Known Calcium Channel α1 Subunits Can Form Low Threshold Small Conductance Channels with Similarities to Native T-Type Channels

The ␣ 1E clone has been put forward as a possible candidate for T-type current because it possesses some of the salient features, including low activation voltage, rapid inactivation, and similar or greater whole-cell current amplitude in Ca 2ϩ compared to Ba 2ϩ (Soong et al., 1993). Nevertheless, major discrepancies exist, the main Summary one being that the single channel conductance reported for ␣ 1E channels is 12–14 pS (Schneider et al., 1994; Native T-type voltage-dependent calcium channels Wakamori et al., 1994). Furthermore, although activation are low voltage–activated and have a small single and steady-state inactivation of expressed ␣ 1E current channel conductance of 5–8 pS, which distinguishes occur at lower voltages than for some other cloned them from any known cloned calcium channels whose VDCCs (Soong et al., 1993; Stephens et al., 1997), their conductances are 12–25 pS. Here, we show that when thresholds are not as low as for native T-type current. ␣ 1B , ␣ 1E , or ␣ 1C are expressed in COS7 cells, which We have expressed ␣ 1B , ␣ 1C , and ␣ 1E VDCCs in COS7 contain no endogenous calcium channel subunits or cells, which contain no endogenous mRNA or protein for calcium channels, they each exhibit a 4–7 pS channel VDCC ␣ 1 or accessory subunits or endogenous calcium as well as a large conductance channel. At low depo-channel currents (Berrow et al., 1997; Brice et al., 1997; larizations, or when the ␣ 1 subunit is expressed in Stephens et al., 1997). This system is therefore advanta-the absence of auxiliary ␣ 2-␦ or ␤ subunits, the small geous for the present study compared to several other conductance channels are seen alone, and their bio-expression systems, including Xenopus oocytes, which physical properties, including voltage dependence possess endogenous ␤ and ␣ 2-␦ subunits and endoge-and kinetics of activation and inactivation, are very nous calcium currents (Singer-Lahat et al., 1992; Tareilus similar to native T-type calcium channels. et al., 1997), or HEK293 cells, which also have endoge-nous calcium currents (Berjukow et al., 1996). For all three ␣ 1 subunits, in addition to large conductance chan-Introduction nels we have also observed low threshold small conduc-tance single calcium channels, which have properties Voltage-dependent calcium channels (VDCCs) in neu-very similar to the native T-type channels that we have rons and other cells have been divided into high and examined for comparison in undifferentiated NG108–15 low voltage–activated (HVA …