Electrophysiological characterisation of the human N-type Ca2+ channel II: activation and inactivation by physiological patterns of activity.

In a cell line (C2D7) stably expressing the human N-type calcium channel encoded by the subunits alpha1B-a, beta1b, alpha2bdelta, we have analysed the Ca2+ currents produced by a range of action potential-like voltage protocols (APVPs). Such protocols consistently produced robust inward currents that could be eliminated by co-application of the Ca2+ channel blocking ions Cd2+ and La3+. The amplitude, latency to peak and area of the current produced by APVPs was dependent on the precise waveform of voltage protocol employed and the temperature. Short bursts of APVPs applied at 100 Hz produced a depression of the Ca2+ current amplitude which was dependent on the half-width of the APVP employed. In contrast, no frequency-dependent changes in the evoked current kinetics were detected. The amount of current depression seen during an 100 Hz 8 APVP burst was greatly enhanced by increasing the temperature from 22 to 37 degrees C. Alterations to the intracellular Ca2+ buffering capacity suggested that the Ca2+ current depression produced during an APVP train arose, at least in part, from a Ca2+-dependent inactivation of the human N-type Ca2+ channel.