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+)].