Molecular and functional properties of the human α1G subunit that forms T-type calcium channels

We describe here several novel properties of the human alpha(1G) subunit that forms T-type calcium channels. The partial intron/exon structure of the corresponding gene CACNA1G was defined and several alpha(1G) isoforms were identified, especially two isoforms that exhibit a distinct III-IV loop: alpha(1G-a) and alpha(1G-b). Northern blot and dot blot analyses indicated that alpha(1G) mRNA is predominantly expressed in the brain, especially in thalamus, cerebellum, and substantia nigra. Additional experiments have also provided evidence that alpha(1G) mRNA is expressed at a higher level during fetal life in nonneuronal tissues (ie. kidney, heart, and lung). Functional expression in HEK 293 cells of a fall-length cDNA encoding the shortest a,, isoform identified to date, alpha(1G-b), resulted in transient, low threshold activated Ca2+ currents with the expected permeability ratio (I-Sr > I-Ca greater than or equal to I-Ba) and channel conductance (similar to 7 pS). These properties, together with slowly deactivating tail currents, are typical of those of native T-type Ca2+ channels, This alpha(1G)-related current was inhibited by mibefradil (IC50 = 2 mu M) and weakly blocked by Ni2+ ions (IC50 = 148 mu M) and amiloride (IC50 > 1 mM). We showed that steady state activation and inactivation properties of this current fan generate a "window current" in the range of -65 to -55 mV. Using neuronal action potential waveforms, we show that alpha(1G) channels produce a massive and sustained Ca2+ influx due to their slow deactivation properties. These latter properties would account for the specificity of Ca2+ influx via T-type channels that occurs in the range of physiological resting membrane potentials, differing considerably from the behavior of other Ca2+ channels.