Fig. 4. Consecutive measurement of L-type Ca²⁺ channel current (I_Ca), 1-25 pulses after caffeine application in a representative rainbow trout atrial myocyte at 21°C receiving square (SQ) stimulation pulses (-80 mV to + 10 mV, 200 ms). The green traces indicate the original curve, and the solid black lines are the double exponential fit (Clampfit, Axon Instruments). The slow inactivation kinetics (_s) for this cell were unchanged by progressive SR Ca²⁺ loading (_s = approximately 25 ms at each pulse). However, the fast inactivation kinetics (_f) became faster with an increasing number of pulses going from 7.1 ms, 6.7 ms, 5.2 ms, 4.5 ms, 3.9 ms to 4.1 ms after 1 pulse, 5 pulses, 10 pulses, 15 pulses, 20 pulses and 25 pulses, respectively.