Study on the neuromuscular action of 4-aminopyridine in the mouse diaphragm.

4-Aminopyridine (4-AP), in concentrations ranging from 0.01 to 0.3 mM, increased indirectly evoked twitches of mouse diaphragm; directly evoked twitches were unaffected. 4-AP did not significantly affect the resting membrane potential, the amplitude and duration of muscle action potentials, the frequency and amplitude of m.e.p.p.s. or the half decay time of e.p.p.s. and m.e.p.p.s. However, e.p.p. amplitude and quantal content were increased by 4-AP in a concentration-dependent manner. On single stimulation of phrenic nerve 0.1 mM 4-AP induced repetitive e.p.p.s. Although 4-AP did not significantly affect compound action potentials evoked in the phrenic nerve, 4-AP caused repetitive firing to appear in phrenic nerve following single indirect stimulation of the nerve-muscle preparation. This repetitive firing was abolished by d-tubocurarine. In low Ca2+ (0.35 mM) Krebs solution 0.1 mM 4-AP induced muscle fasciculation accompanied by spontaneous giant e.p.p.s. and spontaneous muscle action potentials. In addition, 4-AP (0.1 mM) induced spontaneous repetitive firing of phrenic nerve axons; this was not inhibited by d-tubocurarine.