T-type calcium currents in rat cardiomyocytes during postnatal development: contribution to hormone secretion

T-type Ca2+ channels have been suggested to play a role in cardiac automaticity, cell growth, and cardiovascular remodeling. Although three genes encoding for a T-type Ca2+ channel have been identified, the nature of the isoform( s) supporting the cardiac T-type Ca2+ current (I-Ca,I-T) has not yet been determined. We describe the postnatal evolution of I-Ca,I-T density in freshly dissociated rat atrial and ventricular myocytes and its functional properties at peak current density in young atrial myocytes. I-Ca,I-T displays a classical low activation threshold, rapid inactivation kinetics, negative steady-state inactivation, slow deactivation, and the presence of a window current. Interestingly, I-Ca,I-T is poorly sensitive to Ni2+ and insensitive to R-type current toxin SNX-482. RT-PCR experiments and comparison of functional properties with recombinant Ca2+ channel subtypes suggest that neonatal I-Ca,I-T is related to the alpha (1G)-subunit. Atrial natriuretic factor (ANF) secretion was measured using peptide radioimmunoassays in atrial tissue. Pharmacological dissection of ANF secretion indicates an important contribution of I-Ca,I-T to Ca2+ signaling during the neonatal period.