The calcium stored in the sarcoplasmic reticulum acts as a safety mechanism in rainbow trout heart

Cardiomyocyte contraction depends on rapid changes in intracellular Ca2+. In mammals, Ca2+ influx as L-type Ca2+ current (I-Ca) triggers the release of Ca2+ from sarcoplasmic reticulum (SR) and Ca2+ induced Ca2+ release (CICR) is critical for excitation-contraction coupling. In fish, the relative contribution of external and internal Ca2+ is unclear. Here, we characterized the role of I-Ca to trigger SR Ca2+ release in rainbow trout ventricular myocytes using I-Ca regulation by Ca2+ as an index of CICR. I-Ca was recorded with a slow (EGTA) or fast (BAPTA) Ca2+ chelator in control and isoproterenol conditions. In the absence of beta-adrenergic stimulation, the rate of I-Ca inactivation was not significantly different in EGTA and BAPTA (27.1 +/- 1.8 vs. 30.3 +/- 2.4 ms), whereas with isoproterenol (1 mu M), inactivation was significantly faster with EGTA (11.6 +/- 1.7 vs. 27.3 +/- 1.6 ms). When barium was the charge carrier, inactivation was significantly slower in both conditions (61.9 +/- 6.1 vs. 68.0 +/- 8.7 ms, control, isoproterenol). Quantification revealed that without isoproterenol, only 39% of I-Ca inactivation was due to Ca2+, while with isoproterenol, inactivation was Ca2+-dependent (similar to 65%) and highly reliant on SR Ca2+ (similar to 46%). Thus, SR Ca2+ is not released in basal conditions, and ICa is the main trigger of contraction, whereas during a stress response, SR Ca2+ is an important source of cytosolic Ca2+. This was not attributed to differences in SR Ca2+ load because caffeine-induced transients were not different in both conditions. Therefore, Ca2+ stored in SR of trout cardiomyocytes may act as a safety mechanism, allowing greater contraction when higher contractility is required, such as stress or exercise.