Mechanisms of Ca2+ handling in zebrafish ventricular myocytes

The zebrafish serves as a promising transgenic animal model that can be used to study cardiac Ca 2+ regulation. However, mechanisms of sarcoplasmic reticulum (SR) Ca 2+ handling in the zebrafish heart have not been systematically explored. We found that in zebrafish ventricular myocytes, the action potential-induced Ca 2+ transient is mainly (80 %) mediated by Ca 2+ influx via L-type Ca 2+ channels (LTCC) and only 20 % by Ca 2+ released from the SR. This small contribution of the SR to the Ca 2+ transient was not the result of depleted SR Ca 2+ load. We found that the ryanodine receptor (RyR) expression level in zebrafish myocytes was ∼72 % lower compared to rabbit myocytes. In permeabilized myocytes, increasing cytosolic [Ca 2+ ] from 100 to 350 nM did not trigger SR Ca 2+ release. However, an application of a low dose of caffeine activated Ca 2+ sparks. These results show that the zebrafish cardiac RyR has low sensitivity to the mechanism of Ca 2+ -induced Ca 2+ release. Activation of protein kinase A by forskolin increased phosphorylation of the RyR in zebrafish myocardium. In half of the studied cells, an increased Ca 2+ transient by forskolin was entirely mediated by augmentation of LTCC current. In the remaining myocytes, the forskolin action was associated with an increase of both LTCC and SR Ca 2+ release. These results indicate that the mechanism of excitation–contraction coupling in zebrafish myocytes differs from the mammalian one mainly because of the small contribution of SR Ca 2+ release to the Ca 2+ transient. This difference is due to a low sensitivity of RyRs to cytosolic [Ca 2+ ].