Fig. 5. The small conductance channel conducts Ca²⁺, but not Mg²⁺. After vesicle fusion, no spontaneous Ca²⁺ channels are present (+60 mV clamp) (a). After IP₃ addition, the small conductance channel does not conduct Mg²⁺ (+60 mV clamp) (b), but does conduct Ca²⁺ (12 pS) (+60 mV clamp) (c). 2-APB inhibits completely the channel (+50 mV clamp) (d).