Comparison of the Ca2+ currents induced by expression of three cloned α1 subunits, α1G, α1H and α1I, of low-voltage-activated T-type Ca2+ channels

Expression of rat alpha 1G, human alpha 1H and rat alpha 1I subunits of voltage-activated Ca2+ channels in HEK-293 cells yields robust Ca2+ inward currents with 1.25 mm Ca2+ as the charge carrier. Both similarities and marked differences are found between their biophysical properties. Currents induced by expression of alpha 1G show the fastest activation and inactivation kinetics. The alpha 1H and alpha 1I currents activate and inactivate up to 1.5- and 5-fold slower, respectively. No differences in the voltage dependence of steady state inactivation are detected. Currents induced by expression of alpha 1G and alpha 1H deactivate with time constants of up to 6 ms at a test potential of -80 mV, but currents induced by alpha 1I deactivate about three-fold faster. Recovery from short-term inactivation is more than three-fold slower for currents induced by alpha 1H and alpha 1I in comparison to alpha 1G. In contrast to these characteristics, reactivation after long-term inactivation was fastest for currents arising from expression of alpha 1I and slowest in cells expressing alpha 1H calcium channels. The calcium inward current induced by expression of alpha 1I is increased by positive prepulses while currents induced by alpha 1H and alpha 1G show little (< 5%) or no facilitation. The data thus provide a characteristic fingerprint of each channel's activity, which may allow correlation of the alpha 1G, alpha 1H and alpha 1I induced currents with their in vivo counterparts.