New findings for bothropstoxin-I, a myotoxin from Bothropsjararacussu venom

Oshima-Franco, Y.; Leite, G.B.; Belo, C.A.D.; Hyslop, S.; Giglio, J.R.; Cruz-Höfling, M.A.; Rodrigues-Simioni, L.

Department of Pharmacology, Faculty of Medical Sciences, State University of Campinas (UNICAMP), 13083-970, Campinas, SP, Brazil.

Bothropstoxin-I (BthTX-I) from Bothrops jararacussu venom is a Lys49 PLA₂ and has myotoxic and neurotoxic activities. In this study, we investigated the mode of action of BthTX-I in mouse phrenic nerve-diaphragm preparations using myographical and electrophysiological techniques. The effects of Mn²⁺ on the neuromuscular action of BthTX-I were also examined. BthTX-I (1.4 mM) produced 50% neuromuscular blockade in 31 + 6 min whereas Mn²⁺ (0.9 mM and 1.8 mM) produced rapid blockade (50% in less than 4 min) which was spontaneously reversible at the lower concentration. Pre treating preparations with 0.9 mM Mn²⁺ prevented the blockade by BthTX-I. When added after BthTX-I, Mn²⁺ produced its characteristic blockade and, after washing, the twitch tension returned to pre-Mn²⁺ levels. Using d-tubocurarine (5.8 mM) showed that BthTX-I acted directly on muscle cell membranes and not on nicotinic receptors. Unlike 3,4-diaminopyridine (0.09 mM), BthTX-I (1.4 mM) did not block K⁺ channels, nor did it exert a specific sarcolemmal action such as seen with dantrolene(10 mM). Electrophysiological measurements revealed a novel pre synapticaction for BthTX-I (0.35 mM) seen as the appearance of ‘giant’ miniature end plate potentials before any change in resting potential. Preparations preincubated with Mn²⁺ (0.9 mM, 30 min) were protected against the depolarizing action of BthTX-I (0.7 mM). These results indicate that BthTX-I acts on axolemmal and sarcolemmal membranes via Ca²⁺ channels.

CORRESPONDENCE TO:

Rodrigues-Simioni, L., Department of Pharmacology, Faculty of Medical Sciences, State University of Campinas (UNICAMP), 13083-970, Campinas, SP, Brazil, Email: simioni@unicamp.br