Fig. 5. Sarcoplasmic reticulum (SR) Ca²⁺-dependent inactivation of L-type Ca²⁺ channel current (I_Ca) in representative rainbow trout atrial myocytes with square (SQ) and action potential (AP) pulses at 14°C. The green traces indicate the original curve, and the solid black lines are the double exponential fits (Clampfit 6.0, Axon Instruments). (A) I_Ca elicited by a SQ-voltage clamp pulse immediately after caffeine application [SR empty; fast inactivation kinetics (_f)=24.7 ms, and slow inactivation kinetics (_s)=113 ms] and after the SR Ca²⁺ content had reached a steady-state (SR steady-state; _f=10.7 ms and _s=80 ms). (B) The effect of steady-state SR Ca²⁺ content on I_Ca elicited by an AP pulse (green traces). The kinetics of inactivation were best fit with a single exponential (solid black line) over the first 200 ms after peak current. For this cell, decreased from 57 ms after caffeine application to 50 ms with a steady-state SR Ca²⁺ content. Means and statistics for SR-Ca²⁺-dependent inactivation of I_Ca at 7°C, 14°C and 21°C are given in Tables 1, 2.