Fig. 2.

Measuring changes in the efficiency of exocytosis. Increases in membrane capacitance in response to a pair of stimuli that activated the Ca²⁺ current submaximally. The stimuli were 2 msec depolarizations to −25 mV, with an interval of 100 msec. The Ca²⁺ currents are shown superimposed. In each case, Ca²⁺ influx carried 0.25 pC of charge. The first response, ΔC₁, was 37 fF, and the second response, ΔC₂, was 4 fF. The remainder of the rapidly releasable pool of vesicles was exhausted with a third stimulus, a 20 msec depolarization to −10 mV. This emptying stimulus passed 4.14 pC of charge and elicited a capacitance increase, ΔC₃, of 32 fF. After accounting for the reduced size of the RRP, the second response was 21% of the first, i.e., the efficiency of exocytosis was reduced by 79% (see Results).