Mode of neuromuscular blocking action of toxic phospholipases A2 from Vipera ammodytes venom.

The effects of toxic phospholipases A2 ( fraxtions "j", "k1" and "k2") isolated from the venom of Vipera ammodytes were studied on the chick biventer cervicis muscle and the mouse phrenic nerve-diaphragm preparations. In the chick muscle, all of these PLA2s caused neuromuscular (N-M) blockade without producing contracture or affecting the response of the muscle to acetylcholine. In the mouse diaphragm, these PLA2s inhibited completely the indirectly elicited contraction without affecting that evoked directly. The order of their N-M blocking potency is "k2" greater than "k1" greater than or equal to "j". In a low Ca2+ (0.5 mM) medium, they produced a triphasic change in the indirectly elicited contractions: an initial inhibition followed by an enhancement and then a progressive depression leading to complete N-M blockade. The frequency of miniature endplate potentials (m.e.p.p.s) in the mouse diaphragm first increased 2-3 fold and then gradually decreased after "k2" treatment, while the amplitude of m.e.p.p.s did not decrease even after the evoked release of transmitter failed. Giant potentials and bursts of m.e.p.p.s were frequently observed. The quantal content of e.p.p.s was first increased and then decreased gradually. The resting membrane potential was only slightly reduced at 30 micrograms per ml. The ultrastructure of motor nerve terminals in the "k2"-intoxicated mouse diaphragm showed an increase in omega-shaped indentation in the axolemma. The mitochondria in the nerve terminal were swollen and vacuolized. No structural changes were found in the muscle fibers, fibrocytes and myelinated axons in the diaphragm. It is concluded that the toxic PLA2s from Vipera ammodytes venom produce a N-M blockade by acting selectively on the presynaptic site.