Neurotoxic and myotoxic actions of Naja naja kaouthia venom on skeletal muscle in vitro

Abstract

The neuromuscular and skeletal muscle actions of Naja naja kaouthia snake venom were studied in mammalian (rat left hemidiaphragm) and avian (chick biventer cervicis) nerve–muscle preparations. The venom (5 and 10 μg/ml) produced neuromuscular blockade (85% in 36.8±2.0 min, mean±SEM, n=5, and 18±0.6 min, n=3, p<0.01, respectively) in the rat preparation. That the phospholipase A 2 (PLA 2) activity of the venom is involved in this effect was evaluated by inhibiting this enzyme with p-bromophenacyl bromide. This resulted in significantly ( p<0.01) increasing the time required for 85% blockade with 5 and 10 μg/ml to 54±4.6 min ( n=3) and 29±0.6 min ( n=3), respectively. In chick preparations, the venom (5 μg/ml) produced neuromuscular blockade in 14.0±1.8 min ( n=5). The contractures to exogenous acetylcholine were completely inhibited by the venom, whereas those to 134 μM KCl were partially blocked in chick preparations ( n=4, n=3, respectively). The venom (5 μg/ml) produced a progressive decrease in the amplitude of miniature end-plate potentials (m.e.p.ps) in the rat hemidiaphragm, but did not alter the resting membrane potential at 5 μg/ml. Neostigmine (5.8 μM) immediate and partially reversed the 85% blockade produced by venom (61%, n=3) in rat preparations, as did 4-aminopyridine (53 μM) (∼59%, n=3). The 4-aminopyridine and neostigmine also restored the m.e.p.ps to pre-venom (control) values. In rat preparations, the venom damaged 47%±11% and 62.7±3.6% of the muscle fibers at concentrations of 5 and 10 μg/ml, respectively. For venom in which PLA 2 activity was inhibited, the corresponding values were 38±11.8% (5 μg/ml) and 67±9.6% (10 μg/ml). These findings suggest a post-synaptic neurotoxic action for N. n. kaouthia venom, and that inhibiting phospholipase activity of the venom reduces significantly the neuromuscular block but not the direct myotoxicity.