Résumé
The L-type α1C(Cav1.2) calcium channel is the major calcium entry pathway in cardiac and smooth muscle. We inactivated theCav1.2 gene in two independent mouse lines that had indistinguishable phenotypes. Homozygous knockout embryos (Cav1.2−/−) died before day 14.5 postcoitum (p.c.). At day 12.5 p.c., the embryonic heart contracted with identical frequency in wild type (+/+), heterozygous (+/−), and homozygous (−/−) Cav1.2 embryos. Beating of isolated embryonic cardiomyocytes depended on extracellular calcium and was blocked by 1 μm nisoldipine. In (+/+), (+/−), and (−/−) cardiomyocytes, an L-type Ba2+ inward current (IBa) was present that was stimulated by Bay K 8644 in all genotypes. At a holding potential of −80 mV, nisoldipine blocked IBa of day 12.5 p.c. (+/+) and (+/−) cells with two IC50 values of ≈0.1 and ≈1 μm. Inhibition of IBa of (−/−) cardiomyocytes was monophasic with an IC50 of ≈1 μm. The low affinity IBa was also present in cardiomyocytes of homozygous α1D(Cav1.3) knockout embryos at day 12.5 p.c. These results indicate that, up to day 14 p.c., contraction of murine embryonic hearts requires an unidentified, low affinity L-type like calcium channel.