A voltage-gated H+ channel is a powerful mechanism for pH homeostasis in murine osteoclasts

Osteoclasts secrete a large amount of proton (H+) ions and proteolytic enzymes into bone resorption pit to degrade bone matrix. In addition to H+ pumps and exchangers, voltage-gated H+ channels, which are H+ conductive pathways, are expressed in osteoclasts. H+ channels are distinct in their strong H+ extrusion ability, but the functional role is not clear. This is the first study of H+ channels in murine osteoclasts generated from mononuclear precursors in the presence of a soluble form of receptor activator of nuclear factor kappa B ligand (RANKL) and macrophage colony stimulating factor (M-CSF). The H+ channel was characterized by voltage- and pH-dependent activation, slow activation kinetics, and outward rectification. The reversal potential (Vrev) was shifted to more positive potentials by decreasing the pH gradient across the plasma membrane (deltapH). Employing Vrev as a real time monitor of pH in clamped cells, it is revealed that the H+ channel activation could decrease deltapH by approximately 0.43 unit/s. Decline in the current during prolonged depolarizations was accompanied by a positive shift in Vrev. This implies that the H+ channel activity is auto-regulated by sensing deltapH, to compensate for pH imbalance. The H+ current-density in cells with 36 nuclei was significantly smaller than that in cells with < or =5 nuclei, although the activation rate was unaltered. Thus, the H+ channel activity may alter during osteoclastogenesis. These data suggest the H+ channel is a powerful mechanism for pH homeostasis of osteoclasts that are exposed to drastic change in pH environments during the bone resorption cycle.