Figure 4. Variations in the amplitude and kinetics of discrete localized calcium release events.

A, a 3 s line-scan image transversely across the cell (scan rate, 500 Hz). The intensity of fluorescence was normalized to the average fluorescence during the initial 300 lines. Scale bars: x, 3 μm; t, 200 ms. Note that three spontaneous [Ca²⁺]_i release events occur (a, b, c). Ba-c, 3 fragments (12 μm × 280 ms each) of the line-scan image labelled in panel A as a, b and c (respectively) are shown at a higher time scale and magnification. C, time course of the fluorescence changes at the sites indicated by bars (i, ii, iii, iv) in panel B; in the case of ii, iii, and iv the time course of the fluorescence change was obtained by averaging the signal from the pairs of sites labelled in B. Note the small amplitude and rapid time course of the event shown in panel ii compared with that in panel i. Fluorescence measured at a point 1 μm from the centre of the spark (to simulate the kinetics of a spark which is just out of the confocal plane) produces a similar amplitude event (iii) but its time course is much slower. Thus the event in panel ii cannot be explained by an out-of-focus spark. Panel iv shows the background fluorescence signal.