Figure 4. Relationship of α-LT-induced channel activity to exocytosis in physiological and in very low [Ca²⁺]_o.

A, close application of 5 nm α-LT (left traces) in the vicinity of a canine islet cell bathed in 2 mm Ca²⁺ PSS, induces a progressively increasing inward I_m followed by a large increase in C_m once the induced inward current I_m exceeds −100 to −150 pA. In contrast, right-hand traces demonstrate that a similar puff of α-LT in the vicinity of a canine islet cell bathed in no added Ca²⁺/EGTA PSS (< 20 μm free Ca²⁺) produces little or no increase in C_m despite stepwise increase in I_m to −300 to −350 pA. B, demonstrates that in 2 mm Ca²⁺ PSS (▪) the total change in membrane capacitance (ΔC_m) is proportional to the toxin-induced inward membrane current I_m; whereas, in no added Ca²⁺/EGTA PSS (□), ΔC_m is barely detectable despite similarly wide range of increases in I_m.