Molecular and functional properties of the human alpha(1G) subunit that forms T-type calcium channels

We describe here several novel properties of the human alpha(1G) subunit th at 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: al pha(1G-a) and alpha(1G-b). Northern blot and dot blot analyses indicated th at alpha(1G) mRNA is predominantly expressed in the brain, especially in th alamus, 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). Functiona l 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 thre shold 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 p S). These properties, together with slowly deactivating tail currents, are typical of those of native T-type Ca2+ channels, This alpha(1G)-related cur rent 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 pote ntial waveforms, we show that alpha(1G) channels produce a massive and sust ained Ca2+ influx due to their slow deactivation properties. These latter p roperties would account for the specificity of Ca2+ influx via T-type chann els that occurs in the range of physiological resting membrane potentials, differing considerably from the behavior of other Ca2+ channels.