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

Abstract

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 alpha-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) 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 mu mol l(-1), which was 31.7% of the combined Ca2+ influx (129 mu mol l(-1)) via I(CAT )and I(CAL)( ()88.5 +/- 20.5 mu mol 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.