Functional and Molecular Characterization of a T-type Ca2+ Channel During Fetal and Postnatal Rat Heart Development

T-type calcium current (ICaT) is distributed among a large variety of species and tissues. The main functions of ICaT are thought to be related to pacemaker activity and to the cell cycle. Using the whole-cell patch-clamp configuration, we showed that fetal rat ventricular cells exhibit an ICaT with electrophysiological and pharmacological characteristics similar to those already described for this current. We investigated ICaT density and found that this current was mainly expressed in fetal cells and remained stable until birth (3.1±0.3 pA/pF for 18-day-old fetus, n=9). ICaT density decreased soon after birth (2.0±0.3 pA/pF,n =6, 1.1±0.2 pA/pF, n=5, for 1- and 5-day-old rats, respectively) and was no longer detected in 21-day-old rats. The rat ventricular cells express an α 1H isoform in addition to a homologousα 1G variant. Interestingly, the Ni2+ sensitivity of ICaT indicates that in newborn myocytes, ICaT is only generated by α 1G subunits, whereas both α 1G and α 1H subunits participate in the fetal ICaT. Moreover, the relative contribution of each subunit varies during fetal developmental stages, with a major contribution of α 1H in 16-day-old fetuses. Through quantitative RT-PCR we showed that the amount of both α 1G and α 1H transcripts are developmentally regulated. In fetuses of less than 18 days and in newborn rats after 1 day old, the transcriptional levels of α 1G and α 1H subunits clearly mismatch the functional contribution of these subunits to ICaT. However, in perinatal period, the amount ofα 1G mRNA seems to be in accordance to α 1G-related ICaT density. In conclusion, we showed that ICaT is mainly expressed during fetal stages, that α 1G and α 1H differentially participate to ICaT and that α 1G and α 1H isoforms are regulated by both transcriptional and post-transcriptional mechanisms.