EXTRACELLULAR CA2+ SENSING BY THE OSTEOCLAST

An increasing number of cell types appear to detect changes in the ext racellular Ca2+ concentrationand and accordingly modify their function . We review recent evidence for the existence and function of such a m echanism in the osteoclast. Elevated external [Ca2+] in the mM range r educes bone resorption and results in motile changes in the cells. The se changes may partly result from elevations of cytosolic [Ca2+] trigg ered through activation of a surface Ca2+ receptor. Closer analyses of the increases in cytosolic [Ca2+] associated with receptor activation are hindered by the action of this ion both as extracellular agonist and intracellular second messenger. Variations in the peak cytosolic [ Ca2+] response to external Ca2+ with changes in cell membrane potentia l by K+ and valinomycin establish a contribution from extracellular Ca 2+. Use of ClO4 , Ni and Cd as surrogate activators in low extracellul ar [Ca2+] indicate a contribution from Ca2+ release from intracellular stores as well. Such agonists also modify Ca2+ redistribution in othe r systems, such as skeletal muscle. Thus, we may gain insights into os teoclast extracellular Ca2+ detection and transduction from known feat ures of more well-characterised cell systems.