The effect of streptomycin on the neuromuscular junction of the frog.

The neuromuscular block produced by streptomycin was studied in the frog sartorius muscle by means of intracellular recording and iontophoretic application of acetylcholine. Decreases of the end-plate potentials (e.p.p.s) and acetylcholine potentials (ACh potentials) were observed following the application of streptomycin. At a concentration of 10(-4) M, the e.p.p. was 36.8 +/- 5.5% (mean +/- S.E.) (n = 12), and the ACh potential was 53.7 +/- 3.8% (n = 12) of the control size. The resting membrane potential and membrane conductance of the muscle fibers were not affected by streptomycin. Streptomycin did not significantly alter the spontaneous prejunctional activity, but the amplitude of the miniature end-plate potentials (m.e.p.p.s) was decreased. After the application of higher concentrations of streptomycin, the quantum content decreased from the control value. It is suggested that the reduction of the e.p.p. amplitude produced by streptomycin is mainly due to a decrease in the sensitivity of the end-plate membrane. In addition, at higher concentrations, streptomycin reduced the amount of transmitter released from the motor nerve in response to a nerve volley.