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Immunoglobulin G(IgG) prepared from the plasma of patients with a presynaptic disorder of neuromuscular transmission (Lambert‐Eaton myasthenic syndrome, l.e.m.s.), or from normal pooled control human plasma, was injected into mice (10 mg daily) for up to 99 days. Micro‐electrodes were used to record end‐plate potentials from the diaphragm muscle bathed in normal Krebs solution containing tubocurarine (1.0‐4.6 microM). At 0.5 Hz nerve stimulation frequency, the quantal content was significantly reduced (P less than 0.01‐P less than 0.001) in mice treated with six l.e.m.s. patients’ IgG each compared with paired controls. The pooled quantal content was 55 +/‐ 3 (n = 110 end‐plates) for all test animals and 131 +/‐ 9 (n = 47) for all controls (P less than 0.001). During short trains at 20 or 40 Hz nerve stimulation, control muscles showed marked depression, while test muscles showed either facilitation or less marked depression. Quantal content throughout these trains remained lower than in controls. The results indicate that IgG antibody from l.e.m.s. patients can induce a similar physiologic disorder in injected mice, and they support the view that this antibody interferes with evoked release of transmitter in l.e.m.s. by binding to nerve terminal determinants.

1. The presynaptic mechanisms governing the release and recycling of synaptic vesicles have been studied by examining the effects of nerve stimulation, (‐)‐vesamicol (an inhibitor of acetylcholine transport into synaptic vesicles) and troxypyrrolium (an inhibitor of the high‐affinity, sodium‐dependent, choline uptake system) on endplate currents (EPCs) and miniature endplate currents (MECPs) recorded from motor endplates in cut rat hemidiaphragm preparations. 2. In control experiments, 5 min of 10 Hz nerve stimulation had no effect on either the mean or the distribution of MEPC amplitudes. 3. Nerve stimulation in the presence of (‐)‐vesamicol (25 nM‐10 microM) revealed a population of MEPCs that was unaffected by the compound and a population of MEPCs whose mean amplitude was selectively reduced by the compound. 4. Nerve stimulation in the presence of troxypyrrolium (20 microM) produced a uniform reduction in the amplitude of all MEPCs with no change in the coefficient of variance of MEPC amplitudes. 5. The concentration‐dependent effects of (‐)‐vesamicol on the amplitude of the evoked EPCs paralleled the concentration‐dependent effects of the compound on MEPC amplitudes. 6. The results are consistent with the hypothesis that both recycled and performed synaptic vesicles are heterogeneously released from rat motor nerve terminals and that (‐)‐vesamicol acts selectively on recycling vesicles. In addition, a model of vascular loading that accounts for the different effects of nerve stimulation on MEPC amplitudes in the presence of (‐)‐vesamicol and troxypyrrolium is described.

1. The frequency (0.5‐150 Hz) and calcium dependence (0.5‐2.0 mM) of the effects of the nicotinic antagonist tubocurarine (0.2 microM) on acetylcholine (ACh) liberation from motor nerve terminals has been examined using binomial analysis of quantal transmitter release. 2. At an extracellular calcium ion concentration ([Ca2+]o) of 2.0 mM, tubocurarine produced a decrease in the endplate current (EPC) quantal content of approximately 30% at high frequencies of motor nerve stimulation (50‐150 Hz). In contrast, at low frequencies of stimulation (0.5‐1.0 Hz), tubocurarine enhanced the EPC quantal content by approximately 20%. 3. The enhancement of EPC quantal content produced by tubocurarine at low frequencies of motor nerve stimulation was [Ca2+]o dependent, being abolished when [Ca2+]o was lowered from 2.0 to 0.5 mM. In contrast, the decrease in quantal content produced by tubocurarine at high frequencies of motor nerve stimulation was independent of [Ca2+]o, being approximately 30% at all calcium ion concentrations studied. 4. In direct contrast to tubocurarine, the nicotinic antagonist vecuronium (1.0 microM) produced no increase in EPC quantal content at low frequencies of nerve stimulation. However, at high frequencies of nerve stimulation it decreased EPC quantal content to a similar extent to 0.2 microM tubocurarine. The frequency‐dependent decrease in EPC quantal content produced by 1.0 microM vecuronium in 2.0 mM [Ca2+]o was very similar to that seen with 0.2 microM tubocurarine in 0.5 mM [Ca2+]o. 5. Binomial analysis revealed that all the changes in EPC quantal content associated with both nicotinic antagonists were due to changes in the size of the pool of quanta in the nerve terminal available for immediate release with no effect on the probability of release of an individual quantum. 6. The results are interpreted in terms of two separately identifiable prejunctional actions of the nicotinic antagonists, both involving an action at nicotinic ACh receptors situated on the motor nerve terminal. Thus, at high frequencies of motor nerve stimulation tubocurarine and vecuronium produce a [Ca2+]o‐independent decrease in ACh release, probably through an inhibitory action on a positive‐feedback prejunctional nicotinic autoreceptor closely related to the muscle‐type nicotinic ACh autoreceptor. However, at low frequencies of motor nerve stimulation we suggest that tubocurarine, but not vecuronium, produces a [Ca2+]o‐dependent increase in ACh release through an action at a negative‐feedback prejunctional neuronal‐type nicotinic ACh autoreceptor.

1. The action of immunoglobulin G obtained from patients with Lambert‐Eaton myasthenic syndrome (LEMS IgG) was investigated by injecting mice, followed by intracellular recordings from the mouse diaphragm. 2. End‐plate potential quantal content was studied over a range of Ca2+ concentrations. Curves of log quantal content versus log Ca2+ concentration were shifted to the right by LEMS IgG. For low Ca2+ concentrations, release continued to follow Poisson statistics after LEMS IgG treatment. 3. Miniature end‐plate potential (m.e.p.p.) frequency was measured in solutions containing high K+ concentrations. LEMS IgG significantly reduced m.e.p.p. frequency at each K+ concentration studied. 4. M.e.p.p. frequency was measured at fixed high‐K+ concentration (15.9 mM) for a range of Ca2+ concentrations. The log‐log plot of m.e.p.p. frequency versus Ca2+ concentration was shifted downwards throughout by LEMS IgG. 5. M.e.p.p. frequency was not affected by LEMS IgG in Ca2+‐free solutions (K+ concentration 15.9 mM) or in solutions of low Ca2+ concentration (K+ concentration 5.9 mM). 6. At each Ca2+ concentration studied, m.e.p.p. amplitudes were not affected by LEMS IgG. 7. The data suggest that LEMS IgG acts on presynaptic voltage‐dependent Ca2+ channels to cause their loss of function, probably by down‐regulation.