Presynaptic and postsynaptic sites of action of enhydrotoxin-a (EsNTx-a) isolated from Enhydrina schistosa venom

Abstract

Abstract Enhydrotoxin-a (EsNTx-a), one of the three toxins isolated from the venom of Enhydrina schistosa produced an irreversible blockade of indirect stimulation of rat phrenic nerve hemidiaphragm preparation in concentration of 0.5 to 1 μg ml−1. The response to direct stimulation was not affected over a 4 h period. The neuromuscular blockade produced by EsNTx-a was antagonized in so far as twitches evoked by indirect stimulation are concerned, when the preparation was treated with approximately double the concentration of purified sea snake antivenin. The tension of tetanic contractions, although depressed in amplitude by the toxin, was sustained. However, it was converted to a twitch-like response in toxin-antivenin treated preparations. In the presence of eserine (1 × 10−6 g ml−1), the twitch-like tetanus was dramatically restored in toxin-antivenin treated rat phrenic nerve hemidiaphragm preparations. Intracellular recordings showed that the frequency of spontaneous miniature end plate potentials (m.e.p.ps) recorded in EsNTx-a poisoned (1.24 × 10−6 g ml) rat phrenic nerve hemidiaphragm preparations decreased to zero, while the m.e.p.p. amplitude was decreased in the later phase of block. The evoked end plate potentials (e.p.p.) in EsNTx-a poisoned (1.24 × 10−1 g ml−1) preparations progressively diminished to zero. The antivenin molecules may have an easy access to bind to the toxin receptor complexes at the postsynaptic site while the presynaptic binding site by the toxin may be relatively inaccessible due to their being intracellular. The presynaptic effect of the toxin was revealed only when the postsynaptic effect was abolished.