Fig. 4. Effect of an internal Ca²⁺ increase on the osteoclast background current. (a) Effect of 10 µM calcium ionophore A-23187 on holding current (V_holding=-20 mV). The internal calcium rise induced the activation of an outward current `oscillation'. (b) I-V relationships of the current before and after application of Ca²⁺ ionophore. (c) I-V curve of the difference current (A-23187—control). Characteristics of this current were identical to those of the current induced during spontaneous oscillations. (d) Inside-out patch recording showing channel activation by 0.5 mM Ca²⁺ applied at the inner face of the patch membrane. Activation of this channel induced burst openings made by fast switching between close and open states (see magnification in panel d). (Mean unitary conductance was =25±2 pS.)