Voltage-dependent Ca2+ channels are heteromultimeric protein complexes consisting of a pore-forming subunit α1 and accessory β, γ and α2δd subunits. Of the β subunit, 4 isoforms exist, the β2 is most abundant in heart but also expressed in brain and aorta. In order to test the function of the β2 subunit in vivo, we generated mouse models with either β2 null alleles (β2-/-) or with alleles in which exon 4 of the β2 gene is flanked by loxP sites (β2flox/flox) for conditional inactivation using the Cre recombinase transgenes. Previously, we have shown that the L-type Ca2+ channel current (IBa) in primary cultured myocytes isolated from β2-/- embryonic mice (E9.5) is reduced, without a change in the action potential duration (Held et al, 2003). Mice were sacrificed according to German guidelines. Similar results were obtained when Ca2+ (1.8mM) was used as charge carrier. In cardiac myocytes from E10.5 embryos, the ICa (1.8mM Ca2+) at 0mV was reduced from –
10.5 ± 0.71 pA/pF (n=32) in β2+/+ to -4.2 ± 0.42 pA/pF (n=15) in β2-/- cells (p<0.05, ANOVA), no significant differences were observed between β2+/+ and β2+/- cells. To examine whether embryonic lethality is due to cardiac dysfunction, β2flox/flox mice were mated with a mouse line expressing the Cre recombinase under a cardiomyocyte-specific promotor. Mice with a cardiomyocyte-specific inactivation of the β2 gene (β2-/flox; MLC2a-Cretg/0) are not viable. Hearts were isolated at E13.5, separated in ventricle and atrium, and L-type Ca2+ current (ICa, 1.8mM Ca2+) in cardiomyocytes was measured as before. Ventricular cells lacking the β2 gene (β2-/flox,MLC2a-Cretg/0), had with -4.0 ± 0.3 pA/pF (n=29) a reduced L-type Ca2+ current (β2+/flox;MLC2a-Cretg/0: -6.8 ± 0.6 pA/pF, n=18, p<0.001; β2+/flox; MLC2a-Cre0/0: -6.7 ± 1.0 pA/pF, n=6, p<0.05; β2-/flox;MLC2a-Cre0/0: -5.9 ± 0.5, n=29; p<0.01, ANOVA) as observed with the classical knock-out at E9.5 or E10.5. The data indicate that in cardiomyocytes the β2 subunit and the Ca2+ influx through L-type Ca2+ channel are essential for development and survival of the mouse embryo.