NEW SODIUM CHANNEL-BLOCKING CONOTOXINS ALSO AFFECT CALCIUM CURRENTS IN LYMNAEA NEURONS

Two new conotoxins that affect both sodium and calcium currents have b een characterized from the venom of Conus marmoreus, using direct assa ys on voltage-gated currents in caudodorsal neurons (CDC) of the fresh water snail Lymnaea stagnalis. The designations and amino acid sequenc es of the new toxins are MrVIA, ACRKKWEYCIVPIIGFIYCCPGLICGPFVCV, and M rVIB, ACSKKWEYCIVPILGFVYCCPGLICGPFVCV. Both toxins block voltage depen dent sodium currents in snail neurons with ED(50)'s of 0.1-0.2 mu M. E ffects are also observed on the fast-inactivating calcium current subt ype in the CDC at greater than or equal to 1 mu M. At concentrations o f 1-5 mu M, MrVIA acts as a calcium current agonist whereas MrVIB acts as a blocker. At higher doses both toxins block the fast-inactivating calcium current. Almost no effects of MrVIB are seen on the second (s ustained kinetics) CDC calcium current subtype, while MrVIA also sligh tly blocks the sustained current. The calcium current block is rapidly reversible, whereas in contrast recovery of the sodium current requir es extensive wash. MrVIA/B have the same cysteine framework as the ome ga- and delta-conotoxins and a high content of hydrophobic residues, i n common with the delta-conotoxins. There is only one localized concen tration of charged residues in MrVIA/B, in the first intercysteine loo p. These two conotoxins provide unique probes for structure and functi on studies on voltage-gated sodium and L-type calcium channels. Their unusual cross-channel activity suggests they may represent an ''interm ediate'' variant of conotoxin, in the diversification of one conotoxin structural family that selectively targets either sodium or calcium c hannels.