Distinctive functional properties of the neuronal BII (class E) calcium channel.

Functional diversity of voltage-dependent calcium channels (VDCC) is primarily due to the existence of six distinct genes of the channel-forming subunit alpha 1, which can be further classified into the L-type and neuronal non-L-type subfamilies. We have examined functional properties of the calcium channel BII expressed from the cloned cDNA, in Xenopus oocytes, and compared the results with the other members of the non-L-type subfamily, the BI and BIII channels. The BII channel is a high voltage-activated calcium channel pharmacologically features by its unique sensitivity to the inorganic blocker Ni2. The decaying component of the BII current shows high sensitivity to Ni2+ similar to that of the low voltage-activated channels and the R-type channel in cerebellar granule cells, whereas the sustained component is relatively resistant to Ni2+ as are the other high voltage-activated calcium channels. Dihydropyridines, omega-CgTx-GVIA, and omega-Aga-IVA, which have been used to discriminate L-, N-, and P-types, do not affect the BII current. The mode of modulation of the BII channel by auxiliary subunits is strikingly different from that observed in the L-type channels. Both activation and inactivation rates of the BII current are decelerated by coexpression of the beta subunit, and this effect is cancelled by further coexpression of the alpha 2 subunit. In situ tissue distribution studies indicate a higher level of BII mRNA expression in the hippocamus compared to other brain regions, revealing important difference in the relative abundance of BI, BII, and BIII channels in brain tissues. Overall, the results suggest that the BII channel forms a novel functional category of VDCC that is different from T-, L-, N-, and P-type.