Suppression by elevated calcium of black widow spider venom activity at frog neuromuscular junctions

Frog neuromuscular junctions were treated with both concentrated black widow spider venom (BWSV) and elevated extracellular calcium (5-50 mM). This procedure causes a dramatic increase in the frequency of spontaneous miniature endplate potentials (mepps) which persists for only a few minutes. In contrast, BWSV-induced mepp activity, the venom effect (VE), continues for 20 min-1 h at junctions in elevated calcium Ringer solutions treated with doses of dilute venom or at junctions in normal calcium (1.91 mM) Ringer solution treated with concentrated venom. Following the disappearance of the VE in elevated extracellular calcium, only a few normal amplitude mepps and a few giant amplitude mepps are observed. The disappearance of the VE in these preparations is irreversible and occurs whether exposure to elevated extracellular calcium precedes or follows exposure to BWSV. Electron microscopy indicates that the major structural alterations produced by exposure to concentrated BWSV and 20 mM calcium Ringer solution are the swelling of nerve terminal mitochondria and the clumping of synaptic vesicles, large numbers of which remain in the terminals. Exposure to 20 mM calcium Ringer solution alone produces no ultrastructural modifications in these preparations. These observations can best be explained if one of the effects of BWSV is to increase the permeability of the nerve terminal membrane to calcium. Only doses of concentrated venom can sufficiently elevate intracellular calcium to a concentration at which synaptic vesicles clump together, thus interruping the transmitter release process.