δ-philanthotoxin, a semi-irreversible blocker of ion-channels

Abstract

1. The digger wasp, Philanthus triangulum, which preys on honeybees, produces a paralysing venom possessing a wide variety of activities. 2. In insects, the venom has a central as well as a peripheral effect; the latter effect consists of a presynaptic as well as a postsynaptic block of the skeletal neuromuscular transmission. 3. The presynaptic block is probably caused by an inhibition of the re-uptake of the transmitter. The postsynaptic effect probably consists of a block of open ion channels. 4. The venom contains at least four active toxins called α-, β-, γ- and δ-philanthotoxin (PTX). α-PTX blocks transmission in the cockroach CNS. The other three toxins block neuromuscular transmission, δ-PTX being the most active toxin in blocking glutamate evoked postsynaptic depolarizations. 5. In the junctional, as well as in the extrajunctional, muscle fibre membrane δ-PTX blocks ion channels in a use-dependent manner. Once the channel has been blocked, unblocking seems to be semi-irreversible when agonist activation is low (spontaneous release of transmitter and/or leak of glutamate from the pipette). 6. The time constant of blocking is roughly estimated to be in the order of 10 msec, that of unblocking seems to be several hundreds of msec.