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 Ca 2+ channel composition of fish hearts is still largely unknown. To this end, we determined transcript expression of Ca 2+ channels in the heart of zebrafish ( Danio rerio ), a popular model species. Altogether, 18 Ca 2+ channel α-subunit genes were expressed in both atrium and ventricle. Transcripts for 7 L-type (Ca v 1.1a, Ca v 1.1b, Ca v 1.2, Ca v 1.3a, Ca v 1.3b, Ca v 1.4a, Ca v 1.4b), 5 T-type (Ca v 3.1, Ca v 3.2a, Ca v 3.2b, Ca v 3.3a, Ca v 3.3b) and 6 P/Q-, N- and R-type (Ca v 2.1a, Ca v 2.1b, Ca v 2.2a, Ca v 2.2b, Ca v 2.3a, Ca v 2.3b) Ca 2+ 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 Ca 2+ channel transcripts. In the atrium, the relative expression of T-type and L-type Ca 2+ channel transcripts was 64.1% and 33.8%, respectively (others accounted for 2.1%). Thus, at the transcript level, T-type Ca 2+ 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 ) Ca 2+ current were 6.3±0.8 and 7.7±0.8 pA pF -1 , respectively. I CaT mediated a sizeable sarcolemmal Ca 2+ influx into ventricular myocytes: the increment in total cellular Ca 2+ content via I CaT was 41.2±7.3 µmol l -1 , which was 31.7% of the combined Ca 2+ influx (129 µmol l -1 ) via I CaT and I CaL (88.5±20.5 µmol l -1 ). The diversity of expressed Ca 2+ 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.