Expression of calcium channel transcripts in the zebrafish heart: dominance of T-type channels

Calcium (Ca) channels are necessary for cardiac excitation-contraction (e-c) coupling, but Ca channel composition of fish hearts is still largely unknown. To this end, we determined transcript expression of Ca channels in the heart of zebrafish (Danio rerio), a popular model species. Altogether 18 Ca channel α-subunit genes were expressed in both atrium and ventricle. Transcripts for 7 L-type (Cav1.1a, Cav1.1b, Cav1.2, Cav1.3a, Cav1.3b, Cav1.4a, Cav1.4b), 5 T-type (Cav3.1, Cav3.2a, Cav3.2b, Cav3.3a, Cav3.3b) and 6 P/Q-, N- and R-type Ca channels (Cav2.1a, Cav2.1b, Cav2.2a, Cav2.2b, Cav2.3a, Cav2.3b) were expressed. In the ventricle, T-type channels formed 54.9%, L-type channels 41.1% and P/Q-, N- and R-type Ca channels 4.0% of the Ca channel transcripts. In the atrium, the relative expression of T-type and L-type Ca channel transcripts was 64.1% and 33.8%, respectively (others accounted for 2.1%). Thus, at the transcript level T-type Ca channels are prevalent in zebrafish atrium and ventricle. At the functional level, peak densities of ventricular T-type (ICaT) and L-type (ICaL) Ca current were 6.3±0.8 and 7.7±0.8 pA pF−1, respectively. ICaT mediated a sizeable sarcolemmal Ca influx into ventricular myocyte: increment in total cellular Ca content via ICaT was 41.2±7.3 µmol L−1, which was 31.7% of the combined Ca influx (129 µmol L−1) via ICaT and ICaL (88.5±20.5 µmol L−1). The diversity of expressed Ca channel genes in zebrafish heart is high, but dominated by the members of the T-type subfamily. The large ventricular ICaT is likely to play a significant role in e-c coupling.