Fig. 3. Time course of nucleotide release. Exocytosis evoked by voltage-clamp depolarization in a P2X₂-EGFP-expressing Ins1-cell. (A) Membrane currents (I; middle) and increase in cell capacitance (C) elicited by a 500 millisecond voltage-clamp depolarization from -70 mV to 0 mV (V; top). The inset shows the indicated part (highlighted by square) of the current trace after vertical and horizontal expansion. Asterisks indicate current spikes superimposed on the inward Ca²⁺-current (identified by eye). Spikes during the depolarization are smaller than at -70 mV due to the reduced inward driving force. The larger spikes cause brief artifacts on the capacitance trace that are unrelated to exo- or endocytosis. (B) As in A, but suramin (100 µM) was applied to the cell, resulting in nearly complete suppression of spikes in the current trace. (C) Cumulative histogram of the time (t) of the current spikes relative to the onset of the depolarization in eleven experiments as in A (circles). Overlaid are mono-exponential functions fitted to all points (time constant of 0.38 seconds; black line), and to the values greater than 500 milliseconds ( of 0.28 seconds; dotted line). The shaded area indicates the duration of the depolarizing stimulus.