The presynaptic activity of bothropstoxin-I, a myotoxin from Bothrops jararacussu snake venom

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(2+)), a presynaptic blocker, to investigate a possible presynaptic action of bothropstoxin-I. At concentrations of 0.9 mM and 1.8 mM, Mn(2+) produced 50% neuromuscular blockade in less than 4 min., which was spontaneously reversible at the lower concentration. Bothropstoxin-I (1.4 microM) irreversibly inhibited neuromuscular blockade by 50% in 31+/-4 min. (mean+/-S.E.M., n = 9). Pretreating preparations with 0.9 mM Mn(2+) prevented the blockade by bothropstoxin-I. When added after bothropstoxin-I, Mn(2+) produced its characteristic blockade and, after washing, the twitch tension returned to pre-Mn(2+) levels, indicating that bothropstoxin-I caused irreversible damage before the addition of Mn(2+). Electrophysiological measurements showed that a concentration of bothropstoxin-I (0.35 microM), 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(2+) (0.9 mM, 30 min.) were protected against the depolarizing action of bothropstoxin-I (0.7 microM). These results show that, in addition to its well-known myotoxic effect, bothropstoxin-I also has a presynaptic action.