J. C. Cogo, J. Prado-Franceschi, M. A. Cruz-Hofling, A. P. Corrado and L. Rodrigues-Simioni. Effect of Bothrops insularis venom on the mouse and chick nerve-muscle preparation. Toxicon 31, 1237-1247, 1993.-The effects of Bothrops insularis venom were examined in vivo in mice and chicks and in vitro using the mouse phrenic nerve diaphragm and chick biventer cervicis muscle preparations. Incubation of the indirectly or directly stimulated mouse preparation with B. insularis venom (20-80 μg/ml) produced an initial increase in twitch tension followed by irreversible blockade. With direct stimulation in the presence of d-tubocurarine, no increase in twitch tension was observed prior to the onset of blockade. A venom-induced effect on presynaptic activity was suggested by the marked increase in the frequency of the mepps recorded in vitro 5-15 min after venom addition. A direct muscular effect was shown by the dose- and time-dependent reduction in the resting membrane potential of the diaphragm. Chick preparations were more sensitive than those of the mouse. In the isolated chick biventer cervicis muscle preparation, B. insularis venom induced a contracture and a dose-dependent block of responses to indirect stimulation. At low venom concentrations (1-5 μg/ml), no significant release of creatine kinase (CK) was observed from this preparation. However, a dose-dependent release of CK was detected at higher doses (10-80 μg/ml). For morphological studies, B. insularis venom was injected into the chick left pectoralis muscle. At low doses (0.4 μg), only an inflammatory reaction was present, while at high doses (20-80 μg) increasing numbers of necrotic fibers were observed as well as occlusive thrombosis and hemorrhage. 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