Fig. 1. Action potential (AP) waveforms used to elicit L-type Ca²⁺ channel current (I_Ca) and intracellular Ca²⁺ concentration ([Ca²⁺]_i). The APs were recorded by current-clamp in a separate series of experiments (Shiels et al., 2002a). AP waveforms were obtained by averaging recordings from 6-10 cells at each temperature. The relatively depolarized resting potential is typical for isolated atrial myocytes from rainbow trout (Shiels et al., 2000) and reflects both a low density of the inward rectifier K⁺ current (I_K1) and the lack of cholinergic tone (Vornanen et al., 2002). The inter-pulse holding level during subsequent AP stimulation was -80 mV. The solid line shows the dimensions of the 500 ms square (SQ) pulse.