Coexpression of Cloned a1B, b2a, and a2/d Subunits Produces Non-Inactivating Calcium Currents Similar to Those Found in Bovine Chromaffin Cells

Chromaffin cells express N-type calcium channels identified on the basis of their high sensitivity to block by v-conotoxin GVIA (v-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 a1B subunit and accessory b and a2/d subunits. To examine the molecular composition of the non-inactivating N-type calcium channel, we cloned the a1B and accessory b (b1b , b1c, b2a , b2b , and b3a ) subunits found in bovine chromaffin cells. Expression of the subunits in either Xenopus oocytes or human embryonic kidney 293 cells produced high-threshold calcium currents that were blocked by v-CgTx GVIA. Coexpression of bovine a1B with b1b , b1c , b2b, or b3a produced currents that were holding potential dependent. In contrast, coexpression of bovine a1B with b2a produced holding potential-independent calcium currents that closely mimicked native non-inactivating currents, suggesting that non-inactivating N-type channels consist of bovine a1B, a2/d, and b2a.