The Journal of Neurophysiology Vol. 83 No. 2 February 2000, pp. 671-684
Copyright ©2000 by the American Physiological Society

Inactivation Properties of Human Recombinant Class E Calcium Channels

Eli Lilly and Company Limited, Erl Wood Manor, Windlesham, Surrey GU20 6PH, United Kingdom

Jouvenceau, Anne, Federica Giovannini, Cath P. Bath, Emily Trotman, and Emanuele Sher. Inactivation Properties of Human Recombinant Class E Calcium Channels. J. Neurophysiol. 83: 671-684, 2000. The electrophysiological and pharmacological properties of _1E-containing Ca²⁺ channels were investigated by using the patch-clamp technique in the whole cell configuration, in HEK 293 cells stably expressing the human _1E together with _2b and _1b accessory subunits. These channels had current-voltage (I-V) characteristics resembling those of high-voltage-activated (HVA) Ca²⁺ channels (threshold at 30 mV and peak amplitude at +10 mV in 5 mM Ca²⁺). The currents activated and deactivated with a fast rate, in a time- and voltage-dependent manner. No difference was found in their relative permeability to Ca²⁺ and Ba²⁺. Inorganic Ca²⁺ channel blockers (Cd²⁺, Ni²⁺) blocked completely and potently the _1E,/_2b/_1b mediated currents (IC₅₀ = 4 and 24.6 µM, respectively). _1E-mediated currents inactivated rapidly and mainly in a non-Ca²⁺-dependent manner, as evidenced by the fact that 1) decreasing extracellular Ca²⁺ from 10 to 2 mM and 2) changing the intracellular concentration of the Ca²⁺ chelator 1.2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA), did not affect the inactivation characteristics; 3) there was no clear-cut bell-shaped relationship between test potential and inactivation, as would be expected from a Ca²⁺-dependent event. Although Ba²⁺ substitution did not affect the inactivation of _1E channels, Na⁺ substitution revealed a small but significant reduction in the extent and rate of inactivation, suggesting that besides the presence of dominant voltage-dependent inactivation, _1E channels are also affected by a divalent cation-dependent inactivation process. We have analyzed the Ca²⁺ currents produced by a range of imposed action potential-like voltage protocols (APVPs). The amplitude and area of the current were dependent on the duration of the waveform employed and were relatively similar to those described for HVA calcium channels. However, the peak latency resembled that obtained for low-voltage-activated (LVA) calcium channels. Short bursts of APVPs applied at 100 Hz produced a depression of the Ca²⁺ current amplitude, suggesting an accumulation of inactivation likely to be calcium dependent. The human _1E gene seems to participate to a Ca²⁺ channel type with biophysical and pharmacological properties partly resembling those of LVA and those of HVA channels, with inactivation characteristics more complex than previously believed.