Novel omega-conotoxins block dihydropyridine-insensitive high voltage-activated calcium channels in molluscan neurons.

We have identified two novel peptide toxins from molluscivorous Conus species that discriminate subtypes of high voltage-activated (HVA) calcium currents in molluscan neurons. The toxins were purified using assays on HVA calcium currents in the caudodorsal cells (CDCs) of the snail Lymnaea stagnalis. The CDC HVA current consists of a rapidly inactivating, transient current that is relatively insensitive to dihydropyridines (DHPs) and a slowly inactivating, DHP-sensitive L-current. The novel toxins, designated omega-conotoxins PnVIA and PnVIB, completely and selectively block the transient HVA current in CDCs with little (PnVIA) or no (PnVIB) effect on the sustained L-type current. The block is rapid and completely reversible. It is noteworthy that both PnVIA and PnVIB reveal very steep dose dependences of the block, which may imply cooperativity in toxin action. The amino acid sequences of PnVIA (GCLEVDYFCGIPFANNGLCCSGNCVFVCTPQ) and of PnVIB (DDDCEPPGNFCGMIKIGPPCCSGWCFFACA) show very little homology to previously described omega-conotoxins, although both toxins share the typical omega-conotoxin cysteine framework but have an unusual high content of hydrophobic residues and net negative charge. These novel omega-conotoxins will facilitate selective analysis of the functions of HVA calcium channels and may enable the rational design of drugs that are selective for relevant subtypes.