Abstract
Chromaffin cells express N-type calcium channels identified on the basis of their high sensitivity to block by ω-conotoxin GVIA (ω-CgTx GVIA). In contrast to neuronal N-type calcium currents that inactivate during long depolarizations and that require negative holding potentials to remove inactivation, many chromaffin cells exhibit N-type calcium channel currents that show little inactivation during maintained depolarizations and that exhibit no decrease in channel availability at depolarized holding potentials. N-type calcium channels are thought to be produced by combination of the pore-forming α1B subunit and accessory β and α2/δ subunits. To examine the molecular composition of the non-inactivating N-type calcium channel, we cloned the α1B and accessory β (β1b, β1c, β2a, β2b, and β3a) subunits found in bovine chromaffin cells. Expression of the subunits in either Xenopusoocytes or human embryonic kidney 293 cells produced high-threshold calcium currents that were blocked by ω-CgTx GVIA. Coexpression of bovine α1B with β1b, β1c, β2b, or β3aproduced currents that were holding potential dependent. In contrast, coexpression of bovine α1B with β2aproduced holding potential-independent calcium currents that closely mimicked native non-inactivating currents, suggesting that non-inactivating N-type channels consist of bovine α1B, α2/δ, and β2a.