by extracellular cations such as Ca(2+) and H(+). In addition, two classes of Ca(2+)-activated K(+) channels have been described--large and intermediate conductance channels, which are activated by increases of cytosolic Ca(2+) concentration. Other channels include stretch-activated nonselective cation channels and voltage-activated H(+) channels. A recent revelation is the presence of ligand-gated channels in osteoclasts, including P2X nucleotide receptors and glutamate-activated channels. Osteoclasts also exhibit an outwardly rectifying Cl(-) current that is activated by cell swelling. Kir2.1 and Cl(-) channels may be essential for resorptive activity because they provide pathways to compensate for charge accumulation arising from the electrogenic transport of H(+). As in other cell types, osteoclast ion channels also play important roles in setting the membrane potential, signal transduction and cell volume regulation. These channels represent potential targets for the development of antiresorptive drugs.