Neuromuscular effects of three phospholipases A2 from the venom of the Australian king brown snake Pseudechis australis

Three single chain phospholipases A2 (Pa-10A, Pa-11 and Pa-13) isolated from Australian king brown snake (Pseudechis australis) venom were tested for effects on neuromuscular transmission and muscle contractility on chick biventer cervicis and mouse diaphragm preparations. At 1 μg/ml (about 85 nM) and higher, Pa-10A and Pa-11 reduced responses of both preparations to indirect stimulation in a concentration-dependent manner. Responses to direct muscle stimulation were generally reduced more slowly. Pa-11 also decreased membrane potentials of chick biventer muscle fibres and caused damage visible by light microscopy. Pa-13, which is about 50 times less active as a phospholipase A2, was also less potent in its pharmacological effects: 20 μg Pa-13 per ml were required to reduce responses of either preparation. The phospholipases A2 also caused a slow contracture. After block of responses to nerve stimulation, responses of the chick preparation to acetylcholine, carbachol and KCl could be obtained, although they were smaller than control and highly variable in different preparations. It is concluded that Pa-10A and Pa-11 produce muscle paralysis by reducing acetylcholine release and by a direct blockade of muscle fibre contractility. Pa-13 has similar, though less pronounced, activities