[Effect of presynaptic neurotoxins from the bee and cobra venoms on spontaneous mediator secretion from the motor nerve endings in mice].

Phospholipases A2 (PhLA) from the bee and cobra venoms induced the three-phasic changes in miniature end-plate potential (MEPP) frequency recorded in the mouse diaphragm muscle: an initial fall of transmitter release followed by a transient increase before the final complete blockade. The removal of Ca2+ from the perfusing solution (below 10(-9) M) prevented the presynaptic effect of both PhLA. If all PhLA molecules were washed out by the Ca-free solution, the subsequent exposure to a standard solution (2 mM Ca2+) action provoked an increase in MEPP frequency. The agents capable of increasing the axoplasmic Ca concentration (ions K+, hypertonic sucrose and uncoupler--TTFB) caused the usual increase of MEPP frequency in the muscles treated with PhLA. Apparently, the presynaptic biockade induced by PhLA cannot be due to the depletion of transmitter stores.