1. Presynaptic effects of d-tubocurarine on neurotransmitter release were examined at the frog neuromuscular junction, using intracellular and extracellular recording techniques. 2. d-Tubocurarine in concentrations of 10(-7)-10(-6) M decreased the quantal content (m) measured by the coefficient of variation and failure methods. 3. d-Tubocurarine produced a shift to the right of the curve relating log quantal content to log [Ca2+]o without changing the slope. 4. The duration of twin-impulse facilitation was not affected by 5 x 10(-7) M-d-tubocurarine. Early facilitation was higher in d-tubocurarine. 5. d-Tubocurarine altered the synaptic delay histogram. The peak of the histogram was shifted to longer delays. Prolongation of the minimal delay was seen in most but not all experiments. 6. These results suggest that d-tubocurarine inhibits release of neurotransmitter by affecting a stage in the process of release, which occurs after the entry of Ca2+ ions.
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