Action of black widow spider venom on quantized release of acetylcholine at the frog neuromuscular junction: dependence upon external Mg2+.

Black widow spider (Latrodectus tredecimguttatus) venom (BWSV) increases several hundredfold the frequency of occurrance of minature end-plate potentials (Fmepp) at frog neuromuscular junctions bathed in Ringer's solutions containing either Ca2+ of Mg2+, but it has little effect on Fmepp at junctions bathed in modified Ringer's solution containing 1--2 mM ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) but no Ca2+ or Mg2+. When Mg2+ is added to preparations that have been treated with BWSV in the modified solution, Fmepp increases exponentially with time. Fmepp falls again to low values when the Mg2+ is removed. The rate constant of the exponential rise is proportional to [Mg2+]o in the range 1--4 mM, and the threshold [Mg2+]o is 0.1--0.5 mM. Increasing the K+ concentration of the bathing solution decreases the ability of Mg2+ to increase Fmepp. Addition of Ca2+, Co2+, Mn2+, or Zn2+ also leads to a large increase in Fmepp. These results are consistent with the possibility that BWSV increases the permeability of the nerve terminal to divalent cations. BWSV can, however, increase Fmepp in hypertonic solutions in the absence of external divalent cations. This result suggests that the effects of BWSV on the nerve terminal may not be confined to increasing the permeability of the plasma-lemma.