Mechanisms of Ca2+ handling in zebrafish ventricular myocytes
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The zebrafish serves as a promising transgenic animal model that can be used to study cardiac Ca2+ regulation. However, mechanisms of sarcoplasmic reticulum (SR) Ca2+ handling in the zebrafish heart have not been systematically explored. We found that in zebrafish ventricular myocytes, the action potential-induced Ca2+ transient is mainly (80 %) mediated by Ca2+ influx via L-type Ca2+ channels (LTCC) and only 20 % by Ca2+ released from the SR. This small contribution of the SR to the Ca2+ transient was not the result of depleted SR Ca2+ 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 [Ca2+] from 100 to 350 nM did not trigger SR Ca2+ release. However, an application of a low dose of caffeine activated Ca2+ sparks. These results show that the zebrafish cardiac RyR has low sensitivity to the mechanism of Ca2+-induced Ca2+ release. Activation of protein kinase A by forskolin increased phosphorylation of the RyR in zebrafish myocardium. In half of the studied cells, an increased Ca2+ 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 Ca2+ 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 Ca2+ release to the Ca2+ transient. This difference is due to a low sensitivity of RyRs to cytosolic [Ca2+].
KeywordsZebrafish Sarcoplasmic reticulum Ca2+-induced Ca2+ release Ryanodine receptor Phosphorylation Heart
We would like to thank Dr. Olga Alekhina for excellent technical assistance. This work was supported by the National Institutes of Health grants HL62426 and HL75494 to P.d.T., the Research Career Development Award from The Schweppe Foundation to A.V.Z., and American Heart Association Fellowship Grant 14510047 to A.V.D.