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Activation of Cl− channels by extracellular Ca2+ in freshly isolated rabbit osteoclasts

Ionic channels regulated by extracellular Ca2+ concentration ([Ca2+]0) were examined in freshly isolated rabbit osteoclasts. K+ current was suppressed by intracellular and extracellular Cs+ ions. In this condition, high [Ca2+]0 evoked an outwardly rectifying current with a reversal potential of about −25 mV. When the concentration of extracellular Cl− ions was altered, the reversal potential of the outwardly rectifying current shifted as predicted by the Nernst equation. 4′,4‐diisothiocyanostilbene‐2′,2‐disulphonic acid (DIDS) inhibited the outwardly rectifying current. These results indicated that this current was carried through Cl− channels. Cd2+ or Ni2+ caused a transient activation of the Cl− current in contrast to the sustained activation elicited by Ca2+. Intracellular 20 mM ethylene glycol‐bis(β‐aminoethyl ether)‐N,N,N′,N′‐tetraacetic acid (EGTA) inhibited the divalent cation‐induced Cl− current. Either when the osmolarity of extracellular medium was increased, or when 100 μM cAMP was dissolved in the patch pipette solution, high [Ca2+]0 still elicited the Cl− current, indicating that the divalent cation‐induced Cl− current was carried through Ca2+‐activated Cl− channels. Under perforated whole cell clamp extracellular divalent cations evoked the Cl− current, indicating that the activation of Cl− current did not arise from possible leakage of divalent cations from the extracellular medium under the whole cell clamp condition. This experiment further excluded a possible activation of volume‐sensitive Cl− channels under whole cell clamp. Intracellular application of guanosine 5′‐O‐(3‐thiotriphosphate) (GTPγS) activated the Cl− current and it was inhibited by intracellular 20 mM EGTA, suggesting that the activation of Cl− current was mediated through a G protein, and that an increase in [Ca2+]i was critical for the activation of Cl− channels. A protein phosphatase inhibitor, okadaic acid (100 nM), caused an irreversible activation of the Cl− current, suggesting that protein phosphatase 1 or 2A was involved in the regulation of Ca2+‐activated Cl− channels. © 1996 Wiley‐Liss, Inc.