Diversity and Developmental Expression of L-type Calcium Channel β2 Proteins and Their Influence on Calcium Current in Murine Heart

By now, little is known on L-type calcium channel (LTCC) subunits expressed in mouse heart. We show that CaV beta 2 proteins are the major CaV beta components of the LTCC in embryonic and adult mouse heart, but that in embryonic heart CaV beta 3 proteins are also detectable. At least two CaV beta 2 variants of similar to 68 and similar to 72 kDa are expressed. To identify the underlying CaV beta 2 variants, cDNA libraries were constructed from poly(A)(+) RNA isolated from hearts of 7-day-old and adult mice. Screening identified 60 independent CaV beta 2 cDNA clones coding for four types of CaV beta 2 proteins only differing in their 5' sequences. CaV beta 2-N1, -N4, and -N5 but not -N3 were identified in isolated cardiomyocytes by RT-PCR and were sufficient to reconstitute the CaV beta 2 protein pattern in vitro. Significant L-type Ca2+ currents (I-Ca) were recorded in HEK293 cells after co-expression of CaV1.2 and CaV beta 2. Current kinetics were determined by the type of CaV beta 2 protein, with the similar to 72-kDa CaV beta 2a-N1 shifting the activation of I-Ca significantly to depolarizing potentials compared with the other CaV beta 2 variants. Inactivation of I-Ca was accelerated by CaV beta 2a-N1 and -N4, which also lead to slower activation compared with CaV beta 2a-N3 and -N5. In summary, this study reveals the molecular LTCC composition in mouse heart and indicates that expression of various CaV beta 2 proteins may be used to adapt the properties of LTCCs to changing myocardial requirements during development and that CaV beta 2a-N1-induced changes of I-Ca kinetics might be essential in embryonic heart.