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

Calcium channels are necessary for cardiac excitation-contraction (E-C) coupling, but Ca²⁺ channel composition of fish hearts is still largely unknown. To this end, we determined transcript expression of Ca²⁺ channels in the heart of zebrafish (Danio rerio), a popular model species. Altogether, 18 Ca²⁺ channel α-subunit genes were expressed in both atrium and ventricle. Transcripts for 7 L-type (Ca_v1.1a, Ca_v1.1b, Ca_v1.2, Ca_v1.3a, Ca_v1.3b, Ca_v1.4a, Ca_v1.4b), 5 T-type (Ca_v3.1, Ca_v3.2a, Ca_v3.2b, Ca_v3.3a, Ca_v3.3b) and 6 P/Q-, N- and R-type (Ca_v2.1a, Ca_v2.1b, Ca_v2.2a, Ca_v2.2b, Ca_v2.3a, Ca_v2.3b) Ca²⁺ channels were expressed. In the ventricle, T-type channels formed 54.9%, L-type channels 41.1% and P/Q-, N- and R-type channels 4.0% of the Ca²⁺ channel transcripts. In the atrium, the relative expression of T-type and L-type Ca²⁺ channel transcripts was 64.1% and 33.8%, respectively (others accounted for 2.1%). Thus, at the transcript level, T-type Ca²⁺ channels are prevalent in zebrafish atrium and ventricle. At the functional level, peak densities of ventricular T-type (I_CaT) and L-type (I_CaL) Ca²⁺ current were 6.3±0.8 and 7.7±0.8 pA pF^-1, respectively. I_CaT mediated a sizeable sarcolemmal Ca²⁺ influx into ventricular myocytes: the increment in total cellular Ca²⁺ content via I_CaT was 41.2±7.3 µmol l^-1, which was 31.7% of the combined Ca²⁺ influx (129 µmol l^-1) via I_CaT and I_CaL (88.5±20.5 µmol l^-1). The diversity of expressed Ca²⁺ channel genes in zebrafish heart is high, but dominated by the members of the T-type subfamily. The large ventricular I_CaT is likely to play a significant role in E-C coupling.