Résumé
We have cloned and expressed a human α1I subunit that encodes a subtype of T-type calcium channels. The predicted protein is 95% homologous to its rat counterpart but has a distinct COOH-terminal region. Its mRNA is detected almost exclusively in the human brain, as well as in adrenal and thyroid glands. Calcium currents generated by the functional expression of human α1I and α1G subunits in HEK-293 cells were compared. The α1I current activated and inactivated ∼10 mV more positively. Activation and inactivation kinetics were up to six times slower, while deactivation kinetics was faster and showed little voltage dependence. A slower recovery from inactivation, a lower sensitivity to Ni2+ ions (IC50 ∼180 μm), and a larger channel conductance (∼11 picosiemens) were the other discriminative features of the α1I current. These data demonstrate that the α1I subunit encodes T-type Ca2+ channels functionally distinct from those generated by the human α1G or α1H subunits and point out that human and rat α1I subunits have species-specific properties not only in their primary sequence, but also in their expression profile and electrophysiological behavior.