Bothropstoxin-I from Bothrops jararacussu snake venom is a lysine-49 phospholipase A 2 with myotoxic and neurotoxic activities. In this study, we used mouse phrenic nerve-diaphragm preparations in the absence and presence of manganese (Mn 2π ), a presynaptic blocker, to investigate a possible presynaptic action of bothropstoxin-I. At concentrations of 0.9 mM and 1.8 mM, Mn 2π produced 50% neuromuscular blockade in less than 4 min., which was spontaneously reversible at the lower concentration. Bothropstoxin-I (1.4 mM) irreversibly inhibited neuromuscular blockade by 50% in 31∫4 min. (mean∫S.E.M., nΩ9). Pretreating preparations with 0.9 mM Mn 2π prevented the blockade by bothropstoxin-I. When added after bothropstoxin-I, Mn 2π produced its characteristic blockade and, after washing, the twitch tension returned to pre-Mn 2π levels, indicating that bothropstoxin-I caused irreversible damage before the addition of Mn 2π . Electrophysiological measurements showed that a concentration of bothropstoxin-I (0.35 mM), which did not produce neuromuscular blockade, caused the appearance of giant miniature end-plate potentials with no change in the membrane resting potential but increased the quantal content. Preparations preincubated with Mn 2π (0.9 mM, 30 min.) were protected against the depolarizing action of bothropstoxin-I (0.7 mM). These results show that, in addition to its well-known myotoxic effect, bothropstoxin-I also has a presynaptic action.

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In this work, we studied the neuromuscular blockade caused by Micrurus altirostris venom (0.1-10 microg/mL) in indirect stimulated chick biventer cervicis and mouse phrenic nerve-diaphragm preparations and the ability of commercial antivenom (Instituto Butantan) and antiserum raised in rabbits to neutralize neurotoxicity and lethality in chicks and mice (LD(50) 0.042 and 0.255 mg/kg), injected i.m. and i.p., respectively, with venom (5 LD(50)):antivenom or antiserum mixtures (n = 6) of 1:1-1:2.5-1:5-1:10-1:20. The venom caused a complete and irreversible neuromuscular blockade in both preparations, inhibited the acetylcholine and carbachol contractures, without interfering on KCl response. The neuromuscular blockade was not Ca(2+) or temperature-dependent and did not affect the response to direct stimulation. Only a venom:antivenom or antiserum ratio of 1:20 neutralized the neuromuscular blockade in vitro and protected chicks and mice against 5 LD(50) of venom. Our results indicated that Micrurus altirostris venom interferes with postsynaptic neurotransmission and that commercial antivenom and rabbit antiserum have low efficacy in neutralizing the neurotoxicity and lethality of this venom.

Casearia sylvestris Sw., popularly known in Brazil as 'guaçatonga', has been used as antitumor, antiseptic, antiulcer, local anaesthetic and healer in folk medicine. Snakebite envenomation by Bothrops jararacussu (Bjssu) constitutes a relevant public health hazard capable of inducing serious local damage in victims. This study examined the pharmacological action of apolar and polar C. sylvestris leaf extracts in reverting the neuromuscular blockade and myonecrosis, which is induced by Bjssu venom and its major toxin bothropstoxin-I on the mouse phrenic nerve-diaphragm preparations. The polar methanol extract (ME) was by far the most efficacious. ME not only prevented myonecrosis and abolished the blockade, but also increased ACh release. Such facilitation in neuromuscular transmission was observed with ME alone, but was accentuated in preparations incubated with ME plus venom or toxin. This established synergy opens an interesting point of investigation because the venom or toxin in contact with ME changes from a blocking to a facilitating effect. It is suggested that rutin, known to have potent antioxidant properties, and one of the components present in the ME, could have a role in the observed effects. Since commercial rutin did not reproduce the ME effects, it is likely that a rutin-containing phytocomplex is neutralizing the bothropic envenoming effects.

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