Functional and molecular characterization of a T-type Ca(2+) channel during fetal and postnatal rat heart development.

T-type calcium current (I(CaT)) is distributed among a large variety of species and tissues. The main functions of I(CaT) 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 I(CaT) with electrophysiological and pharmacological characteristics similar to those already described for this current. We investigated I(CaT) 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). I(CaT) 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 alpha 1H isoform in addition to a homologous alpha 1G variant. Interestingly, the Ni(2+) sensitivity of I(CaT) indicates that in newborn myocytes, I(CaT) is only generated by alpha 1G subunits, whereas both alpha 1G and alpha 1H subunits participate in the fetal I(CaT). Moreover, the relative contribution of each subunit varies during fetal developmental stages, with a major contribution of alpha 1H in 16-day-old fetuses. Through quantitative RT-PCR we showed that the amount of both alpha 1G and alpha 1H transcripts are developmentally regulated. In fetuses of less than 18 days and in newborn rats after 1 day old, the transcriptional levels of alpha 1G and alpha 1H subunits clearly mismatch the functional contribution of these subunits to I(CaT). However, in perinatal period, the amount of alpha 1G mRNA seems to be in accordance to alpha 1G-related I(CaT) density. In conclusion, we showed that I(CaT) is mainly expressed during fetal stages, that alpha 1G and alpha 1H differentially participate to I(CaT) and that alpha 1G and alpha 1H isoforms are regulated by both transcriptional and post-transcriptional mechanisms.