Fig. 3. The effect of pulse number, temperature, frequency and stimulus waveform on sarcoplasmic reticulum (SR) Ca²⁺ accumulation in rainbow trout atrial myocytes. At each temperature, myocytes were exposed to caffeine to release the SR Ca²⁺ stores (not shown) and then stimulated with an increasing number of stimulation pulses between 1 and 100. Stimulation pulse waveform and frequency are given in each panel. SR Ca²⁺ content is expressed in pC pF^-1, and the corresponding values for Ca²⁺ concentration (µmol l^-1) are given in the text. (A) The effect of temperature and pulse number on SR Ca²⁺ content when cells were stimulated with square (SQ) pulses applied at 1.0 Hz. The results at 21°C are given by a broken line, as only four out of eight cells achieved any SR Ca²⁺ content under these loading conditions (see text for details). (B) The effect of applying SQ pulses at a frequency that corresponds to the heart rate of rainbow trout in vivo at each test temperature on SR Ca²⁺ accumulation. # indicates an increase in SR Ca²⁺ content between 25 and 100 pulses at 7°C, and also a difference between 100 pulses at 0.6 Hz and 100 pulses at 1.0 Hz (part A) at 7°C. (C) The effect of temperature and frequency on SR Ca²⁺ content when action potentials (APs) were applied at a frequency that corresponds to the heart rate of rainbow trout in vivo at each test temperature. ^* indicates that the SR content after AP pulses is significantly greater than after SQ pulses. All values are means ± S.E.M. of 4-16 cells at each temperature. In (A) and (B), the lines were drawn by fitting the data to an asymptotic exponential function. In (C), the line is plotted through the data without a curve fit.

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