Figure 4.

Dependence of spark frequency on cytosolic [Ca²⁺]. (A and C) Frequency of events (number, per second, per 100 μm of line scanned) determined objectively in successive line scans, versus time. In red is a smoothed version of the same frequency. Dashed trace: nominal [Ca²⁺] in perfusing solution. (B and D) Averaged frequency versus nominal cytosolic [Ca²⁺], frog data (B) derived from all experiments listed in Table II. Rat data (D) derived from all experiments in Table III. Frequency in repeated passages at same [Ca²⁺] were averaged first in individual fibers, and then fiber values were averaged with equal weight. Bars represent ± one SEM (range when n = 2). Regression slope for frog data: 0.0052; intercept: 2.43; r²: 0.977. Regression slope for rat: 0.017; intercept: −0.67; r²: 0.911.