AN UNUSUAL PRESYNAPTIC ACTION OF BOTHROPS INSULARIS SNAKE-VENOM MEDIATED BY PHOSPHOLIPASE A(2) FRACTION

A phospholipase A(2)-containing fraction was isolated from the venom o f Bothrops insularis by a combination of gel filtration on Sephadex G- 150 and ion exchange chromatography on DEAE-Sephadex, Peak IV of the l atter chromatography containing all of the phospholipase A(2) (PLA(2)) activity, was assayed on isolated neuromuscular preparations. In the mouse phrenic nerve-diaphragm incubated in Tyrode at 37 degrees C, the PLA(2) fraction produced an initial increase in the twitch tension an d in the frequency of the mepps, followed by a dose-dependent, irrever sible blockade. The replacement of 1.8 mM Ca2+ by 4 mM Sr2+ Inhibited the neuromuscular blocking effect of the fraction. In the chick bivent er cervicis preparation incubated with Krebs solution at 37 degrees C, the PLA(2) fraction induced blockade but did not affect the response to acetylcholine and K+, excluding the involvement of post-synaptic an d direct muscular effects. A low temperature (18-22 degrees C) incubat ion prevented the neuromuscular effect from developing. These results suggest that the PLA(2)-containing fraction acts predominantly at pres ynaptic sites at the neuromuscular junction. This fraction also accoun ts for most of the pharmacological effects of the crude venom. (C) 199 8 Published by Elsevier Science Ltd. All rights reserved.