ω-Agatoxins differentially block calcium channels in locust, chick and rat synaptosomes

Abstract

Three toxins (ω-Agatoxins IA, IIA and IIIA) isolated from the venom of the funnel web spider, Agelenopsis aperta, differentially block depolarization-induced calcium influx in chick, rat and locust synaptosomes. In chick, this block of calcium influx is observed with ω-Agatoxins IIA and IIIA but not with ω-Agatoxin IA. Block by ω-Agatoxin IIA and IIIA is maximal at 70 and 82% respectively of the total depolarization-induced calcium influx; maximal suppression of calcium influx by ω-Conotoxin GVIA (ω-CgTx) is 100%. The IC 50 block with ω-Agatoxin IIA is ca 3 nM as compared with an IC 50 of 38 nM for ω-CgTx. Incomplete block of calcium influx at saturating concentrations of ω-Agatoxins IIA and IIIA (above 100 nM) suggests that both ω-Agatoxin-sensitive and -insensitive calcium channels occur in chick brain synaptosomes. In rat cerebrocortical synaptosomes, ω-Agatoxins IA and IIA are only partially effective at blocking depolarization-induced calcium influx, as is ω-CgTx, whilst IIIA blocks 47% of this influx. In synaptosomes prepared from the CNS of adult locusts, ω-Agatoxins IA and IIA are most effective at blocking depolarization-induced calcium influx; ω-CgTx and ω-Agatoxin IIIA are ineffective. Block of depolarization-induced calcium influx in chick brain synaptosomes by ω-Agatoxins IIA, IIIA and ω-CgTx suggests that the spider toxin interacts directly with the voltage-dependent calcium channel. The ω-Agatoxins are able to define functionally distinct subclasses of calcium channel based on their ability to interact with novel binding sites on neuronal, voltage-dependent calcium channels in the insect, avian and mammalian central nervous sytems. These calcium channel subtypes do not fall into the classifically defined L-, T- and N-type channels.