REVERSAL BY CYSTEINE OF THE CADMIUM-INDUCED BLOCK OF SKELETAL NEUROMUSCULAR-TRANSMISSION INVITRO

1 Neuromuscular transmission in isolated nerve-muscle preparations was blocked by exposure to Cd2+ for less than 30 min or more than 2 h. The abilities of cysteine, Ca2+ or 3,4-diaminopyridine (3,4-DAP) to reverse the blockade induced by Cd2+ were studied. 2 On the mouse hemidiaphragm preparation, exposure to Cd2+ (10-mu-m) for 10 to 20 min induced a blockade which was easily reversed by increasing the extracellular Ca2+ concentration (5-10 mM) or by 3,4-DAP (100-mu-m). Exposure to Cd2+ (3-10-mu-m) for over 2 h led to a blockade which was not reversed by Ca2+ (5-15 mM) or 3,4-DAP (100-mu-m). Cysteine (1 mM) was able to reverse completely the blockade induced by both brief and prolonged exposures to Cd2+. 3 In chick biventer cervicis preparations, Cd2+ (100-mu-m) decreased the twitch height of indirectly stimulated preparations without affecting responses to exogenously applied acetylcholine, carbachol or KCl. Cysteine (1-3 mM) had no appreciable effect on twitch responses to indirect stimulation or to exogenously applied agonists but fully reversed the blockade induced by Cd2+ (100-mu-M). 4 In mouse triangularis sterni preparations, Cd2+ (1-30-mu-m) depressed the evoked quantal release of acetylcholine. Concentrations of Cd2+ which completely blocked endplate potentials (e.p.ps) were without significant effect on miniature endplate potential (m.e.p.p.) amplitude and frequency or time constant of decay. Cysteine (1-10 mM) alone had no effect on e.p.ps or m.e.p.ps, but completely reversed the blockade induced by Cd2+. 5 Extracellular recording of perineural waveforms from triangularis sterni preparations revealed that Cd2+ was able to block the long-lasting positive component that is sensitive to Ca2+ channel blockers and the delayed negative deflection that is related to the Ca2+-activated K+ current (I(K,Ca)) seen in the presence of 3,4-DAP. Cysteine by itself had no effect on any component of the perineural waveform, but promptly reversed the blockade induced by Cd2+. 6 In addition to the competitive blocking action of Cd2+ at the prejunctional Ca2+ channels, long exposure to Cd2+ leads to a blockade that is not competitive. This probably involves binding of Cd2+ at an extracellular thiol site on, or close to, voltage-operated Ca2+channels.