| Abstract |
Effects of the extracellular Ca2+ concentration ([Ca2+]o) on whole cell membrane currents were examined in mouse osteoclastic cells generated from bone marrow/stromal cell coculture. The major resting conductance in the presence of 1 mm Ca2+ was mediated by a Ba2+-sensitive, inwardly rectifying K+ (IRK) current. A rise in -Ca2+-o (5-40 mM) inhibited the IRK current and activated an 4'4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS)-sensitive, outwardly rectifying Cl- (ORCl) current. The activation of the ORCl current developed slowly and needed higher [Ca2+]o than that required to inhibit the IRK current. The inhibition of the IRK current consisted of two components, initial and subsequent late phases. The initial inhibition was not affected by intracellular application of guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) or guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS). The late inhibition, however, was enhanced by GTPgammaS and attenuated by GDPbetaS, suggesting that GTP-binding proteins mediate this inhibition. The activation of the ORCl current was suppressed by pretreatment with pertussis toxin, but not potentiated by GTPgammaS. An increase in intracellular Ca2+ level neither reduced the IRK current nor activated the ORCl current. Staurosporine, an inhibitor for protein kinase C, did not modulate the [Ca2+]o-induced changes in the IRK and ORCl conductances. These results suggest that high [Ca2+]o had a dual action on the membrane conductance of osteoclasts, an inhibition of an IRK conductance and an activation of an ORCl conductance. The two conductances modulated by [Ca2+]o may be involved in different phases of bone resorption because they differed in Ca2+ sensitivity, temporal patterns of changes and regulatory mechanisms.
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