Influence of divalent cations on the phospholipase-independent action of beta-bungarotoxin at frog neuromuscular junctions.

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The influence of different divalent cations on the phospholipase-independent inhibition of transmitter release caused by beta-bungarotoxin (beta-BuTx), has been investigated by measuring the frequency of spontaneous miniature end-plate potentials (m.e.p.p.s) at frog neuromuscular junctions. After adding the toxin to normal calcium Ringer solution the m.e.p.p. frequency fell quickly to very low values. This was followed by an increase in frequency characterized by bursts of m.e.p.p.s. The temperature had a negligible effect on the speed of the first inhibition. In Ringer solutions where calcium had been substituted by other divalent cations (5 mM) in the presence of ethyleneglycol-bis-(beta-aminoethylether)N, N'-tetraacetic acid (EGTA, 1 mM), this beta-BuTx-induced decrease in m.e.p.p. frequency was markedly slower. The potency of cations in promoting the initial phase of toxin action was in the sequence: calcium greater than magnesium greater than strontium congruent to cobalt greater than manganese. This phospholipase-independent inhibition of transmitter release followed approximately first-order kinetics, suggesting that it depends mainly on toxin concentration. In the absence of any divalent cations in the Ringer solution beta-BuTx had practically no effect on m.e.p.p. frequency. It appears that beta-BuTx requires divalent cations in order to bind to motor nerve terminals and exert its initial inhibitory action on spontaneous release of transmitter quanta.

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