Functional embryonic cardiomyocytes after disruption of the L-type α1C (Cav1.2) calcium channel gene in the mouse

The L-type alpha (1c) (Ca(V)1.2) calcium channel is the major calcium entry pathway in cardiac and smooth muscle. We inactivated the Ca(nu)1.2 gene in two independent mouse lines that had indistinguishable phenotypes. Homozygous knockout embryos (Ca(V)1.2-/-) 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 (-/-) Ca(V)1.2 embryos. Beating of isolated embryonic cardiomyocytes depended on extracellular calcium and was blocked by 1 muM nisoldipine, In (+/+), (+/-), and (-/-) cardiomyocytes, an L-type Ba2+ inward current (I-Ba) was present that was stimulated by Bay K 8644 in all genotypes. At a holding potential of -80 mV, nisoldipine blocked I-Ba of day 12.5 p.c. (+/+) and (+/-) cells with two IC50 values of approximate to0.1 and approximate to1 muM. Inhibition of I-Ba of (-/-) camdiomyocytes was monophasic with an IC50 of approximate to1 mum. The low affinity I-Ba was also present in cardiomyocytes of homozygous alpha (1D) (Ca(V)1.3) 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.