Extracellular fluid flow and chloride content modulate H(+) transport by osteoclasts

Bone resorption takes place within the basic multicellular units (BMU), and the surface to be resorbed is isolated from adjacent bone surfaces by a sealing zone between osteoclast membrane and bone matrix, which defines the limits of the resorption lacuna. Considering that the extracellular fluid (ECF) in both BMU and the resorption lacuna can be isolated from its surroundings, I hypothesize that flow and ion composition of the bone ECF in these sites might contribute to the regulation of osteoclast H(+) secretion. To investigate this hypothesis, I evaluated the H(+) secretion properties of individual osteoclasts and osteoclast-like cells (OCL-cells) and investigated whether changes in flow or chloride content of the extracellular solution modify the H(+) secretion properties in vitro.

The results show that 1) osteoclasts are unable to secrete H(+) and regulate intracellular pH (pHi) under continuous flow conditions and exhibit progressive intracellular acidification; 2) the cessation of flow coincides with the onset of H(+) secretion and subsequent progressive intracellular alkalinization of osteoclasts and OCL-cells; 3) osteoclasts exhibit spontaneous rhythmic oscillations of pHi in non-flowing ECF, 4) pHi oscillations are not abolished by concanamycin, NPPB, or removal of extracellular Na(+) or Cl(-); 5) extracellular Cl(-) removal modifies the pattern of oscillations, by diminishing H(+) secretion; 6) pHi oscillations are abolished by continuous flowing of ECF over osteoclasts and OCL-cells.

The data suggest, for the first time, that ECF flow and Cl(-) content have direct effects on osteoclast H(+) secretion and could be part of a mechanism determining the onset of osteoclast H(+) secretion required for bone resorption.