Fig. 3. (A) Heparin does not inhibit the small conductance channel. After vesicle fusion, no spontaneous Ca²⁺ channels are observed in the absence of IP₃ (40 mV clamp) (a). D-IP₃ activates Ca²⁺ channels (12 pS) (+40 mV clamp) (b). The competitive IP₃ channel blocker heparin (50 µM) has no effect on channel activity even after a second treatment (40 mV clamp) (c).(B) Dantrolene does not inhibit the small conductance channel. Prior to IP₃ addition, no spontaneous Ca²⁺ channels are present (+60 mV clamp) (a). IP₃ opens a small conductance channel (11 pS) (+100 mV clamp) (b). Dantrolene does not inhibit the channel (+100 mV clamp) (c).