L-type Ca2+ channels of the embryonic mouse heart

In the heart, where Ca(2+) influx across the sarcolemma is essential for contraction, L-type Ca(2+) channels represent the major entry pathway of Ca(2+). Mice with a homozygous deletion of the L-type Ca(v)1.2 Ca(2+) channel gene die before day 14.5 p.c. Electrophysiological and pharmacological investigations on Ca(v)1.2-/- cardiomyocytes demonstrated that contractions depended on the influx of Ca(2+) through an L-type-like Ca(2+) channel. We analyzed now the expression pattern of various L-type Ca(2+) channels. Amplification of the alternative exons 1a and 1b revealed that embryonic cardiac cells express both Ca(v)1.2a and Ca(v)1.2b subunits. Reverse transcriptase-polymerase chain reaction (RT-PCR) amplifications indicated the expression of Ca(v)1.1 and Ca(v)1.3 in about a 1:10 ratio in Ca(v)1.2-/- embryos. Two different amino termini of the Ca(v)1.3 cDNA were found in the embryonic heart, which both gave rise to functional channels. Ca(v)1.3(1a) and Ca(v)1.3(1b) channels have similar current kinetics and voltage-dependencies as described for Ca(v)1.3(8A) channels [J. Biol. Chem. 276 (2001) 22100], but the properties of Ca(v)1.3(1a) or Ca(v)1.3(1b) channels are different from that of the L-type-like current in Ca(v)1.2-/- cardiomyocytes. The I(Ba) of Ca(v)1.3(1a) was blocked by the dihydropyridine nisoldipine with an IC(50) value of 0.13 microM at a holding potential of -80 mV. In embryonic Ca(v)1.2+/+ cardiomyocytes, I(Ba) was blocked by nisoldipine with an IC(50) value of 0.1 microM. Although the expressed Ca(v)1.3 channel has a similar affinity for nisoldipine as Ca(v)1.2+/+ cardiomyocytes, the L-type-like Ca(2+) channel found in Ca(v)1.2+/+ and -/- cardiomyocytes is not identical with the new Ca(v)1.3 splice variants.