Diversity and properties of calcium channel types in NG108-15 hybrid cells

We used an integral of the current–voltage relation as a new evaluation of Ca 2+ current component composition in NG108-15 hybrid cells. We determined significant changes in the values and composition of Ca 2+ currents during cell differentiation. Only low-voltage-activated Ca 2+ currents could be observed in undifferentiated cells; after cell differentiation, high-voltage-activated currents appeared and the total Ca 2+ current was increased about 30-fold. By pharmacological and biophysical separation, we determined four main types of Ca 2+ channels in differentiated cells: approximately 50%, 20% and 17% of N, T and L types, respectively, and 12% of residual current, which is insensitive to classical blockers of low- and high-voltage-activated currents, with the exception of ω-conotoxin GVIA. All current components displayed kinetics and pharmacological properties similar to neuronal ones. We also established a significant Ca 2+ dependence of ω-conotoxin GVIA to inhibit N-type Ca 2+ channels: 10 mM Ca 2+ in bath solution reduced the toxin efficacy to block N channels three-fold. The residual component fitted the properties of Q-type Ca 2+ channels: it was sensitive to ω-conotoxin GVIA and very similar to the T-type channel with respect to its kinetics; however, the threshold of its activation was closer to the high-voltage-activated component (−40 mV). Our results show the functional diversity of Ca 2+ channels and demonstrate, for the first time, that presumably the Q type of an α 1A family, which has biophysical and pharmacological properties distinct from the previously described T, L and N types in these cells, is co-expressed in NG108-15 cells.