Fig. 4. Depolarization by high potassium elicits quantal release in PC12 cells. A, Amperometric spikes after PC12 stimulation with 80 mm KCl for 6 sec (applied atarrow). This representative control trace represents 15 spikes (Imax > 4.5 rms noise), ranging from 1.25 to 14.0 pA with a mean amplitude of 4.8 ± 1.0 pA and quantal size of 201,000 ± 39,400 molecules. B, The portion above the broken line in A is displayed with an expanded time resolution. The quantal size (#molecules), peak amplitudes (Imax; in picoamperes), and width at half height (t½; in milliseconds) are displayed under four corresponding events. C, For the sample event indicated with the asterisk inB, the maximal amplitude is indicated by the top broken line, and the half height is indicated by themiddle broken line. The foot, filled by an array of points, is estimated by integrating the portion of the event preceding the slope (broken line) between the 60 and 90% rise-times (Chow and von Ruden, 1995). D, For control cells (events in Table 1), the interspike intervals (intspike; in milliseconds) are displayed in 100 msec bins on the x-axis. The percentage of intervals within each bin (n/N) is labeled on the y-axis. The decreased representation at longer interval values is approximated by an exponential decay (see Materials and Methods).